Testing for source-sink population dynamics: an experimental approach exemplified with desert annuals

Theoretical models indicate that natural populations may be structured in s uch a way that many individuals occur in habitats where reproduction is ins ufficient to balance mortality. The persistence of such 'sink' populations depends on immigration from neighboring 'source' habitats where local repro duction exceeds mortality. While source-sink dynamics has become a fundamen tal concept in ecological theory, there has been virtually no experimental test for the existence of sources and sinks in natural populations. This pa per reports the results of a four-year study that was designed to experimen tally test for source-sink population dynamics in desert annual communities . Based on evidence from a variety of desert ecosystems indicating that pat chiness 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 struct uring such annual communities, one would expect that removal of populations from one habitat would lead to extinction of some species in the other hab itat. We tested this prediction using removal experiments. We monitored den sity responses of annual populations inhabiting open areas to the repeated removal of conspecific populations from under shrubs and vice versa. Four y ears 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 an d point to the importance of applying an experimental approach in studies o f source-sink dynamics.