Direct drivers vary in their importance within and among systems and in the extent to which they are increasing their impact. Historically, habitat and land use change have had the biggest impact on biodiversity across biomes. Climate change is projected to increasingly affect all aspects of biodiversity, from individual organisms, through populations and species, to ecosystem composition and function. Pollution, especially the deposition of nitrogen and phosphorus, but also including the impact of other contaminants, is also expected to have an increasing impact, leading to declining biodiversity across biomes. Overexploitation and invasive species have been important as well and continue to be major drivers of changes in biodiversity (C4.3). (See Figure 3.10)
For terrestrial ecosystems, the most important direct driver of change in the past 50 years has been land cover change (C4.3, SG7). Only biomes relatively unsuited to crop plants, such as deserts, boreal forests, and tundra, are relatively intact (C4). Deforestation and forest degradation are currently more extensive in the tropics than in the rest of the world, although data on boreal forests are especially limited (C21). Approximately 10–20% of drylands are considered degraded (medium certainty), with the majority of these areas in Asia (C22). A study of the southern African biota shows how degradation of habitats led to loss of biodiversity across all taxa. (See Figure 3.11)
Cultivated systems (defined in the MA to be areas in which at least 30% of the landscape is in croplands, shifting cultivation, confined livestock production, or freshwater aquaculture in any particular year) cover 24% of Earth’s surface. (See Figure 3.12) In 1990, around 40% of the cropland is located in Asia; Europe accounts for 16%, and Africa, North America, and South America each account for 13% (S7).
For marine ecosystems, the most important direct driver of change in the past 50 years, in the aggregate, has been fishing. Fishing is the major direct anthropogenic force affecting the structure, function, and biodiversity of the oceans (C18). Fishing pressure is so strong in some marine systems that over much of the world the biomass of fish targeted in fisheries (including that of both the target species and those caught incidentally) has been reduced by 90% relative to levels prior to the onset of industrial fishing. In these areas a number of targeted stocks in all oceans have collapsed—having been overfished or fished above their maximum sustainable levels. Recent studies have demonstrated that global fisheries landings peaked in the late 1980s and are now declining despite increasing effort and fishing power, with little evidence of this trend reversing under current practices (C18.3). In addition to the landings, the average trophic level of global landings is declining, which implies that we are increasingly relying on fish that originate from the lower part of marine food webs (C18.3). (See Figures 3.13 and 3.14) Destructive fishing is also a factor in shallower waters; bottom trawling homogenizes three-dimensional benthic habitats and dramatically reduces biodiversity.
For freshwater ecosystems, depending on the region, the most important direct drivers of change in the past 50 years include physical changes, modification of water regimes, invasive species, and pollution. The loss of wetlands worldwide has been speculated to be 50% of those that existed in 1900. However, the accuracy of this figure has not been established due to an absence of reliable data (C20.3.1). Massive changes have been made in water regimes. In Asia, 78% of the total reservoir volume was constructed in the last decade, and in South America almost 60% of all reservoirs were built since the 1980s (C20.4.2). Water withdrawals from rivers and lakes for irrigation or urban or industrial use increased sixfold since 1900 (C7.2.2). Globally, humans now use roughly 10% of the available renewable freshwater supply, although in some regions, such as the Middle East and North Africa, humans use 120% of renewable supplies—the excess is obtained through mining groundwater (C7.2.3). The introduction of non-native invasive species is now a major cause of species extinction in freshwater systems. It is well established that the increased discharge of nutrients causes intensive eutrophication and potentially high levels of nitrate in drinking water and that pollution from point sources such as mining has had devastating impacts on the biota of inland waters (C20.4).