The ecological role of the Florida manatee (Trichechus manatus latirostris) in waterhyacinth-dominated ecosystems / Mark Vincent Lomolino.

Tese (mestre) - University of Florida, 1977

Feeding studies and ecosystem simulations were conducted to investigate the effects of herbivory by Florida manatees (Trichechus manatus latirostris) on the plant communities of ecosystems into which waterhyacinths have invaded, and to suggest some characteristics of ideal biological control agents of waterhyacinths (Eichhornia crassipes) and other aquatic weeds. The mean dry weight consumption rate of an adult manatee eating waterhyacinth was 48 kg per hour. The corresponding daily consumption rate of an adult manatee eating waterhyacinths in a natural ecosystem was estimated to be 2.88 to 3.84 kg (dry weight). The retention time of waterhyacinths was 146 hours. Mean digestive efficiencies for dry weight, energy, and nitrogen in waterhyacinths were 82.6, 80.0, and 78.1 percent, respectively. These values were significantly lower than those of lettuce: 91.4, 88.8, 93.8 percent. There was no significant difference between the mean digestive efficiencies for phosphorus in waterhyacinths and lettuce (82.0 and 70.3, respectively). The high digestive efficiencies were attributed to the long retention time of plant material, the potentially high microfloral populations, and the comparatively low crude fiber content of plants consumed. Assimilation efficiencies for phosphorus in both waterhyacinths and letture were low: 5.1 and 22.1 percent, respectively. Ecosystem simulations that, by reducing the abundance of the dominant plants, i.e., waterhyacinths, herbivory by manatees may reduce net productivity of the plant community, but increase its diversity and, hence, its resistence to future dominance by waterhyacinths. The stocking levels of mantees that would be necessary to reduce waterhyacinths from 80 percent to less than five percent coverage in approximately nine years were 1.6 manatees per hectare if they are continuosly preent in the system, and 3.9 manatees per hectare if they are present late fall and winter. These simulations suggested three important characteristics of an ideal biological control agent for waterhyacinths and other aquatic weeds. 1. Efficiencies of digestion and assimilation, and secondary productivity should be high to prevent substantial contributions to the levels of dissolved nutrients. 2. Abundance or metabolic rate of the organism should be postively correlated with seasonal variations in primary productivity, so that it is less limited by low levels of primary productivity during winter, and its grazing intensity is highest during periods of maximum primary productivity. 3. The agent should be a generalist in feeding habits, so it can utilize different species of aquatic plants and maintain itself by feeding on more abundant producers when weed levels are low. Furthermore, by feeding most heavily on the dominant plants, a generalist herbivore would increase diversity of the plant community and, consequently, its resistance todominance by weed plants.