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.

Methods
We used mark-resight data across the breeding range to interpret the role of immigration on population dynamics. From 1997 through 2015, multiple standardized band-resight surveys (4-6) were conducted during the peak of the snail kite breeding season (March 1 –June 30th) covering the entire known breeding range of the snail kite in Florida (Dreitz et al., 2002; Martin, Kitchens, & Hines, 2007; Reichert, Kendall, et al., 2016). During this time, 3,327 individually banded snail kites were resighted (7,232 total resights, of which 4,522 were in the northern region, 798 in Lake Okeechobee, and 1912 in the southern region). Of the total resighted individuals, 1,289 were banded before 1997, and first observed within this study as adults (>1 year of age). From 1997 to 2015, 2,888 snail kites were banded in the nest just prior to fledging (1,361 in the northern region, 252 on Lake Okeechobee, and 1,275 in the southern region) and 45 were banded as adults (28 in the northern region, 3 in Lake Okeechobee, and 14 in the southern region).