WHY THE WORLD IS GREEN, THE WATERS ARE BLUE AND FOOD WEBS IN SMALL STREAMS IN THE ATLANTIC RAINFOREST ARE PREDOMINANTLY DRIVEN BY MICROALGAE?
Keywords:Food web, Herbivore control, Plant defense, Stable isotopes, Stream cosystem.
The question “Why is the world green?” has been debated and researched ever since the seminal paper of Hairston et al. (1960). Three main hypotheses were generated to explain why a large part of terrestrial plant biomass is not eaten by herbivores: (1) predators control herbivores (especially folivores), (2) plant defenses inhibit herbivores, and render the plant biomass relatively unavailable, and (3) different controls operate in regions of different productivity. Aquatic systems, especially those of plankton, tend to have much less plant biomass than terrestrial systems, and generally a much higher proportion of the living plant biomass is consumed by herbivores. Thus open-water aquatic systems appear transparent blue or green. They also often display cascading relationships in which changes at one trophic level have effects at two or more levels below. The orthodoxy for small streams shaded by forest is that they receive much organic matter from the surrounding forest and this provides the main source of energy and material for the food web. Some recent search in tropical streams shows that microalgae provide a greater proportion of the primary source than does allochthonous material. This may be a specifically tropical phenomenon, or perhaps a tendency that is more accentuated in the tropics. Exclusion experiments show trophic cascades and strong interactions between fauna (particularly shrimps) and allochthonous substrates. But the functional interactions (shredding of litter, removal of benthic material) are not trophic -- the animals are herbivores or predators of herbivores based on microalgae. We can speculate that the apparent wastage of allochthonous material is at least partly due to the inherent costs that a detritivore would have in processing the more intractable and less nutritive food source (litter) in conditions of higher predation and competition, which are possibly more stringent in the tropics.