Community Dynamics and Stability

Focus on ecological networks and temporal change.
Examine diversity and composition.
Analyze environmental change and community response, including resilience and stability.
Consider assembly rules and four main processes.
Building and applying networks (e.g., understanding extinction effects).
Starting point for community ecology: understanding what controls community structure.
Representing biotic interactions at a community-scale.
How do communities respond to environmental change?

Biotic Factors:
- Inter-specific interactions (= selection).
- Demographic stochasticity (= drift).
- Invasive species (= dispersal + selection).
Abiotic Factors:
- Global change: habitat loss & fragmentation, climatic change.
- Eutrophication, pollutants.
- Disturbance: floods, wildfires, treefall, pollutant release.

1. Stationary:
- Broadly constant community structure or other state variable (biomass, etc.).
- Likely to vary around the long-term equilibrium.
- Influenced by intrinsic dynamics and extrinsic disturbance (e.g., drought, flood, wildfire).
2. Long Term, Directional Change:
- Declines, increases, or turnover.
- Natural or anthropogenic change.
- Primary and secondary succession.
- Directional pattern of colonizations and extinctions.
- Move towards a ‘climax’ community.
3. Alternative Stable States:
- Example: eutrophic shallow lakes.
  - Clear state: macrophyte growth, zooplankton suppress phytoplankton.
  - Turbid state: abundant phytoplankton, fish suppress zooplankton due to reduced shelter & re-suspended sediment.
- Dramatic shift in community structure, often linked to small environmental changes.
- Disproportionately large change in the environment required to drive the system back.
4. Collapse:
- Catastrophic biodiversity loss – may be irreversible.
- Examples: fisheries, desertification.
- Australian megafauna after human colonisation (Miller et al. 2005).
  - c. 45–50k years ago.
  - c. 60 spp lost.
  - Changes in vegetation and fire regime.
  - Only adaptable species survived e.g., emu, wombat.
  - Shift in diet apparent from stable isotope analysis (see Zimmo et al. 2012).

Any combination of four properties:
1. Abundance distributions.
2. Evenness (dominance).
3. Richness.
4. Composition.
Dissimilarity measures → quantify overall changes.
Standard methods + QA.
Citizen science.

Diverse responses through time are observed, driven by biotic or abiotic factors.
Responses range from stability to collapse and may be complex and unpredictable.
Monitoring relies upon the four facets of community structure discussed in Lecture 1.

Complex, multifaceted concept.
Broken down into components including:
1. Variability.
2. 'Engineering' resilience.
3. Resistance.
4. ‘Ecological’ resilience.

Are more diverse communities more stable?
- Lower variability, greater resistance, higher resilience…
The diversity-stability debate started in the 1950s.
Reinvigorated by ongoing species loss.
Three phases of research.

Phase 1:
- Up to 1950/60s. Elton, MacArthur.
- More complex = more stable.
- Qualitative reasoning:
  - More routes for energy flow → redundancy.
  - ‘More natural’ = more stable.
  - Pest outbreaks seen in simple, agricultural systems.
Phase 2:
- 1970s – Robert May.
- More complex = less stable.
- Mathematical models.
  - More routes for transmitting disturbance.
  - Stronger species coupling.
Phase 3:
- 1990s – present. Tilman.
- More complex = more stable.
- Long-term experiments.

Tilman (1996) – Cedar Creek grassland experiments.
- 13 yrs, measured overall biomass.
- Increasing diversity increased community-level stability but decreased population level.

Clarified different responses at community- and population-levels.
Reconciled May with earlier theory.
Population level:
- More inter-specific competition as diversity increases.
- Decrease in a species may release its competitors, allowing compensatory increases.
Community level:
- Different niches (biotic and abiotic) → species respond in different ways and at different rates to disturbance.
- Populations are asynchronous.
- These differences average out at the community level = portfolio effect.

Weak interactions in the food web may also help.
Strong consumer-resource interactions encourage variability (McCann et al. 1998).
E.g., Canadian lynx and snowshoe hare.
Adding a weak hare predator (coyote) reduces fluctuations in hare population.

Communities are exposed to a range of disturbances that vary in type, magnitude, and duration.
Stability = umbrella term.
Clear evidence that stability is affected by community structure.
Current thinking = stability increases with:
- Asynchronous species’ responses to disturbance.
- Frequency of weak interactions.
Loss of diversity affects both properties: highlights the risks associated with species loss.

Recommended reading:
- Begon & Townsend (2021) Section 17.2.
- Scheffer, M. et al. (2001) Catastrophic shifts in ecosystems. Nature, 413, 591-596.
- Polis, G.A. (1998) Stability is woven by complex webs. Nature, 395, 744-745
References:
- Defra (2020) Wild Birds Populations in the UK, 1970 to 2019. Department for Environment, Food and Rural Affairs, York.
- Hill, M.F. et al. (2002) Spatio-temporal variation in Markov chain models of subtidal community succession. Ecology Letters, 5, 665-675.
- Lake, P.S. (2000) Disturbance, patchiness and diversity in streams. Journal of the North American Benthological Society, 19, 573-592.
- McCann, K. et al. (1998) Weak trophic interactions and the balance of nature. Nature, 395, 794-798.
- Miller, G.H. et al. (2005) Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction. Science, 309, 287-290.
- Tilman, D. (1996) Biodiversity: population versus ecosystem stability. Ecology, 77, 350-363.
- Vaughan, I.P. & Gotelli, N.J. (2019) Water quality improvements offset the climatic debt for stream macroinvertebrates over twenty years. Nature Communications, 10, 1956.
- Zimmo, S. et al. (2012) The Use of Stable Isotopes in the Study of Animal Migration. Nature Education Knowledge 3(12):3