- Multiple species interactions
• Food webs-feeding relationships within a community
• Interaction webs-non-feeding interactions
• trophic structure-describes the feeding relationships
- describes energy moving among trophic levels
• food chains linked to form food webs
• Direct interactions (solid arrows)-between two species and include trophic and non-trophic interactions
• Indirect interactions (dotted arrow)-relationships between two species is mediated by a third or more species
• Trophic cascade-rate of consumption at one trophic level results in a change in species abundance at another trophic level
- Species interactions influence community structure
• Social web spiders
- >100 species
• Community-two or more different species interacting together in a specific area
- plants, animals, microbes liked by feeding relationships and other interactions
- defined spatially, and by the dominant lifeform
• Why are certain species found together in a community
- they depend on each other
- similar habitat needs
• How are communities structured?
- Closed community-close association between species regulates distribution of whole community
• species depend on each other to exist
• interactions among species determine which species inhabit a community
• Interdependent (discrete) communities
• Frederic Clements-plant ecologist (early 19th century)
- most communities function as interdependent communities
• Interdependent communities-species boundaries are consistent across species within a community
- boundary of each species is not dependent on the boundaries of other species
• repeatable structure
- Open community-species are distributed independently to one another, regulated by environmental conditions
• Species have similar adaptations and nutrient requirements
• each species had different ranges of conditions in which they exist
• community reflects overlapping ranges of species
• Independent communities
- Clements approach
• if species rely on each other to persist, removing a species should cause other species to decline
- Clements VS Gleason conclusion
• species have independent (Gleason) distributions in MOST communities when plotted on abiotic gradients
• Species have dependent (Clements) distributions in communities with harsh environmental conditions
Measuring community structure
• species richness-number of species in a community
• relative abundance-the % each species contributes to the total of all species
• species evenness-a measure of how numerically equal the species are in a community
• Species diversity-number of species (richness) and the evenness of species (abundance)
- Shannon diversity index
• higher, higher diversity
• probability that two randomly selected are the same species
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