An ecological community is a collection of different species that live together in the same geographical area and interact with one another. This grouping can be as large as a vast forest or as small as the organisms living within a single pitcher plant. Community ecology studies the organizational patterns and interactions among these multiple populations, which include plants, animals, fungi, and microorganisms. Understanding the structure and function of these communities is fundamental to grasping the complexity of life on Earth.
Defining the Components and Boundaries
A community is defined by two primary characteristics describing its living components. The first is species richness, which is a straightforward count of the number of different species present in the area. For example, a tropical rainforest has far greater species richness than a tundra environment.
The second characteristic is relative abundance, which measures how common or rare each species is compared to others in the community. A community where all species have roughly equal numbers is considered more diverse than one dominated by only a few species. Ecologists define geographical boundaries based on the specific question being asked.
Many natural boundaries are gradual transitional zones called ecotones, such as the area where a forest slowly gives way to a grassland. Ecotones often host species from both adjacent communities, resulting in an increase in local species richness. For practical study purposes, a researcher may define an arbitrary boundary, such as a square plot of land, to focus on a particular assemblage of species.
Key Interactions Shaping the Community
Populations are interconnected through various interspecific interactions, categorized by the effect they have on the two species involved. Competition is a negative interaction for both parties, as two species vie for the same limited resource, such as plants competing for sunlight (-/-). In predation, one species benefits by consuming the other, like a lion hunting a zebra (+/-). Herbivory is a specific type of predation where an animal consumes a plant, such as a deer grazing on shrubs (+/-).
Parasitism also involves a benefit to one species and harm to the other, but the parasite, like a tick, usually lives on the host without immediately killing it (+/-). Mutualism is a cooperative relationship where both species benefit, such as a bee gaining nectar while pollinating a flower (+/+). Finally, commensalism occurs when one species benefits while the other is neither helped nor harmed, such as barnacles attaching to a whale (+/0). These interwoven relationships fundamentally influence which species can coexist and thrive within the community.
Understanding Community Structure
Community organization is defined by the trophic structure, which describes how energy flows through its members. Organisms are grouped into trophic levels based on their primary source of nourishment. Producers, like plants, convert sunlight into chemical energy. Primary consumers (herbivores) feed directly on producers, followed by secondary and tertiary consumers (carnivores or omnivores).
Energy transfer is highly inefficient; approximately 90% is lost as heat or waste at each step, known as the 10% rule. This massive energy loss explains why food chains rarely extend beyond four or five levels. These feeding relationships form intricate food webs, where most organisms consume or are consumed by multiple species. Within this web, keystone species are disproportionately important in maintaining the community’s stability.
The removal of a keystone species, such as a sea otter controlling the sea urchin population, can trigger dramatic changes across the entire community structure. Decomposers, including fungi and bacteria, play a distinct role by breaking down dead organic matter and waste. This process is crucial for nutrient cycling, transforming complex organic molecules into simpler inorganic nutrients. Without these recyclers, essential elements would remain locked up, unavailable for producers to use in new growth.
How Communities Respond to Change
Ecological communities are constantly responding to environmental shifts and disturbances. Ecological succession is the process of predictable, sequential changes in species composition following a disturbance. Natural disturbances, such as wildfires or volcanic eruptions, act as drivers that reset the community. Primary succession begins in an area completely devoid of life and soil, such as newly formed volcanic rock.
Pioneer species, like lichens and mosses, are the first to colonize these barren areas, slowly building up the first layer of soil. Secondary succession is a faster process occurring where a previous community existed but was disturbed, leaving soil intact, such as a forest recovering after a fire. In secondary succession, fast-growing plants quickly colonize the area, followed by shrubs and various tree species. Eventually, the community may reach a relatively stable state, but these mature communities remain subject to future disturbance.