The demographic contributions of connectivity versus local dynamics to population growth of an endangered bird

1. Conservation and management increasingly focus on connectivity, because
connectivity driven by variation in immigration rates across landscapes is
thought to be crucial for maintaining local population and metapopulation
persistence. Yet, efforts to quantify the relative role of immigration on
population growth across the entire range of species and over time have
been lacking. 2. We assessed whether immigration limited local and
range-wide population growth of the endangered snail kite (Rostrhamus
sociabilis) in Florida, USA, over 18 years using multi-state, reverse-time
modeling that accounts for imperfect detection of individuals and
unobservable states. Demographic contributions of immigration varied
depending on the dynamics and geographic position of the local
populations, were scale-dependent, and changed over time. 3. By comparing
the relative contributions of immigration versus local demography for
periods of significant change in local abundance, we found empirical
evidence for a disproportionately large role of immigration in
facilitating population growth of a centrally-located population—a
connectivity ‘hub’. The importance of connectivity changed depending of
the spatial scale considered, such that immigration was more important
driver of population growth at small versus large spatial scales.
Furthermore, the contribution of immigration was much greater during time
periods when local population size was small, emphasizing
abundance-dependent rescue effects. 4. Our findings suggest that efforts
aimed at improving local breeding habitat will likely be most effective at
increasing snail kite population growth. More broadly, our results provide
much needed information on the role of connectivity for population growth,
suggesting that connectivity conservation may have the greatest benefits
when efforts focus on centrally located habitat patches and small
populations. Furthermore, our results highlight that connectivity is
highly dynamic over time and that interpreting the effects of connectivity
at local scales may not transfer to region-wide dynamics.