Population viability of Seaside and Saltmarsh sparrows in New Jersey
Date
2016
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Publisher
University of Delaware
Abstract
As salt marsh habitat continues to disappear, understanding the population dynamics of salt marsh breeding birds is an important step towards managing declining populations. Among the most prominent threats to salt marsh habitat and its inhabitants is sea-level rise, which is projected to continue at accelerated rates and reduce global salt marsh area 20 – 45% by 2100. Along the Atlantic coast, Seaside (Ammodramus maritimus) and Saltmarsh (A. caudacutus) sparrows are year-round endemic species with declining breeding populations from Maine to Virginia. As sister taxa, these two species share many similarities in life history, however, recent estimates have shown that Saltmarsh Sparrow populations in the northeast are declining annually ~9%, while Seaside Sparrow populations are relatively stable. To address the disparity in the conservation status of these species, it is critical to compare population vital rates when possible. Using five years (2011 – 2015) of demographic data, I studied the current and future status of the Seaside and Saltmarsh sparrow populations within Forsythe National Wildlife Refuge (Forsythe NWR), located in coastal New Jersey.
From May – August each year, I conducted mark-recapture sampling and monitored the nests of Seaside and Saltmarsh Sparrows on three study plots within Forsythe NWR. One of these sites was unditched, while the other two sites had extensive historic grid-ditching throughout, providing a unique opportunity for site comparisons. I first compared adult and nest survival rates between Seaside and Saltmarsh sparrows and between sites with different management histories for each species. The point estimate for adult Seaside Sparrow survival was 22% greater than that for adult Saltmarsh Sparrow survival. Adult survival was constant between site types for both species. I did not detect a difference in nest survival or failure probabilities between site types for either species. There was also no difference in nest survival or failure probabilities between species within each site type. The apparent lack of effect of site management history on adult and nest survival rates indicates that future management of these species may not require differential treatment of unditched and ditched marshes.
I used a metapopulation model to estimate the future viability of these breeding Seaside and Saltmarsh sparrow populations over a 42-year period. In these models I incorporated empirical data on vital rates and abundances of these populations and simulated the effect of habitat loss through low (0.35 m) and high (0.75 m) levels of sea-level rise. I found that the Seaside Sparrow population persists for the modeled time-period under both sea-level rise scenarios. The Saltmarsh Sparrow population reaches a quasi-extinction threshold within eight years under the 0.35 m sea-level rise scenario. Both populations are most sensitive to variation in fecundity, adult survival, and juvenile survival rates. Using the same modeling framework, I compared potential predator management scenarios for the Saltmarsh Sparrow population. I found that targeting predator control at select, high quality breeding habitat, was more effective and would be a better use of resources than attempting to manage predators across the entire refuge. Effective short-term management of these species will require future research into the identity and abundance of predator populations to better target specific nest and juvenile predators.