Plant Community Ecology
An introduction
Ecology as a Science
Study of the relationships
between living organisms and
their environment
Of the interactions of
organisms with one another
Of the patterns and causes of
the abundance and distribution
of organisms
1.1 The scientific method
Patterns
Processes
Theories
Diversity of Ecological Evidence
1) Observations (descriptive data)
Careful monitoring within the natural environment to
detect patterns
Diversity of Ecological Evidence
Sudden Aspen Decline in Southwest Colorado
Worrall et. al 2010, Forest Ecology and Management
1.3 Repeated observations can reveal information not apparent from one or a few observations (1)
Lake Mendota, WI
1.3 Repeated observations can reveal information not apparent from one or a few observations (2)
1.3 Repeated observations can reveal information not apparent from one or a few observations (3)
Diversity of Ecological Evidence
2) Field experiments
Manipulative experiments in the field to establish
cause of observed patterns
Large-scale manipulative experiments at Lower Middle Mountain, SW Colorado
Fule et. al 2009, Forest Ecology and Management
Hand Thinning Treatments
Prescribed Burning
Diversity of Ecological Evidence
3) Laboratory experiments
Controlled conditions
Simplified system
Address specific questions
Diversity of Ecological Evidence
4) Mathematical modeling
Computer-aided
Climate change scenarios
for desert areas. SRES
scenarios show the period
2071 to 2100 relative to the
period 1961 to 1990, and
were performed by AOGCMs.
Scenarios A2 and B2 are
shown as no AOGCM runs
were available for the other
SRES scenarios.
.
1.4 Ecologists study patterns and processes across a wide range of scales in space and time
Scale important because of heterogeneity of habitats
1.5 The environment in a microhabitat can differ from conditions in the surrounding area
Microhabitat: condition in immediate surroundings
for individual plant
H.M.S. Beagle sailed from England December 27, 1831, on a five-year mission
Beginnings of plant ecology as the study of natural history
“A traveller should be a botanist, for in all views plants form the
chief embellishment.”
Plant ecology emerged in mid- to late-1800s
Eugene Warming
Species response curve: a plot of the abundance of a species as a function
of position along an environmental gradient or complex-gradient.
What is a Community?
A group of populations that
coexist in space and time
and interact with one another
directly or indirectly.
The structure of plant communities: the history of the
debate
Frederic Clements
Superorganism Concept; Community
Unit Model
Highly organized entities made up of mutually
interdependent species.
Superorganism: organic entity that is born, develops, grows
into a climax community and dies. Abrupt boundaries
between communities.
Did not view it as complete equilibrium theory;
acknowledged shifts in plant populations and disturbances.
Developed ideas on primary and secondary succession;
succession orderly process that is highly predictable and has a
set endpoint.
Robert Peet, 1991
Species response curves: The theoretical abundance of five different
species along an environmental gradient.
Individualist Concept; Continuum
Model
Interactions between individual species and the
environment (biotic and abiotic) in combination with chance
historical events.
Each species has its own environmental tolerances and
responds in its own way to environmental conditions.
Believed defining a community was an arbitrary human
construct (gradual changes between communities). Species
don’t necessarily interact with one another.
Chance events determine whether a species is actually
found in a given location. Succession leads to different end
points due to differences in initial species pools and
disturbances.
Hierarchical Continuum Model
Species change their distribution and abundance patterns
along gradients in response to environmental fluctuations
Species with similar niches increase their competitive ability
over time (theory of competitive combining ability)
positive correlation between the rank abundance of a
species at a small spatial scale and its rank abundance at a
larger spatial scale
distribution of species across sites in a region will be
polymodal, which reflects hierarchical structure, and that the
distribution and abundance of species within and between
sites will be spatially and temporally dynamic
Collins et al. 1993
Dominant plant species along an
elevation gradient shifted
synchronously with one another over a
30-year span that had a concurrent
temperature increase, based on a new
study by Kelly and Goulden (13). The
ranges of the plant species’
distributions remained the same,
resulting in an overall ‘‘leaning’’ of the
vegetation gradient toward higher
elevation. (Breshears et al. 2008)
Context-specific
and boring
Generalizable
and interesting!
Stochastic yet critical!
Guisan &
Zimmerman (2000)
A general theory of ecological community structure
Species interactions
Latitude/Longitude
Elevation
Topography
Geology
Temperature
Precipitation
Solar radiation
Soil properties
Demographic
Animal community structure
stochasticity
Disturbance
Demographic
Plant community structure
stochasticity
Soil biota
Species interactions
Integrated Community Concept
Basic ecological question:
Species
Species
What controls the distribution
and abundance of species?
Environmental
Gradient
Experimental
Treatments
There are other controls on community structure: e.g., competition
Fundamental Problems
• Communities are comprised of many
species, not just one
• Species abundances are not independent
• Environmental gradients are multifaceted
and intercorrelated
Modern Perspective on the Structure of Plant Communities
 Primary issue of debate is
based on pattern vs. process
Pattern: focuses on how
species and communities are
distributed over the
landscape
Process: focuses on
identifying the processes
that are functioning in
communities and which
processes are most important
for determining patterns
Scale (1m2 experiments to
landscape) is crucial when
thinking about pattern and
process
Plant Community Patterns: ways in which to describe
plant communities
Summary of Plant Community Patterns—how to quantify?
Species
Species
Species
Species
Species
Species
Species
Species
richness
evenness
abundance
diversity
frequency
density
vertical spatial arrangement
horizontal spatial arrangement
The Ecological Niche
• Why
does an organism live where it lives?
• Why does it eat what it eats?
• Which organisms can coexist?
• Why is one organism so abundant and others so
rare?
• How does an organism influence ecosystem
processes?
History of the niche concept
• Grinnell’s (1924) niches were the distributional limits
of a species that are set by physical or climatic factors
- “pre-interactive”
• Elton’s (1927) niches referred to the place of an
organism in its environment, e.g., food web
- “post-interactive”
•Gause’s theory: No two species can occupy the same
ecological niche; therefore, niche theory was
inextricably tied to competition for limiting resources
Niche: multidimensional description of a species’ resource
needs, habitat requires and environmental tolerances
(Hutchinson 1957).
• Fundamental niche: “all aspects of the n-dimensional
hypervolume in the absence of other species”
• Realized niche: “part of the fundamental niche to which a
species is restricted due to inter- and intra-specific interactions.”
Hutchinson’s (1957)
“n-dimensional hypervolume”
Limiting factors!
Theory and our analytical framework
• Niche theory assumes that species response
curves are symmetric Gaussian (bell-shaped)
unimodal curves.
• We need statistical models that accommodate
nonlinearities (at both the population and
community-level).
• We need samples that span beyond the entire
gradient of a species if we have any hope of
successfully modeling its distribution.
Niche Breadth—Range of values along an axis at which the species can
persist
2 types: broad niche and narrow niche breadth
Competitive Exclusion Hypothesis: Gause’s (1934) principle
 Two species competing for the
same resources cannot stably
coexist if other ecological factors
are constant. One of the two
competitors will always overcome
the other, leading to either the
extinction of this competitor or an
evolutionary or behavioral shift
towards a different ecological
niche.
If this hypothesis were true, all communities
would have low species richness. Why doesn’t
this hypothesis work in natural communities?
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Spatial heterogeneity: Many subtly different microhabitats,
in each of which one species excludes all others.
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Temporal variation: All species must exhibit the ability to
increase when rare (invasion criterion).
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Competitive Ability: Species migration between patches in
a heterogeneous environment through dispersal. All
species must exhibit the ability to increase when rare
(invasion criterion).
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Niche Separation: Niches of various species are different
enough to prevent species exclusion (resource
partitioning).
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Herbivory: Various species are palatable which allows
refuge of species from being eliminated from the system.
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Disturbance: Disturbances provide recruitment microsites,
which allows for new species to invade systems.
Why the Competitive Exclusion Hypothesis: Gause’s (1934)
principle doesn’t work for natural plant communities
1) Refuges: Source for future propagule dispersal.