4 Ecology, Concept and Theories in Figure Interaction webs demonstrating three types of indirect interaction: (a) apparent competition; (b) predator-mediated competition; and (c) trophic cascade, as well as (d) a more realistic, species-rich interaction web depicting the major interactors associated with fireweed at Mt St Helens, in Washington competing because, as the population of one species grows, the abundance of the other declines However, these changes in abundance are not caused by direct competition, but rather by a third species that consumes each prey species in proportion to its relative abundance (see Figure 1a) A second type of indirect interaction occurs when a consumer species indirectly affects a species via its competitor For example, a consumer that limits a competitively dominant species may indirectly benefit a competitively inferior species (see Figure 1b) A third type of indirect effect thought to be common and important is a trophic cascade An example of a trophic cascade is when a carnivore species limits the abundance of herbivores, resulting in luxuriant plant growth (see Figure 1c) The presence of a trophic cascade is demonstrated when removal or exclusion of the carnivore results in a dramatic increase in the herbivore population and the subsequent decimation of plant biomass Field studies are increasingly documenting the ubiquity and importance of indirect interactions However, the difficulties encountered when studying the simplified food webs depicted in Figure 1a–c are greatly exacerbated when trying to understand the interactions occurring in more realistic, species-rich food webs (see Figure 1d) The significance of this is that simple ecological experiments that focus on just one or two species may fail to reveal the important determinants of community structure In sum, theories of species interaction have largely focused on the problem of coexistence or, as it has also been called, the paradox of diversity These theories address how so many species manage to coexist in nature, whereas simple lab experiments and mathematical models so often predict extinction A major shift in thinking about this problem was the recognition that natural populations are subjected to environmental variability and other disturbances that can prevent the process of competitive exclusion In addition, the complexities of multispecies interactions, including indirect interactions, can promote the coexistence of species Although the earliest models of species interactions seem abstract and disconnected from reality, ecology has a rich tradition of using experiments to test models For example, the Russian biologist G F Gause pioneered the testing of speciesinteraction models using laboratory cultures of protozoans and yeast in which he could follow population dynamics on the time-scale of weeks, and thus could rapidly see whether or not a species could persist in his experimental systems When examining competition between Paramecium species, Gause often observed that some species could drive others to extinction, a process termed competitive exclusion (and a result predicted in theory by the differential equations that depict continuous reproduction of two protozoans in competition for the same resource) Currently, laboratory microcosms involving protozoan communities are receiving fresh attentionF nowadays these experiments are aimed at testing models of multispecies interactions and interaction webs Theories of Biodiversity Biodiversity is the variety of all living things, and most commonly refers to the number of different species in a particular