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    Biodiversity - Terrestrial Ecosystems
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Pressure indicators

Land-use changes

The pressure exerted by the production of goods and services has led to an increasing loss and degradation of terrestrial ecosystems due to changes in land use. This is perhaps the primary factor that threatens the integrity and permanence of terrestrial ecosystems and their biodiversity in Mexico and worldwide (Vitousek et al., 1997; Walker and Steffen, 1997; Semarnat, 2003; Sánchez Colón et al., 2009; Nellemann et al., 2009). Activities that promote more strongly changes in land use are agriculture and livestock, followed in importance by the growth of urban areas, communications infrastructure and other services (Conabio, 2006). Farming activities lead to the reduction of temperate and tropical forests (deforestation) or other ecosystems (shrubland, for example) through land clearance for cattle-raising (Conabio, 2006; FAO, 2006; OECD, 2008; Semarnat, 2008) or through agricultural practices that demand new areas (PNUMA, 2003; Semarnat, 2008; Nellemann et al., 2009).


The loss of biodiversity that occurs as a result of changes in land use is mainly due to the reduction and fragmentation of ecosystem areas, which lead to the decline of the populations of many wildlife species and thus may promote local and global extinction processes (Forman and Alexander, 1998). Land-use changes also favor environmental degradation, mainly due to the deterioration of the properties and characteristics of soil, as well as by modifying the conditions of temperature, humidity and light, which in some cases may prevent the regeneration of natural vegetation (Wood et al., 2000). The indicator Land use change is used to denote the pressure exerted by this phenomenon on some of Mexico’s main terrestrial ecosystems. This indicator is included in the UN list of Indicators for Sustainable Development (ONU, 2007), in the environmental integration initiative of the UE Statistics Office (Eurostat) and in the Organization for Economic Cooperation and Development (OECD), the Ministry of Environment of Spain and the International Union of Geological Sciences (IUGS, 2000).

 

Population Growth


According to the II Survey on Population and Housing (INEGI, 2006), Mexico belongs to the eleven most densely populated countries, ranking third in the Americas (after the United States and Brazil) and twelfth for their contribution to the world’s population growth (Semarnat, 2003). The rapid population growth, the asymmetric distribution of its population in the territory, the migratory phenomena and the establishment of new communities in uninhabited areas can all have significant consequences in the state of Mexico’s terrestrial ecosystems. Population growth, for example, causes indirect damages to terrestrial ecosystems as a result of the pressure of human populations on natural resources, infrastructure construction and the increase in the generation of municipal and industrial wastes (Groombridge and Jenkins, 2002; UNEP, 2003). This translates in overexploitation of many commercial species by local populations, habitat disruption and destruction, and pollution of soils and surface waters, all with significant impacts on the structure and function of many terrestrial ecosystems and their biodiversity.


There are differences in the impact of urban versus rural populations. Impacts of rural populations are modest (because of their consumption and waste-disposal patterns, etc.); in contrast, urban impacts may be far more important (higher consumption, demand of services and production of wastes). The total population in Mexico reflects the magnitude of the pressure that human populations could exert on various terrestrial ecosystems. This indicator is developed in the chapter on Water, Availability section. Population growth rates are considered as pressure indicators in the UN list of Sustainable Development Indicators (2007) and the Organization for Economic Cooperation and Development (OECD, 2008).

 

Expansion of road infrastructure

One of the basic needs for social and economic development of a country is the construction of a system of roads and highways to enable the transit of persons and transportation of goods and products within its boundaries. However, the growth of this infrastructure may affect the state of biodiversity, at both local and regional levels (Forman and Alexander, 1998). The direct impacts include loss and disruption of habitats, fragmentation of ecosystems and their populations, animals deaths by motor vehicle collisions (mainly mammals and reptiles), increased noise levels (which in some cases may affect wildlife at both sides of the road) and spillage of pollutants and soil pollution (Forman and Alexander, 1998; CE, DFID and UICN, 2001; PNUMA, 2003). Unplanned urban growth, mainly by irregular settlements along roads, also indirectly affect natural resources and ecosystems because of vegetation clearing, overexploitation of resources and pollution of bodies of water, among others (EPA, 1999). The indicator Expansion of road infrastructure shows the pressure exerted on natural terrestrial ecosystems by this expansion. This indicator is part of the OECD environmental indicators (2008), where it is included as an indirect-pressure indicator.

 

Forest fires

Forest fires occur in nature and are key factors in the dynamics of many ecosystems around the world, especially in temperate forests. Fires influence processes that determine the availability of nutrients in soil and promote ecological succession processes that contribute to maintain biodiversity (Matthews et al., 2000; SCBD, 2001; Manson et al., 2009). However, largely due to human activities and control, today’s natural patterns of fire occurrence have changed. Now many forest fires take place in areas not previously affected by fires, but no longer occur in areas where fires occurred regularly (SCBD, 2001, Castillo et al., 2003).


Factors that affect to a greater extent forests fires worldwide are the continued logging of forests and the use of fire in preparing arable land; however, previous fires, the condition of the trees, bonfires and garbage burns also foster the occurrence of fires (SCBD, 2001; Cochrane, 2002; Castillo et al., 2003; PNUMA, 2003; Manson et al., 2009). The effects of fire on ecosystems are diverse and depend on the intensity and frequency of fires. The most important effect is the removal of standing plant biomass, which. along with the removal of seedlings of tree species, delays or disrupts natural regeneration, besides promoting the invasion of forest pests and diseases (Matthews et al., 2000; Castillo et al., 2003). The direct effect of fire on wildlife can be death, while indirect effects may include loss of habitat, territories and feeding grounds, as well as the displacement of terrestrial mammals and birds (SCBD, 2001; Castillo et al., 2003). All the above affects both food chains and the natural balance of ecosystems (SCBD, 2001; Castillo et al., 2003). The indicator Forest fires and area affected is useful to denote the pressure of these events on national terrestrial ecosystems. This indicator is developed in this publication in the chapter on Forest resources. It is also used by the U.S. Environmental Protection Agency (EPA, 2003) and in Greece’s Report on Sustainable Development Indicators (NCESD, 2003).

 

Invasive species

While the movement of species from one region to another may be a natural phenomenon, human activities have increased the frequency of occurrence, largely due to the global movement of vehicles and products. Invasive species are those that can be categorized as new in a given region and which can have a negative environmental, social or economic impact (Mooney, 2000). It is now recognized that the introduction of invasive species, either accidentally or intentionally, is a major cause of biodiversity loss in the planet (EPA-Australia, 2003; Aguirre Muñoz, 2009). Invasive species may compete directly with native species for habitat use and resources, or even act as a new predators in the ecosystem, even leading to the local extinction of native species (Mooney, 2000; Aguirre Muñoz, 2009). The indicator Invasive species in national terrestrial ecosystems denotes the pressure of this problem on terrestrial ecosystems in the country.

 

Global Climate Change

Global climate change resulting from both the internal variability of the climate system and external factors (either natural or derived from of human activities) are evident on land by the rise in surface temperature (0.74 °C in the last hundred years, 1906-2005), as well as by changes in precipitation patterns (for example, 5 - 10% rise in intermediate and high altitudes in the northern hemisphere. and 3% decrease on average in the subtropics; IPCC, 2007). At the same time, the snow cover and ice caps recorded a 10 percent decrease in the northern hemisphere since the late sixties. The El Niño phenomenon became more frequent, persistent and intense since the mid-seventies compared to the previous hundred years (IPCC, 2002).


Some effects associated with climate change that have occurred in species of terrestrial ecosystems are: disrupted periods of invertebrate emergence, growth and reproduction; disrupted egg-laying in birds; changes in the migration season of birds and insects; changes in the distribution ranges of butterflies; increase in pest and disease outbreaks, changes in the morphology, physiology and behavior of some animal species; and earlier flowering and longer growing season of some plant species, among others (IPCC, 2002; Manson, 2009). In the case of vegetation, forests in Alaska have expanded their distribution ranges northward, the composition of tundra vegetation (especially lichens and other herbaceous plants) has changed in some places and the area affected by fire in boreal forests doubled in the last decades of the twentieth century.


The predictions of experts point out that the global climate change might lead to global changes in patterns of distribution and diversity of terrestrial, marine and freshwater ecosystems. The extinction of species unable to become adapted adapt (through migration or other changes) to new environmental conditions has been foreseen. In the case of Mexico, although Townsend and colleagues (2002) have predicted neither massive extinctions nor drastic changes in the distribution ranges of animal species, these authors do stress that the species composition turnover will increase in some communities, temperate pine and oak forests being the most threatened ecosystems in the country (Magaña and Gay, 2002). The indicator Change in global temperature denotes the pressure that this phenomenon may exert on national terrestrial ecosystems. This indicator is included in the Climate Change section under the chapter on Atmosphere.