Coexistence of similar species

While it is no surprise that species with rather different roles coexist within the same community, it is also generally the case that communities support a variety of species performing apparently rather similar roles. The Antarctic seals are an example. It is thought that the ancestral seals evolved in the northern hemisphere, where they are present as Miocene fossils, but one group of seals moved south into warmer waters and probably colonized the Antarctic in the Late Miocene or Early Pliocene (about 5 million years ago). When they entered the Antarctic, the Southern Ocean was probably rich in food and free from major predators, as it is today. It was within this environment that the group appears to have undergone radiative evolution (Figure 1.21). For example, the Weddell seal feeds primarily on fish and has unspe-cialized dentition; the crab-eater seal feeds almost exclusively on krill and its teeth are suited to filtering these from the sea water; the Ross seal has small, sharp teeth and feeds mainly on pelagic squid; and the leopard seal has large, cusped, grasping teeth and feeds on a wide variety of foods, including other seals and, in some seasons, penguins.

Do these species compete with one another? Do competing species need to be different if they are to coexist? If so, how different do they need to be: is there some limit to their similarity? Do species like the seals interact with one another at the present time, or has evolution in the past led to the absence of such interactions in contemporary communities? We return to these questions about coexisting, similar species in Chapter 8.

Even at this stage, though, we may note that coexisting species, even when apparently very similar, commonly differ in subtle ways - not simply in their morphology or physiology but also in their responses to their environment and the role they play within the community of which they are part. The 'ecological niches' of such species are said to be differentiated from one another. The concept of the ecological niche is itself explained in the next two chapters.

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