Page - 27 - in Biodiversity and Health in the Face of Climate Change

27 and diseases in large areas of monoculture crops (Dobson et  al. 2006). In turn, biodiversity ultimately affects human health by making agricultural systems more inherently resilient and less liable to large scale losses (Dobson et  al. 2006). Evidence also suggests a link between biodiversity and the productivity of systems for human use, for example more biodiverse woodlands and fisheries are more productive for fuel and food (Harrison et  al. 2014). In order to understand mechanisms in more detail, it is necessary to unpack the concept of biodiversity and understand how, where and when its different elements are important. Otherwise, there is considerable potential for uncertainty and the potential to equate ‘ecosystem services’ and ‘biodiversity’ so that they are seen as essentially the same thing (Mace et  al. 2012). Indeed, there is still considerable disagreement about which ecosystem and biodiversity metrics should be considered (ibid.), with most reviews considering metrics beyond those implied by the defini- tion used to frame this volume. Figure  2.4 shows two examples of diagrammatic representations of biodiversity metrics and the functions of ecosystems known to influence human health, a number of which relate to the pathways that have already been identified in Sect. 2.2. Figure 2.4 (top) identifies a range of biodiversity metrics of different levels of complexity and summarises the available evidence on how they relate to ecosystem functions that have a useful role for people in urban areas. Some of the connections are identified as being positive (red  – beneficial for functions) while others are nega- tive (blue  – detrimental for functions). For example, Schwarz et  al. (2017) (Fig.  2.4 (top)) reviewed 82 studies that examined taxonomic diversity and its links to useful ecosystem functions in urban areas. The studies identified positive connections through pollination, soil protection and fertility, pest control, fresh water and envi- ronmental regulation. However, the studies also identified some negative connec- tions, even for these same pathways. Therefore, even taking the one example of urban ecosystems, the extent to which there are positive compared to negative effects depends on context and perspective (Díaz et  al. 2018). Some of the biodiver- sity metrics, such as functional identity (associated with 22 studies) were found to have only positive effects on urban ecosystem functions. While it may be assumed that these effects are then positive for human health, this claim cannot be made on the basis of the review findings alone. Figure  2.4 (bottom) identifies ecological ele- ments acting as ‘Ecosystem Service Providers’, i.e. the conduits through which the various biotic attributes listed act to benefit or harm human beings. For example, a wide range of function providers exist for pest regulation, from single species to functional groups and whole habitats. In this case, most studies have connected pest regulation to species within single functional groups. There are fewer studies con- sidering multiple functional groups which makes cross-connections more difficult to determine. Ultimately considering the impacts of environmental stressors, includ- ing climate change, will require the systematic investigation of cross connections and whole ecosystem responses. 2 Biodiversity, Physical Health and  Climate Change: A  Synthesis of  Recent Evidence