A closer look at our research on invasive crabs |
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This video summarizes work that we have conducted to explore why one invasive crab (the European green crab) has been replaced by another (the Asian shore crab) on New England shores. Read below for a more complete description of this research and for links to the pertinent publications.
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Why is the Asian shore (Hemigrapsus sanguineus) crab replacing the European green crab (Carcinus maenas) on New England shores? |
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Figure from Griffen and Delaney 2007 showing that Hemigrapsus has largely replaced Carcinus throughout much of the southern part of its range. The region where Hemigrapsus dominates continues to shift northward and is now into southern Maine. |
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Carcinus is being replaced by Hemigrapsus throughout much of the region where these two species overlap. So one question is why is this species replacement happening? Previous
work by other researchers demonstrates that predation is a contributing mechanism. My work has examined whether there are other contributing mechanisms. Field experiments (figure below on left) and gut content analyses across the region where the two crabs overlap (figure below on right) demonstrated that Carcinus shifts its diet in the presence of Hemigrapsus from consuming primarily mussels to consuming primarily algae. |
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Figure adapted from Griffen et al. 2008 demonstrating that Carcinus shifts its diet in the presence of Hemigrapsus towards greater herbivory. |
Figure adapted from Griffen et al. 2008 demonstrating that the observed diet shift results in less and less mussel consumption by Carcinus as Hemigrapsus becomes more abundant.
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The reason for this diet shift is unclear. But what ever the reason, the shift has clear detrimental impacts on Carcinus reproduction, reducing both current reproductive effort, and energy stored in the hepatopancreas that is used for future growth and reproduction. This discovery came from an experiment that took advantage of an interesting biological feature of many crustaceans, including Carcinus. Reproduction in these crabs is controlled by gonad inhibiting hormone, which is secreted from a gland located in the eyestalk. Removal of the eyestalk (eyestalk ablation) removes this hormone and therefore stimulates reproduction. |
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We fed crabs specific diets that included different proportions of mussels and algae and then performed experimental eystalk ablation. We then quantified the proportion of body mass devoted to the ovaries (GSI, which is directly proportional to egg production) and the proportion of body mass devoted to the hepatopancreas (HSI, a digestive organ that stores energy and nutrients for future growth and reproduction). We found that replacing mussels with algae in the diet had a strong negative effect on both ovary and hepatopancreas biomass. The figure to the left is from Griffen et al. 2011. Finally, in addition to this experimental demonstration that diet influences reproduction and energy storage, Griffen et al. 2011 also used stable isotopes to determine the relative herbivory of naturally foraging crabs and demonstrated that reproductive output (mass of egg clutches) was negatively correlated with herbivory in the diet. Thus, previous work demonstrates that predation is clearly important in this system and is a contributing mechanism to the replacement of Carcinus by Hemigrapsus. However, the work shown here demonstrates that other mechanisms may also contribute to this species replacement. Specifically, the diet shift from animal consumption to plant consumption by Carcinus in the presence of Hemigrapsus reduces reproduction by Carcinus and may therefore limit the ability of Carcinus to successfully reproduce in areas where Hemigrapsus is abundant. |
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What are the implications of replacing one invader with another? |
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With the invasion of Hemigrapsus and the decline of Carcinus, considerable effort has been made to compare the impacts of these two species. Most of this work has compared diets or consumption rates of these two species and have focused on short term patterns, comparing individual crabs. These studies conclude that Hemigrapsus is more herbivorous than Carcinus, but that Carcinus frequently has higher consumption rates. I have focused instead on broad scale impacts across the entire community (shown in the figure to the right) that occur across longer time scales and that account for differences in density and size structure of the populations of these two species. I conducted 5-month field caging experiments that examine both the direct and indirect effects of these two species on each of these members of this community. This experiment used realistic densities and size structure of crabs, reflecting their respective populations in the surrounding area. In addition, I compared their impacts across years to determine temporal variation in their impacts that occur because of annual differences in prey recruitment (bottom-up control of top-down impacts). |
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Results of these experiments (shown below) demonstrated that the impacts of both species differ across years due to strong differences in prey recruitment and thus the ambient
prey community, and are influenced by density differences between the two species and by different strengths and types of indirect effects that each elicits. This included both density-mediated and triat-mediated indirect effects. This study also highlighted the importance of including bottom-up processes (i.e., prey recruitment) when examining the redundancy of consumers. |
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Figure modified from Griffen and Byers 2009 showing the community resulting from consumer impacts of Carcinus and Hemigrapsus in two years that differed in prey recruitment (recruitment of barnacles, mussels, snails, and macroalgae was much higher in 2005 than in 2006). Size of the circle reflects biomass relative to control cages without crabs (shown in gray circle). I summarized my work with these two invasive species, focusing on the implications of replacing Carcinus with Hemigrapsus, in a chapter (Griffen 2011) from a book (B.S. Galil et al. [eds.], In the Wrong Place - Alien Marine Crustaceans: Distribution, Biology and Impacts, Invading Nature - Springer Series in Invasion Ecology 6) that examines invasive marine crustaceans and their impacts. |
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Can injury be used to improve predictions about the impacts of invasive consumers? |
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Given the highly invasive nature of crabs and their often large impacts, being able to predict their impacts could be very beneficial in many systems around the world. Previous attempts to predict the impacts of consumer species have largely used per capita consumption rates combined with population densities to project impact. However, there are many other factors that could determine the impacts of invasive consumers. One such factor for crabs could be the amount of injury. Injury may be important because of the highly aggressive nature of many crab species. Delaney et al. 2011 sampled 30 sites throughout the northeast United States and Canada to determine the population density and demographics of Carcinus maenas and Hemigrapsus sanguineus. We also examined injury at each site. This survey included over 11,000 crabs. We combined this survey with experimental examination of the impacts of injury on consumption of native mussel prey for each species, and then used a consumption model developed by Griffen 2009 for Carcinus to determine how injury altered the impacts of these invaders at each site given the observed amount of injury at each site. |
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We found that injury in the way of a single missing cheliped reduced Carcinus mussel consumption by 21%, and completely eliminated mussel consumption by Hemigrapsus (figure left). While these impacts are quite large at the individual level, when scaled to population levels the overall influence of injury on Carcinus' consumptive impacts should be relatively modest across its range, but may be locally quite high where injury is intense, such as in the southern part of its invaded range in southern New England. For Hemigrapsus the influence of injury on population-level impacts was higher still, and particularly in northern parts of its invaded range where injury was more intense. |
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Current work in this area |
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Work with these invasive crabs is ongoing. Hemigrapsus has now essentially replaced Carcinus in bolder and cobble habitats on the New Hampshire coast and is now present at very high densities (~80 per square meter). Current work continues to track population changes on the New Hampshire coast and is also exploring the link between diet composition, consumption rates, and reproductive potential for these species. |
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