Replacing Sources with Sinks: When Do Populations Go Down the Drain?

We investigate the scenario in which some amount of higher quality habitat is destroyed and is then replaced by some undetermined amount of lower quality habitat. We examined how much low‐quality habitat would need to be created to maintain the equilibrium population abundance in the entire geographic area. Using a source–sink model, we find that (1) the number of hectares of created habitat per hectare of destroyed habitat must equal the ratio of the high‐quality habitat's productivity to the low‐quality habitat's productivity, however, (2) if the created habitat is a sink, then there is a threshold fraction of destroyed high‐quality habitat below which the initial population abundance cannot be maintained through the creation of habitat. We illustrate these results using data on Red‐winged Blackbirds (Agelaius phoeniceus) in two different regions where high‐quality habitat is being replaced by or converted into lower quality habitat.     

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.     

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‐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.     

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.     

生境破坏的空间结构对集合种群续存的影响

生境破坏及其影响是生态学亟待解决的问题之一,目前的研究主要集中在破坏数量,即遭破坏生境的比例,对物种续存的影响方面;其中最主要的结论是Levins原理和适合生境斑块最小数量(MASH),而关于生境破坏的空间结构的研究却比较稀少,在本文中,我们首先将偶对近似引入到集合种群的研究当中,并替代原有的均匀场假设.然后我们对生境破坏导致的集合种群大小、空间结构以及分布等做了全面讨论.结果显示:随破坏比例的增加,集合种群大小将下降并且其分布将远离破坏生境.进一步聚集式分布结构将瓦解.随着破坏规则化的下降,集合种群将萎缩并使其聚集结构崩溃,在破坏生境周围集合种群起初将增加然后迅速消失.根据这些结果,我们可以对边界效应进行分析:不能用破坏比例描述生境破坏的程度和影响,而只能用破坏区域边界的长短来描述.根据边界效应,我们可以得出在一连通生境上物种保护的条件是生境破坏后剩余的适合生境比例应该大于破坏前原始生境的一半.居住在斑块环境中的物种比连续生境中生存的物种可以更好地抵抗生境破坏带来的影响.     

Can the source–sink hypothesis explain macrofaunal abundance patterns in the abyss? A modelling test

Low food availability is a major structuring force in deep-sea benthic communities, sustaining only very low densities of organisms in parts of the abyss. These low population densities may result in an Allee effect, whereby local reproductive success is inhibited, and populations are maintained by larval dispersal from bathyal slopes. This slope–abyss source–sink (SASS) hypothesis suggests that the abyssal seafloor constitutes a vast sink habitat with macrofaunal populations sustained only by an influx of larval ‘refugees'' from source areas on continental slopes, where higher productivity sustains greater population densities. Abyssal macrofaunal population densities would thus be directly related to larval inputs from bathyal source populations. We evaluate three predictions derived from the SASS hypothesis: (i) slope-derived larvae can be passively transported to central abyssal regions within a single larval period, (ii) projected larval export from slopes to the abyss reproduces global patterns of macrofaunal abundance and (iii) macrofaunal abundance decreases with distance from the continental slope. We find that abyssal macrofaunal populations are unlikely to be sustained solely through influx of larvae from slope sources. Rather, local reproduction probably sustains macrofaunal populations in relatively high-productivity abyssal areas, which must also be considered as potential larval source areas for more food-poor abyssal regions.     

Source–sink dynamics sustain central stonerollers (Campostoma anomalum) in a heavily urbanized catchment

1. The influence of spatial structure on population dynamics within river–stream networks is poorly understood. Utilizing spatially explicit analyses of temporal genetic variance, we tested whether persistence of central stonerollers (Campostoma anomalum) reflects differences in habitat quality and location within a highly modified urban catchment in southwestern Ohio, U.S.A. 2. Estimates of genetic diversity did not vary with habitat quality. Nevertheless, evidence of weak but temporally stable genetic structure, location‐dependent effective population sizes and rates of immigration among sites, together suggest that persistence of central stonerollers within the catchment may be attributable to source–sink dynamics driven by habitat heterogeneity. 3. Under this scenario, migrant‐pool colonization from areas of relatively high habitat quality in the upper catchment sustains the presence of central stonerollers at degraded sites in the main stem and dampens population subdivision within the catchment. However, because intact habitat is restricted to the upper portion of the catchment, it is not possible to preclude net downstream dispersal as a mechanism contributing to source–sink dynamics. The slight genetic structure that persists appears to reflect weak isolation by distance diminished by high rates of immigration. 4. This study suggests that without a systems perspective of the conditions that sustain populations in degraded waterways, environmental assessments may underestimate levels of impairment. Conservation and management of stream fishes could be improved by maintaining habitat in areas that are net exporters of migrants or by remediation of impaired habitat.     

Status of the Black muntjac, Muntiacus crinifrons, in eastern China

A detailed survey was conducted to show the relative abundance and current known distribution of the Black muntjac population. New information on the distribution, abundance and status of Black muntjac shows it has a small population within a restricted geographical area and habitat range, which covers an area of approximately 76 , 500 km². The relative abundance of the Black muntjac at different locations was divided into three categories, depending on the percentage composition in hunting returns; these correlated with the amount of habitat destruction and hunting pressure. With the additional consideration of reproductive performance, the Black muntjac is doing well in some suitable areas. Data from historical records, and the new discovery of an isolated population in 1980 and 1981 indicated that habitat destruction and hunting greatly reduced the range and numbers of Black muntjac in the last hundred years. The paper also suggests that the current status of the Black muntjac should be given in the Red Data Book as a rare species.     

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.     

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.     

不同栖息地破坏格局下具捕食偏爱似然竞争结局的模拟分析

使用元胞自动机模型,对具有捕食偏爱、不同栖息地破坏比例和不同空间破坏格局条件下的捕食-食饵系统中各物种的变化动态进行了模拟分析。在捕食者和两猎物物种共存时:栖息地破坏比例、栖息地破坏的聚集度对猎物物种间强弱关系产生相反的作用,若增加栖息地破坏比例不利于某一猎物生存,则提高聚集度对其有利;适当提高适宜栖息地的聚集度,对所有物种都有利,若聚集度过高,效果相当于减少了栖息地的破坏比例,可能对某些猎物物种不利,但对整体系统有利;被破坏栖息地的聚集度发生变化时,捕食者的反应更敏感;在一定条件下,增强弱势种群的捕食偏爱会有助于其生存。     

Experimental evidence that source genetic variation drives pathogen emergence

A pathogen can readily mutate to infect new host types, but this does not guarantee successful establishment in the new habitat. What factors, then, dictate emergence success? One possibility is that the pathogen population cannot sustain itself on the new host type (i.e. host is a sink), but migration from a source population allows adaptive sustainability and eventual emergence by delivering beneficial mutations sampled from the source''s standing genetic variation. This idea is relevant regardless of whether the sink host is truly novel (host shift) or whether the sink is an existing or related, similar host population thriving under conditions unfavourable to pathogen persistence (range expansion). We predicted that sink adaptation should occur faster under range expansion than during a host shift owing to the effects of source genetic variation on pathogen adaptability in the sink. Under range expansion, source migration should benefit emergence in the sink because selection acting on source and sink populations is likely to be congruent. By contrast, during host shifts, source migration is likely to disrupt emergence in the sink owing to uncorrelated selection or performance tradeoffs across host types. We tested this hypothesis by evolving bacteriophage populations on novel host bacteria under sink conditions, while manipulating emergence via host shift versus range expansion. Controls examined sink adaptation when unevolved founding genotypes served as migrants. As predicted, adaptability was fastest under range expansion, and controls did not adapt. Large, similar and similarly timed increases in fitness were observed in the host-shift populations, despite declines in mean fitness of immigrants through time. These results suggest that source populations are the origin of mutations that drive adaptive emergence at the edge of a pathogen''s ecological or geographical range.     

Movements and source–sink dynamics of a Masai giraffe metapopulation

Spatial variation in habitat quality and anthropogenic factors, as well as social structure, can lead to spatially structured populations of animals. Demographic approaches can be used to improve our understanding of the dynamics of spatially structured populations and help identify subpopulations critical for the long-term persistence of regional metapopulations. We provide a regional metapopulation analysis to inform conservation management for Masai giraffes (Giraffa camelopardalis tippelskirchi) in five subpopulations defined by land management designations. We used data from an individual-based mark–recapture study to estimate subpopulation sizes, subpopulation growth rates, and movement probabilities among subpopulations. We assessed the source–sink structure of the study population by calculating source–sink statistics, and we created a female-based matrix metapopulation model composed of all subpopulations to examine how variation in demographic components of survival, reproduction, and movement affected metapopulation growth rate. Movement data indicated no subpopulation was completely isolated, but movement probabilities varied among subpopulations. Source–sink statistics and net flow of individuals indicated three subpopulations were sources, while two subpopulations were sinks. We found areas with higher wildlife protection efforts and fewer anthropogenic impacts were sources, and less-protected areas were identified as sinks. Our results highlight the importance of identifying source–sink dynamics among subpopulations for effective conservation planning and emphasize how protected areas can play an important role in sustaining metapopulations.     

The Effects of Density Dependence and Immigration on Local Adaptation and Niche Evolution in a Black-Hole Sink Environment

We examine the effects of density dependence and immigration on local adaptation in a "black-hole sink" habitat, i.e., a habitat in which isolated populations of a species would tend to extinction but where a population is demographically maintained by recurrent one-way migration from a separate source habitat in which the species persists. Using a diploid, one-locus model of a discrete-generation sink population maintained by immigration from a fixed source population, we show that a locally favored allele will spread when rare in the sink if the absolute fitness (or, in some cases, the geometric-mean absolute fitness) of heterozygotes with the favored allele is above one in the sink habitat. With density dependence, the criterion for spread can depend on the rate of immigration, because immigration affects local densities and, hence, absolute fitness. Given the successful establishment of a locally favored allele, it will be maintained by a migration-selection balance and the resulting polymorphic population will be sustained deterministically with either stable or unstable dynamics. The densities of stable polymorphic populations tend to exceed densities that would be maintained in the absence of the favored allele. With strong density regulation, spread of the favored allele may destabilize population dynamics. Our analyses show that polymorphic populations which form subsequent to the establishment of favorable alleles have the capacity to persist deterministically without immigration. Finally, we examined the probabilistic rate at which new favored alleles arise and become established in a sink population. Our results suggest that favored alleles are established most readily at intermediate levels of immigration.     

Molecular dissection of the genetic relationships of source,sink and transport tissue with yield traits in rice

Source, sink, and translocation capacity of assimilates play important roles during the formation of grain yield. The present study was conducted to characterize the genetic bases of traits representing source, sink and transport tissue, and their relationships with yield traits in rice, by analyzing QTLs for these traits and various ratios among them. The genetic materials were a recombinant inbred population derived from a cross between two indica cultivars Zhenshan 97 and Minghui 63, the parents of the most-widely grown hybrid rice in China. Using a linkage map that covers a total of 1,796 cM based on 221 molecular marker loci, a total of 81 QTLs were identified for the 15 traits studied (three leaf areas as the source, total spikelets per panicle as the sink, the number of large vascular bundles in the stem as transport tissue, three source to sink ratios, three transport tissue to source ratios, one transport tissue to sink ratio and three yield traits). The amount of variation explained by individual QTLs ranged from 1.12% to 24.14%. Five QTLs were identified to show interaction effects with the environment, which explained from 3.19% to 9.15% of the variation. The results showed that close linkage or pleiotropy is the genetic basis for the correlations of grain yield traits with source, sink, transport tissue and the various ratios among them. Of the 25 QTLs identified for source-sink-transport tissue trait, and 43 for various ratios, 8 and 22 QTLs, respectively, were mapped to the similar genomic blocks harboring QTLs for yield traits, especially for grain weight. Co-location of QTLs for yield traits with those for ratios among source, sink and transport tissue may provide a genetic explanation for the physiological expression of yield traits, and also suggest that improvement in ratios among source, sink and transport tissue may result in improvement in yield potential.     

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.     

Predicting the time to colonization of the parasitoid Diadegma semiclausum: The importance of the shape of spatial dispersal kernels for biological control

The time at which natural enemies colonize crop fields is an important determinant of their ability to suppress pest populations. This timing depends on the distance between source and sink habitats in the landscape. Here we estimate the time to colonization of sink habitats from a distant source habitat, using empirical mark-capture data of Diadegma semiclausum in Broccoli. The data originated from experiments conducted at two locations and dispersal was quantified by suction sampling before and after a major disturbance. Three dispersal kernels were fitted to the dispersal data: a normal, a negative exponential, and a square root negative exponential kernel. These kernels are characterized by a thin, intermediate and a fat tail, respectively. The dispersal kernels were included in an integro-difference equation model for parasitoid population redistribution to generate estimates of time to colonization of D. semiclausum in sink habitats at distances between 100 and 2000 m from a source. We show that the three dispersal kernels receive similar support from the data, but can produce a wide range of outcomes. The estimated arrival time of 1% of the D. semiclausum population at a distance 2000 m from the source ranges from 12 days to a length of time greatly exceeding the life span of the parasitoid. The square root negative exponential function, having the thickest tail among the tested functions, gave the fastest spread and colonization in three of the four data sets, but it gave the slowest redistribution in the fourth. In all four data sets, the rate of accumulation at the target increased with the mean dispersal distance of the fitted kernel model, irrespective of the fatness of the tail. This study underscores the importance of selecting a proper dispersal kernel for modelling spread and colonization time of organisms, and of the collection of pertinent data that enable kernel estimation and that can discriminate between different kernel shapes.     

Population dynamics in two-patch environments: Some anomalous consequences of an optimal habitat distribution

The effect of dispersal on population size and stability is explored for a population that disperses passively between two discrete habitat patches. Two basic models are considered. In the first model, a single population experiences density-dependent growth in both patches. A graphical construction is presented which allows one to determine the spatial pattern of abundance at equilibrium for most reasonable growth models and rates of dispersal. It is shown under rather general conditions that this equilibrium is unique and globally stable. In the second model, the dispersing population is a food-limited predator that occurs in both a source habitat (which contains a prey population) and a sink habitat (which does not). Passive dispersal between source and sink habitats can stabilize an otherwise unstable predator-prey interaction. The conditions allowing this are explored in some detail. The theory of optimal habitat selection predicts the evolutionarily stable distribution of a population, given that individuals can freely move among habitats so as to maximize individual fitness. This theory is used to develop a heuristic argument for why passive dispersal should always be selectively disadvantageous (ignoring kin effects) in a spatially heterogeneous but temporally constant environment. For both the models considered here, passive dispersal may lead to a greater number of individuals in both habitats combined than if there were no dispersal. This implies that the evolution of an optimal habitat distribution may lead to a reduction in population size; in the case of the predator-prey model, it may have the additional effect of destabilizing the interaction. The paper concludes with a discussion of the disparate effects habitat selection might have on the geographical range occupied by a species.