Question

Explain how species diversity varies with latitude and area, and discuss the key factors that explain those gradients, recognizing that none of them on their own explains all patterns of global diversity  

Differentiate between primary and secondary succession, and explain the processes of facilitation, inhibition and tolerance in succession patterns

Answer 1

1)The increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal diversity gradient (LDG), is one of the most widely recognized patterns in ecology. Put another way, in the present day localities at lower latitudes generally have more species than localities at higher latitudes.

The key factors that explains the gradients depends upon Species richness ultimately depends on whatever proximate factors are found to affect processes of :

• speciation,
• extinction,
• immigration, \
• emigration.

2)Differentiate between primary and secondary succession

1. Primary succession takes a long time for completion,1000 years or more where as secondary succession takes a less time for completion,50 to 200 years
2. Primary succession occurs in an area which is lifeless where as secondary succession occurs in an area which has been previously inhabited
3. Soil is absent at the beginning of Primary succession where as soil is present at the beginning of secondary succession
4. Intermediary seral communities are many in Primary succession as it takes long time where as in secondary succession communities are a few
5. Pioneer community in Primary succession comes from outside where as in secondary succession Pioneer community develops partly from previous occupants and partly from migrants
6. In Primary succession ,Reproductive structures of any previous community are absent where as in secondary succession it is present.

THE FACILITATION MODEL.

Whether the conditions are those of primary or secondary succession, the outcome of the preceeding disturbance is such that resources are now widely available, but there is little competition for them. One way of describing this situation is through what is known as the facilitation model, which identifies "pioneer species" as those life-forms most capable of establishing a presence on the site of the disturbance.

Pioneers modify a site by their presence, for instance, by regenerating the soil with organic material, thus making the area more attractive for invasion by other species. Eventually, new species move in, edging out the pioneers as they do so. This process may repeat itself several times, until the ecosystem reaches the climax stage. At the climax stage, there are few biological "openings" for further change, and change is only very slight and slow—at least until another disturbance arises and starts the process over again.

THE TOLERANCE MODEL.

The tolerance model is another possible mechanism of succession. According to this concept, all species involved in succession are equally capable of establishing themselves on a recently disturbed site, but those capable of attaining a large population size quickly are most likely to become dominant. Unlike the facilitation model, the tolerance model does not depict earlier inhabitants as preparing the site biologically for new invader species; rather, this model is more akin to natural selection.

According to the tolerance model, some species will prove themselves more tolerant of biological stresses that occur within the environment as succession proceeds. Among these stresses is competition, and those species less tolerant of competition may succeed earlier on, when there is little competition for resources. Later in the succession process, however, such species will be eliminated in favor of others more capable of competing.

THE INHIBITION MODEL.

Yet another model of succession is the inhibition model, which, like the tolerance model, starts with the premise of an open situation at the outset: in other words, all species have equal opportunity to establish populations after a disturbance. In the inhibition model, however, some of the early species actually make the site less suitable for the development of other species. An example of this is when plants secrete toxins in the soil, thus inhibiting the establishment and growth of other species. Nevertheless, in time the inhibitory species die, thus creating opportunities that can be exploited by later successional species.