October 21, 2014 Role of Animals in Salt Marsh Restoration

EEOS 660

Role of Animals in Salt Marsh Restoration

Salt marshes along the eastern United States are “among the most productive in the world” and are important both environmentally and economically (Silliman and Bertness, 2002). Animals are an important part of ecosystem functions, found in every coastal habitat, and can either inhibit or enhance primary production in salt marshes. Not only do animals affect primary production, they can also be key indicators of unhealthy ecosystems. As many of the animals in the salt marsh have evolved adaptations to cope specifically with the conditions of the salt marsh environment, they are dramatically affected by the changes that occur in the systems. Changes in population density or biodiversity is often an indication of a failing ecosystem and can lead to damaging trophic cascades, which effect the ecosystem services provided by the salt marsh.

Salt marsh habitats are subjected to extreme changes in environmental conditions as temperature and salinity change with tidal fluctuations. Most marine animals can deal with differences in the concentration of individual salts in their environment but not the total concentration of salts (Teal and Teal, 1969). Animals in salt marshes have adapted, both physiologically and behaviorally, to combat these changing conditions. Some of the adaptations include closable shells that prevent desiccation during low tide, a primitive lung in the fiddler crab to cope with being out of water, ability to regulate temperature and osmotic balance, burrowing behavior, and niche selection. For example, coffee bean snails (Melampus bidentatus) will climb up stems to avoid inundation and mollusks tend to occupy the low tide zone to limit their time out of water (Shumway, 2008).

Salt marsh restoration efforts are important in maintaining the biodiversity of life within the system and maintaining the balance of predator-prey relationships. Not only is diversity important to ecosystem functioning, the percent-cover of species is also fundamental. Cardinale et al (2012) explained that the “loss of diversity across trophic levels has the potential to influence ecosystem functions even more strongly than diversity loss within trophic levels.” The loss of diversity across trophic levels can cause a trophic cascade, in which the removal or loss of predators causes a top-down effect on the population density of consumers. Increasing the population density of consumers, such as periwinkle or purple marsh crabs, have caused a loss in plant biomass in salt marshes (die-offs), impacting the ability of the marsh to provide crucial ecosystem services.

Silliman and Bertness (2002) conducted a study on the effects of trophic cascades and the impacts on primary production, which concluded that the “overexploitation of a major predator may indirectly alter the structure and function of intertidal marsh habitats.” The observations showed that an increase in the population density of Littoraria irrorata (common periwinkle) exerted a strong top-down control on plant biomass production for Spartina. Although periwinkles don’t directly eat Spartina, as they graze on algae and fungus on the stems of the plant they leave behind scars that become colonized by fungi. An increase in the population density of the snails increases the amount of scaring which cause Spartina to become susceptible to infection (Shumway, 2008). Altieri et al (2012) also reported evidence of recreational fishing along salt marshes having an impact on the population of herbivorous purple marsh crabs (Sesarma reticulatum).

Salt marsh ecosystems are more sensitive to predator removal because of the limited number of secondary consumers that are adapted to the conditions in the marsh (mainly small fish and carnivorous crabs). However, grazing of herbivores has also been shown to increase with bottom-up influences such as nitrogen availability. An increase in nutrients, found in the tall form Spartina near the creek banks, had shown an increase in grazing in Silliman and Bertness (2002) study. The increased grazing resulted in a dramatic decrease in biomass in these areas, also indicating the importance of maintaining predation on periwinkles. Even though other factors may have influenced the results of the studies, aside from over-fishing, the lack of predators in the area still had a top-down trophic cascade on the population density of both the periwinkle and the purple marsh crab.

Another study by Bertness and Coverdale (2013) shows the importance of top-down predator control. Although invasive species are typically thought to be harmful to ecosystems, the European green crab (Carcinus maenas) may be facilitating the recovery of salt marsh areas that have been destroyed by the purple marsh crab in Cape Cod. Not only do green crabs prey on purple marsh crabs, their presence alone may be enough to drive out the purple marsh crab and decrease their effect on Spartina, regardless of how many are actually consumed.

Animals can also be indicators of unhealthy salt marsh ecosystems. A rapid decline in the population density of native species can indicate toxic or unhealthy conditions because they were unable to tolerate the change in conditions. Even though Bertness and Coverdale (2013) showed the benefit of an invasive species, some species can still be harmful to ecosystem functions. DDT (dichlorodiphenyltrichloroethane) is now banned in the United States but the effect that it had on bird populations is an effective example of animals as indicators. The breakdown product of DDT is DDE (dichlorodiphenyldichloroethylene), which inhibits calcium deposition important for the formation of eggshells. Bioaccumulations of DDE lead to a decline in shorebird populations by approximately 90% throughout the east coast (Shumway, 2008). Also, declining water conditions, such as increased turbidity, can make the marsh habitat unsuitable for filter feeders such as mussels.

Understanding how salt marshes respond to disturbances is key to restoration efforts, both long-term and short-term, and disturbances from animals are primarily driven by population density. Short-term efforts may be to determine what population densities are healthy for maintaining ecosystem functions and services. Long-term conservation efforts of salt marshes would need to include maintaining top-down control of herbivores by avoiding overexploitation of predators that are important to the system, whether they are primarily found in the marsh, introduced to the system, or just use it as a nursery.

As shown by Bertness and Coverdale (2013), invasive species, specifically green crabs, can contribute to salt marsh restoration by exerting top-down control of the purple marsh crab. There is an upper limit to the population densities that salt marshes can support and knowing those limits is important to both long- and short-term restoration efforts. However, there is also a minimal limit of secondary consumers that needs to be maintained in order to sustain the predator-prey relationships important to the health of the salt marsh. Biodiversity and the impact it has on trophic cascades are crucial to the restoration process of salt marshes, and determining healthy population densities that promote the maintenance of biodiversity and trophic interactions would benefit restoration efforts.

References:

Altieri, A., Bertness, M., Coverdale, T., Herrmann, N., & Angelini, C. (2012). A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing. Ecology, 93(6), 1402-1410.

Bertness, M., & Coverdale, T. (2013). An invasive species facilitates the recovery of salt marsh ecosystems on Cape Cod. Ecology, 94 (9), 1937-1943. http://www.esajournals.org/doi/abs/10.1890/12-2150.1

Cardinale, B., et al (2012). Biodiversity loss and its impact on humanity. Nature, 486, 59-67.

Shumway, S. (2008). Salt Marshes. In The naturalist's guide to the Atlantic seashore: Beach ecology from the Gulf of Maine to Cape Hatteras. Guilford, Conn.: Globe Pequot Press.