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Modification of food webs is a frequent cause of shifts in ecosystem states that resist reversal when the food web is restored to its original condition. We used the restoration of the large carnivore guild including gray wolves (Canis lupis), cougars (Felis concolor), and grizzly bears (Ursus arctos horribilis) to the northern range of Yellowstone National Park as a model system to understand how ecosystems might resist reconfiguration after the restoration of apex predators to the food web. The absence of wolves, cougars, and grizzly bears for nearly a century from the northern range was the primary cause of dramatic changes in riparian plant communities. Willows (Salix spp.) were suppressed in height by intense browsing by the dominant herbivore, elk (Cervus canadensis). The loss of activity by beavers (Castor canadensis) coincided with the loss of tall willows. We hypothesized that intense elk browsing interrupted the mutualism between willow and beavers: ecosystem engineering by beavers was a critical component of willow habitat and tall willows were a critical component of habitat for beavers. This interruption made riparian communities resilient to the disturbance caused by the restoration of apex predators. We hypothesized further that reductions in elk browsing attributable to reductions in elk population size were not sufficient to prevent the suppression of willow growth. To test these hypotheses, we conducted a 20‐year, factorial experiment that crossed simulated beaver dams with the exclusion of browsing. We found that willows grew to heights expected for restored communities only in the presence of dams and reduced browsing. Willows experiencing ambient conditions remained well below this expectation. We found no difference in heights or growth rates of willows in experimental controls and willows in 21 randomly chosen sites, confirming that the results of the experiment were representative of range‐wide conditions. A reorganized community of large herbivores was implicated in the suppression of willow growth. We conclude that the restoration of large carnivores to the food web failed to restore riparian plant communities on Yellowstone's northern range, supporting the hypothesis that this ecosystem is in an alternative stable state caused primarily by the extirpation of apex predators during the early 20th century. [ABSTRACT FROM AUTHOR]

Alpine ecosystems are under increasing pressure from land‐use change and road construction, resulting in habitat fragmentation and declines in biodiversity and ecosystem function. Ecosystem restoration can mitigate the impacts from human land‐use change by assisting the recovery of natural ecosystems. We developed an assessment framework using established international standards and monitored alpine vegetation recovery within restored zones along a hydropower‐dam road built in 2013 in the Knutshø landscape protected area, Norway. Using data collected in 2016, 2018 and 2021, we compared abiotic variables and vegetation indicators (cover, and community composition) in restored zones against directly adjacent intact zones within three different habitat types. After eight years, wetland habitat had similar cover of plant functional types and abiotic variables within restored and intact zones, while ridge and willow‐heath habitats had more cover of graminoids and less cover of shrubs in restored compared to intact zones. In addition, community composition in restored wetland habitat zones was similar to intact zones, while the community composition of restored zones in ridge and willow‐heath habitats was significantly different from adjacent intact zones. This suggests that restored wetland habitat in our study system is on a trajectory of recovery, but that recovery of other plant community types may require longer periods of time or, in some cases, may not be possible without further intervention because of alternative stable states. Our results underscore the need for long‐term monitoring and adaptive management to insure restoration success in alpine environments. [ABSTRACT FROM AUTHOR]

Background: Due to the effects of climate change and overgrazing in recent decades, alternative stable states in the alpine Kobresia meadow degradation process have coexisted in the same geographical and climatic environment, with variations occurring among microsites. Methods: We used a space‐for‐time substitution approach to explore the synergies of microsite variation according to its numerical characteristics and the proportion of each stable state at various stages of succession in alpine Kobresia meadows on the Qinghai–Tibetan Plateau. Results: (1) The highest average aboveground biomass in summer was 196.2 ± 20.3 g m−2, with significantly higher levels of biomass in ≤3.65 sheep unit ha−1 than in other levels of grazing intensity, while the parameters showed no significant differences among grazing intensity levels in >3.65 sheep unit ha−1. (2) The importance of plant functional groups, aboveground biomass, and niche breadth of Poaceae and Cyperaceae significantly decreased as the grazing intensity increased. (3) The effects of ≥0°C accumulated temperature, total precipitation, altitude, longitude, and latitude cumulatively contributed less than 20% of the variation in the distribution of functional group characteristics across microsites. Conclusions: (1) Overgrazing decreases primary production in alpine Kobresia meadows, but ecosystem responses regulate plant community structure and botanical components so as to partially counteract grazing disturbance. (2) Overgrazing changed the proportion of microsites, which in turn led to regime shift in the plant community and subsequent synergies between the microsites of plant communities and their stable states. [ABSTRACT FROM AUTHOR]

The extensive coastal forests in eastern southern Africa persist as a putative alternative stable state in an open ecosystem mosaic that includes grassland and savanna. We examine two current hypotheses of the state-transition origins of these forests: (1) facilitation—where light-demanding pioneer cohorts facilitate establishment by species that tolerate shade and whose functional traits align with forest; and (2) nucleation—where tree species of savanna origin establish on fire refugia in grassland, and where forest develops from the coalescence of these bush-clumps. We compared tree species diversity and composition on old-lands colonized by the savanna pioneer Vachellia kosiensis, with bush-clumps established on old-lands and on untransformed grassland, to determine their resemblance to intact coastal forest. The facilitation pathway in V. kosiensis woodland comprised impoverished tree assemblages differing markedly in richness, diversity, and composition from the nucleation pathway. By contrast, the bush-clump nucleation pathway comprised random assemblages of savanna species. Overlap in savanna tree composition between the bush-clump pathway and forest was notable. A suite of functional traits related to growth, reproduction and dispersal revealed that the life histories of most coastal forest tree species (~ 88%, n = 83 species) correspond with those expected from savanna. A combination of shade intolerance, multi-stemmed architecture, and growth form plasticity indicate species origins and persistence under frequent fire and variable light regimes typical of savanna environments. While these coastal woody formations satisfy the structural definition of forest, their constituents are clearly derived from savanna and questions the efficacy of their management as successional forest. [ABSTRACT FROM AUTHOR]

The cause of large areas of treeless Sedgeland and Scrub communities in western Tasmania, one of the wettest regions of Australia, has long puzzled ecologists, given the climatic suitability for temperate Eucalyptus and rainforests. A pervasive theory, known as the ecological drift model, is that landscape fires have created a dynamic mosaic of fire-adapted and fire-sensitive vegetation. A contrary view, known as the fire cycle model, asserts that fire patterns are a consequence, not a cause, of the mosaics, which are edaphically determined. We leveraged the opportunity presented by a large wildfire that occurred in a Sedgeland tract surrounded by Eucalyptus forest in the Huon Valley in 2019 to help discriminate between these competing models. Specifically, we sought to determine whether there was any evidence that the Sedgeland was becoming infilled with Scrub prior to the 2019 fire, and whether the fire caused the Scrub community to convert to Sedgeland. A field survey was used to assess the mortality of shrubs and their regeneration following the 2019 fire, and we used dendrochronology to determine the age of the fire-killed shrubs. We also used historical aerial photography since the 1980s to map fire scars and the distribution of Sedgeland and Scrub. We found that fire killed most shrubs in the Sedgeland and Scrub communities and initiated a cohort of shrub regeneration. Dendrochronological analysis of the fire-killed shrubs revealed that most were established approximately 40 years ago, following a fire that is apparent from aerial photography and most likely occurred around 1983. An analysis of aerial photography revealed that since 1980, the distribution of the Scrub community has remained stable, although the density of shrubs declined following the 1983 fire. The recovery of the burned Scrub areas in 1983 and the rapid regeneration of the shrubs following the 2019 fire is more consistent with the fire cycle model than the ecological drift model. These findings concord with the demonstrated stability of the Eucalyptus forest boundary at this site revealed by a separate study. The slow growth of the shrubs cautions against frequently burning Sedgelands, because it could cause the collapse of shrub populations by killing the immature cohort initiated by fire. [ABSTRACT FROM AUTHOR]

Forest removal for livestock grazing is a striking example of human‐caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience to the alternative grass state can greatly benefit managers when planning forest restoration. We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawaiʻi Island, USA. Since the 1980s, over 400,000 Acacia koa (koa) trees have been planted across degraded pasture, and invasive grasses still dominate the understory with no native woody‐plant recruitment. Between this koa/grass matrix are remnant native Metrosideros polymorpha (ʻōhiʻa) trees beneath which native woody plants naturally recruit. We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and ʻōhiʻa) with a range of native woody understory basal area (BA). We found a positive relationship between total seed rain (but not bird‐dispersed seed rain) and native woody BA and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (six levels) of two common understory species in plots under koa (n = 9) and remnant ʻōhiʻa (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities two to 75 times higher than naturally occurring. However, seedling recruitment increased two to three times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses is not necessary to achieve system responses: Even moderate reductions (~50%) can increase rates of native woody recruitment. The nonlinear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources. [ABSTRACT FROM AUTHOR]

Regime shifts from submersed macrophyte dominance to phytoplankton dominance have been widely reported in small- to medium-sized shallow lakes. However, alternative stable states in large shallow lakes (surface area >500 km2) remain unconfirmed. To understand the alternative stable states and the main influencing factors of submersed macrophytes in large lakes, the ecosystem states from monitoring data from 1959 to 2019 in large shallow Taihu Lake (2 338 km2 in average depth of 2.12 m) in China were examined. Changes in submersed macrophyte coverage (CMac) and phytoplankton chlorophyll a (Chl a) in the time series and their relationships with environmental factors were analyzed. During the field investigation from August 2018 to May 2019, nutrients and Chl a showed obvious heterogeneity across the lake, being generally higher in the western and northern areas and lower in the southeast area, while CMac was only observed in the eastern areas, e.g., East Taihu Lake, Xukou Bay, and Gonghu Bay. During the long-term monitoring from 1959 to 2019 in the Central Region, Meiliang Bay, and East Taihu Lake, Chl a increased significantly in the time series. CMac varied slightly among different subareas, always at low levels (<10%) in the Central Region and Meiliang Bay but at relatively high levels in East Taihu Lake (10%–90%). Frequency distributions of response variables had no multimodality except for CMac in East Taihu Lake, with two peaks between 15% and 20% and between 55% and 60%. A dual relationship was found between Chl a and total phosphorus (TP) in the areas with and without macrophytes, while CMac showed no relationship with TP, and submersed macrophytes did not flourish in the Central Region and Meiliang Bay even when TP was at very low levels (≈10 mg/m3). Taihu Lake had similar algal turbidity (TurbAlg) as small- to medium-sized lakes but generally presented with higher values of nonalgal turbidity (TurbNonAlg), as did their contribution to total turbidity as a percentage. This study suggested that large shallow Taihu Lake may have no alternative stable states, but more evidence is needed for East Taihu Lake, which was dominated by macrophytes, as it remains unknown whether hysteresis occurs between the processes of eutrophication and oligotrophication. Unfavorable conditions caused by wind might be the main reason due to the absence of submersed macrophytes in Taihu Lake. These results demonstrate that stricter nutrient control is needed to maintain a healthy state or to recover from a decayed state for large lakes. [ABSTRACT FROM AUTHOR]

Ecosystems with forest and grassland patches as alternative stable states usually contain various closed, semi‐open and open habitats, which may be aligned along a vegetation cover gradient. Taxonomic diversity usually peaks near the middle of the gradient, but our knowledge on functional and phylogenetic diversity trends along gradients is more limited.We investigated the eight main habitats of Hungarian forest‐grassland mosaics, representing various vegetation cover values, and compared their species composition as well as their taxonomic, functional and phylogenetic diversity.We found a compositional gradient ranging from large forest patches through smaller‐sized forest patches and edges to closed and open grasslands. Species richness peaked at the middle of the gradient (at edges). Shannon diversity was high near the middle and at the open end of the gradient. Functional diversity was high throughout woody habitats (in forests and at edges) and was significantly lower in grasslands. When considering all species, phylogenetic diversity tended to peak at north‐facing forest edges. When excluding non‐angiosperms, this peak disappeared.The high taxonomic diversity at the middle of the gradient is in line with the edge‐effect theory. Our results suggest that community assembly in grasslands may be dominated by environmental filtering, while competition may be decisive in woody habitats. The low phylogenetic diversity of grassland habitats can be explained by their young evolutionary age compared to forests.Synthesis. Functional and phylogenetic diversity do not necessarily coincide with taxonomic diversity along vegetation cover gradients. In ecosystems where forest and grassland patches represent alternative stable states, the trends of taxonomic diversity may be similar to those revealed here, but functional diversity patterns may be more system‐specific for some traits. Trends in phylogenetic diversity may vary according to the evolutionary age of the habitats. [ABSTRACT FROM AUTHOR]

Influenced by human activities and natural interference, the worldwide distribution of coastal wetlands is now undergoing rapid evolution. The prediction on the locations of vegetation conversion is greatly important for the management of these coastal ecosystems in terms of early warning. In this paper, a series of waterlines extracted from multiple satellite images were used to generate a high-precision digital elevation model (DEM) in the intertidal zone of the Liaohe estuary. Based on the characteristics of the alternative stable states in elevation and normalized difference vegetation index (NDVI), the Logistic Regression Model was applied to predict the potential locations of vegetation expansion with geomorphic factors, such as elevation, slope, and annual changing rate of elevation. Before the prediction, the existence of two stable states in the landscape was confirmed in the study area, i.e., low-lying tidal flats and high-lying saltmarshes. When the geomorphic parameters exceeded the thresholds, the stable state transition would occur. By using the Logistic Regression Model, the elevation was the best explainer for determining the vegetation conversion in the single-factor simulation, while the slope was the worst. When multiple factors were integrated in simulations, the prediction with the elevation, slope, and annual elevation change rate was the best, with R2 = 0.739, and the overall accuracy of prediction reached 88.6%. The prediction result indicated that the area of saltmarshes in the Liaohe estuary increased by 16.7 km2 at a rate of 0.8% per year between 2011 and 2015. When considering the popularization in restoration practice, it is necessary to evaluate the reliability and flexibility of the Logistic Regression Model in predicting vegetation conversion in more types of estuarine wetlands.

Anthropogenic impacts on ecosystems across spatiotemporal scales are expanding globally, undermining ecosystem resilience and increasing the risk of regime shifts within ecosystems. Governance incorporating social–ecological considerations has become essential. Here, we investigate two kinds of spatiotemporal multiscale impacts of ecotourism: "fast-acting and spatially-limited" impacts, and "slow-acting and large-scale" impacts. We showed that high levels of ecotourism impacts operating at multiple scales can generate an alternative stable state, and a potential simplification of our discussion resorting to the existence of multiple scales in the system. Our study provides insights into resilient ecotourism under the multiscale dynamics that can inform management decisions at appropriate scales. [ABSTRACT FROM AUTHOR]

Prey state and prey density mediate antipredator responses that can shift community structure and alter ecosystem processes. For example, well-nourished prey at low densities (i.e., prey with higher per capita predation risk) should respond strongly to predators. Although prey state and density often co-vary across habitats, it is unclear if prey responses to predator cues are habitat-specific. We used mesocosms to compare the habitat-specific responses of purple sea urchins (Strongylocentrotus purpuratus) to waterborne cues from predatory lobsters (Panulirus interruptus). We predicted that urchins from kelp forests (i.e., in well-nourished condition) tested at low densities typically observed in this habitat would respond more strongly to predation risk than barren urchins (i.e., in less nourished condition) tested at high densities typically observed in this habitat. Indeed, when tested at densities associated with respective habitats, urchins from forests, but not barrens, reduced kelp grazing by 69% when exposed to lobster risk cues. Barren urchins that were unresponsive to predator cues at natural, high densities suddenly responded strongly to lobster cues when conspecific densities were reduced. Strong responses of low densities of barren urchins persisted across feeding history (i.e. 0–64 days of starvation). This suggests that barren urchins can respond to predators but typically do not because of high conspecific densities. Because high densities of urchins in barrens should weaken the non-consumptive effects of lobsters, urchins in these habitats may continue to graze in the presence of predators thereby providing a feedback that maintains urchin barrens. [ABSTRACT FROM AUTHOR]

The cause of large areas of treeless Sedgeland and Scrub communities in western Tasmania, one of the wettest regions of Australia, has long puzzled ecologists, given the climatic suitability for temperate Eucalyptus and rainforests. A pervasive theory, known as the ecological drift model, is that landscape fires have created a dynamic mosaic of fire-adapted and fire-sensitive vegetation. A contrary view, known as the fire cycle model, asserts that fire patterns are a consequence, not a cause, of the mosaics, which are edaphically determined. We leveraged the opportunity presented by a large wildfire that occurred in a Sedgeland tract surrounded by Eucalyptus forest in the Huon Valley in 2019 to help discriminate between these competing models. Specifically, we sought to determine whether there was any evidence that the Sedgeland was becoming infilled with Scrub prior to the 2019 fire, and whether the fire caused the Scrub community to convert to Sedgeland. A field survey was used to assess the mortality of shrubs and their regeneration following the 2019 fire, and we used dendrochronology to determine the age of the fire-killed shrubs. We also used historical aerial photography since the 1980s to map fire scars and the distribution of Sedgeland and Scrub. We found that fire killed most shrubs in the Sedgeland and Scrub communities and initiated a cohort of shrub regeneration. Dendrochronological analysis of the fire-killed shrubs revealed that most were established approximately 40 years ago, following a fire that is apparent from aerial photography and most likely occurred around 1983. An analysis of aerial photography revealed that since 1980, the distribution of the Scrub community has remained stable, although the density of shrubs declined following the 1983 fire. The recovery of the burned Scrub areas in 1983 and the rapid regeneration of the shrubs following the 2019 fire is more consistent with the fire cycle model than the ecological drift model. These findings concord with the demonstrated stability of the Eucalyptus forest boundary at this site revealed by a separate study. The slow growth of the shrubs cautions against frequently burning Sedgelands, because it could cause the collapse of shrub populations by killing the immature cohort initiated by fire.

Fire in tropical forests jeopardizes natural regeneration and can lead to the establishment of an alternative stable state. To address the effects of fire on natural regeneration in a lowland tropical forest, we performed a time series study over 12 years, focusing on the impact of fire frequency and post-fire regeneration time on vegetation structure, diversity, and functional composition in secondary forests. The study was conducted in the Brazilian Atlantic Forest at sites subjected to different fire frequencies (never, once, three, and four times over 44 years) and old-growth forests. During 12 years of monitoring time, no significant difference was observed in community structure and diversity among sites subjected to fire, which suggests an arrested succession pattern and corroborates with the establishment of an alternative stable state. Functional traits associated with fire resistance, including wood density and bark thickness, were higher in areas with fire history compared to old-growth forests. Therefore, our results showed that recurrent fires can lead communities toward an arrested succession pattern with a high divergence in community structure and functional composition compared to old-growth forest. These results have several practical implications for restoration and conservation programs in the Brazilian Atlantic Forest. [ABSTRACT FROM AUTHOR]

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The factors that determine why ecosystems exhibit abrupt shifts in state are of paramount importance for management, conservation, and restoration efforts. Kelp forests are emblematic of such abruptly shifting ecosystems, transitioning from kelp-dominated to urchin-dominated states around the world with increasing frequency, yet the underlying processes and mechanisms that control their dynamics remain unclear. Here, we analyze four decades of data from biannual monitoring around San Nicolas Island, CA, to show that substrate complexity controls both the number of possible (alternative) states and the velocity with which shifts between states occur. The superposition of community dynamics with reconstructions of system stability landscapes reveals that shifts between alternative states at low-complexity sites reflect abrupt, high-velocity events initiated by pulse perturbations that rapidly propel species across dynamically unstable state–space. In contrast, high-complexity sites exhibit a single state of resilient kelp–urchin coexistence. Our analyses suggest that substrate complexity influences both top-down and bottom-up regulatory processes in kelp forests, highlight its influence on kelp-forest stability at both large (island-wide) and small (<10 m) spatial scales, and could be valuable for holistic kelp-forest management. [ABSTRACT FROM AUTHOR]

Multiple stressors such as climate change and eutrophication are responsible for the global decline in macrophytes in lakes. The loss of this key component can result in turbid conditions and a loss of important ecosystem functions and services, particularly in shallow lakes. Benthivorous fish, which can increase in abundance during eutrophication, can adversely affect macrophytes through physical disturbance, cascading effects on turbidity, suspended and attached algae (phytoplankton and periphyton) and direct consumption. However, whether warming amplifies their effects on macrophytes and can trigger regime shifts remains unexplored.Here, we tested the single and combined effects of warmer water (+4.5°C) and the widespread benthivorous juvenile common carp Cyprinus carpio on two different types of aquatic macrophytes in 24 mesocosms (2,500 L each). We monitored phytoplankton, periphyton, turbidity and the abundance of the submerged curly leafed pondweed Potamogeton crispus and the floating‐leaved water chestnut Trapa bispinosa during their growing season. These species dominated successively in spring and summer.Warming alone advanced the growing season of P. crispus by 17 days. Juvenile carp decreased the abundance of the more palatable P. crispus, but promoted the abundance of T. bispinosa, supporting an ecosystem shift to a dominance of floating‐leaved macrophytes. Fish also substantially increased water turbidity and the biomass of phytoplankton and periphyton. Warming amplified juvenile carp effects on turbidity and submerged macrophytes, but also decreased the abundance of floating‐leaved macrophytes leading to an overall macrophyte decline and increase in water turbidity.Synthesis and applications. Our study provides the first experimental evidence for a warming‐induced regime shift from clear‐water conditions dominated by submerged or floating/floating‐leaved macrophytes to a turbid state in shallow aquatic ecosystems. The regime shift was triggered by the impacts of warming on benthivorous fish (juvenile common carp) rather than on macrophytes. Lowering nutrient loading and other measures to reduce the abundance of benthivorous fish (e.g. fish removal and piscivorous fish restocking) thus may become increasingly important for the management of shallow lakes under global climate change. [ABSTRACT FROM AUTHOR]

Following a long-term herbivore-exclusion study (2003-2008) in the mid-eulittoral zone at Kalk Bay, Cape Town, South Africa, the post-herbivore-exclusion effect (12 years later) of the limpet Cymbula oculus, the dominant herbivore in the community, was examined after it had recolonised the area. Despite the limpet having recruited into all plots (previous exclusion plots and control plots), the species richness, percentage cover of sessile organisms, invertebrate densities and invertebrate biomass were all significantly higher in previous exclusion plots compared with in control plots. Cymbula oculus density was higher in previous exclusion plots (11.11 [SD 12.54] ind. m−2) than in control plots (9.26 [SD 7.97] ind. m−2) (p = 0.038); however, its biomass was similar between the previous exclusion plots and control plots (366.39 [SD 421.63] vs 367.96 [SD 285.44] g m−2, p = 0.179), indicating that limpets outside exclusion plots were generally larger. This was likely because of younger C. oculus individuals, heightened interspecific competition, or insufficient space in previous exclusion plots. The observed differences might be attributed to a combination of factors, including the persistence of species that have achieved a size refuge from grazing and bulldozing by C. oculus, as well as the establishment of invasive mussels Mytilus galloprovincialis which offer secondary substrates and microhabitats for the establishment of macroalgae and other invertebrates. The recruitment of M. galloprovincialis into the previous exclusion plots is likely to be the cause of the alternative community state observed. Unless some extreme disturbance event occurs, the community from which C. oculus was previously excluded is unlikely to revert to its pre-C. oculus exclusion state. [ABSTRACT FROM AUTHOR]

Questions: Species defined as ecosystem engineers (e.g. elephant) are able to strongly shape their habitat. In African savannas, elephants have often been shown to reduce woody‐plant abundance and diversity. However, recent studies highlight more complex elephant‐induced effects on vegetation. Here, we assessed if long‐term high elephant densities (>2 km−2) in a large open landscape resulted in the depletion of savanna woodland woody communities or if it led to a new alternative equilibrium. Location: Woodland savanna of Hwange National Park, Zimbabwe. Elephant densities at the study site have remained high for the past two decades (>2 km−²). Methods: We measured long‐term (>15 years) elephant utilisation of woody‐plant communities and their effects on vegetation structure, species composition and functional traits (e.g. N leaf concentration, specific leaf area) in 12 vegetation plots. Results: We observed opportunistic foraging behaviour by elephants with only a slight temporal shift in species composition, mainly explained by changes in rare species. Further, we did not observe any modification in mean functional trait values, overall height, and stem diameters of the woody‐plant communities. However, we found differential changes in woody‐plant abundance according to the height layer (decrease in the number of tall plants [>200 cm] and increase in the number of short plants [<50 cm]) and a strong reduction in crown diameter for plants in the 50–200 cm height class. Conclusion: Our study strongly suggests that long‐term high elephant densities have led to a stable state in savanna woodland vegetation in terms of plant community composition and their functional traits. However, high elephant densities did affect vegetation structure, which would have several important indirect effects on this ecosystem (e.g. predator–prey interactions). We hope that this study stimulates more work on the long‐term effects of ecosystem engineers in large and open ecosystems. [ABSTRACT FROM AUTHOR]

Biomes of the world have long been assumed to be determined by climate. Major disparities, where open low biomass systems occurred in the same climate zone as closed forests have been dismissed as products of deforestation. Many of these open ecosystems of the world, have been shown to be of ancient origins, stable alternatives to forests, and typically maintained by disturbance regimes. Open ecosystems include some of the most biodiverse regions in the world. They are often consumer-controlled by large mammal herbivores or fire. Mosaics of closed forest and open ecosystems have been interpreted as alternative stable states with each state maintained by positive feedbacks to environmental conditions that maintain that state. For example, flammable grasses maintain fires which consume woody plants, while closed forests exclude flammable grasses by shading them out. Understanding open ecosystems may therefore require some radical revision of familiar ecological concepts, starting with the hypothesis that climate largely determines world vegetation patterns. Open ecosystems function differently from forests in an earth system context affecting the hydrological cycle, rates of rock weathering, and presenting a different planetary surface to solar radiation reaching the earth's land surface. Open ecosystems require explicit attention in conservation policy and management. [ABSTRACT FROM AUTHOR]

Ungulate herbivory is a key natural disturbance in many ecosystems. In forest ecosystems, ungulate herbivory often co‐occurs with other disturbances yet there are few studies looking at the cumulative impacts of ungulates and other natural disturbances on forest dynamics.We report on an 18‐year experiment to investigate the combined effects of introduced, non‐native moose and native spruce budworm herbivory on balsam fir forest recruitment and regeneration. Specifically, we measured inputs into early life‐history components of balsam fir dominant boreal forest ecosystems such as seed density, seedbed quality, seedling and sapling height and density, as well as adult density in eight paired moose exclosure and control sites across a spruce budworm disturbance gradient (i.e. no outbreak, recent outbreak, old outbreak) that resulted in closed, partially open and open canopies.Piecewise structural equation models (SEMs) revealed a strong negative effect of spruce budworm disturbance on balsam fir adult density and indirect effects of this budworm disturbance on balsam fir seedling and sapling density mediated through adult density. We observed a similar pattern of budworm disturbance effects on balsam fir seed density with the lowest densities in sites with open canopy, then partially open and closed canopies. As expected, the SEM showed a positive effect of bryophyte seedbed cover on balsam fir seedling density. The SEM showed support for a positive effect of moose exclusion on balsam fir sapling height and density. Specifically, saplings were on average taller and denser in exclosure than control plots and these differences are most pronounced at sites with partially open canopies created by spruce budworm disturbance and undisturbed or closed canopy sites. Overall, we show evidence for cumulative and interactive effects of spruce budworm and moose herbivory on all life stages of balsam fir and such effects are negatively affecting regeneration of balsam fir forests.Human activities are creating conditions that facilitate the expansion and increased impacts of herbivores on boreal ecosystem functioning. A greater understanding of how ungulate herbivory interacts with other disturbances is needed to improve our predictions of forest ecosystem dynamics under global change. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Macroalgal forests play a key role in shallow temperate rocky reefs worldwide, supporting communities with high productivity and providing several ecosystem services. Sea urchin grazing has been increasingly influencing spatial and temporal variation in algae distributions and it has become the main cause for the loss of these habitats in many coastal areas, causing a phase shift from macroalgae habitats to barren grounds. The low productive barrens often establish as alternative stable states and only a major reduction in sea urchin density can trigger the recovery of macroalgal forests. The present study aims to assess if the 2018 disease outbreak, responsible for a strong reduction in the sea urchin Diadema africanum densities in Madeira Island, was able to trigger a reverse shift from barren grounds into macroalgae-dominated state. By assessing the diversity and abundance of benthic sessile organisms, macroinvertebrates and fishes before, during and after that particular mass mortality event, we evaluate changes in benthic assemblages and relate them to variations in grazer and herbivore densities. Our results revealed a clear shift from barren state to a macroalgae habitat, with barrens characterized by bare substrate, sessile invertebrate and Crustose Coralline Algae (CCA) disappearing after the mortality event. Overall variations in benthic assemblages was best explained by four taxa (among grazers and herbivores species). However, it was the 2018 demise of D. africanum and its density reduction that most contributed to the reverse shift from a long stable barren state to a richer benthic assemblage with higher abundance of macroalgae. Despite this recent increase in macroalgae dominated habitats, their stability and persistence in Madeira Island is fragile, since it was triggered by an unpredictable disease outbreak and depends on how D. africanum populations will recover. With no control mechanisms, local urchin populations can easily reach the tipping point needed to promote a new shift into barren states. New conservation measures and active restoration are likely required to maintain and promote the local stability of macroalgal forests.

Abstract Predation can affect prey behavior, demography, abundance, and distribution, particularly in lentic freshwater ecosystems. Fish are predators known to reduce the abundance of their prey and to restrict the distribution of species. Using time series which spanned 43 and 22 yr, respectively, we analyzed the effect of a change in the fish predator community on the dynamics of two pond‐breeding amphibian populations (Rana temporaria and Rana dalmatina). Specifically, we used a state‐space time series model which allows for density dependence and observation error, to ask whether the change in predation risk affects population growth rate and the return point around which the populations fluctuate. The results showed that the type of observation error assumed did not affect the biological parameters. We found evidence for density dependence in both populations. The effect of the change in fish predation on population growth rate and the return point was strong in the population where fish invaded a previously fish‐free pond. The effect was weaker in the population where the change was from cyprinid fish to pike. The results showed that fish predation can have strong effects on amphibian population dynamics. The observed population dynamical pattern is phenomenologically similar to alternative stable states.

Predation can affect prey behavior, demography, abundance, and distribution, particularly in lentic freshwater ecosystems. Fish are predators known to reduce the abundance of their prey and to restrict the distribution of species. Using time series which spanned 43 and 22 yr, respectively, we analyzed the effect of a change in the fish predator community on the dynamics of two pond‐breeding amphibian populations (Rana temporaria and Rana dalmatina). Specifically, we used a state‐space time series model which allows for density dependence and observation error, to ask whether the change in predation risk affects population growth rate and the return point around which the populations fluctuate. The results showed that the type of observation error assumed did not affect the biological parameters. We found evidence for density dependence in both populations. The effect of the change in fish predation on population growth rate and the return point was strong in the population where fish invaded a previously fish‐free pond. The effect was weaker in the population where the change was from cyprinid fish to pike. The results showed that fish predation can have strong effects on amphibian population dynamics. The observed population dynamical pattern is phenomenologically similar to alternative stable states. [ABSTRACT FROM AUTHOR]

Abstract In recent decades, conservation objectives have driven changes to the management of some pine forests in the southeastern United States. Forest thinning and frequent burning of old‐field and plantation pine forests have resulted in an open loblolly–shortleaf pine forest community which resembles the original longleaf pine forest. It is, however, unclear how the structure, composition, and function of the loblolly–shortleaf forest compare to natural longleaf pine forest, and whether it represents an alternative stable state, or simply a transitional state. Understanding the stability of open loblolly–shortleaf pine forest is critical, particularly because several threatened and endangered species are now reliant on it for habitat. The structure and composition of loblolly–shortleaf forest and natural longleaf pine forest were compared using data from permanent forest plots at Fort Benning, Georgia, USA. To assess the stability of the loblolly–shortleaf pine forest and determine whether it is an alternative stable or transitional state, the LANDIS‐II forest landscape simulation model was used to simulate changes in forest type cover under no disturbance, and a frequent‐fire regime at Fort Benning. Under both management scenarios, nearly all loblolly–shortleaf pine forest converted to mixed hardwood forest over the course of the simulation, with most conversion occurring within 60 yr. In contrast, longleaf pine forest cover increased under frequent fire. Several important structural and compositional differences may have contributed to the instability of loblolly–shortleaf pine forest compared to longleaf pine forest. These include, among other factors, higher densities of resprouting hardwood trees and shrubs in loblolly–shortleaf pine forest, including sweetgum, a resilient broadleaf species capable of transforming ecosystem structure. These results highlight the instability of the open loblolly–shortleaf pine forest community and confirm that is a transitional state, destined for mixed hardwood forest in the coming decades under either no disturbance or frequent fire alone. Future forest planning should consider an active transition from the loblolly–shortleaf pine forest in the coming decades if open pine forest is to be conserved for wildlife and conservation objectives.

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In recent decades, conservation objectives have driven changes to the management of some pine forests in the southeastern United States. Forest thinning and frequent burning of old-field and plantation pine forests have resulted in an open loblolly-shortleaf pine forest community which resembles the original longleaf pine forest. It is, however, unclear how the structure, composition, and function of the loblolly-shortleaf forest compare to natural longleaf pine forest, and whether it represents an alternative stable state, or simply a transitional state. Understanding the stability of open loblolly-shortleaf pine forest is critical, particularly because several threatened and endangered species are now reliant on it for habitat. The structure and composition of loblolly-shortleaf forest and natural longleaf pine forest were compared using data from permanent forest plots at Fort Benning, Georgia, USA. To assess the stability of the loblolly-shortleaf pine forest and determine whether it is an alternative stable or transitional state, the LANDIS-II forest landscape simulation model was used to simulate changes in forest type cover under no disturbance, and a frequent-fire regime at Fort Benning. Under both management scenarios, nearly all loblolly-shortleaf pine forest converted to mixed hardwood forest over the course of the simulation, with most conversion occurring within 60 yr. In contrast, longleaf pine forest cover increased under frequent fire. Several important structural and compositional differences may have contributed to the instability of loblolly-shortleaf pine forest compared to longleaf pine forest. These include, among other factors, higher densities of resprouting hardwood trees and shrubs in loblolly-shortleaf pine forest, including sweetgum, a resilient broadleaf species capable of transforming ecosystem structure. These results highlight the instability of the open loblolly-shortleaf pine forest community and confirm that is a transitional state, destined for mixed hardwood forest in the coming decades under either no disturbance or frequent fire alone. Future forest planning should consider an active transition from the loblolly-shortleaf pine forest in the coming decades if open pine forest is to be conserved for wildlife and conservation objectives. [ABSTRACT FROM AUTHOR]

Small bowel transplants provide an exceptional opportunity for long-term study of the microbial ecology of the human small bowel. The ileostomy created at time of transplant for ongoing monitoring of the allograft provides access to samples of ileal effluent and mucosal biopsies. In this study, we used qPCR to assay the bacterial population of the small bowel lumen of 17 small bowel transplant patients over time. Surprisingly, the posttransplant microbial community was found to be dominated by Lactobacilli and Enterobacteria, both typically facultative anaerobes. This represents an inversion of the normal community that is dominated instead by the strictly anaerobic Bacteroides and Clostridia. We found this inverted community also in patients with ileostomies who did not receive a transplant, suggesting that the ileostomy itself is the primary ecological determinant shaping the microbiota. After surgical closure of the ileostomy, the community reverted to the normal structure. Therefore, we hypothesized that the ileostomy allows oxygen into the otherwise anaerobic distal ileum, thus driving the transition from one microbial community structure to another. Supporting this hypothesis, metabolomic profiling of both communities demonstrated an enrichment for metabolites associated with aerobic respiration in samples from patients with open ileostomies. Viewed from an ecological perspective, the two communities constitute alternative stable states of the human ileum. That the small bowel appears to function normally despite these dramatic shifts suggests that its ecological resilience is greater than previously realized.

Ecological shifts, between a clear macrophyte-dominated state and a turbid state dominated by phytoplankton and high inorganic suspended solids, have been well described in shallow lake ecosystems. While few documented examples exist in rivers, models predict regime shifts, especially in regulated rivers with high water retention time. Here I quantified ecological shifts in a large, semi-regulated floodplain river during a transition from a turbid- to a clear-water state using water quality, aquatic vegetation and fisheries data from a rigorous, standardized long-term data set. My findings indicate that significant changes occurred in total suspended solids concentration, aquatic macrophyte abundance, native and non-native fish biomass, fish functional feeding guild patterns, fish habitat guild assemblages and fish spawning guild assemblage patterns over a nearly 20-year period in Navigation Pool 8 of the Upper Mississippi River. Transitions in physical and biological indicators were examined to identify mechanisms underlying the ecological shifts. Environmental variables driving fish assemblage changes were identified (total suspended solids and aquatic vegetation) and management-relevant thresholds are presented. Awareness of management thresholds is critical for resource managers to implement measures to prevent the river from moving to a degraded state characterized by high non-native fish abundance and low predatory fish species abundance.

Future changes in climate are widely anticipated to increase fire frequency, particularly in boreal forests where extreme warming is expected to occur. Feedbacks between vegetation and fire may modify the direct effects of warming on fire activity and shape ecological responses to changing fire frequency. We investigate these interactions using extensive field data from the Boreal Shield of Saskatchewan, Canada, a region where >40% of the forest has burned in the past 30 years. We use geospatial and field data to assess the resistance and resilience of eight common vegetation states to frequent fire by quantifying the occurrence of short‐interval fires and their effect on recovery to a similar vegetation state. These empirical relationships are combined with data from published literature to parameterize a spatially explicit, state‐and‐transition simulation model of fire and forest succession. We use this model to ask if and how: (a) feedbacks between vegetation and wildfire may modify fire activity on the landscape, and (b) more frequent fire may affect landscape forest composition and age structure. Both field and GIS data suggest the probability of fire is low in the initial decades after fire, supporting the hypothesis that fuel accumulation may exert a negative feedback on fire frequency. Field observations of pre‐ and postfire composition indicate that switches in forest state are more likely in conifer stands that burn at a young age, supporting the hypothesis that resilience is lower in immature stands. Stands dominated by deciduous trees or jack pine were generally resilient to fire, while mixed conifer and well‐drained spruce forests were less resilient. However, simulation modeling suggests increased fire activity may result in large changes in forest age structure and composition, despite the feedbacks between vegetation–fire likely to occur with increased fire activity. Here, we study fire and forest dynamics in Saskatchewan's Boreal Shield to better understand the resilience and resistance of North American boreal forests to projected increases in fire activity. Both field and GIS data show evidence for self‐regulation of fire, where young, recently burned forest stands are more resistant to fire than older stands, and immaturity risk, where short‐interval fires reduce forest resilience to fire. However, simulation modeling suggests increased fire activity may result in large changes in forest age structure and composition, despite the feedbacks between vegetation–fire likely to occur with increased fire activity. [ABSTRACT FROM AUTHOR]

Globally, many shallow lakes have shifted from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state due to eutrophication. Such shifts are often accompanied by toxic cyanobacterial blooms, with specialized traits including buoyancy regulation and nitrogen fixation. Previous work has focused on how these traits contribute to cyanobacterial competitiveness. Yet, little is known on how these traits affect the value of nutrient loading thresholds of shallow lakes. These thresholds are defined as the nutrient loading at which lakes shift water quality state. Here, we used a modelling approach to estimate the effects of traits on nutrient loading thresholds. We incorporated cyanobacterial traits in the process-based ecosystem model PCLake+, known for its ability to determine nutrient loading thresholds. Four scenarios were simulated, including cyanobacteria without traits, with buoyancy regulation, with nitrogen fixation, and with both traits. Nutrient loading thresholds were obtained under N-limited, P-limited, and colimited conditions. Results show that cyanobacterial traits can impede lake restoration actions aimed at removing cyanobacterial blooms via nutrient loading reduction. However, these traits hardly affect the nutrient loading thresholds for clear lakes experiencing eutrophication. Our results provide references for nutrient loading thresholds and draw attention to cyanobacterial traits during the remediation of eutrophic water bodies.

Regime shifts, i.e., large, persistent, and usually unexpected changes in ecosystems and social-ecological systems, can have major impacts on ecosystem services, and consequently, on human well-being. However, the vulnerability of different regions to various regime shifts is largely unknown because evidence for the existence of regime shifts in different ecosystems and parts of the world is scattered and highly uneven. Furthermore, research tends to focus on individual regime shifts rather than comparisons across regime shifts, limiting the potential for identifying common drivers that could reduce the risk of multiple regime shifts simultaneously. Here, we introduce the Regime Shifts Database, an open-access database that systematically synthesizes information on social-ecological regime shifts across a wide range of systems using a consistent, comparative framework, providing a wide-ranging information resource for environmental planning, assessment, research, and teaching initiatives. The database currently contains 28 generic types of regime shifts and > 300 specific case studies. Each entry provides a literature-based synthesis of the key drivers and feedbacks underlying the regime shift, as well as impacts on ecosystem services and human well-being, and possible management options. Across the 28 regime shifts, climate change and agriculture-related activities are the most prominent among a wide range of drivers. Biodiversity, fisheries, and aquatic ecosystems are particularly widely affected, as are key aspects of human well-being, including livelihoods, food and nutrition, and an array of cultural ecosystem services. We hope that the database will stimulate further research and teaching on regime shifts that can inform policy and practice and ultimately enhance our collective ability to manage and govern large, abrupt, systemic changes in the Anthropocene. [ABSTRACT FROM AUTHOR]

Abstract: In the context of ongoing climatic warming, forest landscapes face increasing risk of conversion to non‐forest vegetation through alteration of their fire regimes and their post‐fire recovery dynamics. However, this pressure could be amplified or dampened, depending on how fire‐driven changes to vegetation feed back to alter the extent or behaviour of subsequent fires. Here we develop a mathematical model to formalize understanding of how fire–vegetation feedbacks and the time to forest recovery following high‐severity (i.e. stand‐replacing) fire affect the extent and stability of forest cover across landscapes facing altered fire regimes. We evaluate responses to increasing burn rates while varying the direction (negative vs. positive) of fire–vegetation feedbacks under a continuum of values for feedback strength and post‐fire recovery time. In doing so, we determine how interactions among these variables produce thresholds and tipping points in landscape responses to changing fire regimes. Where the early‐seral vegetation was less fire‐prone than older forests, negative feedbacks limited the reductions in forest cover in response to higher fire frequency or slower forest recovery. By contrast, positive feedbacks (more flammable early‐seral vegetation) produced a tipping point beyond which increases in burn rates or a slowing of forest recovery drove extensive forest loss. With negative feedbacks, the rates of forest loss and expansion in response to variation in fire frequency were similar. However, where feedbacks were positive, the conversion from predominantly forested to non‐forested conditions in response to increasing fire frequency was faster than the re‐expansion of forest cover following a return to the initial burn rate. Strengthening the positive feedbacks increased this asymmetry. Synthesis. Our analyses elucidate how fire–vegetation feedbacks and post‐fire recovery rates interact to affect the trajectories and rates of landscape response to altered fire regimes. We illustrate the vulnerability of ecosystems with positive fire–vegetation feedbacks to climate change‐driven increases in fire activity, especially where post‐fire recovery is slow. Although negative feedbacks initially provide resistance to forest loss with increasing burn rates, this resistance is eventually overwhelmed with sufficient increases to burn rates relative to recovery times. [ABSTRACT FROM AUTHOR]

Abstract: Climate change indirectly affects forest ecosystems through changes in the frequency, size, and/or severity of wildfires. In addition to its direct effects prior to fire, climate also influences immediate postfire recruitment, with consequences for future vegetation structure and fire activity. A major uncertainty, therefore, is if, when and where vegetation shifts will occur. With an emphasis on species traits, we use a demographic framework to examine how the interaction of changing climate and fire will affect postfire woody vegetation recruitment and the likelihood of vegetation shifts. Each demographic stage – adult mortality, propagule availability, seed germination, seedling establishment, and seedling survival – serves as a filter through which a species must pass for establishment and recovery to occur. We apply this framework to case studies in western North American forests, including boreal and southwestern U.S. Pinus ponderosa forests, to help understand the mechanisms behind recent postfire vegetation changes. The case studies highlight how changes in climate and fire properties will make it increasingly difficult for some species to pass through each demographic filter in the future. As climate warming continues, we expect increased dominance of species that resprout following fire, maintain canopy or soil seed banks, have long distance seed dispersal, produce drought‐tolerant seedlings, and/or reach reproductive maturity quickly. The persistence of postfire vegetation shifts will depend the on ratio of recovery time to disturbance interval(s). An advantage of the demographic‐filter framework is that it places emphasis on mechanisms, thus improving our ability to anticipate future vegetation shifts. As such, it highlights the clear need for more mechanistic studies of postfire recruitment to disentangle the relative effects of multiple drivers in postfire environments. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Abstract: Hysteresis linked to alternative stable states may explain delays in water quality recovery despite reduced nutrient loadings in shallow lakes. Because deep lakes are assumed to be less prone to critical transitions, similar delays are attributed to the confounding effects of additional environmental disturbances, such as climate warming. Herein, we hypothesised that the lack of evidence of nutrient‐driven alternative stable states in a deep lake arises from the fact that the nutrient threshold that causes the critical transition is lower than the nutrient threshold in shallow lakes. Thereby, it might have been crossed much earlier in the lake history. To test this hypothesis, we focused on the palaeo‐ecological trajectory of Lake Varese, which is a deep, hypereutrophicated peri‐alpine lake undergoing restoration. Proxies for drivers of ecological state (i.e. total phosphorus—TP—through diatoms and pigments) and ecological responses (Cladocera), as well as a repeatable analysis, were used to identify transitions and to distinguish hysteretic delays from those of the ecosystems responding to additional constraints over the past century. Our results suggest spatial heterogeneity in the ecological response. The littoral habitats changed abruptly and prematurely for a low TP threshold, causing a shift that met many criteria of a flickering‐type critical transition. Soon after the littoral shift, a striking increase in the lake phosphorous concentration was recorded and drove the pelagic assemblage towards a new state. This transition was abrupt, and the pelagic communities exhibited limited evidence of recovery; however, we found no evidence of hysteresis. Therefore, the modern ecological trajectory of the pelagic communities is currently driven by climate warming. This detailed analysis allowed us to go beyond the general pattern that links ecological responses to drivers and suggest that a nonlinear transition following eutrophication can take place in a deep lake synchronously with linear transitions. Instead of triggering a new regime shift, climate warming, to which pelagic habitats are more sensitive than littoral ones, has driven the lake further from its safe operating space. [ABSTRACT FROM AUTHOR]

The Wetland Continuum is a conceptual framework that facilitates the interpretation of biological studies of wetland ecosystems. Recently summarized evidence documenting how a multi-decadal wet period has influenced aspects of wetland, lake and stream systems in the southern prairie-pothole region of North America has revealed the potential for wetlands to shift among alternate states. We propose that incorporation of state shifts into the Wetland Continuum, as originally proposed or as modified by Hayashi et al., is a relatively simple matter if one allows for shifts of wetlands along the horizontal, groundwater axis of the framework under conditions of extreme and sustained wet or dry conditions. We suggest that the ease by which state shifts can be accommodated within both the original and modified frameworks of the Wetland Continuum is a testament to the robustness of the concept when it is related to the alternative-stable-state concept. [ABSTRACT FROM AUTHOR]