An Introduction to

Community Assembly

What is Community Assembly?

“the process by which species from a regional

pool colonize and interact to form local

communities”

HilleRisLambers et al. (2012)

HilleRisLambers et al. (2012)

Long History

What a wondrous problem it is,—what a play of

forces, determining the kinds & proportions of

each plant in a square yard of turf!”

C.R. Darwin in letter to J.D. Hooker (1857)

Progression of Key Ideas

• Principle of Competitive Exclusion – Superior competitors persist

– Niche hyper-volumes

– Stabilizing-Niche Differences

• Assembly Rules – Competition results in limited sets of species composition

• Gradient Analysis – Community structure reflects environmental gradients

• Neutral Theory – Species have equivalent niches

– Dynamics driven by speciation, dispersal, and random birth-death processes

Competitive Exclusion

(Gause's law - 1934)

• Two species growing in the same location and

using the same resources will compete

• superior competitor will exclude the inferior

competitor

The Niche

• Hutchinsonian Niche Hypervolume (1959)

– n-dimensional set of resources and environments

that a species requires to persist

Prey length

Tem

pera

ture

Coexistence Theory

(Chesson)

• Stabilizing-Niche Differences

– Differences that cause a species to more strongly limit themselves than other species

– e.g., pathogen load, specific resource requirements

• Relative-Fitness Differences

– Differences that drive competitive exclusion

• Species will coexist if

– Stabilizing forces > Fitness differences

Stable Coexistence

HilleRisLambers et al. (2012)

Stable Coexistence

Palmer (1994)

Assembly Rules

(sensu Diamond)

• Competition structures which combinations of

species are observed in nature

• First examined with bird co-occurrence

patterns on island archipelagos

Competition or Chance?

• Simberloff

– Random immigration could have driven patterns

– Pioneers ecological null randomization tests

Gradient Analysis

(Whittaker)

• Species have different optima along

environmental gradients

Neutral Theory

(Hubbell)

• Dispersal, Ecological Drift, and Speciation

– Stochastic demographic processes result in stable

coexistence.

• Stabilizing differences absent

• Fitness differences are all equal

speciation

A change in Terminology

HilleRisLambers et al. (2012)

Our goals this semester!

1. Stochastic and deterministic mechanisms of assembly

2. Functional-trait based assembly

3. Phylogenetic constraints on assembly

4. Role of dispersal limitation

5. Global change and environmental filtering

6. Tipping points and multiple steady states

7. Importance of spatial and temporal scale

8. Experimental community assembly

9. Big data approaches to understanding assembly