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

PatternsProcessesTheories

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 experimentsManipulative 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 experimentsControlled conditionsSimplified systemAddress specific questions

Diversity of Ecological Evidence

4) Mathematical modelingComputer-aided

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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)

Guisan &Zimmerman (2000)

Context-specificand boring

Generalizableand interesting!

Stochastic yet critical!

Latitude/LongitudeElevation

TopographyGeology

TemperaturePrecipitation

Solar radiationSoil properties

Plant community structure

Animal community structure

Disturbance

Species interactions

Species interactionsSoil biota

Demographicstochasticity

Demographicstochasticity

A general theory of ecological community structure

Integrated Community Concept

Basic ecological question:

EnvironmentalGradient

Sp

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ExperimentalTreatments

Sp

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What controls the distributionand abundance of species?

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 richnessSpecies evennessSpecies abundanceSpecies diversitySpecies frequencySpecies densitySpecies vertical spatial arrangement Species 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.