ch 18 : community structure Flashcards
an assemblage of species that occur together in the same place
community
what were the two parts of the historical debate in community ecology? who were the important people involved?
- Superorganism / Holistic concept
- (Frederick) Clements
- Individualistic concept
-(Henry) Gleason
what was the holistic concept?
communities are a distinct ecological units or organizations having recognizable boundaries and whose structure and functioning are regulated by interaction among their component species
what did the holistic concept require?
requires that communities be DISCRETE entities and distinguishable from one another.
what was the individualistic concept?
a loose assemblage of species that can tolerate the conditions of a particular habitat
what did the individualistic concept not require?
distinct boundaries
how do we describe/characterize communities?
- dominant species
- environment/biomes
- relationships
- stability
- control
- riparian community - named for ???
- deciduous hardwood forest community - named for ???
- riparian community - named for environment
- deciduous hardwood forest community - named for the behavior of the dominant tree species
distributions of species sets closely coincide and are separated from other sets of species
closed communities
species are independently distributed along environmental conditions
open communities
what do closed communities result in?
ecotones (regions of rapid replacement of species)
which are less common: open or closed communities? when?
closed communities are less common when there are gradual changes in environment–gradient
what is the continuum concept?
when species are distributed along a gradient
what were some conclusions that were made after Robert Whittaker surveyed the distributions of plant species in the Great Smoky Mountains?
- different tree species appeared and disappeared at different elevations, corresponding to changes in moisture.
- as environmental conditions change, plants will show a gradual and independent change in abundance
what might abrupt changes in species distributions be a result of?
however, abrupt changes in species distributions may be the result of abrupt changes in abiotic conditions (e.g., moving from lake to land), and do not necessarily suggest that species are interdependent.
the number 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 _____________; most species have intermediate abundance; they follow a _____________
low or high abundance; log-normal distribution
a normal, or bell-shaped, distribution that uses a logarithmic scale on the x-axis
log-normal distribution
what can the species richness of a community be affected by?
the amount of available resources
how do ecologists understand the influence of resources?
ecologists have examined the relationship between productivity and species richness
across aquatic and terrestrial environments, the correlation between richness and productivity exhibits a wide range of patterns. what are the observed patterns?
U-shaped
negative
neutral
positive
hump-shaped correlations
across studies of vertebrates and invertebrates in aquatic and terrestrial systems, what is the most commonly observed relationship between diversity and productivity? what does it indicate?
- a hump-shaped curve is the most commonly observed relationship between diversity and productivity.
- this indicates that a site with medium productivity has a higher species richness than sites with either low or high productivity.
how is productivity manipulated in experiments?
adding nutrients (e.g., nitrogen) to an ecosystem
what does added fertility do to the species richness of producers?
added fertility commonly causes a decline in the species richness of producers (e.g., plants and algae).
communities with a higher diversity of habitats should offer what?
more potential niches (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
what would happen if there was the removal of 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
what happens when disturbances are rare?
populations grow until resources are scarce, and competitively superior species become dominant
what happens when disturbances are frequent?
habitats typically support a small number of species that are adapted to disturbances.
what happens 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
what do the arrows in a food web indicate?
indicate consumption and the movement of energy and nutrients
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
who forms the first trophic level of a food web?
producers
a species that eats producers
primary consumers
a species that eats primary consumers
secondary consumers
species that eats secondary consumers
tertiary consumers
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
what are the problems with trophic levels
- informality of construction
- lumping species into guilds or categories
- ignore changing roles (catfish: detritivore/predator)
- all links equal, although may vary in strength
what are the general conclusions of trophic levels?
- most are fairly short (average = 4 links in previous)
- most are open (receive significant input from and export to adjoining systems)
effect of structure on diversity?
- predators can maintain high diversity
- apparent competition
- keystone species—one species that has a disproportionately high impact on a community
- top predators—wolves (reintroduction to Yellowstone); otters—urchins and kelp
- resource populations–krill
how does control of communities work?
population regulation—effects on adjacent trophic levels
what about more distant effects within communities?
trophic cascades—indirect effect of consumer-resource interactions on additional trophic levels
what happens when one trophic level effects its adjacent level?
top-down vs bottom-up (consumer vs resource)
- can also lead to trophic cascades
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
top down control effects if food webs have 3 trophic levels
top-down control by predators would reduce the abundance of herbivores, leading to an increase in vegetation
1960; what did researchers Hairston, Slobodkin, and Smith suggest?
- suggested that since communities contain an abundance of vegetation, food webs must be controlled from the top down.
- in fact, many communities are simultaneously controlled from the top down by predators and from the bottom up by resources.
the ability of a community to maintain a particular structure.
community stability
the amount a community changes when acted upon by a disturbance (e.g., addition or removal of a species)
community resistance
the time it takes after a disturbance for a community to return to its original state.
community resilience
________ may promote community stability
(**review slide 42)
diversity
