Habitat selection model explains source-sink paradox

Habitat quality for a species in a particular area can be inferred from population growth rate. Areas with increasing populations (i.e., reproduction greater than mortality) are considered good habitat and called sources because surplus individuals can emigrate to other habitats including those where populations are declining (often called sinks).

Ecological theory suggests that free distribution of identical animals should produce stable populations in all occupied habitats at equilibrium. The existence of sources and sinks is a paradox conflicting with the free distribution model and implies that some individuals make habitat choices that lower their fitness.

In a recent paper, Dr. Craig Loehle (NCASI Principal Scientist) shows that that the source-sink paradox “can be resolved by considering individual decisions in a conditional choice model with non-identical individuals that differ in competitive ability and current expected reproductive output. Individuals that are more mature, healthier, and/or arrive earlier to the source will acquire territory at lower cost and will defend it more vigorously and effectively. For other individuals, costs (including running out of time for breeding) of acquiring territory in the source become so high that moving to the sink increases their fitness.”

Dr. Loehle’s article is titled “A conditional choice model of habitat selection explains the source-sink paradox” (Ecological Modelling Vols. 235-236:59-66). This paper has important implications for modeling population trends in context of wildlife conservation and regulation. For example, the paper shows that sink habitats are not necessarily “ecological traps” that reduce the long-term viability of species, but rather can make positive contributions to viability that depend on the relative abundance and spatial arrangement of sources and sinks across landscapes.  

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