BIOTIC RESISTANCE TO INVASION: NATIVE PREDATOR LIMITS ABUNDANCE AND DISTRIBUTION OF AN INTRODUCED CRAB

Abstract

Introduced species frequently escape the natural enemies (predators, com- petitors, and parasites) that limit their distribution and abundance in the native range. This reduction in native predators, competitors, and parasites may result in ecological release in the introduced range. However, biological interactions also can limit the establishment and spread of nonnative populations. The extent to which such biotic resistance occurs is poorly resolved, especially for marine ecosystems. Here we test whether a native predator, the blue crab Callinectes sapidus, affects the abundance and geographic range of the in- troduced European green crab Carcinus maenasin eastern North America. Both crab species occur in shallow, soft-sediment habitats of bays and estuaries, and their ranges overlap in eastern North America. First, we tested for a negative relationship in the abundances of the two species from trap samples across a 640-km (5.788 latitude) coastal transect. Second, we estimated variation in predation pressure on tethered Carcinus maenas across latitude and as a function of Callinectes sapidus abundance. Third, we measured predation rates on Carcinus maenasby Callinectes sapidusin field and laboratory experiments. Our results support the hypothesis that the native predator Callinectes sapidusprovides biotic resistance to invasion and prevents the southward spread and establishment of Carcinus maenas. Within and across bays, Carcinus maenas were significantly less abundant at sites and depths with Callinectes sapiduscompared with areas lacking Callinectes sapidus. Moreover, no Carcinus maenaswere found in Chesapeake Bay, where Callinectes sapidus were most abundant. Predation of tethered Carcinus maenasincreased with Callinectes sapidusabun- dance. In laboratory and field experiments,Callinectes sapidus preyed readily on Carcinus maenas. Thus, we conclude the predation by Callinectes sapidus, alone or in combination with other factors, limits the abundance and geographic range of an invasive marine species.