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 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.
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.
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.
Secondary succession occurs in areas that have soil and have supported a community previously.
TIME AND DIRECTION IN SUCCESSION
Time is the integral component of succession.
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.
It is a steady state of competition, structure and energy flow.
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.
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.
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.
Fluctuations may involve the replacement of one age class by another within the same species.
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.
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.
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:
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 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.
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.
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 nature of vegetational communities today reflects the evolutionary impact of changing conditions during the Pleistocene.
Web site to visit: http://facstaff.cbu.edu
Author of the text: indicated on the source document of the above text
If you are the author of the text above and you not agree to share your knowledge for teaching, research, scholarship (for fair use as indicated in the United States copyrigh low) please send us an e-mail and we will remove your text quickly. Fair use is a limitation and exception to the exclusive right granted by copyright law to the author of a creative work. In United States copyright law, fair use is a doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders. Examples of fair use include commentary, search engines, criticism, news reporting, research, teaching, library archiving and scholarship. It provides for the legal, unlicensed citation or incorporation of copyrighted material in another author's work under a four-factor balancing test. (source: http://en.wikipedia.org/wiki/Fair_use)
The information of medicine and health contained in the site are of a general nature and purpose which is purely informative and for this reason may not replace in any case, the council of a doctor or a qualified entity legally to the profession.
The texts are the property of their respective authors and we thank them for giving us the opportunity to share for free to students, teachers and users of the Web their texts will used only for illustrative educational and scientific purposes only.
All the information in our site are given for nonprofit educational purposes