Читать книгу Ecology - Michael Begon - Страница 33

Although a particular type of organism is often characteristic of a particular ecological situation, it will almost inevitably be only part of a diverse community of species. A satisfactory account, therefore, must do more than identify the similarities between organisms that allow them to live in the same environment – it must also try to explain why species that live in the same environment are often profoundly different. To some extent, this ‘explanation’ of diversity is a trivial exercise. It comes as no surprise that a plant utilising sunlight, a fungus living on the plant, a herbivore eating the plant and a parasitic worm living in the herbivore should all coexist in the same community (food webs will be discussed in Chapter 17 and the flow of energy and nutrients through ecosystems in Chapters 20 and 21). On the other hand, most communities also contain a variety of different species that are all constructed in a fairly similar way and all living (at least superficially) a fairly similar life. We have seen excellent examples among the finches of the Galápagos (Figure 1.9), the cichlid fish of Lake Apoyo (Figure 1.11), the Howea palms of Howe Island (Figure 1.12), and the picture‐winged fruit‐flies of Hawaii (Figure 1.16). There are several elements in an explanation of this diversity.

environments are heterogeneous

A completely homogeneous environment might well become dominated by one or a very few species that are well adapted to the conditions and resources there. But there are no homogeneous environments in nature. Even a continuously stirred culture of microorganisms is heterogeneous because it has a boundary – the walls of the culture vessel – and cultured microorganisms often subdivide into two forms: one that sticks to the walls and the other that remains free in the medium.

The extent to which an environment is heterogeneous depends on the scale of the organism that senses it. To a mustard seed, a grain of soil is a mountain; and to a caterpillar, a single leaf may represent a lifetime’s diet. A seed lying in the shadow of a leaf may be inhibited in its germination while a seed lying outside that shadow germinates freely. What appears to the human observer as a homogeneous environment may, to members of species within it, be a mosaic of the intolerable and the adequate.

There may also be gradients in space (e.g. altitude) or gradients in time, and the latter, in their turn, may be rhythmic (like daily and seasonal cycles), directional (like the accumulation of a pollutant in a lake) or erratic (like fires, hailstorms and typhoons).

Heterogeneity crops up again and again in later chapters – in part because of the challenges it poses to organisms in moving from patch to patch (Chapter 6), in part because of the variety of opportunities it provides for different species (Chapters 2 and 3), and in part because heterogeneity can alter communities by interrupting what would otherwise be a steady march to an equilibrium state of a few species (Chapters 8 and 18).

pairs of species

It is important to note that 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 parts (e.g. skin or 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, behaviour 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 8–10 and 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 specialisation – an ever closer match.

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. 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 Darwin's finches 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, and take them up again in Chapters.

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.