For a marsh willow-herb (Epilobium palustre), its only habitat is in slightly acidic ponds. In contrast, a species such as a blackberry (Rubus fruticosus) may be found in more than one habitat, e.g. heath land, woodland and in hedges. The common rat (Rattus norvegicus), often associated with humans, is also seen in various habitats (e.g. farms, sewers, hedgerows and food stores).
Within the term 'habitat', distinction can be drawn between closed plant communities and open plant communities.
These two terms can only be used in a relative way because the radiation from the sun, the gases in the atmosphere, and migrant species prevent a true closed system being established within the natural environment.
A couple of general points may be added about the vegetation of the British Isles. The British Isles at present has about 1700 plant species. Fossil and pollen evidence suggests that before the Ice Age there was a much larger number of plant species, possibly comparable to the 4000 species now seen in Italy (a country with a similar land area to that of Britain). In Neolithic times, when humans began occupying this area, most of Britain was covered by mixed oak forest. Since that time progressive clearing of most of the land has occurred, especially below the altitude limits for cattle (450 m) and for crops (250m).
In natural habitats, it is seen that a number of plant species (and associated animals) are grouped together, and that away from this habitat they are not commonly found. Two habitat examples can be given. In south-east Britain, in a low rainfall, chalk grassland habitat there will often be greater knapweed (Centaurea scabiosa), salad burnet (Poterium sanguisorba) and bee orchid (Ophrys apifera). In the very different high rainfall, acid bogs of northern Britain, cotton grass (Eriophorum vaginatum), bog myrtle (Myrtus gale) and sundew (Drosera anglica) are commonly found together. Other habitat species such as bluebell (in dense broadleaved woodland), bilberry (in dry acid moor), mossy saxifrage (in wet north-facing cliffs), broom (in dry acid soils) and water violet (in wet calcareous soils) can be mentioned. It should be noted that successful weeds such as chickweed are not habitat-restricted (see Chapter 13) in this way.
For a Sphagnum moss, its niche would be as a dominant species within an acid bog. The term 'niche' carries with it an idea of the specialization that a species may exhibit within a community of other plants and animals. A niche involves, for plants, such factors as temperature, light intensity, humidity, pH, nutrient levels, etc. For animals such as pests and their predators, there are also factors such as preferred food and chosen time of activity determining the niche. The niche of an aphid is as a remover of phloem sugars from its host plant.
The term is sometimes hard to apply in an exact way, since each species shows a certain tolerance of the factors mentioned above, but it is useful in emphasizing specialization within a habitat. The biologist, Gause, showed that no two species can exist together if they occupy the same niche. One species will, sooner or later, start to dominate.
For the horticulturalist, the important concept here is that for each species planted in the ground, there is an ideal combination of factors to be considered if the plant is to grow well. Although this concept is an important one, it cannot be taken to an extreme. Most plants tolerate a range of conditions, but the closer the grower gets to the ideal, the more likely they are to establish a healthy plant.
A major regional or global community of organisms, such as a grassland or desert, characterized by the dominant forms of plant life and the prevailing climate.
This term refers to a wider grouping of organisms than that of a habitat. As with the term habitat, the term 'biome' is biological in emphasis, concentrating on the species present. This is in contrast to the broader ecosystem concept described below. Commonly recognized biomes would be 'temperate woodland', 'tropical rainforest', 'desert', 'alpine' and 'steppe'. About 35 types of biome are recognized worldwide, the classification being based largely on climate, on whether they are land-or water-based, on geology and soil, and on altitude above sea level. Each example of a biome will have within it many habitats. Different biomes may be characterized by markedly different potential for annual growth. For example, a square metre of temperate forest biome may produce ten times the growth of an alpine biome.
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