Transcript Powerpoint

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
Habitat?
Niche
• Alpha Niche: the
resources used within a
community/site
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–
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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
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–
–
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Climate
Soil
Geomorphology
Other Organisms
(facilitators and herbivores)
– Co-occurance of species
with same beta niche
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”
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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:
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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-chemicalbiological 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?