Biodiversity is being threatened at an unprecedented scale by global environmental change brought about by human societies. In addition to the many moral reasons to preserve it for its own sake, biodiversity provides numerous ecosystem services that are crucial to human well-being at present and in the future. Ecosystem services (also called environmental services or nature’s services) are benefits provided by ecosystems to humans, that contribute to making human life both possible and worth living. Biodiversity can affect ecosystem services directly. For example, humans derive most of their essential food and fibers from animals and plants. Certain plants and animals are at the core of traditional knowledge systems. In many areas of the world, fields covered in colorful flowers provide enjoyment to locals and visitors, and support important tourist industries (Figure 1). Particular configurations of vegetation, bird songs, and scents are crucial to human sense of place. In addition, biodiversity can affect the provision of ecosystem services indirectly, through its influence on ecosystem processes that are essential to Earth’s life support systems. By affecting the magnitude, pace, and temporal continuity by which energy and materials are circulated through ecosystems, biodiversity influences the provision of regulating ecosystem services, such as pollination and seed dispersal of useful plants, regulation of climatic conditions suitable to humans and the animals and plants they consider important, the control of agricultural pests and diseases, and the regulation of human health. Also, by affecting nutrient and water cycling, and soil formation and fertility, biodiversity indirectly supports the production of food, fiber, potable water, shelter, and medicines.
Biodiversity has several components, such as the number, abundance, composition, spatial distribution and interactions of genotypes, populations, species, functional types and traits, and landscape units in a given system. All these components may play a role in maintaining life support systems in the long term. However, some of these components are more important than others in influencing specific ecosystem services. For example, functional composition, that is, the identity, abundance and range of species traits, appears to be considerably more important than species number in determining the effects of biodiversity on many ecosystem services, such as plant and animal biomass production, soil formation and retention, pollination, climatic regulation, and protection against natural hazards. At least among species within the same trophic level (e.g., plants), rarer species likely have small effects at any given point in time. Therefore, the most dramatic changes in ecosystem services are likely to come from altered functional composition of communities and from the loss, within the same trophic level, of locally abundant species rather than from the loss of already rare species. For example, accelerated land use change in the last decade (e.g. replacement of dry forest by soybean fields in Southern South America, replacement of lowland rainforest by oil palm plantations in South East Asia) will likely increase the risk of total extinction of critically endangered plant and animal species native to those ecosystems (Figure 2). However, most of the ecosystem-service consequences are likely to arise from the decreased abundance of locally dominant species -which are unlikely to become totally extinct- rather than from the complete disappearance of rare species, many of which might be already “functionally extinct”. The presence of several plant species and genotypes within the same trophic level appear to play a significant role in some ecosystem services, for example in the protection against agricultural pests and diseases. Some of the most dramatic examples of effects of biodiversity changes on ecosystem services have been the consequence of the alteration of local food-web diversity through indirect interactions and trophic cascades. Most of these “ecological surprises” have been the consequence of intentional or accidental removal or addition of certain predator, pathogen, herbivore, or plant species to ecosystems. The result has often been disproportionately large, unexpected, irreversible, alterations of ecosystem services, with high environmental, economic and cultural losses. The vast literature on biological invasions provides dramatic examples of the potential consequences of the alteration of local interactions between species, without necessarily involving initially large alterations in species number. Well-known examples include the cascading effects of decreases in sea otter (Enhydra lutris) populations that led to coastal erosion in the North Pacific, and the loss of large expanses of Hawaiian native vegetation to the lantana shrub (Lantana camara), whose expansion over the landscape is promoted by the myna bird (Acridotheres tristis) originally introduced to control a pest of sugarcane.
On the basis of the available evidence, we are unable to establish how many species are needed to preserve different ecosystem services, although moral reasons and the precautionary principle suggest that in all ecosystems as many existing species as possible should be preserved. In general, because most ecosystem services are provided at the local scale, if we are to preserve the regulating and supporting services that ecosystems provide to humans, conservation efforts need to focus on preserving or restoring their biotic integrity (whether inherently species-poor or species-rich), rather than on simply maximizing the number of species present.
Decreased provision of ecosystem services as a result of biodiversity loss is not expected to affect all peoples in the same way. People that rely most directly on ecosystem services, such as subsistence farmers and fishers, the rural poor, and traditional societies, face the most serious and immediate risks (Figure 3). This is because they are the ones who rely the most on the “safety net” provided by the biodiversity of natural ecosystems in terms of food security and sustained access to medicinal products, fuel, construction materials, and protection from natural hazards such as storms and floods. Also, because of their low economic and political power, the less privileged sectors cannot substitute purchased goods and services for the lost ecosystem benefits and they typically have little influence on national policy. Therefore the loss of biodiversity-dependent ecosystem services is likely to accentuate inequality and marginalization of the most vulnerable sectors of society.
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