9.1: Ecosystem Diversity
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Measuring biodiversity on a large scale involves measuring ecosystem diversity, the number of different ecosystems on Earth or in a geographical area as well as their relative abundances (figure \(\PageIndex{a}\)). The loss of an ecosystem means the loss of the interactions between species and the loss of biological productivity that an ecosystem is able to create. An example of a largely extinct ecosystem in North America is the prairie ecosystem (figure \(\PageIndex{a}\)). Prairies once spanned central North America from the boreal forest in northern Canada down into Mexico. They are now all but gone, replaced by crop fields, pasture lands, and suburban sprawl. Many of the species survive, but the hugely productive ecosystem that was responsible for creating our most productive agricultural soils is now gone. As a consequence, their soils are now being depleted unless they are maintained artificially at great expense.
The soil productivity described above is an example of an ecosystem service. These are the products and processes associated with biological systems and are directly or indirectly of immense value to the well-being of people. Some ecosystem services are processes such as the regulation of climate, flooding, and disease. Nutrient cycling, pollination, and regulation of crop pests are ecosystem services important to food production (see the Food Production and Ecosystem Services box below). The water cycle provides fresh water, and photosynthesis adds oxygen to our air. Other ecosystem services are human provisions including food, fuel, and fiber (such as cotton for clothing or timber). Medicines are another important provision (see Importance of Species Diversity). Furthermore, healthy ecosystems allow for recreational activities, such as hiking, kayaking, and camping, and educational opportunities, such as field trips. Nature is also the basis for a significant part of aesthetic and spiritual values held by many cultures.
In 1997, Robert Costanza and his colleagues estimated to annual value of ecosystem services to be $33 trillion dollars ($53 trillion in 2019 dollars), and many consider this to be an underestimation. For comparison the gross domestic product of the United States in 2020 was $21 trillion. Valuing ecosystem services can be difficult, particular for those services that are processes rather than provisions. One strategy is to calculate replacement cost. For example, how much would it cost to control a pest population if it was not regulated by natural processes? Figure \(\PageIndex{b}\) illustrates the value of a few ecosystem services.
Food Production and Ecosystem Services
Food production relies on several interacting ecosystem services. Most soils contain a huge diversity of organisms that maintain nutrient cycles, breaking down organic matter into nutrient compounds that crops need for growth. These organisms also maintain soil texture that affects water and oxygen dynamics in the soil that are necessary for plant growth. Replacing the work of these organisms in forming arable (farmable) soil is not practically possible. They occur within ecosystems, such as soil ecosystems, as a result of the diverse metabolic activities of the organisms living there, but they provide benefits to human food production, drinking water availability, and breathable air.
Other key ecosystem services related to food production are plant pollination and crop pest control. It is estimated that honeybee pollination within the United States brings in $1.6 billion per year; other pollinators contribute up to $6.7 billion. Over 150 crops in the United States require pollination to produce. Many honeybee populations are managed by beekeepers who rent out their hives’ services to farmers. Honeybee populations in North America have been suffering large losses caused by a syndrome known as colony collapse disorder, a phenomenon with complex and interacting causes.
Other pollinators include a diverse array of other bee species and various insects and birds. Loss of these species would make growing crops requiring pollination impossible, increasing dependence on other crops. A 2002 study by Claire Kremen and colleagues found that native pollinators in Central California (those that historically occurred there; figure \(\PageIndex{c}\) and d) provided full pollination to watermelon crops. This was only true on organic farms that were located near the natural habitat for these pollinators, highlighting the importance of sustainable farming practices and habitat conservation in preserving ecosystem services. Essentially, in a healthy ecosystem, native pollinators eliminated the need for rented honeybee hives. Note that while honeybees are valuable in an agricultural setting, they are not native to North America and can disrupt ecosystems by competing with native bees.
Finally, humans compete for their food with crop pests, most of which are insects. Pesticides control these competitors, but these are costly and lose their effectiveness over time as pest populations adapt. They also lead to collateral damage by killing non-pest species as well as beneficial insects like honeybees, and risking the health of agricultural workers and consumers. Moreover, these pesticides may migrate from the fields where they are applied and do damage to other ecosystems like streams, lakes, and even the ocean (see Industrial Agriculture and Environmental Toxicology). Ecologists believe that the bulk of the work in removing pests is actually done by predators and parasites of those pests, but the impact has not been well studied. A review found that in 74 percent of studies that looked for an effect of landscape complexity (forests and fallow fields near to crop fields) on natural enemies of pests, the greater the complexity, the greater the effect of pest-suppressing organisms. Another experimental study found that introducing multiple enemies of pea aphids (an important alfalfa pest) increased the yield of alfalfa significantly. This study shows that a diversity of pests is more effective at control than one single pest. Loss of diversity in pest enemies will inevitably make it more difficult and costly to grow food. The world’s growing human population faces significant challenges in the increasing costs and other difficulties associated with producing food.
References
Costanza, R., d'Arge, R., de Groot, R. et al. The value of the world's ecosystem services and natural capital. Nature 387, 253–260 (1997). DOI
Kremen, C., Williams, N. M., and Thorp, R. W. Crop pollination from native bees at risk from agricultural intensification. PNAS 99, 6812-16816 (2002). DOI
Attributions
Modified by Melissa Ha from the following sources:
- Importance of Biodiversity from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)
- Biodiversity, Species Loss, and Ecosystem Function from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (licensed under CC-BY). Download for free at CNX.