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Concept 15.1: Communities are groups of interacting species
that occur together at the same place and time.
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Concept 15.2: Species diversity and species composition are
important descriptors of community structure.
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Concept 15.3: Communities can be characterized by complex networks of
direct and indirect interactions that vary in strength and direction.
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A ---------------- defined community might encompass all the species in a sand dune, a mountain stream, or a desert.
physically
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A ------------- defined community might include all species associated with a kelp forest, coral reef or a salt marsh wetland.
biologically
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Subsets can be defined in several ways:Taxonomic affinity—Guilds—Functional group—
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Taxonomic affinity—
a study might be confined to all bird species in a community.
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Guilds—
- groups of species that use the same
- resources.
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Functional group—
species that function in similar ways, but do not necessarily use the same resources.
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Trophic levels are groups of species that
have similar ways of obtaining energy (e.g., primary producers, primary consumers).
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4 trophic levels
- primary producer
- primary consumer (Herbivores)
- Secondary consumer (carnivores)
- Tertiary consumer
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Community structure
is the set of characteristics that shape communities.
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Species richness—
the number of species in a community.
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Species evenness—
relative abundances compared with one another.
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Species diversity
combines species richness and species evenness.
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Shannon index
pi =
s=
- =proportion of individuals in the ith species
- =number of species in the community
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Shannon index has ----------- not ----------------
proportions, not percentages
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Biodiversity =
diversity of important ecological entities at multiple spatial scales, from genes to species to communities. Interconnectedness of all components of diversity.
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Rank abundance curves plot
the proportional abundance of each species (pi) relative to the others in rank order.
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Species accumulation curves =
species richness is plotted as a function of the total number of individuals that have been counted with each sample.These curves can help determine when most or all of the species in a community have been observed.
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Species composition—
the identity of species present in the community.Two communities could have identical species diversity values, but have completely different species.The identity of species is critical to understanding community structure.
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Direct interactions
occur between two species (e.g., competition, predation, and facilitation).
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Indirect interactions
occur when the relationship between two species is mediated by a third (or more) species.
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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.
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Aphids had more difficulty finding ...... but
- Iva in the presence of Juncus, but when they did, population growth rates were significantly higher.
- study this more
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Trophic facilitation
occurs when a consumer is indirectly facilitated by a positive interaction between its prey and another species.In New England salt marshes, two plants—a sedge (Juncus gerardii), and a shrub (Iva frutescens)—have a commensalistic relationship.When Juncus is removed, Iva growth rate decreases, but removing Iva had no effect on Juncus.
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Competitive interactions occur
in a network fashion (i.e., every species negatively interacts with every other species).competitive networks, by fostering diffuse and indirect interactions, can promote diversity in communities.
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Dominant species (foundation species) can have a large effect on other species and species diversity by virtue of
high abundance or biomass.Dominant species may also be dominant by virtue of being good competitors for space, nutrients, or light.
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Some dominant species are ecosystem engineers—
they create, modify, or maintain physical habitat for themselves and other species.Example: Trees—provide habitat and food; reduce light, wind and rainfall, which changes temperature and moisture conditions; roots increase weathering and soil aeration
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Keystone species have a strong effect because of
their roles in the community.Their effect is large in proportion to their biomass or abundance.They usually influence community structure indirectly, via trophic means, as in the case of sea otters
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Context-dependent species interactions
can change under different environmental conditions.Some keystone species play important roles in their communities in one context, but not in another.
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