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Spatial dynamic theories such as source-sink models frequently describe habitat-specific demographies, yet there are surprisingly few field studies that have examined how and why interacting species vary in their dynamics across multiple habitat types. We studied the spatial pattern of interaction between a chewing herbivore and its primary larval host plant in two habitat types. We found that the interaction between an arctiid caterpillar (Platyprepia virginalis) and its host (Lupinus arboreus) differed in wet vs. upland dry habitats, as did yearly population dynamics for the caterpillar. In upland sites, there was a strong positive relationship between lupine cover and the abundance of caterpillars although this relationship was not apparent in wet sites. Additionally, in wet sites, caterpillar populations were larger and less variable across years. Caterpillars appeared to exhibit source-sink dynamics, with the time-averaged finite growth rate λ > 1 in wet sites (sources), λ <1 in upland dry sites (sinks), and predominant source-to-sink movement of late-instar caterpillars. Populations in upland dry sites also went locally extinct in years of low regional abundance. Emigration from wet sites could potentially explain the lack of coupling of herbivore and host plant dynamics in these sites. These results indicate that movement and other factors affecting demography are habitat-specific and have important implications for trophic control. Acknowledging such complexity makes simple models of trophic control seem overly general but may allow us to formulate more broadly applicable ecological models. [ABSTRACT FROM AUTHOR]

Understanding positive interactions among species is integral to predicting the outcomes of community-level dynamics. Few studies have explored intraspecific variation in positive associations. We investigated facilitation between two coastal dune shrubs in California, Baccharis pilularis and the nitrogen-fixing species, Lupinus arboreus . L. arboreus seedlings and adults were non-randomly distributed, occurring most frequently in association with B. pilularis . The relationship depended on the genotype of B. pilularis ; only the prostrate architectural form of this species was positively associated with L. arboreus . To evaluate the nature of this association, we assessed the influence of three microhabitat types (prostrate B. pilularis , Ammophila arenaria (an introduced dune grass), and exposed sand) on four life history stages of L. arboreus . Prostrate B. pilularis benefited L. arboreus seedling emergence, survival, and growth but had no effect on post-dispersal seed predation or adult establishment. Enhanced resistances to granivory by rodents or to seedling predation by voles were not important mechanisms underlying facilitation. In addition, an experiment manipulating herbivory by cutworms found no effects of microhabitat on susceptibility to damage. Evidence supports the hypothesis that the prostrate form of B. pilularis facilitates L. arboreus germination, seedling growth, and establishment as compared to exposed sand and dune grass. Furthermore, in combination with prior research, the results suggest that by facilitating an important nitrogen-fixer, B. pilularis may have effects that cascade to other members of the coastal plant community. [ABSTRACT FROM AUTHOR]

Insect herbivory is common, but the conditions under which it reduces the fitness and population size of plants remain poorly understood. We quantified populationlevel impacts of floral herbivory by specialized insects on cobweb thistle (Cirsium occidentale var. occidentale) in a California coastal dune ecosystem, and then compared these demographic effects to those published for Platte thistle (Cirsium canescens) in similarly designed exclusion experiments in the continental sand dune ecosystem in the Great Plains. As a separate test of the strength of the seed-to-seedling linkage, we quantified seedling establishment rates in seed addition plots and compared these results to those obtained in herbivore exclusion experiments. This is the first test of direct vs. indirect methods of evaluating the potential impact of seed loss in plant dynamics. Floral herbivory on cobweb thistle in coastal dunes substantially reduced key components of plant fitness. Reduction of insect feeding within flower heads with insecticide increased mean per capita seed production by 144-316% and led to a 130-196% increase in cumulative seedling recruitment in the next generation, depending on dune habitat. Juvenile plant densities around insecticide-treated plants subsequently increased by >50%. Both seed and flower head addition experiments corroborated the seed limitation of recruitment and juvenile plant establishment for cobweb thistle; exclusion of postdispersal seed predators did not substantially alter this relationship. While seed addition results were qualitatively similar to herbivore exclusion results, seed addition overestimated the seed-seedling transition. These results are broadly parallel to those found previously for Platte thistle in continental dunes, where reduction of insect feeding in Platte thistle flower heads increased mean per capita seed production by 37-240%, depending on year and microhabitat. This reduction in seed translated into a 33-580% increase in... [ABSTRACT FROM AUTHOR]

The article presents the study on the restoration of enriched grasslands in California. It states that the mowing and removal of biomass resulted in changes in species assemblage. It notes that species richness was greater in treated plots compared to control plots. It mentions that the removal of plant biomass through mowing shifted plant community to perennial forb assemblage.

Exotic plant invasions are thought to alter productivity and species richness, yet these patterns are typically correlative. Few studies have experimentally invaded sites and asked how addition of novel species influences ecosystem function and community structure and examined the role of competitors and/or consumers in mediating these patterns. We invaded disturbed and undisturbed subplots in and out of rodent exclosures with seeds of native or exotic species in grasslands in Montana, California and Germany. Seed addition enhanced aboveground biomass and species richness compared with no-seeds-added controls, with exotics having disproportionate effects on productivity compared with natives. Disturbance enhanced the effects of seed addition on productivity and species richness, whereas rodents reduced productivity, but only in Germany and California. Our results demonstrate that experimental introduction of novel species can alter ecosystem function and community structure, but that local filters such as competition and herbivory influence the magnitude of these impacts., (© 2014 John Wiley & Sons Ltd/CNRS.)

Legacy effects occur when particular species or their interactions with others have long-lasting impacts, and they are increasingly recognized as important determinants of ecological processes. However, when such legacy effects have been explicitly explored, they most often involve the long-term direct effects of species on systems, as opposed to the indirect effects. Here, we explore how a legacy of small mammal exclusion on the abundance of a shrub, bush lupine (Lupinus arboreus), influences the abundance of a native land snail (Helminthoglypta arrosa) in coastal prairie and dune habitats in central California. The factors that limit populations of land snails are very poorly known despite the threats to the persistence of this group of species. In grasslands, prior vole (Microtus californicus) exclusion created long-lasting gains in bush lupine abundance, mediated through the seedbank, and was associated with increased snail numbers (10×) compared to control plots where mammals were never excluded. Similar plots in dune habitat showed no difference in snail numbers due to previous mammal exclusion. We tested whether increased competition for food, increased predation, and/or lower desiccation explained the decline in snail numbers in plots with reduced lupine cover. Tethering experiments supported the hypothesis that voles can have long-lasting impacts as ecosystem engineers, reducing woody lupine habitat required for successful aestivation by snails. These results add to a growing list of studies that have found that non-trophic interactions can be limiting to invertebrate consumers.

For nearly 30 years, ecologists have argued that predators of seeds and seedlings seldom have population-level effects on plants with persistent seed banks and density-dependent seedling survival. We parameterized stage-based population models that incorporated density dependence and seed dormancy with data from a 5.5-year experiment that quantified how granivorous mice and herbivorous voles influence bush lupine (Lupinus arboreus) demography. We asked how seed dormancy and density-dependent seedling survival mediate the impacts of these consumers in dune and grassland habitats. In dune habitat, mice reduced analytical lambda (the intrinsic rate of population growth) by 39%, the equilibrium number of aboveground plants by 90%, and the seed bank by 98%; voles had minimal effects. In adjacent grasslands, mice had minimal effects, but seedling herbivory by voles reduced analytical lambda by 15% and reduced both the equilibrium number of aboveground plants and dormant seeds by 63%. A bootstrap analysis demonstrated that these consumer effects were robust to parameter uncertainty. Our results demonstrate that the quantitative strengths of seed dormancy and density-dependent seedling survival--not their mere existence--critically mediate consumer effects. This study suggests that plant population dynamics and distribution may be more strongly influenced by consumers of seeds and seedlings than is currently recognized.

The enemy release hypothesis (ERH), which has been the theoretical basis for classic biological control, predicts that the success of invaders in the introduced range is due to their release from co-evolved natural enemies (i.e. herbivores, pathogens and predators) left behind in the native range. We tested this prediction by comparing herbivore pressure on native European and introduced North American populations of Hypericum perforatum (St John's Wort). We found that introduced populations occur at larger densities, are less damaged by insect herbivory and suffer less mortality than populations in the native range. However, overall population size was not significantly different between ranges. Moreover, on average plants were significantly smaller in the introduced range than in the native range. Our survey supports the contention that plants from the introduced range experience less herbivore damage than plants from the native range. While this may lead to denser populations, it does not result in larger plant size in the introduced versus native range as postulated by the ERH.