Source-sink Population Research Articles

MODELING THE ECOLOGICAL TRAP HYPOTHESIS: A HABITAT AND DEMOGRAPHIC ANALYSIS FOR MIGRANT SONGBIRDS

Most species occupy both high- and low-quality habitats throughout their ranges. As habitats become modified through anthropogenic change, low-quality habitat may become a more dominant component of the landscape for some species. To conserve species, information on how to assess habitat quality and guidelines for maintaining or eliminating low-quality habitats are needed. We developed a source-sink population model that depicted the annual cycle of a generalized migratory songbird to address these questions. We determined how demographic factors, landscape composition (the percentage of highand low-quality habitat), and habitat selection interacted to promote population persistence or extirpation. Demographic parameters, including adult and juvenile survival, nesting success (probability of a nest successfully fledging one or more young), number of nesting attempts, and number of young fledged per nest, interacted to affect population growth. In general, population growth was more sensitive to adult and juvenile survival than to fecundity. Nevertheless, within typically observed survival values, nest success was important in determining whether the population increased, decreased, or was stable. Moreover, the number of nest attempts by females and the number of young fledged per nesting attempt influenced population stability. This highlights the need to obtain more complete demographic data on species than simple nest success to assess habitat quality. When individuals selected high- and low-quality habitats in proportion to habitat availability, populations persisted as long as low-quality habitat did not make up >40% of the landscapes. However, when individuals preferred low-quality habitats over high-quality habitats, populations were extirpated in landscapes with >30% low-quality habitat because low-quality habitat functioned as an ecological trap, displacing individuals from high-quality to low-quality habitat. For long-term conservation, we emphasize the need for basic information on habitat selection and life-history characteristics of species throughout their range.

Maximum likelihood estimation of a migration matrix and effective population sizes in n subpopulations by using a coalescent approach.

A maximum likelihood estimator based on the coalescent for unequal migration rates and different subpopulation sizes is developed. The method uses a Markov chain Monte Carlo approach to investigate possible genealogies with branch lengths and with migration events. Properties of the new method are shown by using simulated data from a four-population n-island model and a source-sink population model. Our estimation method as coded in migrate is tested against genetree; both programs deliver a very similar likelihood surface. The algorithm converges to the estimates fairly quickly, even when the Markov chain is started from unfavorable parameters. The method was used to estimate gene flow in the Nile valley by using mtDNA data from three human populations.

Spatial Heterogeneity and Critical Patch Size: Area Effects via Diffusion in Closed Environments

We describe a class of mathematical models for critical patch size in which the mechanisms inducing area effects are based on source–sink population dynamics arising from dispersal throughout a closed, finite, but spatially heterogeneous environment. Our models are reaction–diffusion equations, but unlike classical KISS models for area effects they do not assume that there is dispersal across the boundary of the environment into a hostile exterior. We observe that simple rescaling has the same effects in our models as in KISS models and hence predicts the same sort of area effects, but that other sorts of rescaling may not predict area effects. The models considered here provide an alternative to the KISS models used in our previous work on species–area relationships in island biogeography.

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. Theper capitarecruitment 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.

Differential growth of crayfish Procambarus alleni in relation to hydrological conditions in marl prairie wetlands of Everglades National Park, USA

Environmental conditions influence crustacean growth by affecting molt intervals and incremental increases in length and weight. In the seasonally-flooded marl prairie wetlands of eastern Everglades National Park, U.S.A., hydropattern exerts considerable influence on aquatic primary productivity, and so may influence the availability of food resources for higher trophic levels. The seasonal hydroperiod has been drastically altered by anthropogenic factors, but the impacts on the aquatic community are not well known. We studied whether differences in growth of crayfish Procambarus alleni could be detected in habitats with different hydroperiods. We first described growth patterns based on incremental increases in length and weight of crayfish on a high protein diet in the laboratory. Regression analyses indicated that growth patterns in males and females were similar. Although the intermolt period increased with age, the proportional increases in length and weight were similar through successive molts. The relationship between length and weight of crayfish was best described by a power equation for allometric growth. We then compared growth curves for crayfish subpopulations from different areas of the marl prairie. In habitats with the longest hydroperiods, crayfish weight-at-size was not significantly different from that in laboratory crayfish on the high protein diet. However, weight gain per unit increase in length in short hydroperiod sites was significantly less than in long hydroperiod sites or in the laboratory. These results indicate that crayfish productivity may be associated with hydroperiod in these stressed wetlands, and this may contribute to observed source-sink population regulation.

Testing for Source-Sink Population Dynamics: An Experimental Approach Exemplified with Desert Annuals

Theoretical models indicate that natural populations may be structured in such a way that many individuals occur in habitats where reproduction is insufficient to balance mortality. The persistence of such sink' populations depends on immigration from neighboring source' habitats where local reproduction exceeds mortality, While source-sink dynamics has become a fundamental concept in ecological theory, there has been virtually no experimental test for the existence of sources and sinks in natural populations. This paper reports the results of a four-year study that was designed to experimentally test for source-sink population dynamies in desert annual communities. Based on evidence from a variety of desert ecosystems indicating that patchiness caused by the presence of shrubs is important in structuring desert annual communities, we distinguished between two types of habitats: areas beneath the canopy of shrubs and the open areas between the shrubs. If, as suggested in previous studies, source-sink dynamics are important in structuring such annual communities. one would expect that removal of populations from one habitat would lead to extinction of some species in the other habitat. We tested this prediction using removal experiments. We monitored density responses of annual populations inhabitremoval experiments. We monitored density responses of annual populations inhabit ing open areas to the repeated removal of conspecific populations from under shrubs and vice versa. Four years after establishment of the experiment, none of the 34 species studied responded to the removal treatments with habitat-specific extinction. Only one species exhibited a significant habitat-specific decrease in density in response to the removal of conspecific populations from the other habitat. These findings contradict our expectations based on conventional theory and point to the importance of applying an experimental approach in studies of source-sink dynamics.

Spatial Variation in Population Dynamics of Sitka Mice in Floodplain Forests

Population dynamics and demography of the Sitka mouse, Peromyscus keeni sitkensis, were studied by mark-recapture live-trapping over a 4-year period in four floodplain and upland forest habitats: old-growth Sitka spruce (Picea sitchensis) floodplain; red alder (Alnus rubra) floodplain; beaver-pond floodplain; and nearby old-growth Sitka spruce-western hemlock (Tsuga heterophylla) upland forest. We did not find the expected source-sink population dynamics resulting from flood disturbance or between-habitat variation in food resources. We found significant between-year differences (P < 0.05) in population densities, age and sex ratios, survival rates, growth rates, and movements. Within-year differences in all demographic variables were consistently greater between replicate trapping grids within habitat types than between habitat types, and there was no year-to-year consistency in any demographic difference between replicates or habitat types. We conclude that floodplain forests, per se, do not provide unique habitat for Sitka mice in either a strongly positive or negative sense compared with upland old-growth forests.

Regional Dynamics of Plants: A Review of Evidence for Remnant, Source-Sink and Metapopulations

Despite a long tradition in plant ecology of studies of patch dynamics, recent developments of models for large scale dynamics in source-sink and metapopulations have largely focused on animals. In contrast to mobile unitary animals, many plants resist extinction. even under conditions where only a part of the life cycle can be maintained. This model of remnant population dynamics adds to the two commonly recognized source-sink and metapopulations dynamics. A review of the literature suggests that all three types of dynamics are common in plants. Regional dynamics are related to life-cycle characteristics determining dispersal ability and longevity of life cycle stages. Short-lived or highly habitat specialized plants with good dispersal tend to build up metapopulations, i.e. systems of local populations and non-occupied but potentially suitable sites, interconnected with dispersal. and in a continuous flux of local colonization and extinction. At the other extreme, long-lived plants with clonal propagation. or plants with extensive seed banks, tend to build up remnant population systems, in which many local populations persist over periods long enough to bridge unfavourable phases of successional development, intervening periods of favourable conditions. Source-sink populations are a special case of metapopulations, in the sense that they comprise both persistent refuge populations and ephemeral populations maintained by dispersal. It is suggested that a concept of local population inertia in remnant population systems, scales to higher level phenomena of vegetation inertia, and to community stabilization (through enhanced recovery after perturbations). Such an inertia may contribute to explain cases of exceptionally high species diversity, and lack of pronounced mass extinctions of plants in the fossil record.

Source‐Sink Population Dynamics may Complicate the Interpretation of Long‐ Term Census Data

We examined the reproductive success and long—term population dynamics of Neotropical migrant birds in the fragmented landscapes of Illinois. Our primary objective was to assess whether annual variation in abundances and persistence of populations within woodlots was related to variation in reproductive success. Nesting success of migrants was so low in the woodlots where we sampled that it is unlikely that these populations are self—sustaining. Moreover, populations frequently disappear from and recolonize these woodlots. These data suggest that the source—sink metaphor for population dynamics within spatially structured populations is applicable to migratory birds in Illinois. The scale of this dynamic is unknown, but may be regional (i.e., midwestern U.S.). An important implication of this result is that census data must be interpreted carefully. For one woodlot, we detected no systematic trend in the overall abundances of migratory birds over much of the 20th century. Yet the viability of populations within that woodlot likely decreased dramatically over that time owing to increasing rates of nest predation and brood parasitism. Lack of data on dispersal is a major gap in understanding the population dynamics of Neotropical migrants and prescribing effective conservation measures.

Sources, Sinks and Pseudo-Sinks

It has been suggested that the habitats which a species occupies can be divided into sources and sinks, depending on whether or not local reproduction is sufficient to balance mortality. Source populations are those where reproduction exceeds mortality, surplus individuals dispersing to sink populations where mortality exceeds local reproduction. Sink populations would not be viable in the absence of immigration. A difference equation model is constructed to show that sources and sinks cannot be identified from a simple comparison of the demographic rates between populations, as measured by the numbers of births and deaths. Viable populations may appear to be non-viable simply because the dispersal of individuals into them depresses fecundity or increases mortality as a result of density-dependence. The consequence is that local recruitment appears insufficient to balance local mortality. Viable populations that appear as sinks, as a result of the dispersal of individuals into them, are termed here as 'pseudo-sinks'. They will clearly be difficult to distinguish from genuine sinks on the basis of a simple comparison of the numbers of births and deaths in different populations. Examples of source and genuine sink populations and the data required to establish them are discussed.

Habitat matching: Alternatives and implications to populations and communities

I evaluate habitat matching rules based on ideal distribution models of density-dependent habitat use. Recent approaches and the ideal free continuous input matching rule on which they depend, are restricted to only those habitats that are jointly occupied across the full range of population sizes. These assumptions may often be inappropriate to field applications of habitat matching. I develop alternatives that can be applied to a wide array of ideal forms of habitat selection, including the ideal free, continuous input example. Input matching can be distinguished from assumptions of consumer-resource models and preemptive habitat use by regressions of density between paired habitats (isodars). Isodars for continuous input models should be linear on a logarithmic scale, while those for consumer-resource models should be linear on an arithmetic scale. Pre-emptive isodars can be distinguished from the others by dramatic non-linearities at both low and high densities. Field data on white-footed mice support the consumer-resource theory. Implications of the rules for population regulation and community organization are highlighted by new models that specify how the fitness of pre-emptive habitat selectors should decline with increasing density. Strong non-linearities produced by comparisons between variable and homogeneous habitats produce reversing source-sink population regulation and a new form of cyclical community dynamics. Variable habitats act as a source of emigrants at low density and a sink for immigrants at high density. Subordinate species may occupy only the variable habitat at both low and high density.

On the Evolutionary Stability of Dispersal to Sink Habitats

On the Evolutionary Stability of Dispersal to Sink Habitats