Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.
Each habitat usually supports a fairly diverse group of different organisms (plants and animals), called a community.
A community is made up of several populations, which are the numbers of individual organisms of one type or species.
List some examples of British wildlife habitats, together with some representative species (both plants and animals) forming communities in them:
> heathland > "wild woods"
> grassland > ponds and marshes
Each living organism is in some way adapted to suit its lifestyle in some way. These are Biological specialisations, such as different ways of feeding, moving, reproducing, etc. This allows organisms to compete, for food, shelter or to find a mate.
At the most basic level, adaptation, competition, and predation are factors which may be said to influence the distribution and population sizes of organisms in various environments.
Explain what is meant by predation, using the term trophic level.
> the way one animal lives by preying on other animals - i.e consuming one from a lower trophic level
Explain what is meant by the distribution of an organism.
> the (usually geographical) area where the organism lives, i.e. its range - possibly within a larger area
> climate (temperature, rainfall, sunshine)
> landscape (local differences in climate)
> soil - sandy/clay - pH
Man is apparently an exception to this situation of stability, since the population of Homo sapiens is on the increase. Man is also the main species on this planet to have a major influence on the environment and this impact may be related to the actual size of the human population, as well as to economic factors and industrial requirements, all of which vary from country to country. The population of the world has shown a fairly dramatic increase in recent times, and shows little likelihood of decreasing.
When did the world's population reach 1 billion?
When did the world's population reach 2 billion?
When did the world's population reach 4 billion?
Agriculture, Horticulture and "Harvesting the Wild" Most methods of food production or exploiting natural resources involve management of ecosystems, or the creation of "artificial" ecosystems. Examples include arable and animal farming (including fish farming), production of glasshouse crops, as well as fishing and hunting. Such management may be said to impose a duty of care, but misunderstanding or misinterpretation of of the Biological principles may have far-reaching consequences. This may be illustrated by reference to information on catches of fish which show fairly clear evidence of over-fishing.
It is said that it is best to let stocks of cod reach 6 years of age before being caught, so that they can breed for about 3 seasons. They thus maintain the biomass of a fish stock by adding to the recruitment of new individuals into the population. Natural mortality decreases the number of older fish, so that in a healthy balanced population there are fewer larger fish as well as more younger smaller fish.
The effects of air pollutants e.g. sulphur dioxide, carbon monoxide and water pollutants, e.g. organic pollution from sewage, and inorganic pollution due to nitrates and phosphates have already been mentioned. The correlation between large populations, industry and damage to the environment is now clearly established. For example, lichens have been shown to decrease in abundance in proportion to their exposure to atmospheric pollution by sulphur dioxide which is a by-product of many industrial processes. Although seemingly insignificant on their own account, these are other good examples of indicator organisms which give information about the differing degrees of air pollution in the environment.
What is meant by the term correlation?
> statistical connection between 2 factorsWhat industrial processes might result in the production of sulphur dioxide?
> power generation/ burning coal/oil / flaring off XS gas from oil
How are these related to the country's industrial requirements?
> industry needs power, fuel, petroleum products
What might be the economic consequences of measures to reduce these emissions?
> costs of "scrubbing" may add marginally to production costs, reducing profits, but there may be other savings
The use of chemical pesticides (insecticides to kill insects, herbicides to kill weeds, fungicides to kill fungi) effectively alters the "balance of nature" exemplified by the various models (food chains, webs, etc) used to explain the transfer of organic material from one trophic level to another, but due to concentration effects it may have extremely profound effects on whole ecosystems.
Pesticides are effective at extremely low concentrations - parts per million (ppm) or parts per billion (ppb), and often chemically very stable. However they may build up in food chains, and become lethal to animals at a considerable distance from the site of pesticide application.
The same principles apply to other forms of pollution, such as industrial wastes containing heavy metals.
See previous notes.
There are numerous examples of human activities, such as tree felling, mining, road construction, etc., which have major impacts on wildlife as a result of large or small changes in their environment.
Even though some habitats have been effectively established and maintained by Man's activities over the years, many are facing considerable pressures, mostly commercial. Conservation of habitats and their special wildlife communities has consequently become a major political issue. However, habitats naturally change over a period of time, so it is not simply a matter of preventing change. It is also difficult to reconcile the needs of other organisms with those of Man, and to establish priorities.
In any habitat, some organisms which have previously been successful gradually become replaced by others which are more able to exist there, presumably because their specialisation is better, or because environmental conditions have altered. This change in a community is calle succession, and so some other organisms are said to succeed. In fact, many well-defined habitats are in a state of transition.
Give an example of a type of habitat which is in such a transition state.
There are many examples of animals (usually carnivores) which have been hunted and persecuted, perhaps because of their effects (real or imagined) on livestock, or, more likely, for "sport":
e.g. foxes, wolves, otters, snakes, and even spiders - the list is practically endless!
In some cases, reduction in "pest" numbers has not had the desired effect of raising numbers of livestock.
Releasing novel species
Sometimes, Man has introduced new species into areas where they did not previously exist. Similarly, exotic) crops or animals grown under protected conditions may become infested with pests, sometimes from the areas where the crops or animals originally came from. This has frequently resulted in great andunforeseen population explosions, because normal prey-predator control mechanisms have not operated. Some of these "releases" have had dire consequences for the native populations of plants and animals. In some cases, e.g. glasshouse crops, it has been necessary to resort to Biological Control which makes use of natural "enemies" which either feed on or parasitise the introduced pest.
Although farmers' and growers' first reaction in such a situation may be to use chemical methods to kill these pests (using chemical pesticides), there are several risks:
- the (food) product may be contaminated, e.g. pesticides on/in food
- pesticides may be passed up the food chain, with unforeseen consequences, contaminating other environments
- pesticides are non-specific, and may affect "useful" species
- pests may still survive, e.g. due to being missed by the pesticide spray
- pests may develop resistance to pesticides, so they are unaffected and pass this on to their offspring
- pesticides cost money, which adds to the cost of the food
Reaction to these drawbacks forms the basis of "organic farming" - which involves no use of artificial pesticides and (inorganic, chemical) fertilisers.
Biological control involves using the principles of ecology to bring pests - target pests - under control, for example by encouraging the growth of controlling agents - "natural" predators and parasites of pest species, or by manipulating the life-cycle of the pest.
In other situations, economically important organisms may depend on other species for the completion of their life-cycle, so this information can be put to good use by applied biologists.
There are many examples of the useful application of ecological knowledge to commerce, and several different mechanisms of control:
The whitefly Trialeurodes vaporariorum - a sap-sucking pest of glasshouse crops e.g. tomato, cucumber - can be controlled by a parasitic wasp Encarsia formosa
Aphids - "blackfly" and "greenfly" - may be controlled by the larvae of hoverflies, as well as a whole host of other "beneficial" insects, such as the larvae of ladybirds.
Another example involves the use of predatory mites Phytoseilus persimilis to combat "red spider mites" Tetranchus urticae which are pests of cucumbers and other glasshouse crops.
Why is it not possible to combine conventional treatment of insects with Biological Control?
> pesticides would kill both pests and control agents