Cellulose Centered Perspective on Terrestrial Community Structure

Abstract

How can the earth be so green when it teems with plant-eating animals ranging in size from aphids to elephants, including half of the ca. 800,000 species of insects which accounts for one-quarter of living species excluding algae and microorganisms (Strong et al. 1984)? Plant-eating animals kill more plants than drought or logging does, yet they do not wipe out the entire plant community (Howe and Westley 1988). As for the evolution of plants in relation to phytophagous animals, especially insects, much attention has been paid to the chemical defenses that plants exert with their secondary compounds (Ehrlich and Raven 1964, Whittaker and Feeny 1971, Feeny 1975, Rosenthal and Janzen 1979, Strong et al. 1984, Howe and Westley 1988). In contrast, very little attention has been paid to the fact that most animals, unlike certain microorganisms (Nielsen 1962, Janzen 1981, Begon et al. 1986), lack the ability to produce the enzymes necessary for decomposing cellulose, the primary cell-wall component of higher plants and the most abundant organic compound on earth. The limited ability to digest cellulose places a definite limit on the consumption of plants. Thus, cellulose provides a basic defense for plants. The green on earth would be best preserved if a living plant is not consumed by heterotrophs, rather consumed promptly upon death thereby returning nutrients back to the living portion of plants. Therefore, the spatial distribution pattern of cellulolytic organisms (organisms that can digest cellulose) plays a crucial role. In terrestrial communities, microorganisms that can produce cellulases, the cellulose-decomposing enzymes, are largely confined to the soil and thus, in most cases, they decompose the cellulose not in living plants but in fallen dead plants; the important exception being fungi that break down the roots of living trees (Bell 1981). This restricted distribution and feeding habit of cellulose-digesting organisms, supplemented with chemical defenses by secondary compounds, may possibly provide one answer to the question: how can the earth be so green? Hairston et al. (1960) addressed this question of a green earth and concluded that green plants of the earth are abundant because populations of herbivores are controlled by predation. This general conclusion was criticized by Murdoch (1966) and Ehrlich and Birth (1967); in particular, Murdoch pointed out that all of the green plant material may not be edible. Those recent studies on the chemical defenses of plants to which we have already referred support Murdoch's point. The argument that we have just presented is one possible answer to the question of a green earth. It extends Murdoch's view by identifying cellulose as a capsule that protects plants while alive and lets them dissolve upon death. The real situation is, however, more complicated than the general picture that we have described, mainly because of the associations that microorganisms make with some animals, such as termites and other cellulosedigesting insects, which expand beyond soil the distribution of microorganisms. There are also those exceptional microbes that break down living plant tissue. After examining the significance of cellulose, we will return in the last section with a new insight to this basic problem of the maintenance of a green earth. Here a remark is in order: there may not necessarily be a single answer to this problem, and those different hypotheses that have been proposed and the one that we will present in the last section are not mutually exclusive. Thus, it is possible, and we suspect very likely, that these causes work together for the maintenance of the green plants of the earth, or different causes act in varied habitats on the earth.