Just one of the amazing aspects of life on Earth is that it spreads to all areas where the habitat will allow it to survive. In hostile environments, where the soil is infertile, and therefore cannot sustain life in a sustainable fashion, small plants and bacteria begin to colonise the area, which recognise the area as a new ecological niche without competition.
While these small organisms begin to occupy the area, the pioneers, the organisms that first colonised the area begin to die out. This essential process allows the dead mass to decompose into the ground, and essentially provide nutrients to plants that continue to live in the habitat. Now that their is more nutrients available in the local environment, larger species are able to be supported, and this process can have a multiplying effect over the longer term. This process is called succession.
Succession usually occurs in areas where no other species offer competition in the area. Succession, however, can occur in changing climates, where less suited species give way to the more evolutionary adapted species of the area. In a more stable environment, a climax community develops, the climax being that the community stays in a relative equilibrium over a long period of time. Succession is no longer an issue as the most advanced species have established their presence and lesser organisms cannot compete.
The type of organisms' that occupy areas in such circumstances depends on a number of factors;
Mainly temperature and precipitation, varying species require various amounts of sunlight and temperature to operate at their optimum level. The organisms' most suited to a particular environment, and the ones that can be found in the vicinity are most likely to be the pioneers, or at the climax of succession
Soil also plays a major factor in what organisms' occupy an ecosystem. pH is one major factor, as some plants prefer and acidic environment to alkaline, or vice versa. The composition of the soil also plays a role in determining which species' are suitable, as the soil may be sandy, loamy or clay.
Humans' have altered the worldwide environment on a huge scale, particularly since the industrial age. Factors such as pollution, like acid rain, can alter the way an ecosystem operates and effect factors within it. With this in light, humans play a major role in which organisms' are likely to succeed in an ecosystem.