Community Ecology

1. Community Ecology

2. Climate and the Distribution ofEcological Communities • Communities are assemblages of large numbers of speciesthat all interact with each other. • Areas with different climatic characteristics contain different ecological communities. • Climate types are classified using the Koeppen Classification System. • Categorizes climate types based on annual temperatureand precipitation, as well as variations occurring in these two variables. • Examples: tropical wet forests, subtropical deserts, temperate grasslands, temperate forests, boreal forests, and tundra.

4. Climate and the Distribution ofEcological Communities • Productivity is positively correlated with temperature and humidity. • Communities have a characteristic pattern or type of disturbance.

5. How Predictable Are Community Assemblages? • Two views of community dynamics exist. • Clements believed that communities are stable, integrated, orderly, and predictable entities. • Gleason believed communities are neither stable nor predictable, but a matter of history and chance. • Historical and experimental data support Gleason’s view.

6. Climate characteristics 40 Belém, Brazil Temperature (ºC) 30 Very low variation Average: HIGH 20 50 Variation: VERY LOW 40 30 Variation Precipitation (cm) 20 Annual total: HIGH 10 Variation: HIGH 0 J F M A M J J A S O N D Month

7. Appearance

8. Climate characteristics 40 Yuma, Arizona Temperature (ºC) 30 Average: HIGH 20 Variation Variation: MODERATE 10 (Freezing) 0 Precipitation (cm) 20 Annual total: VERY LOW 10 Variation: LOW 0 J F M A M J J A S O N D Month

9. Appearance

10. Climate characteristics 40 Denver, Colorado Temperature (ºC) 30 Average: MODERATE 20 Variation: MODERATE 10 0 (Freezing) Precipitation (cm) 20 Annual total: LOW 10 Variation: MODERATE 0 J F M A M J J A S O N D Month

11. Appearance

12. Climate characteristics 40 Chicago, Illinois Temperature (ºC) 30 Average: MODERATE 20 Variation: MODERATE 10 0 (Freezing) Precipitation (cm) 20 Annual total: MODERATE 10 Variation: LOW 0 J F M A M J J A S O N D Month

13. Appearance

14. Climate characteristics 40 Dawson, Yukon, Canada Temperature (ºC) 30 Average: LOW 20 Variation: VERY HIGH 10 0 (Freezing) Precipitation (cm) 20 –10 Annual total: LOW –20 10 Variation: LOW 0 –30 J F M A M J J A S O N D Month

15. Appearance

16. Climate characteristics 40 Barrow, Alaska Temperature (ºC) 30 Average: VERY LOW 20 Variation: HIGH 10 0 (Freezing) Precipitation (cm) 20 –10 Annual total: VERY LOW –20 10 Variation: LOW 0 –30 J F M A M J J A S O N D Month

17. Appearance

18. EXPERIMENT TEST ON COMMUNITY STRUCTURE 1. Construct 12 identical ponds. Fill at the same time with sterile water so that there are no preexisting organisms. 2. After one year, examine water samples from each pond under the microscope. Count the number of plankton species in each sample. 10 Ponds 11 1234 12 3. Plot results. 1 2 3 4 5 6 7 8 9 Plankton species (numbered rather than named, for simplicity) 1 10 20 30 Total species in each pond 40 Gleason hypothesis: The composition of biological communities is largely a matter of chance. Clement hypothesis: Biological communities have a predictable composition. 50 60 35 31 38 35 39 31 35 30 31 37 33 34 REJECTED SUPPORTED

19. 10 Ponds 11 1234 12 1 2 3 4 5 6 7 8 9 1 10 20 Plankton species (numbered rather than named, for simplicity) 30 Total species in each pond 40 Gleason hypothesis: The composition of biological communities is largely a matter of chance. Clement hypothesis: Biological communities have a predictable composition. 50 60 35 31 38 35 39 31 35 30 31 37 33 34 REJECTED SUPPORTED EXPERIMENT TEST ON COMMUNITY STRUCTURE 1. Construct 12 identical ponds. Fill at the same time with sterile water so that there are no preexisting organisms. 2. After one year, examine water samples from each pond under the microscope. Count the number of plankton species in each sample. 3. Plot results.

20. How Predictable Are Community Assemblages? • Disturbance and change in ecological communities. • Disturbance is any event that removes some individuals or biomass from a community. • The characteristic type of disturbance found in a community is known as its disturbance regime. • Important management decisions hinge on understanding the disturbance regimes of any community.

21. Giant sequoias after a fire

22. Fire scars in the growth rings

23. Reconstructing history from fire scars 50 40 30 Number of fires per century 20 10 0 0 400 800 1200 1600 2000 Years A.D.

24. How Predictable Are Community Assemblages? • Succession • Succession is the recovery and development of communitiesafter a disturbance occurs. • Primary succession removes all organisms and soil, while secondary succession leaves soil intact. • A distinct sequence of communities develops as succession proceeds. • Succession is greatly impacted by the particular traits of the species involved, how species interact, the short-term weather conditions, and the overall environmental conditions. • Glacier Bay, Alaska provides an excellent case study in succession.

25. Old field Disturbance ends, site is invaded by short-lived weedy species. Pioneering species Weedy species replaced by longer-lived herbaceous species and grasses. Early successional community Shrubs and short-lived trees begin to invade. Mid-successional community Short-lived tree species mature; long- lived trees begin to invade. Late-successional community Long-lived tree species mature. Climax community

26. Old field Disturbance ends, site is invaded by short-lived weedy species. Pioneering species Weedy species replaced by longer-lived herbaceous species and grasses. Early successional community Shrubs and short-lived trees begin to invade. Mid-successional community Short-lived tree species mature; long- lived trees begin to invade. Late-successional community Long-lived tree species mature. Climax community

27. Hypothesis 1: Only one successional pathway occurs in Glacier Bay. Soils exposed less than 20 years: willow and Dryas Proposed successionalpathway: 20 km Soils exposed 45-80 years: sitka alder, scattered cottonwood N Soils exposed 100 years: sitka alder, scattered spruce Soils exposed 150-200 years: dense sitka spruce and western hemlock Glacier Bay Alaska Glacier Bay Direction of glacial retreat

28. Hypothesis 2: Three distinct successional pathways occur in Glacier Bay. Spruce PATHWAY 1 Alder Early-mid successional Late-mid successional Climax Hemlock Cottonwood PATHWAY 2 No hemlock? ? Early successional Climax Mid-successional Late-successional PATHWAY 3 No hemlock? ? Early successional Climax Mid-successional Late-successional

29. Hypothesis 2: Three distinct successional pathways occur in Glacier Bay. Spruce PATHWAY 1 Alder Early-mid successional Late-mid successional Climax Hemlock Cottonwood

30. Hypothesis 2: Three distinct successional pathways occur in Glacier Bay. PATHWAY 2 No hemlock? ? Early successional Climax Mid-successional Late-successional

31. Hypothesis 2: Three distinct successional pathways occur in Glacier Bay. PATHWAY 3 No hemlock? ? Early successional Climax Mid-successional Late-successional

32. Recent data indicate that all three pathways exist. • In pathway 1, spruce arrive early and in high densities. Alder never forms continuous groves and western hemlock arrive after spruce. • In pathway 2, alder arrives early and forms continuous thickets. Spruce arrives early as well but develops low-density stands under the alder thicket. Western hemlock is absent. • In pathway 3, alder arrives early and forms continuous thickets. Cottonwood and spruce form a low-density forest int eh alder thickets. Western hemlock is absent.

33. Species Diversity in Ecological Communities • Quantifying diversity can be simple or complex. • Research has focused on why some communities are more diverse than others and why diversity is important.

34. Tropical forest Boreal forest Canopy Canopy Subcanopy Epiphytes Vines Understory trees and shrubs Understory shrubs

35. Species Diversity in Ecological Communities • On a global scale, a latitudinal gradient of species diversity exists for most taxa. • Species diversity declines as latitude increases. • Several hypotheses have been proposed to explain thisgradient, but no simple answer exists.

36. 10000 60º 8000 30º 0º Equator 6000 Number of vascular plant species per 10,000 km2 30º 4000 60º 2000 0 0 10 20 30 40 50 60 Latitude (degrees North or South)

37. Species Diversity in Ecological Communities • Communities with high diversity are more productive, more resistant, and more resilient than those with low diversity.

38. Shading indicates burned areas Lake Yellowstone Park boundary