Biotic factors are conditions created by living things that influence other organisms within the same ecosystem. Ecology scientists study these different types of interactions. Biotic factors include interactions, conditions and energy created or changed by living things.
Keystone species are the species that "regulate" ecosystems. Additionally, their effect on an ecosystem is disproportionate to their mass. Usually, the keystone species is an apex predator. Apex predators are large predators with no predators of their own. In the absence of apex predators, competition tends to favor one prey species over another, leading to less diversity in the environment. In many freshwater systems, the pikeminnow (Ptychocheilus oregonensis) acts as an apex predator.
Autotrophs produce their own energy from abiotic, or nonliving, factors. They are also called producers since they "produce" energy for an ecosystem. In freshwater systems, they are almost always plants or algae. They produce sugars and oxygen from sunlight, carbon dioxide and abiotic nutrients. More rarely, some ecosystems start with bacteria or archaeans that produce energy through chemical reactions. This is not the case in most freshwater enviroments, but in some freshwater hot springs, heat-tolerant microorganisms form the base of the food chains. Autotroph productivity is an important biotic factor as it supplies energy for the entire ecosystem.
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Competition between organisms is another important biotic factor in an ecosystem. This competition can take several forms. But in all situations, two different organisms fight over the same resources. This can include competition between different species. It can also take the form of competition between individuals within the same species. Competition is the driving force behind evolution and shapes all ecosystems.
Competition is not the only form of interaction between species. Mutualism, or cooperation, is another important biotic factor, though competition tends to steal the spotlight. Sometimes different species cooperate to improve their chances of survival. One freshwater example is a highly adapted form of mycorrhizal fungus that works together with the roots of swamp plants. These fungi help the plant liberate abiotic nutrients more easily. In return, the plant provides the fungus with energy in the form of sugar. Similarly, the freshwater hydra, a tiny sea-anemone-like animal, has symbiotic algae living within its cells. These algae produce energy, while the hydra protects the algae with its stinging cells.