Autore: Elīna Brice
Augu un dzīvnieku izplatīšanās
Plant distribution limiting factors
Climate. Climate is generally accepted as being the dominant factor affecting plant geographic ranges. It has many different forms and affects different species in different ways, but ultimately the distribution of plant species on Earth is controlled by the climate. The most important climatic factors are temperature and precipitation, temperature includes mean summer and winter temperatures, yearly maximum and minimum temperatures, the length of the growing season (above a certain temperature), frosts, soil freezing, or the length of the thaw in cold environments. Precipitation principally limits distribution by its absence, i.e. a droughts, but can also have an effect in the form of snow cover, flooding, etc. It is likely that the distribution of most northern hemisphere plants is ultimately (i.e if not held earlier by another factor,) limited by cold temperatures in the north and summer drought in the south, though obviously different plants are better than others at coping with one or the other or both Other climatic variables include: Day length, light intensity, humidity, and wind speeds. each may be required in different intensities by different species, or act as a trigger for a process such as flowering that may never occur in some climates. For example, many plants use a set day length as a trigger for flowering,but this will occur at different times of the year at different latitudes. Climate can have big effects in very small areas, the most dramatic indication of the effects of the climate are arctic and mountain treelines, Here the edge of a species range is clearly visible. Similar 'drought lines' can occur at the edges of deserts or 'wet lines' near a marsh, where the ground gets too wet for trees (or many other plants) to tolerate.
Land use and habitat. Land use and habitat are broadly the same thing and have an obvious effect on plant ranges: A species will not be found in unfavourable habitat, for example, agricultural weeds are found on agricultural land, Bluebells and understory shade plants in woodland, etc.... The prescence or absence of habitat is usually fairly obvious, if a species range follows the available habitat closely then this is often apparent on maps. Habitat fragmentation may be less obvious, Good dispersal ability is important in avoiding the effects of Habitat fragmentation.
Competition. Competition with other species can influence plant ranges: In the absence of competitors most species would be found over much wider areas. In fact competition interacts with all the other factors, most species ranges are limited when the climate, or soil conditions, becoming increasingly more adverse reach the point where the species is unable to thrive enough to stave off its competitors. A spot free of competition is very rare in the wild, but management in gardens allows many species to be grown far outside natural ranges and habitats.
Herbivory. Herbivory can be, like competition, an interaction with another factor, reducing a species ability to survive until it cannot cope with herbivory. Remove this or competitors and most ranges would shift outwards a fair bit. A mountain Herb of the Alps, Arnica montana, is restricted to higher altitudes by slug herbivory. On lower slopes the new shoots of the plant coincide with peak slug activity, higher up where it is colder the slug activity is reduced and the plant is better sable to survive. Note the indirect effect of climate. This effect may be common on mountain slopes, and Insect herbivores can also have a significant effect.
Dispersal ability. Another major restriction on plant ranges is the species own ability to travel to new sites for colonisation. Plants have a great many methods of dispersal, but great mountain range or ocean will prevent dispersal completely for most species. A great any species are restricted to their continent of origin, lacking the means to leave it. Any area of unsuitable habitat too large to cross can act as a 'barrier to dispersal', e.g. an area of intense herbivory or competition. Many plants are dispersed by animals, and the ranges, dispersal ability, and any extinction of these will have a profound effect on dispersal. The effect of climate, and many of the other factors affects dispersal ability first, -in the two examples above it is reproductive structures that are affected, preventing the plant from dispersing further, and this has long been known to almost always be the case. These structures are sensitive, and take a second priority to survival under adverse conditions. Many plant species are self incompatible, meaning an individual is unable to fertilise its own seed, this can effectively stop dispersal except by vegetative growth, if there are not enough individuals, This will be discussed in more detail later!
Genetic factors. Low genetic diversity can lead to inbreeding depression, the over expression of harmful deleterious mutations, as the number of individuals with two copies (homozygous) of these genes increases. The harmful effects of inbreeding can reduce a species ability to compete or cope with adverse conditions at a range edge, but for this to occur there needs to be almost no pollen or seed flow into these populations, and this is probably going to be due to something else. All populations get sparser towards range limits, as a species can only survive in progressively more ideal locations to cope with the stresses, making dispersal between them more difficult, rarer plants support less pollinators, etc.. 
|Dzīvnieku pielāgojumi (eng)|
|Augu pielāgojumi (eng)|
|Augu izplatīšanās (eng)|
Pēdējo reizi labots: 21.01.2007
Autore: Elīna Brice