Ecological communities are not fixed collections of species. They shift as births, deaths, movement, and species interactions alter which populations are present and how abundant they are across seasons, years, and longer time scales.

What a community is (and why it changes)

A community is defined by which populations co-occur in the same place and time and by the ways those populations influence one another. Community change is often measured as changes in species presence/absence, relative abundance, and dominance (which species are most influential due to high biomass or numbers).

Rank-abundance (Whittaker) curves plot species rank (most to least abundant) against relative abundance, summarizing both community richness and evenness. A steep initial decline indicates dominance by a few species, while a flatter slope indicates more even relative abundances across species. Comparing curves across times or sites provides a compact visualization of community turnover in abundance structure. Source

Community composition changes over time because:

• Populations fluctuate (reproduction and mortality vary with conditions).

• Individuals move (immigration and emigration alter local membership).

• Interactions shift (as one population changes, others respond through altered survival or reproduction).

• The physical environment varies (seasonality, resource pulses, and disturbances create new conditions).

Time scales of community dynamics

Short-term change (days to seasons)

Short-term dynamics often reflect predictable cycles and rapid responses.

• Seasonal phenology (timing of flowering, breeding, migration) can temporarily reshape community interactions and resource use.

• Population pulses (e.g., sudden increases in a prey population) can cause short-lived changes in which species are common or rare.

• Behavioral shifts (e.g., altered foraging times) can change encounter rates among populations, indirectly changing community structure.

Medium-term change (years to decades)

Over multiple generations, community membership can reorganize as:

• Local extinctions occur when a population can no longer persist under the current biotic context.

• Colonisation brings in new populations that establish and persist.

• Feedbacks among populations amplify or dampen change (for example, a population increase may reduce a competitor, allowing a third population to expand).

Long-term change (decades to centuries)

Long-term change often involves directional trends driven by persistent conditions, repeated disturbances, or slow environmental shifts. Community trajectories may be gradual, step-like, or cyclical depending on how often conditions reset.

Disturbance, succession, and community turnover

Disturbances (events that remove biomass and alter resources) can rapidly change community composition, creating opportunities for different populations to establish. After disturbance, communities often undergo succession, where the identity and abundance of populations change in a somewhat predictable sequence.

Key features to recognise:

• Early succession: typically dominated by fast-colonising populations with high dispersal and rapid growth; community membership can change quickly.

• Later succession: populations that are better competitors under stable conditions may increase; turnover often slows as niches become filled.

• Alternative stable states: similar starting conditions can lead to different community outcomes if early colonists or key interactions differ.

How interactions drive change over time

Communities change “based on interactions among those populations” because interactions alter fitness (survival and reproduction) and therefore population growth in context.

• Direct effects: one population immediately affects another’s growth (e.g., reducing survival, limiting recruitment).

• Indirect effects: one population changes a second population by first altering a third population or a shared resource.

• Interaction strength varies over time: a weak interaction can become strong when conditions change (resource scarcity, new arrivals, or shifting abundances).

Community change is therefore a product of:

• Who is present (membership)

• How many of each population exist (abundance)

• How strongly populations affect one another (interaction network)

• When interactions occur (timing and sequence)