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By Phil Williams
Salt marshes are among the most biologically interesting places on earth. The waters and soil teem with plants and animals bound up in the cycles of life and death. For plant-eating
animals known as herbivores, salt marshes are a garden of delights, a salad bar with savory selections.
Picky eaters
Herbivores can, however, be picky eaters, depending on how palatable salt-marsh plants are. New research by UGA’s Marine Institute on Sapelo Island, in collaboration with Brown University in Providence, R.I., strongly suggests that salt-marsh plants nearer the equator are less  palatable to herbivores than ones considerably farther away. It is the first time such a preference by herbivores for northern versus tropical plants has been conclusively demonstrated.
“There has long been a biogeographic theory that the pressure of predators and prey defenses is greater in the tropics than in higher latitudes,” says Steven Pennings, an assistant research scientist at the Marine Institute. “But there hasn’t been much work examining similar patterns in plant communities.”
Pennings collaborated on the research with Mark Bertness and Erin Siska of Brown University. Their work, which was supported by a grant from the National Geographic Society, was presented at the 84th annual meeting of the Ecological Society of America.
The researchers selected 10 salt-marsh plants common both to the Northeast coast (Maine and Rhode Island) and to sites in Georgia and Florida. These plants make up more than 85 percent of total plant biomass in all the marsh zones under study. Plant material from Rhode Island was collected, packed on ice and immediately flown to Sapelo Island, where it was exposed to a group of herbivores on the same day.
Likewise, southern plants were flown to Rhode Island for use with northeastern herbivores. Leaf beetles, grasshoppers, moth larvae and crabs were used to test palatability of the plants. In all cases, the herbivores preferred the northern plants.
“One difficulty with this kind of research is that it must be done on a large spatial scale,” says Pennings, “but Mark Bertness, my collaborator at Brown, smoothed the way for all the work in New England.”
The researchers conducted feeding trials for all 10 marsh plants using herbivores that feed naturally on each species. Twenty herbivores were housed individually in appropriate chambers with a water source. The scientists then added leaves to the herbivore chambers, one leaf from the north and one of the same species from the south. (Oddly, Atlantic coastal salt marshes are ideal for examining plant-animal interactions because salt marshes actually consist of very few species of plants, making it logistically possible to study virtually the entire plant community.)
Feeding trials
Feeding trials were checked twice a day for three days, and individual testing was stopped when the herbivore had eaten at least 30 percent of one leaf. Consumption was measured in terms of square millimeters of leaf area lost.
“We conducted trials using plants from multiple sites within each geographic location to provide spatial replication,” says Pennings. “We conducted feeding trials at least twice during the growing season for two different years [1997 and 1998] to determine if seasonal differences exist.
“We found that for every plant species tested, there was a striking trend for herbivores to consume greater quantities of plants from the north when given a choice, and this was true regardless of plant species, herbivore used or time of year.”
The reasons why herbivores prefer northern plants are not yet clear and will require further study, Pennings says. Still, some intriguing possibilities exist. Scientists know that herbivore food preferences are influenced by physical and chemical defenses, such as plant toughness and secondary metabolites. Broad-leafed trees in the tropics, for example, contain more crude fiber and are tougher than temperate species.
These two characteristics provide structural support for the leaf and may also add significantly to its defense. Scientists also know that some salt-marsh plants have effective chemical defenses. For instance the leaves of the groundsel tree, Baccharis haimifolia, are toxic and may cause death if eaten by cattle. The common cattail, Typha latipholia, contains anti-algal organic steroid toxins.
Geese, snails, crabs and other animal species have all demonstrated decreased feeding in response to the presence of defensive chemicals in salt-marsh plants. Feeding pressures in the tropics may have played an evolutionary role in the development of defenses by salt-marsh plants.
Latitudinal differences in consumer-prey interactions were first proposed in the 1970s. This research, however, suggests new possibilities for study.
“We believe this is an important finding, because it may be true for other plant communities as well,” says Pennings. “Our next goal is to find out what plant traits differ between the north and south to cause these dramatic differences.”
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