Community Assembly Notes

Community structure refers to the observed patterns in abundance, richness, and composition of species within a community.
Understanding what explains community structure is a central question in community ecology.

Community ecology is a complex subject with a vast and intricate literature.
There are numerous published theories and mechanisms.
- Palmer (in Vellend 2010) noted over 100 hypotheses to explain species diversity patterns.
Contradictory and system-specific results are common.
- Lawton (1999) questioned why ecology lacks universal laws.
- Findings often depend on the specific organisms and environment studied.
- This problem is most pronounced at the community scale compared to smaller or larger scales.

Vellend (2010) proposed a framework distinguishing between 'low-level' and 'high-level' processes to simplify the teaching of community ecology.
- Low-level processes are detailed hypotheses/mechanisms.
- These processes can be grouped into four high-level processes.
- This framework can be applied from local to continental scales.

The four high-level processes that influence community structure are:
1. Drift: Stochastic processes
2. Selection: Relative fitness of different species
3. Dispersal: Multi-scale (e.g., meta-community)
4. Speciation: Evolutionary component
Drift and selection can reduce richness, while dispersal and speciation can increase richness.

Drift involves random fluctuations in population sizes, independent of species identity or traits.
These fluctuations are due to chance effects on mortality and reproduction.
Eventually, drift can lead to a single species dominating the community as others drift to extinction.
This process can be lengthy.
Rarer species or smaller populations are at higher risk.
Simulations demonstrate the effect of drift on communities of increasing size, starting with a 50:50 species ratio.

Selection occurs when some species in a community have higher fitness than others.
This is analogous to natural selection acting on individuals within a species.
It is linked to niche concepts where species' fitness varies depending on environmental/biotic conditions.
Simulations show that selection, combined with drift, leads to a smooth transition when one species has higher fitness than another, again starting with a 50:50 ratio.

Dispersal is the movement of individuals among locations.
Immigration is one way species are added to a community.
Local communities are not independent.
Dispersal leads to more similar composition and dynamics among communities.
To some extent, connected communities act like a single, large community.
Simulations demonstrate the effects of no dispersal, moderate dispersal, and high dispersal on species frequencies in neighboring communities.

Speciation is another way species can be added to a community.
It is mainly relevant at larger spatial scales.
Recent research shows that (evolutionary) selection and speciation can occur rapidly.
In microbial communities, new species interactions can evolve very quickly (Hansen 2007).

The four processes (drift, selection, dispersal, speciation) link to community structure by influencing:
- Abundance distributions
- Composition
- Richness
- Evenness & dominance
These high-level processes influence community ecology's 'black box'.
Multiple routes can produce community structure.

Starting with a two-species community:
- Dispersal (immigration) can increase species richness (e.g., from 2 spp to 3 or 4 spp).
- Speciation can also increase species richness.
- Drift and selection can decrease species richness.

Alien species arrive via dispersal, increasing richness.
Selection alters the abundance distribution of the community over time.
Local extinctions can decrease richness due to competition and other inter-specific interactions.
Drift may:
- reduce the likelihood of establishment.
- interact with selection to increase the risk of local extinction.

The traditional view of ecology emphasizes selection due to local abiotic factors and inter-specific interactions.
This view may ignore dispersal, drift, and speciation.
It aligns with the 'niche' view (Elton/Hutchinson).
More recently, 'neutral' models have been proposed, ignoring selection but including speciation, dispersal, and drift.
This creates a niche-neutrality continuum.

Hubbell (2001) proposed the Unified Neutral Theory of Biodiversity and Biogeography.
This model explains community structure with drift, dispersal, and speciation, assuming all species are equivalent ('neutral').
Selection is considered absent.
This theory was controversial but fits a range of community data well.
It's an example of a scientific approach using a simple model to question accepted ideas and make testable predictions, advancing the science by clarifying the roles of different processes.

Current thinking integrates elements of both niche and neutral processes.
Real patterns fall between niche and neutral extremes.
Research is focusing on when niche and neutral components are more important.
This emphasizes the value of an overall framework.
Factors like species richness, niche breadth, and dispersal distance influence the relative importance of niche and neutral processes.

Evidence supports the influence of all four processes (drift, selection, dispersal, speciation) on community structure.
Selection probably plays the most significant role overall.
Dispersal is also important, particularly for introducing new species and countering drift.
Drift is generally less important but may play a big role in certain circumstances, such as small populations or scales.
The role of speciation is not fully understood but is certainly important at larger scales.
Determining the relative importance of these processes is an ongoing challenge.