A man in a boat on the river watched a blue heron wade along the bank. Suddenly, its head darted down. When the heron's beak cleared the water, frog legs dangled. The man witnessed biotic factors in the freshwater ecosystem. Understanding the interactions of the organisms in an ecosystem helps people better manage it. Freshwater ecosystems are important because they act as resources for potable water, food and entertainment.
Freshwater Ecosystem Biotic Factors Defined
Freshwater ecosystems include lakes, ponds, streams, rivers, swamps, marshes and bogs. Biotic factors for freshwater and other ecosystems share the same concepts. In their book "Environment: The Science Behind the Stories," Jay Withgott and Scott Brennan provide a basic definition for biotic factors: any living component of the environment. Mohan Arora, in his book "Ecology," takes it one step further by designating living organisms as producers, consumers and decomposers. Christer Bronmark and Lars-Anders Hansson, in their book "Biology of Lakes and Ponds," offer a more complex definition. Biotic factors are the interactions between living things in the ecosystem. Organisms affect each other in one of three ways, positive, negative or no effect.
Herbivores eat only plants. In terrestrial ecosystems herbivores consume only leaves, not the entire plant, so their impact on the plant is neutral. In a freshwater ecosystem, herbivores may eat the entire plant, or algae. Freshwater herbivores impact the plant negatively. In a freshwater ecosystem, herbivores and predators are viewed the same because both destroy the organism they are consuming. Hippos and manatees are freshwater herbivores that don't follow this pattern since they graze like cattle and don't feed on algae. The American flagfish is an example of an herbivore that feeds exclusively on algae.
Predators consume other animals. Another name for predator is carnivore. They gain through their interaction with other organisms, but the other organisms are irrevocably harmed. By definition, biotic factors include the interactions or effects of one organism on another organism. When a bass eats a minnow, the minnow has a positive effect on the bass, but the bass has a negative effect on the minnow.
Parasitism, a type of symbiosis, occurs between two organisms in which one organism, the parasite, requires another organism, a host, to survive. The parasite gains through the relationship, and the host suffers. Several freshwater parasites exist, including protozoa, leeches, nematodes and flukes. Ichthyophthirius multifiliis, a protozoa, causes white spot disease. Its more common name is ich. Fish will have swollen gills and lesions on their bodies and fins. The microbe benefits from living off the fish, but the fish suffers.
Mutualism and Commensalism
Two more types of symbiosis are mutualism and commensalism. In mutualism, both organisms benefit from the relationship. With commensalism, one organism benefits, but the other organism is neither helped nor harmed. In a case of freshwater ecosystem mutualism, Hydra viridis has a green algae, chlorella, living in its digestive cells. Both organisms benefit from this union. The algae growing on the back of a manatee would be an example of commensalism. The manatee is not harmed or helped, but the algae has a place to grow relatively free of predation.
Competition not only refers to interactions between different kinds of organisms, but between organisms of the same species. In competition, organisms compete for resources such as mates, food, water and homes. Competition involves the interaction of biotic and abiotic factors. Abiotic factors are the interactions of living organisms with the physical world. In freshwater ecosystems this includes clearness of water, depth in the water column, temperature and pH, just to name a few.