Community ecology

1. Community structure and diversity

An ecological community is an assemblage of populations of different species occurring together and interacting, directly or indirectly, within a shared area. A pond community includes algae, aquatic plants, zooplankton, insects, fish and microorganisms. Water, temperature, light and nutrients influence this assemblage but are abiotic components of the ecosystem, not members of the community. Community boundaries are often chosen for a study rather than being sharply defined in nature.

Community structure describes species composition, abundance, spatial arrangement and trophic relationships. Species richness is the number of species present; evenness describes how uniformly individuals are distributed among those species. Two forests may have equal richness but different diversity if one is overwhelmingly dominated by a single species. Diversity indices such as the Shannon index incorporate both richness and relative abundance.

Alpha diversity refers to diversity within a habitat or local community. Beta diversity describes differences or turnover in species composition among communities. Gamma diversity is the total diversity across a larger region. Sampling area, effort and season influence observed richness; therefore, comparisons require comparable methods.

Communities also show vertical stratification and horizontal patchiness. Forest canopies, understories and forest floors provide different light and moisture conditions. Organisms occupying these layers may use different resources, allowing many species to coexist within the same broad habitat.

• Population: individuals of one species in an area.
• Community: populations of multiple species in an area.
• Ecosystem: the community together with its abiotic environment and their interactions.

2. Habitat, niche and species interactions

Habitat is the place where an organism lives; niche encompasses how it uses resources, tolerates environmental conditions and interacts with other organisms. The fundamental niche represents the potential range of conditions and resources a species could occupy without restricting biotic interactions. Its realised niche is the range actually occupied under existing ecological conditions, often narrowed by competition or predation.

Competition occurs when organisms use a limiting resource, reducing the performance of both relative to conditions without competition. It can occur within or between species. The competitive exclusion principle states that species competing for the same limiting resources in effectively identical ways cannot coexist indefinitely under stable conditions. Resource partitioning can promote coexistence through differences in food, feeding location or activity time.

Predation and herbivory benefit the consumer while harming the prey or plant. Parasitism benefits a parasite at the host’s expense, usually through a relatively prolonged association. Parasites generally do not kill hosts immediately, although some cause fatal disease. Parasitoids differ because their development ultimately kills the host.

Mutualism benefits both partners, as in mycorrhizal associations: fungi improve plant access to soil resources while receiving organic carbon. Commensalism benefits one partner without a measurable effect on the other; epiphytic orchids using trees for support are a standard example. Amensalism harms one participant while leaving the other unaffected. Interaction outcomes can change with environmental conditions, so these categories are useful models rather than immutable labels.

• Competition: negative effect on both participants.
• Predation, herbivory and parasitism: positive effect on one and negative effect on the other.
• Mutualism: positive effects on both; commensalism: positive and neutral; amensalism: negative and neutral.

3. Ecological roles and food-web effects

A dominant species contributes a large share of community abundance, biomass or cover. A foundation species creates or strongly structures habitat, as reef-building corals do for coral-reef communities. A keystone species has an ecological effect disproportionately large relative to its abundance. These roles may overlap, but the terms are not interchangeable.

An ecosystem engineer changes the physical environment and thereby modifies resource availability for other species. Beavers create wetlands by building dams; mangroves trap sediment and alter coastal habitat. An indicator species provides information about environmental conditions, but its interpretation requires context. Lichen communities, for example, can indicate air-quality differences, although species vary in pollutant sensitivity.

Food webs link species through multiple feeding pathways. A trophic cascade occurs when a change at one trophic level indirectly influences other levels. Predator removal may increase herbivore abundance and reduce vegetation. Bottom-up regulation begins with resource availability, such as nutrients affecting primary production; top-down regulation operates through consumers. Both processes can act simultaneously.

Community assembly reflects environmental filtering, dispersal, species interactions and chance events. An invasive alien species may reorganise these relationships by competing with native organisms or changing disturbance regimes. Alien origin alone does not establish invasiveness: invasion involves establishment and spread with harmful effects.

• A rare species can be a keystone species; numerical dominance is not required.
• Flagship and umbrella species are conservation concepts, not synonyms for keystone species.

4. Succession, disturbance and stability

Ecological succession is directional change in community composition through time, often following the creation of a new surface or a disturbance. Primary succession begins on substrates without developed soil, such as fresh lava or recently exposed rock. Pioneer organisms depend on local conditions: lichens and mosses are common textbook examples, but they are not obligatory first colonisers everywhere.

Secondary succession follows disturbance where soil, seeds, roots, microorganisms or other biological legacies persist. Abandoned agricultural land and many burned forests illustrate this process. It is generally faster than primary succession because these legacies support recolonisation. Colonists can facilitate later arrivals by improving conditions, inhibit them through competition, or be replaced by species better able to tolerate changing conditions.

Classical descriptions often present succession as movement towards a stable climax community. Modern ecology recognises multiple possible trajectories shaped by climate, soils, land use, dispersal and recurring disturbance. Ecosystems do not always converge on one permanent endpoint. Fire-adapted grasslands, for example, need not represent degraded or incomplete forests.

Resistance is the capacity to remain relatively unchanged during disturbance; resilience concerns recovery or reorganisation afterwards, although definitions vary. Diverse communities can sustain functions through complementary resource use and differing species responses, but diversity does not guarantee every form of stability. The intermediate disturbance hypothesis proposes peak diversity at intermediate disturbance levels under some conditions; it is not a universal ecological law.

• Soil and surviving biological legacies are the key clues distinguishing secondary from primary succession.
• Repeated disturbance or invasive species can redirect succession rather than merely delay it.

5. Ecotones, fragmentation and conservation relevance

An ecotone is a transition zone between neighbouring ecological communities, such as a forest–grassland boundary. It may contain species from both adjoining communities and species associated specifically with transitional conditions. An edge effect is a change in environmental conditions, species abundance or ecological interactions near a boundary. Ecotones can be natural; edges can also be created by roads, clearing and settlement.

Edges may receive more light and wind and experience altered temperature, moisture, predation or invasion. Local richness can rise when generalist species arrive, while interior specialists decline. Consequently, increased richness along a fragmented forest edge does not necessarily mean improved biodiversity conservation. Patch size, isolation, surrounding land use and connectivity all influence community composition.

Community ecology supports management that goes beyond protecting individual species. Maintaining pollinators, seed dispersers, predators and decomposers preserves ecological processes. Restoration should recover appropriate native communities and their interactions, not simply maximise tree cover. Afforesting natural grasslands or savannas can damage habitat for open-country species even when the intervention increases woody biomass.

• Assess biodiversity using composition and ecological function as well as species counts.
• Conserve habitat mosaics and connectivity while recognising that different species require different habitats.

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Practice questions