Lecture 19: Community Dynamics (Non-Equilibrium)

1. Lecture 19: Community Dynamics (Non-Equilibrium) EEES 3050

2. Equilibrium vs. non-equilibrium • Equilibrium: • Define: communities in which species abundances remain constant or stable over time. • Nature is in a “state of balance”. • Non-equilibrium: • Define: communities in species abundances do not remain constant. Abundances continually change.

3. Non-equilibrium systems: • Non-equilibrium: • Define: communities in species abundances do not remain constant. Abundances continually change. • What mechanisms can cause species abundances to change? • Heterogeneity • Succession • Disturbance • Immigration • Extinction

4. Equilibrium systems. • Classical Competition theory • Population growth deterministic • Homogeneous environment • Competition is only significant biological interaction • Coexistence requires a stable equilibrium point. • Non-equilibrium systems: • Environments are heterogeneous • Environments change, i.e. disturbance • These are two of the main themes of Landscape Ecology.

5. Heterogeneity • The environment has pattern and the configuration and composition effect the processes within an ecological system. • Typically thought of in terms of patches • Configuration • Connectivity • Dominance • Edges

6. Heterogeneity • What does heterogeneity effect? • Abundance • Distribution of resources • Flow of nutrients, water, carbon • Movement of organisms

7. How is local movement affected by heterogeneous landscapes? • Example: • Animal movement in heterogeneous landscapes: an experiment with Eleodes beetles in shortgrass prairie. • By Crist, Guertin, Wiens and Milne

8. Animal movement in heterogeneous landscapes • What were the basic questions? • What is the effect patch type on animal movement? • Patches can be bare ground, grass, shrub or cactus • What is the effect of grazing intensity on animal movement?

9. Animal movement in heterogeneous landscapes • Methods: • 3 species • 3 habitat types based on grazing intensity • Beetle paths: • Followed beetles for 100 time steps (5 sec steps) • Marked location after every 5 seconds. • Mapped in a GIS

10. Animal movement in heterogeneous landscapes • Quantifying paths: • Mean step length • Mean turning angle • Mean vector length • Net displacement • Fractal dimension

11. Animal movement in heterogeneous landscapes

12. Animal movement in heterogeneous landscapes • Cover type matters for net displacement.

13. Animal movement in heterogeneous landscapes • Grazing matters for some species.

14. Animal movement in heterogeneous landscapes • Conclusions: • Beetle movement affected at 2 different scales. • Vegetation type and grazing treatment

15. Disturbance • Disturbances are relatively discrete events in time that disrupt ecosystem, community or population structure and change resources, substrate availability or the physical environment.

16. Disturbance terminology • Distribution: • Spatial distribution • Frequency: • Mean number of events per time period • Return Interval: • 1/frequency • Rotation period: • Mean time need to disturb area equivalent of study area • Predictability: • An inverse function of variance of the return interval

17. Disturbance terminology • Area or size: • Area disturbed (can be described in many ways compared to event or time period) • Magnitude: • Intensity • Physical force • Severity • Effects on the community • Synergism • Effects on the occurrence of other disturbances. • Recovery time • Amount of time to return to previous condition.

19. Local vs. Global Stability

20. Non-equil. Example 1: Fire

22. Disturbance Example 1: Fire • Observation: • Yellowstone fire of 1988 affected >250,000 ha, creating a mosaic of burn severities. • Main question: • How did fire size affect post fire succession?

23. Disturbance Example 1: Fire • Hypotheses:

24. Disturbance Example 1: Fire • Hypotheses:

25. Disturbance Example 1: Fire • Methods: • At each of three sites, different sized patches were analyzed. • Small (1-2 ha) • Medium (80-200 ha) • Large (480 – 3698) • Response variables: • Biotic cover • Lodgepole pine seedling density • Opportunistic species

26. Results: Size of fire • Prediction: • Greater in smaller patches

27. Results: Burn severity • Prediction: • Decreasing cover with increasing burn severity

28. Results: Richness • Prediction: • Burn severity: • Greatest is severe-surface. • Lower in light-surface. • Lowest in crown fire. • Patch size: no effect

29. Results Yes Yes No Yes & No

30. Non-equil Example 2: Coral Reefs • Observations: • Coral reefs have a very high diversity • At the northern edge of the Great Barrier Reef, over 1500 species of fish have been recorded. • Question: • Q1: Are corals in equilibrium when they are impacted frequently by tropical storms? • Q2: How do these fish species coexist?

31. Q1: Are corals in equilibrium when they are impacted frequently by tropical storms? • Hypothesis: • In an equilibrium system, recruitment rates should be… • consistent, or at least density dependent. • Methods: • Examine recruitment rates through time in areas protected from storms compared to areas exposed to storms.

32. Results: Recruitment was highly variable. • Supports non-equilibrium hypothesis.

33. Non-equil Example 2: Coral Reefs • Q2: How do these fish species coexist? • Two competing hypothesis: • Equilibrium Hypothesis: • Populations controlled by density dependent processes • Each species has a highly specialized niche. • Niche diversification hypothesis. • Non-equilibrium hypothesis: • Larval recruitment is unpredictable. • Competition is present, but winner cannot be predicted. • Lottery hypothesis

34. Non-equil Example 2: Coral Reefs • How to test these hypotheses? • Test the specialization of fish species • Feeding • Habitat • Method: • Talbot et al. (1978) put out artificial reefs. • Measured colonization by different species • Measured turnover rates.

36. Non-equil Example 2: Coral Reefs • Results: Similarity was low, ranging from 14 to 47% similar. • Supports lottery hypothesis.

37. Lottery hypothesis • Three requirements: • Environmental variation such that it permits each species to have high recruitment rates at low population densities. • Generations must overlap • Adult death rates should be unaffected by competition. • When this occurs: reef communities retain high regional diversity in a continually fluctuating environment.

38. Theoretical models for non-equilibrium systems • Four types of models: • Fluctuating environment • Density independent • Directional changing environment • Slow competitive displacement.

39. Fluctuating environment • Based on classical equilibrium model • Adds temporal variability • Competition still major biological interaction. • Seasonal fluctuations do not allow one species to dominate.

40. Fluctuating environment • Density of Daphnia in a Eunice Lake. • Changes with season.

41. Density independent • Assume population densities changes • These changes are density independent. • That is birth and death rates do not depend on density • Competition for resources is rare.

42. Directional changing environment • These models consider history: • Changes in climate • Previous land use

43. Directional changing environment • Example: current and potential range of American beech under 2 climate-change scenarios.

44. Slow competitive displacement. • If competitive abilities are nearly equal, competitive exclusion will be very slow. • In this situation, community structure is strongly affected by chance. • Example: Tree in tropical forests

46. Community persistence in non-equilibrium systems.

47. Community persistence in non-equilibrium systems. • Mechanisms • Stabilizing mechanisms • Disturbance • Compensatory • Integration in larger landscape