Community ecology
Community – group of interacting species that occur together at the same time and place. Interactions among these species and their physical environment give communities their character and function. The relative importance of species interactions and the environment can vary. Communities can be defined at any scale
Ecologists can use subsets of species to define communities e.g.
- Study of forest community may be limited to all the birds (evolutionary lineage/taxonomic affinity)
- Study of species using the same resource (taxonomically distinct)
- Study of species that function in similar ways e.g nitrogen fixing plants
Species diversity combines the number of species (species richness – count no. of species) and species abundance in comparison to other species (species evenness). Species abundance requires the knowledge of abundance relative to other species.
CASE STUDY: SUDDEN OAK DEATH
Symptoms include
- Bleeding or sapping from main stem/trunk
- Wilted shoots
- Bark beetle infestation
- Rapid change in foliage colour (green->brown)
It was first reported in 1995. The causal agent identified as Phytophthora ramorum. Conducted an oak removal experiment to quantitatively monitor what happens to the communities that rely on oak if you remove them (oak). Hypothesised that there are many species that rely on oak - focused on ants. They found 9 species, in different numbers. They ranked them from the most common species -> least common and created a rank-abundance diagram (RAD). RADs are typically a ‘lazy J’ shape. Species richness (S) – the number of bars on the RAD. Almost all communities have this ‘skewed’ RAD.
When describing community structure by only the number of species, you would overlook the fact that soe species are rare and others are common. In the RAD, species evenness = the variation among the bars on the RAD
RABINOWITZ ‘RARITY’ CRITERIA:
- Size of geographic area (large/small)
- Habitat specificity (generalised/specialised)
- Local population density (dense/sparse)
Species rare by all criteria are the most vulnerable to extinction
DIVERSITY INDICIES: QUANTIFYING DIVERSITY
- Diversity incorporates relative abundance
- Diversity indices focus on one aspect of the taxa abundance and emphasise either richness (weighting towards uncommon taxa), or dominance (weighting towards abundant taxa)
- Evenness is a measure of how evenly individuals are distributed across the sample
QUANTIFYING BIODIVERSITY: SPECIES RICHNESS
Sestimated = Sobserved + Sundiscovered
Species accumulation curve:
Number of species vs sampling effort (or species-effort curve). The curve will eventually flatten. The curve can be extrapolated to a flat-line to estimate Sundiscovered
CHOA1 ESTIMATOR
Chao1 estimator is for abundance data and helps to estimate the number of undiscovered species by using the ratio of singletons to doubletons
Sundiscovered = a2/2b
A = the number of species represented by a single individual (singletons)
B = the number of species represented by two individuals (doubletons)
A ‘stopping rule’ - Choa1 reports no undiscovered species when the collection has no singletons
SPECIES-ABUNDANCE DISTRIBUTION (SAD)
SAD is a description of the commonness and rarity of species in a community by means of a frequency distribution of species abundance.
NEUTRAL THEORY
Neutral theory makes a controversial ‘neutrality assumption’ – all individuals within a particular trophic level have the same chances of reproduction and death regardless of their species identity.
Classic neutral theory has a metacommunity (well-mixed source pool of potential immigrants) and a local community. Within the local community, local extinction and immigration from the metacommunity are in equilibrium.
It provides a quantitative null model for assessing the role of adaption and natural selection. It highlights the importance of dispersal limitation, speciation, and ecological drift in the natural world.
MACARTHUR’S ‘BROKEN STICK’ MODEL:
Mathematically the same as randomly breaking a linear resource (stick) into N sections
STUDY TASK 1:
- Neutral theory highlights the importance of dispersal limitation, speciation and ecological drift in the natural world and provides quantitative null models for assessing the role of adaptation and natural selection.
- Classic neutral theory includes several ‘auxiliary assumptions’, unrelated to neutrality, which can cause the model to fail for reasons other than its neutrality and draw attention away from the key issues
- The original neutral model assumes a ‘point mutation’ mode of speciation, where, with each birth in the metacommunity, there is a small probability n that the newborn founds a new species
- Point mutation speciation produces many species whose lifetimes are far too short, whereas random fission speciation produces mean species lifespans that are too long These problems were initially linked to the neutrality assumption
- Rosindell et al. recently proposed a model of ‘protracted speciation’, in which speciation is not instantaneous, but a gradual process beginning with the creation of an ‘incipient species’ or ‘variant’.
- Neutral theory can be used as a foundation for more advanced speciation models that include explicit genetic details
- Dispersal limitation is central to neutral theory and important for determining species abundance distributions even in niche-based models
- A neutral model can be used conveniently to study the effect of species being dispersal limited where all species have the same dispersal abilities.
- Theory of island biogeography introduced the revolutionary idea that community species composition is not static, but in continual dynamic turnover. The neutral theory of biodiversity can replicate these results, but in addition incorporate species abundances
- The temporally explicit nature of neutral theory might make it useful in palaeontological applications. Neutral theory could help to address the complex taxonomic and other sampling issues that palaeontologists routinely face.
- The possibility that neutral theory could contribute significantly to the fields of phylogenetics, population dynamics, island biogeography, paleobiology and conservation has been suggested from the start, but hardly any work has been done in these areas.
SIMPSON’S DIVERSITY INDEX:
Simpsons Diversity Index is the simplest measure of the character of a community. It considers both abundance (or biomass) patterns and species richness. High scores indicate high diversity. Low scores indicate low diversity.
Simpson’s Index, D =
S = the total number of species in a community (i.e. richness), and the proportion of individuals that the species contributes to the community (Pi for the i species)
Pi2 – proportion of number of (x) squared
Equitability can itself be quantified (between 0 and 1) by expressing Simpon’s Index, D, as a proportion of the maximum possible value D would assume if individuals were completely evenly distributed amongst the species
Equitability, E = = x
Dmax = S
S is the total number species in a community (i.e., richness), and the proportion of individuals that the species contributes to the community, (Pi for the i species)
DIVERSITY INDIVIES: SHANNON INDEX:
Diversity indices are essential for making assessments of habitat quality in relation to human activities or impact. The Shannon index is a measure of community diversity that takes into account both abundance and species richness patterns
S – the total number of species in a community
Pi – proportion of each species in the sample/community
InPi – natural log of Pi
Real world data ranges between 0 and 5, where 0 = all groups have the same frequency
Equitability is simply the Shannon diversity index divided by the maximum diversity. It ranges between 0 and 1 – the lower the evenness, the higher the diversity
Equitability
S – total number of species in a community
Pi - proportion of each species in the sample / community
ln Pi - natural log of Pi
Hill numbers or the effective number of species are increasingly used to quantify species diversity of an assemblage. Recently extended to phylogenetic diversity, which incorporates species evolutionary history, as well as to functional diversity, which considers the differences among species traits.
For a landscape of habitat patches containing different sets of species,
- total species richness of the region, γ‐richness
- average species richness within patches (α‐richness) and
- between‐patch component of regional richness (β‐richness)
γ‐richness is the sum of α‐richness and β‐richness.
If every patch has identical species, β‐richness = 0, γ‐richness = α‐richness.
But β‐richness will make a contribution to γ‐richness wherever there is heterogeneity in the distribution of species among patches.
PHASE 1 HABITAT SURVEY
The Phase 1 Habitat Survey represents a standard system for classifying and mapping wildlife habitats in the UK. JNCC Phase 1 Survey Handbook states:
“The aim of Phase 1 survey is to provide, relatively rapidly, a record of the semi-natural vegetation and wildlife habitat over large areas of countryside.”
This is information is critical for ecologists – vegetation can be easy to observe, mobile, small or fugitive species (animals) can require comprehensive surveys, not so practical or easy on a large scale.
Phytosociology is a basis for the study of plant matter, it’s a perfect knowledge of the floristic composition of the synecological unit
Succession – the developmental study of vegetation rests upon the assumption that the unit or climax formation is an organic entity. Each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development. The end of the process of stabilisation is a climax community. The cause is dominance. This may mean control of soil factors, air factors, etc