Community Ecology Flashcards
Interspecific competition
Competition between species
Intraspecific competition
Competition within a species
How are competing species quantified
Lotka-Volterra model
3 main forms of competition
Exploitation
Interference (direct inhibition)
Apparent
Interference competition
2 competitions species interfering directly
Exploitation competition
2 species consuming a common resource
Apparent competition
2 species being attacked by a common predator
The prey are competing for ‘enemy-free space’
Environmental structuring alleviates apparent competition and facilitates co-existences of all prey in the larger habitat
may also be driven by shared parasites or pathogens
Red vs grey squirrel - forms of competition
Interference = grey bigger and steal/destroy red nests
Exploitation = acorns, seeds, food, space
Apparent = squirrel pox virus. Grey has fewer predators than red
Competitive exclusion principle
Better competing species will eliminate or exclude less competitive species
If two competing species coexist in a stable environment, then they do so as a result of realised niche differentiation.
If, however, there is no such differentiation, or if it is precluded by the habitat, then one competing species will eliminate or exclude the other.
Two species can compete for two resources and coexist as long as two conditions are met.
- The habitat (i.e. the supply point) must be such that one species is more limited by one resource, and the other species more limited by the other resource.
- Each species must consume more of the resource that more limits its own growth.
Resource competition theory
predicts that the number of coexisting species should increase with the number of resources that are at physiologically limiting levels
Fundamental niche
combination of conditions and resources that allow that species to exist, grow and reproduce
Realised niche
combination of conditions and resources that allow it to exist, grow and reproduce including the presence of interspecific competitors (and other species that might be harmful to its existence).
Even though competitors may have the same fundamental niche, they may coexist if they are provided with different realised niches. E.g. for the charr, the different temperatures in the stream provided different realised niches for each species.
Explanation to why there are so many co-existing species ?
Idea 1: If species have nearly the same competitiveness in a realised niche – if identical then the ratio of each would not change.
If almost identical then the rate of change would be extremely slow – they may appear to co-exist
Time scales – very long-lived species may appear to us to co-exists by it might just be the time scales are very long
Idea 2: Habitats / realised niches change through time
competitors may coexist because they live in a fluctuating environment that first favours one species, then the other, then the first (or another), and so on
Other aspects that may allow competing species to co-occur
Temperature-mediated competition
Competitive release (realised niche expansion)
Community
An assemblage of a species populations that occur together in space and time
Sampling a community
The more you sample, the more species* you can see
(if they are there)
Rare species will only be recorded with intensive sampling efforts
The above is true for each individual sample taken, thus …
You need to REPLICATE samples to determine any patterns
Why do you have to repeat samples
Species may not be regularly distributed
Environments may be patchy
There may be clines
Species may vary through time
How to describe communities
Species richness
Diversity indices
Species richness
This ignores each species abundance: it is just a list and count of species present in the sub-samples
From this alpha-diversity can be calculated: the biodiversity property at ANY ONE SITE (within sample diversity)
Rarefraction curves
Keep sampling until you recover no new species
- time consuming
Chao1
Mathematically estimates species richness
Alpha-diversity
The biodiversity property at any one site
a list of species present for any one community
Beta diversity
how biodiversity differs at different sites, treatments (or times), etc
You MUST use a statistical test to determine if any differences are significant
this COMPARES presence and absence of each species in two or more communities.
Gamma diversity
describe the diversity of the larger area and is the sum of individual sites and how these differ (⍺ + β = γ)
Rarefy data
Ideally samples for comparison have the same sample effort.
If not, then can ‘rarefy’ the data: artificially randomly sub-sample from the sampled data to the same depth across all. This makes them comparable!
Species-area relationship
It is well established that the number of species on ‘islands’ decreases as island area decreases
Diversity indices
measures that also incorporate abundances and thus how ‘even’ communities are
Simpsons index
the proportion of individuals or biomass that it contributes to the total in the sample (the proportion is Pi for the ith species) for each species (or group). S is the total number of species in the community
D increases with evenness/equitability (same richness for both)
Evenness / equitability
can be quantified (between 0 and 1) by expressing Simpson’s index as a proportion of the maximum possible value D would assume if individuals were completely evenly distributed amongst the species. Therefore, Dmax = S
D/10
Shannon diversity (H)
Shannon is more sensitive to species richness, while Simpson is more sensitive to evenness (or dominance).
H and J increase with evenness/equitability (same richness for both)
Equitability = J
Dissimilarity
dissimilarity (distance) is calculated – this produces a numbers for each comparison that can be statistically analysed (and plotted)
1 = completely dissimilar
(no species overlap)
Zero = not dissimilar at all
(identical types of species present)
Similarity = 1-dissimilarity
Significance of differences
If mean BETWEEN is not statistically different from mean
WITHIN, then no difference between sites
Get a p value by comparing to a ‘null distribution of no difference:
randomising the site locations of each sample then recalculating within v between difference (100 – 1,000 times)
PermANOVA test
This produces an objective evaluation of whether species types differ between locations
How might you visualise the differences in many species between sites?
Ordination methods
R^2
Absolute species richness
Number of species is the same
Relative species richness
Number of species is the same but the types of species differ- only 1 overlaps
Community composition
The same species might be present but their abundance might differ
Bray and Jaccard calculations
Community dissimilarity
Bray-curtis
B = (A+B-2X) / (A+B-X)
A = number of species in community A
B = number of species in community B
X = number of species in both sites
Jaccard
J = (2B) / (1+B)
Which of the following could qualify as a top-down control on a grassland community?
Effect of grazing intensity by bison on plant species diversity.
