Species Diversity in Communities

1. Species Diversity inCommunities

2. Chapter 19 Species Diversity in Communities • CONCEPT 19.1 Species diversity differs among communities due to variation in regional species pools, abiotic conditions, and species interactions. • CONCEPT 19.2 Resource partitioning is theorized to reduce competition and increase species diversity.

3. Chapter 19 Species Diversity in Communities • CONCEPT 19.3 Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity. • CONCEPT 19.4 Many experiments show that species diversity is positively related to community function.

4. Powered by Prairies? A Case Study Dwindling supplies of fossil fuels has led to development of biofuels—liquid or gas fuels from plant material (biomass). In the United States, ethanol is made from corn, and biodiesel is made from soybeans.

5. Powered by Prairies? A Case Study Ideally, biofuels are carbon neutral—the amount of CO2 produced by burning them is equal to the amount taken up by the plants from which they are made. They are a nearly limitless renewable resource, as long as the crops can be grown.

6. Powered by Prairies? A Case Study Biofuels have many downsides as well. Growing corn and soybeans for biofuels competes for land and water that could be used for growing food. Fossil fuels, in the form of fertilizers and pesticides and used for farm work, are required to grow these crops.

7. Powered by Prairies? A Case Study Another option is to use non-edible plants (or parts), such as corn stalks, straw, or waste wood, to make biofuels. Biofuel crops could be grown on degraded land that is no longer suitable for high-yield food crops.

8. Powered by Prairies? A Case Study David Tillman has studied prairie plant species diversity in abandoned agricultural land at Cedar Creek, Minnesota. He has shown that plots with more species produce more biomass than plots with few species.

9. CONCEPT 19.1 Species diversity differs among communities due to variation in regional species pools, abiotic conditions, and species interactions.

10. Concept 19.1Community Membership Membership in a community depends on: 1. Regional species pools and dispersal ability 2. Abiotic conditions 3. Species interactions These factors act as “filters,” which exclude or include species in particular communities.

11. Figure 19.4 Community Membership: A Series of Filters

12. Concept 19.1Community Membership 1. The regional species pool provides an upper limit on the number and types of species that can be present in a community. The importance of dispersal can be seen in cases of non-native species invasions.

13. Concept 19.1Community Membership Humans have greatly expanded regional species pools by serving as vectors of dispersal. Example: Aquatic species travel around the world in ballast water carried by ships. Ships are now larger and faster, so trans-ocean trips take less time—species are more likely to survive.

14. Concept 19.1Community Membership Zebra mussels (Dreissenapolymorpha), arrived in the Great Lakes in ballast water in the late 1980s and have had major impacts on aquatic communities. The comb jelly Mnemiopsisleidyi was also introduced via ballast water, into the Black Sea.

15. Figure 19.5 Humans Are Vectors for Invasive Species

16. Concept 19.1Community Membership 2. Abiotic conditions: A species may be able to get to a community but be unable to tolerate the abiotic conditions. For example, a lake might not support organisms that require fast-flowing water.

17. Concept 19.1Community Membership 3. Species interactions: Coexistence with other species is also required for community membership. Other species may be required for growth, reproduction, or survival. Species may be excluded by competition, predation, parasitism, or disease.

18. Concept 19.1Community Membership Some non-native species do not become part of the new community. Biotic resistance occurs when interactions with the native species exclude the invader. Example: Native herbivores can reduce the spread of non-native plants.

19. CONCEPT 19.2 Resource partitioning is theorized to reduce competition and increase species diversity.

20. Concept 19.2Resource Partitioning Resource partitioning: Competing species are more likely to coexist if they use resources in different ways. We can think of each type of resource as varying along a “resource spectrum,” representing different nutrients, prey sizes, habitat types, etc.

21. Concept 19.2Resource Partitioning In a simple model, each species’ resource use falls on a spectrum of available resources. Figure 19.7 A Resource Partitioning

22. Concept 19.2Resource Partitioning Assumption: The greater the overlap of resource use, the more competition between species. The less overlap, the more specialized species have become, and the less strongly they compete.

23. Concept 19.2Resource Partitioning If species have a high degree of specialization, it can result in less competition and high species richness. More species can be “packed” into a community if overlap is small. Or, if the resource spectrum is broad, a diversity of resources would be available for use by a wide variety of species.

24. Figure 19.7 Resource Partitioning

25. CONCEPT 19.3 Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity.

26. Concept 19.3Resource Mediation and Coexistence If disturbance, stress, or predation keeps the dominant competitor from reaching carrying capacity, competitive exclusion cannot occur, and coexistence will be maintained.

27. Figure 19.12 The Outcome of Competition under Constant and Variable Conditions

28. Concept 19.3Resource Mediation and Coexistence G. E. Hutchinson considered the idea in his paper “The Paradox of the Plankton” (1961). Lake phytoplankton communities have very high diversity (30–40 species), all using the same limited resources, in a homogeneous environment.

29. Figure 19.13 Paradox of the Plankton

30. Concept 19.3Resource Mediation and Coexistence His explanation was that conditions in the lake changed seasonally, which kept any one species from outcompeting the others. As long as conditions changed before competitively superior species reached carrying capacity, coexistence would be possible.

31. Concept 19.3Resource Mediation and Coexistence Intermediate disturbance hypothesis, first proposed by Connell (1978): Species diversity will be greatest at intermediate levels of disturbance. At low levels of disturbance, competition regulates diversity. At high disturbance levels, many species cannot survive.

32. Figure 19.14 The Intermediate Disturbance Hypothesis

33. Concept 19.3Resource Mediation and Coexistence There have been many tests of this hypothesis. Sousa studied communities on intertidal boulders in southern California that were overturned by waves. Small boulders were overturned frequently (disturbance), large boulders were overturned less often.

34. Concept 19.3Resource Mediation and Coexistence After two years: Most small boulders had one species living on them (frequent disturbance). Most large boulders had two species (rare disturbance). Intermediate sized boulders had four to seven species.

35. Figure 19.15 A Test of the Intermediate Disturbance Hypothesis

36. Concept 19.3Resource Mediation and Coexistence Huston (1979) added competitive displacement to the intermediate disturbance model: The best competitor uses the limiting resources, reducing the weaker competitor’s population growth to the point of extinction.

37. Concept 19.3Resource Mediation and Coexistence Hacker and Gaines (1997) incorporated positive interactions into the intermediate disturbance hypothesis. Evidence suggests that positive interactions are more common under relatively high levels of disturbance, stress, or predation.

38. Concept 19.3Resource Mediation and Coexistence At low levels of disturbance, competition reduces diversity. At intermediate levels, species involved in positive interactions are released from competition and can increase diversity. At high levels, positive interactions are common and help to increase diversity.

39. Figure 19.17 Positive Interactions and Species Diversity

40. Concept 19.3Resource Mediation and Coexistence The intermediate disturbance hypothesis considers disturbance and predation to be similar—a dominant competitor is killed or damaged, creating opportunities for other species. Menge and Sutherland (1987) argue that because predation is a biological interaction, it should be considered separately.

41. Concept 19.3Resource Mediation and Coexistence Their model predicts that predation is most important when environmental stress is low. As stress increases, importance of predation decreases, and competition increases in importance. At high stress levels, neither are important.

42. Figure 19.19 The Menge–Sutherland Model

43. Concept 19.3Resource Mediation and Coexistence The above theories assume an underlying competitive hierarchy. What if species have equivalent interaction strengths? Lottery models and neutral models emphasize the role of chance in maintaining species diversity.

44. Concept 19.3Resource Mediation and Coexistence In lottery models, all species have equal chances of obtaining resources that were made available by disturbances, and this allows coexistence. Species must have similar interaction strengths and growth rates and be able to respond quickly to disturbances that free up resources.

45. Concept 19.3Resource Mediation and Coexistence The lottery model may be most relevant in very diverse communities where many species overlap in their resource requirements. Its relevance decreases in communities in which species have large disparities in interaction strength.

46. CONCEPT 19.4 Many experiments show that species diversity is positively related to community function.

47. Concept 19.4The Consequences of Diversity A central idea in ecology is that species diversity can control community functions, such as plant productivity, soil fertility, water quality, etc.

48. Concept 19.4The Consequences of Diversity Many community functions also provide valuable services to humans: Food and fuel production Water purification O2 and CO2 exchange Protection from catastrophic events, such as floods

49. Concept 19.4The Consequences of Diversity The Millennium Ecosystem Assessment (2005) predicts that if current losses of species diversity continue, human populations will be severely affected by the loss of services those species provide.

50. Concept 19.4The Consequences of Diversity The Diversity–Stability Theory A long-standing idea in ecology is that species richness is positively related to community stability— The tendency of a community to remain the same in structure and function, or to return after a disturbance.