9.5 Community Ecology

6 min read•august 22, 2024

Community ecology explores how species interact and coexist in shared environments. It examines factors like species richness, evenness, and interactions such as competition and predation. These concepts help us understand the complex web of relationships that shape ecological communities.

This topic connects to the broader study of ecology by highlighting the importance of species interactions in shaping ecosystems. It shows how individual organisms influence each other and their environment, contributing to the dynamic nature of ecological systems studied throughout the chapter.

Ecological Community Structure

Species Richness and Evenness

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  • Ecological communities are assemblages of different species living in the same area and interacting with one another and their environment
  • The structure of a community is determined by the number of species present (species richness) and their relative abundances (species evenness)
  • Communities with high species richness have a greater variety of species, while those with high species evenness have a more balanced distribution of individuals among species
  • The distribution and abundance of species within a community can be represented by rank-abundance curves, which plot the relative abundance of each species against its rank in abundance
    • Steep rank-abundance curves indicate low species evenness, with a few dominant species and many rare species
    • Shallow rank-abundance curves indicate high species evenness, with a more even distribution of individuals among species

Factors Shaping Community Structure

  • Biotic factors, such as competition and predation, and abiotic factors, such as temperature and rainfall, shape the composition and structure of ecological communities
  • Niche partitioning allows multiple species to coexist in a community by reducing competition for resources
    • Species occupy different ecological niches based on their resource requirements and interactions with other species
    • Examples of niche partitioning include different bird species feeding on different parts of a tree (canopy, trunk, or understory) or different sizes of seeds
  • Environmental gradients, such as elevation, soil moisture, or salinity, can influence the distribution and abundance of species within a community
    • Species adapted to specific environmental conditions will be more abundant in areas where those conditions prevail
    • Ecotones, transitional zones between two distinct communities (forest and grassland), often have high species richness due to the overlap of species from both communities

Species Interactions in Communities

Competition and Predation

  • Interspecific competition occurs when two or more species compete for the same limited resources, such as food, water, or space
    • The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely in the same community
    • Competition can lead to resource partitioning, where species specialize in using different resources or occupy different microhabitats to reduce competition
  • Predation is an interaction in which one species (the predator) feeds on another species (the prey)
    • Predator-prey relationships can influence the population dynamics of both species and shape community structure
    • Predators can exhibit prey specialization (feeding on a narrow range of prey species) or generalization (feeding on a wide range of prey species)
    • Prey species may evolve various defenses against predation, such as camouflage (leaf insects), warning coloration (monarch butterflies), or chemical defenses (poison dart frogs)

Symbiotic Relationships

  • Mutualism is a symbiotic relationship in which both species benefit from the interaction
    • Examples include pollination, where plants provide nectar and pollen to pollinators (bees, hummingbirds) in exchange for pollination services, and nitrogen fixation in legume-rhizobium associations
  • Commensalism is an interaction in which one species benefits while the other is unaffected
    • Examples include barnacles growing on whales, gaining a substrate for attachment and access to food particles, while the whale is unaffected, and epiphytic plants (orchids) growing on trees, benefiting from increased light and moisture without harming the tree
  • Parasitism is a relationship in which one species (the parasite) benefits at the expense of another species (the host)
    • Parasites can influence host population dynamics and community structure by reducing host fitness and altering host behavior
    • Examples include mistletoes parasitizing trees, obtaining water and nutrients from the host, and tapeworms living in the digestive tracts of animals, absorbing nutrients from the host's food

Ecological Succession and Community Change

Succession Processes

  • Ecological succession is the gradual process of change in the species composition and community structure over time, often following a disturbance or the colonization of a new area
  • Primary succession occurs when a community develops in a previously uninhabited area, such as a newly formed volcanic island or a glacial moraine
    • Pioneer species are the first to colonize the area, followed by a predictable sequence of species replacements
    • Examples of primary succession include the colonization of bare rock surfaces by lichens and mosses, followed by herbaceous plants, shrubs, and eventually trees
  • Secondary succession occurs when a community recovers after a disturbance, such as a fire or clear-cutting, that removes the existing vegetation but leaves the soil intact
    • The community undergoes a series of changes, eventually reaching a relatively stable state
    • Examples of secondary succession include the regrowth of a forest after a wildfire or the recovery of a grassland after abandonment of agricultural land

Successional Stages and Climax Communities

  • Successional stages include the pioneer stage, characterized by fast-growing, opportunistic species; the intermediate stage, with increasing species diversity and competition; and the climax stage, representing a relatively stable community composition
  • Climax communities are the final, self-perpetuating stage of succession, characterized by a diverse assemblage of species well-adapted to the prevailing environmental conditions
    • The concept of climax communities has been debated, as communities are often subject to ongoing disturbances and may not reach a true equilibrium state
    • Alternative stable states may exist, where a community can shift between different compositional states depending on the type and intensity of disturbances
  • Succession can be influenced by various factors, such as the nature and severity of the disturbance, the availability of propagules (seeds, spores, or other reproductive structures), and the biotic and abiotic conditions of the site
    • Facilitation, where early successional species modify the environment in ways that favor the establishment of later successional species (nitrogen-fixing plants enriching the soil), can influence the trajectory of succession

Keystone Species and Ecosystem Stability

Keystone Species Concept

  • Keystone species are species that have a disproportionately large influence on the structure and functioning of an ecological community relative to their abundance
  • The removal or significant decline of a keystone species can lead to cascading effects on other species and alter the overall community composition and ecosystem processes
  • Keystone species play critical roles in maintaining the stability and diversity of ecological communities

Types and Examples of Keystone Species

  • Keystone predators, such as wolves in Yellowstone National Park, can regulate the populations of their prey species (elk) and indirectly influence the abundance and distribution of other species in the community (aspen, beavers)
    • The reintroduction of wolves in Yellowstone led to a trophic cascade, with reduced elk populations, increased aspen and willow growth, and the return of beavers and other species
  • Keystone modifiers, such as beavers, physically alter the environment through their activities (dam building), creating habitats for other species and influencing ecosystem processes like nutrient cycling and water flow
    • Beaver dams create wetlands that support a diverse array of plant and animal species, and their ponds trap sediments and nutrients, improving water quality downstream
  • Other examples of keystone species include:
    • Sea otters in kelp forest ecosystems, where they control the population of sea urchins and maintain the balance between kelp and urchin-dominated states
    • African elephants in savanna ecosystems, where they shape the vegetation structure through their foraging and dispersal of seeds
    • Reef-building corals in coral reef ecosystems, providing habitat and shelter for a wide variety of marine species

Implications for Conservation and Management

  • The concept of keystone species highlights the importance of species interactions and the role of certain species in maintaining the stability and functioning of ecological communities and ecosystems
  • Identifying and protecting keystone species can be crucial for the conservation and management of biodiversity and ecosystem services
  • The loss of keystone species can have far-reaching consequences for the entire community and ecosystem, emphasizing the need for their conservation and the maintenance of species interactions in natural systems
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