Source–sink dynamics: how sinks affect demography of sources

Models of source–sink population dynamics have to make assumptions about whether, and eventually how, demographic parameters in source habitats are dependent on the demography in sink habitats. However, the empirical basis for making such assumptions has been weak. Here we report a study on experimental root vole populations, where estimates of demographic parameters were contrasted between source patches in source–sink (treatment) and source–source systems (control). In the presence of a sink patch (simulated by a pulsed removal of immigrants), source‐patch populations failed to increase over the breeding season, mainly due to a high spatially density‐dependent dispersal rate from source to sink patches. The per capita recruitment rate was almost two times higher in source–sink than in the source–source systems, but this did not compensate for the loss rate due to dispersal from source to sink patches. Sex ratio in the source–sink systems became less female biased, probably as a result of an enhanced frequency of dispersal movements in females. Good knowledge of the degree of density‐and habitat‐dependent dispersal is critical for predicting the dynamics of source–sink populations.     

Source-sink populations in Mediterranean Blue tits: evidence using single-locus minisatellite probes

Long term studies on population biology of Blue tits (Parus caeruleus L.) in Mediterranean habitats have shown that in patchy landscapes life-history traits seem to be adapted to the predominant type of habitat, where reproductive success is higher. The “source-sink hypothesis” suggests that differences in the local production of fledglings result in an asymmetrical gene flow from rich deciduous habitats (“source”) to evergreen poor habitats (“sink”), preventing local adaptation in evergreen habitats. In this study we used single-locus minisatellite DNA probes to test the following predictions of the source-sink hypothesis: 1) source and sink populations are not genetically differentiated; 2) amount of gene flow is ranked in the following decreasing order: between source and sink habitats, among source habitats and among sink habitats; and 3) linkage disequilibrium is higher in sink than in source populations. Results were consistent with these three predictions, and with previous results obtained using other approaches. Results clearly support a source-sink functioning of Blue tit populations in southern France mosaic landscapes, and emphasise the need of combining genetic and ecological studies to understand the functioning of natural populations.     

On the evolutionary stability of sink populations

The evolution of adaptive behaviours can influence population dynamics. Conversely, population dynamics can affect both the rate and direction of adaptive evolution. This paper examines reasons why sink populations – populations maintained by immigration, preventing local extinction – might persist in the habitat repertoire of a species over evolutionary time-scales. Two such reasons correspond to standard explanations for deviations from an ideal free habitat distribution: organisms may not be free to settle in whichever habitat has the highest potential fitness, and may be constrained by costs, perceptual limitations, or mode of dispersal in the acuity of their habitat selectivity. Here, I argue that a third general reason for persistent sink populations is provided by unstable population dynamics in source habitats. I present a simple model illustrating how use of a sink habitat may be selectively advantageous, when a source population has unstable dynamics (which necessarily reflects temporal variation in local fitnesses). Species with unstable local dynamics in high-quality habitats should be selected to utilize a broader range of habitats than species with stable local dynamics, and in particular in some circumstances should utilize sink habitats. This observation has implications for the direction of niche evolution, and the likelihood of niche conservatism.     

Habitat selection,interspecific interactions and landscape composition

Summary I argue here that, from the perspective of any individual, most landscapes are composed of only three basic types of habitats. These are: (1) source habitat in which reproduction exceeds mortality and the expected per capita growth rate is greater than one; (2) sink habitat, in which limited, reproduction is possible but will not on average, compensate for mortality and the per capita rate of growth is between zero and one; and (3) unusable habitat, which comprises the matrix of all habitats that are never exploited by the species in question, and in which patches of source and sink habitats are embedded. Unlike earlier source-sink models, this model explicitly considers the effects that substituting one type of habitat for another has on the equilibrium size of a population and the interactions between species which can use both source and sink habitats. The model demonstrates that the equilibrium size of a species' population can sometimes be increased by substituting unusable habitat for sink habitat. Thus, even though the average patch quality in the landscape may be decreased, the overall quality of the landscape can increase. For two species with distinct habitat preferences, interactions between species can vary qualitatively as well as quantitatively as a function of the relative abundances of each of the habitat types. The model also shows that the interactions between species are particularly sensitive to the relative costs of moving between patches and sampling patches to determine their quality. Recent fragmentation of natural landscapes may increase the cost of searching for usable (source or sink) patches. Under some conditions, the interspecific interactions may be substantially more negative (competitive) than the interactions that evolved in the original natural landscape, further reducing population sizes and increasing the likelihood of competitive exclusion in fragmented modern landscapes.     

Self‐organization of background habitat determines the nature of population spatial structure

A substantial literature treats the dynamics of populations in response to spatial patterning of underlying habitats, the most common formulations being source/sink populations and metapopulations living in a patchwork of habitats. A separate and growing literature on the self‐organization of spatial pattern focuses on how local interactions give rise to regional patterns that can be described with scale‐free distributions. Motivated by the potential ubiquity of the coupling of these processes (spatial self organization and the dynamics of organisms in spatially structured habitats), we link these two lines of thought into a theory of populations in fragmented habitats. Using a combined analytical and computational approach, we show that self organization can generate a background into which an independent population fits, and that the scale‐free nature of such habitat creates conditions that influence both the persistence versus extinction properties of the embedded population and where it lies on the continuum between source/sink populations and metapopulations. Both the analytical framework and the computer simulations demonstrate the potential for populations to persist on either end of the metapopulation–source/sink continuum while failing to persist under intermediate conditions. These results provide a new perspective on ecological theory related to habitat fragmentation and population persistence, suggesting that under some conditions intermediate states between these two extremes may maximize risk of extinction of the population. These implications could be of particular importance for managing self‐organized systems of conservation concern.     

Allee effects, immigration, and the evolution of species' niches

Theoretical studies of adaptation to sink environments (with conditions outside the niche requirements of a species) have shown that immigration from source habitats can either facilitate or inhibit local adaptation. Here, we examine the influence of immigration on the evolution of local adaptation, given an Allee effect (i.e., at low densities, absolute fitness increases with population density). We consider a deterministic model for evolution at a haploid locus, and a stochastic individual-based model for evolution of a quantitative trait, and several kinds of Allee effects. We demonstrate that increased immigration can greatly facilitate adaptive evolution in the sink; with greater immigration, local population sizes rise, and because of the Allee effect, there is a positive indirect effect of immigration on local fitness. This makes it easier for alleles of modest effect to be captured by natural selection, transforming the sink into a locally adapted population that can persist without immigration.     

Source–sink dynamics explain the distribution and persistence of an invasive population of common carp across a model Midwestern watershed

Source–sink theory is an ecological framework that describes how site and habitat-specific demographic rates and patch connectivity can explain population structure and persistence across heterogeneous landscapes. Although commonly used in conservation planning, source–sink theory has rarely been applied to the management of invasive species. This study tested whether the common carp, one of the world’s most invasive species, exhibits source–sink dynamics in a representative watershed in the Upper Mississippi River Basin comprised of a dozen interconnected ponds and lakes. To test for source–sink population structure, we used standard fish sampling techniques, tagging, and genetic assignment methods to describe habitat-specific recruitment rates and dispersal. Five years of sampling revealed that while adult carp were found across the entire watershed, reproductive success (the presence of young carp) was restricted to shallow ponds. Additionally, nearly a third of the carp tagged in a representative pond dispersed into the connected deeper lakes, suggesting that ponds in this system serve as sources and lakes as sinks. This possibility was confirmed by microsatellite analysis of carp tissue samples (n = 1041) which revealed the presence of two distinct strains of carp cohabitating in the lakes, whose natal origins could be traced back to one of two pond systems, with many adult carp attempting to migrate back into these natal ponds to spawn. We conclude that the distribution and persistence of invasive carp in complex interconnected systems may often be driven by source–sink dynamics and that their populations could be controlled by suppressing reproduction in source habitats or by disrupting dispersal pathways, instead of culling individuals from sink habitats.     

Empirical evidence for source–sink populations: a review on occurrence,assessments and implications

Assessing the role of local populations in a landscape context has become increasingly important in the fields of conservation biology and ecology. A growing number of studies attempt to determine the source–sink status of local populations. As the source–sink concept is commonly used for management decisions in nature conservation, accurate assessment approaches are crucial. Based on a systematic literature review of studies published between 2002 and 2013, we evaluated a priori predictions on methodological and biological factors that may influence the occurrence of source or sink populations. The review yielded 90 assessments from 73 publications that included qualitative and quantitative evidence for either source or sink population(s) for one or multiple species. Overall, sink populations tended to occur more often than source populations. Moreover, the occurrence of source or sink populations differed among taxonomic classes. Sinks were more often found than sources in mammals, while there was a non‐significant trend for the opposite to be true for amphibians. Univariate and multivariate analyses showed that the occurrence of sources was positively related to connectivity of local populations. Our review furthermore highlights that more than 25 years after Pulliam's widely cited publication on ‘sources, sinks, and population regulation’, in‐depth assessments of the source–sink status of populations based on combined consideration of demographic parameters such as fecundity, survival, emigration and immigration are still scarce. To increase our understanding of source–sink systems from ecological, evolutionary and conservation‐related perspectives, we recommend that forthcoming studies on source–sink dynamics should pay more attention to the study design (i.e. connectivity of study populations) and that the assessment of the source–sink status of local populations is based on λ values calculated from demographic rates.     

Are urban habitats ecological traps for a native songbird? Season‐long productivity,apparent survival,and site fidelity in urban and rural habitats

The northern mockingbird Mimus polyglottos is a native species that is more abundant in urban than non‐urban habitats (i.e. an urban‐positive species). Abundance alone, however, is not an accurate index of habitat quality because urban habitats could represent ecological traps (attractive sink habitat) for urban‐positive species. We compared mockingbird nesting productivity, apparent survival, and decision rules governing site fidelity in urban and rural habitats. If the higher abundance of mockingbirds in urban habitats is driven by higher quality urban habitat, then we predicted that productivity of urban mockingbirds would exceed the estimated source‐sink threshold and productivity of non‐urban mockingbirds. If, on the other hand, urban habitats act as ecological traps, productivity would be lower in urban habitats and would fall below the estimated source‐sink threshold. Productivity of urban pairs exceeded that of non‐urban pairs and more than offset estimated adult mortality, which makes urban habitat a likely source habitat. Apparent adult survival was higher in urban habitats than in non‐urban habitats, although this could be driven by dispersal more than mortality. Decision rules also appeared to differ between urban and non‐urban populations. Females in urban habitats with successful nests were more likely to return than those with unsuccessful nests, whereas return rates of females in nonurban habitats were unrelated to nesting success and may be more related to nesting habitat availability. We conclude that urban habitats do not act as ecological traps that lure mockingbirds into sink habitat and that increased breeding productivity contributes to their success in urban habitats.     

Habitat‐specific demography,source‐sink dynamics,and the niche of a common shrub in a heterogeneous and fluctuating environment

The occurrence of a species in habitats of varying quality connected through migration can only be understood by detailed investigation of itsdemography. In the Chihuahuan Desert, the common shrub Flourensia cernua is found in both productive and unproductive areas. In the former, both growing and senescent populations are regularly found, while in the latter a low density scattered population persists indefinitely. While precipitation (and its annual stochastic variation) is the same in both habitats, their geomorphological differences produce a sharp difference in the availability of the limiting resource, water. This produces different population dynamics in F. cernua, but also radically different plant communities. Counterintuitively, the low‐resource habitat (LR) supports a scattered, slightly increasing or stable population that coexists with its neighbors and acts as exporter of seeds (source population). In contrast, the high‐resource habitat (HR) allows sporadic recruitment of locally dense patches that tend towards extinction (sink population). The latter is accounted for by the increasing dominance of the grass Pleuraphys mutica. The different dynamics and regulatory mechanisms in each habitat allow the species to occupy a wider distribution than it would have in their absence. The higher abundance of F. cernua in the sink habitat, together with its consequences on community composition and dynamics, questions the idea proposed in the literature that a sink population lives outside its fundamental niche. The study provides support to the notion that the ecological niche of a species cannot be completely characterized by its requirements (e.g. as they relate to physiology), but must also include the complex demographic responses to a spatially and temporally variable environment, which may often include substandard conditions. For the niche concept to retain its usefulness, it must incorporate the demographic response of populations to spatially and temporallyvariable resource supply.     

The role of sink to source re-colonisation in the population dynamics of insects living in unstable habitats: an example of terrestrial chironomids

Certain species of terrestrial chironomids (Diptera) are specialised on open patches in initial stages of primary or secondary succession (early fallow, lichens and mosses on rocks, etc.). These "source" habitats provide good quality food for their larvae and most offspring are produced here, but they are sensitive to summer desiccation. This often results in extinction of the summer larval population, followed in winter by re-colonisation from less suitable, but more stable "sink" habitats in the surrounding landscape. Soil dwelling and long-lived larvae are poor migrants; short-lived, winged adult females select patches for their development. Proper choice of oviposition sites and consequent distribution of eggs among individual habitats is thus critical for the success of these species. A mathematical model was developed in order to find out whether this re-colonisation strategy could ensure population persistence at the landscape level. The model was verified using long-term data on Smittia atterima abundance in old fields. The results indicate that even a small proportion of eggs laid in a sink habitat can ensure a successful re-colonisation of the source habitat. Thus, re-colonisation of source habitats from sink habitats is concluded to be one of the reasons for persistence of the latter. The model indicates that this re-colonisation may ensure population persistence even in conditions when exclusive use of only one habitat leads to population extinction either due to environmental stress or to a negative growth rate.     

Evolutionary consequences of asymmetric dispersal rates

We study the consequences of asymmetric dispersal rates (e.g., due to wind or current) for adaptive evolution in a system of two habitat patches. Asymmetric dispersal rates can lead to overcrowding of the "downstream" habitat, resulting in a source-sink population structure in the absence of intrinsic quality differences between habitats or can even cause an intrinsically better habitat to function as a sink. Source-sink population structure due to asymmetric dispersal rates has similar consequences for adaptive evolution as a source-sink structure due to habitat quality differences: natural selection tends to be biased toward the source habitat. We demonstrate this for two models of adaptive evolution: invasion of a rare allele that improves fitness in one habitat but reduces it in the other and antagonistic selection on a quantitative trait determined by five additive loci. If a habitat can sustain a population without immigration, the conditions for adaptation to that habitat are most favorable if there is little or no immigration from the other habitat; the influence of emigration depends on the magnitude of the allelic effects involved and other parameters. If, however, the population is initially unable to persist in a given habitat without immigration, our model predicts that the population will be most likely to adapt to that habitat if the dispersal rates in both directions are high. Our results highlight the general message that the effect of gene flow upon local adaptation should depend profoundly on the demographic context of selection.     

Impacts of environmental variability in open populations and communities: "inflation" in sink environments

Ecological communities are typically open to the immigration and emigration of individuals, and also variable through time. In this paper we argue that interesting and potentially important effects arise when one splices together spatial fluxes and temporal variability. The particular system we examine is a sink habitat, where a species faces deterministic extinction but is rescued by recurrent immigration. We have shown, using a simple extension of the canonical exponential growth model in a time-varying environment, that variation "inflates" the average abundance of sink populations. We can analytically quantify the magnitude of this effect in several special cases (square-wave temporal variation and Gaussian stochastic variation). The inflationary effect can be large in "intermittent" sinks (where there are periods with positive growth), and when temporal variation is strongly autocorrelated. The effect appears to be robust to incorporation of demographic stochasticity (due to discrete birth-death-immigration processes), and to direct density dependence. With discrete generations, however, one can observe a wide range of effects of temporal variation, including depression as well as inflation. We argue that the inflationary effect of temporal variation in sink habitats can have important implications for community structure, because it can increase the average abundance (and hence local impacts) of species that on average are being excluded from a local community. We illustrate the latter effect using a familiar model of exploitative competition for a single limiting resource. We demonstrate that temporal variation can reverse local competitive dominance, even to the extent of allowing an inferior competitor maintained by immigration to exclude a competing species that would be locally superior in a constant environment.     

Competition along a spatial gradient of resource supply: a microbial experimental model

In a set of laboratory experiments, we examined competition for phosphorus between algae and bacteria under various carbon:phosphorus (C:P) supply ratios in spatially homogeneous and heterogeneous microcosms. Experimental results were compared to those predicted by theoretical models of resource competition. In the spatially heterogeneous microcosm, algae that were inferior competitors for P persisted in vessels with high local C:P supply ratios that would cause exclusion in the spatially homogeneous microcosms. Resource competition theory, adapted to this system, provided a starting point for explaining these results. Spatial structure can enhance local diversity because locally inferior competitors are transported from source habitats into sink habitats where they would otherwise be excluded. Such local sources were determined by their resource supply ratios. These results verify the hypothesis that spatial processes enhance local diversity when a system of local habitats is divided into sources and sinks in such a way that each persisting species has at least one source within the system. However, existing theoretical models did not accurately predict distributions of competitor abundance within this experimental system.     

Understanding the contribution of habitats and regional variation to long‐term population trends in tricolored blackbirds

Population trends represent a minimum amount of information required to assess the conservation status of a species. However, understanding and detecting trends can be complicated by variation among habitats and regions, and by dispersal connecting habitats through source‐sink dynamics. We analyzed trends in breeding populations between habitats and regions to better understand the overall dynamics of a species' decline. Specifically, we analyzed historical trends in breeding populations of tricolored blackbirds (Agelaius tricolor) using breeding records from 1907 to 2009. The species breeds itinerantly and ephemerally uses multiple habitat types and breeding areas, which make interpretation of trends complex. We found overall abundance declines of 63% between 1935 and 1975. Since 1980 overall declines became nonsignificant and obscure despite large amounts of data from 1980 to 2009. Temporal trends differed between breeding habitat types and were associated with regional differences in population declines. A new habitat, triticale crops (a wheat‐rye hybrid grain) produced colonies 40× larger, on average, than other breeding habitats, and contributed to a change in regional distribution since it primarily occurred in a single region. The mechanism for such an effect is not clear, but could represent the local availability of foodstuffs in the landscape rather than something specific to triticale crops. While variation in trends among habitats clearly occurred, they could not easily be ascribed to source‐sink dynamics, ecological traps, habitat selection or other detailed ecological mechanisms. Nonetheless, such exchanges provide valuable information to guide management of dynamic systems.     

The phenomenology of niche evolution via quantitative traits in a 'black-hole' sink

Previous studies of adaptive evolution in sink habitats (in which isolated populations of a species cannot persist deterministically) have highlighted the importance of demographic constraints in slowing such evolution, and of immigration in facilitating adaptation. These studies have relied upon either single-locus models or deterministic quantitative genetic formulations. We use individual-based simulations to examine adaptive evolution in a 'black-hole' sink environment where fitness is governed by a polygenic character. The simulations track both the number of individuals and their multi-locus genotypes, and incorporate, in a natural manner, both demographic and genetic stochastic processes. In agreement with previous studies, our findings reveal the central parts played by demographic constraints and immigration in adaptation within a sink (adaptation is more difficult in environments with low absolute fitness, and higher immigration can accelerate adaptation). A novel finding is that there is a 'punctuational' pattern in adaptive evolution in sink environments. Populations typically stay maladapted for a long time, and then rapidly shift into a relatively adapted state, in which persistence no longer depends upon recurrent immigration.     

Hamilton's rule confronts ideal free habitat selection

If individuals occupy habitats in a way that maximizes their fitness, if they are free to occupy the habitats they choose and if fitness declines with population density, then their abundance across habitats should follow an ideal free distribution. But, if individuals are genetically related, this simple fitness-maximization mechanism breaks down. Habitat occupation should obey Hamilton's rule (natural selection favours traits causing a loss in individual fitness as long as they result in an equal or greater gain in inclusive fitness) and depends more on inclusive fitness than it does on individual fitness. We demonstrate that the resulting inclusive-fitness distribution inflates the population density in habitats of poorer inherent quality, creating pronounced source sink dynamics. We also show that density-dependent habitat selection among relatives reinforces behaviours such as group defence and interspecific territoriality, and that it explains many anomalies in dispersal and foraging.     

Demographic constraints in evolution: Towards unifying the evolutionary theories of senescence and niche conservatism

Summary I suggest that there may be a fundamental conceptual unity between two seemingly disparate phenomena: (1) senescence (the progressive deterioration in physiological function and, thus, individual fitness with age) and (2) niche conservatism (the observation that species often seem rather fixed over evolutionary time in their basic niche properties). I argue that both phenomena arise from demographic asymmetries. The evolutionary theory of ageing rests on the observation that the force of selection declines with age, reflecting the basic demographic facts that in persistent populations there are always fewer individuals in old than in young age classes and these individuals tend to have lower reproductive value. A similar demographic asymmetry arises when populations inhabit environments with source habitats (i.e. where conditions are within the species' niche) and sink habitats (where conditions lie outside the niche): there tend to be more individuals in sources than in sinks and individuals in sources have relatively higher reproductive values. These demographic asymmetries should often imply that the force of selection is greater in sources than in sinks, leading automatically towards niche conservatism. I suggest that niche evolution is most likely in circumstances where these demographic asymmetries in space weaken or reverse.     

Temporal autocorrelation can enhance the persistence and abundance of metapopulations comprised of coupled sinks

In spatially heterogeneous landscapes, some habitats may be persistent sources, providing immigrants to sustain populations in unfavorable sink habitats (where extinction is inevitable without immigration). Recent theoretical and empirical studies of source-sink systems demonstrate that temporally variable local growth rates in sinks can substantially increase average abundance of a persisting population, provided that the variation is positively autocorrelated--in effect, temporal variation inflates average abundance. Here we extend these results to a metapopulation in which all habitat patches are sinks. Using numerical studies of a population with discrete generations (buttressed by analytic results), we show that temporal variation and moderate dispersal can jointly permit indefinite persistence of the metapopulation and that positive autocorrelation both lowers the magnitude of variation required for persistence and increases the average abundance of persisting metapopulations. These effects are weakened--but not destroyed--if variation in local growth rates is spatially synchronized and dispersal is localized. We show that the inflationary effect is robust to a number of extensions of the basic model, including demographic stochasticity and density dependence. Because ecological and environmental processes contributing to temporally variable growth rates in natural populations are typically autocorrelated, these observations may have important implications for species persistence.     

Effects of an attractive sink leading into maladaptive habitat selection

Habitat sinks can attract dispersing animals if high mortality or breeding failure are difficult to detect (e.g., when due to human hunting or pollution). Using a simple deterministic model, we explore the dynamics of such source-sink systems considering three scenarios: an avoided sink, no habitat preference, and an attractive sink. In the second two scenarios, there is a threshold proportion of sink habitat above which the whole population decreases to extinction, but this extinction threshold varies with habitat preference and the relative qualities of the two habitat types. Hence, it would be necessary to know the habitat preferences of any species in a source-sink system to interpret data on population increases and declines. In the attractive sink scenario, small changes in the proportion of sink habitat may have disproportionate effects on the population persistence. Also, small changes in growth rates at the source and the sink severely affect the threshold and the time of extinction. For some combinations of demographic parameters and proportion of habitat sink, the decline affects the source first; thus, during some time, it will be hidden to population monitoring at the sink, where numbers can even increase. The extinction threshold is also very sensitive to the initial population sizes relative to carrying capacity. Attractive sinks represent a novel aspect of source-sink dynamics with important conservation and management implications.