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 dynamics 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 inhabiting 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.