The ecologist William J. Bond (in Journal of Vegetation Science 16, p. 261-266, 2005 „Large parts of the world are brown or black: A different view on the ‚Green World‘ hypothesis„) suggests asking these questions, when we next step into a grassland, savanna or shrubland: „What is the potential ecosystem here? Which consumers might prevent it from reaching climate potential? Which species exist in the assembly because of their ability to tolerate consumers? What are the key traits that enable them to do so?“At the beginning of his article he asks: „What are the primary determinants of vegetation types at a regional, or global scale? Why do forests dominate in some landscapes, grasslands in others? “ He insists that the usual answer is climate, especially availability of moisture and energy for plant growth. Soils modify plant available moisture and vary in nutrient supply and are also important regional determinants. This perspective is mentioned as the ‚green world‘ hypothesis. Polis (1999) argued that terrestrial vegetation is largely determinated by climate, locally modified by low-nutrient soils, with consumer-control sometimes occurring but localized in space and time.
Bond, contrary to Polis‘ analysis, believes that very large areas of the world are ‚consumer-controlled‘. He focuses on large mammals as biotic consumers (–> ‚brown world‘) and fire as an abiotic consumer (–> ‚black world‘). He draw upon Hairston et al. (1960) who proposed that patterns of vegetation are determined by consumption by animals. Herbivores are regulated by carnivores, pests and pathogens. If not, they would have major impacts on vegetation. Without this regulation „herbivore impacts would explode and the world would no longer be ‚green'“ (p. 261). According to Bond, this should be „a shock for a vegetation scientist, trained to analyse plant/soil/climate interactions as major structuring forces“.
Bond assesses the extent of consumer-controlled ecosystems, and therefore he uses the potential vegetation. He measures the vegetation of actual ecosystems against the ‚carrying capacity‘ of trees for a given climate and soil type. He searches for outliers in climate/vegetation correlations. He reminds us of Whittaker’s ordination of world ecosystems on a precipitation/temperature plane (p. 262). A small region of this plane supports grasslands, shrublands, woodlands or forests – i.e. ‚ecosystems uncertain. Ecosystem uncertain are climate zones in which either grassland, or one of the types dominated by woody plants occur (see Whittaker 1975). But above all, its spatial extent is large. He shows a map with the areas of the world falling into the ‚ecosystem uncerteain‘ envelope of Whittaker’s global biome ordination on temperature/precipitation gradients. These are parts of the world where strikingly different natural vegetation occurs in the same climate: large parts of the USA, Mexico, Europe and Africa (except the rain forests of the Kongo Basin, Northern Africa and the Sahara, large parts of South America south of the equator, India, large parts of East Asia, North- and East-Australia, also Southwest-Australia and parts of New Zealand.
According to Bond, the magnitude of consumer-control can be measured by the difference between actual and climate-limited potential biomass (p. 263). Bond et al. (2005) have simulated global vegetation using a Dynamic Global Vegetation Model (DGVM) and found „large disparities between climate-limited and actual vegetation.“ According to their model simulations, the world has the climate potential to support closed forests in an area about 56% of the terrestrial world. Currently cclosed forest vegetation covers 25%. „The areas most at variance with climate conform quite well with the tropical grasslands and savanna regions of Whittaker’s ‚ecosystem uncertain‘.“ Bond concludes: „The world is not as green as it should be“ (p. 263)
The next question Bond asks: „Which consumers are most influential in preventing ecosystems from reaching their climate-potential?“ (p. 263).
The first possible answer: megaherbivores as prime candidates for significant consumer-control of vegetation. Owen Smith (1988) suggested that megaherbivores would be particularly influential in shaping vegetation structure. According to Bond (p. 263), it’s possible because large animals can consume low quality diets (food quality requirements vary with body mass) and they are difficult and dangerous prey for predators. Critics of this hypothesis have questioned that the consumers can ever reduce plant biomass enough to significantly effect vegetation structure and composition. They argue, that the world is green because most plants are inedible, full of indigestable components such as cellulose and lignin (see Polis 1999).
The key point of Bond is the suggestion of fire as an alternate consumer of vegetation (p. 264), because it has properties and effects similar to those of its animal counterparts. „Fire consumes large amounts of plant material in irregular ‚bites‘ of varying intensity and in preferred seasons. Fire differs from mammalian herbivory in the important respect that it defoliates vegetation irrespective of quality […]. Fire thrives on just those features that make plants inedible to herbivores – high cellulose and lignin content and low nitrogen content. Indigestible plants are the ‚food‘ that feeds fires.“
After fire is included as a consumer, Bond calls the world „multicoloured“: „‚brown ‚where intense mammal grazing and browsing controls vegetation; ‚black‘ where vegetation is largely controlled by fire, or ‚green‘ where climate, and not consumers, limit woody biomass“ (p. 264). Because fire and large mammal grazers are competing consumers of vegetation, Bond raises the questions: Has more of the world become ‚black‘ since extirpation/extinction of the megafauna? Is the current extent of fire-controlled ecosystsms an artefact of megafaunal extinction?
Finally Bond’s last question (p. 264): How do community and assembly rules differ in consumer-controlled ecosystems? Following Hairston et al. (1960) who predict little competition between plant species in consumer-controlled ecosystems, Bond supposes that the presence of a species depend on its ability to persist in the face of losses to the dominant consumer. This contrasts with the generally sought answer of differing abilities to compete for resources or to escape competitors by dispersal. Life history attributes, together with the patterns of fire or herbivore cosumption, escpecially their frequency, are the structuring factors, at the same time interactions with other plant species are seldom positioned to explain community membership in frequently burnt ecosystems. „In short, the central concerns of ecologists in consumer-controlled ecosystem (where fire is the consumer) seem to be entirely different from resource-controlled ecosystems. An obsession with competition as the structuring force is replaced by an obsession with key life history traits and their fit to the prevailing fire regime.“
I am not sure, if Bond is absolutely right. If so, for example the vegetation in Southwest-Australia would be consumer-controlled. I have doubts that the heath-vegetation there is not at all climate-controlled. I can imagine, that anywhere (outside this region) could be some trees which could grow and reproduce there. But in respect of the actually existing species-pool, there are climatic constraints, wich prevent its tree species to grow in areas dominated by heath vegetation. In effect of a species-pool dependent climatic barrier, fire functions as the major structuring factor in vegetation types, which are assemblages of more or less discrete portions of the regional species-pool. Furthermore in Southwest-Australia are Eucalypt-Forests, which are ressource-controlled (e.g. the Karri). And the Jarrah-forest is a forest type where fire is an important ecofactor.
It doesn’t matter to me in what way Bond is right, but I like his questions: „What is the potential ecosystem here? Which consumers might prevent it from reaching climate potential? Which species exist in the assembly because of their ability to tolerate consumers? What are the key traits that enable them to do so?“ (p. 265).
Facing the cultural landscapes of Europe, the forests, pastures, meadows, more or less green urban spaces, gardens, parks and last but not least the national parks, this questions don’t lead me to appropriate answers. I could consider the humans as a very effective consumer, but this assumption would push me in a very ecocentric position. The anthropogenic factor is not only destructive or disturbing, even if it is very transformative. Humans do not only devour natural ecosystems driving them back to ever-decreasing isolated remnants. The world is not antagonistically polarized in a grey (cultural) world and a green, brown or black (natural) world. Through human agency ecoystems are changed, managed, transformed, also preserved. Novel (cultural-determined) ecosystems are created. What about these human-controlled, managed ecosystems? The vegetation structure of many landscapes in the world is altered by human behavior. Even the national parks and wilderness areas are an outcome of human agency, a product of a negotiation table, an issue once entered into negotations about preservation/conservation vs. exploitation/alteration. A world without human being would be fundamentally either green, brown or black, but cultural as the landcapes are, their ecosystem and vegetation are either more volatile or artificially stabilized, in most cases in one way or another, they are hybrid. So the world is tinted greyish-blue (see Fig. 1).
Fig. 1: Parts of the world are green, brown or black, large parts of the world are tinted greyish-blue.
Therefore I want to supplement some questions concerning the human dimension: Do we face an ecosystem which is either ressource-controlled or consumer-controlled? Is this ecosystem prevented by being ressource- or consumer-controlled through human behavior? In what way is this ecosystem influenced by humans? Why is the human factor prevalent? What are the motivations (pragmatic, economic, cultural, aesthetic, spiritual or religious reasons) to alter or control this ecosystem?
Fig. 2: A multi-colured view of the world (after Bond 2005, p. 264, altered). According to Bond, communities in any locality may have elements of all three possible ecosystems states depending on the history, magnitude, and type of consumer-control. The resource base influences the probability of transitions from one state to another. For example, nutrient-poor soils would tend to reduce mammal herbivory, favouring fire and ‚black world‘ species. I have supplemented the human factor so that there appears a fourth ecosystem state, the human altered- or controlled-ecosystem.
Bond, W. J. (2005): Large parts of the world are brown or black: A different view on the ‚Green World‘ hypothesis. Journal of Vegetation Science 16: 261-266. This artile is available for free (here is the link)
Bond, W. J.; Woodward, F. I. & Midgley, G. F. (2005): The global distribution of ecosystems in a world without fire. New Phytologist 165: 525-538.
Hairston, N.; Smith, F. & Slobodkin, L. (1960): Community structure, population control and competition. American Naturalist 94: 421-425.
Owen-Smith, R. N. (1988): Megaherbivores: the influence of very large body size on ecology. Cambridge University Press, Cambridge, UK.
Polis, G. A. (1999): Why are parts of the world green? Multiple factors control productivity and the distribution of biomes. Oikos 86: 3-15.
Whittaker, R. H. (1975): Communites and ecosystems. 2nd. ed. Collier MacMillan, London UK.