Habitat Change
Habitat degradation
Habitat degradation is human-caused change (e.g. logging) that reduces habitat quality for certain species. As it affects some species more than others, it results in changes in evenness and thus a loss of biodiversity (see 3.1). Habitat degradation also leads to increased venerability to invasive species (Cain, Bowman, & Hacker, 2014, p. 530).
Habitat fragmentation
The effects of habitat fragmentation, the division of originally continuous habitat into smaller, separated areas due to human impacts such as road building, as per the island biogeography theory, depend on island size and separation distance. Larger ‘islands’ or fragments, that are closer together, will have higher species richness than smaller, more isolated fragments (Cain, Bowman, & Hacker, 2014, p. 420). This is because smaller habitats cannot support species that require a larger range, or as many individuals as larger fragments. A locally declining population is more likely to be resuscitated by a closer nearby population. Furthermore, habitat fragmentation results in isolation, meaning that species cannot disperse across a landscape in order to find resources. Finally, more fragmented areas have higher proportions of edge areas that have differences in climate and where access is easier for invasive species and pollutants (Rei, et al., 2010).
Habitat loss
Habitat loss is the complete conversion of habitat into other use areas by humans, leading to loss of entire ecosystems and death of organisms previously inhabiting the area. It has been uneven between ecosystem types, for example effecting 50% of wetlands in the 20th century (Rosser & Walpole, 2012). Cultivated systems such as livestock production areas cover at least quarter of the earth’s terrestrial surface and 10-20% more grassland and forestland is expected to be converted by 2050 (Millennium Ecosystem Assessment, 2005).
Climate change
Climate change, a DPSIR pressure on biodiversity, leads to extinctions because species (especially specialists) are unable to adapt to rapid changes in temperature, shifts in climactic zones or extreme weather events such as droughts and floods (Omann, Stocker, & Jäger, 2009). These changes in climate can lead to deaths due to trauma from extreme conditions or decreases in water availability and quality (Millennium Ecosystem Assessment, 2005).
Invasive species
Invasive alien species (introduced, non-native species with growing populations that have large effects on communities) can result in large declines or extinctions of native species by either out-competing them, preying on them, or changing the properties of the ecosystem around them (Cain, Bowman, & Hacker, 2014, p. 530). Additionally, diseases can cross over from domesticated animals, leading to deaths in wildlife.
Overexploitation
Overexploitation, harvesting at a rate not sustainable for the population, leads to species extinctions and thus a loss of biodiversity as above. Harvesting food and other resources for fueling a growing population, from a diminishing natural area is one example of this, but it can also include harvesting live or dead animals for trade, or overhunting. Rare species can become more sought after, raising their value and leading to an overexploitation vortex towards extinction (Tournant, Joseph, Goka, & Courchamp, 2012). Overexploitation can also lead to habitat degradation (Cain, Bowman, & Hacker, 2014, p. 531).
Pollution
Pollutants cause physiological stresses on organisms along with habitat degradation and biodiversity loss. For example, persistent organic pollutants such as DDT or flame retardants get bioaccumulated and biomagnified, causing immune, reproductive, and developmental problems in mammals (Cain, Bowman, & Hacker, 2014, p. 533). Human-caused increases in pollutants are considered one of the most important pressures on terrestrial ecosystems; their importance is expected to increase in the future. (Millennium Ecosystem Assessment, 2005). More biologically available nitrogen is now produced by humans than nature; the deposition of nitrogen into terrestrial ecosystems leads directly to lower plant diversity. Phosphorus has similar effects to those of nitrogen; its use has tripled between 1960 and 1990 (Millennium Ecosystem Assessment, 2005).
*These pressures all lead directly to the state of terrestrial biodiversity: general biodiversity loss, (or to deaths and extinctions of individuals or populations, which constitutes a reduction in biodiversity due to loss in species richness, changes in distributions of species, or resulting ecosystem collapses).*
