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Haeckel (1866) first defined Ecology as 'the science of relations between organisms and their environment' (Bramwell 1989). The study of ecology has developed over the years from an initial descriptive field of study in the 19th century to a more quantitative, experimental and analytical discipline in the 21st century (Mayorga et al. 2002). The definition has also developed. Krebs (1972) defined it as 'Ecology is the scientific study of the interactions that determine the distribution and abundance of organisms'. It is clear however that the interactions to which he referred are the very factors which create the environment and so a more complete definition which marries the definitions given by Haeckel (1866) and Krebs (1972) is suggested as follows by Begon et al. (2006): 'Ecology is the scientific study of the distribution and abundance of organisms and the interactions that determine distribution and abundance.'
It is necessary to take a historical aspect to understand the present distribution of plants and animals within the United Kingdom. The initial event is generally seen to be the movement of land masses followed by several periods of glaciation. Pleistocene glaciation is largely the event determining the present day patterns in distribution of Flora and Fauna in the UK (Begon et al 2006). Most of the plants and animals were pushed further south due to the icy conditions. Recolonisation of plants and animals is dependent on distance from other populations and the ability of some animals to disperse and reproduce, grow and survive, once they arrive in a suitable environment. There are two notable contrasts with continental Europe, 'Britain has a relatively impoverished mammal fauna as several species, such as the garden dormouse Eliomys quercinus and the beech marten Murtesfuina, now found on the western edge of the continental mainland, failed to recolonise Britain after the last ice age' (Mitchell-Jones 1996). Reptiles and amphibians species are also more diverse on the continent than in the UK. There are only 6 native species in the United Kingdom, by comparison with a single species in Ireland which is further from Europe, Europe itself boasts over 87 reptilian species (Silva et al. 2009).
To discuss the ecological factors affecting the distribution of plants and animals in the United Kingdom it is necessary to look at all aspects. The scope of the interactions which will be discussed here are outlined in figure 1.
Figure 1 Influencing distributing factors of plants and animals.
A niche is a summary of the organism's tolerances and requirements, how they interact to define the conditions and resources needed by an individual or species in order to practice its way of life (Begon et al. 2006), and the time it occurs there (Mackenzie et al. 1998). The niche of an animal is generally larger than that it actually inhabits, This is the fundamental niche which is characterised by conditions (temperature, relative humidity amongst others), within the tolerable limits of a species provided that there are enough resources available i.e. food, accommodation and that it is not limited by interactions with other organisms such as predation and competition and that it is not prevented from getting to the suitable area (dispersal). Hutchinson (1957) identified the Fundamental niche from the realised niche with the latter being "the more limited spectrum of conditions and resources that allow an animal to persist even in the presence of competition and predation.
Conditions are defined as variable environmental factors which organisms respond to. They are non depletable, the organism cannot use them up (Mackenzie et al. 1998) These are portrayed as one dimensional but clearly the effect of one condition may be altered by another i.e. Cool winds can high temperatures more bearable, thus making the effects often two or indeed multidimensional.
Temperature is a condition which affects the rate of development in organisms, in fact many cold blooded animals incubation and development is given in degree days rather than in actual time. An example of how temperature affects not one but ultimately two species in their realised niche and distribution is given by Randall (1982). The rush moth (Coleophora alticolella) lays its eggs on the flowers of the rush Junctus squarrosus and the caterpillar uses the ripe seeds as its food resource. The moths and the larvae are little affected by low temperatures so there is no reason why they can't extend their niche further up in altitude however at the lower temperatures above 600m the seed of the rush fails to ripen and so there is no food resource for the caterpillar. The temperature related limit of tolerance has been reached for the rush, which in turn limits the niche of the caterpillar giving us the realised niche. Relative humidity plays a very important part in the distribution of animals but it is impractical to discuss it in isolation due to the large influence that temperature has on it. Relative humidity affects the rate at which we lose water. Increasing temperature increases humidity, the physiology of animals is important with this limiting condition. 'The animals most affected by humidity are those 'terrestrial' animals that are actually, in terms of the way they control their water balance' 'aquatic'. (Begon et al. 2006).
The pH of soil and water can have a strong influence on plant and animal communities. Plant roots tend to be damaged in soils below pH3 and above pH 9 due to the pathogenic effect of toxic levels of H+ or OH- ions. Soil pH also has an effect on the uptake of nutrients and the concentration of toxins, tolerance levels vary for pH but only a minority of plants can grow at pH less than 4.5 (Begon et al. 2006). Kidd and Proctor (2001) observed separate adaptations in the various populations of the grass Holcus lanatus L. (Yorkshireâ€fog) and the tree Betula pendula Roth (Silver Birch) in very acid soils at some Scottish sites to H+ or Al3+ toxicity. These were closely related to the edaphic characteristics of the original site from which they were collected. 'These adaptations support the view that this is an important factor in very acid soils' (Kidd and Proctor 2001).
Increased salinity in the soil water offers osmotic resistance to water uptake.
Wind plays a major role in plant dispersal. Small light seeds have developed special adaptations which facilitates there dispersal by the winds. These adaptations are to enable the seeds to remain airborne longer so they can be carried greater distances. These adaptations increase the area to catch the wind (see plates 1, 2. and 3. below.)
Plate 1: Pepper-pot type: Red Campion The ovary which holds the seeds becomes a dry hollow vessel with openings. When they are blown in the wind, the seeds are dispersed through the openings, spreading them throughout the vicinity.
Plate 2: Parachute type
Ragwort Feathery hairs help the seed to be carried long distances by the wind
Plate 3: Winged type
The fruit (which contains the seed) delays its fall so that the wind may carry it a far distance away. (Photo:http://www.countrysideinfo.co.uk/seed_dispersl/wind.htm)Wind also has a negative effect on the distribution of organisms in that it has been implicated in the problem of soil erosion of arable lands and of sand dunes, thereby reducing their capacity to sustain plant communities.
. While Man has a positive impact in the management of resources and in the study of ecosystems, man is also the perpetrator of the majority of effects leading to environmental pollution. These include the clearing of land for operations totally wiping out local habitats e.g. Corn Crake (RSPB 2010), the leaching of metals into the environment and the dumping of copper, zinc and lead around mines, elevating their presence to lethal levels (Begon et al. .2006). Power plants and other factories may emit sulphur dioxide and nitrogen oxide which facilitate the problems with acid rain. Researchers now know that acid rain causes slower growth, injury, or death of forests. It is practical to assume that if it has this effect in forests it may also retard the growth of other plants which affects a food resource (National Geographic 2010).
When pollution occurs, organisms often find away to combat the stress and overcome the effects this is evidenced in the well known example of the peppered moth. However while the development is a reaction to industrial pollution there are many other factors at play, such as genetic variability. With the increasing industrialisation in Britain, the peppered moth survived by developing a darker coloured form which was better camoflaged from predators when it landed on the soot darkened trees after the lichens had died off (Majerus and Stevens 2006).
Environmental pollution leads us then to consider the changes we are making that ultimately influence these conditions termed as global warming. This is occurring at a rate that will not allow flora or fauna to adapt and thus could change living conditions so to be intolerable to many species. Berry et al. (2003) have undertaken a study which shows the vulnerability of terrestrial habitats and species distribution in Britain to climate change which is essentially the temperature increase of 0.6Â°C over the past century. They contend that with such changes that it is not safe to assume that a species historical range of distribution will remain suitable.
Resources are all things used by an organism, some are infinite, others may involve competition, but in all cases resources that are used up by the organism are not available to others. Resources can be places to live and many other specific items used by organisms but for the present we will discuss
Energy: Solar radiation
. Plants are the primary producers and are the foundation of the food chain/web for all animals. Plants need energy; they can only get this from solar radiation. Radiation must be captured and chemically fixed by plants or it is lost forever as energy (Begon et al.). When radiation is used by one plant it is no longer available to others. It is not recycled like nutrients, carbon and minerals. Radiation is important to most plants along with CO2 and water for photosynthesis. It is difficult to discuss radiation separate to Water and CO2 as one often influences the other.
Water is a vital resource essential for all organisms. It is essential for photosynthesis in plants and for transportation of nutrients throughout the plant. It is critical for animals for respiration and again for transportation and for many of the metabolic processes. The degree of morphological and physiological specialization that has occurred in plants to adapt to radiation, intensity and water availability is vast and too extensive for present discussion.
CO2 is vital for photosynthesis by plants and is drawn from the air it is released by animals after respiration and by plants at night. Levels in a terrestrial environment vary greatly (Begon et al. 2006) and so organisms have developed different ways to enhance its utilization.
Carbon dioxide is the single resource which is increasing globally, due to man's clearing of forests and burning of fossil fuels.
Oxygen is critical for all plants and animals. Only a small number of bacteria can survive without it.
When resources are limited or affected by other factors they produce stress in organisms. This generally leads to a reduction in abundance at the extremes of tolerance. Tolerance can be explained as the peak of endurance of species to an extreme bearable condition. When this situation avails, the species limit of tolerance in which the organism can survive and grow in that environment has been stretched. This can also be regarded as ecological stress zone. 'As conditions approach the upper and lower extremes of environmental conditions which individuals can survive the species of tolerance and no growth occurs'(McKenzie et al. 1998). Grime (1979) divides plants in to three categories depending on how well they survive at various intensities of stress and disturbance, competitor (C), stress tolerator (S), and ruderal (R) . (Fig 2.)
Figure 2. http://www.sidthomas.net/Ecology/heisenberg.htm
There are many other factors which influence the fundamental niche, and the distribution of organisms in the UK, these are interactions with other organisms such as predation and competition. Where a natural predator is removed the prey can thrive so well that it may be in danger of destroying its own environment. Such is the case with the Stag in the highlands. With no natural predator its population is at its highest level in over a thousand years (Postnote 2009). If an organism is introduced that is similar to a pre-existing organism it may out compete it and cause patchiness in its distribution, this is evident in the competition between the introduced grey squirrel and the native red squirrel (Okubo et al. 1989). As all organisms are a potential food source for others, many animals defend themselves through physical, chemical, camouflage,
The main ecological factors effecting the distribution of organisms in the UK are as outlined above. The temperate nature of the United Kingdom appears not to be extreme; however it is pertinent to note that the relativeness of the ecological interraction of the organisms on above factors make it more complex and to the effects of the distribution of terrestial plants and animals inÂ United Kingdom. A single leaf is important to an insect and may possibly sustain the larva for its whole existence but it is of no effect to a cow. We have seen from the earlier example of the rush moth how distribution of one species can be a determining limit tolerance factor level of another species.