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Herbivores | Invasive species | Climate ChangeLarval Dispersal

Herbivores:: Seaweed communities are among the most productive on earth: their rates of productivity rivals or exceeds that for tropical rain forests.  Like forests on land, seaweeds add three-dimensional structure and complexity, providing shelter for 100s if not 1000s of associated organisms.  Herbivores are animals that consume seaweeds as their primary diets.  Larger herbivores (mostly fishes and urchins) and smaller herbivores (also termed ‘mesograzers’) have dramatic effects on seaweed communities, extirpating some seaweeds from the seabed altogether.  Herbivores maintain ecosystem services by 1) determining whether non-native species establish themselves, 2) altering the flux of materials and nutrients through food webs, 3) facilitating or impeding recruitment by other species, and 4) serving as prey items for the predatory fishes and invertebrates that have commercial and recreational value.

Our ability to manage these ecosystem services is limited by key knowledge gaps of herbivores. Currently, we cannot predict whether herbivore species diversity or functional diversity is more important to maintaining healthy ecosystems. Our laboratory focuses on the ecology and evolution of herbivores and their functional diversity. Specifically, we document the ultimate and proximate mechanisms that underlie herbivore feeding responses.

Invasive species: Species invasions may alter ecosystem functions via multiple ecological mechanisms.  The most profound of these effects are generated by a relatively small number of invaders that create physical structure, including important biogenic habitat de novo.  By altering physical structure, these non-native ecosystem engineers alter local abiotic conditions, interactions between species, and species composition.  Highly influential invaders may also change food web structure and trophic flow of energy and materials.  Unfortunately, for most invasions, we lack mechanistic understanding of these multiple cascading impacts.

Currently, our laboratory works on the non-native seaweed Gracilaria vermiculophylla within South Carolina and Georgia estuaries. Originally from east Asia, Gracilaria has colonized the U.S. west and east coasts and northern Europe. The seaweed invasion in the southeastern U.S. (i.e., the Carolinas) is noteworthy because these mudflats were historically devoid of macrophyte-based primary production and structure. Gracilaria succeeds where benthic seaweeds and seagrasses largely fail because it is more tolerant of salinity and temperature variation. Because Gracilaria has few to no native analogues in this region, it represents a unique opportunity to examine the consequences of invasions within a previously unexploited niche.

Climate change: Sustaining ecosystems over time will be challenging given the recent acceleration of climate changes.  During the 20th century, oceans rose in elevation and became hotter and more acidic.  Changes in physical traits are predicted to accelerate in the 21st century, and over the next 50 years, seawater temperatures will exceed those under which nearshore systems have flourished over the past half million years.  Predicting the effects of a warming sea on ecosystem function remains elusive because of a lack of mechanistic understanding of the effect of temperature on individuals, populations and communities.

First, we are exploring the effect of temperature on herbivores and their interaction with seaweeds. Temperature alters herbivore feeding rates, feeding choices and fitness. These herbivore responses also depend on the food quality of seaweeds (i.e., their nutritional contents and chemical defenses), and there are virtually no studies that simultaneously test for the effects of both temperature and food quality.

Second, we are documenting microevolutionary responses of marine populations to warming seas. Despite the fact that microevolution to temperature can occur, we currently cannot predict whether or how quickly a species adapts (via natural selection) to a warming sea. Our laboratory documents geographic variation in thermal responses within a single species, because spatial variation can serve as a proxy for temporal changes in water temperature (“Space for time”). We also document microevolutionary responses by re-examining geographic clines at genes that mediate temperature tolerance. One take-home message from this work is that predicting the response of populations to water temperature over space and time will be difficult, as it depends on direct thermal effects, as well as interactions between temperature and interactions with competitors, predators and prey.

Dispersal: Most marine organisms produce hundreds to thousands of larvae that are pelagically dispersed. These larvae can live in the water column for days to weeks before settlement, during which time they may move hundreds to thousands of kilometers. However, despite this dispersal potential, recent evidence indicates that the realized dispersal distances can be far more restricted than the life history of the organism would predict. The exact degree of spatial spread is species-specific, and will depend on local oceanographic circulation patterns, the local ecology of the animal, the behavior of their larvae and the local availability of suitable habitat.

The vexing nature of larval dispersal requires an interdisciplinary approach. We genotype and analyze genetic markers in order to understand connectivity and demographic history. We have experience analyzing nuclear and mitochondrial DNA sequences, microsatellites and single nucleotide polymorphisms (or SNPs) that range from 1-100s of loci per species. We collaborate with computer modelers to quantify the relative roles of dispersal, historical demography and natural selection and with oceanographers to understand the role of surface currents in driving patterns of population genetic structure.

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