CONICET | Buscador de Institutos y Recursos Humanos

Biodiversity, including the number, abundance, and composition of genotypes, populations, species, functional types, communities, and landscape units, strongly influences the provision of ecosystem services and therefore human well-being (high certainty). Processes frequently affected by changes in biodiversity include pollination, seed dispersal, climate regulation, carbon sequestration, agricultural pest and disease control, and human health regulation. Also, by affecting ecosystem processes such as primary production, nutrient and water cycling, and soil formation and retention, biodiversity indirectly supports the production of food, fiber, potable water, shelter, and medicines. Species composition is often more important than the number of species in affecting ecosystem processes (high certainty). Thus, conserving or restoring the composition of communities, rather than simply maximizing species numbers, is critical to maintaining ecosystem services. Changes in species composition can occur directly by species introductions or removals, or indirectly by altered resource supply due to abiotic drivers (such as climate) or human drivers (such as irrigation, eutrophication, or pesticides). Although a reduction in the number of species may initially have small effects, even minor losses may reduce the capacity of ecosystems for adjustment to changing environments (medium certainty). Therefore, a large number of resident species, including those that are rare, may act as insurance that buffers ecosystem processes in the face of changes in the physical and biological environment (such as changes in precipitation, temperature, or pathogens). Productivity, nutrient retention, and resistance to invasions and diseases tend to increase with increasing species number in experimental ecosystems that have been reduced to a small number of species (10 or fewer). This is known with high certainty for experimental herbaceous ecosystems and with low certainty for natural ecosystems, especially those dominated by longlived species. In natural ecosystems these direct effects of biodiversity loss may often be masked by other environmental changes that are caused by the factors that resulted in the loss of biodiversity (such as eutrophication or climate change). Nevertheless, human activities that cause severe reductions in species number can directly impair these ecosystem services. Preserving interactions among species is critical for maintaining longterm production of food and fiber on land and in the sea (high certainty). The production of food and fiber depends on the ability of the organisms involved to successfully complete their life cycles. For most plant species, this requires interactions with pollinators, seed disseminators, herbivores, or symbionts. Therefore, land use practices that disrupt these interactions will have a negative impact on these ecosystem services. Intended or accidental changes in the composition of ecological communities can lead to disproportionately large, irreversible, and often negative alterations of ecosystem processes, causing large monetary and cultural losses (high certainty). In addition to direct interactions, the maintenance of ecosystem processes depends on indirect interactions, whose disruption can lead to unexpected consequences. These consequences can occur very quickly; for example, in a wide range of terrestrial, marine, and freshwater ecosystems, the introduction of exotic species by humans has altered local community interactions. Alternatively, these consequences may be manifest only after a long time. For example, the intraspecific genetic diversity of certain plant species decreases when the populations of their animal pollinators or dispersers are reduced. Invasion by exotic species, facilitated by global trade, is a major threat to the biotic integrity of communities and the functioning of ecosystems. Empirical evidence suggests that areas of high species richness (such as hot spots) are more susceptible to invasion than species-poor areas. On the other hand, within a given habitat the preservation of its natural species pool appears to decrease its susceptibility to invasions. On the basis of our present theoretical knowledge, however, we still cannot predict with accuracy whether a certain organism will become a serious invader in a given ecosystem. The extinction of local populations, or their reduction to the point that they become functionally extinct, can have dramatic consequences in terms of regulating and supporting ecosystem services. Local extinctions have received little attention compared with global extinctions, despite the fact that the former may have more dramatic ecosystem consequences than the latter. Before becoming extinct, species become rare and their ranges contract. Therefore their influence on ecosystem processes decreases, even if local populations persist for a long time, well before the species becomes globally extinct. We do not have sufficient knowledge to predict all the consequences of these local extinctions. However, because they tend to be biased toward particular organisms that depend on prevailing land uses and types, rather than occurring at random, we can anticipate some of the most obvious impacts. The properties of species are more important than species number in influencing climate regulation (medium certainty). Climate regulation is in- fluenced by species properties via effects on sequestration of carbon, fire regime, and water and energy exchange. The traits of dominant plant species, such as size and leaf area, and the spatial arrangement of landscape units are particularly important in climate regulation. The functional characteristics of dominant species are thus a key element determining the success of mitigation practices such as afforestation, reforestation, slowed-down deforestation, and biofuel plantations.

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