Chapter 19: Community Structure

community

a community is an assemblage of species living together in an area

zonation

species occur in different locations across an environmental gradient

terrestrial communities are categorized by

dominant plants

aquatic communities are categorized by

physical characteristics or by dominant organisms

ecotone

a boundary created by sharp changes in an environmental conditions causing a major change in species composition, species differ in distribution across an ectone and ecotones can have higher diversity

interdependent communities

communities in which species depend on each other to exist

independent communities

communities in which species do not depend on each other to exist

what are communities

communities are generally independent

species richness

the number of species in a community

relative abundance

the proportion of individuals in a community represented by each species

in a typical community

most species have intermediate abundance: log normal distribution

rank abundance curves

a curve that plots the relative abundance of each species in a community in rank order from the most abundance species to the least abundant species

species evenness

a comparison of the relative abundance of each species in a community

diversity indices

take into account both abundance and richness

relationship between resources and diversity

are found in natural communities

communities with a higher diversity of habitats should offer more what

potential niches and a higher diversity of species

keystone species

a species that is not necessary numerous but substantially affects the structure of a community, removal of a keystone species can substantially alter diversity

ecosystem engineers

are keystone species that affect communities by influences the structure of a habitat

intermediate distance hypothesis

the hypothesis that diversity is greatest at intermediate levels of disturbance

how are species connected

species are connected through feeding relationships

food chain

a linear representation of feeding relationships

food web

a network representation of feeding relationships in which arrows indicate consumption and movement of energy and nutrients

trophic level

a level in a food chain or food web of an ecosystem

producers

autotroph that conduct photosynthesis, produces form the first trophic level of a food chain or web

primary consumer

a species that eats producers

secondary consumer

a species that eats primary consumers

tertiary consumers

a species that eats secondary consumers

omnivore

a species that eats produces and consumers

guild

a group of species that feeds on similar items

bottom up control

when the abundances of trophic groups are determined by the amount of energy available from producers

top down control

when the abundance of trophic groups are determined by the existence of predators at the top of the food web

the world is green hypothesis

herbivores are controlled by predators (top down control) bc plants are so abundant they are unlikely to be a limiting factor

direct effects

one species directly influences another species

indirect effect

one species indirectly influences another species through an effect on another species or resource

species are lined in networks of what

direct and indirect interactions

density mediated indirect effects

one species indirectly affects another through changes in density

trait mediated indirect effects

one species indirectly affects another through changes in traits, including behaviors

trophic cascade

when indirect effects are initiated by a predator, it is called a trophy cascade. effects reverberate down the food chain

how wolves change rivers

wolves are keystone species, wolves cause trophic cascades, wolves have direct effects and also density mediated and trait mediated indirect effects, wolves indirectly increase vegetation, which stabilizes the soil and changes the course of rivers

the fact that a community can lose a species to extinction yet still persist without loss of other species provides evidence that species in many communities are

independent

an ecotone is an area that

is found between two community types

a species that plays an important role in affecting the structure of a community, despite its low abundance is called an

keystone species

what does it mean when a community is said to exhibit top down control

the abundance of trophic groups is determined by the presence of predators