Causes of threat and extinction

What are the major factors that lead the species to extinction? Several statistical analyses, though approximate, were made. The main cause of the extinctions occurred so far is the change of land use, which leads to the degradation and destruction of habitats. Additional causes are the introduction of alien species, the harvest of animals and plants and pollution. Fig. 10 shows for example the main causes of extinction and threat to the bird populations at the global level.

Figure 10: Percentage importance of the causes of extinction of and threat to bird species worldwide (Szabo et al., 2012). Abbreviations: R & C stays for Residential and Commercial, CC for Climate Change and T & S for Transport and Services.

There are many examples of habitat degradation, but nowadays the most impressive is the destruction of tropical forests. While Europe's forests have been destroyed and fragmented in the course of centuries and with the aim of building towns and cities that are part of our culture, tropical forests (see Fig. 11) are destroyed over much shorter time scales (tens of years) and basically in order to produce resources for what we call the developed countries. Often the fragmentation is linked to the construction of roads as is the case for the Trans-Amazonian Highway road network. Sometimes there is a conversion of land use in favour of agricultural crops such as soy-bean that are primarily intended for export. This conversion is financed by international loans. The destruction is unfortunately very fast and creates an unnatural mosaic of fragments at different territorial scales. Fig. 12 shows an example of a Landsat7 image taken in August 2000 that frames the new agricultural settlements east of Santa Cruz de la Sierra, Bolivia, in an area of ​​tropical dry forest.

Figure 11: Annual deforested area of the Amazon forest in the last three decades (thousands of km$^2$).

Figure: Landsat7 image, taken on August 1, 2000, showing new agricultural settlements east of Santa Cruz de la Sierra, Bolivia, in an area of ​​tropical forest. Since the 1980s, this region has been rapidly deforested as a result of a re-settlement from the Andes highlands and an extensive agricultural development project called Tierras Baja. The radial pattern of fields is part of the re-settlement of San Javier. At the center of the settlement there is a small community which includes a church, a coffee bar, a school and a soccer field - the essentials of rural life in Bolivia. Straightlined, clear-colored areas are soybean fields cultivated for export and mainly financed by foreign capital. The dark stripes across the fields are windbreaks which are necessary because the soils of these areas are fine-textured and subject to erosion.

Many models have been proposed to understand and predict the loss of biodiversity due to habitat fragmentation. To better define the problem, it is worthwhile to remark that even undisturbed natural populations are in many cases divided into subpopulations. The butterflies Melitaea cinxia of Alan Islands in Finland (Hanski et al., 1995) and Euphydryas editha in California (Harrison et al., 1988) are two famous examples of this kind. The first species lives in a landscape made up of hundreds of pieces of vegetation located in many islands of the Alan archipelago, while the second requires a particular habitat (grasslands with serpentine soil) which is patchily distributed in California. As we will better specify in Chapter , the famous ecologist Richard Levins (1970) was the first to coin the term metapopulation, or “population of populations”, to indicate a set of local populations where local extinctions are balanced by immigration due to emigration from other subpopulations that are present in fragments of still occupied habitat. The metapopulation is the current paradigm not only for populations inhabiting naturally inhomogeneous landscapes but also for populations threatened by habitat fragmentation.

A major extinction driver, though little known to the general public, is the introduction of alien species. These have not evolved in the environment in which they are introduced, therefore they neither are adapted to it nor co-evolved together with the indigenous species. They often have a competitive advantage over native species, or carry parasites and diseases to which native species have not adapted, or act as new predators against which native species are defenceless. Examples abound. The grey squirrel (Sciurus carolinensis) was introduced from North America to Britain in late 1800. It has few natural predators and is bigger and more athletic than the native species, i.e. the red squirrel (Sciurus vulgaris). Additionally, it is the vector of Parapoxvirus, which is lethal to the red squirrel, and better resists the fragmentation of forest habitat. The American species has replaced the European one almost anywhere in Britain. The distribution area of the red squirrel is now reduced to the island of Anglesey and the North of Scotland. The comb jelly Mnemiopsis leidyi, a marine ctenophore (see Fig. 13), native to the western Atlantic coast, was accidentally introduced into the Black Sea in 1980 probably through the ballast waters of merchant ships. Here it greatly multiplied; as M. leidyi is a carnivore that feeds on eggs and larvae of pelagic fish, it has caused a dramatic decrease of the harvest of Black Sea fisheries, especially of the anchovy Engraulis encrasicholus. The Nile perch (Lates niloticus, see Fig. 13) was introduced into Lake Victoria in East-Central Africa to give a boost to the local fishing industry. Being a large predator it has caused the extinction or near extinction of more than 200 species of cichlid fish that are found only in the largest African lake.

Figure 13: Examples of invasive alien species discussed in the text.

A further cause of threat to many species lies in overexploitation due to fishing and hunting. The harvest can be the primary cause or aggravate situations already at risk because of habitat degradation. The species most threatened by hunting and fishing are not only those whose flesh is edible (typically game and fish stocks), but also those whose skin and whose horns, tissues and organs have a high commercial value (like the elephant tusks, from which humans obtain ivory, or the rhino horn to which aphrodisiac properties are falsely ascribed to). Hunting and fishing do not always affect the ecosystem diversity, but become a serious extinction threat for species that are exploited excessively, i.e. whenever the catch rate is larger than the renewal rate of the species biomass or numbers.

Great importance as an extinction or threat cause can be attributed to pollution. Human activities, in fact, have profoundly altered the biogeochemical cycles which are critical to the overall functioning of ecosystems. Sources of pollution are, in addition to industries and civil waste, also the agricultural activities that utilize insecticides, pesticides and herbicides, thus profoundly altering the soil and the food chains. It is worthwhile to remind the reader about the phenomenon of biomagnification, which consists in the amplification of the concentration of toxic substances in the food webs from the lower levels (primary producers) to the highest levels (top predators). The consequence of this process is the accumulation of significant quantities of harmful chemicals (especially heavy metals) in organisms that are at the top of the food chain (raptors, large carnivores).

In addition to analysing the reasons for the loss of biodiversity in the past we can also wonder what causes will mainly drive the loss of biodiversity in the future. According to Sala et al. (2000) the major impacts on biodiversity between now and 2100 will be those listed and compared in Fig. 14. The main cause will still be land-use change, leading to the degradation and destruction of habitats, followed by global climate change (GCC), the alteration of the nitrogen cycle, the introduction of alien species, the increase of carbon dioxide (CO$_2$). If we consider that global climate change is mainly due to rising concentrations of carbon dioxide and methane, we can state that the increase in greenhouse gases will therefore be in the future the most important cause of biodiversity loss, followed by land-use change on land and by human pressure (fishing and coastal urbanization) in the marine environment. The effects of GCC are now particularly severe in the arctic, boreal and alpine biomes, where climate is the main factor regulating the ecosystems, but will be increasingly important in changing ecosystems functioning almost everywhere in the next 50 years and beyond (IPCC-WGII, 2014).

Figure 14: Estimate for the year 2100 of the relative effect on biodiversity of the main drivers of change; the thin bars represent the variability of the impact in the different terrestrial biomes (after Sala et al., 2000).

The golden toad (Bufo periglenes) in the mountains of Costa Rica is probably the first species that was officially declared extinct because of GCC. It is estimated that 67% of the 110 species of harlequin frogs (Atelopus sp.), which are endemic to the American tropics are going to meet the same fate (Pounds et al., 2006). Other species have escaped GCC moving towards the poles or to higher altitudes. Still others, with possible effects on human health, such as Anopheles mosquitoes that carry malaria, will find more favourable conditions in warmer environments. In oceans the coral reefs are the ecosystems that harbour the majority of biodiversity. They are very sensitive to temperature because corals can survive only within a narrow temperature range. Several episodes of coral bleaching have already been occurring, and they are certainly linked to the increasing temperature of ocean waters as documented by NOAA-NESDIS (National Oceanic and Atmospheric Administration, National Environmental Satellite Data and Information Service) and UNEP-WCMC (United Nations Environment Programme - World Conservation Monitoring Centre).

It should be noted that organisms are among the best indicators of global warming. The “ecological footprints” of GCC, as they are called, are very clear. Very apparent is the phenological shift of plants and animals (typically, earlier breeding and flowering seasons). In recent decades, the average shift has been five days per decade and the birds were the most sensitive to the climatic signal (Root et al., 2003). Researchers have also found a shift towards the poles of indicator species such as butterflies (Parmesan et al., 1999). Very clear is also the shift towards higher altitudes. For example, the response to global warming of alpine vegetation in Europe has been a shift upwards of several meters per decade (Grabherr et al., 1994).