Page - 161 - in Biodiversity and Health in the Face of Climate Change

161 interact as a functional unit. Therefore, most parks and gardens require constant human interference (maintenance and wild management) to remain in existence in their present (desired) state. Actually, human influence is virtually omnipresent, but the level of this influence differs. Consequently, it is a matter of choice how much human interference is deemed acceptable to still consider a collection of plants, animals, insects and micro-organisms a natural ecosystem, or an ecosystem at all. Biodiversity  – here limited to species diversity  – has its primary focus on vari- ability in the biotic part of nature, the living nature. But even the concept of vari- ability may be interpreted differently. One interpretation is in terms of species richness, usually defined as the number of species in a certain area. This implies the notion that more (richer) is better. From an ecological perspective this interpretation has little value. Ecologists study communities that are linked to the ecosystem type present in the study area. Diversity here is seen more from a functional perspective. The question is to what extent the species diversity of an area contributes to the health of the ecosystem, with ecosystem health being defined in terms of sustain- ability and resilience (Costanza 2012). This leads to the following more specific questions: Is the species community complete, or are (key) species missing? Are population levels of the species above the viability level, so the species may expect to survive in the defined area in the long run? Are there sufficient species  – and suf- ficient individuals per species  – within a functional group (e.g. pollinators) to ensure functional traits (e.g. pollination) continue to be present, even under changing con- ditions (e.g. climate change)? Thus, desired diversity here is seen as a combination of species diversity (number of species, within each functional group) and species abundance (number of individuals per species). If key species are missing and popu- lation levels of existing species are below the viability level, one may define the ecosystem as degraded. If the diversity within a functional group is small, the eco- system may be considered vulnerable. It is important to note that some ecosystems require a higher number of species to be present in order to be considered healthier than others. Adhering to a strict, functional definition of biodiversity would require that first the applicable ecosystem is determined, and only subsequently the level of biodiversity at the species diversity level of those ecosystems is assessed. We already mentioned that the concept ‘biodiversity’ has its origin in ecology. A priori, there is no reason why it should be as relevant from a human health perspec- tive as it is from a nature conservation perspective. It may be too specifically geared towards its ecological purpose, as well as too crude from a public health perspec- tive. With regard to the latter, the composition of species that hides behind a certain level of biodiversity may be relevant with regard to its influence on mental health. This latter argument is similar to one made in a more advanced field of environmen- tal epidemiology, that on air pollution. It is not only the level of air pollution that matters, but also the precise pollutants that make up the air pollution, with some being more harmful than others for human physical health. Also from an ecological perspective all animals are equal but some more than others: rare species tend to be more valued than very common species. But this does not necessarily mean that the presence of rare species will coincide with higher mental health benefits. 8 Biodiversity in  the  Context of  ‘Biodiversity  – Mental Health’ Research