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Communities do not stay constant. Instead, they change over time in response to interactions between populations. These interactions determine how individuals and species access resources within a community. Relationships among populations can have both positive and negative effects and these effects can be modeled as scientists use them to understand how the community functions.
Competition
Competition occurs when multiple species utilize the same limited resources. These resources can be habitat, food, or anything else each species needs to survive. Competition plays a significant role in communities because these interactions shape the structure of ecosystems. Competition can also push species into niche partitioning. Niche partitioning is when species evolve to occupy slightly different niches. This reduces direct competition because the species change physically or behaviorally to utilize different resources. This division of resources helps organisms obtain the resources they need more efficiently and conserve energy.
Predation
Predation is the interaction between predator and prey, and it plays a large role in regulating the population size of prey species. This helps maintain balance across all trophic levels. The ecological phenomenon where a predator indirectly impacts organisms in lower trophic levels is called a trophic cascade. For example, wolves prey on elk that eat grass and other plants. When the wolf population decreases, the elk population increases, which will result in increased herbivory of the plants in the ecosystem. Even though wolves do not primarily consume plants, their role in the ecosystem is critical to the primary producer population.
Symbioses
Symbioses are unique types of relationships between species in an ecosystem. These relationships help shape population dynamics due to their influence on resource availability and their involvement in ecological dynamics. Some of these interactions have led to evolutionary adaptations as species evolve together or specialize to take advantage of other species. There are three main forms of symbiotic relationships – parasitism, mutualism, and commensalism. Parasitism is a relationship where one species benefits at the expense of another. Mutualism is a relationship where both species benefit. An example of this are lichen. A lichen is a partnership between a fungus and an alga. The fungus provides a structure for the algae to live on and the algae photosynthesize and provides the fungus with food. Cooperation between species can result in improved resource availability. Commensalism is a relationship where one species benefits but the other is not affected positively or negatively.
