Mechanisms of human impacts on biodiversity have been grouped into overexploitation of wild living resources; expansion of agriculture, forestry, or aquaculture; habitat loss and fragmentation; indirect negative effects of species introduced by humans; pollution; and global climate change (McNeely et al. 1995). Because HANPP is an indicator for changes in terrestrial ecosystems caused by land use, it refers mostly to expansion of agriculture, forestry, or aquaculture and habitat loss and fragmentation, which are closely related.
On an abstract level it is obvious why HANPP is relevant for biodiversity. Biomass is the mass of living or dead organisms present in a system. The very idea of the production—ecological (or trophic—dynamic) process in ecosystems (Lindemann 1942) is an abstract notion for organisms coming into being, growing, and dying. This process is fueled by various metabolic processes taking place within organisms. Energy enters organisms above all through two processes: photosynthesis and ingestion of dead or living organisms or parts thereof. Human-induced changes in this process affect patterns
(including biodiversity), processes, functions, and services of ecosystems almost by definition.
Vitousek et al. (1986:368) put it as follows: "Homo sapiens is only one of perhaps 5-30 million animal species on Earth, . . . yet it controls a disproportionate share of the planet's resources. . . . NPP provides the basis for maintenance, growth, and reproduction of all heterotrophs (consumers and decomposers); it is the total food resource on Earth. We are interested in human use of this resource . . . for what it implies for other species, which must use the leftovers." Discussing their finding that humans appropriate about 40 percent of global terrestrial NPP, they add, "People and associated organisms use this organic material largely, but not entirely, at human direction, and the vast majority of other species must subsist on the remainder. An equivalent concentration of resources into one species and its satellites has probably not occurred since land plants first diversified. The co-option, diversion, and destruction of these terrestrial resources clearly contributes to human-caused extinctions of species and genetically distinct populations."
It has been stated that "'sustainability' might be interpreted as the maintenance of a level of biological diversity that will guarantee the resilience of ecosystems that sustain human society. The goal of a conservation strategy should be to protect not all biodiversity in some areas, but biodiversity thresholds in all areas" (Folke et al. 1996:1021). Theoretical considerations indicate that a sufficient amount of energy remaining in the ecosystem is necessary for ecosystems to be resilient (Kay et al. 1999). HANPP might impede ecosystem services and thus sustainability: "To the extent that . . . natural systems, species and populations provide goods or services that are essential to the sustainability of human systems, their shrunken base of operations must be a cause of concern" (Vitousek and Lubchenko 1995:60).
It is not easy to be more specific, though: How exactly are biodiversity, resilience, or other properties of ecosystems related to HANPP? Almost 20 years after Vitousek's famous article, disappointingly little is known, mostly because most ecological work is focused on systems with little human impact (McDonnell and Pickett 1997) and because of the lack of generally agreed-upon ecological theories in that field (Brown 1995). Nevertheless, attempts have been made, based on the so-called species-energy hypothesis, to evaluate the potential effect of HANPP on species richness (Wright 1987, 1990).
The species-energy hypothesis (Brown 1981, 1995; Gaston 2000; Hutchinson 1959; Wright 1983; Wright et al. 1993) suggests that more available energy should allow more species to coexist, resulting in a positive relationship between energy availability and species diversity. Mechanisms behind this pattern could be a finer subdivision of resources (specialization) in richer environments, density-dependent regulation of population size (costs of commonness; Brown 1981), or the fact that more resources allow more organisms to live in a defined location. This greater number of organisms is more likely to belong to a larger number of species than fewer organisms would (Hubbell
2001). Irrespective of the mechanism, the species—energy hypothesis implies that the number of heterotroph species present in an ecosystem is related to the amount of energy remaining in the system (i.e., NPPt) because this is the amount of energy potentially available for all food chains. According to the species—energy hypothesis, HANPP contributes to species loss because it reduces NPPt (Wright 1987, 1990).
One problem is that the specific mathematical relationship between species diversity and energy flows is uncertain. Some believe that there is a monotonous relationship such as S = c. E z between energy flow (E) and species richness (S) (Currie and Paquin 1987; Lennon et al. 2000; Weiher 1999; Wright 1983; Wright et al. 1993), whereas others favor unimodal (hump-shaped) species—energy curves (Rosenzweig and Abramsky 1993; Rapson et al. 1997). A recent review found that linear and unimodal patterns seem to be found about equally often (Waide et al. 1999). In the first case, HANPP should always result in species loss, whereas in the second case intermediate levels of HANPP could increase species richness.
Although the ability of HANPP to aggregate various processes increases its utility for many purposes, it also means that it is associated with a host of different changes in ecosystems. For example, in central Europe, where climax vegetation is mostly forest, introducing agriculture increases habitat diversity and thus should favor species richness according to the habitat diversity hypothesis. This hypothesis claims that environmental heterogeneity promotes species richness (Gaston and Blackburn 2000; Hubbell 2001; MacArthur and MacArthur 1961; Levin and Paine 1974). The interpretation of correlations between HANPP and species richness is hampered by such effects, which are difficult to control.
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