The Biodiversity Crisis
Scientists estimate that species extinctions are currently 500–1000 times the normal, or background, rate seen previously in Earth’s history. The current high rates will cause a precipitous decline in the planet’s biodiversity in the next century or two. The loss of biodiversity will include many species we know today. Although it is sometimes difficult to predict which species will become extinct, many are listed as endangered (at great risk of extinction). However, many extinctions will affect species that have not yet been discovered. Most of these “invisible” species that will become extinct currently live in tropical rainforests like the Amazon basin. These rainforests are the most diverse ecosystems on the planet and are being destroyed rapidly by deforestation. Between 1970 and 2011, almost 20 percent of the Amazon rainforest was lost.
Biodiversity is a broad term for biological variety, and it can be measured at a number of organizational levels. Traditionally, ecologists have measured biodiversity by considering the number of species and the number of individuals of each species (known as relative abundance). However, scientists are using different measures of biodiversity, including genetic diversity, to help focus efforts to preserve the biologically and technologically important elements of biodiversity.
Biodiversity loss refers to the reduction of biodiversity due to displacement or extinction of species. The loss of a particular individual species may seem unimportant to some, especially if it is not a charismatic species like the Bengal tiger or the bottlenose dolphin. However, the current accelerated extinction rate means the loss of tens of thousands of species within our lifetimes. Much of this loss occurs in tropical rainforests like the one pictured in Figure 1, which are very high in biodiversity but are cleared for timber and agriculture. This is likely to affect human welfare through the collapse of ecosystems dramatically.
Environmental scientists recognize that human populations are embedded in ecosystems and depend on them, just as is every other species on the planet. Agriculture began after early hunter-gatherer societies settled in one place and heavily modified their immediate environment. This cultural transition has made it difficult for humans to recognize their dependence on living things other than crops and domesticated animals. Today our technology smooths out the harshness of existence and allows many of us to live longer, more comfortable lives. Still, ultimately the human species cannot exist without its surrounding ecosystems. Our ecosystems provide food, medicine, clean air and water, recreation, and spiritual and aesthetical inspiration.
Types of Biodiversity
A common meaning of biodiversity is simply the number of species in a location or on Earth; for example, the American Ornithologists’ Union lists 2078 species of birds in North and Central America. This is one measure of the bird biodiversity on the continent. More sophisticated measures of diversity take into account the relative abundances of species. For example, a forest with ten equally common species of trees is more diverse than a forest with ten species, wherein just one of those species makes up 95 percent of the trees. Scientists have also identified alternate measures of biodiversity, some of which are important in planning how to preserve biodiversity.
Genetic diversity is one alternate concept of biodiversity. Genetic diversity is the raw material for evolutionary adaptation in a species and is represented by the variety of genes present within a population. A species’ potential to adapt to changing environments or new diseases depends on this genetic diversity.
It is also useful to define ecosystem diversity: the number of different ecosystems on Earth or in a geographical area. The loss of an ecosystem means the loss of the interactions between species and the loss of biological productivity that an ecosystem can create. The prairie ecosystem is an example of a large extinct ecosystem in North America (Figure 2). 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 species survive, but the hugely productive ecosystem 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 decline in soil productivity occurs because the interactions in the original ecosystem have been lost.
Current Species Diversity
Despite considerable effort, knowledge of the planet’s species is limited. A recent estimate suggests that only 13% of eukaryotic species have been named (Table 1). Estimates for the number of prokaryotic species are largely guesses, but that science has only begun cataloging their diversity. Given that Earth is losing species at an accelerating pace, science knows little about what is being lost.
| Estimated Numbers of Described and Predicted species | ||||||
|---|---|---|---|---|---|---|
| Source: Mora et al., 2011 | Source: Chapman, 2009 | Source: Groombridge and Jenkins, 2002 | ||||
| Described | Predicted | Described | Predicted | Described | Predicted | |
| Animals | 1,124,516 | 9,920,000 | 1,424,153 | 6,836,330 | 1,225,500 | 10,820,000 |
| Photosynthetic protists | 17,892 | 34,900 | 25,044 | 200,500 | — | — |
| Fungi | 44,368 | 616,320 | 98,998 | 1,500,000 | 72,000 | 1,500,000 |
| Plants | 224,244 | 314,600 | 310,129 | 390,800 | 270,000 | 320,000 |
| Non-photosynthetic protists | 16,236 | 72,800 | 28,871 | 1,000,000 | 80,000 | 600,000 |
| Prokaryotes | — | — | 10,307 | 1,000,000 | 10,175 | — |
| Total | 1,438,769 | 10,960,000 | 1,897,502 | 10,897,630 | 1,657,675 | 13,240,000 |
There are various initiatives to catalog described species in accessible and more organized ways, and the internet is facilitating that effort. Nevertheless, at the current rate of species description, which according to the State of Observed Species1 reports is 17,000–20,000 new species a year, it would take close to 500 years to describe all of the species currently in existence. The task, however, is becoming increasingly impossible over time as extinction removes species from Earth faster than they can be described.
Naming and counting species may seem an unimportant pursuit given the other needs of humanity, but it is not simply an accounting. Describing species is a complex process by which scientists determine an organism’s unique characteristics and whether or not that organism belongs to any other described species. It allows scientists to find and recognize the species after the initial discovery to follow up on questions about its biology. That subsequent research will produce the discoveries that make the species valuable to humans and our ecosystems. Without a name and description, a species cannot be studied in depth and coordinated by multiple scientists.
The International Union for the Conservation of Nature (IUCN), which coordinates efforts to catalog and preserve biodiversity worldwide, defines biodiversity as “the variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which they are part; this includes diversity within species, between species, and of ecosystems.” Rather than just species, biodiversity, therefore, includes variation from the level of genes and genomes to that of ecosystems to biomes.
Even within a single ecosystem, the number of species can be impressive. For example, Brazil has a large dry forest and savanna region known as the Cerrado (see Figure Cerrado Forest (Figure 4.5)). This ecosystem alone hosts over 10,000 species of plants, almost 200 species of mammals, over 600 species of birds, and about 800 species of fish.
Patterns of Biodiversity
Biodiversity is not evenly distributed on the planet. Lake Victoria contained almost 500 species of cichlids (just one family of fishes present in the lake) before introducing an exotic species in the 1980s and 1990s caused a mass extinction. All these species were found only in Lake Victoria, which is to say they were endemic. Endemic species are found in only one location. For example, the blue jay is endemic to North America, while the Barton Springs salamander is endemic to the mouth of one spring in Austin, Texas. Endemic species with highly restricted distributions, like the Barton Springs salamander, are particularly vulnerable to extinction.
Lake Huron contains about 79 species of fish, all of which are found in many other lakes in North America. What accounts for the difference in diversity between Lake Victoria and Lake Huron? Lake Victoria is a tropical lake, while Lake Huron is a temperate lake. Lake Huron, in its present form, is only about 7,000 years old, while Lake Victoria, in its present form, is about 15,000 years old. These two factors, latitude and age, are two of several hypotheses biogeographers have suggested explaining biodiversity patterns on Earth.
Biogeography is the study of the distribution of the world’s species in the past and present. The work of biogeographers is critical to understanding our physical environment, how the environment affects species, and how changes in the environment impact the distribution of a species.
There are three main fields of study under the heading of biogeography: ecological biogeography, historical biogeography (called paleobiogeography), and conservation biogeography. Ecological biogeography studies the current factors affecting the distribution of plants and animals. Historical biogeography, as the name implies, studies the past distribution of species. On the other hand, conservation biogeography is focused on protecting and restoring species based on known historical and current ecological information.
One of the oldest observed patterns in ecology is that biodiversity typically increases as latitude declines. In other words, biodiversity increases closer to the equator (Figure 4). Generally, biodiversity is greatest in tropical areas, especially “rainforests,” but terrestrial biodiversity “hotspots” exist on all the major continents.
Visit this website to view an interactive map of hotspots (https://www.conservation.org/priorities/biodiversity-hotspots).
It is not yet clear why biodiversity increases closer to the equator. Still, hypotheses include the greater age of the ecosystems in the tropics versus temperate regions, which were largely devoid of life or drastically impoverished during the last ice age. The greater age provides more time for speciation, the evolutionary process of creating new species. Another possible explanation is the greater energy the tropics receive from the sun. But scientists have not been able to explain how greater energy input could translate into more species. The complexity of tropical ecosystems may promote speciation by increasing the habitat complexity, thus providing more ecological niches. Lastly, the tropics have been perceived as being more stable than temperate regions, which have a pronounced climate and day-length seasonality. The stability of tropical ecosystems might promote speciation. Regardless of the mechanisms, it is certainly true that biodiversity is greatest in the tropics. There are also high numbers of endemic species.
Importance of Biodiversity
Loss of biodiversity may have reverberating consequences on ecosystems because of the complex interrelations among species. For example, the extinction of one species may cause the extinction of another. Biodiversity is important to the survival and welfare of human populations because it impacts our health and our ability to feed ourselves through agriculture and harvesting populations of wild animals.
Human Health
Many medications are derived from natural chemicals made by diverse organisms. For example, many plants produce compounds meant to protect the plant from insects and other animals that eat them. Some of these compounds also work as human medicines. Contemporary societies that live close to the land often have a broad knowledge of the medicinal uses of plants growing in their area. For centuries in Europe, older knowledge about the medical uses of plants was compiled in herbals—books that identified the plants and their uses. Humans are not the only animals to use plants for medicinal reasons. The other great apes, orangutans, chimpanzees, bonobos, and gorillas have all been observed self-medicating with plants.
Modern pharmaceutical science also recognizes the importance of these plant compounds. Significant medicines derived from plant compounds include aspirin, codeine, digoxin, atropine, and vincristine (Figure 5). Many medications were once derived from plant extracts but are now synthesized. It is estimated that, at one time, 25 percent of modern drugs contained at least one plant extract. That number has probably decreased to about 10 percent as synthetic versions of the plant compounds replace natural plant ingredients. Antibiotics, which are responsible for extraordinary improvements in health and lifespans in developed countries, are compounds largely derived from fungi and bacteria.
Aside from representing billions of dollars in profits, these medications improve people’s lives. Pharmaceutical companies are looking for new natural compounds that can function as medicines. It is estimated that one-third of pharmaceutical research and development is spent on natural compounds. About 35 percent of new drugs brought to market between 1981 and 2002 were from natural compounds.
Finally, it has been argued that humans benefit psychologically from living in a biodiverse world. The chief proponent of this idea is famed entomologist E. O. Wilson. He argues that human evolutionary history has adapted us to living in a natural environment and that built environments generate stresses that affect human health and well-being. There is considerable research into the psychologically regenerative benefits of natural landscapes, suggesting the hypothesis may hold some truth.
Agricultural
Since the beginning of human agriculture, more than 10,000 years ago, human groups have been breeding and selecting crop varieties. This crop diversity matched the cultural diversity of highly subdivided populations of humans. For example, potatoes were domesticated beginning around 7,000 years ago in the central Andes of Peru and Bolivia. The people in this region traditionally lived in relatively isolated settlements separated by mountains. The potatoes grown in that region belong to seven species, and the number of varieties likely is in the thousands. Each variety has been bred to thrive at particular elevations and soil and climate conditions. The diversity is driven by the diverse demands of the dramatic elevation changes, the limited movement of people, and the demands created by crop rotation for different varieties that will do well in different fields.
Potatoes are only one example of agricultural diversity. Every plant, animal, and fungus cultivated by humans has been bred from original wild ancestor species into diverse varieties arising from the demands for food value, adaptation to growing conditions, and resistance to pests. The potato demonstrates a well-known example of the risks of low crop diversity: during the tragic Irish potato famine (1845–1852 AD), the single potato variety grown in Ireland became susceptible to a potato blight—wiping out the crop. The crop loss led to famine, death, and mass emigration. Disease resistance is a chief benefit to maintaining crop biodiversity, and the lack of diversity in contemporary crop species carries similar risks. Seed companies, the source of most crop varieties in developed countries, must continually breed new varieties to keep up with evolving pest organisms. These same seed companies, however, have participated in the decline of the number of varieties available as they focus on selling fewer varieties in more areas of the world, replacing traditional local varieties.
The ability to create new crop varieties relies on the diversity of varieties available and the availability of wild forms related to the crop plant. These wild forms are often the source of new gene variants that can be bred with existing varieties to create varieties with new attributes. Loss of wild species related to a crop will mean the loss of potential for crop improvement. Maintaining the genetic diversity of wild species related to domesticated species ensures our continued food supply.
Since the 1920s, government agriculture departments have maintained seed banks of crop varieties as a way to maintain crop diversity. This system has flaws because, over time, seed varieties are lost through accidents, and there is no way to replace them. In 2008, the Svalbard Global Seed Vault on Spitsbergen Island, Norway (Figure 6) began storing seeds worldwide as a backup system to the regional seed banks. If a regional seed bank stores varieties in Svalbard, losses can be replaced from Svalbard should something happen to the regional seeds. The Svalbard seed vault is deep into the rock of the Arctic island. Conditions within the vault are maintained at ideal temperature and humidity for seed survival. Still, the deep underground location of the vault in the Arctic means that failure of the vault’s systems will not compromise the climatic conditions inside the vault.
Although crops are largely under our control, our ability to grow them depends on the biodiversity of the ecosystems in which they are grown. Crops are grown in soil, and although some agricultural soils are rendered sterile using controversial pesticide treatments, most 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, which are necessary for plant growth. Replacing the work of these organisms is not practically possible. These kinds of processes are called ecosystem services. They occur within ecosystems, such as soil ecosystems, due to the diverse metabolic activities of living organisms. Still, they benefit 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 annually; 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 suffered large losses caused by a syndrome known as colony collapse disorder, a new phenomenon with an unclear cause. Other pollinators include many other bee species and various insects and birds. Loss of these species would make growing crops requiring pollination impossible, increasing dependence on other crops.
Finally, humans compete for their food with crop pests, most of which are insects. Pesticides control these competitors, which are costly and lose their effectiveness over time as pest populations adapt. They also lead to collateral damage by killing non-pest species and 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 damage other ecosystems like streams, lakes, and even the ocean. Ecologists believe that predators and parasites of those pests actually do the bulk of the work in removing pests, but the impact has not been well studied. A review article found that 74 percent of studies that looked for an effect of landscape complexity (forests and fallow fields near 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) significantly increased alfalfa yield. This study shows that a diversity of enemies is more effective at controlling than one single enemy. Loss of diversity in pest enemies will inevitably make growing food more difficult and costly. The world’s growing human population faces significant challenges in the increasing costs and other difficulties associated with producing food.
Wild Food Sources
In addition to growing crops and raising food animals, humans obtain food resources from wild populations, primarily wild fish populations. For about one billion people, aquatic resources provide the main source of animal protein. But since 1990, production from global fisheries has declined. Despite considerable effort, few fisheries on Earth have managed sustainability.
Fishery extinctions rarely lead to the complete extinction of the harvested species but rather to a radical restructuring of the marine ecosystem in which a dominant species is so over-harvested that it becomes a minor player, ecologically. In addition to humans losing the food source, these alterations affect many other species in difficult or impossible ways to predict. The collapse of fisheries has dramatic and long-lasting effects on local human populations that work in the fishery. In addition, losing an inexpensive protein source to populations that cannot afford to replace it will increase the cost of living and limit societies in other ways. In general, the fish taken from fisheries have shifted to smaller species, and the larger species are overfished. The ultimate outcome could clearly be the loss of aquatic systems as food sources.