Professor Timothy Schowalter argues that a better understanding of insect contributions to ecosystem services will improve our ability to sustain delivery of ecosystem services.
Our increasingly urban population is losing appreciation for our dependence on the services ecosystems provide. Human survival and well-being depend on ecosystems for provisioning services (such as food, fiber and fresh water), cultural services (such as recreation and spiritual retreat) and supporting services necessary for resource production (such as primary production, pollination, decomposition, soil formation and biological control of ‘pests’). These services are provided at no cost, but their value is difficult to assess because only provisioning and some cultural and supporting services have market values. However, global value of ecosystem services has been estimated at US$33 trillion (~€28.7tr) annually. Our dependence on these services becomes apparent when delivery is threatened. For example, much attention is now being paid to the global decline of insect pollinators that are responsible for 35% of global food production. The cost of hand pollinating crops that require insect pollinators would be prohibitive.
Insects deserve particular attention because they are largely unrecognised regulators of ecosystem processes that sustain the delivery of services. While the critical role of pollinators in supporting global food supply has become widely recognised, insects are critical to virtually all other ecosystem services, as well. However, insects are also targets of pest control efforts that may have unintended consequences for ecosystem services.
How do insects contribute to ecosystem services?
Managing insects and ecosystem services in ways that ensure the sustainability of ecosystem services is not optional, but rather is critical to human survival. The consequences of undermining the sustainable delivery of ecosystem services include famine, water shortages, threats to human health and economic disruption. Social unrest and population displacement increase the likelihood of epidemics of crowd diseases, often vectored by insects.
Insects or their products are valuable food resources in many cultures. Honey has been among the most important commercial trade products for millennia. Grasshoppers, cicadas, caterpillars, beetles, and other insects are important sources of protein and revenue in many parts of the world. About 1,500 edible insect species are consumed by 3,000 ethnic groups in 113 countries. Because insects require less energy for metabolism than do warm-blooded vertebrates, insects produce protein 300-fold more efficiently than cattle. Investigators have found that harvesting grasshoppers for food during an outbreak in Mexico substantially reduced grasshopper abundance and reproduction and provided US$3,000 revenue per family, compared to US$150 cost per family for insecticide treatment had control tactics been implemented. Furthermore, replacing vertebrate protein with grasshopper protein would significantly reduce greenhouse gas emissions and the amount of grain required for protein production, thereby increasing grain availability for human consumption.
A number of insect species provide widely-used medical or industrial products. Blow fly maggots have been used for wound healing for thousands of years. Their medical value lies in their selective feeding on necrotic tissue, leaving clean tissue when they depart. Their use is seeing a renaissance because, unlike more invasive surgical techniques, maggots improve wound healing with less risk of infection. Insects also provide several important pharmaceutical compounds, such as cantharidin (from blister beetles) for wart removal, alloferon (from blow fly larvae), a powerful antimicrobial compound, and promising anti-cancer compounds from wasp venom. Insects are also a source of several important industrial products, especially silk and cochineal (red) dye. Silkworms are the only source of commercial silk, still among the most valued and widely-traded commercial products.
Insects are often the objects of tourist destinations. Many people travel to visit overwintering aggregations of monarch butterflies in Mexico, for instance, and related species in tropical Asia. Butterfly houses are popular in many areas, offering tourist opportunities to enjoy representative butterflies from around the world.
Supporting services include ecosystem processes necessary for the delivery of provisioning or cultural services. Insects are integral components of ecosystems and often provide important feedbacks and indirect effects that maintain consistency in rates of ecosystem processes in a changing environment.
Pollination services have generated the greatest attention to ecosystem services. Bees and other pollinators are necessary for the pollination of 60-90% of plant species and are critical to 35% of global crop production. Although honey bees are most often cited, thousands of native bee species and other pollinators are more efficient pollinators for many crops and other plants. The fruit set of highland coffee increases with increasing bee diversity. Insect pollinators (including hover flies, bumble bees and other insects, as well as honey bees) increase oilseed rape seed weight by 18% and market value by 20% per plant. Consequently, pollination services have recognised monetary value. Declining pollinator abundance in many areas has prompted efforts to implement land use and pest management tactics that promote pollinator activity and to protect or restore native habitats necessary to sustain pollinator diversity and abundance.
Although the current focus is on pollinators, insects are instrumental in maintaining other ecosystem services, as well, and their disappearance would threaten our quality of life. The decomposition of plant and animal detritus is necessary for the release of nutrients that become available for new plant growth and for the removal of disease organisms in carcasses. Termites and dung beetles, in particular, provide a major agricultural service by removing and burying livestock dung, thereby preventing the fouling of pasture forage by dung accumulation, increasing carbon and water storage in soil, reducing nitrogen loss via erosion and volatilisation, and reducing livestock losses due to blood-feeding flies.
An example from Australia illustrates the importance of dung beetles. Australia has native dung beetles adapted to feed on the dung of native marsupials, but they do not feed readily on the dung of introduced livestock. When accumulating livestock dung fouled pastures and threatened livestock production, the government launched an expensive, but ultimately successful, research programme to find, test and release dung beetle and predaceous mite species from other continents to remove dung and kill blood-feeding fly larvae that develop in dung. This programme illustrates the expense of replacing ecosystem services that may be lost.
Sustaining human survival and well-being depends on the consistent delivery of ecosystem services. Obviously, variation in supply can lead to overabundance or famine. Other organisms face similar challenges. Species that compose ecosystems are adapted to regulate each other’s populations through feedback mechanisms that minimise variation. Feedback works like the thermostat of an air conditioning system: when room temperature departs from a set temperature, the thermostat triggers an electrical feedback that turns on the heat or the air conditioner, as necessary, to return the room temperature to the set point. In ecosystems, similar feedback regulation occurs when a population increases above its carrying capacity (the maximum population size at which available resources can sustain the population). Overabundance and stress resulting from resource limitation initiate the population growth of predators and parasites, which reduce population growth of prey. As population growth declines, predation eases, and the population returns to carrying capacity.
A diversity of predaceous arthropods and insectivorous vertebrates provides the important regulation of prey populations, including many pest species. Biological control has been valued at US$5.4 billion per year in the USA. The economic benefit of natural biological control of coffee pests in Costa Rica has been estimated at US$75–US$310 ha-1 year-1, a benefit per plantation equivalent to the average annual income of Costa Ricans. Predation also controls abundances of disease vectors. The complex interactions that limit tick abundance and spread of Lyme disease depend on the diversity of small mammal species. Similarly, bird species diversity limits the abundance of reservoir hosts for West Nile Virus and reduces the incidence of human cases.
Although most people believe that herbivores are bad for plants, in fact native herbivores regulate primary production near carrying capacity in the same way that predators and parasites regulate populations of their hosts. Outbreaks are most likely when environmental changes increase host plant density or stress, creating favorable conditions for herbivore population growth. Outbreaks typically remove stressed plants and reduce the density of host plants, thereby increasing vegetation diversity and stimulating nutrient cycling, improving conditions for plant growth. Obviously, this may not meet management goals to maximise resource production, but would help meet management goals to sustain ecosystem services.
Insects contribute to ecosystem services in significant ways, in some cases mitigating their effects as pests. Managing insects and ecosystem services in ways that ensure the sustainability of ecosystem services is not optional, but rather is critical to human survival. Better understanding of insect contributions to ecosystem services will improve our ability to sustain delivery of ecosystem services.
Professor Timothy Schowalter
Louisiana State University
+1 225 578 1827