A closer look at our research on biodiversity |
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Why is species functional redundancy important? |
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Human activities are driving rapid changes in species diversity in a wide range of habitats globally. Changes to species diversity take mostly one of two forms: increases to diversity due to species invasion, or decreases in diversity due to population extinction. These changes in species diversity raise questions about the ability of altered systems to continue to offer valuable ecosystem services. Maintenance of ecosystem services under changing biodiversity depends largely on the ability of persisting species to fill the functional gaps left by species in decline, and thus on the ecological or functional redundancy of species. My work and work in my lab has examined several aspects of functional redundancy. These are outlined below. |
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Species functional redundancy depends on the trophic structure of the predator guild |
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Species that perform the same function in a system are often lumpted together into a guild, such as a predator guild, which implies functional redundancy. One of the primary influences of a predator are direct consumptive effects. However, abundant work has demonstrated that consumers do more than simply eat food. They also interact with one another in ways to alter each other's impacts, creating emergent multiple predator effects (Click here to read about my research on multiple predator effects). This often occurs through interference effects between predators. Consumers may therefore be redundant in their consumptive effects, their interference effects, both type of effects, or neither. |
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The invasion of two species of crab to New England shores provided an opprotunity to examine redundancy of these two seemingly similar consumers. These two species each have overlapping generations with adults and juveniles coexisting on intertidal rocky beaches. The figure to the left demonstrates that large individuals of both species will readily consume smaller individuals of both species. Predation is much less common between similarly sized individuals. The dotted lines indicate that cannibalism is generally weaker than intraguild predation. Both large and small crabs of both species also share some of the same prey items, such as amphipods shown here and used in this study. |
I conducted an experiment that examined the trophic redundancy and interference redundancy of these two species in all possible combinations of large and small individuals of these two species. The figure to the right comes from Griffen and Byers 2006 and shows the interference effect and the trophic effect of different combinations of these two species. Symbols that overlap or are close to each other are redundant. The overall pattern is one of trophic and interference redundancy between the two species when all individuals are small and when all individuals are large. However, the four different treatments that combined large and small individuals together separated widely along both axes, indicating low redundancy. The overall impact of including intraguild predation by including multiple sizes of individuals was therefore to increase interference among these predators and to decrease the redundancy of these two invasive species. |
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Another project in this same system by Griffen and Byers 2009 examined the redundancy in the community impacts of these two invasive speceis in the Gulf of Maine. In addition, I provide a summary of the redundancy of these two crab species from several perspectives in a chapter (Griffen 2011) of 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 on invaded systems. |
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Moving species redundancy towards a more predictive framework |
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Previous work suggests that the concept of species redundancy holds little applied
value because, among other reasons, this concept is highly context dependent, and therefore holds little general predictive power. Griffen et al. 2010 is a collaborative project with several participants from the 2008 EcoDAS symposium. Our goal was to
demonstrate a conceptual framework for determining conditions or situations in which redundancy should play a significant role. We did this by examining the prevalence and importance of redundancy across example environmental and biological gradients that are common to many systems. By exploring general conditions that should elevate the importance or
prevalence of redundancy, we hoped to demonstrate that this concept can be used predictively, despite its context-dependent nature. |
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Current work in this area - intraspecific diversity |
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My current work in this area focuses on diversity within a species. This intraspecific diversity is an understudied aspect of biodiversity. I currently have several ongoing projects that examine the importance of intraspecific diversity of the mud crab Panopeus herbstii in intertidal oyster reefs on the South Carolina coast. For example, one of my current graduate students, Ben Toscano, is exploring the role of body size diversity of crabs and how this influences trophic dynamics within oyster reefs. Understanding the role of body size diversity in this system is important because harvesting of oyster reefs degrades their complex, three dimensional structure and thereby eliminates critical habitat for large mud crabs. We recently demonstrated (Toscano and Griffen 2012) that crab body size diversity is positively correlated with reef height. Further, we showed that loss of large mud crabs from oyster reefs alters the trophic interactions within the reef because only large mud crabs can effectively consume large bivalves. |
