Download Gleason

Successful Ecologists Biography Schedule
Organismal Ecology, William Murdoch - Tim Assal group - Oct 7
Population Ecology, ??? - ??? - Oct 14
Community Ecology, ??? – Jeremy Sueltenfuss group - Oct 21
Ecosystems Ecology, Dave Schimel – Kevin Wilcox group - Oct 28
Nov 9, 11
Dan Simberloff – Jared Stabach group
Unknown Ecosystem Ecologist – Ed Gage group
Unknown ecologist – Nell Campbell group
Gene Likens – Ben Gannon group
1. Approach
How Ecology is “done”
What is the relationship
between diversity and
invasion?
Large scale,
Observational
Small scale,
Experimental
3. Perspective
Vegetation
Patterns &
Succession
2. Process
Kaibab population crash
Succession &
Chronosequences
Gleason
Clements
Equilibrium
Nonequilibrium
Inference
Synthesis
Understanding
Re-examination
Alternative
Ecology & paradigm shifts – an “old” example
(Clements/Gleason) and a new challenge?
(succession)
----------------------------“Balance of Nature” vs. “Non-equilibrium view”
In Chemistry – what is the difference between a compound
and a mixture?
• The properties of compounds are different from those of their constituent elements.
This is one of the main criteria for distinguishing a compound from a mixture.
• Another criterion for distinguishing a compound from a mixture is that the
constituents of a mixture can usually be separated by simple means, but the
components of a compound can only be separated by a chemical reaction.
• Conversely, mixtures can be created by mechanical means alone, but a compound
can only be created by a chemical reaction.
Q. Are natural ecosystems/communities more like compounds
or mixtures?
The “Balance of Nature” as the classical ecological paradigm
• Both western and eastern roots
Eastern philosophy – nature seen in terms of unity,
interplay, harmony…
Western society – divine order, earth is a fellow creature,
nature is a finely tuned machine…
Early ecologists – concepts of stability and equilibrium
embedded in the view that populations, communities and
ecosystems are self-regulating systems, kept in stable
equilibrium if left alone…
Many early ecologists seemed to believe that nature
could be understood in terms of a balance of
destructive and conservative forces, but that if left
undisturbed, nature would maintain a permanence of
structure and function…
(Thus, “no management” is good management? Is this the view
today?)
Concepts derived from this view:
Steady-state, Stability, Homeostasis
View that at an equilibrium state, all factors or
processes leading to change are resisted or balanced
by the system…
Diversity – stability debate has its roots in this
perspective…
Concepts in literature:
• Resistance – the capacity of an ecological system to
resist external perturbation
• Resilience – the rate at which an ecological system
returns to its equilibrium state after a perturbation
• Persistence – the capacity of an ecological system
to stay within some predefined bounds despite
perturbations
• Constancy – the degree of variation in system
properties over time
Support for the “Balance of Nature” or Equilibrium paradigm
•
Density-dependence and negative feedback mechanisms
•
Particularly important in population ecology
• Virtually all of the mechanisms proposed to explain
long-term coexistence of species can be related to
trade-offs in some components of their biology at
equilibrium (Chesson 1991)…Tilman’s
Resource Ratio hypothesis
In sum:
Equilibrium paradigm focused on closed, selfregulating systems and the factors that allow
populations to persist, and species to coexist in stable
communities and downplayed the importance of spatial
and temporal variation and history.
Non-equilibrium perspective
Emphasis on:
Openness
Transient dynamics
Stochastic processes
Fluctuations in population densities are an
essential feature of non-equilibrium communities
with chance and historical contingency important
Non-equilibrium systems are also characterized
by non-linear interactions and thresholds such that
they can be quite sensitive to small perturbations
Non-equilibrium systems may exhibit abrupt,
discontinuous changes in system behavior as a
result of certain parameters crossing an apparent
boundary (Wu and Loucks 1995)
A key point: Non-equilibrium systems are less
predictable, but not necessarily less persistent…
F. Clements
F.E. Clements (1916) conceived of the
community as a sort of superorganism
• Species were tightly bound together both now
and in their common evolutionary history
• Mutualism and coevolution play an important
role in the evolution of species making up the
association
• Individuals, populations, and communities bore
a relationship to each other which resembled that
between cells, tissues and organisms
•Species found in a community have similar
distributional limits
• Thus community boundaries are clearly
delineated
• Communities, not species, are the essential
unit
On the landscape, associations of species would appear along
environmental gradients as groups of species with high fidelity to specific
associations, with little overlap between associations, and narrow
ecotones (boundaries between adjacent communities). Figure 22.1a is
from (Smith and Smith 2001).
Clements' theories on communities were also
linked to succession.
Nature of succession-The community can
reproduce itself, “repeating with essential fidelity the
stages of its development” (Clements 1916)
• Each stage of succession represented a step in the
development of the superorganism
• Seral stages modify the environment so that
species of that stage can no longer exist there
and they prepare the site for replacement by
species of the next stage
• The process continues until the vegetation
arrives at the self-reproducing climax
According to Clements:
• Succession was a predictable, directional,
inevitable process
• The climax represented a community at
some equilibrium or steady state with the
physical and biotic environment
• Only one climax would exist for a region
whose characteristics were determined by
climate and edaphic conditions.
Clements presented a deterministic
unidirectional view of succession
The philosophical structure of Clements's holistic
approach was similar to advances in other
sciences of that time.
In geology and geomorphology theorists
presented views that contained various stages of
maturity, and compared landscape evolution to a
developing organism.
H. Gleason
Gleason viewed the community as consisting of individual
species that respond independently to environmental
conditions.
• Nature of the community-“the vegetation of an area is
merely the resultant of two factors, the fluctuation and
fortuitous immigration of plants and an equally fluctuating
and variable environment” (Gleason 1926).
• Species, not communities, are the essential unit.
• Species change in abundance along environmental
gradients so gradually that it is not practical to divide
vegetation into associations.
• The relationship between coexisting species within a
community is a result of similarities in their requirements
and tolerances, not as a result of strong interactions or
common evolutionary history
• What is referred to as a community is merely the
group of species found to coexist under any particular set
of environmental conditions
On the landscape, such communities of plants
would appear to not be clearly associated with
each other and there would be no distinct
boundaries between one community and an
adjacent one.
Gleason –
Nature of succession-succession results from the
individual responses of different species to the prevailing
environmental conditions.
Plants involved in succession are those that arrive first
on the site and are able to establish themselves under
prevailing environmental conditions.
As time passes, plants modify the environment and
competition and other interactions among species
determine the final outcome
Gleason’s views, even though they included less
determinism (more chance) and holism than
Clements, still had many equilibrium
characteristics…
Competition is an equilibrium concept…
A nonequilibrium view:
Populations, communities and ecosystems may be far
from equilibrium because of variable frequencies of
disturbance and climatic fluctuations
Main forces determining community membership are
colonization and extinction rates of species and the
time since disturbance, not species interactions
The metapopulation viewpoint (systems are open,
not closed) and spatial structure of the environment
(both abiotic and biotic) play important roles in
determining community membership
There is a strong historical component to
community membership…Assembly order and
priority effects are important
For many years, Clementsian views dominated
Ecology and Gleason was ignored…
Today? Recognition that both perspectives are
important
Non-equilibrium
focus
Equilibrium
focus
The future? Will ecological dynamics
be driven primarily by disturbance
and random weather fluctuations?
Are contemporary ecosystem dynamics being driven more
by chronic resource alterations (CO2, N, H2O), instead of
discrete disturbances?
Simple model of directional change
Resource alteration
Response
Community change
Community re-ordering
Physiological response
Time
A. Disturbance
B. Global Change
Biota
Biota
Resources
Resources
Discrete
disturbance
Chronic
resource
alteration
Smith et al. 2009 (Ecology 90:3279-3289)