Search

Ecological traps can be caused when partial restoration leads organisms to make maladaptive habitat choices. One example of this is fishways (e.g., fish ladders) that provide upstream passage at dams but are not paired with adequate downstream passage. We tested the hypothesis that attracting anadromous fishes to spawn above a dam, but blocking downstream passage of their offspring leads to an ecological trap. Using passive integrated transponder (PIT) tags, we monitored the movements of steelhead ( Oncorhynchus mykiss) at a dam and fishway on the central California coast. We found that downstream passage for juveniles and kelts was limited by four factors: migration delay, loss in the reservoir, avoidance of the downstream bypass, and water depths on the spillway. Based on the spillway-passage depth thresholds, we estimated that the ability for fish to pass downstream was limited to only half of the migration season in 55% of the past 20 years (2002–2021). Our results support the ecological trap hypothesis, which may explain why restoration using fishways has failed to produce recovery gains in this population and elsewhere.

Recovery of anadromous salmonid populations is complicated by their complex life histories. We examined the spatiotemporal interplay of stream temperature, geomorphic features, and a species’ thermal sensitivity mediated by biological interactions in a case study of steelhead trout (Oncorhynchus mykiss) and Chinook salmon (Oncorhynchus tshawytscha) in California’s Eel River watershed. We estimated habitat suitability and fish capacity for each salmonid run and freshwater life stage during average, cool, and warm years in each of the watershed’s subbasins, including a historically occupied high-elevation subbasin upstream of an impassable dam. Our estimates varied depending on whether we accounted for exposure to the Sacramento pikeminnow (Ptychocheilus grandis), an introduced predator and competitor. Our results indicate that the dammed subbasin has substantial salmonid capacity relative to the rest of the watershed and could provide an important cool-water refuge during warm years and from pikeminnow, potentially improving the productivity and resilience of multiple anadromous salmonid populations. Our approach can be applied in any setting where spatially explicit habitat metrics can be estimated and population-specific and life-stage-specific habitat criteria can be specified.

Steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) near the southern limit of the species’ range commonly use shallow alluvial rivers for migration, spawning, and rearing. These rivers have been widely modified for water management, and an enduring question is whether their rehabilitation would create summer nursery habitat for steelhead. We used process-based models to evaluate the thermal potential for steelhead nursery habitat in the Santa Ynez River, California, a regulated alluvial river that currently supports few steelhead. We assessed (1) how well a calibrated model of river heat fluxes predicted summer temperature patterns for a warm year and an average year; (2) whether those patterns created thermal potential for the rapid growth that is characteristic of steelhead nursery habitat; and (3) whether manipulation of flows from an upstream dam significantly altered thermal potential. In the heat flux model, the root mean square error for 15-min temperatures was 1.51°C, about three tim...

Climate change affects seasonal weather patterns, but little is known about the relative importance of seasonal weather patterns on animal population vital rates. Even when such information exists, data are typically only available from intensive fieldwork (e.g., mark-recapture studies) at a limited spatial extent. Here, we investigated effects of seasonal air temperature and precipitation (fall, winter, and spring) on survival and recruitment of brook trout (Salvelinus fontinalis) at a broad spatial scale using a novel stage-structured population model. The data were a 15-year record of brook trout abundance from 72 sites distributed across a 170-km-long mountain range in Shenandoah National Park, Virginia, USA. Population vital rates responded differently to weather and site-specific conditions. Specifically, young-of-year survival was most strongly affected by spring temperature, adult survival by elevation and per-capita recruitment by winter precipitation. Low fall precipitation and high winter precipitation, the latter of which is predicted to increase under climate change for the study region, had the strongest negative effects on trout populations. Simulations show that trout abundance could be greatly reduced under constant high winter precipitation, consistent with the expected effects of gravel-scouring flows on eggs and newly hatched individuals. However, high-elevation sites would be less vulnerable to local extinction because they supported higher adult survival. Furthermore, the majority of brook trout populations are projected to persist if high winter precipitation occurs only intermittently (≤3 of 5 years) due to density-dependent recruitment. Variable drivers of vital rates should be commonly found in animal populations characterized by ontogenetic changes in habitat, and such stage-structured effects may increase population persistence to changing climate by not affecting all life stages simultaneously. Yet, our results also demonstrate that weather patterns during seemingly less consequential seasons (e.g., winter precipitation) can have major impacts on animal population dynamics.

Threshold effects are common in ecosystems and can generate counterintuitive outcomes in management interventions. A threshold effect proposed for steelhead trout (Oncorhynchus mykiss) is size-conditional smolting and marine survival. Steelhead are anadromous, maturing in the ocean but migrating to freshwater to spawn, where their offspring reside for one or more years before smolting-physiologically transforming to a saltwater form-and migrating to the ocean. In conditional smolting, juveniles transform only if growth exceeds a threshold body size prior to migration season, and subsequent marine survival correlates with size at ocean entry. Conditional smolting suggests that efforts to improve freshwater survival of juveniles may reduce smolt success if they increase competition and reduce growth. Using model-selection techniques, we asked if this effect explained declining numbers of adult Carmel River steelhead. This threatened population has been the focus of two decades of habitat restoration, as well as active translocation and captive-rearing of juveniles stranded in seasonally dewatered channels. In the top-ranked model selected by information-theoretic criteria, adult decline was linked to reduced juvenile growth rates in the lower river, consistent with the conditional smolting hypothesis. According to model inference, since 2005 most returning adult steelhead were captively-reared. However, a lower-ranked model without conditional smolting also had modest support, and suggested a negative effect of captive rearing. Translocations of juvenile fish to perennial reaches may have reduced the steelhead run slightly by raising competition, but this effect is confounded in the data with effects of river flow on growth. Efforts to recover Carmel River steelhead will probably be more successful if they focus on conditions promoting rapid growth in the river. Our analysis clearly favored a role for size-conditional smolting and marine survival in the decline of the population, but did not definitively rule out alternative explanations.

Major ecological realignments are already occurring in response to climate change. To be successful, conservation strategies now need to account for geographical patterns in traits sensitive to climate change, as well as climate threats to species-level diversity. As part of an effort to provide such information, we conducted a climate vulnerability assessment that included all anadromous Pacific salmon and steelhead (Oncorhynchus spp.) population units listed under the U.S. Endangered Species Act. Using an expert-based scoring system, we ranked 20 attributes for the 28 listed units and 5 additional units. Attributes captured biological sensitivity, or the strength of linkages between each listing unit and the present climate; climate exposure, or the magnitude of projected change in local environmental conditions; and adaptive capacity, or the ability to modify phenotypes to cope with new climatic conditions. Each listing unit was then assigned one of four vulnerability categories. Units ranked most vulnerable overall were Chinook (O. tshawytscha) in the California Central Valley, coho (O. kisutch) in California and southern Oregon, sockeye (O. nerka) in the Snake River Basin, and spring-run Chinook in the interior Columbia and Willamette River Basins. We identified units with similar vulnerability profiles using a hierarchical cluster analysis. Life history characteristics, especially freshwater and estuary residence times, interplayed with gradations in exposure from south to north and from coastal to interior regions to generate landscape-level patterns within each species. Nearly all listing units faced high exposures to projected increases in stream temperature, sea surface temperature, and ocean acidification, but other aspects of exposure peaked in particular regions. Anthropogenic factors, especially migration barriers, habitat degradation, and hatchery influence, have reduced the adaptive capacity of most steelhead and salmon populations. Enhancing adaptive capacity is essential to mitigate for the increasing threat of climate change. Collectively, these results provide a framework to support recovery planning that considers climate impacts on the majority of West Coast anadromous salmonids.

Aquatic species are threatened by climate change but have received comparatively less attention thanterrestrial species.Wegleanedkey strategiesforscientistsandmanagers seekingtoaddressclimatechange in aquatic conservation planning from the literature and existing knowledge. We address 3 categories of conservation effort that rely on scientific analysis and have particular application under the U.S. Endangered Species Act (ESA): assessment of overall risk to a species; long-term recovery planning; and evaluation of effects of specific actions or perturbations. Fewer data are available for aquatic species to support these analyses, and climate effects on aquatic systems are poorly characterized. Thus, we recommend scientists conducting analyses supporting ESA decisions develop a conceptual model that links climate, habitat, ecosystem, and species response to changing conditions and use this model to organize analyses and future research. We recommend that current climate conditions are not appropriate for projections used in ESA analyses and that long-term projections of climate-change effects provide temporal context as a species-wide assessment provides spatial context. In these projections, climate change should not be discounted solely because the magnitude of projected change at a particular time is uncertain when directionality of climate change is clear. Identifying likely future habitat at the species scale will indicate key refuges and potential range shifts. However, the risks and benefits associated with errors in modeling future habitat are not equivalent. The ESA offers mechanisms for increasing the overall resilience and resistance of species to climate changes, including establishing recovery goals requiring increased genetic and phenotypic diversity, specifying critical habitat in areas not currently occupied but likely to become important, and using adaptive management.

This study considered whether different population viability analyses give similar estimates of extinction risk across management contexts. We compared the performance of population viability analyses developed by numerous scientific teams to estimate extinction risk of anadromous Pacific salmonids listed under the U.S. Endangered Species Act and challenged each analysis with data from 34 populations. We found variation in estimated extinction risk among analytical techniques, which was driven by varying model assumptions and the inherent uncertainty of risk forecasts. This result indicates that the scientific teams developed techniques that perform differently. We recommend that managers minimize uncertainty in risk estimates by using multiple models tailored to the local ecology. Assessment of relative extinction risk was less sensitive to model assumptions than was assessment of absolute extinction risk. Thus, the former method is better for comparing population status and raises caution about...

A strategy for recovering endangered species during climate change is to restore ecosystem processes that moderate effects of climate shifts. In mid-latitudes, storm patterns may shift their intensity, duration, and frequency. These shifts threaten flooding in human communities and reduce migration windows (conditions suitable for migration after a storm) for fish. Rehabilitation of historic floodplains can in principle reduce these threats via transient storage of storm water, but no one has quantified the benefit of floodplain rehabilitation for migrating fish, a widespread biota with conservation and economic value. We used simple models to quantify migration opportunity for a threatened migratory fish, steelhead (Oncorhynchus mykiss), in an episodic rain-fed river system, the Pajaro River in central California. We combined flow models, bioenergetic models, and existing climate projections to estimate the sensitivity of migration windows to altered storm patterns under alternate scenarios of floodplain rehabilitation. Generally, migration opportunities were insensitive to warming, weakly sensitive to duration or intensity of storms, and proportionately sensitive to frequency of storms. The rehabilitation strategy expanded migration windows by 16-28% regardless of climate outcomes. Warmer conditions raised the energy cost of migrating, but not enough to matter biologically. Novel findings were that fewer storms appeared to pose a bigger threat to migrating steelhead than warmer or smaller storms and that floodplain rehabilitation lessened the risk from fewer or smaller storms across all plausible hydroclimatic outcomes. It follows that statistical downscaling methods may mischaracterize risk, depending on how they resolve overall precipitation shifts into changes of storm frequency as opposed to storm size. Moreover, anticipating effects of climate shifts that are irreducibly uncertain (here, rainfall) may be more important than anticipating effects of relatively predictable changes such as warming. This highlights a need to credibly identify strategies of ecosystem rehabilitation that are robust to uncertainty. Rehabilitacion de Planicies Inundables como Cerco contra la Incertidumbre Hidroclimatica en un Corredor Migratorio de Oncorhynchus mykiss, Especie Amenazada.

[1] We address the growing need for accurate water temperature predictions in regulated rivers to inform decision support systems and protect aquatic habitats. Although many suitable river temperature models exist, few simultaneously model water temperature dynamics while considering uncertainty of predictions and assimilating observations. Here, we employ a stochastic dynamics approach to water temperature modeling that estimates both the water temperature state and its uncertainty by propagating error through a physically based dynamical system. This method involves converting the governing hydrodynamic and heat transport equations into a state space form and assimilating observations via the Kalman Filter. This model, called the River Assessment for Forecasting Temperature (RAFT), closes the heat budget by tracking heat movement using a robust semi-Lagrangian numerical scheme. RAFT considers key thermodynamic processes, including advection, longitudinal dispersion, atmospheric heat fluxes, lateral inflows, streambed heat exchange, and unsteady nonuniform flow. Inputs include gridded meteorological forecasts from a numerical weather prediction model, bathymetric cross-sectional geometry, and temperature and flow measurements at the upstream boundary and tributaries. We applied RAFT to an ∼100 km portion of the Sacramento River in California, downstream of Keswick Dam (a regulatory dam below Shasta Dam), at a spatial resolution of 2 km and a temporal resolution of 15 min. Model prediction error over a 6 month calibration period was on the order of 0.5°C. When temperature and flow gage data were assimilated, the mean prediction error was significantly less (0.25°C). The model accurately predicts the magnitude and timing of diel temperature fluctuations and can provide 72 h water temperature forecasts when linked with meteorological forecasts and real-time flow/temperature monitoring networks. RAFT is potentially scalable to model and forecast fine-grained one-dimensional temperature dynamics covering a broad extent in a variety of regulated rivers provided that adequate input data are available.

The number of individuals in a spawning run of anadromous fish can be estimated by tagging juveniles with passive integrated transponders during the freshwater phase and the subsequent monitoring of adult upstream migrants by use of instream tag readers. The method may enable monitoring of steelhead Oncorhynchus mykiss in systems where other methods are intractable. I developed a hierarchical capture–resighting model and applied it to simulated data to develop a relation between the number of marked juveniles and the precision of run size estimates. Precision is primarily controlled by the number of tagged spawners that ultimately return and are detected; only 30–90 tagged spawners are required to obtain relatively precise estimates of run size. For typical marine survival rates, 0.0033–0.0330, this translates to a tagging effort of between 3,400 and 45,000 juveniles/cohort. Estimates are robust to imperfect detection of tagged fish if at least two instream readers independently scan for tags. Re...

– The salmonid Oncorhynchus mykiss tends to inhabit forested or snow-fed streams having cold reliable flows, but in the California chaparral they inhabit rain-fed stream networks with extensive areas of intermittent flow. We hypothesised that hydrological mechanisms in such watersheds tend to spatially segregate spawning and over-summering habitats, and tested the hypothesis using observations from a series of tributaries in a pristine watershed. Consistent with the hypothesis, reaches with suitably sized spawning gravels tended to occur in intermittent tributaries, and also the perennial mainstem (which was too warm for over-summering). In early summer, juvenile O. mykiss (

Genetic analyses of coastal Oncorhynchus mykiss, commonly known as steelhead/rainbow trout, at the southern extreme of their geographic range in California are used to evaluate ancestry and genetic relationships of populations both above and below large dams. Juvenile fish from 20 locations and strains of rainbow trout commonly planted in reservoirs in the five study basins were evaluated at 24 microsatellite loci. Phylogeographic trees and analysis of molecular variance demonstrated that populations within a basin, both above and below dams, were generally each other’s closest relatives. Absence of hatchery fish or their progeny in the tributaries above dams indicates that they are not commonly spawning and that above-barrier fish are descended from coastal steelhead trapped at dam construction. Finally, no genetic basis was found for the division of populations from this region into two distinct biological groups, contrary to current classification under the US and California Endangered Species Acts.

Summary 1. We asked whether an increase in food supply in the field would increase the ability of fish populations to withstand climate warming, as predicted by certain bioenergetic models and aquarium experiments. 2. We subsidised the in situ food supply of wild juvenile steelhead (Oncorhynchus mykiss) in a small stream near the species’ southern limit. High-quality food (10% of fish biomass per day) was added to the drift in eight in-stream enclosures along a naturally-occurring thermal gradient. 3. The temperatures during the experiment were well below the upper thermal limit for the species (means of enclosures ranged from 15.1 to 16.5 °C). Food supplements had no discernible effect on survival, but raised mean (± SD) specific growth rate substantially, from 0.038 ± 0.135 in controls to 2.28 ± 0.51 in feeding treatments. Food supplements doubled the variation in growth among fish. 4. The mean and variance of water temperature were correlated across the enclosures, and were therefore transformed into principal component scores T1 (which expressed the stream-wide correlation pattern) and T2 (which expressed local departures from the pattern). Even though T1 accounted for 96% of the variation in temperature mean and variance, it was not a significant predictor of fish growth. T2 was a significant predictor of growth. The predicted time to double body mass in an enclosure with a large T2 score (cool-variable) was half that in an enclosure with a low T2 score (warm-stable). 5. Contrary to expectation, temperature effects were neutral, at least with respect to the main axis of variation among enclosures (cool-stable versus warm-variable). Along the orthogonal axis (cool-variable versus warm-stable), the effect was opposite from expectations, probably because of temperature variation. Subtle patterns of temperature heterogeneity in streams can be important to potential growth of O. mykiss.

A new method for designing subsurface drip irrigation (SDI) systems, referred to as the "soil pit method", is presented in this paper. The new method involves the installation of a trial irrigation system using thin-diameter polyethylene tube emitters. Soil water flow is then monitored by observation of the wetting front (WF) on the face of a soil pit. The soil pit method was applied at two field sites located at Forbes and Warren, New South Wales, Australia. Good agreement was found between the WF observed using the soil pit method and neutron moisture meter measurements of soil water content taken at both of the field sites. Data from the soil pit method was used to derive design parameters for drip irrigation systems at both field sites. These parameters were compared to those based on soil texture. At one site (Forbes), the two methods gave similar design parameters. A comparison of emitter lateral spacings that was conducted at this site confirmed that the lateral spacing suggested by the two methods was appropriate for the site. At the other site (Warren), however, the design parameters derived from the soil pit method were considerably different from those based on soil texture, with inadequate watering and water surfacing problems experienced from the SDI system designed using soil texture. Given the simplicity of the new design method and the minimal equipment and operator expertise that it requires, the soil pit method has potential to be a valuable tool in the design of SDI systems.

Despite a growing consensus on the importance of floods in structuring river ecosystems, predicting the geomorphic and habitat response to specific flood pulses across a range of scales remains challenging. We used a large reservoir release in a semiarid river to characterize geomorphic and habitat responses to a flood pulse, using an integrated field, remote sensing, and modeling approach. Large-scale geomorphic changes were observed as a result of the flood, including lateral migration of the river channel, gravel bar formation, and development of off-channel chutes. Spatial patterns of gravel storage varied with downstream distance from a large dam, with the upper 15 km experiencing a net sediment deficit and the lower 65 km undergoing net deposition. The longitudinal trends in gravel transport and storage reflected differences in channel gradient and predicted values of sediment mobility. The flood lowered the channel by an average of −0.5 m and roughly doubled the areal extent of pools, by incising new pools in curved reaches and in areas where the river abutted valley walls and terraces. The increased pool abundance provided greater habitat connectivity and was predicted to have positive impacts on anadromous steelhead, providing up to a 3-fold increase in the number of juvenile fish the river could support. Results from this study highlight the value of using flood pulses as opportunities to learn about river behavior, and for testing the degree to which physical processes can help restore the form and function of river ecosystems.

Regional vulnerability assessment, or ReVA, is an approach to place-based ecological risk assessment that is currently under development by the Office of Research and Development of the U.S. Environmental Protection Agency (EPA). The assessment is done at the scale of EPA regions and builds on data collected for the Environmental Monitoring and Assessment Program (EMAP) of the EPA. The pilot ReVA is being developed for the U.S. mid-Atlantic region to identify those ecosystems, together with the ecological goods and services they provide, that are most vulnerable to being lost in the next 20 years. The project is currently exploring different conceptual approaches to integrated assessment. In this article, we give an operational approach to estimating ecosystem vulnerability and discuss important issues arising from it. The first issue is estimating vulnerability at the regional scale as opposed to the more familiar local scale. The second issue is integrating information about different sorts of risks in order to prioritize them at the regional scale. The challenge of integration is considerable because of the possibility of synergistic (mutually reinforcing) interactions between different environmental stresses. Synergistic effects are often too poorly known to include, yet potentially too important to ignore. Vulnerability at the regional scale may provide a pragmatic, middle-road approach to this problem by highlighting and characterizing geographic areas that are expected to change the most in the future. The goal is not exact predictions, but a first-cut early warning system to identify and prioritize the risks of undesirable environmental changes over the next few decades.

Increasing emphasis is being placed on the large-scale and long-term dynamics of populations. A butterfly (Euphydryas editha) metapopulation that was naturally restricted to rocky outcrops in an area of coniferous forest suffered two major perturbations in 30 yr. First, humans clear-cut patches of forest in about 1967. The butterfly colonized the clear-cuts, in which it began to oviposit on a novel host plant, Collinsia torreyi. Breeding success was high, and the clear-cut habitat supported a population source; in the mid-1980s, there was net movement of adults from clear-cut to outcrop, and butterfly densities were elevated on outcrops close to population sources. The second major perturbation came in 1992, when a severe summer frost killed C. torreyi plants (outcrop hosts were not damaged). As a result, E. editha larvae starved in clear-cuts, and the source populations became extinct. This occurrence allowed us to test several predictions of source-sink theory. As predicted, densities declined on outcro...

[1] Statistical and heat budget methods for analyzing temperature dynamics of creeks are limited by the ability to resolve thermal processes and fine-grained thermal structures, respectively. Here we describe a hybrid method that identifies distinct thermal components in a stream's heat budget using only temperature data and an algorithm that employs mutual information to “unmix” signals in the temperature data. Spatial resolution is limited only by the number of temperature-logging sensors, which can be quite high for distributed-temperature sensors. Process resolution is at the level of thermal components, defined as distinct collections of heat flux elements sharing coordinated (nonindependent) dynamics. Inference can be used to relate thermal components to meteorological forcing and structural heterogeneity in the fluvial system and to suggest novel hypotheses for further testing with targeted heat budget studies. Applying the method to a small, arid-land creek produced two novel hypotheses: (1) lateral conduction of heat from adjacent dry land (bed, terraces) appeared to cause a substantial heating of the stream, augmented by off-channel flow paths, and (2) riparian vegetation was associated with a subtraction of heat from the stream at a rate proportionate to solar insolation, exceeding the maximum decoupling effect of shade by at least 2°C at midday, and suggesting upwelling heat flux from water to tree canopy proportional to sunlight. The method appears useful for generating new hypotheses, for selecting informative sites for detailed heat budgets, for determining the dimensionality of heat budgets in natural streams, and more broadly for associating thermal components to fluvial structure and processes.

A strategy for recovering endangered species during climate change is to restore ecosystem processes that moderate effects of climate shifts. In mid-latitudes, storm patterns may shift their intensity, duration, and frequency. These shifts threaten flooding in human communities and reduce migration windows (conditions suitable for migration after a storm) for fish. Rehabilitation of historic floodplains can in principle reduce these threats via transient storage of storm water, but no one has quantified the benefit of floodplain rehabilitation for migrating fish, a widespread biota with conservation and economic value. We used simple models to quantify migration opportunity for a threatened migratory fish, steelhead (Oncorhynchus mykiss), in an episodic rain-fed river system, the Pajaro River in central California. We combined flow models, bioenergetic models, and existing climate projections to estimate the sensitivity of migration windows to altered storm patterns under alternate scenarios of floodplain rehabilitation. Generally, migration opportunities were insensitive to warming, weakly sensitive to duration or intensity of storms, and proportionately sensitive to frequency of storms. The rehabilitation strategy expanded migration windows by 16-28% regardless of climate outcomes. Warmer conditions raised the energy cost of migrating, but not enough to matter biologically. Novel findings were that fewer storms appeared to pose a bigger threat to migrating steelhead than warmer or smaller storms and that floodplain rehabilitation lessened the risk from fewer or smaller storms across all plausible hydroclimatic outcomes. It follows that statistical downscaling methods may mischaracterize risk, depending on how they resolve overall precipitation shifts into changes of storm frequency as opposed to storm size. Moreover, anticipating effects of climate shifts that are irreducibly uncertain (here, rainfall) may be more important than anticipating effects of relatively predictable changes such as warming. This highlights a need to credibly identify strategies of ecosystem rehabilitation that are robust to uncertainty. Rehabilitación de Planicies Inundables como Cerco contra la Incertidumbre Hidroclimática en un Corredor Migratorio de Oncorhynchus mykiss, Especie Amenazada.

The jaw morphology of needlefishes (Beloniformes : Belonidae) passes through distinctive ontogenetic stages. When hatched, larvae have short jaws of equal length. The lower jaw first elongates to produce a halfbeak form, then the upper jaw elongates to produce the adult needlenose morphology. Onset and duration of the halfbeak stage vary among belonid species. We tested the hypothesis that heterochronic evolution produced this diversity in jaw morphology

We explore the thesis that stochasticity in successional-disturbance systems can be an agent of species extinction. The analysis uses a simple model of patch dynamics for seral stages in an idealized landscape; each seral stage is assumed to support a specialist biota. The landscape as a whole is characterized by a mean patch birth rate, mean patch size, and mean lifetime for each patch type. Stochasticity takes three forms: (1) patch stochasticity is randomness in the birth times and sizes of individual patches, (2) landscape stochasticity is variation in the annual means of birth rate and size, and (3) turnover mode is whether a patch is eliminated by disturbance or by successional change. Analytical and numerical analyses of the model suggest that landscape stochasticity is the most important agent. Landscape stochasticity increases the extinction risk to species by increasing the risk that the habitat will fluctuate to zero, by reducing the mean abundance of species, and by increasing the variance in species abundance. The highest risk was found to occur in species that inhabit patches with short lifetimes. The results of this general model suggest an important mechanism by which climate change threatens biodiversity: an increase in the frequency of extreme climate events will probably cause pulses of disturbance during some time periods; these in turn would cause wider fluctuations in annual disturbance rates and thus increase the overall level of landscape stochasticity. However, the model also suggests that humans can manipulate landscape stochasticity to reduce risk. In particular, if managed disturbances were more evenly distributed in time, attrition of the regional biota might be prevented. Other work on the connection between patch dynamics and extinction risk assumes the absence of landscape stochasticity and thus overlooks an important component of risk to biodiversity.

Theory predicts source-sink dynamics can occur in species with the ideal preemptive distribution but not with the ideal free distribution. Source-sink dynamics can also occur in species with passive dispersal, in which a fixed fraction of the population disperses each generation. However, in nature, dispersal often approximates random diffusion rather than ideal choices or fixed probabilities. Here, I ask which dispersal system occurred in a butterfly (Euphydryas editha) known to have source-sink dynamics. The study used 13 experimental sites, where vacant and occupied habitat patches were juxtaposed. I estimated movement during the flight season and tested hypotheses about the type of dispersal system. Ideal free and ideal preemptive models were rejected because per capita movement rates were density independent. Passive dispersal was rejected because per capita rates were related to patch area and habitat preference. The diffusion model best explained the data because it predicted both the area relationship and an odd feature of the habitat preference: immigration was not higher in preferred habitat; rather, emigration was lower. The diffusion model implied that source-sink dynamics were driven by diffusion from areas of high to low population density. Existing source-sink theory assumes fine-scale patchiness, in which animals have perfect knowledge and ease of mobility. The results from the butterfly suggest that source-sink dynamics arise at coarser spatial scales, where diffusion models apply.

In heterogeneous landscapes, a species’ habitat may be partitioned into sources and sinks. Conceptually, three kinds of habitat have been described: (1) “sources” are consistent net exporters of organisms; “true sinks” are net importers, and without immigration their populations go extinct; and (3) “pseudosinks” are also net importers, but without immigration they can sustain populations and sometimes can even become net exporters (sources). Previously, I have described sources and pseudosinks in a metapopulation of the butterfly Euphydryas editha and reported the extinction of populations in sources due to an unusual frost. Here, I describe the recolonization of the former sources by migrants from extant populations in the former pseudosinks. For comparison, a series of vacant patches was created in the former pseudosinks. Recolonization was tracked by sampling the population density in the vacant patches. Patches were sampled for 5 years. Contrary to theoretical expectations, the establishment rate of new populations was 10 times higher in outcrops, the former pseudosinks, than in clearings, the former sources. Initial population density was 150 times higher in outcrops (measured as number of larval webs per square meter). Yet in clearings, only the immigrants had poor reproductive success. On those occasions when a resident population was established, the residents had high reproductive success, and the population grew rapidly. The low recolonization rate of clearings could not be attributed to effects of patch size or spatial barriers. A temporal barrier was hypothesized, in which immigrants arrived too late each year to reproduce successfully on host plants in clearings. The host plant in clearings, Collinsia torreyi, typically senesced during the breeding season, but the host plant in outcrops, Pedicularis semibarbata, did not. The hypothesis proposed that immigrants oviposited later than residents because the immigrants originated in outcrops, which had a different microclimate that delayed adult eclosion. The hypothesis was supported: the newly established resident populations in clearings tended to oviposit 10 d earlier than populations in nearby outcrops. An experiment showed earlier eclosion times in clearings than in outcrops. Another experiment showed much higher larval survival in early clearings than in late clearings. Mortality was correlated with host-plant senescence. The temporal barrier was strong enough to keep clearings as net importers of butterflies from outcrops, meaning that the net flow of butterflies reversed after the frost destroyed the resident populations in clearings. Thus, “early” clearings had been sources, but “late” clearings were true sinks, giving the system two kinds of source–sink relationships. After the frost, the metapopulation as a whole underwent a demographic source–sink inversion from one locally stable state to the other. The findings suggest that asymmetric constraints on dispersal, due to temporal structure, may be a mechanism for complex source–sink dynamics in herbivorous insects and possibly other animals.