Community dynamics

Chapter 21                                  COMMUNITY DYNAMICS

Communities change in time and space.

The birth and death rates of species changes in response to environmental conditions, resulting in a shifting pattern of species dominance and diversity.

The result is a dynamic mosaic of communities on the landscape.

ZONATION

Zonation is distribution of organisms into distinct areas, layers or zones. It is the pattern of variation in community structure.

Each zone has its own plant composition.

It is found in all environments.

SUCCESSION: TEMPORAL VARIATION IN COMMUNITY STRUCTURE

Succession is the gradual and predictable community replacement leading towards a climax community. It is the temporal change in community structure through time.

Succession involves the colonization of the area by a group of species, establishment of these species for a period of time, and eventually their replacement by a new group of colonists. It usually follows a disturbance.

The sequence of communities has been called a sere. A sere is the characteristic sequence of biotic communities that successively occupy and replace each other in a particular environment over time following disturbance of the original community.
http://wlapwww.gov.bc.ca/sir/fwh/wld/glossary/glossary.html

A seral stage is a stage or recognizable community structure of a plant community that occurs at a point in time during its development from bare ground to climax. It is a point in continuum.

A seral stage may last one or two years or several decades.

Succession occurs in all terrestrial and aquatic environments.

• Pioneer community is the community that initially develops after a disturbance.

 

• High growth rate, small size, wide dispersal, and high population growth characterize pioneer or early successional species. These are r-selected species.

• Climax community is the stable community that is balance with the climate of the region and remains unchanged indefinitely. These are K-selected species.

 

There are two types of succession: primary and secondary.

Primary succession takes place on areas that are bare and have not supported a community previously.

• E.g. newly exposed rock, newly deposited mudflat, lava flow, sand dunes.

 

Secondary succession occurs in areas that have soil and have supported a community previously.

• E.g. abandoned cropland.
• Secondary succession has been observed in seaweed, salt marsh, mangrove, seagrass, and coral reef communities.

 

TIME AND DIRECTION IN SUCCESSION

Time is the integral component of succession.

CLIMAX

The process of succession is often represented as unidirectional, with the community progressing to some defined endpoint or climax that reflects the local environment.

The climax community is in equilibrium with its environment, especially the climate.

• This is the view proposed by Clements in 1916.
• He recognized on climax community for the region, a large scale dynamics.
• The climate determined the characteristics of the community.
• Given sufficient time, all seral communities in a region will converge to and stabilize at a single climax.

 

It is a steady state of competition, structure and energy flow.

CYCLIC REPLACEMENT

The small-scale dynamics within the community leads to a cyclic view of succession rather than to a linear process leading to an end-point community.

• Proposed by Watt in 1947.
• Stages that appear to be directional are in reality phases in a cycle of vegetation replacement.
• Death and periodic disturbance starts regeneration again at some stage.
• Such changes occur continuously within the community.
• These changes are repeated throughout the entire community over the course of time.
• Reciprocal replacement is another name given to small-scale succession.

 

SHIFTING MOSAIC

Recurrent disturbances result in a landscape with greater diversity than in the absence of disturbance.

Equilibrium landscape is used to refer to the mosaic formed in a forest due to constant disturbances like windthrows or localized fires.

• The term shifting-mosaic steady state is used to refer to a community made of patches in different successional stages.
• The shifting-mosaic refers to the mosaic of patches, each in a phase of successional development.
• Each patch is continuously changing, going though some successional sequence.
• Late successional stages revert back to early stages due to the death of canopy trees.
• The average forest composition as a whole remains the same: steady state.
• The term steady state refers to a statistical description of the collection of patches, the average state of the forest.
• Proposed by Bormann and Likens in 1979.

 

AUTOSUCCESSION

Some communities do not follow the usual sequence of seral stages when a disturbance eliminates the dominant vegetation.

This is common in communities found in extreme environments.

After a disturbance, plants belonging to the mature community removed by the disturbance are found in the first year of recovery. This is called autosuccession.

Succession here is more a gradual elimination of individuals rather than a replacement of initial plants by new species.

 

FLUCTUATIONS: NON-SUCCESSIONAL DYNAMICS

Fluctuations are short-term reversible changes.

Fluctuations differ from succession in that although the relative abundance of the species making up the community may change over time, the species composing the community remain the same.

Fluctuations in the community can be the result of changes in habitat factors.

• E.g. drop in the water table results in drying the edges of pond.
• Fluctuations differ from succession in that the species composition remains the same.
• There is a change of dominants over time that is reversible.
• It is the result of seasonal or annual variations in environmental conditions e.g. soil moisture, preferential selection of one species by grazers.

 

Fluctuations may involve the replacement of one age class by another within the same species.

DEGRATIVE SUCCESSION

Succession has been presented in the above discussion as occurring in communities where photosynthetic plants regulate the structure dynamics of the community. These are autotrophic communities.

Heterotrophic communities derive their energy through the decomposition and utilization of organic substances found in living and dead plant and animal tissues.

• These communities are dominated by autotrophs.
• Many of these communities are involved in the process of decomposition.
• Energy and nutrients are more abundant in the earliest stages of succession and decrease gradually as succession progresses.

 

Examples of these heterotrophic communities are dead trees, animal carcasses, droppings, caves, and benthic zone of the ocean.

The organisms that first invade the dead tissue are those that feed on fresh organic matter.

Their feeding activities bring about physical and chemical changes then they disappear.

The organisms that follow are able to extract energy found in a less accessible form.

Each wave of invaders changes the substrate to the point that they cannot survive there any longer and then disappear.

Eventually all the organic matter is degraded and incorporated into the humus.

Organisms involved in this process are different species of fungi, moth and fly larvae, herbivorous mites and collembolans (springtails), and carnivorous mites that feed on the consumer invertebrates. In the humus layer, bacterial activity follows.

SUCCESSION AND ANIMAL LIFE

As vegetational communities change, animal life dependent on each seral stage also changes.

Many forms of animal life are specific to each stage.

A succession progresses, animals characteristic of earlier stages or communities disappear.

Early successional stages support animals of the grasslands and old fields, like meadowlarks, meadow voles and grasshoppers.

PALEOSUCCESSION

The study of the relationships of ancient flora and fauna to their environment is paleoecology.

It is interdisciplinary including mostly biology and geology.

Assumptions of paleoecology:

1. Ecological rules observed today are similar or the same as those in the past.

 

2. Environmental relations of fossil organisms resembled those or their close relatives today, e.g. a present-day genus of salt water clams probably lived in salt water in the geologic past

3. Organisms were morphologically adapted to their environment.

 

THE PLEISTOCENE EPOCH

The Pleistocene began 2 million years ago and ended about 10,000 years ago.

The Pleistocene marks the beginning of the Quaternary period.

The Pleistocene follows the Pliocene epoch and is followed by the Holocene epoch. The Pleistocene is the first of the two epochs of the Quaternary period or 6th epoch of the Cenozoic era.

The history of North America since the Cretaceous extinction 65 million years ago can be divided into three major episodes.

Dinosaurs became extinct at the end of the Cretaceous. This marks the rise of the mammals.

At the beginning of the Cenozoic era (Tertiary period, 65 m.y.a.), most of present day North America and Europe was joined by land.

1. The Early Tertiary

Climate was probably warmer and the difference between summer and winter was less pronounced.

There were land connections with Europe and Siberia. The isthmus of Central America did not exist yet.

Three regional floras have been described from the Paleocene (65 m. y. a.) and Eocene (54 m. y. a.).

2. The Late Tertiary

Beginning with the Oligocene (38 m. y. a.) climate started to change becoming cooler and drier.

By the Miocene epoch (26 - 7 m.y.a.) major geofloristic changes have occurred due to a cooling of the climate, and the Rocky Mountains began to rise.

In the Miocene,

• The Neotropical-Tertiary geoflora retreated south of the United States into Central America. The Great Plains became savannas and grasslands in the rain shadow of the Rocky Mountains.

 

• The Arcto-Tertiary geoflora moved south and occupied most of present day United States. As the Rocky Mountains rose the Arcto-Tertiary forest gave way to grasslands in the central part of the United States.

• The Madro-Tertiary geoflora extended northward in the southwest of United States to form sclerophyllous woodland ancestral to the pine-oak woodland of today. Several other types of vegetation developed and are present to this day: chaparral, desert and woodland. This change was brought on by the uplift of the southwestern region of North America.

 

The mesophytic deciduous forest that was continuous with Eurasia became fragmented as a result of continental drift in the last 20 million years. Some species became extinct, and other disappeared in one region of the world but not in another.

• Magnolias, sweet gum, hickory, walnuts, etc. disappeared in Europe but not in North America; Metasequouias and ginkgoes disappeared in North America but still exist in China.

 

• A relic of the Arcto-Tertiary forest, poor in species was left in California and the Pacific Northwest, e.g. sequoias.

In the Pliocene (7-2 m.y.a.) continuous climatic cooling and mountain building brought on continental glaciations.

3. The Pleistocene in North America

The Tertiary ended about 2 million years ago with the Pliocene.

The Quaternary is divided into Pleistocene from 2 million years ago to 10,000 years ago, and the Holocene from 10,000 years ago to the present.

The climatic changes that began in the Oligocene (38 m.y.a.) reached their climax with the glaciations of the Pleistocene.

The Pleistocene was a period of great climatic fluctuations throughout the world.

• Four or more times glaciers move south in North America and three times in Europe.
• Communities retreated and advanced, each advance having a different mixture of species.

 

Each glacial period was followed by an interglacial period. The climate at each stage oscillated between cold and temperate.

Tundra vegetation and boreal species of fir and spruce dominated the landscape south of the glaciers.

The Wisconsin glaciation was the last one and it began 70,000 years ago and reached its peak about 18,000 years ago, and began to retreat about 16,000 y. a.

Canada was completely under the ice sheet.

A narrow belt of tundra 60 to 100 km wide bordered the edge of the ice sheet.

The boreal forest covered most of eastern and central United States as far as western Kansas.  Its southern limit was about 1200 km south of the modern southern border of boreal forest in Canada.

South of the boreal forest was a mixed forest of conifers and hardwoods.

Mesic, temperate hardwood species (e.g. oak, basswood, walnut, yellow poplar, hickory, chestnut, beech) found refuge in the Mississippi valley, in dissected valley slopes along major southern rive systems, ravines, and perhaps along the exposed coastal lands. The sea level was 300 m (~1000 feet) lower than today.

The ice edge was located...

15,000 y.a. in southern Michigan.
9 - 10,000 y.a. north of the Great Lakes.
4 - 5,000 y.a. the ice sheets had disappeared from the continent.

Pleistocene communities were probably richer. That is true for the large mammals of North America.

The shifts of communities to the south cause changes in the range of many species and species reassorted into new communities.

• The western spruce forests were replaced by grassland after the ice retreated.
• Oak and pine replaced spruce in the southern Appalachians.
• Pines moved northward rapidly.
• Some species of hardwood, e.g. chestnut, moved northward very slowly.

 

The nature of vegetational communities today reflects the evolutionary impact of changing conditions during the Pleistocene.