Biological invaders, non-native species introduced into areas beyond their historical ranges, often have an advantage over native species because of the absence of the predators and parasites that limit them in their home ranges. With such an advantage, invasive species can dominate an environment. However, new evidence published in the Dec. 1 issue of Ecology shows that, at least in some cases, the locals not only hold their own but prevent invasive species from spreading unchecked.
Working at the Marine Invasive Species Lab at the Smithsonian Environmental Research Center, Catherine deRivera and her colleagues have shown that the voracious native blue crab (Callinectes sapidus) is limiting the spread of the European Green Crab in U.S. East Coast waters.
Native to the coasts of Europe, the green crab (Carcinus maenas) has spread throughout the world, colonizing both coasts of North America, Australiaand South Africa. It became established along eastern North Americain the 1800s and now exists from Nova Scotiato Maryland.
The ranges of both crab species overlap in eastern North America, but deRivera and her colleagues, Gregory M. Ruiz, Anson H. Hines and Paul Jivoff, found that predation by native blue crabs is a key factor limiting the abundance of green crabs where they co-exist and even controlling the southern limit of the green crab’s range. By surveying bays from Maineto Virginia, the researchers found green crab population declines in direct proportion to the abundance of blue crabs. In the Chesapeake Bay, where blue crabs are most abundant, there were no European green crabs at all.
The researchers then performed a variety of experiments in the lab and in the field to identify the reason for this relationship. They found blue crabs eat green crabs whenever they can. Regardless of temperature, habitat or presence of predaceous fish and other crabs, the one factor that remained consistent with the shift in green crab abundance was predation by blue crabs.
“Our results support the hypothesis that the native predator C. sapidus provides biotic resistance to invasion and prevents the southward spread and establishment of C. maenas,” deRivera said.
According to deRivera, this is the first documented example of predation by a native species limiting the range of a marine invader. The extent to which biological interactions such as these help an ecosystem resist invasion by non-native species is poorly documented, especially for marine environments. This study helps shed light on the process and offers even more impetus for improved management of blue crabs in the Chesapeake Bay where populations have dwindled dramatically in recent decades.
Catherine E. Derivera, Gregory M. Ruiz, Anson H. Hines and Paul Jivoff. (2005) “Biotic Resistance to Invasion: Native Predator Limits Abundance and Distribution of an Introduced Crab”; Ecology.