6 Humanity and biodiversity: the risk of extinction of species in terrestrial ecosystems
- Maria Alice Dos Santos Alves is an associate professor at the Ecology Department of Rio de Janeiro State University (UERJ), a National Council for Scientific and Technological Development (CNPq) researcher, and a holder of “Cientista do Nosso Estado” and “Prociência” research scholarships, granted by the Rio de Janeiro State Research Foundation (Faperj) and UERJ, respectively. She has an undergraduate degree in biology from the University of Brasília (UnB), a M Sc in ecology from Campinas State University (Unicamp), a Ph D in ecology from the University of Stirling (U.K.) and a postdoc from Duke University (USA). She has previously served as a committee member of the Brazilian Ornithological Society, a member of the CNPq advisory committee on ecology-limnology), and a member of the Rio de Janeiro State Atlantic Forest Biosphere Reserve Committee.
Our planet, Earth, the only one in this immense Universe known to harbor life, contains an enormous variety of living beings, which we call biological diversity, or biodiversity. It is estimated that there are around 10 million species currently living on Earth, not including microbes and underestimating small species, such as those that live in places that people find hard to reach, like the oceans. The total number of identified species, with scientific names, is roughly 1.5 million, although some recent estimates put this figure at 1.75 million (including around 100,000 terrestrial vertebrates, flowering plants and invertebrates with wings or shells). Of this total, birds and mammals are relatively well known, amounting to around 10,000 and 4,300 species, respectively, although new species continue to be discovered. Regarding marine species, just 250,000 to 300,000 have been described, and there are still many to discover. [1].
When it comes to us, human beings, although we appeared recently on this planet’s evolutionary timescale, we have occupied virtually all the land environments. The Industrial Revolution’s process of economic and social change (in the 18th and 19th centuries), which led to an increase in food production and life expectancy, also triggered rapid population growth. In recent decades, this greater presence of people on Earth has caused an intensification of human action on nature, resulting in accelerated environmental destruction and degradation, and consequent strong pressure for loss of biodiversity.
If we consider the last 500 years, 844 species have gone extinct, according to the International Union for Conservation of Nature (IUCN). Of this total, although extinctions have also often occurred on continents in the last two decades, the majority of them have taken place on oceanic islands. [2]
To find a local extinction, we do not have to go very far. One example in the state of Rio de Janeiro is the tropical mockingbird, Mimus gilvus, known for its song, greatly appreciated by humans. At the time Brazil was colonized, this bird was found throughout the state’s coast, but the species now only has established populations in four areas of restinga (sandy coastal vegetation) in the region, where it is endangered by the loss of its habitat and illegal capture.[3]
Scientists believe the current rate of species extinction is, on average, between 100 and 1,000 times higher than pre-human levels, and it is on the way to being 10,000 higher. [4] These figures are considered to be very high and indicate that the situation in recent years is one of rapid extinction, with an accelerating trend. Given this reality, one may say that the extinction of species, despite being a naturally occurring and irreversible event, has been taking place on an unprecedented scale because of human pressure.
The current rate of species extinction is, on average, between 100 and 1,000 times higher than pre-human levels, and it is on the way to being 10,000 higher. These figures are considered to be very high and indicate that the situation in recent years is one of rapid extinction, with an accelerating trend.
As well as already frequent tsunamis, tornados and hurricanes, we have witnessed ever more climate change resulting from environmental alterations brought about by human beings, causing disasters such as extreme flooding and droughts. In addition to loss of habitat (and its consequent fragmentation) and climate change, which are threatening many species (particularly endemic ones), [5] another aggravating factor now poses a great threat: exotic and invasive species. They may have a very negative impact for the survival of many species, especially native ones that make restricted use of habitat.
It is well known that ecosystems are critically dependent on biodiversity, meaning the species and populations that make them up, and their good functioning is vital to the maintenance of species on the planet, allowing them to provide environmental goods and services. When a species is extinguished, the same thing is likely to happen to many others, which interact in ecosystems, forming food chains, for example.
To understand what an ecosystem is, one should take into account not only the set of living beings but also the interactions they establish between themselves (such as the effects that different populations cause on one another) and with the physical environment (such as temperature, precipitation, rain and wind). In other words, ecosystems are composed of all the parts of the physical and biological world they interact with.
Among these interactions, those that take place between organisms may be considered positive or negative if they increase or decrease their population sizes, respectively. Therefore, interactions between species (such as competition, predation, parasitism, mutualism and commensalism) are multiple and permit there to be a network between them. They are shaped by evolution and take place naturally in an ecosystem.
Most of the time, positive human interventions seek to reverse or neutralize negative interventions carried out directly or indirectly by human beings themselves, such as environmental destruction and degradation, which lead species to become endangered, and therefore loss of biodiversity.
External interventions, such as those imposed by human beings, may also be positive or negative. However, the reality is that most of the time, positive human interventions seek to reverse or neutralize negative interventions carried out directly or indirectly by human beings themselves, such as environmental destruction and degradation, which lead species to become endangered, and therefore cause loss of biodiversity. An example of a positive intervention aimed at reversing a negative situation is management to increase the size of a threatened species’ population, or to reduce or control an exotic or invasive species.
If we look to the future, given the threat to biodiversity now posed by human action, we can predict changes that may be gradual or sudden over the next 50 years. Changes that may occur gradually include alterations to the distribution of species due to changes in climate, such as rises in temperature. In addition, even for widely distributed species, there may be a loss of biodiversity within their distribution boundaries, through local extinction (of part of a species’ populations, with a consequent loss of genetic diversity). As the majority of environmental or ecosystem services (benefits provided by nature) depend on biodiversity, the loss of local populations may lead to a reduction in these services, such as pollination and seed dispersal performed by different animal groups, like birds and mammals.
As for sudden changes, over the next five decades we may lose a large share of species that are endangered and whose distribution is heavily restricted. This may largely be caused by habitat loss, but also the introduction of exotic and invasive species (whose effects may be devastating) and climate change (increases in temperature and a consequent rise in sea level).
Although it is hard to make predictions about species extinctions because of the numerous variables involved, researchers have shown that global climate change will result in the extinction of a considerable number of species in the coming decades. For many groups of plants and animals investigated, estimates have been made that 15% to 37% of species will become extinct as a result of the direct or indirect effects (habitat alterations) of the temperature increase projected for 2050. [6] Some species of flora and fauna, particularly endemic ones that are restricted to small portions of coastal environments, for example, may became extinct in this period.
One of the questions that arises in this context is: how much longer will our species be able to live in a sustainable manner on this planet if we continue to alter nature the way we are doing now? To reverse this situation, we will need initiatives such as awareness raising in society about these threats, government programs to monitor species – especially endangered and/or endemic ones – and also activities to manage exotic and invasive species.
In response to the question “what will tomorrow be like?” we can respond that it will be the consequence of what we do today. If we reduce the current negative pressures, we may be able to avoid the most pessimistic scenarios in scientific forecasts. Attitudes such as think, plan and act locally may be reflected in global actions. Raising the awareness of human beings so they feel like just one of many other living beings is vital for us to be able to preserve the most precious resource we have on the planet, namely the biodiversity of which we are a part.
[1] For the mentioned estimates, see Stuart Leonard Pimm et al., “What is Biodiversity?” in Eric Chivian and Aaron Bernstein (orgs.), Sustaining Life: How Human Health Depends on Biodiversity, Oxford: Oxford University Press, 2008, p. 3-27.
[2] Idem.
[3] Mariana S. Zanon et. al., “Missing for the Last Twenty Years: The Case of the Southernmost Populations of the Tropical Mockingbird Mimus gilvus (Passeriformes: Mimidae)”, in Zoologia, vol. 32, 2015, p. 1-8.
[4] Stuart Leonard Pimm et al., op. cit.
[5] Endemic species are those restricted to a certain habitat, and therefore more susceptible to extinction.
[6] Chris D. Thomas et al., “Extinction Risk from Climate Change”, Nature, vol. 427, 2004, p. 145-148.