Source-sink dynamics are a cornerstone of theory for spatially structured populations. Despite long-standing interest, understanding temporal variation in source-sink dynamics in wild populations across entire geographic ranges remains rare. Biological invasions have the potential to alter such dynamics for native species, which may change over time as invasions proceed. We used 26 years of data on reproduction, movement, and survival to estimate annual source-sink dynamics across the entire range of the endangered Everglade snail kite (Rostrhamus sociabilis plumbeus) during the invasion of a novel prey species, the island apple snail (Pomacea maculata). Snail kite populations underwent striking changes in source-sink dynamics with time since invasion. We found that some initial benefits of increased prey availability on snail kite demography vanished in the long-term. Populations invaded by P. maculata impacted uninvaded populations via changes in snail kite retention and emigration across the metapopulation. These findings illustrate how fluctuating emigration and retention alter long-term interpretations of source-sink dynamics through variation in local vs landscape contribution of populations to the metapopulation. Our results highlight that the status of ‘source’ or ‘sink’ can be highly variable through time and long-term effects of invasions may take decades to fully emerge. 

Empirical investigations of source-sink dynamics are needed to relate metapopulation theory to spatially-structured, temporally varying population dynamics in the real world. The difficulty of acquiring the data needed to estimate vital rates has often constrained analyses of source-sink dynamics to static or simulated systems. We used 26 years of data on reproduction, movement, and survival to estimate annual source-sink dynamics across the entire range of the endangered Everglade snail kite (Rostrhamus sociabilis plumbeus) during the invasion of a novel prey species. Populations underwent striking changes in source-sink dynamics over time, varying with time since invasion. Both source and sink populations depended on immigration to offset high emigration rates. Source populations are often prioritized for conservation, but in a metapopulation composed largely of sinks, or sources with high emigration and immigration, an emphasis on maintaining connectivity or expanding available breeding habitat may be more important.