THE INTERRELATIONS OF LIVING THINGS

UP to the middle of the 19th century the work of biologists was mainly concerned with the classification of animals and plants and the structure and function of their various organs. But the study of any organism is incomplete without reference to its habits and the part it plays in nature, so that within the last sixty years or so a new branch of biology dealing with this aspect of animal and plant life has arisen. Perhaps it would be more true to say that the old-fashioned Natural History has become the more precise science called Ecology. Ecology deals with the habits of animals and plants existing under wild conditions, and with the relations these living things bear to one another and to their environment. It is concerned with their home life and is therefore an outdoor study, for the mode of life of an organism cannot be understood by examining an isolated individual, kept under artificial conditions, such as a rabbit in a cage or a woodland plant cultivated in a garden. Under natural conditions the life of a rabbit or wood anemone will be influenced by other living things, and also by such non-living features of the environment as temperature and rainfall.

Although ecology as an important branch of biology has only been studied scientifically for a comparatively short time, farmers all over the world have unconsciously been ecologists for generations. The fanner has always been concerned with the choice of the most suitable animals and crops for his particular locality. He knows that wheat grows better on a clay than on a sandy soil, and that certain breeds of live stock will thrive better on his land than others. Many of the numerous breeds of cattle have originated in certain parts of the country, and possess features which enable them to sucoeed best in their own locality. There is a great difference between a Highland and a Guernsey cow, but each is suited to its own type of environment. Sheep breeders know that the hardy Welsh mountain sheep can thrive under conditions of weather and food supply that would not suit a South Down or Lincoln sheep.

Widely different types of environment occupied by living organisms are to be found in the sea, in fresh water and on land, and it must not be forgotten that the living things found in such places themselves provide the locality required for parasites. In any locality there may be many modes of life, but the animal and plant population is not an affair of chance, for the habits and structure of the members of a community must be suited to the particular conditions of that environment. In general, in a community of organisms living, for example, in a wood, meadow or pond, there must always be green plants to provide food for herbivorous animals, which in turn are preyed upon by carnivorous foes. Bacteria are also present and help to break down the dead bodies of animals and plants, and convert them into the raw materials used by plants. There is a delicate balance between the members of an environment which may easily be disturbed. Should one species become extinct, or greatly increase in numbers, this balance is upset and many organisms are affected.

As an illustration of the interrelation between animals and plants, Darwin ’s example of the connection between village cats and red clover may be stated. The fertilization of red clover can only be effected by bumble bees, which are solitary bees building their nests in holes in banks. These nests are frequently destroyed by field mice, and if the mice are allowed to multiply to a considerable extent they will tend to exterminate the bees, and so prevent the setting of the clover seed. On the other hand, if the cats of the villagers are very numerous they will keep down the numbers of the field mice, and so indirectly permit the pollination of the red clover. Hence there is a relation between the number of cats and the amount of red clover in the locality.

Another instance showing how the activities of animals and plants are often intimately related may be seen in the case of the Norfolk Breckland. The Breckland country is an area occupied by pine and heather, with a large rabbit population. Owing to the preservation of game, the natural enemies of rabbits, such as stoats and weasels, have been partly exterminated by gamekeepers, and therefore the number of rabbits is increasing. Since the pine seedlings are eaten by rabbits, the pine woods are not regenerating themselves and are slowly disappearing as the old trees die. Their place is being taken by bracken and heather, but the heather is eaten by rabbits, and gives place to rush and short grass. Even the rushes are eaten and tend to die out, but the grass survives, although it is cropped very short. Rabbits do not eat bracken unless compelled to for want of a better food, and so this plant is slowly spreading over the Breckland. Where land is fenced off from the rabbits, the heather and bushes and trees begin to grow-again. Thus the alteration in the balance between the rabbits and their natural enemies has caused a slow change in the appearance of the Breckland country.

Other examples which show that there is normally a relationship between the numbers of an organism and those of its natural enemies are afforded by the cases of the rabbit and of the prickly pear in Australia, the thistle in New Zealand and the Canadian water weed in England. These species were thoughtlessly introduced by man, and owing to the absence of natural enemies have multiplied at such a rate as to become pests.

The Meaning of Habitat

Observation shows that plants are not uniformly distributed over the earth ’s surface but that an individual or a species as a whole occurs in a certain locality which is characterized by a number of external conditions. This locality, together with all the conditions that affect it, is called the habitat of the individual or species. The external conditions are called habitat factors, and can be placed in three classes :—

Climatic factors.

Physiographic factors.

Biotic factors.

Climatic Factors

The chief climatic factors of a habitat are temperature, rainfall, wind and illumination. Temperature and rainfall are together responsible for the distribution of the largest plant communities, such as forests, prairies, deserts and tundra. Great Britain lies in the zone of the temperate deciduous forest. The regional variation of temperature in Britain is small, but it is sufficient to affect the distribution of plants slightly. Wheat, for example, does not thrive in the north of Scotland, and the oak is not plentiful north of the Scottish Lowlands.

Winds are of greatest importance to plants as rain bearers, but apart from this effect may influence plants in two ways. In the first case, if the habitat is exposed to strong winds the vegetation will be stunted or of a prostrate habit. This is because the rate of transpiration is increased by wind, and a balance between the water lost in this way and that gained by root absorption can only be maintained in trees and shrubs of a dwarfish, xcrophytic nature.

Secondly, trees become lopsided in exposed situations, partly on account of the mechanical pressure which bends the twigs in the ,4. direction of the wind, and partly because the greater transpiration on the windward side stunts the shoots.

Since all green plants are dependent on the sun ’s energy for the manufacture of their food, the strength and duration of the sunlight will affect the distribution of plants. Most plants have definite light requirements : trees, for example, needing full sunlight, while other plants such as the dog-violet and ground ivy can grow in the shade cast by trees.

Physiographic Factors: the Plant Association

Physiographic factors relate to the structure of the earth ’s surface, and include such features as rivers, mountains, hills and valleys, as well as the character and distribution of the soil. It is mainly due to these factors that local variations are found in the population of the large plant communities determined by climate. England lies in the zone of the cool, temperate forest, but although pre-Roman Britain was extensively forested, it has never been entirely covered by trees. There have always been areas occupied by fen and moor, heath and grassland. To each of the smaller plant populations occurring in the zone of the deciduous forest, the name of plant association has been given. Thus in Great Britain, among others, we have the moor association, the heath association and the deciduous and coniferous forest associations.

The plant associations are related to the physiographic factors in many ways. Soil differences are a common cause of local variation in the nature of the vegetation. For instance, the predominant tree in the deciduous forest association is oak, ash or beech, according as to whether the soil is clay, limestone or chalk. The difference between the vegetation of clay and sand areas is mainly due to the different capacities of the soils for holding water. Many plants require a more or less neutral soil, but some, such as ling, foxglove and rhododendron, thrive best in an acid soil, while others, such as lady ’s fingers and box, prefer a chalky soil.

As a mountain is ascended the plant population slowly changes with the altering climate. In this country deciduous woods may be found up to a height of about 1,600 ft. ; at higher altitudes this woodland gives way to a vegetation of shrubs and stunted trees, and these in turn are replaced by ling and bilberry. The degree of slope of a hillside influences the occurrence of plants, for a steep slope may either prevent the accumulation of soil altogether or result in a soil which is too dry, owing to the rain running down the slope without sinking into the ground. Further, the difference in temperature between the north and south sides of a hill running cast and west may cause a difference between the plant population on cither side.

Biotic Factors

These include the influence of plant on plant, animal on animal and the relation existing between animals and plants, but by far the most important biotic factor is the activity of man. He has changed the nature of many habitats by both planting and clearing forests, and by draining and cultivating land. He has also been responsible for the extinction of many animals, such as the dodo and great auk, and in Great Britain, the wolf. The interaction among plants is seen mainly in the effect of the shade cast by the taller species of an association, and in the root competition that takes place for soil water. By eating seedlings, animals may change the nature of a habitat, and they also assist in seed dispersal and in the pollination of flowers. Worms have a beneficial effect in aerating and draining the soil by the tunnels which they make, and bacteria help to convert dead tissue into the raw materials taken up by plant roots. The chief interrelation between different animals is that which exists between hunter and hunted.

Definitions employed in Plant Ecology.

Plant Community

This is a collection of plants growing together in a certain habitat and having a certain individuality as a whole, e.g. marsh, forest, heath, meadow.

The main divisions of a plant association are :— Consociation, I.e. a community formed by one or two dominant species together with their associated plants, e.g. beech, oak-birch and Calluna ding consociations.

Local societies, e.g. gorse and its undergrowth on a heath.

Occasionals, e.g. isolated plants or clumps of plants.

Strata

Tree, shrub and herb layers.

Sucoession

A scries of plants sucoeeding one another in occupation of a particular habitat.

Climax

A habitat in which sucoession has concluded and which shows a stable type of vegetation.

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