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Lecture 14

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Biological Sciences
Marc Cadotte

BIOB50Winter2012 Lecture 14 Chapter 15 What Are CommunitiesCommunities are groups of interacting species that occur together at the same place and timeInteractions among multiple species give communities their character and function They make communities into something more than the sum of their parts In practical terms defining a community requires using biological or physical guidelines Figure 153 Defining Communities Ecologists often delineate communities by their physical or biological characteristics A community may be defined by the physical characteristics of its environment A physically defined community might encompass all the species in a sand dune a mountain stream or a desert A biologically defined community might include all the species associated with a kelp forest a freshwater bog or a coral reef A common species such as kelp wetland plants or coral is the basis for the community delineation Counting all the species in a community is difficult to almost impossible especially if small or relatively unknown species are considered Ecologists usually consider a subset of species when they define and study communities Figure 154 Subsets of Species in Communities Ecologists may use subsets of species to define communities One way of subdividing a community is based on Taxonomic Affinity Example a study of a forest community might be limited to all the bird species within that community All the bird species grouped by taxonomic activity Another useful subset of a community is a Guild a group of species that use the same resources even though they might be taxonomically differentFor example some birds bees and bats feed on flower pollen thus forming a guild of polleneating animals A Functional Group is a subset of a community that includes species that function in similar ways but do not necessarily use the same resources For example mosquitoes and aphids both have stylet mouthparts although one feeds on plant phloem Especially in the case of plants some functional groups use similar resources for example nitrogenfixing plants function similarly and also use the same set of resources Food webs allow ecologists to organize species based on their trophic or energetic interactions Trophic Levels are groups of species that have similar ways of obtaining energy eg primary producerslowest trophic level primary consumers Figure 155 Food Webs and Interaction Food webs describe trophic interactions among species Interaction webs include both trophic interactions vertical arrows and nontrophic horizontal competitive and positive interactions1 The lowest trophic level contains primary producers which are autotrophs such as plants The primary producers are fed on by organisms at the second level the primary consumers which are herbivores The third level contains secondary consumers which are carnivores which are fed on in turn by tertiary consumers also carnivores In food webs trophic levels may cause some confusion for reasons which may include some species can span two trophic levels corals can be classified as herbivore or carnivore due to symbiotic algae they contain some species change their feeding status as they mature and some species may be Omnivores feeding on more than one trophic level Lastly food webs do not include nontrophic interactions horizontal arrows such as competition and some positive interactionsCommunity StructureSpecies Diversity and Species Composition are important descriptors of community structure Communities vary significantly in the number of species they contain Community Structure is the set of characteristics that shape communities Species Richness is the number of species in a communitySpecies Evenness is the relative abundances compared with one anotherSpecies Diversity combines species richness and species evenness Figure 156 Species richness and species evenness The two hypothetical mushroom communities shown each have the same number of species species richness but different relative abundances species evenness Community A has lower species evenness than community B thus species diversity is lower in community A In community A the abundance of one species the yellow mushrooms is high relative to the other species so this community has low species evenness In community B each species has the same abundance so this community has high species evenness2
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