Biotic Functional Components

Transcription

1 1 Module # 10 Component # 3 Biotic Functional Components Introduction The biotic structural components which comprise the living organisms in the ecosystem are the plants and animals. Individuals are grouped together with other members of their species to form a population. Populations are in turn collectively called communities comprising all the living organisms in the ecosystem. Another way in which the biotic components can be structured is based on the method that the organisms use to obtain their nutrition. Producers These are organisms such as plants that can synthesize or manufacture their own food. This is done by the process of photosynthesis whereby the sun s energy is converted into a useable form of chemical energy. The ability of plants to produce their own food in this way is called autotropism and hence plants are called autotrophs. Consumers Consumers are organisms that are unable to synthesize their own food and rely on other organisms for their nutrition. Examples include animals that feed on plants (primary consumers) and animals that feed on the tissues of other animals (secondary consumers). Decomposers Decomposers are those organisms that can break down existing organic material such as dead wood or dead animal tissue. This is called heterotrophism and animals are therefore referred to as heterotrophs These heterotrophic organisms rely on consuming material synthesized by other organisms for their nutrition. The means whereby decomposers obtain their food is by absorption. Particles of food may be absorbed through the cell membranes of bacteria or digested outside the decomposer and then absorbed into the organism as in the case of saprophytic fungi. Other examples of decomposers are insects such as termites and certain beetles.

2 2 Communities A community is a collection of all the plant and animal populations in each area and is inclined to have a relatively uniform appearance. An example could be a vast forest or the community of insects and fungi found inside a rotting tree stump. These are referred to as major communities and minor communities respectively. A major community is generally thought of as not requiring any input from outside the community, apart from solar radiation. The minor community, on the other hand must receive inputs, other than solar radiation, from outside to function. The community is named according to the dominant features found within the community. For example, a community could be named Marula-Combretum veld. The dominant features of this community are therefore Marula trees and bushwillows. The dominant features are determined by their abundance (how many there are), the size of the area they occupy or the biomass (the amount of living material). In some instances, other non-living components may be included in naming the community. An example would be a Sandveld community. Obviously, the non-dominant members of the community are also of importance and should not be overlooked, as they play an important role in the community. Generally, the dominant species are only a few species whilst the non-dominant species make up a greater number of species.

3 3 Species diversity The number of species residing within a specific ecosystem is an important characteristic of the community. Communities with a greater species diversity (number of different species) are inclined to be more stable than communities with a lower species diversity. If a physically controlled community (such as a desert) is compared with a biologically controlled community (such as a rainforest) we can see why this is so. A biologically controlled community is dependent on the living organisms to play a major role in the community as the species diversity is high. A tropical forest has a high species diversity and the living organisms control themselves more actively (for example by competition) than the physical environment does. A desert is physically controlled by the temperatures and low rainfall. Therefore, the species in the desert need to be better adapted to their environment to survive.

4 4 Patterns Patterns are frequently found in almost all communities. Patterns arrange the members of the community in a specific way. It is the result of interactions between organisms and their environment. Patterns may take on various forms. Stratification is the layering of members of the community, as we see in the savanna areas where the vegetation is found in a herbaceous layer and an overlaying woody layer. Zonation is also seen, for example, where two different soil types are found as the substrate of a single community. Plants growing on one soil type will be different from plants growing on the other. Food webs will influence the structure of communities. Vast herds of migrating herbivores moving to preferred grazing areas will be followed by large numbers of predators. As the herbivores move they will induce the predators to follow suit. Social and reproductive patterns are also seen in communities. An example is the social arrangement of zebras. A dominant stallion will defend his harem (permanent group of females and offspring) from other stallions. Young males will be forcibly ejected from these herds when they reach sexual maturity to join bachelor herds. These harems will remain as distinct groupings even if they join other harems as a larger aggregation of zebras. Other examples of social groupings are schools of fish and flocks of birds. Activity patterns will also play a role in the arrangement of members within the community. Some members of the community will be more active at certain times than others. Predators such as lions and leopards are more active during the night when they catch their prey species. A species like giraffe will be inclined to feed mainly during the day and rest at night. Plants are likely to be most active during the summer months when temperatures are higher and moisture is not limiting. During the winter months, deciduous trees lose their leaves, thereby changing the pattern of the community. The edge of a community is called an Ecotone. It is usually not a distinct linear region separating two communities, but is a zone where the two adjoining communities overlap. It is inclined to consist of members of both communities. The Ecotone therefore may have a greater variety of species than either community. This increase in species diversity is called the edge effect.

5 5 Populations In this component, only the interactions between different populations will be discussed. The effect that the interaction has on the members of a population will be mentioned. An example illustrating the interaction will be given. The equally important study of population dynamics will be discussed in a later module. There are several important terms that describe animal interactions that are frequently used and mis-used.

6 6 Symbiosis This is defined as two or more species or populations living together for the benefit of both. Their relationship is, however, compulsory. Neither species could survive without the other. An example of this is a ruminant and the bacteria that live inside their body. Without the environment that the ruminant provides for the bacteria, the bacteria could not survive. Without the bacteria to digest cellulose for the ruminant, the ruminant could not survive. The emphasis of this kind of relationship is on its compulsory nature. The term symbiosis is also often misused as simply two or more organisms living together. This is strictly incorrect.

7 7 Predation Predation is an interaction between a predator and its prey species. The predator tends to be larger than the prey species in most cases. This interaction can also be thought of as a carnivore-herbivore interaction or even a herbivoreplant interaction. In most cases the interaction is beneficial to the predator and is detrimental to the prey species.

8 8 Parasitism Parasitism is like predation. One of the populations (the parasite) benefits from the interaction, whilst the other population (the host) is usually detrimentally affected. The difference between predation and parasitism, amongst other things, is that the parasite is always smaller than the host and the host may not be killed as is the case with the prey in predation. Examples of parasitism are the interaction between an animal and a tick, a parasitic plant growing on another plant and you and a mosquito. An interesting behaviourism exhibited by most ticks, is the fact that should the animal they are feeding on die, they will soon leave the carcass. This is a result of the host s blood becoming cold apparently unappetising to ticks!

9 9 Antibiosis This unusual interaction occurs where a plant gives off an inhibiting substance into the soil that prevents other plants growing in its vicinity. A chemical substance is released from the plant which will inhibit the other plant. Obviously, the plant releasing the inhibitor will benefit, whilst the other plant will be negatively affected. Marigolds (genus Tagetes) suppress populations of soil endopathogenic nematodes such as Pratylenchus penetrans and Meloidogyne species. Nematode suppression by marigolds is thought to be due to thiophenes, heterocyclic sulfurcontaining molecules abundant in this plant. When activated, thiophenes such as α-terthienyl produce oxygen radicals. If marigold roots release such a powerful biocidal agent and it is activated in soil, microbial populations in the marigold rhizosphere should be substantially perturbed. - Springer-Verlag Berlin Heidelberg 1998.

10 10 Mutualism This entails the voluntary relationship where both species benefit from the interaction. An example includes the relationship between oxpeckers and buffalo. This benefits both parties as the buffalo have external parasites and loose hair removed and the Oxpeckers receive a food source and nest building material. Another example is the relationship between buck and baboon. They both share common predators. The baboon benefits from the antelope s acute hearing while the buck benefits from the baboon s superior vision. Their mutualism is that they both benefit from a warning that one may give the other. This relationship, however, does not need to occur for the survival of either species.

11 11 Commensalism In this interaction, only one population benefits and the other is not affected either positively or negatively. Examples include the epiphytic orchid using a tree for an attachment substrate and birds using trees for a place to construct nests. This may not be a true example, as the tree may be disadvantaged by either the added weight of the nest or orchid, or either of the two may be situated on a growing point (node) of the tree. One further example may be of an animal such as primate which, while feeding in a tree, inadvertently knocks food (fruit, berries) to the ground where another animal eats it. This can only be considered commensal if the primate has no intention of going to collect what it had dropped. True commensal relationships are in fact very rare. The following case however is both true and frequently seen. The best animal example occurs between buffalo and cattle egrets (insectivorous birds). The buffalo graze normally and in doing so scatter / disturb insects from the grass. The egrets spend time following the buffalo and catching the inadvertently flushed insects. The buffalo remain completely oblivious to both insects and the egrets. It is also interesting to note, that the a commensal, is literally one who eats at the same table and thus the word Commensalism.

12 12 Competition Some degree of competition exists between most species that live close together. Generally speaking, one of the species will benefit from the interaction whilst the other may be detrimentally affected. An example is the feeding relationship between bushbuck and nyala. Both species occupy the same habitat and eat the same resource (leaves). As the nyala is taller than the bushbuck, the nyala can browse more effectively than the bushbuck as it has a height advantage. This could cause the numbers of bushbuck to become reduced as they cannot compete with the nyala. Another example is the shading effect of one plant on another. A taller plant will shade the smaller plant, thereby preventing it from receiving sunlight. This could mean that the smaller plant could die off. In extreme conditions, competition could lead to the loss of one species. These examples demonstrate the competitive exclusion principle. The Competitive Exclusion Principle is defined as follows: When two or more species compete for the same LIMITED RESOURCE, one species will outcompete the other. The outcompeted species will either die out, move away or continue to exist in association with its competitor species, but in a stunted form. Not only does this form the corner stone of a sub discipline of ecology known as competition ecology / competition theory, but it also has enormous relevance for Darwinism and its Natural Selection (Survival of the fittest) paradigm.

13 13 Individuals An individual is a single member of a population. The characteristics of an individual are therefore different to those of a population. Here the life cycle and behaviour of the organism are of greater importance. Sometimes the words species or organism are used instead of individual. Two important concepts need to be understood. The first is the ecological niche or simply niche. This is the specific location and /or role that the organism plays in the ecosystem. This could be seen from a couple of points of view. There is the functional role the organism plays in the community. For example, a hyaena plays the role of a scavenger. There is also the trophic role. The hyaena for example is considered a secondary consumer as it feeds on other organisms. The trophic role of an organism is the position it holds in a food chain; that is, whether it is eaten or eats others. The niche can also be thought of as the physical, chemical and biological conditions a species requirements, to live and reproduce in an ecosystem. A further perspective is based on the role the organism plays because of the adaptations, physiology and behaviour it possesses. The second important concept is habitat. The organism s habitat is the place where the organism lives. For example, a wildebeest s habitat is open grasslands while a hippo s habitat is a river or large dam. On the individual level of organisation four main types of species is recognised. Native species are those species which are normally found in a particular habitat, where they thrive and reproduce successfully. Alien species are those species which are not normally found in a particular habitat. Acacia longifolia (The Australian Black wattle) is an example of an alien plant species that has invaded South Africa (from Australia), displacing native species of plants. Indicator species are species which can be used as a warning that something is amiss in the ecosystem. Frogs are good indicators of water quality. Therefore, as the water quality becomes poorer, the numbers of frogs will also become reduced. The silver cluster leaf is a tree that serves as an indicator of where a seep-line begins. See more about seep-lines in the next module.

14 14 Keystone species are those species which may affect the future of other species in the ecosystem. Examples of a keystone species include a species of wasp that pollinate the flowers of fig trees indigenous to South Africa. Should these wasps die off for any reason, such as the misuse of pesticides, the fig trees would be without a pollinator and would also eventually die off. Species can also be broadly classified as being specialist species or generalist species. Specialist species are those species which have a narrow niche. An example could be the sable antelope which requires specific environmental conditions, such as sufficiently long grass to hide its new-born calf and suitable grass species for grazing. If these conditions are not met the sable will be forced to leave the area to find a more suitable habitat or simply die out. The impala is an example of a generalist species. It can adapt well to most conditions, as even overgrazed conditions will be suitable. It can feed on a wide variety of grasses and browses on trees and shrubs.

15 15 Ecological Equivalents These are two species which occupy different geographical regions naturally, but which have the same or similar niche. Examples of ecological equivalent species are the impala and the springbok. These two antelope are found in totally different habitats (under natural conditions) and both play a similar role in the regions they are found.