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Lecture 15

Lecture 15: "The Nature Of Communities"

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Western University
Biology 2483A
Hugh Henry

Ecology Lecture No. 15: The Nature Of Communities th Tuesday October 30 , 2012 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. Communities: -Communities are groups of interacting species that occur together at the same place and time (ecologists usually define communities based on physical or biological characteristics). Physically- defined communities might encompass all the species in a sand dune, a mountain stream, or a desert. Biologically-defined communities might include all the species associated with a kelp forest, a freshwater bog, or a coral reef (emphasizes the importance of abundant species). A study of marine invertebrates in seagrasses might restrict the definition of the community to that interaction, and not include mussel-eating birds (definition of communities is somewhat arbitrary). Subsets: -Ecologists usually consider a subset of species when they define and study communities as counting all species in a community is near impossible. Subsets of species can be defined by: Taxonomic affinity (e.g., all bird species in a community), guild (group of species that use the same resources) and functional group (species that function in similar ways, but do not necessarily use the same resources). Food Webs: -Food webs organize species based on trophic or energetic interactions. Different trophic levels consist of: Primary producers (autotrophs like plants and algae), primary consumers (herbivores), secondary consumers (carnivores), and tertiary consumers (carnivores). -Food webs tell little about the strength of interactions or their importance in the community. Some species span two trophic levels, and some species change feeding status as they mature. Some species are omnivores, feeding on more than one trophic level. Food webs do not include nontrophic interactions (horizontal interactions, such as competition). Interaction webs more accurately describe both the trophic (vertical) and non-trophic (horizontal) interactions than a traditional food web. Community Structure: -Community structure is the set of characteristics that shape communities. These include: Species richness (the number of species in a community), species evenness (relative abundances compared with one another) and species diversity (combines species richness and species evenness). Diversity Of Species: -The most commonly used species diversity index is the Shannon index, of which the equation is as follows: ∑ where p is the proportion of individuals in the ith species and s is the i number of species in the community. Biodiversity describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the interconnectedness of all the components. Genetic diversity affects the viability of populations, which in turn affects species diversity in a community. The number of community types in an area is critical to diversity at larger regional and latitudinal scales. Rank Abundance: -Graphical representations of species diversity can give an explicit view of commonness or rarity. Rank abundance curves plot the proportional abundance of each species (p) relaiive to the others in rank order. 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. Species diversity and rank abundance curves were determined for two soil bacteria communities in pastures. One pasture had been fertilized regularly. Both pastures had similar community structure. A few species were abundant, most species were rare. Species Composition: -Species composition is the identity of a species in a community. Two communities could have identical species diversity values, but have completely different species. The identity of species is critical to understanding community structure. Species Accumulation Curves: -Species accumulation curves are where species richness is plotted as a function of the total number of individuals that have been counted. These curves can help determine when most or all of the species in a community have been observed. The communities varied greatly in the amount of sampling effort necessary to determine species richness. The temperate forest and tropical bird community were adequately represented before half the individuals were counted. For tropical soil bacteria, more effort was needed to sample this extremely diverse community. -Spatial scale is also important. If we sampled bacteria in tropical soils at the same scale as Costa Rican moths, the bacterial diversity would be immense in comparison. The study highlights how little we know about community structure of rarely studied assemblages, such as microbial communities. Interactions Of Multiple Species: -Communities can be characterized by complex networks of direct and indirect interactions that vary in strength and direction. In a community, multiple species interactions generate
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