J. Bastow Wilson’s Chapter 1 Round 2: Comments, Niches, & Communities.

J. Bastow Wilson’s Chapter 1

Round 2:

Comments, Niches, & Communities

Summary of Comments

• Many big statements made and issues raised without sufficient backing or explanation.– “Theories have failed”– “Concept of ‘individual’ doesn’t exist for plants”– “Plants move, animals don’t”

• Some of these and other statements appear to have been made to intentionally ruffle feathers rather than simply make a scientific point– Cover measurements

Summary of Comments continued

• In other instances, generalizations are made that overlook a great deal of variation/nuance.– Plant growth forms– Entropy changes

• The niche and guild discussion was the portion of the chapter that evoked the most “non-critical” discussion.

Niche Theory

Boo

Historical steps in niche theory development

• Defining “niche” as a species concept

• Using niches to explain competitive exclusion and species distributions

• Quantitatively measuring niches

• Redefining the niche

• Poking holes in the theory

• Reconciling niche and neutral theory

1. Each species has a niche

• Grinnell (1917): – First to use term niche to describe the current habitat

and geographic distribution of a single species. – Species are limited by physical and climatic factors. – Not much emphasis on interspecific interactions

• Elton (1927):– Animal’s niche related to it’s food habits and it’s actual

distribution (function in the community)

2. No two species can have the same niche

• Gause (1934): Competitive exclusion prevents species from using the same resources

• MacArthur (1958): Spatial arrangement of niches

And species niches are affected by interspecific interactions

• Hutchinson (1957): – Hypervolume: Niches can be represented in

terms of some combination of abiotic and biotic variables arranged in multivariate space

– Fundamental vs. realized niche: Competition/predation/mutalisms can reduce or expand a species’ niche.

– Coexistance occurs due to differences on some number of dimensions in niche space

– Niches still species specific

3. Quantitatively measuring the niche

• Connell (1961): Distribution of barnacles a result of abiotic and biotic factors in this simple system

4. Trying to figure out what we mean….

• “Niche”: A term used to describe a species’ habitat and distribution or a place in the environment that can be empty or filled?

• Whittaker et al 1973: – Functional niche (role of a species)– Place niche (Distribution of a species)– Habitat+niche

• What is the difference between niche and habitat?

Whittaker et al definitions

• Niche: “Intracommunity role” (population specific): The species’ realized niche with a response curve

• Ecotope: “Species’ relation to the full range of environmental and biotic variables affecting it”

• Habitat: The environment of a community and it’s physical attributes (ordination space can be “habitat hyperspace”)

• Species can overlap along niche axes. There’s no such thing as an empty niche, since it’s a species specific concept

• But…ecologists still wanted to talk about species packing, empty niches and saturation!

5. Poking holes in the species niche concept

• Confusion as to whether niches are species or community specific (Schoener 1989)

• Empty niches (advocated by Tilman and Bastow Wilson): Communities will exclude an invader if its niche is already filled

• Neutral theory: species’ characteristics, and therefore its niche, don’t determine it’s presence in a community. Competitive exclusion often takes so long to occur that other processes – i.e., speciation, dispersal, and “random ecological drift”in population size – come to dominate.

6. Reconciling species and community based niches

• Chase and Leibold (2003): – the requirements of a species for existence in

a given environment and – its impacts on that environment

– A niche axis is a resource, predator, or “stress agent” that both affects and is affected by the species of interest

– Mechanistic based niche theory

Revisiting Wilson’s Alpha vs. Beta Niche

• Alpha Niche: the resources used within a community/site

– Morphology – Growth phenology – Chemical functioning– Plant/plant interactions

(competition)

– Exclusion of species with the same alpha niche

• Beta Niche: the range of physical environmental conditions under which the fitness of a species is maintained

– Climate – Soil– Geomorphology– Other Organisms

(facilitators and herbivores)

– Co-occurance of species with same beta niche

Habitat?

Discussion Questions

• Where is niche theory heading and what classical definitions are still useful?

• How are plant niches different from animal niches?

• Bastow Wilson doesn’t mention neutral theory in his section on niches. Why don’t you think he included this viewpoint?

Communities

• Bastow 1991 JVS editorial:– Do communities exist in any more meaningful

sense, as integrated, discrete entities?– Are there phytosociological limitations on the

occurrence or abundance of plant species?– Are there ‘assembly rules’ (Diamond 1975)?

• His conclusion: No convincing evidence

Paradox of the Plankton

• Hutchinson 1961– How is it possible for a

number of species to coexist in a relatively isotropic or unstructured environments all competing for the same sorts of materials?

– B. Wilson: “Disturbance is currently fashionable”

• Niche diversification• Pest Pressure• Equal chance• Gradual climate change• Intermediate-timescale

disturbance• Life History differences• Initial Patch composition• Spatial Mass Effect• Circular competitive networks• Cyclic succession• Aggregation• Stabilizing co-evolution

Keddy’s (1993) response

• It might be possible to list criteria for the existence of communities:

• What properties must they have?• How is each property intertwined?• How are they measured?• What values are sufficient for ea. measurement to be

considered as evidence for or against existence?

• BUT this debate is an ontological and epistemological game peripheral (and even harmful) to scientific progress.

Palmer and White’s (1994) response

• “We suggest that community ecology define community operationally, with as little conceptual baggage as possible, so that we can out the debate about their existence behind us.”

The new definition of Community

• Linderman (1942)– Defined ecosystem

as” the system composed of physical-chemical-biological processes active within a space-time unit of any magnitude.”

• Palmer and White – Define a community

as “the living organisms present within a space-time unit of any magnitude.”

Turning B. Wilson on his head

• Wilson:Do plant communities

exist in a more meaningful sense, as integrated, discrete entities?

• Becomes:– Are there any

communities which are integrated and discrete?

Q: How does this affect our discussion of communities? Are there integrated and discrete communities? What new questions can we ask with this definition?