The Nature of Communities

1. The Nature of Communities

2. Chapter 16 The Nature of Communities • CONCEPT 16.1 Communities are groups of interacting species that occur together at the same place and time. • CONCEPT 16.2 Species diversity and species composition are important descriptors of community structure. • CONCEPT 16.3 Communities can be characterized by complex networks of direct and indirect interactions that vary in strength and direction.

3. Introduction Although so far we have considered species interactions in two-way relationships, in reality, species experience multiple interactions that shape the communities in which they live. THIS is the dominant direction of community ecology.

4. CONCEPT 16.1 Communities are groups of interacting species that occur together at the same place and time.

5. Concept 16.1 What Are Communities? In practical terms, ecologists usually define communities based on physical or biological characteristics. A physically defined community might encompass all the species in a sand dune, a mountain stream, or a desert.

6. Concept 16.1 What Are Communities? A biologically defined community might include all the species associated with a kelp forest, a freshwater bog, or a coral reef. This approach emphasizes the importance of an abundant species, such as kelp.

7. Figure 16.3 Defining Communities

8. Concept 16.1 What Are Communities? Ecologists often define a community arbitrarily, based on the questions they are posing. Example: A study of marine invertebrates in seaweed might restrict the community to that interaction, and not include mussel-eating birds, etc.

9. Concept 16.1 What Are Communities? Counting all the species in a community is essentially impossible, especially if small or unknown species are considered. Ecologists usually consider a subset of species when they define and study communities.

10. Concept 16.1 What Are Communities? Subsets of species can be defined by: Taxonomy (e.g., all bird species in a community) Guild—group of species that use the same resources Functional group—species that function in similar ways

11. Figure 16.4 Subsets of Species in Communities

12. Concept 16.1 What Are Communities? Food webs organize species based on trophic or energetic interactions. Trophic levels: Primary producers—autotrophs Primary consumers—herbivores Secondary consumers—carnivores Tertiary consumers—carnivores

13. Figure 16.5 Food Webs and Interaction Webs (Part 1)

14. Concept 16.1 What Are Communities? Food webs tell little about the strength of interactions or their importance in the community. Some species span two trophic levels, and some change feeding status as they mature. Some species are omnivores, feeding on more than one trophic level.

15. Concept 16.1 What Are Communities? Food webs do not include nontrophic interactions (horizontal interactions, such as competition). Interaction webs more accurately describe both trophic (vertical) and non-trophic (horizontal) interactions.

16. Figure 16.5 Food Webs and Interaction Webs

17. CONCEPT 16.2 Species diversity and species composition are important descriptors of community structure.

18. Concept 16.2 Community Structure Community structure is the set of characteristics that shape communities. Community structure is descriptive in nature, but provides the basis for generating hypotheses and experiments to understand how communities work.

19. Concept 16.2 Community Structure Species diversity combines species richness and species evenness. Species richness—the number of species in a community. Species evenness—relative abundances compared with one another.

20. Concept 16.2 Community Structure Compare 2 communities with 4 species each (species richness equal). Community A: One species constitutes 85% of the individuals, the other species each constitute 5%. Community B: Abundance is equally divided, each species constitutes 25%. This community has higher diversity.

21. Figure 16.6 Species Richness and Species Evenness

22. Concept 16.2 Community Structure The most commonly used species diversity index is the Shannon index: pi= proportion of individuals in the ith species s = number of species in the community

23. Concept 16.2 Community Structure Species diversity (and biodiversity) are often used more broadly to mean the number of species in a community. Biodiversity describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the interconnectedness of all the components.

24. Figure 16.7 Biodiversity Considers Multiple Spatial Scales

25. Concept 16.2 Community Structure Graphical representations of species diversity show commonness or rarity. Rank abundance curves plot the proportional abundance of each species (pi) relative to the others in rank order.

26. Figure 16.8 Are Species Common or Rare?

27. Concept 16.2 Community Structure Relative abundances can suggest what species interactions might be occurring. In Community A, the dominant species might have a strong negative effect on the three rare species. Experiments that add or remove species are used to explore these relationships.

28. Concept 16.2 Community Structure Species diversity and rank abundance curves were determined for soil bacteria communities in two pastures. One pasture had been fertilized regularly. Bacteria species can be identified using DNA sequencing of 16S ribosomal DNA and grouped using phylogenetic analysis.

29. Concept 16.2 Community Structure 20 phylogenetic groups of bacteria were found in the pastures. Both had similar community structure. A few species were abundant; most species were rare.

30. Figure 16.9 Bacterial Diversity in Pastures in Scotland

31. Concept 16.2 Community Structure Species accumulation curves: Species richness plotted as a function of total number of individuals counted. These curves can help determine when most or all of the species in a community have been observed.

32. Figure 16.10 When Are All the Species Sampled?

33. Concept 16.2 Community Structure In reality, the threshold where no new species are counted never occurs because new species are constantly being found. Hughes et al. (2001) compared species accumulation curves for five different communities.

34. Figure 16.11 Communities Differ in Their Species Accumulation Curves

35. Concept 16.2 Community Structure The communities varied greatly in the amount of sampling effort needed to determine species richness. Temperate forest and tropical bird species were adequately represented before half the individuals were counted. For tropical soil bacteria, more effort was needed to sample this extremely diverse community.

36. Concept 16.2 Community Structure Species composition: Identity of species in a community. Two communities could have identical species diversity values, but completely different species. The identity of species is critical to understanding community structure.

37. CONCEPT 16.3 Communities can be characterized by complex networks of direct and indirect interactions that vary in strength and direction.

38. Concept 16.3 Interactions of Multiple Species In a community, multiple species interactions generate a multitude of connections. Direct interactions:Between two species (e.g., competition, predation, facilitation). Indirect interactions: The relationship between two species is mediated by a third (or more) species.

39. Figure 16.12 Direct and Indirect Species Interactions

40. Concept 16.3 Interactions of Multiple Species Indirect effects are often discovered by accident when species are experimentally removed to study the strength of direct interactions.

41. Concept 16.3 Interactions of Multiple Species Trophic cascade: A carnivore eats an herbivore (a direct negative effect on the herbivore). The decrease in herbivore abundance has a positive effect on a primary producer.

42. Concept 16.3 Interactions of Multiple Species In kelp forests, sea otters feed on sea urchins, which feed on the kelp. Sea otters have a positive indirect effect on kelp. Kelp, in turn, can positively affect abundance of other seaweeds, which serve as habitat and food for marine invertebrates and fishes.

43. Figure 16.13 Indirect Effects in Interaction Webs (Part 1)

44. Concept 16.3 Interactions of Multiple Species Trophic facilitation: A consumer is indirectly facilitated by a positive interaction between its prey and another species. Example: Interactions between salt marsh plants affect aphids.

45. Figure 16.13 Indirect Effects in Interaction Webs

46. Concept 16.3 Interactions of Multiple Species A sedge (Juncus) and a shrub (Iva) have a commensalistic relationship. Juncus shades the soil surface, decreasing evaporation and salt buildup. Juncus also has aerenchyma, tissue that allows oxygen to move to the roots. Some oxygen moves into the soil where other plants can use it.

47. Concept 16.3 Interactions of Multiple Species Experimental removal of Juncus decreased the growth rate of Iva, but removing Iva had no effect on Juncus. Population growth rates of aphids on Iva were significantly higher when Juncus was present. Juncus has an indirect positive effect on the aphids.

48. Figure 16.14 Results of Trophic Facilitation in a New England Salt Marsh (Part 1)

49. Figure 16.14 Results of Trophic Facilitation in a New England Salt Marsh (Part 2)

50. Figure 16.14 Results of Trophic Facilitation in a New England Salt Marsh (Part 3)