chapter 18 Flashcards
an assemblage of species living together in an area
community
Community zonation also occurs in
aquatic communities.
Communities are often categorized by their
dominant organisms or by physical conditions that affect the distribution of species.
Aquatic systems are often categorized by
physical characteristics (e.g., stream or lake communities) or by dominant organisms (e.g., coral reef communities).
Ecologists rarely study every species in a community; rather, they
focus on a subset of species that live in an area
a boundary created by sharp changes in environmental conditions over a relatively short distance, accompanied by a major change in the composition of species.
Ecotone
Some species move between
adjacent communities
most species live in
one of the communities and spread into the ecotone.
Ecotones support a large number of species
including those from adjoining habitats, and species specifically adapted to the ecotone
communities in which species depend on each other to exist.
Interdependent communities
communities in which species do not depend on each other to exist.
Independent communities
If species are interdependent, removing a species should cause
other species to decline
if species are independent, removing a species should cause
neutral or positive changes in other species’ fitness
the # of species in a community.
Species richness
the proportion of individuals in a community represented by each species.
Relative abundance
In a typical community, only a few species have
low or high abundance
most species have intermediate
abundance
if a species has intermediate abundance they follow a
log normal distribution
a curve that plots the relative abundance of each species in a community in rank order from the most abundant species to the least abundant species.
Rank-abundance curves
a comparison of the relative abundance of each species in a community.
Species evenness
The species richness of a community can be affected by
the amount of available resources.
To understand the influence of resources, ecologists have examined the relationship between
productivity and species richness.
observed patterns across aquatic and terrestrial environments
u shaped negative neutral positive hump shaped
what is the most commonly observed relationship between diversity and productivity
hump shaped curve
Experiments have manipulated productivity by
adding nutrients (e.g., nitrogen) to an ecosystem.
Added fertility commonly causes a decline in the species richness of
producers (e.g., plants and algae).
The reason species richness declines with increased habitat fertility
is unclear.
For plant communities, increased fertility may cause
dominant plants to cast more shade on competitively inferior plants.
Communities with a higher diversity of habitats should offer
more potential niches (e.g., places to feed and breed) and a higher diversity of species.
a species that substantially affects the structure of communities, although species might not be particularly numerous
Keystone species
Removal of a keystone species can cause
a community to collapse
keystones species that affect communities by influencing the structure of a habitat.
Ecosystem engineers
the hypothesis that more species are present in a community that experiences occasional disturbances than in a community with either frequent or rare disturbances.
Intermediate disturbance hypothesis
When disturbances are rare, populations
grow until resources are scarce, and competitively superior species become dominant
When disturbances are frequent, habitats
typically support a small number of species that are adapted to disturbances.
When disturbances occur at an intermediate frequency,
both types of species can persist
a linear representation of how different species in a community feed on each other.
Food chain
a complex and realistic representation of how species feed on each other in a community.
Food web
a level in a food chain or food web of an ecosystem.
Trophic level
the autotrophs that convert light energy and CO2 into carbohydrates through photosynthesis.
Producers
a species that eats producers.
Primary consumer
a species that eats primary consumers
Secondary consumer
a species that eats secondary consumers
Tertiary consumer
a species that feeds at several trophic levels.
Omnivore
within a given trophic level, a group of species that feeds on similar items (e.g., guilds of leaf eaters); members of the group are not necessarily related
Guild
an interaction between two species that does not involve other species
Direct effect
The direct effect of one species often sets off a chain of events that
affect other species in the community
an interaction between two species that involves one or more intermediate species.
Indirect effect
indirect effects in a community that are initiated by a predator.
Trophic cascade
Indirect effects can occur
between communities.
when the abundances of trophic groups are determined by the amount of energy available from producers
Bottom-up control
when the abundances of trophic groups are determined by the existence of predators at the top of the food web
Top-down control
If food webs have three trophic levels, top-down control by predators would
reduce the abundance of herbivores, leading to an increase in vegetation.
researchers (Hairston, Slobodkin, and Smith) suggested that since communities contain an abundance of vegetation
food webs must be controlled from the top-down.
