Species extinction is a natural process and would occur without the impact of humankind. In fact species extinction has played a role in the evolution of life on earth since the dawn of time. The Holocene can be defined as the period of time in the last 10,000 years when the earth-system processes have remained at a constant and is ‘the stable desired state within which humanity wants to stay, and has stayed, supporting civilisations as we know it today’. We are now however, entering the Anthropocene where humankind is putting pressure on the hard-wired earth systems at a global scale.
Biodiversity loss in the Anthropocene has accelerated greatly and species are becoming extinct at a rate that has not been seen since the last global mass-extinction event. The fossil record shows that the background extinction rate for mammals is 0.2-0.5 extinctions per million species per year. Today, the rate of extinction of species is estimated to be 100-1,000 times more than what could be considered natural and human activities are the main cause of this acceleration.
Accurately estimating the number of different species on earth is a near impossible task. Current estimates range from 10 million to 100 million different species. There is no doubt that since the Anthropocene the earth’s biodiversity has been in decline but it is not limited to increased rates of species extinction, but also includes the loss of genetic and functional diversity across population, community, ecosystem, landscape and global scales. This decline in biodiversity is due to a broad range of human activities such as habitat modification and destruction, increased rates of introduced non-native species, over exploitation of natural resources etc etc. Even at the lowest estimated rate of extinction about half of all species could be extinct within 100 years and such an event would be similar in magnitude to the five mass extinctions in the 3.5 billion years of earth’s existence.
Biodiversity loss deserves serious consideration because it is still unclear as to the consequences such declines may have for human wellbeing in the future. However, immediate consequences are all too obvious. Firstly, humans derive goods and products from biodiversity essential to life, including food, medicine, industrial products, genetic resources for crop breeding and natural pest control services. These benefits are the market values of biodiversity because they are readily tied to our economy and a price can often be directly applied to these benefits. Biodiversity provides non-market values as well in the form of knowledge, aesthetic and existence values. They are difficult to quantify but for many people they provide sufficient justification for preserving biodiversity alone. ‘Ecosystem services’ is a term used to describe another value that can be placed on biodiversity. The organisms that live, grow, reproduce and interact within ecosystems help to maintain local and regional flows of energy and materials.
Energy and material flows within ecosystems contribute to many ecological or life support services that benefit human welfare such as greenhouse gas regulation, water treatment, erosion control, soil quality control and plant growth.
It is difficult to determine whether biodiversity is important to prime ecosystem functioning because many of the factors that reduce local biodiversity such as habitat conversion also directly affect many ecological processes, masking the more subtle impacts of species loss on functioning. Despite this, as I will explain later in this article, some experiments around the world have proved that biodiversity loss does directly affect ecosystem functioning for the worst and that ecosystems are indeed sensitive to changes in the numbers and kinds of species found in their communities. Ecosystem functioning can be expressed as the collective life activities of plants, animals and microbes and the effects these activities (feeding, growing, moving excreting waste) have on the physical and chemical conditions of their environment. For example, a functioning forest exhibits rates of plant production, carbon storage and nutrient cycling that are characteristic of most forests. Furthermore, if the forest is converted to an agroecosystem (converted for agricultural purposes) its functioning changes.
Although every organism contributes to ecosystem functioning some play a more important role than others and contributions to ecosystem processes vary. It is however, very difficult to assess the individual contributions of certain species or populations. Groups of species can be placed into categories known as functional types. Species within groups such as grazing mammals, large predators, perennial grasses or nitrogen fixing microbes may be functionally similar despite their uniqueness in genes, life history and other traits. Species that play an important role in the regulation of ecosystem processes can be known as keystone species or ecosystem engineers and have high importance in the community. The loss of such species can cause an obvious impact on the ecosystem. For example, a species of nitrogen fixing tree, Myrica faya, introduced to the Hawaiian Islands has had large scale effects on nitrogen cycling, greatly increasing the amount of this essential plant nutrient in soils where the tree invades. The Moose Alces alces, through their dietary preferences, greatly reduce soil nitrogen levels and also influence the succession of trees in the forests they inhabit. Through feeding and dam building beavers not only alter soil fertility and forest succession but increase the diversity of ecosystems on a landscape. Termite species across many of the world’s great grasslands also play critical roles in soil fertility.
Since Darwin, many biologists have hypothesized about the relationship between biodiversity and ecosystem functioning but it is only recently that the importance of biodiversity to maintaining healthy ecosystems throughout the world has come to light.
Scientific research has attempted to answer two questions about the link between biodiversity and ecosystem functioning. First, how are levels of ecosystem functioning affected by changes in biodiversity, particularly species richness? Second, how are the dynamics of ecosystem functioning, particularly the resilience and stability of processes, affected by changes in biodiversity?
Results from many experimental studies demonstrate that ecosystem productivity increases with species richness. These studies range from large outdoor experiments to controlled laboratory experiments conducted in growth chambers, greenhouses or small containers. Experimenting with these variables, however, is difficult because stability (ecosystem) is a long term attribute of a system and testing for it requires either long running experiments or experiments with short lived organisms. BIODEPTH, a pan-European biodiversity-ecosystem functioning experiment has shown that reductions in plant species richness lowered the resistance of grassland production to drought. This is all too obvious in Africa where clearing of native plant life for crop land has led to desertification. Furthermore, lower year to year fluctuations in community productivity was also much lower at lower diversity. This does suggest that for some ecosystems a linear relationship between biological diversity and ecosystem functioning can be observed, more noticeably with plants. Other research has also provided evidence that loss of functioning groups from a food web can cause declines in ecosystem functioning, although, within these functioning groups competing species can replace or compensate for one another and thus minimise, at higher diversity, the ups and downs in functioning. This would suggest that at least one species per functional group is essential to ecosystem functioning, but having more will insure the continuous functioning of the ecosystem in times of disturbance. Declining species richness can lead to declines in ecosystem functioning especially at lower levels of diversity and this knowledge is particularly relevant to current ecological change since most ecosystems are being transformed into managed systems.
So if ecosystem processes can falter in the face of reductions in biodiversity it will be important for future research to look at landscape and even biome scale processes and how they could react to the increasing rate of extinction and subsequent loss of biodiversity. Although biodiversity loss may have only small impacts on an ecosystem, it may reduce its capacity to adjust to changing environments in the future.
So if ecosystem functioning continues to decline due to direct threats or from threats posed by a loss of biodiversity how will this affect human wellbeing in the future?
Biodiversity plays a role in ecosystem functions that provide supporting, provisioning, regulating and cultural services. These services are essential for human wellbeing. Substantial ecosystem services are provided by coral reefs such as habitat construction, nurseries and spawning grounds for fish, nutrient cycling, carbon and nitrogen fixing and wave buffering and sediment stabilisation. The total economic value of reefs and their associated services are highly valued. All coral reefs require a symbiotic relationship with algae to survive, yet ocean acidification, climate change and increased co2 emissions threaten the survival of coral reefs by directly affecting their relationship with algae. Within a given habitat the preservation of its native species and the way they affect the functioning of an ecosystem seems to increase the resistance to invasions from non-native species. Such invasions can extirpate certain key species that may be of value to human wellbeing in that habitat and help to maintain prime ecosystem functioning allowing the continuing use of the ecosystem for human benefits. Pollination is essential for the survival of plant-derived ecosystem services yet worldwide declines in pollinator diversity is occurring due to habitat destruction and the use of pesticides. Fruits and vegetables that supply humans with food require pollinators especially in subsistence farming in developing nations. Natural pest control species can benefit food security to these farmers as well. This saves money on expensive biocides and increases cultural and aesthetic values of the agroecosystem. Biodiversity influences climate at local, regional and global scales and so changes in land use that affect biodiversity can affect climate. Plants function within ecosystems and affect their capacity to influence carbon storage, albedo, evapotranspiration, temperature and fire regime; all of which affect climate. For example, in the Amazon basin, 60% of precipitation comes from water transpired by upwind forest ecosystems. Biodiversity determines how much carbon is taken up from the atmosphere and how much is released back into it depending on the species involved. Forest ecosystems for example contain a lot of carbon, its stored in woody plants, with long life spans which decompose slowly. Forest fires, harvesting and wind-throw temporarily change forests from accumulating carbon to releasing it. Marine biodiversity transfers carbon from marine photosynthesising species through the food web to grazers, which then excrete carbon to the deep ocean as faecal pellets and dead cells. Marine microbial communities provide detoxification services but this process is still not well understood. An example of this service was the American oysters in Chesapeake Bay which were once highly abundant. They filtered out toxins from the Bay and helped with water quality.
Biodiversity loss will affect human wellbeing though degradation of ecosystem functioning. The global climate can be affected by loss of forests, freshwater ecosystems can be affected by eutrophication, the nitrogen and phosphorus cycles can be affected by the loss of biodiversity, and change in land use can not only affect biodiversity directly, but on the flip side, can be caused by the loss of biodiversity within ecosystems.
Vital links between the living natural world and humankind’s wellbeing are clearly visible. Are they likely to fade away before they are fully understood?
Key Reference: Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes http://www.cricyt.edu.ar/institutos/iadiza/ojeda/BiodiFuncio.htm