4.2: Regulating Services
Regulating services maintain nature’s ability to provide material contributions, usually in indirect ways. For example, our ability to obtain food indirectly depends on a variety of subtle yet important ecosystem processes (e.g. energy cycling), as well as more observable services (e.g. pollination). For that reason, regulation services are sometimes called indirect use values . Regulating services (together with nonmaterial contributions, discussed below) are also sometimes referred to as non-consumptive use values because they provide economic benefits without needing to be collected, harvested, consumed, converted, or destroyed during use.
The economic benefits we gain from regulating services are estimated to be larger than all the different kinds of material contributions together.
The economic benefits we gain from regulating services are estimated to be larger than all the different kinds of material contributions together, especially in areas where ecosystems are intact (Costanza et al., 2014). Even so, these benefits do not always appear in descriptions of national economies because those statistics generally focus on material contributions. Nonetheless, maintaining regulating services is very important. When damaged ecosystems cannot provide these benefits, substitute resources must be found—often at great expense—to avoid economic collapse. In Section 4.2.4, we discuss one such example, by considering the value and replacement costs of water maintenance services obtained from forests.
Regulating services can be subdivided into many different subcategories depending on context, each overlapping to varying degrees with one another. Following is a discussion of some prominent subcategories of regulating services.
Maintaining ecosystem stability
Perhaps the most important indirect contribution we gain from biodiversity is its ability to maintain conditions that enable life on Earth to persist. This principle complements the Gaia hypothesis, which proposes that all the biological, physical, and chemical properties on Earth interact to form a complex, self-regulating superorganism, and that these interactions maintain the conditions and processes necessary for life to persist (Lovelock, 1988).
There are two complementary theories that explain the importance of maintaining a variety of different species if one is to conserve this superorganism (Ehrlich and Walker, 1998). Originally proposed by American ecologist Paul Ehrlich, the rivet-popper hypothesis compares biodiversity to the rivets (some of which may be redundant) that hold an aeroplane together. Just as an aeroplane can only lose so many rivets before it falls apart, so will the progressive loss of species systematically weaken an ecosystem until the entire system collapses. A well-known example of the rivet-popper hypothesis is the mutualistic relationships many plants have with all the various pollinators and seed dispersers (Section 4.2.5), in this context representing the rivets holding the system together. We might not immediately notice the systematic loss of pollinators we are currently experiencing (Gallai et al., 2008; Dirzo et al, 2014), but eventually these losses will catch up with us, perhaps in the form of food insecurity.
Keystone species provide such an outsized contribution to ecosystem functioning that their loss will greatly alter ecosystem composition and functioning.
The species redundancy hypothesis, proposed by African ecologist Brian Walker, holds that biodiversity and ecosystem stability is best maintained not by focussing on preserving individual species, but by preserving redundancy in ecosystem functioning, by ensuring that each ecosystem is composed of a variety of (seemingly redundant) species performing similar roles. In other words, we should not focus our efforts on protecting just one or two important pollinating species, but a variety of them, to ensure that a variety of plants (and hence entire ecosystems) can also continue to survive. In this way, if one pollinator is lost due to an environmental disturbance or disease, the system will not collapse because other pollinating species will be able to compensate for the loss of that one species.
It is important to note that there are some individual species that provide such an outsized contribution to ecosystem functioning that their loss will greatly alter ecosystem composition and functioning. These “pilots” of natural ecosystems are generally known as keystone species (Figure 4.2). The keystone species concept was originally proposed after scientists observed that removing sea stars from intertidal zones allowed their prey (mussels) to increase uncontrollably which, in turn, pushed species, such as sea urchins and other shellfish, away, leaving an overall poorer ecosystem (Paine, 1969). Apex predators , such as lions (Panthera leo, VU) and cheetahs (Acinonyx jubatus, VU), are also keystone species because of their role in keeping herbivore populations under control. If these apex predators were to disappear, increasing herbivore populations would lead to overgrazing, and ultimately also herbivore declines. This top-down control predators exert on herbivores also answers one of modern ecology’s oldest questions: “why is the world green?” (Hairston et al., 1960).
An ecosystem engineer is a special type of keystone species that extensively modifies the physical environment, thereby creating and maintaining habitats for other species. Mount-building termites are important ecosystem engineers in many African ecosystems because their activities alter physical, chemical, and biological soil properties (Jouquet et al., 2011), and their massive mounts (some mounts are 10 m high, 20 m across, and may be over 2,000 years old) support distinctive ecological communities and serve as refuges for a large variety of animals and even plants (Loveridge and Moe, 2004; van der Plas et al, 2013). Elephants are also ecosystem engineers; their dramatic foraging habit of pushing over trees provides suitable habitats to countless small animals (Pringle, 2008). Elephants also open up dense vegetation, which allows grasses to thrive, in turn providing food for grazing antelope (Valeix et al., 2011). Holes dug by elephants sometimes make water more accessible, while elephant dung provides food for butterflies and dung beetles and creates an important germination environment for seeds and fungi. But too many elephants can also damage ecosystems by reducing the number of large trees on which other species depend (Cumming et al., 1997). It is important to remember that water is an important limiting resource for elephants (Chamaillé-Jammes et al., 2008), so there is a greater risk for elephants to become overly destructive in areas where humans artificially increase aboveground water availability.
Because so many species depend on ecosystem engineers and other keystone species for survival, their disappearance from an ecosystem can create an extinction cascade—a series of linked extinction events following one another. A related phenomenon known as a trophic cascade describes the situation where one keystone species’ loss has rippling effects at other trophic levels. Some of the best-studied trophic cascades involve apex predators and their role in suppressing prey populations (Estes et al., 2011), but disease pathogens can also be a keystone species that leads to trophic cascades. For example, the introduction of rinderpest from Asia to Africa in the late 1800s caused catastrophic ungulate population declines in East Africa through the early 1900s. With no primary consumers, grasslands were encroached by woody plants; these changes in the primary producer community also increased the intensity and frequency of wildfires, leading to cascading impacts throughout these savannah communities. An extensive vaccination program finally saw the disease eradicated in the 1960s, allowing ungulate population and grasslands to recover; and wildfires to become less destructive (Holdo et al., 2009).
The loss of keystone species from an ecosystem may create an extinction cascade—a series of linked extinction events following one another.
Maintaining ecosystem productivity
Plants and algae—in this context known as primary producers—use photosynthesis to capture and store energy from sunlight in their living tissue. This ability of ecosystems to generate living biomass, starting with plants trapping the sun’s energy, is known as ecosystem productivity. Primary consumers (i.e. herbivores) can then harvest this captured energy by eating plant material. The energy (and nutrition) obtained from plants enable herbivores to generate their own living biomass, in the form of growth and reproduction, before they, themselves, are consumed by secondary consumers (e.g. carnivores, predators, omnivores). This cycle ends (or starts, depending on one’s perspective) when decomposers and detritivores (e.g. fungi, earthworms, and millipedes) that break down complex plant and animal tissues into simple compounds such as nitrates, and phosphates. These simple compounds are then released into the soil and water, from where primary producers can take them up again.
Climate regulation
Many of us were taught from a young age that plants are the “lungs of the planet” (Figure 4.3) because they convert carbon dioxide (CO 2 ) into breathable oxygen (O 2 ) during photosynthesis. This contribution, whereby plants regulate the atmosphere’s CO 2 /O 2 balance through carbon absorption and storage (termed carbon sequestration) forms part of the atmospheric carbon cycle and plays a major role in regulating global climate patterns. The reduction in plant life through deforestation or other human activities is thus of major concern because of the reduced capacity of plants to sequester atmospheric carbon dioxide, a greenhouse gas that contributes to climate change (Chapter 6). The important role of plant communities in the atmospheric carbon cycle is now even being recognized by global markets. For example, the carbon-storing capacity of the Congo Basin’s forests has an estimated value at over US $2.5 billion per year (Hughes, 2011). As part of the worldwide effort to reduce carbon dioxide emissions and address climate change, industrial countries and corporations have started paying some landowners to preserve and restore ecosystems that store significant amounts of carbon (Section 10.4).
Plants are also important in regulating regional climate conditions by influencing both the water cycle via transpiration, and local heating and cooling via solar radiation absorption. For example, forests and other vegetation often absorb more heat than bare soil due to their respective albedos. Because heat rises, heat absorbed by vegetation enables water vapor released by plants via transpiration to rise higher into the atmosphere, where it subsequently condenses and falls as rain. In contrast, the loss of vegetation is often associated with reduced rainfall (Garcia-Carreras and Parker, 2011), which can in turn reduce agricultural productivity and biodiversity (Lawrence and Vandecar, 2015).
Lastly, trees keep local areas cool by providing shade and releasing water vapor into the atmosphere (Morakinyo et al., 2013; Kardan et al., 2015). This cooling effect increases people’s comfort and work efficiency, and reduces the need for fans or air conditioners, leading to higher productivity and cost savings (Balogun et al., 2014; Ogueke et al., 2017). Trees also act as windbreaks, thereby reducing evaporation and erosion in agricultural areas, and reducing the loss of heat from homes and other buildings in cold weather. The value of shade trees is also recognized in agro-ecosystems, as a strategy for coffee and cacao farmers to increase crop yields (Section 14.1.1) and to adapt to increasing temperatures due to climate change (Jaramillo et al., 2011).
Conserving soil and water quality
Wetlands play a prominent role in flood control. They are also very effective in immobilizing pathogens and toxic pollutants released into the environment by human activities.
Wetlands play a prominent role in regulating soil and water quality, as well as flood control. During heavy rains, wetlands slow the speed of rushing floodwater, which lowers flood height and reduces erosion. Wetlands also act as natural sponges: they absorb vast amounts of floodwater during heavy rains, which is then released more slowly and evenly afterwards, thereby maintaining water sources used for drinking, irrigation, hydropower generation, and transport. Wetlands are also very effective in breaking down and immobilising pathogens, toxic pollutants, and excess nutrients released into the environment from agricultural activities, sewage, industrial wastes, and pesticides. One study from South Africa found that wetlands were almost 100% effective in preventing further spread of highly toxic organophosphorus pesticides (Schulz and Peall, 2001).
Wetlands are, however, not the only ecosystem that maintain soil and water quality and quality. In fact, maintaining complex and adaptive ecological communities of all kinds are of vital importance in buffering ecosystems against flooding and drought, protecting fertile soils, and maintaining water quality (see also Section 10.2.1). In intact ecosystems, plant foliage and dead leaves intercept rain, which slows the flow of water from upper reaches of catchment areas into streams and rivers; this allows for a slow release of water for days or even weeks after rains have ceased. Soil is anchored in place by plant roots and aerated by soil organisms; this combination increases the soil’s capacity to absorb water and hold nutrients. All these aspects together reduce flooding and limit erosion of fertile topsoil which, in turn, limits loss of essential nutrients that would otherwise occur after heavy rains.
The economic benefits of water quality maintenance services provided by intact plant communities are enormous. In the late 1980s, the New York City administration paid US $1.5 billion to local authorities in rural New York State to protect their water supplies by maintaining forests in the catchment area that surrounded the city’s reservoirs, and by improving agricultural practices in the catchment area. While US $1.5 billion may seem like a lot of money, at the time it was considered a pittance compared to the US $9 billion that the man-made water filtration systems—doing the same job—would have cost over just the first 10 years in operation (NRC, 2000).
A situation very similar to the one in New York is currently playing out in Kenya. The Mau Forest Complex is one of East Africa’s largest montane forests and serves as the principle catchment area for waters that flow into the famed Mara River and Lake Victoria. But large-scale deforestation in the Mau Forest Complex over the past few decades (Figure 4.4) has resulted in reduced water storage, flow regulation, groundwater discharge, and water purification, causing annual economic losses of over US $65 billion to Kenya’s energy, tourism, and agricultural sectors (UNEP, 2012). The situation in Kenya was so severe that the 2008 inauguration of a hydropower station was postponed due to low water levels; this station later achieved only 50% of its production capacity as a result of deforestation in the Mau Complex. To avoid further losses, the Kenyan government initiated a multi-stakeholder taskforce to investigate options to restore the Mau complex’s degraded forests (Prime Minister’s Task Force, 2009). Since then, tens of thousands of trees have been planted to reverse deforestation in the area.
Pollination and seed dispersal
Pollination describes the transfer of pollen grains from male parts of a flower to female parts to allow fertilization and production of offspring. Some plants can be pollinated by wind, but others require animals to pollinate their flowers; examples include birds, bats, bees, flies, butterflies, and other insects (Figure 4.5). These pollination services are important for the persistence of many wild plants, as well as for many fruit, seed, and vegetable crops that we utilise as food (Box 4.2). Research from The Gambia has shown that management practices that increase the abundance of bats and bees to contribute to increased yields and sweetness of African locust bean (Parkia biglobosa) crops (Lassen et al., 2012). In contrast, work done in Zambia, Mozambique, and Uganda showed that pollinator collapse could increase malnutrition rates by over 50% which, in turn, could increase death rates among children and mothers during childbirth (Ellis et al., 2015). Luckily, many agricultural systems in Africa are still friendly to pollinators (see Box 7.4). Given the dependency on animal-assisted pollination in many agricultural systems, it is critical to maintain or expand pollinator-friendly practices. Our ability to continue benefitting from these services will depend on our ability to maintain and expand on those pollinator-friendly activities.
Abraham J. Miller-Rushing
Acadia National Park, US National Park Service,
Bar Harbor, ME, USA.
Pollinators and food security are so closely tied to one another they should almost be considered synonymous terms. But when people think of pollination, they often only think of honeybees, which people domesticated more than 8,500 years ago for honey production. However, wild pollinators, which include a variety of insects, birds, and mammals, are often more effective at pollinating than honeybees are. One estimate suggests wild pollinators can double fruit production compared to honeybees (Garibaldi et al., 2013). This is most likely because the morphological and behavioral diversity of wild pollinators allow for more specialised pollination relationships with plants. For example, some wild pollinators have longer proboscis (i.e. insect tongues) that enable them to pollinate deeper flowers (Figure 4.A), something honeybees cannot do. African crops rely even more on wild pollinators than do crops in other areas of the world because it can be difficult to maintain aggressive African honeybee hives and prevent them from being damaged by wild animals (African Pollinators Initiative, 2007).
Eggplant, papaya, coffee, and palm oil—crops of huge economic and cultural importance—highlight the value of wild pollinators to local and global economies. Eggplants are hermaphroditic; in other words, they can self-pollinate. Even so, pollination from two wild bee species, namely the doubleband carpenter bee (Xylocopa caffra) and a type of sweat bee (Lipotriches rufipes), increase fruit production far beyond that of self-pollination (Gemmil-Herren and Ochieng, 2008). In contrast, papaya trees are dioecious (i.e. they have separate male and female trees) and thus depend on crosspollination (i.e. pollinators take pollen grains from male flowers on one tree to female flowers on another tree) to produce fruit. While a wide variety of wild bees and butterflies visit papaya flowers, only some hawkmoths and skipper butterflies are effective papaya pollinators, probably because they have long proboscises that can penetrate the deep papaya flowers (African Pollinators Initiative, 2007). A healthy and diverse pollinator community also help coffee plants (which relies on a variety of pollinators, Samnegård et al., 2014) and oil palm (which requires cross pollination by specialist oil palm weevils, African Pollinators Initiative, 2007) produce more fruits, thereby increasing their economic value.
Despite their value to natural ecosystems and food security, wild pollinator populations are declining worldwide (Gallai et al., 2008; Dirzo et al., 2014). To avoid losing them forever, it is important to preserve wild pollinators through the conservation and restoration of native ecosystems (Chapter 10), sustainable agricultural practices, such as the reduced use of pesticides and herbicides (Section 14.1.1), and by communicating the value of pollinators to the general public, land managers, and politicians. Additionally, monitoring and research programs aimed at pollinators could enhance our understanding of their value, ecology, and conservation.
Many fruit and seed-bearing plants also depend on a process called seed dispersal to reproduce, colonise vacant habitats, and avoid competing with parent plants for limiting resources. Seed dispersal describes the physical movement of seeds by fruit-eating and seed-eating birds, large herbivores, primates, and a range of other animals away from the parent plant. Due to specialised features, some seeds can stick to animals’ fur, allowing them to be carried along much further distances than wind could, and different directions than water could. Many animals also consume seeds and fruits, providing opportunities for dispersal when the consumer moves off looking for more food, a resting spot, or mates to interact with. For some plants, seed dispersal involves a critical step required for germination, namely seed scarification . One method of scarification involves an animal breaking the seed’s hard coat by biting it. Alternatively, stomach acids may weaken the consumed seed’s hard coat while it passes through the animal’s digestive tract. Without this step, seeds requiring scarification may not be able to germinate; those plants’ persistence thus depends upon the animals that consume them. While the importance of pollination for food security is well known, the importance of seed dispersal should not be underestimated. A study from Côte d’Ivoire found that primates provided necessary seed dispersal services for at least 25 fruiting plant species important to humans (Koné et al., 2008).
Hazard detection and mitigation
When intact, nature is our first line of defence against many natural disasters. Consider, for example, the contribution of mangrove swamps in protecting us from cyclones/hurricanes (van Bochove et al., 2014), or the contribution of wetlands in flood control (Section 5.3.3). In contrast, degrading the natural environment can have severe consequences. For example, a 2010 landslide in Uganda that buried three villages, killing over 300 people and displacing 8,000 more, was attributed to deforestation activities three years earlier (Gorokhovich et al., 2013). To prevent such disasters, and harness all the other contributions of forests, there are numerous projects across Africa working to reverse deforestation (Section 10.3). Unfortunately, Africa’s tropical forests regenerate very slowly—sometimes requiring more than 100 years (Bonnell et al., 2011). It is thus critical to prevent ecosystem degradation in the first place, rather than having to resort to costly restoration projects.
In addition to keeping us safe, biodiversity can also be used to help track environmental changes. Species used for this purpose, called indicator species or environmental monitors are, by definition, associated with unique environmental conditions or sets of ecosystem processes. Tracking changes in their population sizes, distributions, and behavior of can thus serve as a substitute for expensive detection equipment (Section 10.1). Aquatic filter feeders, such as mussels and clams are particularly useful in this regard because their tissues accumulate chemical pollutants. A study from Senegal’s mangrove swamps detected heavy metal pollution using clams, mussels, and snails after tests barely detected those pollutants in the area’s sediments (Bodin et al., 2013). But even common everyday species can serve as indicator species: for example, conservation authorities around the world are using bird abundances and behaviors to better understand the impact of climate change ( http://climatechange.birdlife.org ).
Sentinel species are a special type of indicator species that can act as an early warning system for environmental hazards because they are more sensitive to certain conditions than humans are. Lichens are particularly well-known sentinel species. Being sensitive to air pollution and chemicals in rainwater, some lichens cannot survive in polluted areas. Thus, their presence is generally a sign of good air quality, while their absence may signal air pollution (Bako et al., 2008). Another example is seabirds, whose declining populations can serve as an early-warning system for overfishing (Paiva et al., 2015). Some sentinel species can even be used directly for human health purposes. For example, the non-profit NGO APOPO has been taking advantage of the incredibly fine sense of smell of southern giant pouched rats (Cricetomys ansorgei LC)—affectionately called HeroRATs—to detect landmines (Figure 4.6), tuberculosis (Reither et al., 2015), salmonella infections (Mahoney et al., 2014), and even people trapped under collapsed structures (LaLonde et al., 2015).
Lastly, some species can be used to mitigate various sources of pollution. For example, through a process called biosorption, the superior absorption capabilities of some lichens, plants, fungi, and microorganisms offer some of the cheapest and most effective methods for removing toxic heavy metals (Fosso-Kankeu and Mulaba-Bafubiandi, 2014) from the environment. Scientists also recently discovered a plastic-eating fungus (Khan et al., 2017) that may provide a potential solution to plastic pollution.
Pest and disease control
Every day, predators, such as owls and bats, keep us healthy by controlling populations of disease vectors, such as rats and mosquitoes. This process, where predatory (and parasitic) species regulate populations of pests and other nuisance species, is known as biological control, or biocontrol in short (Box 4.3). The use of insectivores (i.e. insect-eating species), such as bats and birds, to control crop pests is well established in traditional farming systems (Abate et al., 2000). But even on commercial crop farms, natural enemies, such as bats and birds, play an important role in keeping pests under control (Taylor et al., 2018). Some plants also play a part in biocontrol efforts: recent research found that some native plants used for intercropping in traditional agricultural systems emit chemical signals that kill and drive pest species away from crops (Khan et al., 2010). With an increasing number of studies illustrating the significant benefits gained from natural pest control systems, enhanced farming practices that facilitate greater ecosystem complexity (Section 14.1.1) will hopefully play a bigger role in food security in future.
Meg Boeni and Richard Primack
Biology Department,
Boston University,
Boston, MA, USA.
As it stands along the farm-plot boundary,
its base appears beautiful like a bride’s feet…
O cassava to whom the bembe drum beats a salute
that never reaches an end…
It is no small service the cassava renders us in this our land
Yoruba Poem (Babalola, 1966)
This traditional song from Nigeria praises the cassava, a South American crop brought to tropical Africa in the 16th century, and upon which millions of Africans have since relied for food and income.
Disaster struck in the 1970s, when an agricultural scientist that brought a new variety of cassava from South America to Africa also accidentally introduced a new pest: the cassava mealybug (Phenacoccus manihoti) (Neuenschwander, 2001). Previously unknown to science, the bug attacked the new shoots of cassava plants, laying its eggs at their tips and stripping them of their leaves. As it spread through Central and West Africa, the mealybug wiped out 80–90% of the productivity of most cassava fields, threatening large parts of Africa with famine.
With so many Africans relying on the cassava as a primary food source, scientists had to find a solution, and quickly. The bug’s waxy coating that protected it from pesticides complicated this effort. With conventional pest-control methods failing, scientists turned to biological control, hoping that introducing a natural predator would counteract the spread of the invasive insect. Researchers searching for the source of the mealybug finally found a candidate in the fields of Paraguay, where cassava, known locally as mandioca, was also an important food staple. Here, investigators discovered that mealybug numbers were kept low by a tiny wasp called Anagyrus lopezi that attack the mealybugs’ eggs and larvae (Figure 4.B). A. lopezi passed laboratory tests for host specificity—it fed and bred exclusively on cassava mealybugs and would not attack other African insects. And so, the International Institute of Tropical Agriculture (IITA) began field tests using the wasp as a biological control agent.
Results were astounding; the quick-spreading Paraguayan wasp reduced crop losses by an impressive 95% (Neuenschwander, 2001), all without the danger of pollution and poisoning associated with traditional pesticides. While identifying and introducing the biocontrol agent required significant resources, estimates suggest gains of 370–740 times the original investment, depending on the region considered (Zeddies et al., 2009), making it well worth the cost. Today, A. lopezi is found everywhere where the cassava mealybug survives in Africa. Bolstered by this success, the IITA has subsequently expanded its biological control programs to fight tropical pests on crops, such as cowpeas, maize, and bananas.
In 2008, the cassava mealybug was discovered in Southeast Asia, where it repeated the damage inflicted in Africa (Graziosi et al., 2016). Scientists are now replicating Africa’s biocontrol efforts to reduce crop failure in Vietnam, Thailand, Cambodia, and China. In conjunction with a number of local parasites, they hope that A. lopezi will halt the spread of the mealybug before it reaches even larger fields in India (Parsa et al., 2012). The control of the cassava mealybug is certainly one case where biological control was able to achieve great success.
Most Africans are familiar with scavengers, such as jackals and vultures, that work as nature’s clean-up crew, picking at food scraps left in the field by large predators. Together with the range of flesh-eating insects, detritivores, and decomposers, scavengers play a crucial role in keeping us healthy by sanitising the environment (O’Bryan et al., 2018). While it is all too easy to take these activities for granted, some people actively welcome these services. For example, in northern Ethiopia, spotted hyenas ( Crocuta crocuta , LC) are tolerated in urban settlements because they consume livestock carcasses and sometimes even human corpses, which pose a disease risk (Yirga et al., 2015).
Recent experiences have shown that without proper care, the sanitary services provided by wildlife can collapse over a very short time. For example, during what is known as the Asian vulture crisis of the 1990s, vulture populations in India, Pakistan, and Nepal declined precipitously in a matter of years from secondary poisoning after eating carcasses of dead animals treated with the anti-inflammatory drug diclofenac. With nothing else available to remove carcasses of dead animals as efficiently as vultures, rotting flesh contaminated drinking water and allowed populations of rats and feral dogs (Canis familiaris) to proliferate. While vultures have stomach acids which kill pathogens, dogs and rats do not and thus became major pathogen vectors, spreading deadly diseases such as rabies, anthrax, and plague. The estimated healthcare costs in the face of Asia’s vulture crisis amounted to over US $1 billion per year (Markandya et al., 2008). Today, Africa is facing its own vulture crisis. But instead of one threat, Africa’s vultures face a multitude of human-made threats, making solving this crisis much more complex (Box 4.4).
Scavengers such as vultures and jackals are nature’s clean-up crew; they keep us healthy by sanitising the environment.
Ara Monadjem
Department of Biological Sciences, University of Eswatini,
Kwaluseni, Eswatini.
A common perception among laypeople and conservationists alike is the idea of safety in numbers for wildlife species. After all, is a widely distributed and abundant species not safe from the threats of extinction? The answer is a firm no! As the collapse of central Asia’s vulture populations (Oaks et al., 2004) demonstrates, species numbering in the millions can disappear in the space of just a few years.
The Asian vulture crisis shares some similarities with the demise of the passenger pigeon (Ectopistes migratorius, EX) in North America a century ago. This pigeon was once the most abundant bird on Earth; yet, despite numbering in the billions, it was driven to extinction in a short span of time, primarily due to hunting over a 20-year period in the late 1800s. In the case of Asian vultures, the threat was not hunting, but rather a nonsteroidal anti-inflammatory drug (NSAID)—diclofenac—which is fed to sick cattle and then ingested by vultures when they feed on dead livestock. As diclofenac is deadly toxic to vultures, the widespread use of this treatment on the Indian subcontinent (which includes India, Nepal, and Pakistan) has seen catastrophic vulture population declines. With one of Asia’s major natural trash disposal systems gone, the area experienced a human health crisis from widespread drinking water contamination and increased incidence of diseases carried by ubiquitous and increasing rat and feral dog populations (Markandya et al., 2008).
The Asian vulture crisis is instructive on several grounds. First, it took a long time to detect and confirm the vulture declines because regular and standardised monitoring of the three affected vulture species had not been conducted. Second, the extent of the decline was extreme, with vulture numbers declining by over 95% within a decade. Third, the declines were due to a single threat—contamination from diclofenac, which were subsequently found to be deadly-poisonous to vultures (Oaks et al., 2004). Thanks to the concerted efforts of conservationists and politicians, and the rapid reactions of the governments of India, Pakistan, and Nepal, diclofenac was removed from the market in 2006. Vulture populations in Asia have since stabilised, with even a cautious suggestion of an increase.
Now, Africa faces its own vulture crisis (Ogada et al., 2015). However, in contrast to the Asian crisis, Africa’s crisis involves a greater number of species, and spans a larger geographical area. Importantly, it also includes a greater number of threats, including poisoning, harvesting for traditional medicine and for food, and electrocution following contact with power lines. Many vultures also die when they scavenge on poisoned carcasses meant to kill problem predators (Figure 4.C). To this list of lethal causes, one should also add the universal threats of habitat loss and persecution of birds of prey.
Thanks to long-term monitoring, the collapse of African vulture populations has been well documented. Of the 95 vulture populations being monitored, 89% were either extirpated or experienced severe declines. Across eight study species, the mean rate of decline is estimated at 4.6% per year (i.e. one out of 20 birds that are dying per year are not being replaced). The charismatic Rüppell’s vulture ( Gyps rueppellii , CR) has declined by 85% across its range; consequently, this species is now considered highly threatened by the IUCN, as are the hooded vulture ( Necrosyrtes monachus , CR), white-headed vulture ( Trigonoceps occipitalis , CR), and African white-backed vulture (Gyps africanus, CR). Only slightly better off, at least for now, are the lappet-faced vulture ( Torgos tracheliotos , EN) and Egyptian vulture ( Neophron percnopterus , EN), the Cape vulture (Gyps coprotheres, VU), and the bearded vulture ( Gypaetus barbatus , NT).
The collapse of Africa’s vulture populations is cause for serious concern among conservation biologists, wildlife and livestock managers, and public health officials. Unlike in Asia, however, workable solutions to Africa’s vulture crisis have not yet been found. This may be due to the multitude of threats, and the complexity of the problem exacerbated by the involvement of individual poachers, local communities, and government structures across more than 40 countries. If conservationists and governments can work together, as they did in Asia, then perhaps Africa’s vultures and the ecosystem services that they provide can still be saved.