This is a cloud forest in Puerto Rico. Although these trees are quite old, they are short, and their leaves are small and leathery (sclerophyllous). Note the presence of numerous epiphytes. How does growing at the top of a mountain in the tropics result in stunted growth and small leaves?

I. What is a community?

A. The definition must contain a limit.

1. Spatially and temporally defined. A group of populations that occur together. eg, Oak Community, Sagebrush Community, Tall grass prairie community.

2. Functional definition: Communities can be defined on the basis of interactions among associated organisms.

eg, Detritivore community, herbivorous insect community.

Communities may be more difficult to delimit when defined in this way. For example, O₂ produced by a tree in Brazil could ultimately be taken up by a squirrel in the US.

3. Communities and associations: All communities generally have some overlap with all others.

Frequently the terms Community and Association mean the same thing. The boundaries of most terrestrial associations of plants and animals are distinct compared to the turnover of organisms and flux of materials and energy within the association.

An extreme case: The ecosystems of caves; all energy flux is from outside.

4. Community Structure: A loosely defined term referring to the types, forms and trophic status of the interacting species.

The maintenance of community structure depends on a complex array of interactions, directly or indirectly tying all its members together in an intricate web.

The influence of a population extends to ecologically distant parts of the community through its competitors, predators and prey.

eg. insectivorous birds do not eat leaves, but influence the tree through herbivores

Bob Marquis has shown that exclusion of insectivorous birds results in increased herbivory. This suggests that birds can limit the population size of herbivorous insects. If it can be shown that this happens in a density dependent manner, then birds may regulate insect density.

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II. The community as a natural unit

Question: Does a community have emergent properties? Is it a form of organism? Is it a unit of adaptation?

A. Clements - communities are closed such that relationships among the plant species are interdependent and the product of extensive coevolution.

B. Gleason - communities are open because the composition of a community is largely determined by stochastic processes and each species' physiological tolerances.

C. Margaret Davis demonstrated the migration of vegetation during the Pleistocene.

Vegetation migrated northward after the last retreat of the glaciers. Each species migrated in accordance with its own physiological tolerances and rate of dispersal. This suggests that there is little linkage among the organisms that we see now in a given association.

FYI: The debate between Clements and others (Gleason was just one of the skeptics), played a pivotal role in the development of modern ecology. Much of our present approach to the study of ecosystems is derived from this debate about the nature of communities.

The maturation of modern ecology. Although Clements’ work continued to be enormously influential, its stress on deterministic climax communities drew increasing criticism from ecologists in the 1930s, especially from those who were interested in adding the study of animals to ecology. The English ecologist A.G.Tansley provided the most cogent attack in an article int he new journal Ecology in 1935, challenging his colleagues to adopt the term ecosystem, a reference stressing the dynamic nature of community structure rather than Clements’ goal-directed climax stage. When G. Evelyn Hutchinson and his student Raymond Lindeman provided clear, albeit highly complicated, mathematical models to depict the various interacting component parts of the ecosystem, ecologypromised to become a fully mathematized and experimental discipline. Lindeman, in particular, contributed to this important change when he published a paper in 1942 that synthesized the work of Clements, Elton, Tansley and his mentor Hutchinson by speaking of biogeochemical cycling, energy flow through trophic levels and dynamic succession. Even more importantly, he saw the continuous cycling of material through the ecosystem as an energy-driven process that included producers (organisms that fixed the energy from the sun), consumers and decomposers, which cycled material back to the producers as energy from the sun continued its one-way flow through the ecosystem. By the end of World War II, ecology had become thoroughly transformed from scientific natural history to
ecosystem ecology.

Keith R. Benson; Endeavour Vol. 24(2) 2000

D. Causes of ecotones

1. Physical Boundaries

eg,
a. Serpentine, north-facing and south-facing slopes, intertidal zonation, tree-line.

b. Broad-leaved to coniferous forest - accompanied by an abrupt change in soil acidity.

c. Grassland and shrubland - sharp changes in surface temperature, soil moisture, and light intensity.

2. One species or life form dominates up to the limit of its range.

There are often sharp boundaries between grasses and shrubs because one type of vegetation holds a competitive edge.

Generalization: Sharp physical boundaries create sharp ecotones. Biologically generated ecotones are frequently not as clean.

3. Edge species add to regional diversity

e.g., Serpentine barrens. Only a very few species are found to exclusively inhabit the intermediate regions.

Old field - forest edge. Frequently old fields and their edges are refugia for prairie species which have been competitively displaced by the European grasses that dominate in the old field. There is less than 1% of the Prairie Ecosystem left.

White-tailed deer specialize on forest edge plant species.

Recent studies of bird coloration has provided evidence that edge habitats are a hotbed of evolution.

E. Gradient analysis.

Most large scale changes are more gradual than the abrupt changes caused by sharp physical boundaries.

Robert Whittaker: the concept of the Continuum. Changes along gradients occur gradually over vast geographical distances. Axes for Continuum might be cold to warm, wet to dry, seasonal to moderate, soil factors, etc.

III. The community as a unit of adaptation

A. Emergent properties

Do the attributes of a community represent more than the evolved properties of the individuals?

Individual selection should predominate even though may result in communities that are unstable.

If evolutionary adjustments by chance tend to stabilize relationships among organisms, then these adaptations should improve the efficiency of ecosystem function and enhance community stability.

A possible ecological principle: Community efficiency and stability increase in direct proportion to the degree of evolutionary adjustment between associated populations.

Note that introductions of alien species rarely work, but when they do, they identify open niches with often catastrophic results with respect to energy flow through the ecosystem.

B. Evidence

1. Mutualisms: the evolutionary adjustment of populations to one another depends on the degree of association.

Plant - animal interactions -> discussed previously
Leguminous plants
Mycorrhizal associations.

These coevolved relationships may play an important role in the maintenance of community structure and biodiversity.

Conclusion: although coevolution has produced a number of striking mutualisms and many non-coevolved mutualisms may exist, this does not mean that the community is a super-organism.

2. Convergence - many examples.

hummingbirds and sunbirds

woodpeckers and hornbills

Australian. and North American lizards, despite 100 million years isolation

Mediterranean climates of Chile and California

Tropical Alpine Habitats in New Guinea, Africa, Andes

Conclusion: To the extent that species associations remain constant, the the selective pressures generated by the organisms themselves should remain constant.

Do communities remain constant through time?

ie. are communities closed biological units?

The ultimate extension of this reasoning is Gaia Hypothesis.

IV. Community stability

The stability of an ecosystem or community is important for two reasons:

1. Man-made perturbations are often unlike any that the species of a community would have experienced in evolutionary time.

2. Stable communities are those that persist. Why do some communities last longer than others?

A. Definitions:

1. resilience: speed with which a community returns to its former state after perturbation.

2. resistance: ability to avoid displacement in the first place.

3. local stability: tendency of community to return to its original state when subjected to a small perturbation.

4. global stability: response when subjected to large perturbation.

5. dynamically fragile: stable only within a narrow range of environmental conditions.

6. dynamically robust: stable within a wide range of conditions.

Question: what is the response variable?

a. Demography of the species in the community (most typical)

b. More global features like biomass, or nitrogen contained in the community, etc.

B. The myth of stability in the tropics

Thought Experiment: If you perturb a temperate-zone habitat and a tropical habitat, which requires longer to regenerate?

Population stability: Andrews, 1991. Lizard population stability.

No outbreaks in the tropics? Wong et al. 1990, Insect defoliation of a tropical tree.

For a review of herbivory in tropical ecosystems, see Coley & Barone 1996

C. Complexity and stability

During the 50's and 60's, conventional wisdom was that increased complexity within a community leads to increased stability.

Complexity means more species and more interactions between species

1. Observations:

a. Mathematical models of interactions between two or a few species are inherently unstable. (true, but do math models with multi-species show greater stability?)

b. Simple laboratory communities go to extinction without elaborate controls (Huffaker's experiment)

true, but no evidence that multi-species lab communities would be more stable. Virtually impossible to recreate environmental complexity

c. Islands with few species are more vulnerable to invading species than are continents. (possibly true but -> several well documented examples of continental invasion)

d. Crop monocultures are particularly vulnerable to invasions and destruction by pests. (possibly true, but no evidence that this is a function of diversity of species planted, could be due to lack of time for coevolution between crop and pests).

e. Species-rich tropical communities are not noted for insect outbreaks and other dramatic perturbations found in temperate communities.

This turns out to be false: seasonal variation in insects in the tropics is extreme. Resilience of tropical communities is poor, but resistance may be great as a function of environmental constancy. Tropical communities appear to be dynamically fragile.

2. MacArthur's argument: the more possible pathways there are for energy to be passed through in a community, the less likely that the densities of constituent species would change in response to an abnormally raised or lowered density of one of those species.

Following a disturbance, more pathways usually result in less numerical change in the species' populations of a community.

MacArthur's argument assumes that perturbation comes from below (ie, removal of a food source from a heterotroph). If perturbation comes from above, quite the opposite conclusion may be reached, especially if a keystone predator is removed. In this instance, complexity is more unstable.

3. Present wisdom: There seems to be little reason to accept the argument that complexity causes stability. This hypothesis may deseve reconsideration if the way in which complex communities are structured enhances stability.

Thus, stability should vary between environments. For example, in a stable and predictable environment, a community will experience a limited range of conditions, and thus may not evolve the ability to be dynamically robust.

Predictions:

a. complex communities are fragile and exist in stable and predictable environments, with simple and robust communities in variable and unpredictable environments

b. Overall, we expect to see essentially the same recorded variation in all communities, since this is a function of both inherent stability and the constancy of the environment.

Implications:

Impact of man-made disturbance is greatest on fragile, complex communities of stable environments.

Evidence:

The classic contrast is between tropical rainforest and temperate forest suggests that in some way the harsher environment has selected for a resilience in temperate communities.

McNaughton tested this hypothesis. He allowed grazers to perturb artificially-created simple and complex communities. The effect was greatest in the complex community. Diversity of grasses decreased in the species-rich community.

Henk Wolda (Smithsonian): Looked at population variation in an array of tropical and temperate and arctic insect faunas. He found them to be statistically no different from each other, with periodic insect outbreaks (i.e., population boom and bust cycles) occuring in each group.

4. Impact of species' life history characteristrics

So-called "K - selected" organisms inhabiting highly constant and complex tropical communities are inherently unstable because they may lack the so-called "r-selected" characters for rapid regeneration following a disturbance.

Community Development

Satellite composite picture of Australia showing the different plant communities. Note that most of this continent is desert. The rainforest is on the far eastern shore in the bands of deep green.

I. Definitions

After a community is disturbed it is slowly rebuilt. Pioneering species adapted to the disturbed habitat are successively replaced by others until community attains its former structure and composition.

Succession: the sequence of biotic and abiotic changes on a disturbed site.

Each system has initial colonizers and then later colonizers

The concept of r and K selection was partially developed from observing this process

Fugitive species concept: species that must disperse before the next stage of succession renders the habitat unusable.

Climax: The final association of species. Note that we may not know what "final" is for many communities.

Types of diversity

Alpha diversity: The number of species in a local area. Usually within a specific habitat.

Beta diversity: The variety of organisms within a region arising from turnover of species among habitats.

Gamma diversity: The number of species in a broad geographical region, for example, the Isthmus of Panama (just to pick a random location).

Sere: transition from abandoned field to mature forest is only one of several successional sequences leading to the same climax. Each of these possible sequences is called a sere. An ecocline is a chronosequence of successional stages observed spatially.

Examples of succession:

old field

Old field in North Carolina: annuals-> herbaceous perennials > shrubs -> pines -> hardwoods

Old field in the Midwest: end point is Oak/Hickory forest on south-facing slopes, and Oak/Maple forest on north-facing slopes. Why does the aspect (direction) of a slope matter?

sand dune

Classic studies of dunes on the shore of Lake Michigan:

The dunes are first invaded by marram grass and bluestem grass. This stabilizes the soil and allows the growth of numerous annuals and perennial grasses. Soil is enriched and stabilized. Gradually creating conditions for first shrubs. Sand cherry dune willow, bearberry, and juniper. Pines persist for only two generations. The apparent endstage, or putative climax, is beach-maple-oak-hemlock forest.

elephant dung

decaying carcass

coral reef

treefall light gap

estuaries

Halophytic or salt tolerating plants will eventually lead to terrestrial communities as silt builds up and frequency of inundation decreases.

Point: Clements outlined the basic features of succession in 1916. Although we now think that he was largely wrong and that communities are not closed, the terms associated with his formulation persist.

II. Primary Succession

A. Seres are classified into two groups

1. Primary succession: This is succession originating from habitats previously without plants or substrate derived from living things.

Examples:

Receding glaciers in boreal North America

mosses-> sedges-> prostrate willows-> shrubby willows -> alder -> sitka spruce -> spruce & hemlock

Krakatau eruption

2. Secondary succession: The return of an area to its natural vegetation following a major disturbance. This process occurs on substrate already modified by living things.

B. Xerarch succession

This is the development of vegetation on bare rock or sand from inorganic sediments or substrate in a regions where water is scarce.

C. Hydrach succession

Hydarch succession begins in the open water of a shallow lake, bog or marsh. Anything which results in the drying of the lake will promote hydrach succession.

Bog succession in Northern Michigan

Aquatic plants at the edge of a pond sedges from mats on water surface-> increasing sedimentation and detritus accumulation-> (no Oxygen -> no decomposition -> peat) -> sphagnum moss and bog shrubs and eventually, -> black spruce and large -> birch maple or fir (depending on the region)

This is only one of several seres which lead to climax forest of spruce fir and birch. eg. after a fire , succession passes through intermediate grass and aspen stages.

III. Ecoclines: a chronosequence

A "sere" occurs through time, but parallel changes in space are called "ecoclines" and these suggest the causes of succession.

eg., xerarch succession from rock surface to forest in the Eastern US corresponds in structure to the ecocline in vegetation from nearly bare rock surfaces of the western deserts:

e.g., dry grasslands, prairie, shrubby oak woodland, to tall mixed hardwood forest. Occurs along increasing moisture gradient from east to west.

The ecocline represents a series of stages of community development that may be stopped at different points by, say, lack of moisture.

Hydrach succession is mirrored spatially as you go from center of pond to forest.

IV. Climax

To Clements, it appeared that the many seres found within a regions where each developing under a particular set of local environmental conditions towards the same climax.

A. The mature community thought to be a closed system

Clements' view of the world: The climate-determined climax.

"The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity." [The life history of this entity is comparable to life history of any organism.]

14 Climaxes in North America: 2 grassland, 3 scrub, 9 forest each corresponding to regional climates.

He argued that the nature of the local climax was determined solely by climate. Examples that didn't fit his concept were thought to be immature communities, arrested at an earlier stage due to edaphic conditions, topography, fires, etc.

B. Open community:

Gleason greatly modified the concept of closed community. He eventually rejected the Clementsian view.

Whittaker developed the Continuum Index in Wisconsin forest. The CI is calculated from the species composition of each forest type. Values varied between arbitrarily set extremes of 300 for pure Burr Oak to 3,000 for a pure stand of Sugar Maple. In the Clementian view, increasing values of the CI correspond to seral stages leading to the Sugar Maple "climax", they also represent local "climax" communities determined by soil.

Thus some botanists use "climatic climax" and "subclimax" to describe these compositions.

Ultimately, any climax must be self perpetuating.

C. Present view: Margaret Davis

Because of Davis' study of lake and bog sediments, our understanding of succession now incorporates biogeographic processes occurring over millenia.

This process is a mix of organic and physical factors.

The map in the previous section shows sites for pollen core analysis used in were used for the study of prehistorical distributions of forest. Davis was one of several researchers to use this approachl. For an example of tree migration following the Pleistocene, see the figure for Picea in the Lecture on Climate Change.

V. Causes of succession

Two factors determined the positions of species in a sere:

1. rate at which new species invade. Note that in addition to local processes, this may be a function of large-scale, long-term biogeographical processes such as glaciation and other forms of climate change.

With respect to such large-scale processes, we have recently learned that Amazonia was not fragmented during the Pleistocene, as was previously thought.

2. changes in the environment as a function of succession itself.

eg, dispersal ability varies greatly.

Point: Each species in the sequence is adapted to that part. In overview, the self perpetuation of climax is not possible either because of competition from other species, or because the environment is changed by the very nature of the community. Does this mean that a community is not a collection of highly co-evolved species?

VI. Endpoint of Succession

Succession continues until the addition of new species to the sere and the exclusion of established species no longer change the environment of the developing community over a time that it can be witnesses.

Post-climax: subtle and slow changes after the traditional climax is formed.

Early in succession, conditions of light, temperature, moisture, and soil nutrients change quickly with the progression of different forms. To see what a community may become, look at the seedlings in the understory and the seeds in the soil seed bank. Note that the soil seed back is very small in the Tropics. Why?

Fire is extremely important in maintaining some climaxes, such as Giant Redwood forest.

VII. Local climax

Clement's insistence on the idea that a region had only one true climax (the monoclimax) forced botanists to come up with the following entities:

subclimax

preclimax

postclimax

Eventually these terms converged on the idea of a polyclimax, or as Whittaker called it the "pattern-climax"

All of these terms came about because the composition at any one locality depends on the particular environmental conditions at that point. Thus, these terms are generally worthless for understanding the causes and consequences of succession at that point. These terms do not inform us about the mechanism or the process of succession.

A. Factors

soil nutrients, slope, aspect, temperature, moisture, exposure, fire

B. Examples

the prairie peninsula of the Midwest (recall the four factors that cause prairie - rainfall, claypan, fire, grazing) - Once prairie is established fires are more frequent and thus prairie seems to be perpetual.

glade moisture, exposure and soil

great pine forests of SE: perpetuated by fire. fire is necessary to the life history of certain species pine that do not shed their seeds unless triggered by the heat of a fire passing through the understory. Thus after fire, seedlings can become established without competition from understory species. Serotinous cones.

chaparral: seasonally dry central valley of California. A fire-maintained climax that is replaced by oak woodland when fire is prevented. Serotinous cones.

C. Transient Climaxes

not all climaxes persist.

development of animal and plant communities in seasonal ponds.

small bodies of water that dry up in summer and/or freeze solid in the winter. Communities must become reestablished each year.

Ultimate extension of this concept is the idea of transient succession: sequence of detritivores on a carcass.

D. Cyclic climaxes

The particular life histories of a few dominate species results in a cyclical replacement series.

A can germinate only under species B, B can germinate only under C, but C only under A, etc. Dominance that would result is A->C->B, A->C->B etc.

Classic example of this is wind or frost heaving producing bare earth as one of the stages.

Heath: wind produces bare ground, germination is only possible on the leeward (protected) side of the bare patch. Thus, the bare patch tends to migrate.

Almost any habitat is a mosaic of vegetation types

Ricklefs: "Cyclic patterns of changes and mosaic patterns of distribution must be incorporated into the concept of the community climax" -> Persistence is the key to the climax. If a cycle persists, it is inherently as much a climax an an unchanging steady state.

If we add to this mess the idea that communities change naturally through long time spans (possibly without any significant change in the environment driving this change), then the term "climax" has little or no meaning. The endpoint for coniferous forest in the Pacific Northwest may be a bog.

Point: Thus we should emphasize the process rather than the static composition. A snapshot of the community at any given time give you only a portion of the story.

VIII. Conclusions

1. Succession is one-directional

This is a function of the different life history features of the species inhabiting each stage. We should be warned against making generalizations. -> Tropical trees have some r-selected characteristics (ie rapid growth rate) early in their life, but K-selected characters late in life.

2. Successional species alter the environment in sequence, thus driving succession.

There is general trend toward higher higher diversity, but the "final" community may have relatively low diversity.

3. The climax community is not an endpoint, but rather part of a continuum of possible formations.

4. The climax is almost always a changing mosaic of successional stages.