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Lecture 15

Lecture 15 - The Nature of Communities

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Western University
Biology 2483A
Hugh Henry

LECTURE 15 – THE NATURE OF COMMUNITIES 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 What are Communities?  Communities are groups of interacting species that occur together at the same place and time  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  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 trees  Ecologists often define a community somewhat arbitrarily, based on the questions they are posing  Example: A study of marine invertebrates in seagrasses might restrict the definition of the community to that interaction, and not include mussel-eating birds, etc.  Counting all the species in a community is difficult to impossible, especially if small or unknown species are considered  Ecologists usually consider a subset of species when they define and study communities  Subsets of species can be defined by: o Taxonomic affinity — e.g., all bird species in a community o Guild — group of species that use the same resources o Functional group — species that function in similar ways, but do not necessarily use the same resources  Food webs organize species based on trophic or energetic interactions (where energy is being transferred from one organism to another through feeding)  Trophic levels: o Primary producers (autotrophs)—plants and algae o Primary consumers—herbivores o Secondary consumers—carnivores o 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 o For example, a coral can be both a consumer and a primary producer as it has a photosynthetic component as well as an animal component  Some species are omnivores, feeding on more than on trophic level  Food webs do not include non-trophic 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 – the set of characteristics that shape communities  Species richness – the number of species in a community  Species evenness – relative abundances compared with one another  Species diversity combines species richness and species evenness  Community A: H = 0.589  Community B: H = 1.388  Richness is the same for both communities, but evenness is higher in community B – thus, the diversity index is higher in community B  Biodiversity – describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the interconnectedness of all 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  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)irelative to the others in rank order (highest to lowest) o Often done on a log scale, because abundances can differ to a great extent  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  Species diversity and rank abundance curves were determined for two soil bacteria communities in pastures. One pasture has been fertilized regularly  Bacteria species can be identified quickly using DNA sequencing of 16S ribosomal DNA. They are then grouped using phylogenetic analysis  Both pastures had similar community structure. Few species were abundant, most species were rare  Species composition – identity of species in a community  Two communities could have identical species diversity values, but have different species  The identity of species is critical to understanding community structure  Species accumulation curves – species richness is plotted as a function of the total number of individuals that have been counted overall  These curves can help determine when most or all of the species in a community have been observed  Lots of new species are found in every sample at the beginning of collection, but a point (saturation point) will be reached within that region where same species are encountered over and over again where the curve will plateau  Hughes et al. (2001) compared species accumulation curves for five different communities o The communities varied greatly in the amount of sampling effort necessary to determine species richness o The temperate forest and tropical bird community were adequately represented before half the individuals were counted o For tropical soil bacteria, more effort was needed to sample this extremely diverse community  Spatial scale is also important o If we sampled bacteria in tropical soils at the same scale as Costa Rican moths, the bacterial diversity would be immense in comparison o 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 directi
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