© Marcel Burkhardt
Source and sink populations in reality
From a conservation point of view, it is desirable to identify those habitats and local populations, where individuals of a target species attain high reproductive and survival rates. The project examined under which circumstances source and sink populations had been found in previous studies.
In conservation planning, source habitats (and source populations) are important as they represent priority areas for species conservation and should therefore be promoted accordingly. To distinguish source from sink habitats, a local population growth rate is usually calculated based on reproductive and survival rates. If the growth rate is larger than 1, the habitat or population is considered to be a source, otherwise it is deemed to be a sink. Although easy to apply, this approach may lead to an insufficient assessment of the source-sink-status, because the exchange among habitats or populations is not taken into account. As a an example, a local population may be classified as a sink based on its population growth rate, even though it provides individuals to other populations in a network, for which it may therefore act as a source.
Based on a comprehensive literature review, the project evaluated predictions on possible causes for the presence (or absence) of source populations in terrestrial vertebrates and invertebrates.
The literature review followed the guidelines for systematic reviews outlined by Pullin & Stewart (2006). On the basis of the existing overview from Runge et al. (2006), a search on pre-defined terms was conducted in several online databases. In addition, relevant studies cited in articles from the literature search were included in the review. We will then extract from these articles the information needed to test our predictions.
The assignment of funds for species conservation and/or habitat protection rests upon a variety of background information. The source-sink-status of a site for target species may be one of the criteria. However, if the source-sink-status cannot be adequately assessed, funds for conservation may be spent at unsuitable sites, which in the end could affect the conservation goals adversely. Therefore, a critical evaluation of the utility of the source-sink-concept appears to be advisable.
The review yielded 90 assessments of the source-sink-status from73 publications. Overall, sink populations tended to occur more often than source populations. Moreover, the occurrence of source or sink populations differed among animal 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. No difference in source–sink occurrence was found for insects, ray-finned fishes (Actinopterygii, a class of the bony fishes) and birds, even though the latter showed a non-significant accumulation of source populations.
Several analyses showed that sources were more likely to occur, the better the local populations were connected. However, there was no significant difference in the occurrence of source and sink populations in relation to spatial scale of the study, number of local populations studied, population trend, migratory status, dispersal ability, habitat specialization and range size of the study species.
Our review furthermore highlights that 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, forthcoming studies should pay more attention to the study design (i.e. connectivity of study populations) and base the assessment of source–sink status of local populations on several demographic rates jointly.
Empirical evidence for source-sink populations: a review on occurrence, assessments and implications.