Top-down control of ecosystems and the case for rewilding: does it all add up?

In its simplest form, ‘top-down’ control refers to directional regulation within an ecosystem, where species occupying higher trophic levels exert controlling influences on species at the next lower trophic level (Terborgh et al., 1999). Thus, top-down control can describe top predators controlling smaller predators or prey, or herbivores exerting a controlling influence on plant biomass. By contrast, ‘bottom-up’ control refers to abiotic resources and species at the lowest trophic level (producers) regulating the abundance of species at the next highest trophic level (herbivores), which in turn can influence species at higher trophic levels (predators). The top-down control hypothesis was first proposed by Camerano (1880) but refined by Hairston and colleagues (1960), who argued that herbivores, under usual conditions, cannot be limited by either weather or food, and must therefore be limited by predation (i.e. top-down control). This somewhat simplistic view triggered much debate (e.g. Murdoch, 1966; Ehrlich and Birch, 1967; Slobodkin et al., 1967), highlighting many exceptions to, and logical gaps in, the original hypothesis. To this day, the relative contributions of both top-down and bottom-up forces in shaping terrestrial, marine, and freshwater aquatic ecosystems are hotly debated (Sih et al., 1985; Linnell and Strand, 2000; Elmhagen and Rushton, 2007; Laundré et al., 2014), illustrating that causal relationships between species and their limiting factors are far more complex than the simplistic construct first suggested.