Can an Invasive Species of Crayfish Help Save a Population of a Threatened Species of Bird, the King Rail?

Abstract

ABSTRACT - Invasive species are frequently harmful to native species and to ecosystem stability. Yet, in a few cases, alien species have been found to benefit native residents. I investigated the relationship between a threatened species of marsh bird and an invasive species of crayfish on which it feeds. The Red Swamp Crayfish, Procambarus clarkii, was deliberately introduced into an impoundment at Mackay Island National Wildlife Refuge (NWR) 25 years ago. The refuge hosts one of the largest breeding populations of King Rails Rallus elegans on the east coast of the United States. This secretive marsh bird is globally Near Threatened (Birdlife International). The King Rail is mainly carnivorous and feeds in the shallows on fish, crustaceans, and other aquatic animals, with crayfish being a preferred food. The invasive crayfish is fast-growing and a prolific breeder. I investigated whether the crayfish could be providing these rare birds a resource that allows them to prosper here relative to other sites. Data were collected over the summer breeding season on both crayfish and rail abundance. I determined the distribution of crayfish among ten predetermined sites that were being surveyed for rail breeding density as part of an ongoing study. Crayfish were caught using food-baited traps. Carapace remains from consumed crayfish were collected as a representative sample of the segment of the population that fell prey. The rail population was surveyed via passive recording of calls using autonomous recording units (Wildlife Acoustics). Both species preferred areas of natural marsh compared to impoundments. I investigated whether peak numbers of the largest size class of the invasive species, P. clarkii, coincided with the brood rearing period, when King Rails would likely be most nutritionally stressed. The temporal data revealed that King Rail hatching dates peaked when the largest crayfish size classes were most abundant. That King Rails timed their breeding so that hatching coincided with larger sizes of the invasive crayfish suggests that P. clarkii may have a positive effect on King Rail population growth at this site. Relative rail abundance based on calling rates among sites was then compared to crayfish abundance at the same sites based on trap data to see if these were correlated. The spatial results of the study showed that when comparing P. clarkii abundance and King Rail relative density at ten locations, there was no significant relationship between higher numbers of crayfish and where rails chose to nest. Dietary constraints may have contributed to the decline of King Rail populations across its range, and its extirpation from marshes where vegetation, water depth and other habitat variables appear suitable. Further research will be needed to reveal what proportion of the diet of Mackay Island King Rails the crayfish represent, and whether the invasive species, P. clarkii, is a contributing substantially to their reproductive success.