The preservation of the number, types, and relative abundance of resident species can enhance invasion resistance in a wide range of natural and semi-natural ecosystems (medium certainty). Although areas of high species richness (such as biodiversity hot spots) are more susceptible to invasion than species-poor areas, within a given habitat the preservation of its natural species pool appears to increase its resistance to invasions by non-native species. This is also supported by evidence from several marine ecosystems, where decreases in the richness of native taxa were correlated with increased survival and percent cover of invading species (C11.3.1, C11.4.1).
Pollination is essential for the provision of plant-derived ecosystem services, yet there have been worldwide declines in pollinator diversity (medium certainty). Many fruits and vegetables require pollinators, thus pollination services are critical to the production of a considerable portion of the vitamins and minerals in the human diet. Although there is no assessment at the continental level, documented declines in more-restricted geographical areas include mammals (lemurs and bats, for example) and birds (hummingbirds and sunbirds, for instance), bumblebees in Britain and Germany, honeybees in the United States and some European countries, and butterflies in Europe. The causes of these declines are multiple, but habitat destruction and the use of pesticide are especially important. Estimates of the global annual monetary value of pollination vary widely, but they are in the order of hundreds of billions of dollars (C11.3.2, Box C11.2).
Biodiversity influences climate at local, regional, and global scales, thus changes in land use and land cover that affect biodiversity can affect climate. The important components of biodiversity include plant functional diversity and the type and distribution of habitats across landscapes. These influence the capacity of terrestrial ecosystems to sequester carbon, albedo (proportion of incoming radiation from the Sun that is reflected by the land surface back to space), evapotranspiration, temperature, and fire regime—all of which influence climate, especially at the landscape, ecosystem, or biome levels. For example, forests have higher evapotranspiration than other ecosystems, such as grasslands, because of their deeper roots and greater leaf area. Thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems. In the Amazon, for example, 60% of precipitation comes from water transpired by upwind ecosystems (C11.3.3).
In addition to biodiversity within habitats, the diversity of habitats in a landscape exerts additional impacts on climate across multiple scales. Landscape-level patches (>10 kilometers in diameter) that have lower albedo and higher surface temperature than neighboring patches create cells of rising warm air above the patch (convection). This air is replaced by cooler moister air that flows laterally from adjacent patches (advection). Climate models suggest that these landscape-level effects can substantially modify local-to-regional climate. In Western Australia, for example, the replacement of native heath vegetation by wheatlands increased regional albedo. As a result, air tended to rise over the dark (more solar-absorptive and therefore warmer) heathland, drawing moist air from the wheatlands to the heathlands. The net effect was a 10% increase in precipitation over heathlands and a 30% decrease in precipitation over croplands (C11.3.3).
Some components of biodiversity affect carbon sequestration and thus are important in carbon-based climate change mitigation when afforestation, reforestation, reduced deforestation, and biofuel plantations are involved (high certainty). Biodiversity affects carbon sequestration primarily through its effects on species characteristics, which determine how much carbon is taken up from the atmosphere (assimilation) and how much is released into it (decomposition, combustion). Particularly important are how fast plants can grow, which governs carbon inputs, and woodiness, which enhances carbon sequestration because woody plants tend to contain more carbon, live longer, and decompose more slowly than smaller herbaceous plants. Plant species also strongly influence carbon loss via decomposition and their effects on disturbance. Plant traits also influence the probability of disturbances such as fire, windthrow, and human harvest, which temporarily change forests from accumulating carbon to releasing it (C11.3.3).
The major importance of marine biodiversity in climate regulation appears to be via its effect on biogeochemical cycling and carbon sequestration. The ocean, through its sheer volume and links to the terrestrial biosphere, plays a huge role in cycling of almost every material involved in biotic processes. Of these, the anthropogenic effects on carbon and nitrogen cycling are especially prominent. Biodiversity influences the effectiveness of the biological pump that moves carbon from the surface ocean and sequesters it in deep waters and sediments. Some of the carbon that is absorbed by marine photosynthesis and transferred through food webs to grazers sinks to the deep ocean as fecal pellets and dead cells. The efficiency of this trophic transfer and therefore the extent of carbon sequestration is sensitive to the species richness and composition of the plankton community (C11.4.3).
Pest, disease, and pollution control
The maintenance of natural pest control services, which benefits food security, rural household incomes, and national incomes of many countries, is strongly dependent on biodiversity. Yields of desired products from agroecosystems may be reduced by attacks of animal herbivores and microbial pathogens, above and below ground, and by competition with weeds. Increasing associated biodiversity with low-diversity agroecosystems, however, can enhance biological control and reduce the dependency and costs associated with biocides. Moreover, high-biodiversity agriculture has cultural and aesthetic value and can reduce many of the externalized costs of irrigation, fertilizer, pesticide, and herbicide inputs associated with monoculture agriculture (C11.3.4, Boxes C11.3 and C11.4).
The marine microbial community provides critical detoxification services, but how biodiversity influences them is not well understood. There is very little information on how many species are necessary to provide detoxification services, but these services may critically depend on one or a few species. Some marine organisms provide the ecosystem service of filtering water and reducing effects of eutrophication. For example, American oysters in Chesapeake Bay were once abundant but have sharply declined—and with them, their filtering ecosystem services. Areas like the Chesapeake might have much clearer water if large populations of filtering oysters could be reintroduced. Some marine microbes can degrade toxic hydrocarbons, such as those in an oil spill, into carbon and water, using a process that requires oxygen. Thus this service is threatened by nutrient pollution, which generates oxygen deprivation (C11.4.4).