Community Structure. Community An assemblage of all the populations interacting in an area

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1 Community Structure Community An assemblage of all the populations interacting in an area

2 Community Ecology The ecological community is the set of plant and animal species that occupy an area Questions central to ecological studies: What controls the relative abundance of species within the community? How do the component species interact with each other? How do communities change through time? How do different communities on the larger landscape interact?

3 Community Structure A broad definition of community is a group of species that occupy a given area, interacting either directly or indirectly A spatial concept A more restrictive definition of community is a subset of species, such as a plant, bird, small mammal, or fish community Food chain vs. food web

4 Community Structure A community has attributes that differ from those of its components Number of species Relative abundance of species Nature of species interactions Physical structure

5 Relative Abundance and Diversity Species richness (S) is the count of the number of species occurring within the community Relative abundance represents the percentage each species contributes to the total number of individuals of all species

6 Relative Abundance and Diversity The patterns of species richness and relative abundance can be compared between communities

7 Relative Abundance and Diversity A common method for comparing patterns of species richness and abundance between communities is to plot the relative abundance of each species against rank (called a rankabundance diagram) Species evenness indicates the distribution of species richness A community with a greater species evenness would have a more gradual slope of the rankabundance curve

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11 Relative Abundance and Diversity Diversity indexes provide a way to quantify the relationship between species number and relative abundance Simpson s index (D) = (n i /N) 2 = summation for all species n i = number of individuals of species i N = total number of individuals of all species D ranges between 0 and 1 and as both species richness and evenness increase, the value approaches 0

12 Relative Abundance and Diversity Simpson s index (D) = (n i /N) 2 Simpson s index of diversity = 1 D The greater the value of D, the lower the diversity Simpson s reciprocal index or Simpson s diversity index = 1/D The lowest possible value is 1, representing a community containing only one species The maximum value is the number of species in

13 Relative Abundance and Diversity Shannon (or Shannon-Weiner) index = H = (p i )(log 2 p i ) Relative abundance of each species = p i = n i / N p i = proportion of species i In the absence of diversity, where only one species is present, H = 0 H max = ln S, occurs when all species are present in equal numbers

14 Species Dominance When a single or few species predominate within a community, these species are referred to as dominants Dominance is the converse of diversity. When the basic Simpson index (D) is 1, the 1 represents total dominance only one species present in the community Dominant species are usually defined separately for different taxonomic or functional groups of organisms within the community (e.g., tree versus herbaceous plant species)

15 Species Dominance Dominance can reflect the number of individuals, size of individuals, or some combination of characteristics that include both the number and size of individuals Dominant species are typically the dominant competitors under the prevailing environmental conditions Other factors may determine dominance within communities

16 Structure of Communities The adaptation of organisms to the physical environment and species interactions need to be integrated to explain the processes that control community structure

17 Communities are affected by: Available living space habitat Resource Availability niche Species interactions Symbiosis living together commensalism mutualism parasitism Competition Predation Coevolution

18 Available Living Space Habitat an organism s mailing address the type of place where the individuals normally live

19 Resources Resources: Features of the environment required for growth, survival, or reproduction, and that can be consumed to the point of depletion.

20 Resources Examples of resources: Food Water in terrestrial habitats Light for plants Space, especially for sessile organisms For mobile animals, space for refuge, nesting, etc.

21 Space Can Be a Limiting Resource

22 Habitat and Resources Species are also influenced by features of the environment that are not consumed, such as temperature, ph, salinity. These factors are not consumed and are not considered to be resources. Competition reduces availability of resources.

23 Resource Availability Niche an organism s profession (role) in the community

24 Niche Constrains Community Structure All living organisms have a range of environmental conditions under which they can successfully survive, grow, and reproduce This range of environmental conditions is not the same for all organisms

25 Niche Constrains Community Structure Environmental conditions vary in both time and space The fundamental niches of species vary along environmental gradients The distribution of fundamental niches along the environmental gradient represents a primary constraint on community structure

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27 Niche Constrains Community Structure As environmental conditions change from location to location, the possible distribution and abundance of species will change in turn changing the community s structure Geographic distributions of species reflect the occurrence of suitable environmental conditions

28 Niche Constrains Community Structure The null model assumes that the presence and abundance of species are solely a result of the independent responses to the prevailing physical environment Interactions among species have no significant influence on community structure

29 Niche Constrains Community Actual community patterns are compared to the null model to establish the role (if any) of species interactions A great deal of evidence indicates that species interactions influence both the presence and abundance of species within a wide variety of communities Interspecific competition Facilitation Mutualism Structure

30 Species Interactions Are Diffuse Ecological studies underestimate species interactions because such interactions are often diffuse, involving a number of species

31 Species Interactions Symbiosis Living together for at least some part of the life cycle Most interactions are neutral; they have no effect on either species Commensalism Mutualism Parasitism

32 Commensalism One species benefits and has no effect on the other

33 Mutualism Both species benefit Some are obligatory; partners depend upon each other (coevolution) Yucca plants and yucca moth Mycorrhizal fungi and plants Anemone fish and anemone

34 Mutualism Protection

35 photograph Alex Wild 2004

36 Mutualism Birds get food, help plant disperse seeds

37 Mutualism Pollination

38 Yucca and Yucca Moth Example of an obligatory mutualism Each species of yucca is pollinated only by one species of moth Moth larvae can grow only in that one species of yucca

39 Yucca Moth Yucca Plant Obligate Mutualism

40 Mycorrhizae Obligatory mutualism between fungus and plant root Fungus supplies mineral ions to root Root supplies sugars to fungus

41 Parasitism Parasites draw nutrients from hosts live on or in host body Vectors Convey a parasite from host to host Parasitoids Develop inside another species Consume and kill host

42 Protozoan - flagellate Micro Parasites bacteria fungus

43 Macro parasites tick flea

44 Macro parasite lamprey

45 Parasites

46 Parasites as biological controls Some parasites and parasitoids are used as biological controls Adapted to specific host and habitat Good at locating host High population growth rate Offspring disperse

47 Biological Controls Wasp and aphid

48 Social Parasite Brown-headed cowbird

49 Competition Interspecific: among different species Exploitative competition Interference competition Intraspecific: between members of the same species Intraspecific competition is most intense Territoriality Allelopathy Some species have eliminated ways of avoiding competition: Resource partitioning

50 Interspecific Competition

51 Intraspecific Competition

52 Intraspecific Competition Territoriality

53 Intraspecific Competition Allelopathy (sibling harming) Eucalyptus oils harm young saplings of same species. Can also affect other species

54 Forms of Competition Exploitative competition Species have equal access to resource; compete to exploit resource Interference competition One species prevents another from using resource usually through aggressive behavior

55 Interference Competition

56 Competitive Exclusion Principle When two species compete for identical resources, one will be more successful and will eventually eliminate the other

57 Gause s Experiment Species grown together Paramecium caudatum Paramecium aurelia

58 Resource Partitioning Apparent competitors may have slightly different niches May use resources in a different way or time Minimizes competition and allows coexistence

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61 Predation Predators animals that feed on other living organisms free-living do not reside on their prey Carnivores and omnivores

62 Predator Prey Relationships: Canada Lynx and the Showshoe Hare Species are limited by the number of available prey In some cases predators limit a prey species

63 Coevolution Two or more species exert selection pressure on each other Prey defenses evolve Predator responses to prey evolve

64 Prey Defenses Camouflage Warning coloration Mimicry

65 Camouflage

66 Warning Coloration & Mimicry

67 Predator Responses Predators counter prey defenses with new adaptations stealth camouflage avoidance of chemical repellents

68 Predator Responses