Sink Populations Research Articles

Population trends of northern fur seals (Callorhinus ursinus) from a metapopulation perspective

A metapopulation model was created to explore source-sink population dynamics in northern fur seals (Callorhinus ursinus), for which the largest source population is declining, but smaller, newly established populations are either stable or growing. A declining source population could therefore slow down the growth of newly established populations. We also investigated changes in mortality and birth rates that could stabilize the declining population trend on the Pribilof Islands. The six main breeding populations—in the Pribilof Islands, Commander Islands, Kuril Islands, Robben Island, San Miguel Island, and Bogoslof Island—were modeled as individual age-structured populations, that were linked together using a migration model. The migration model component allowed movement of subadults among the six populations, with rules based on distances between populations, average travel speeds, habitat preferences, and El Nino events. The metapopulation model revealed the importance of immigration to newly established populations during the first 8years of population growth. After the initial growth period, additional immigration had no noticeable effect on population growth in sink populations. The population trend on the Pribilof Islands was difficult to stabilize, and immigration from smaller populations could not slow the population decline. Consequently, conservation of the Pribilof Islands population likely requires a long recovery period following reductions in population-specific mortality rates for adult females, juveniles and pups.

Intensive nest predation by crabs produces source–sink dynamics in hosts and parasites

Nest predation is a common cause of reproductive failure. Here, we show for the first time unusually high rates of nest predation on eggs and chicks in nests of the Oriental Reed Warbler Acrocephalus orientalis and the brood parasitic Common Cuckoo Cuculus canorus caused by mudflat crabs Chiromantes dehaani. Crabs and the Common Cuckoo greatly reduce reproductive success of hosts at a site in coastal habitat, but only Common Cuckoos did so at an inland habitat. Thus, the parasitic Common Cuckoo suffered an indirect high predation cost by using bird species affected by crabs as hosts. The difference in the relationship between mudflat crabs, cuckoos and hosts between these two habitats can be explained by the source–sink dynamics of an ecological trap. The coastal habitats constitute sink populations for both host and brood parasite, while inland habitats with cuckoos only constitute sink populations for hosts. Because the population densities of both hosts and parasites are similar in the two kinds of habitats, these similarities might be maintained by immigration from source populations such as those without mudflat crabs and Common Cuckoos, respectively.

Experimental demonstration of accelerated extinction in source-sink metapopulations.

Population extinction is a fundamental ecological process which may be aggravated by the exchange of organisms between productive (source) and unproductive (sink) habitat patches. The extent to which such source-sink exchange affects extinction rates is unknown. We conducted an experiment in which metapopulation effects could be distinguished from source-sink effects in laboratory populations of Daphnia magna. Time-to-extinction in this experiment was maximized at intermediate levels of habitat fragmentation, which is consistent with a minority of theoretical models. These results provided a baseline for comparison with experimental treatments designed to detect effects of concentrating resources in source patches. These treatments showed that source-sink configurations increased population variability (the coefficient of variation in abundance) and extinction hazard compared with homogeneous environments. These results suggest that where environments are spatially heterogeneous, accurate assessments of extinction risk will require understanding the exchange of organisms among population sources and sinks. Such heterogeneity may be the norm rather than the exception because of both the intrinsic heterogeneity naturally exhibited by ecosystems and increasing habitat fragmentation by human activity.

HABITAT USE OF AN ENDEMIC MOLLUSC ASSEMBLAGE IN A HYDROLOGICALLY ALTERED REACH OF THE SNAKE RIVER, IDAHO, USA

Major waterways have been dramatically altered by dams some of which have operated with consistent and significant flow alterations annually since the early 1900s. Re-operation strategies are increasingly being considered for such dams in regions where biodiversity hotspots of sensitive taxa remain. Understanding the impacts of current dam operations on sensitive species is vital to designing re-operations that effectively promote species conservation. Our objectives were to examine the effects of consistent and dramatic flow alterations from a major dam on the distribution of a highly diverse endemic mollusc assemblage in the middle Snake River, Idaho, USA. Quarter metre plots were surveyed along transects with a Venturi suction dredge and classified as permanently or seasonally wetted habitat by depth. Species were classified by degree of endemism. Chi-square analysis was used to compare species presence between habitat types and analysis of variance was used to assess the significance of habitat type as a predictor of mollusc abundance, species richness and Shannon's H. Multivariate analyses were used to investigate mollusc assemblage composition and correlations with environmental parameters. The majority of mollusc species, including species of conservation concern, were found more frequently and in greater abundance in permanently wetted habitat. In addition, the mollusc assemblage in permanently wetted habitat was more diverse. Species presence and abundance patterns were consistent with source sink population dynamics with permanently wetted habitat serving as a source for seasonally wetted habitat. Results indicate that most mollusc species do not use seasonally wetted habitat even four months after it becomes available despite viable environmental conditions, highly mobile species and colonization sources. Our findings suggest that dam re-operations which reduce the magnitude and duration of seasonal low flows may improve habitat availability for sensitive benthic assemblages. Copyright © 2013 John Wiley & Sons, Ltd.

Demographic network and multi-season occupancy modeling of Rana sylvatica reveal spatial and temporal patterns of population connectivity and persistence

Many populations are spatially structured with frequent extinction–colonization events. A clear understanding of these processes is necessary for making informed and effective management decisions. Due to the spatially and temporally dynamic nature of many systems, population connectivity and local extinction–colonization processes can be difficult to assess, but graph theoretic and occupancy modeling approaches are increasingly being utilized to answer such vital ecological questions. In our study, we used 6 years of egg mass counts from 34 ponds for Rana sylvatica to parameterize spatially explicit demographic network models. Our models revealed that the studied populations have spatial structure with strong source–sink dynamics. We also assessed the colonization and persistence probability of each pond using multi-season occupancy modeling. We observed extreme fluctuation in reproductive effort among years, resulting in variable levels of connectivity across the landscape. Pond colonization and persistence were most influenced by local population dynamics, but colonization was also affected by precipitation. Our demographic network model had moderate ability to predict reproductive effort, but accuracy was hindered by variation in annual precipitation. Source populations had higher colonization and persistence rates as well as a greater proportion of ravine habitat within 1,000 m than sink populations. By linking a spatially explicit connectivity model with a temporal occupancy/persistence model, we provide a framework for interpreting patterns of occupancy and dispersal that can serve as an initial guide for future habitat management and restoration.

Minimum Annual Survival Estimates of Male Cerulean Warblers (Setophaga cerulea) In Southwestern Michigan

The Cerulean Warbler (Setophaga cerulea) is a North American landbird showing a high rate of population decline. Estimates of its annual adult survival rate are needed to evaluate its current population status. Annual return rates of 104 color-banded adult male Cerulean Warblers were used to estimate minimum annual survival from 2008–2011. The study site was a large oak-hickory forest bordered by a mosaic of forest tracts and agricultural land in southwestern Michigan. Cormack-Jolly-Seber (CJS) live recapture models yielded an annual survival estimate (Φ) of 0.59. This estimate lies in the range of available CJS survival estimates for other Neotropical migratory species with different population trajectories. In addition, this survival value confirms sink population status for Cerulean Warbler breeding populations studied at widespread locations in the breeding range (all λ < 1.0). Inter-year breeding dispersal was frequently <1.0 km, but seven banded males made breeding dispersal movements among years >1.0 km. Previous studies have suggested that some adult male Cerulean Warblers make very long-distance breeding dispersal movements among years, suggesting realized annual survival rates are higher than reported. The annual adult male survival rate reported here is similar to the only other value obtained from a breeding population of this species. In aggregate, these findings suggest that reduced breeding ground productivity is a probable cause of the long-term population decline of the Cerulean Warbler. Conservation efforts focused on this imperiled songbird species might focus on increasing this demographic factor, perhaps via reforestation of the breeding grounds.

Genetic diversity, structure and conservation of the endangered Cantabrian Capercaillie in a unique peripheral habitat

Populations at the rear edge of the species’ range are often at a high risk of extinction due to their isolation, fragmentation and small population sizes. However, these populations also play a relevant role in the conservation of biodiversity since they may represent a valuable genetic resource. The endangered Cantabrian Capercaillie (Tetrao urogallus cantabricus) inhabits deciduous forests of the Cantabrian Mountains of Spain, at the southwestern limit of the species’ range. Recently, nine Cantabrian Capercaillie leks were discovered in Pyrenean oak forests of the southern slope of the Cantabrian range, where the subspecies historically occurred. To elucidate if the origin of this peripheral population nucleus is the result of a historical range contraction or a recent re-colonization from the core population, we sampled moulted feathers from all the known leks in the southern peripheral forests and from the adjacent main core population, based on nine microsatellite loci genotypes. No significant genetic differentiation was detected between main core and peripheral forests suggesting that gene flow is not interrupted between these nuclei. Contrary to expected, peripheral forests did not represent sink populations, since gene flow mainly occurred from southern peripheral to northern main core forests. Therefore, the origin of these birds inhabiting the peripheral nucleus seems not to be a recent colonization but relicts from the former distribution range that have remained unnoticed in a drier and warmer environment than described so far for the species. Cantabrian Capercaillie faces a high risk of extinction in the southernmost forests of its distribution, not only because of its peripheral location but also due to its small population size, low genetic diversity and low incoming gene flow. According to our results, this peripheral nucleus could represent an expanding edge for the population if Pyrenean oak forests continue to spread out southwards and consequently stress the need for conservation programs to preserve habitat availability and forest connectivity.

Population genetics of the invasive ctenophore Mnemiopsis leidyi in Europe reveal source–sink dynamics and secondary dispersal to the Mediterranean Sea

Repeated invasions of European waters by the ctenophore Mnemiopsis leidyi offer a unique opportunity to study population dynamics and dispersal in gelatinous zooplankton. Here we followed population establishment in two recently invaded areas, the North and Baltic Sea, and analysed changes in population structure during a 3-yr interval using 7 highly polymorphic microsatellites (representing 191 alleles). A second goal was to reconstruct routes of recent invasive range expansion into the Mediterranean Sea During the study period (2008-2010) populations in North Sea and Western Baltic Sea maintained their allelic composition with virtually unchanged levels of genetic diversity and between-population differentiation. This demonstrates that gene flow between the two regions was limited and indicates successful reproduction in both areas. In contrast, at the eastern distribution limit in the central Baltic (Bornholm Basin) the same measures fluctuated between years and genetic diversity decreased from 2008-2010. In concordance with prior ecological observations, this supports the view that here M. leidyi formed a sink population. In the area of recent range expansion (Mediterranean Sea) we observed high population differentiation for a holoplanktonic species. Among Mediterranean samples collected at sites in Spain, France and Israel pairwise differentiation was between Fst = 0.04-0.16. Despite such differentiation, Bayesian clustering and phylogeographic analysis support the hypothesis that all Mediterranean M. leidyi result from a secondary introduction originating in the Black Sea. Our study contributes to growing evidence that multiple invasions of the same species can vary in their degree of genetic diversity and demonstrates how genetic markers can help to resolve whether gelatinous plankton species form self-sustaining populations.

Evaluating the Effects of Anthropogenic Stressors on Source‐Sink Dynamics in Pond‐Breeding Amphibians

Although interwetland dispersal is thought to play an important role in regional persistence of pond-breeding amphibians, few researchers have modeled amphibian metapopulation or source-sink dynamics. Results of recent modeling studies suggest anthropogenic stressors, such as pollution, can negatively affect density and population viability of amphibians breeding in isolated wetlands. Presumably population declines also result in reduced dispersal to surrounding (often uncontaminated) habitats, potentially affecting dynamics of nearby populations. We used our data on the effects of mercury (Hg) on the American toad ( Bufo americanus) as a case study in modeling the effects of anthropogenic stressors on landscape-scale amphibian dynamics. We created a structured metapopulation model to investigate regional dynamics of American toads and to evaluate the degree to which detrimental effects of Hg contamination on individual populations can disrupt interpopulation dynamics. Dispersal from typical American toad populations supported nearby populations that would otherwise have been extirpated over long time scales. Through support of such sink populations, dispersal between wetland-associated subpopulations substantially increased overall productivity of wetland networks, but this effect declined with increasing interwetland distance and decreasing wetland size. Contamination with Hg substantially reduced productivity of wetland-associated subpopulations and impaired the ability of populations to support nearby sinks within relevant spatial scales. Our results add to the understanding of regional dynamics of pond-breeding amphibians, the wide-reaching negative effects of environmental contaminants, and the potential for restoration or remediation of degraded habitats.

Testing common assumptions in studies of songbird nest success

We studied Ovenbird Seiurus aurocapilla and Golden-winged Warbler Vermivorachrysoptera populations in northern Minnesota, USA, to test two common assumptions instudies of songbird nest success: (1) that the condition of an empty nest on or near itsexpected fledge date is an indicator of nest fate; and (2) that the presence of a fledgling orfamily group within a territory confirms a successful nest in that territory. We monitoredthe condition of nests and used radiotelemetry to monitor juveniles through the expectedfledging date and early post-fledging period. Of nests that contained nestlings 1–2 daysbefore the expected fledge date, fates were misidentified using nest condition alone for9.5% of Ovenbird nests, but those misidentifications were made in both directions (suc-ceeded or failed), yielding only a small bias in estimated nest success. However, 20% ofGolden-winged Warbler nests were misidentified as successful using nest condition duringthe final visit interval, biasing the nest success estimate upward by 21–28% depending onthe treatment of uncertain nest fates. Fledgling Ovenbirds from 58% of nests travelledbeyond their natal territory within 24 h, rising to 98% after 5 days, and those fledglingstravelled up to 390 m from nests within 10 days of fledging. Fledgling Golden-wingedWarblers from 13% of nests travelled beyond their natal territory within 24 h, rising to85% after 5 days, and those fledglings travelled up to 510 m from nests within 10 days offledging. We conclude that nest condition and fledgling presence can be misleading indica-tors of nest fate, probably commonly biasing nest success estimates upward, and we rec-ommend that these assumptions should be tested in additional species.Keywords: fledgling, Golden-winged Warbler, Ovenbird, Seiurus aurocapilla, telemetry,Vermivora chrysoptera.Estimates of songbird reproductive success, typi-cally limited to nest data, are used to assess habitatquality (e.g. Weinberg & Roth 1998), model pop-ulation dynamics (e.g. Podolski et al. 2007), iden-tify source and sink populations (e.g. Donovan etal. 1995), and inform conservation and manage-ment plans (e.g. Woodworth 1999). Althoughsongbird population growth may be generally moresensitive to adult annual survival and fledgling sur-vival (Donovan & Thompson 2001, Streby &Andersen 2011), population growth is also sensi-tive to variation in nest success (Donovan et al.1995), and nest success is the only directly esti-mated parameter in most studies of songbirdreproductive success (Anders et al. 1997). Manypopulation models account for re-nesting (birdsnesting again after initial failure) and estimates ofnest productivity (number of young produced persuccessful nest). All such studies require accuratefield identification of whether each monitored nestsucceeded or failed in producing young. However,observational studies of songbird nests often

Daily Survival Rate for Nests and Chicks of Least Terns (Sternula antillarum) at Natural Nest Sites in South Carolina

Although a species of conservation concern, little is known about the reproductive success of Least Terns (Sternula antillarum) throughout the southeastern USA where availability of natural beaches for nesting is limited. Daily survival rate (DSR) of nests and chicks was examined at four natural nesting sites in Cape Romain National Wildlife Refuge, South Carolina, 2009–2010. Measures of nest success (n = 257 nests) ranged from 0–93% among colony sites. The DSR of nests was primarily related to colony site, but year and estimates of predation risk also were related to DSR. Predation was the principal cause of identifiable nest loss, accounting for 47% of nest failures when the two years of data were pooled. The probability (± SE) of a chick surviving from hatching to fledging = 0.449 ± 0.01 (n = 92 chicks). DSR of chicks was negatively related to tide height and rainfall. Therefore, productivity of Least Terns is being lost during both the nesting and chick stage through a combination of biotic and abiotic factors that may prove difficult to fully mitigate or manage. Although natural nesting sites within Cape Romain National Wildlife Refuge intermittently produce successful nests, the consistency of productivity over the long term is still unknown. Given that the long term availability of anthropogenic nest sites (e.g., rooftops, dredge-spoil islands) for Least Terns is questionable, further research is required both locally and throughout the region to assess the extent to which natural sites act as population sources or sinks.

Where do you come from, where do you go? Directional migration rates in landscape genetics

Identifying landscape elements that influence gene flow and migration in wild species is the current main topic of landscape genetics. Most landscape genetic studies infer gene flow and migration from genetic distances among populations or individuals and statistically relate these measurements to landscape composition and configuration. This approach assumes symmetrical gene flow between pairs of populations. Such an assumption, however, will often be violated, especially in source–sink systems. Source populations provide more emigrants than they receive immigrants, and sink populations get many immigrants, but release few emigrants. Source–sink dynamics cannot be explored using common landscape genetic approaches relying on genetic distances. In this issue of Molecular Ecology, Andreasen et al. (2012) apply an alternative approach allowing them to infer asymmetrical migration. They use a Bayesian assignment test among objectively defined populations of mountain lions (Puma concolor) in western USA to estimate recent and directional migration rates. The study shows that an area with a high amount of wildlife refuges and low hunting pressure harbours a source population for mountain lion dispersal, while areas with high hunting pressures form sink populations; a result helpful in making informed decisions in conservation management.

Invasibility as an emergent property of native metapopulation structure

Biological invasions constitute major threats to global biodiversity. Eco‐evolutionary considerations highlight the importance of contemporary evolution in community responses to bioinvasions. However, effects of metapopulation structure on invasion success have been mostly overlooked even though metapopulation structure determines gene flow and is likely to affect evolutionary processes. Here, we investigate a stepping‐stone model with evolving alien native interaction strengths. We demonstrate analytically that the site of invasion can determine the success of an invading consumer because gene flow and demography of a local resource species interact to obstruct local resource adaptation. Our main results are 1) that invasion success is more likely in genetic sink populations of the native species and 2) that invasion is more likely to occur against the migrational flow of native species. These findings suggest that invasibility is best regarded as an emergent property not only of communities but of entire metapopulations. Since migration networks of aliens and natives are often mismatched due to anthropogenic interference, our results indicate how population structure eases the spread of invasives against the migrational flow of natives.

Identification of source‐sink dynamics in mountain lions of the Great Basin

Natural and anthropogenic boundaries have been shown to affect population dynamics and population structure for many species with movement patterns at the landscape level. Understanding population boundaries and movement rates in the field for species that are cryptic and occur at low densities is often extremely difficult and logistically prohibitive; however genetic techniques may offer insights that have previously been unattainable. We analysed thirteen microsatellite loci for 739 mountain lions (Puma concolor) using muscle tissue samples from individuals in the Great Basin throughout Nevada and the Sierra Nevada mountain range to test the hypothesis that heterogeneous hunting pressure results in source-sink dynamics at the landscape scale. We used a combination of non-spatial and spatial model-based Bayesian clustering methods to identify genetic populations. We then used a recently developed Bayesian multilocus genotyping method to estimate asymmetrical rates of contemporary movement between those subpopulations and to identify source and sink populations. We identified two populations at the highest level of genetic structuring with a total of five subpopulations in the Great Basin of Nevada and the Sierra Nevada range. Our results suggest that source-sink dynamics occur at landscape scales for wide-ranging species, such as mountain lions, and that source populations may be those that are under relatively less hunting pressure and that occupy refugia.

How to understand species’ niches and range dynamics: a demographic research agenda for biogeography

AbstractRange dynamics causes mismatches between a species’ geographical distribution and the set of suitable environments in which population growth is positive (the Hutchinsonian niche). This is because source–sink population dynamics cause species to occupy unsuitable environments, and because environmental change creates non‐equilibrium situations in which species may be absent from suitable environments (due to migration limitation) or present in unsuitable environments that were previously suitable (due to time‐delayed extinction). Because correlative species distribution models do not account for these processes, they are likely to produce biased niche estimates and biased forecasts of future range dynamics. Recently developed dynamic range models (DRMs) overcome this problem: they statistically estimate both range dynamics and the underlying environmental response of demographic rates from species distribution data. This process‐based statistical approach qualitatively advances biogeographical analyses. Yet, the application of DRMs to a broad range of species and study systems requires substantial research efforts in statistical modelling, empirical data collection and ecological theory. Here we review current and potential contributions of these fields to a demographic understanding of niches and range dynamics. Our review serves to formulate a demographic research agenda that entails: (1) advances in incorporating process‐based models of demographic responses and range dynamics into a statistical framework, (2) systematic collection of data on temporal changes in distribution and abundance and on the response of demographic rates to environmental variation, and (3) improved theoretical understanding of the scaling of demographic rates and the dynamics of spatially coupled populations. This demographic research agenda is challenging but necessary for improved comprehension and quantification of niches and range dynamics. It also forms the basis for understanding how niches and range dynamics are shaped by evolutionary dynamics and biotic interactions. Ultimately, the demographic research agenda should lead to deeper integration of biogeography with empirical and theoretical ecology.

Mad cow policy and management of grizzly bear incidents

AbstractProtection of humans and livestock from disease has been used to justify many aggressive and costly wildlife control programs. Recent regulatory changes on livestock carcass disposal aimed at controlling the spread of bovine spongiform encephalopathy in Canada have led to substantial increases in exposed livestock carcass dumps. Such “boneyards” are known to attract grizzly bears (Ursus arctos), which leads to human–bear conflict. We compiled data on human–grizzly bear interactions in an agricultural landscape in southwestern Alberta over a 12‐year time period (1999–2010) overlapping regulatory changes. Boneyards increased markedly after regulations were enacted and grizzly bear incidents increased correspondingly, particularly those related to dead livestock. The high rate of conflict results in frequent management captures, relocations, and translocations that create a likely population sink. Although work is underway to reduce human–bear interactions, revisions are needed to recent regulatory changes, such that they take wildlife into account. When combined with programs aimed at ensuring proper storage of attractants, we believe that such policy reforms will make it possible for humans to coexist with grizzly bears in southwestern Alberta. © 2012 The Wildlife Society.

MIGRATION ENHANCES ADAPTATION IN BACTERIOPHAGE POPULATIONS EVOLVING IN ECOLOGICAL SINKS

Migration between populations can be a major evolutionary force. However, some disagreement exists as to precisely how migration affects population adaptation. Some theories emphasize the inhibitory effects of gene flow between locally adapted populations, whereas others propose that migration can enhance adaptation. Migration has also been theorized to rescue sink populations from extinction. In our experiments, we serially passaged bacteriophage Φ6 host range mutants under sink conditions on a novel host while manipulating the source and number of migrants into these experimental populations. Migrants from two sources were used: mutant Φ6 phage able to infect a novel host (treatment) and wild-type Φ6 phage unable to infect a novel host (control). We used quadratic regressions to determine the relationship between the number of migrants per passage and the absolute fitnesses of experimental populations following 30 passages. Our results showed that migration from a control population had no effect on absolute fitnesses of our serially passaged populations following 30 passages. By contrast, the relationship between migrants per passage and absolute fitnesses for populations receiving migrants able to infect the novel host was best described by an upwardly concave curve. These results suggest that intermediate levels of migration can have favorable impacts on evolutionary adaptation.

Distribution of the apple snail Pomacea canaliculata in Pampean streams (Argentina) at different spatial scales

Studies dealing with the distribution of freshwater mollusks in the Pampean plains are very scarce and have all focused on the determinants of their presence among different waterbodies. The aims of this study were to investigate the distribution of the apple snail Pomacea canaliculata among and within Pampean streams. The main sampling scheme included heterogeneous sections within different sites belonging to all the streams of the Encadenadas del Oeste basin (Buenos Aires Province), in which apple snails’ presence and abundance as well as several environmental variables were recorded. P. canaliculata was present in long streams originating in the Piedmont area, with the exception of one that suffered extensive dredging works but it was absent in short streams originating in the plains. Lower altitudes and higher organic matter contents characterized the inhabited sites among the long streams. At a smaller scale, higher concentrations of Na+ and lower levels of organic suspended matter characterized the inhabited sections. Within the short streams P. canaliculata was only found at sites located downstream from a connection to an inhabited long stream and hence constituted sink populations; short streams presented alkaline waters with high conductivity, very slow currents and no trees. The microhabitat of P. canaliculata was investigated at two sites from two different streams in which environmental variables were registered at individual snails’ locations and at systematically distributed points. Apple snails were more frequently found in places located close to the shore, where current velocity was usually low, sediments fine and rich in organic matter, and macrophytes abundant. These variables were all correlated at this small scale thus making their individual interpretation difficult. All the streams in this basin appear to be habitable for P. canaliculata, but the populations would only persist autonomously in the medium and low reaches on the long streams. The distribution within long streams is probably governed by stochastic patterns of extinction-colonization as a consequence of the variability of the climatic and hydrological conditions in this region.

Demographic stochasticity reduces the synchronizing effect of dispersal in predator–prey metapopulations

Dispersal may affect predator-prey metapopulations by rescuing local sink populations from extinction or by synchronizing population dynamics across the metapopulation, increasing the risk of regional extinction. Dispersal is likely influenced by demographic stochasticity, however, particularly because dispersal rates are often very low in metapopulations. Yet the effects of demographic stochasticity on predator-prey metapopulations are not well known. To that end, I constructed three models of a two-patch predator-prey system. The models constitute a hierarchy of complexity, allowing direct comparisons. Two models included demographic stochasticity (pure jump process [PJP] and stochastic differential equations [SDE]), and the third was deterministic (ordinary differential equations [ODE]). One stochastic model (PJP) treated population sizes as discrete, while the other (SDE) allowed population sizes to change continuously. Both stochastic models only produced synchronized predator-prey dynamics when dispersal was high for both trophic levels. Frequent dispersal by only predators or prey in the PJP and SDE spatially decoupled the trophic interaction, reducing synchrony of the non-dispersive species. Conversely, the ODE generated synchronized predator-prey dynamics across all dispersal rates, except when initial conditions produced anti-phase transients. These results indicate that demographic stochasticity strongly reduces the synchronizing effect of dispersal, which is ironic because demographic stochasticity is often invoked post hoc as a driver of extinctions in synchronized metapopulations.

Large forests enhance songbird nesting success in agricultural‐ dominated landscapes of the Midwestern US

We studied the effects of habitat composition and distance from edges on nesting success and brood parasitism of forest birds in the Kaskaskia River Bottoms, one of the largest remaining tracts of floodplain forest in the agricultural Midwestern United States. Our goal was to help the private landowners, who have maintained this region in forest cover, enhance the value of these forests for nesting birds. We measured nest predation rates and levels of brood parasitism of four species, the indigo bunting Passerina cyanea, Acadian flycatcher Empidonax virescens, northern cardinal Cardinalis cardinalis and prothonotary warbler Protonotaria citrea in relation to distances from natural and anthropogenic edges and proportion of natural and anthropogenic habitats within fixed radii around nests. We predicted that nesting success would increase with increasing distance from anthropogenic habitats and with increasing land cover in natural habitats. Our results showed no strong effect of any of these variables on avian nesting success, although parasitism levels increased slightly with increasing proportion of agricultural land around nests for two of the species. Nevertheless, nesting success for at least three of these species was much higher than in more fragmented forest tracts elsewhere in the agricultural Midwest where most forest tracts appear to be population sinks for most species. These results suggest that forest tracts in the Kaskaskia may be saturated with nest predators and brood parasites, but are not super‐saturated in a way that would cause these tracts to become ‘black hole’ population sinks. Our data further suggest that, as long as landowners maintain their private landholdings in forest cover, the details of how they manage their land may have little effect on songbird nesting success. These results also suggest that reforestation efforts in areas with many openings may still benefit forest birds.