With the exception of humans and some of the species associated with us, like rats, all species are limited in their distribution. Tigers are only found in Asia, kangaroos in Australia. Some species are limited to single, small islands. The objective today is to investigate why this is so. In detail:
If we choose a particular species, plot a point on a map of the world for every place in which it is found, and then draw a line surrounding all of the point, we will have delimited its geographical range. Some species, such as the Devil's Hole pupfish that only lives in one particular desert spring, have a geographical range of just a few square meters; others (like humans and their rat and cockroach friends) range over most of the land area of the earth. Most species, though, have distributions somewhere between these two extremes
Usually distributions are plotted on a map with a grid, like this, showing the distribution of Agenium leptocladum:
(from http://www.virtualherbarium.org/biog/bioglec3.html, © G.F. Guala.)
This is mapping on a fairly large scale. Smaller scales are possible. In Britain, the tradition is to map plant distribution in 2km x 2km squares within a county, as you will see in the assignment which looks at distribution in Shropshire.
Here is an example showing the distribution of Impatiens glandulifera.
(from Sinker CA (1985) An Ecological Flora of the Shropshire region.)
The simple answer to this, of course, is that species are only found where environmental conditions are favourable to them - where the amount of rainfall is suitable, for example, or the temperature. This helps to explain the wide distribution of humans: we have largely freed ourselves from these constraints.
Less obvious points are also important. Firstly, no matter how favourable an area is, a species can only occupy it if it can get there - a question of dispersal.
Secondly, the biological components of an area must also be suitable. Predation, competition and food supplies are all important, as is the presence of a host species for a parasitic organism.
If we concentrate for now on the first point, that of environmental conditions (also called environmental factors) we can ask which conditions are important.
Heinz Ellenberg, who died in 1997, was a German botanist who devoted much of his life to investigating the environmental factors which affected Central European plants. His scheme has recently been adapted for British species, and allows us to identify a list of important factors.
The five factors used in the British scheme are Light, Moisture, Reaction (pH), Nitrogen (a measure of soil fertility) and Salt.
What Ellenberg did was to test the tolerance of plant species to each of these factors and give them a value on a scale. The values are indicator values, and they indicate the ecological conditions favoured by a particular species. A single plant species can only tolerate a certain range of temperature or soil pH or light levels. This tolerance can be represented by a number, and this is the Ellenberg value. For example, on the scale for pH, Rhododendron ponticum has the value 3, and Scabiosa columbaria has the value 8. This tells us that Rhododendron prefers an acid soil, and small scabious an alkaline soil.
It is very important to note that the values are numbers on an arbitrary scale. They do not represent actual pH values.
Each of the other factors has its own scale, and a plant is given a value on each. A single plant species therefore ends up with five numbers. As an example, here is the set of values for Sycamore, Acer pseudoplatanus.
| Light | Moisture | Reaction | Nitrogen | Salt |
| 4 | 5 | 6 | 6 | 0 |
So, as a first level of understanding distribution, we could say that a plant like sycamore will only occur where each of these environmental factors is suitable for it. A simple example for Agenium leptocladum is shown here.
How do these factors or conditions operate ? As before, there is a simple answer. Any individual which is dispersed to an unfavourable area is killed. This can certainly happen, and the factor is then operating as a lethal factor. A good example is frost, which will kill any non-tolerant plant species growing in the wrong place.
However, there are also more subtle effects. Sub-lethal conditions can also limit a species range. This can happen
because growth or reproduction rate is so limited that species can't maintain a viable population. Example, wild madder and the 4.5ºC January isotherm. Here
growth or reproduction is so limited that a species becomes a poor competitor. Whilst it could survive in the absence of competitors, it is not able to compete effectively.
This is a common phenomenon with garden plants.
Also seen in the barnacle Balanus balanoides. This is killed by summer air temperatures of 25ºC. On the Pacific coast of America it lives 1600km north of such a limit. Further movement south is thought to be limited by competition - Balanus becomes a poorer competitor as temperatures rise.
Indirect effects are also possible. The moth Coleophora altiocoletta feeds on the seeds of the heath rush as a larva. It is not found above 600m. Above this the rush produces flowers and the moth lays eggs on them. But it is too cold for seed production, so the larvae starve. The moth is being indirectly limited by temperature.
A further subtlety is that the effect of a factor differs as a species
approaches the edge of its range. As this happens, it may become more and more restricted to favourable patches or microclimates.
The rufous grasshopper reaches its northern limit in southern Britain. Here it can live only on steep, south-facing slopes. In Europe it can live on any kind of slope.
Mountain avens is a mountain-top plant in North Wales. In Northern Scotland, where the climate is colder, it can live at any height.
The existence of environmental conditions and their effects on distribution and abundance led to a second definition of the ecological niche in addition to that of Elton.
Elton's niche, remember, considered the role of a species in the community.
Hutchinson's niche, named after the American ecologist GE Hutchinson, (photo
taken from
http://www.nceas.ucsb.edu/~alroy/lefa/Hutchinson.html) considers the distribution limits of a species.
Its formal definition is: a multidimensional hypervolume of resource axes.
This can be explained with diagrams.
Each condition is treated as the axis of a graph.The combination of all conditions which affect distribution represents a volume on the graph which defines where the species can live.
Hutchinson's niche defines where a species is physically able to live. Whether it actually occupies that space depends on two things:
firstly,
whether it can reach it, given that there are often barriers separating one
suitable habitat area from another. One factor that limits the distribution of a species is
its dispersal ability, i.e., how well individuals or their offspring can
move from place to place. Obviously in the case of the Devil's Hole pupfish, the only route to other waters is overland
through the Mohave desert:an unlikely journey for a fish. Humans, on the other hand, have learned to transport themselves quickly to virtually any spot
on (and recently off) the planet. Desert bighorn sheep fall somewhere in between these extremes.
They prefer
rugged desert terrain and thus inhabit many of the hundreds of small, isolated mountain ranges of the southwestern United
States. Although bighorn are known to move across the flats between close mountain ranges, they don't really like flat open
areas and thus are less likely to move between mountain ranges that are far apart.
So the availability of suitable
habitats within a potentially traversable distance affects the distribution of a
species.
This allows us to illustrate the difference between fundamental and realised niche. Look at the handout showing rabbits in Australia, starlings and Mynah birds in North America
secondly, whether enemies (competitors or predators) prevent it from living there (example with the barnacles Chthamalus stellatus and Semibalanus balanoides)
Species therefore often occupy a smaller area than purely physical conditions allow. The possible area where they could live is called the fundamental niche, the actual area where they do live is called the realised niche. The realised niche can never be bigger than the fundamental niche, the fundamental niche is often bigger than the realised niche.
