Date of Award

Spring 2007

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

James E Byers


With continued globalization, species are being transported and introduced into novel habitats at an accelerating rate. As invasive species become more common, interactions between invasive species will also increase and may alter the way that these species impact invaded communities. The European green crab Carcinus maenas is an aggressive predator that was introduced to the east coast of North America in the mid 1800s and often has detrimental impacts on prey communities. A newer invasive predator, the Asian shore crab Hemigrapsus sanguineus, was first discovered on the Atlantic coast in the 1980s, and now inhabits many of the same regions as C. maenas within the Gulf of Maine. It too can have significant negative impacts on prey communities. Interactions between these species are often aggressive and may alter their influences on native prey.

I used field and laboratory experiments together with spatial patterns in the field to investigate the impacts of these species and examine how interactions between them alter these impacts. My study focused on three interrelated areas of community ecology: multiple predator effects, prey dependent vs. ratio dependent foraging, and species redundancy. I demonstrate that aggressive interactions between and within these species strongly influence their impacts on native communities. The result is that when both species are present, their combined impacts are less than the sum of their individual impacts. However, the strength of interference varied with habitat type, prey density, predator density, and size of predators that interacted.

Interference also affected predation by the two species differently, resulting in very different community impacts in areas where C. maenas dominates (northern Gulf of Maine) and areas where H. sanguineus has replaced C. maenas as the dominant predatory crab (Long Island Sound and southern Gulf of Maine, but progressing northward). Both conspecific and heterospecific interference strongly affected C. maenas, likely limiting its population size and subsequent impacts. In contrast, interference had little influence on predation by H. sanguineus. The lack of interference effects likely have allowed H. sanguineus to achieve very high densities observed in many areas, resulting in large population impacts on the native community.