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45.5A: The Role of Species within Communities - Biology

45.5A: The Role of Species within Communities - Biology


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Communities are shaped by foundation species and keystone species, while invasive species disrupt the natural balance of an area.

Learning Objectives

  • Distinguish between foundation, keystone, and invasive species

Key Points

  • A community is defined by the structure of different species that occupy it and how those structures change over time.
  • Foundation species change the environment where other species live, modifying it to benefit the organisms that live there.
  • Keystone species maintain biodiversity; their removal can greatly alter the dynamics within the community.
  • Invasive species are non-native organisms introduced into an area that may be better competitors and reproduce faster than native species; they tend to upset the natural balance.

Key Terms

  • invasive species: any species that has been introduced to an environment where it is not native and has since become a nuisance through rapid spread and increase in numbers, often to the detriment of native species
  • community: a group of interdependent organisms inhabiting the same region and interacting with each other
  • keystone species: a species that exerts a large, stabilizing influence throughout an ecological community, despite its relatively small numerical abundance

Characteristics of Communities

Communities are complex entities that can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). Understanding community structure and dynamics enables community ecologists to manage ecosystems more effectively. There are three main types of species that serve as the basis for a community. These include the foundation species, keystone species, and invasive species. Each of these has a specific role in how communities are formed.

Foundation Species

Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, a brown algae, is a foundation species that forms the basis of the kelp forests off the coast of California.

Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef. Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.

Keystone Species

A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species. Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.

Invasive Species

Invasive species are foreign species whose introduction can cause harm to the economy and the environment. These species have many ways of entering foreign environments, including through ship’s ballast water: when planes take off, organisms can sometimes become stuck in the cargo area. When the plane arrives in its destination, the organisms are now in a foreign environment. Travelers sometimes illegally smuggle items, such as fruits, plants, or even animals as pets, from one state or country to another..

Invasive species are often better competitors than native species, resulting in population explosions. These new species usually overtake the native populations, driving them to localized extinctions.

One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations. Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of the fish escaped, however, and by the 1980s, they had colonized many waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.

Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially leading to native species extinctions. For example, black carp are voracious eaters of native mussels and snails, limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails feed upon, reducing this food source by a different alteration of the food web. In some areas of the Mississippi River, Asian carp species have become predominant, effectively outcompeting native fish for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community’s biomass. Although edible, the fish is bony and not a desirable food in the United States. Moreover, their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the boaters.

One infested waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of electric barriers have been successfully used to discourage their migration; however, the threat is significant enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species, such as the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of the world.


Biotic Community: Structure and Characteristics &ndash Explained!

Ecologists have given variety of meanings to the term community (Schoener, 1986 Fauth et al., 1996).

The term has been used by many of them for associations of plants and animals occurring in a particular locality and dominated by one or more prominent species or by some physical characteristic (Daubermiire, 1968).

A community, technically often referred to as biota or biotic community is a local association of several species populations. According to Krebs (1994), a community is an assemblage of the populations of living organisms in a prescribed area or habitat.

According to Clarke (1954), a group of mutually adjusted plants and animals inhabiting a natural area is known as a community. He prefers to use the term biocenose (a shortened form of the word “biocoenosis” coined by Mobius in 1880) for community. When territorial ranges of several species populations overlap, they may coexist within a given limited area.

The organisms in such an area represent a community (Fig. 4.1). A community always contains plants as well as animals, because both are very necessary for the survival of the community. Just as populations possess characteristics above and beyond those of their component organisms, the community also exhibits characteristics above and beyond those of its constituent populations.

A major community is the smallest ecological unit that is self-sustaining and self-regulating. It is made up of a large number of smaller minor communities that are not altogether self-sustaining. For example, forests and ponds are major communities decaying logs and ant hills are minor communities. Members of a major community are relatively independent of other communities, provided they receive radiant energy from the sun. These members normally show a similarity in their physiological make-up, behaviour, and mode of life.

A number of parameters can be estimated in a community, but their interpretation depends on nature of the community. In plant ecology, two schools have developed over the question of nature of the community. The organismic school holds that communities are integrated units with discrete boundaries. The individualistic school holds that communities are not integrated units but collections of populations that require the same environmental conditions. However, the individualistic interpretation of the community is favoured more.

Structure and Characteristics of a Community:

All communities have certain general characteristics in common, and their maintenance is governed by similar forces. Communities do not have exact limits but tend to overlap each other. Animals frequently shift from one community to another because of seasonal or other variations. Communities may be widely separated, but if the environmental factors are the same, similar kind animals may be found in them.

Some characteristics of the community are as follows:

Communities that possess a large biomass usually exhibit stratification, that is, the populations they contain are spaced out or distributed into definite horizontal or vertical strata.

Five vertical subdivisions of the forest community are:

Similarly, there are animals that live on the forest floor, others on shrubbery and low vegetation, and still others in the treetops. Many forms shift from one substratum to another, especially in a diurnal manner. Many of the adjustments and requirements of a particular stratum are very similar in forests widely separated from each other in many parts of the world. The animals that occupy such similar strata, or ecological niches, although geographically separated, are called Ecological equivalents.

There may be an intermediate transitional zone between two distinct communities. This is called an Ecotone, or a tensional zone. An ecotone can be defined as a zone of transition between adjacent ecological systems having a set of characteristics uniquely defined by space and time scales and by the strength of interactions between adjacent ecological systems (Holland, 1988).

An example may be the marginal region between a forest and a pasture or open land. Several examples of aquatic- terrestrial ecotones may be given, such as wetland ecotones (ecotones between wetlands and other types of ecosystems), lotic ecotones (fluvial boundaries of rivers and stream ecotones). As a rule, the ecotone contains more species and often a denser population than either of the neighbouring communities and this is known as the Principle of edges.

Recently is has been recognised that ecotones are not simply static zones where two communities join but are dynamic and have unique properties (Risser, 1990). They provide unique habitats for biota they regulate inter-patch dynamics they may serve as early indicators of hydro-climatic change and they have strong visual quality. These characteristics of ecotones are significant for aquaitc landscape management (Petts, 1990).

(C) Ecological Dominance:

In every community some plants and animals exert a dominant influence, in determining the nature and function of the whole community, because of their numbers, activities, or other important reasons. They are called Ecological dominants. An ecological dominant may be defined as a species population that exercises a major controlling effect on the nature of the community.

Thus, each community usually contains one or two species populations at the producer, herbivore, carnivore, and reducer levels that are recognizable by the controlling influence they exert on the community. These dominant species populations are the members of the community through which a major portion of the energy transfer is affected.

In land communities, plants are usually the dominants and some communities are named according to their dominant vegetation. However, in plant communities abundance is rarely correlated with dominance. For example, in a forest, an herbaceous species may be very abundant while a free species is hardly so, in terms of the numbers of individuals present but it is the tree species thus determines the nature of such a community and is therefore the dominant.

In plant communities the dominants are simply those plants that overtop all others in the community. In doing this, they modify the amount of light the subordinate species receive, the humidity, the amount of precipitation, the extent of air movement, and the composition and temperature of the air. Some plant communities have only a single dominant for example, pine trees in a pine forest, or saal trees in a saal forest. But in a mixed forest there may be several codominants. In the ecological cycle of a community, the removal of a dominant species causes disturbances and changes in the character of the community.

(D) Seasonal and Diurnal Fluctuation:

The component populations of a community may succeed one another in time as well as in space. The most conspicuous temporal variation communities are seasonal. E.J. Salisbury (1925) was actually the first to demonstrate such temporal separation of plant communities in temperate woodland. Local fluctuation of populations in communities, similar to seasonal migrations but over much more restricted distance, result from diurnal responses.

The active day time mammalian population of a wooded area may include squirrels, but by night there may appear reccoons, opoossums and wood rats. These temporal relations between populations in a community, like the spatial ones, must be considered in terms of the niche concept.

(E) Pattern Diversity:

Pattern diversity is the basic characteristic of community organization. There are several patterns. Such as stratification patterns (vertical layering), zonation pattern (horizontal segregation), activity patterns (periodicity), food-web patterns (network organization in food chains), reproductive patterns (parent- offspring association and plant clones), social patterns (flocks and herds), coactive patterns (resulting from competition, antibiosis, mutualism), and stochastic patterns (resulting from random forces).

Periodicity (activity patterns) refers to the rhythmic patterns of organisms in search for mates, food, and shelter. Some community periodicities are correlated with the daily rhythms of day and night, some are seasonal, and others represent tidal or lunar events.

In a community, as on all other levels of living organization, turnover occurs continuously. Individuals of the various populations emigrate or die out and are replaced by others. The important point is that this flux is automatically self-adjusting. As a result of this, the community- remains internally balanced and exhibits a numerical steady-state, i.e., in all populations making the community, the numbers of individuals remain relatively constant.

For example, in a large permanent pond the number of algae, frogs, fish and any other plants or animals, will be more or less the same from decade to decade. Annual fluctuations are common but over longer periods of time constancies of numbers are characteristic to most natural communities.

Various kinds of interactions bring about a certain degree of integration within the members of a community. Food, reproduction and protection are the principal links which make the members of a community interdependent.

According to Krebs (1994), there are five characteristics of communities that can be studied:

This means what species of animals and plants live in a particular community. An important characteristic of the community is its diversity, which is a function of the number of different species it contains and their abundance. Diversity actually depends on Species richness and on the evenness (equitability) of species abundance. Two hypothetical communities comprising the same species may differ in their structure and diversity, depending on their relative abundance distributions.

Communities in which the species are all more or less equal in abundance exhibit evenness, whereas communities with one or a few abundant species and many rare ones show dominance. However, communities possess both taxonomic diversity (diversity of species) and ecological diversity, which reflects the variation in ecological roles of species in the community.

(ii) Growth Form and Structure:

A community may be described by major categories of growth forms such as trees, shrubs, herbs and mosses. These different growth forms determine the ratification of the community.

This refers to the fact that all species in the community are not equally important. Of the hundreds of species, relatively few exert a major controlling influence by virtue of their size, numbers, or activities. Thus, dominant species are those which are highly successful ecologically and largely determine the conditions under which the associated species have to grow or live.

This emphasizes the relative proportions of different species in Ac community.

The feeding relations of the species in the community determine the low of energy and materials from plants to herbivores to carnivores. Thus, “who eats whom” decides many things in the community. These characteristics can be studied in all the communities that are in equilibrium and also in communities that are changing. The changes may be spatial along environmental gradients such as altitude, temperature and moisture. The changes may also be temporal, which are called succession and may lead to a stable community or the so-called climax community.


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