Award: OCE-1129166

Individuals within natural populations often differ considerably in their diets because of individual differences in food preferences or due to differences through space or time in the availability of food.  This project examined the reasons behind individual diet variation and the consequences of that variation for reproductive success.  We used crabs in this study because they are abundant, easy to use in experiments, live in a wide range of ocean habitats, and have a broad range of natural diets (from herbivores to carnivores).  We examined several different species of crab found on the shore along the United States east coast.  For each of these, we examined their diets and the factors that caused differences in dietary intake between individuals.  We also examined their metabolic rates and ability to assimilate consumed food into their bodies, both of which vary with diet.  Then for each species we experimentally measured the impacts of dietary intake on reproductive success. 

We found that the diet of each of the species we examined was influenced by changes to their environment brought about by humans.  Each species experienced a different type of human-caused stress, and this meant that each species had its own unique mechanisms that caused differences between individuals in their diets.  A simple example of this can be seen by contrasting two of the species we examined.  The mangrove tree crab in Florida is responding to climate change by moving further northward.  Its mangrove habitat is also creeping northward, but not as fast.  As a consequence, some of the tree crabs now live in salt marshes that are found north of the mangroves.  Crabs in the salt marsh have a different diet and also experience much less shading than those in the mangroves.  As a result, the balance between the energy intake from food and the energy expenditure (which is determined by temperature for these cold-blooded animals) has shifted so that crabs in the marsh have less energy available for reproduction.  They are producing smaller clutches of lower quality eggs than their counterparts in the mangrove.  As a second example, mud crab lives in oyster reefs on the southeast coast that are often degraded for several reasons (oyster harvesting, disease, lack of oxygen in the water, getting buried by sediment, etc.).  Mud crabs eat oysters, so as oyster reefs degrade, mud crabs have less to eat.  However, the reduction in food consumption depends on the personality of the individual crab.  Bold crabs will readily leave degraded reefs to find better ones that offer more food, while shy crabs are more likely to remain on degraded reefs rather than take the risk of trying to move between reefs.    This results in bold crabs living on healthy reefs where they eat a lot of food and shy crabs living on low quality reefs where they eat smaller amounts of food.  The amount of food consumed by individual crabs in turn controls how many eggs they produce. 

As illustrated by these two examples, each system we examined had very different mechanisms that altered dietary food intake, but in each case the end result was the same – as crabs ate less food, and especially as they ate less animal food (even if they still ate lots of plants), their reproductive success dropped off quickly.  This was even true for species that are generally considered to be primarily herbivorous – consumption of animal prey was necessary for successful reproduction.  Since the diet changes in each of the systems examined were caused by human impacts, results of this project will help us to better understand the consequences of human impacts on the animals that live in our coastal oceans.  Results of this project may also be used to help improve fisheries, since two of the species included in this study (the Florida stone crab and the blue crab) support active commercial and recreational fisheries.

This project involved 8 graduate students and more than 2 dozen undergraduate students who participated and received training and experience in all aspects of the scientific process.  Results from this project have also had important impacts on the nonscientific community.  Specifically, results here were used as the basis for 3^rd and 5^th grade lesson plans that were developed by local elementary school teachers here in Columbia, SC, and are available for general use.  In addition, the results of this project served as the basis for more than 130 education outreach visits by me and members of my lab to K-12 classrooms throughout South Carolina.  These presentations focused on introducing kids to ocean sciences and on enhancing their quantitative and critical thinking skills.

Data collected as part of this project are freely available to the public on the web at http://www.bco-dmo.org/project/562104.

 

Last Modified: 09/08/2016
Modified by: Blaine Griffen