55-communities Flashcards
Community
A group of species living together in the same place and time
- vary in size and scope
- can be small/contained or cover a large geographic area
Biologists use subsets to organize communities based on…
- taxonomy (similar species)
- guilds (use similar resources)
- functional group (perform similar tasks)
*trophic structure (food webs)
Trophic levels
-Contain species which have a similar way of interacting & obtaining energy
-depicted in food webs
Primary producer —> primary consumer —> secondary consumer —> tertiary & higher consumers
*detrivores/decomposers & scavenges
Interaction webs
-include non-trophic interactions such as competition and positive interactions (more realistic)
Species composition
Kinds of species
Species diversity
- combines species richness (# of species) & species evenness (relative abundance)
- Shannon diversity index
H = -(sum of s) [pi x ln(pi)] i = 1
- s= #of species - pi= relative abundance - H=Shannon index value - the higher H, the higher species diversity of the community
Community membership depends on 3 factors…
1) species supply
- regional species pool and dispersal ability
2) abiotic conditions
- physical and chemical environmental conditions
3) biotic conditions
- species interactions
Species supply
- regional species pools provide upper limits on # of species that can live within communities
- first step to community membership is the species being present at the regional level with the ability to disperse to the community
Abiotic conditions
- “abiotic filters” restrict species that are physiologically incapable of serving in the community
- physical environment can be significant barrier for non-native species
- community membership can change with environmental conditions
Biotic conditions
-species must be able to live with the other species in the community
(Obtain food, survive without being eaten, cope with competition)
-resident species ca promote or restrict species membership
Biotic resistance
The ability of resident species to exclude or slow the population growth of non-native species
Direct interactions
Occurs between two species
Indirect interactions
When the direct interactions between two species re mediated by a 3rd
- often occur via trophic interactions
- can be traced to a trophic level several away from where it is seen
trophic cascade
When the rate of consumption at one trophic level results in a change in species abundance at lower trophic levels
Interaction strength
-the affect one species has on the abundance of another species
- can be measured experimentally by removing the “interactor” species, and observing what happens to the target species
- positive interaction if target species declines
- negative interaction if target species increases
- no interaction if no change in target species
Resource-mediated coexistence
When competition among species i mediate by factors that affect the dominant species’s ability to acquire resources
Intermediate disturbance hypothesis
Describes how varying degrees of disturbances (abiotic process that kills or injures individuals) affect species diversity
- high diversity with intermediate disturbance levels - low diversity with high or low disturbance levels
Keystone species
- have strong effects in community; not because of size or abundance, but because of the role they play
- mostly act by creating trophic cascades
Foundation species
- have large effect on community because of their size and abundance
- Trees
Ecosystem engineering species
- create, modify, or maintain a physical habitat fo themselves and other species
- don’t necessarily have great size or abundance
Succession
How a community changes in species composition over time
-result of biotic and abiotic factors
Disturbance
An abiotic event that physically or chemically injuries or kills individuals
- creates opportunities for other individuals to grow and reproduce
Stress
When some abiotic factor reduces growth, reproduction, and/or survival of some individuals
Climax community
Stable assemblage of species that experience little change until an intense disturbance wipes out the community
Primary succession
-result of a disturbance killing all of the organisms in a community
-sometimes caused by glaciers, landslides, volcanic activity and floods
-slow because early arrivals must deal with extreme conditions
(Pioneer species/early successional species)
Secondary succession
The reestablishment of a community when most, but not all organisms have been destroyed
- more common
- progresses more rapidly
Alternative states
The different community assemblages that could develop at the same location under similar environmental conditions
Hysteresis
When the community is unable to shift back to its initial state, even if original conditions are restored
Stability
The communities resistance to disturbance
-communities with more species are generally more stable, ie. use resources efficiently
