Felicia Woods
EEOS 660
October 21, 2014
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 topdown 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, 5967.
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.
Silliman, B., & Bertness, M. (2002). A trophic cascade regulates salt marsh primary
production. PNAS, 99(16), 10500-10505
http://www.pnas.org/content/99/16/10500.full.pdf+html?sid=dd3ef75d-f088-4e489ff0-803f42b63479
Teal, J., & Teal, M. (1969). Marsh Animals. In Life and death of the salt marsh. New
York: Ballantine.