Maintaining biodiversity underpins the stability of ecosystems and the services that they supply to the community, such as food, drinking water, clean air, control of disease and raw materials for the development of medicinal drugs. These are essential to human health.
In 2010, Comunità Ambiente carried out a literature study on on the impact of biodiversity changes on human health, on behalf of the Directorate General Environment of the European Commission (contract number 07-0307/2009/533527/ETU/B3).
The objective of the study was to provide an overview of existing information concerning the impacts of changes in biodiversity and ecosystems on two services related to human health: regulation of infectious diseases and provision of medicines.
Human infectious diseases
The review focuses on infectious diseases in the human population, in particular vector-borne diseases (VBD), because, as pointed out in the Millennium Ecosystem Assessment:
• these diseases are highly sensitive to changes in the natural environment, i.e. environmental conditions affect both the infectious pathogens and the insects and other intermediate hosts that transmit them;
• many such infections are related to specific ecosystems (such as forests and wetlands);
• VBDs are major killers, causing approximately 1.4 million deaths per year worldwide. Due to the increasing impact of biodiversity changes they are expected to represent the largest share of the future disease burden.
The VBDs included in the review were selected on the following criteria:
• the direct impact of biodiversity and changes in ecosystems on their spread;
• their current high frequency and incidence on human health (VBDs affect over 700 million people every year worldwide);
• the occurrence of outbreaks outside their traditional areas (the so-called emerging diseases) and reappearance in areas where these diseases were considered eradicated or contained (the so-called re-emerging diseases).
The diversity of species of intact ecosystems can protect mankind against the emergence and spread of infectious diseases. Disease transmission cycles are generally kept in equilibrium by population limiting processes (such as acquired immunity to infectious disease, predation and competition for food) and by the carrying capacity limits of habitats for hosts and vectors.
In stable ecosystems each species occupies a particular position or niche and in so doing impedes the invasion of “foreign” species which may form part of an infectious disease cycle, either as predators, prey, hosts, vectors or parasites.
There is increasing evidence that greater species richness may decrease the spread of pathogens to humans. Species rich communities are more likely to be populated by highly competitive species which leave fewer vacant niches for possible invasion by species carrying infectious agents. Recent data indicate that higher host diversity (synonymous with species richness) may decrease the risk of disease through a “dilution effect”, i.e. a reduced likelihood that “vectors” (organisms which carry pathogens) come in contact with pathogen hosts.
However, although a greater diversity of hosts can reduce transmission rates of particular diseases, they may also harbour additional pathogens. The relative role of species richness versus species composition remains to be clarified, as changes in the level of biodiversity affect not the number of species but also their composition.
Alteration of natural ecosystems through human activity influences the distribution and incidence of vector-borne infectious diseases. Alterations to ecosystem are diverse and often interrelated. They include introduction of alien species; loss, fragmentation and deterioration of habitats; changes in the distribution and availability of surface water; changes in agricultural practice, urbanisation and other changes in land use. Ecological alterations directly or indirectly affecting the populations of the pathogen, the vector, or the nonhuman hosts of the pathogen and the context within which they interact, may disrupt their complex relationships, destabilise natural equilibrium and alter the epidemiology of vector-borne diseases. Conditions for disease transmission may be enhanced or transmission cycles disrupted.
Links between biodiversity change and infectious diseases of humans occur at all levels of biology, from genetics of individual organisms to the structural diversity of habitats.
Any disturbance in an ecosystem can induce:
• genetic changes in disease pathogens (e.g. change in pathogen virulence),
• changes in population dynamics of vectors or hosts species (abundance, diversity, composition, distribution), changes in the community (predation, competition, population density, etc.),
• changes in structural diversity (structure, complexity of habitats, size, fragmentation and distribution, area- species relationships).
Changes to an environment brought about by human activity can drive selection processes of vectors and pathogens leading to the expansion of those vector and strains suited to the new environmental conditions. An example of newly evolved pathogens include newly re-assorted influenza strains. The potential for mutability allows pathogens to switch hosts migrating into a new ecological niche. This ‘‘host transfer” is easier at the interface between wild communities and agricultural communities with high population densities of humans, domestic animals, and crops, where higher is the vector-host contact rate.
Some human infectious diseases are linked to population dynamics of vectors, hosts and pathogens, e.g. high risk or incidence of Lyme disease and West Nile virus may be closely associated with changes in the diversity or composition of animal hosts, which in turn is associated with certain types of habitat destruction and fragmentation.
The transmission of major human infectious vector-borne diseases such as malaria and yellow fever can be fairly described as connected to structural diversity. Changes in plant and habitat complexity, habitat fragmentation and alteration, particularly of forests and wetlands, linked to human settlements and activities, can create new breeding sites for the vector or alter the distribution, density and behaviour of reservoir host and their interactions with humans and increase contact with vectors.
Vector-borne infections of humans create the highest disease burden and will continue to do so in the future. This disease burden is concentrated in the poorest regions of the world, where VBDs are not only a result but also a cause of poverty. Malaria alone is responsible for approximately 11% of the total disease burden in Africa, while all VBDs combined account for less than 0.1% of the burden in Europe.
VBDs have long-term negative consequences on the social well-being and economic performance of low income countries, affecting human resources, inequality, education, productivity and learning capacity. Such diseases incur wide social and economic costs and prevent economic development, perpetuating the “poverty trap”.
In developed countries infectious diseases of humans imply high costs resulting from absence from work, medical care, hospitalisation and rehabilitation. Of particular note is the high cost of vaccination programmes to prevent possible pandemic outbreaks from new influenza strains.
Biodiversity loss diminishes the supplies of raw materials for traditional medicine and for drug discovery. Biological diversity, particularly plants, is a key source of medical products. Between 50,000 and 70,000 plant species are known to be used in traditional and modern medicine worldwide and almost every class of drug includes a model structure derived from nature. The loss of species could have immediate negative effects if it involves species currently used for medicinal purposes, and could also reduce the opportunity for the future discovery of new natural products which have medicinal properties if it involves species not yet studied for their pharmaceutical potential or even undiscovered. For example, tropical rainforests contain at least half of all world species, but less than 5% of tropical plant species have been studied for their pharmaceutical potential. This leaves great potential for even more discovery, but also the potential for great loss as rainforests are felled around the globe and unstudied species are lost to extinction. It is estimated that biodiversity loss is leading to the loss of about three potential new medicines each year.
Valuing biodiversity as a source of medicines could help towards economic sustainability of nature conservation. The economic value of medicines is considerable and in many parts of the world expenditure on traditional and complementary medicine is not only significant but growing rapidly. Moreover, herbal treatments are internationally highly lucrative. Although not included in formal national accounting, the use of medicinal plants makes a significant contribution to productive activities, incomes, and well-being in some communities. Biodiversity also remains a major source of bioactive compounds for modern medicine.
The links between biodiversity, ecosystem functioning and the spread of infectious diseases are complex and knowledge is still fragmentary. However, there are indications that the current decline in biodiversity and the widespread changes in ecosystems may generate increased risks of spread of several major human diseases. Maintaining biodiversity could also allow us to continue to obtain and develop medicines from natural products.
At present, a number human vector-borne diseases are returning to areas where they had been previously eradicated, and are on the increase in endemic areas as well as emerging in new countries. The serious health and economic impact of infectious diseases is expected to continue and even increase in the near future.
Better understanding of the links between changes in biodiversity, epidemiology of infectious diseases and availability of natural resources for medicines would be necessary for administrators to make informed decisions regarding the management of biodiversity concerning human health risks and to reduce the incidence of infectious diseases in humans.
Human infectious diseases
Infectious diseases prevention and treatment and biodiversity conservation decisions are generally considered separately, despite the links.
Although several studies indicate that changes in biodiversity affect the rate of transmission of infectious diseases to humans, the connections between infectious diseases and biodiversity are poorly understood and have only been partially studied and documented. Part of the reason is that the relation between the relevant components within ecosystems is still unclear. For example, the consequences of changes in a host or vector to the spread of an infectious disease are often still unknown. This shortfall in information makes estimates of future incidence of infectious diseases due to changes in biodiversity very difficult.
A health care strategy for a vector-borne disease must take into account its biology and ecology, the possible range of adverse effects, the risks and the costs and benefits of protective actions, in decision making.
Data connecting the socio-economic impact of changes in biodiversity and ecosystem services on human infectious diseases are very scarce. They are also difficult to compare, since they measure disease burden in different ways. Moreover, studies are generally at local level and do not allow comparison of results between developing and developed countries. As a consequence, it is still not possible to put a value on the loss of an infectious disease regulation “service” provided by an ecosystem.
However, studies analysing the relation between infectious diseases and incomes do propose public actions to combat infectious diseases which take into account development of immunity, disease ecology, changes in ecosystems and management of diseases.
Since the publication of the Millennium Ecosystem Assessment in 2005, there has been an increasing attention to the links between biodiversity and transmission of infectious diseases.
In recent years, and in particular during 2009, there have been numerous studies on the link between biodiversity changes and the spread of infectious diseases, an indication that this is becoming a “hot” topic of research.
Further interdisciplinary research (combining ecology, biology, epidemiology, pharmacology, medicine, social sciences and economics) is needed in order to better understand:
• the role of biodiversity in the emergence, spread and transmission of infectious diseases;
• the relations between man-made changes in ecosystems, biodiversity and transmission of infectious diseases to humans, including their economic impact;
• the value of biodiversity in protecting against infectious diseases.
Development of an integrated approach requires agreement on common terms and definitions.
Biodiversity provides an essential source of medicines, and will continue to do so in the future. With the development of new techniques in combinatorial chemistry (the rapid synthesis of drugs using large numbers of different but structurally related molecules) and chemical modification in the development of new drugs, one would expect a decline in the interest in natural products as a chemical source for medicines.
However, the same techniques also facilitate the screening of natural products and consequently revive the search for basic chemical material in organisms, which have never been used in traditional medicine. Thus, maintenance of biodiversity will be important in preserving a source of raw material for new drugs. Of particular interest are marine ecosystems and microorganisms.
Although there is no doubt as to the current reliance of traditional and modern medicines on biodiversity, only recently have scientists and economists combined forces to quantify biodiversity as a provider of medicines.
Further research is needed to evaluate biodiversity as a source of natural products for health care.