Multi-Scale Habitat Associations of Shorebirds in Connection with Land Cover Change in the San Elijo Lagoon, CA. USA
Habitat loss is thought to be the biggest factor contributing to the decline of shorebirds worldwide. The California coast provides many of the water rich stopovers, wintering and breeding sites that North American shorebird populations depend on during their annual migration cycle. However, most of these sites are heavily managed for human uses, which results in habitat loss and alteration. Even state protected reserves that implement restoration techniques to preserve or restore natural habitat still cause shorebirds to repeatedly return to altered habitat. Losses in shorebird habitat will affect shorebird distribution and abundance at a site. Alternatively, changes in shorebird numbers locally could be due to declines in the species regionally from a loss of habitat in other areas along their migratory route, which causes declines at a local site as a side-effect of declines in regional numbers. This study measured the loss of mudflat habitat in the San Elijo Lagoon in southern California from 2002 - 2012 after tidal flushing was restored in 2001 and related mudflat loss to changes in eight shorebird populations abundance and community composition using monthly bird surveys. Five out of the eight shorebird species analyzed showed decline, but surprisingly only one of those declining species, the American Avocet (Recurvirostra americana), could be explained by the loss of mudflats in the lagoon. Regional numbers did not predict local numbers in San Elijo. Bird species composition among the different survey zones in San Elijo significantly changed over time as species abundances changed at different rates, even though relative numbers were not explainable by changes in mudflat. This suggests that other factors resulting from the change in habitat after the restoration in the lagoon, possibly food abundance and distribution, were affecting the shorebirds there. Based on these results, the species most negatively affected by loss of mudflat are species that breed at the reserve, particularly the American Avocet, and habitat management efforts should be directed at the needs of these species. Given the dependence of shorebird numbers on habitats found within the reserve and the lack of evidence of regional effects, managing habitat for shorebirds at the reserve is likely to increase the numbers there, and benefit the species as a whole.