Community ecology is the scientific study of the interactions that occur between different species living together within a particular geographic area. This field investigates how these relationships shape the distribution, abundance, and overall structure of biological communities. Community ecology provides a framework for understanding the complex web of life that sustains ecosystems and offers insight into the organization and health of natural environments.
How Ecologists Define a Community
A biological community is defined by its structure, which ecologists characterize using metrics describing the variety and relative numbers of species present. The simplest measure is species richness, which is a count of the total number of distinct species found in an area. For instance, a forest plot might contain 50 different species of trees, birds, and insects, resulting in a species richness of 50.
A more comprehensive measure is species diversity, which incorporates both species richness and relative abundance (evenness). Relative abundance quantifies how equally individuals are distributed among the different species. A community where all species have roughly the same number of individuals is considered highly diverse. In contrast, a community dominated by a single or a few species is considered less diverse.
Ecologists use mathematical indices, such as the Shannon-Wiener index, to quantify species diversity for comparison between habitats. These measures allow scientists to track changes in community composition over time or assess the impact of environmental factors like pollution or habitat fragmentation. Understanding community composition is the foundational step before analyzing the processes that maintain or alter its structure.
Major Types of Species Interactions
The structure of a community is influenced by direct interactions between member species, categorized based on whether they affect each species positively (+), negatively (-), or neutrally (0). Competition occurs when two species vie for the same limited resource, such as food or space, resulting in a negative effect on both parties (-/-). This interaction can lead to the local elimination of one species or the evolutionary divergence of resource use.
Predation and herbivory are interactions where one species benefits at the expense of another, categorized as (+/-). Predation involves a predator killing and consuming prey, while herbivory involves an animal consuming parts of a plant; both drive the coevolution of offensive and defensive traits. Parasitism is also a (+/-) relationship, where the parasite lives on or in its host, obtaining nourishment without immediately killing the host.
Mutualism is a relationship where both interacting species receive a fitness benefit, denoted as (+/+). An example is the relationship between flowering plants and their pollinators, where the plant receives reproductive assistance and the animal receives nectar. Commensalism describes an interaction where one species benefits while the other is neither harmed nor helped (+/0), such as a bird nesting in a tree.
Understanding Community Dynamics and Change
Biological communities are not static; their composition changes over time through ecological succession. This directional change in species structure is often initiated by a disturbance event. Primary succession begins in an environment devoid of life and soil, such as on volcanic rock or retreating glaciers. Pioneer species, like lichens and mosses, colonize the barren substrate and gradually create the first layers of soil.
Secondary succession occurs when a disturbance, such as a wildfire or logging, removes existing vegetation but leaves the soil intact. This process is faster than primary succession because the established soil and seed bank allow for quicker colonization by early-successional species. Over decades, these species are replaced by later-successional species, eventually leading to a community dominated by long-lived trees.
Disturbance plays a significant part in maintaining the variety of life within a landscape. Intermediate levels of disturbance can prevent a few species from dominating a community, thereby increasing overall species diversity. The community’s response to these events, known as resilience, determines how quickly it can return to its previous state. While older theory described a stable end-point called a climax community, modern ecologists recognize that most communities are in a continuous state of flux due to ongoing disturbances.
Energy Flow and Trophic Structure
Energy enters most communities through the first trophic level, composed of primary producers, typically photosynthetic organisms like plants or algae. Consumers acquire this energy by feeding on organisms from other levels, establishing the community’s trophic structure. Primary consumers (herbivores) feed on producers, secondary consumers (carnivores) feed on primary consumers, forming pathways known as food chains and complex food webs.
Energy transfer between trophic levels is highly inefficient, described by the 10% rule. On average, only about ten percent of the energy stored in the biomass of one trophic level is passed on to the next. The remaining ninety percent is lost primarily as heat during metabolic processes and waste production. This energy loss is why food webs rarely have more than four or five trophic levels; there is not enough energy remaining to support higher consumers.
Within a food web, certain keystone species have a disproportionately large impact on community structure relative to their abundance. Often occupying a high trophic level, the removal of a keystone predator can trigger a trophic cascade that fundamentally alters the abundance of other species, demonstrating its structural control over the community. These species show how the flow of energy and matter is tightly regulated by specific interactions.