Ecology Quiz 7 Flashcards
What are communities? Definition
In practical terms, ecologists usually define communities based on: (can’t study all of communities bc there are too many so must split it up)
Communities: are groups of interacting species that occur together at the same place and time.
So far we have considered species interactions in two way relationships. In reality, species experience multiple interactions that shape the communities in which they live.
Physical characteristics: e.g., all species in a sand dune, mountain stream.
Biological characteristics: e.g., all the species associated with a kelp forest or a coral reef; implies importance of abundant species.
Ecologists usually consider a subset of species when they define and study communities.
Subsets can be defined by:
- Taxonomic affinity: Example: a study might be confined to all bird species in a community
- Guild: groups of species that use the same resources.
- Functional group: species that function in similar ways, but do not necessarily use the same resources.
Community structure:
Community structure: Set of characteristics that shape communities. Provides a basis for generating hypotheses and experiments to understand how communities work.
Community structure is usually characterized by:
- Species diversity
Species diversity combines species richness and species evenness - Species composition (identities of species in community)
- Species diversity
How do ecologists determine when most or all of the species in a community have been observed?
Species diversity combines species richness and species evenness
Species richness: the total number of species in a community.
Question? Species accumulation curves: Species richness plotted as a function of total number of individuals counted. The threshold where no new species are counted never occurs in natural systems because new species are constantly being found.
Species evenness: (horizontal) (proportion of species in a given community) Describing relative abundance - Rank abundance curves plot the proportional abundance of each species (pi ) relative to the others, in rank order.
Suggests possible species interactions:
E.g dominant species might have a strong negative effect on less abundant species.
Rank-abundance curves for four closed-canopy tree communities spanning a large latitudinal gradient, from boreal to equatorial Amazonian forest (after Hubbell 1996).
The most commonly used species diversity index is the Shannon index: species richness and evenness
Equation
pi = proportion of individuals in the ith species - abundance/total - higher pi means more diverse based on advance and evenness - mor evenness will give you higher diversity pi number
s = number of species in the community
H= - S Pi ln (Pi)
- Species composition
Species composition: the identities of species present in the community. Two communities could have identical species diversity values but have completely different species.
Species diversity VS Biodiversity VS Environmental complexity
Species diversity = the number of species in a community.
Biodiversity describes diversity at multiple spatial scales, from genes to species to communities. Implicit is the interconnectedness of all the components. - genes all the way to ecosystem
Environmental complexity: In general, species diversity increases with environmental complexity or heterogeneity.
Robert MacArthur found warbler diversity increased as vegetation stature increased.
Many studies have shown a positive relationship between environmental complexity and species diversity.
Interactions of Multiple Species:
Buss and Jackson (1979) hypothesized that competitive networks allow the
Interactions of Multiple Species: Communities can be characterized by complex networks of direct and indirect interactions varying in strength and direction - More than two species! –> Competitive networks: Interactions among multiple species in which every species has a negative effect on every other species.
May be hundreds of species within a community.
Many, indirect interactions.
Many types of interactions (competition, predation, mutualism).
Buss and Jackson (1979) hypothesized that competitive networks allow the coexistence of competitors and maintain species richness.
Test of the competitive network hypothesis:
Algae and invertebrate species compete for space on coral reefs by overgrowing one another.
In areas of overlap, researchers determined the proportion of wins (species on top) to losses (species on bottom).
No species won consistently. species interacted in a circular network rather than a linear hierarchy.
Results support the idea that competitive networks, by fostering diffuse and indirect interactions, promote diversity in communities
Trophic cascade:
Trophic cascade: how nature is organized - indirect positive interaction - keystone
Rate of consumption at one trophic level results in change in species abundance or composition at lower trophic levels:
- A carnivore eats an herbivore (a direct negative effect on the herbivore).
- The decrease in herbivore abundance has a positive effect on a primary producer.
- Indirect effects in interaction webs.
Foundation versus Keystone Species
Foundation species have large effects on other species, and thus species diversity, by virtue of their considerable abundance and biomass, e.g., trees, corals.
Some dominant species are ecosystem engineers: they create, modify, or maintain physical habitat for themselves and other species.
Keystone species have a low relative abundance and biomass, but a strong effect on other species, despite their rarity and low biomass.
Keystone Species: Large effect, disproportionate to their biomass or abundance.
Beavers: Keystone Species and Ecosystem Engineers - Wetlands increased with beaver recovery, and regional biodiversity increases
Some species play only a small role in community structure and function:
Contribute to overall diversity and their presence or absence has little significance for the ultimate regulation of the community
Some might be redundant bc they have similar functions as other species but might be important bc redundant can help when species are disturbed
Theory of succession:
Factors -
Primary and Secondary
Change in species composition in communities over time, resulting from both biotic and abiotic factors. Succession involves colonization and extinction due to abiotic and biotic agents of change. Although climax communities are assumed to be the endpoint of ecological succession, some believe that there is no such thing… that a community never actually reaches a climax! - succession theory comes from plants bc sessile.
Abiotic agents of change fall into two categories:
Disturbance: Events that injure or kill some individuals and create opportunities for other individuals, e.g., tsunami.
Stress: Abiotic factors that reduce growth, reproduction, or survival of individuals, e.g., increasing temperature.
Biotic agents of change:
Species interactions can result in species replacements
Diseases can cause death or slow growth of a species.
Ecosystem engineers or keystone species can influence community change.
Primary succession: Starts off with no life and progress to climax stage
Secondary succession: Either in an intermediate or climax stage and then some disturbance where you have to start off from scratch
Spectrum of disturbance:
Clements:
Gleason:
Spectrum of disturbance:
Agents of change vary in frequency and intensity.
Clements:
Community has a predictable life history
Ultimately reaches a stable end point called the “climax community.
Climax communities have dominant species that persist over many years and provide stability
Superorganism
Gleason:
“Individualist concept” that succession is the result of environmental requirements of the individual species
He also noted that “. . . no two species make identical environmental demands.”
Succession reflected the interactions of individuals with the environment
Henry Cowles:
Henry Cowles: Studied succession on sand dunes along Lake Michigan. - replacing time with space and observe succession without having to wait for hundreds of years
Plant assemblages farthest from the lake’s edge were the oldest;
Plants nearest the lake were the youngest,
Representing a time series of successional stages.
“Succession was a variable chasing a variable” – the changes in vegetation chase the changes in the climate.
Predict how communities would change over time without actually waiting for the pattern to unfold (“space for time substitution”)
