As we have already noted, the existence of one type of organism in an area immediately diversifies it for others. Over its lifetime, an organism may increase the diversity of its environment by contributing dung, urine, dead leaves and ultimately its dead body. During its life, its body may serve as a place in which other species find homes. Indeed, some of the most strongly developed matches between organisms and their environment are those in which one species has developed a dependence upon another. This is the case in many relationships between consumers and their foods. Whole syndromes of form, behavior and metabolism constrain the animal within its narrow food niche, and deny it access to what might otherwise appear suitable alternative foods. Similar tight matches are characteristic of the relationships between parasites and their hosts. The various interactions in which one species is consumed by another are the subject matter of Chapters 9-12.
Where two species have evolved a mutual dependence, the fit may be even tighter. We examine such 'mutualisms' in detail in Chapter 13. The association of nitrogen-fixing bacteria with the roots of leguminous plants, and the often extremely precise relationships between insect pollinators and their flowers, are two good examples.
When a population has been exposed to variations in the physical factors of the environment, for example a short growing season or a high risk of frost or drought, a once-and-for-all tolerance may ultimately evolve. The physical factor cannot itself change or evolve as a result of the evolution of the organisms. By contrast, when members of two species interact, the change in each produces alterations in the life of the other, and each may generate selective forces that direct the evolution of the other. In such a coevolutionary process the interaction between two species may continually escalate. What we then see in nature may be pairs of species that have driven each other into ever narrowing ruts of specialization - an ever closer match.
Figure 1.21 Antarctic seals, similar species that coexist: (a) the Weddell seal, Leptonychotes weddellii (© Imageshop - zefa visual media uk ltd/Alamy), (b) the crab-eater seal Lobodon carcinophagus (© Bryan & Cherry Alexander Photography/Alamy), (c) the Ross seal, Omatophoca rossii (© Chris Sattlberger/Science Photo Library), and (d) the leopard seal, Hydrurga leptonyx (© Kevin Schafer/Alamy).
Was this article helpful?