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29 terms

Communities

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What is a community?
• An assemblage of species occurring in one place
• These species interact with each other (consumers, resources, competitors)
The community approach to ecology
• We ask questions about feeding relationships, patterns in time & space, & the diversity & relative abundance of species living together.
**Can be Closed or Open
Info on closed community concept
Communities are distinct units
Info on open community concept
o Communities are composed of species that are associated by chance; they share adaptations that allow them to live together in a particular area
Info on how the discrete nature of communities is controversial...
o When we look at closed communities (on graph), each community is separated by ecotones (regions of rapid replacement of species along a gradient) (boundary)
o When we look at open communities, species are distributed independently with respect to each other, and ecotones don't exist
What concept - open or closed - is correct?
o Integration (neither are wrong; today we see integration)
• Most species assemblages lack distinct boundaries (part of the open community concept)
• Interactions among species determine community structure & function (part of the closed community concept); species aren't together only by chance.
Info on biological community is an association of interacting populations..
Species aren't just together by chance read
Measures of community structure include number of species and trophic levels
read
Some communities are discrete units...
• Are referred to as closed communities
• Are separated by ecotones
Info on Ecotones..
Boundaries/transition between two areas, communities, biomes, etc.
*Vary in their degree of development

• E.g., grasses next door to a tree community (upper left)
Distinction is very clean cut between x and y
Argued it represents an edge and not an ecotone
• Community y invades community x (upper right)
See ecotype; environmental gradient; invading/merging of communities
• Community x invades community y (lower left)
See ecotype; environmental gradient; invading/merging of communities
• Communities invading each other even more (lower right)
See ecotype; environmental gradient; invading/merging of communities
How do ecotone edges vary in contrast?
Can see very clear cut edges
Can see fuzzy edge with merging
Why are ecotones important?
They connect communities: materials, energy, & organisms move through them
Ecotone characteristics influence gradients of wind flow, moisture, temperature, & solar radiation
They restrict or facilitate the movement of animals & seed (represent boundaries)
Some species, such as indigo bunting, are _______ (ecotone) __________. Info.
Edge specialists
Some organism that wants the variety of features in close proximity of each other.
• The males want tall trees/perches from which to sing and advertise their territories, where as the females want shrubs to build their nest
In open communities, species are distributed _________ of each other
independently
• Variety of species distributed across various gradients (e.g., moisture gradient where some prefer dry and some prefer wet)
More info on open communities..
• In open communities (the absence of ecotones), separations among these communities occur along an environmental gradient (a continuum; e.g., moisture continuum), which is subject to gradient analysis
Info on gradient analysis
• Gradient analysis: the abundance of each species is plotted on a continuous gradient of one or more physical conditions.
o E.g., figure with moisture gradient (we can look at the distribution of organisms along a gradient; looking at the presence/abundance of a species along a gradient of environmental conditions)
Info on quantifying community structure..
Difficult because some communities are very complex/species rich
*Can quantify in 3 ways
**1)Species richness
**2) Diversity Indices
**3) Trophic relationships
Info on species richness.
The number of species present in that community
Info on diversity indices
• Mathematical formulas that take into account not only the number of species (species richness), but also relative abundance.
• E.g., two communities having the same species richness, but one community (b) shows more of a variety and higher evenness (more abundance of several different kind of species), whereas the other community (a) may be dominated by a particular species)
Info on trophic relationships
Feeding relationships
Feeding relationships organize communities into _____ _____.
Food webs
info on food webs and community structure
• Food webs are used to show/quantify community structure
• More complex if the feeding relationship is more complex
o E.g., community with omnivores (feed on a variety of resources) will be more complex
In general, the number of trophic levels is what?
Positively associated with species richness

o As the food web gets more complex (more trophic levels), the species richness increases
• Guilds also increase
What is a guild?
Group of species with similar ecological positions
• E.g., certain insects feed on certain parts of a grass blade as larvae
o Guild shows the intensity of competition among species
READ
What are keystone species (consumer)?
A species who's presence determines the community structure

• Have significance to the community out of proportion to their abundance
o E.g., relationship between sea otters and the sea urchins on which they feed
• As long as the sea otters are protected, we have healthy kelp cores, coral controlled, shellfish controlled.
• Introduction of humans
Took commercial shellfish
Took the sea otters
• Deprivation of fish → decrease* in seal and sea lions → Killer whales had to begin taking sea otters
• The sea otter was the corner stone of its community


o E.g., Pisaster (sea star) is another keystone species
• Work by Robert Paine
In the absence of Pisastar, 1 mussel dominates the community & crowds out other organisms
Community diversity and complexity are reduced
Pisastar determined community structure
What are the three relationships food webs can show?
1) Feeding relationships among organisms
o Who eats who
2) Energy flow
o How much energy is flowing from species to species
3) Influence of one population on the growth rate of another (functional web)
Info on food webs and their influence on the stability of communities.
• Resilience vs. constancy

• Resilience
o Ability of a community/system to return to some reference state after a disturbance (i.e., to 'bounce back' after flood, fire, etc.)
o Enhanced by complex food webs
• More complex food webs → more resilience
• Constancy (resistance)
o The ability of a system to resist change
o Relationship with food webs is more complex
Info on communities switching between alternative stable states..
o Rockweed alga removed (mimicking ice scouring - ice rubbing off plants - of this intertidal community)
o Clearings were colonized by brown alga & by barnacles
o Which community got established depended on whether site was northward facing
o New communities replaced the original

READ
Info on trophic levels being influenced from top and bottom.
• In some communities, increased production of producers → greater productivity in the levels above it.
o Bottom up control
• In some communities, the introduction of a predator → might cause a trophic cascade (effecting trophic levels below it causing decrease/alteration of biomass at all levels below it)
o Relative biomass alternates (e.g., large → small → large → small)
• Top down control