The role of biodiversity in providing ecosystem services is actively debated in ecology. The diversity of functional groups (groups of ecologically equivalent species (Naeem and Li 1997)), is as important as species diversity, if not more so (Kremen 2005), and in most services a few dominant species seem to play the major role (Hooper et al. 2005). However, many other species are critical for eco system functioning and provide “insurance” against disturbance, environmental change, and the decline of the dominant species (Tilman 1997; Ricketts et al. 2004; Hobbs et al. 2007). As for many other ecological processes, it was Charles Darwin who first wrote of this, noting that several distinct genera of grasses grown together would produce more plants and more herbage than a single species growing alone (Darwin 1872). Many studies have confirmed that increased biodiversity improves ecosystem functioning in plant communities (Naeem and Li 1997; Tilman 1997). Different plant species capture different resources, leading to greater efficiency and higher productivity (Tilman et al. 1996). Due to the“sampling-competition effect” the presence of more species increases the probability of having a particularly productive species in any given environment (Tilman 1997). Furthermore, different species’ ecologies lead to complementary resource use, where each species grows best under a specific range of environmental conditions, and different species can improve environmental conditions for other species (facilitation effect; Hooper et al. 2005). Consequently, the more complex an ecosystem is, the more biodiversity will increase ecosystem function, as more species are needed to fully exploit the many combinations of environmental variables (Tilman 1997). More biodiverse ecosystems are also likely to be more stable and more efficient due to the presence of more pathways for energy flow and nutrient recycling (Macarthur 1955; Hooper et al. 2005; Vitousek and Hooper 1993; Worm et al. 2006). Greenhouse and field experiments have confirmed that biodiversity does increase ecosystem productivity, while reducing fluctuations in productivity (Naeem et al. 1995; Tilman et al. 1996).
Although increased diversity can increase the population fluctuations of individual species, diversity is thought to stabilize overall ecosystem functioning (Chapin et al. 2000; Tilman 1996) and make the ecosystem more resistant to perturbations (Pimm 1984). These hypotheses have been confirmed in field experiments, where speciesrich plots showed less yearly variation in productivity (Tilman 1996) and their productivity during a drought year declined much less than species poor plots (Tilman and Downing 1994). Because more species do better at utilizing and recycling nutrients, in the long-term, species-rich plots are
better at reducing nutrient losses and maintaining soil fertility (Tilman et al. 1996; Vitousek and Hooper 1993). Although it makes intuitive sense that the species that dominate in number and/or biomass are more likely to be important for ecosystem function (Raffaelli 2004; Smith et al. 2004), in some cases, even rare species can have a role, forexample, in increasing resistance to invasion (Lyons and Schwartz 2001). A keystone species is one that has an ecosystem impact that is disproportionately large in relation to its abundance (Hooper et al. 2005; Power et al. 1996;). Species that are not thought of as “typical” keystones can turn out to be so, sometimes in more ways than one (Daily et al. 1993). Even though in many communities only a few species have strong effects, the weak effects of many species can add up to a substantial stabilizing effect and seemingly “weak” effects over broad scales can be strong at the local level (Berlow 1999). Increased species richness can “insure” against sudden change, which is now a global phenomenon (Parmesan and Yohe 2003; Root et al. 2003). Even though a few species may make up most of the biomass of most functional groups, this does not mean that other species are
unnecessary (Walker et al. 1999).
better at reducing nutrient losses and maintaining soil fertility (Tilman et al. 1996; Vitousek and Hooper 1993). Although it makes intuitive sense that the species that dominate in number and/or biomass are more likely to be important for ecosystem function (Raffaelli 2004; Smith et al. 2004), in some cases, even rare species can have a role, forexample, in increasing resistance to invasion (Lyons and Schwartz 2001). A keystone species is one that has an ecosystem impact that is disproportionately large in relation to its abundance (Hooper et al. 2005; Power et al. 1996;). Species that are not thought of as “typical” keystones can turn out to be so, sometimes in more ways than one (Daily et al. 1993). Even though in many communities only a few species have strong effects, the weak effects of many species can add up to a substantial stabilizing effect and seemingly “weak” effects over broad scales can be strong at the local level (Berlow 1999). Increased species richness can “insure” against sudden change, which is now a global phenomenon (Parmesan and Yohe 2003; Root et al. 2003). Even though a few species may make up most of the biomass of most functional groups, this does not mean that other species are
unnecessary (Walker et al. 1999).
Species may act like the rivets in an airplane wing, the loss of each unnoticed until a catastrophic threshold is passed (Ehrlich and Ehrlich 1981b). As humanity’s footprint on the planet increases and formerly stable ecosystems experience constant disruptions in the form of introduced species, pollution , climate change, excessive nutrient loads, fires, and many other perturbations, the insurance value of biodiversity has become increasingly vital over the entire range of habitats and systems, from diverse forest stands sequesterin CO2 better in the long-term (Bolker et al. 1995; Hooper et al. 2005; but see Tallis and Kareiva 2006) to forest-dwelling native bees’ coffee pollination services increasing coffee production in Costa Rica (Ricketts et al. 2004;). With accelerating losses of unique species, humanity, far from hedging its bets, is moving ever closer to the day when we will run out of options on an increasingly unstable planet.
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