“The most wonderful mystery of life may well be the means by which it created so much diversity from so little physical matter. The biosphere, all organisms combined, makes up only about one part in ten billion of the earth’s mass. It is sparsely distributed through a kilometer-thick layer of soil, water, and air stretched over a half billion square kilometers of the surface.”
The variety of life on Earth is immense and wondrous, as this quote by famed ecologist E.O. Wilson suggests. About two million species have been described by scientists. On an average day, about 300 new species are documented. Some scientists estimate that there are as many as 50 million species alive in the world today.
Biodiversity is a measure of variation and richness of living organisms at a particular scale. It can be measured on an extremely small scale, such as the number of organisms living in a spoonful of soil, or on a large scale, such as the whole earth. Biodiversity can also be thought of on several levels of biological variation, ranging from genetic diversity within a species to species richness within whole biomes. The biodiversity of a particular place, region, or landscape is influenced by climate, topography, and geologic history, as well human and non-human disturbances.
Why biodiversity matters
Humans have many reasons to value biodiversity, including anthropocentric reasons and ecocentric reasons.
Anthropocentric reasons to value biodiversity include the many potentials for different lifeforms to provide scientific information, recreational benefits, medicine, food, or other materials that are useful to us. Even if we don’t know what exactly some species or ecological community might be useful for, we may choose to protect it, just in case, it turns out to be useful.
Ecosystem services are the services that ecosystems perform for humanity. They are a popular way of characterizing the variety of anthropocentric values surrounding parts of nature, including biodiversity. Animals, plants, and other components of every ecosystem do many things for humans such as purifying water and air, pollinating crops, maintaining a proper heat balance in the atmosphere, and cycling critical nutrients.
When we speak of these natural processes as ecosystem services, often we are imagining them in an economic context. For example, we might consider how much it would cost for flood control and soil erosion prevention if the vegetation in a particular region wasn’t providing the “services” of absorbing much of the rainfall, slowing down runoff, and holding the soil in place with its roots. In the case of biodiversity, one team of researchers from Minnesota recently showed that prairies with high species richness are more drought-resistant than those with fewer species. In many cases, high biodiversity enables a region to be more resilient, and to continue providing important basic “ecosystem services.”
Ecocentric reasons to value biodiversity are based on the idea of biodiversity having intrinsic value irrespective of any potential human uses (refer back to Module 3 for a refresher). An ecocentric perspective on valuing biodiversity would include conserving coral reefs or redwood forests on the basis that these ecosystems have a right to exist, irrespective of how, if at all, they might benefit human society.
Given the value of biodiversity, its protection has become a primary conservation concern during the past several decades. As you work through this module, consider whether you think that anthropocentric or ecocentric arguments are more likely to be successful in conserving biodiversity. What are the strengths and weaknesses of each? What type of argument would you construct if you wanted to save a particular species or protect a natural area?
Conserving biodiversity is the main issue driving the rapid growth of protected areas around the world, places like biosphere reserves, national parks, and wildlife refuges. Sometimes protected areas are developed to protect one particular species, or to keep certain types of habitat intact. Learning about protected areas is important not only because they are important for protecting biodiversity, but also because these areas are often laboratories for studying human-environmental change. These are the sites where new ideas about environmental management are tried out, where mistakes are made, and where lessons are learned about how to balance the needs of humans with the values of biodiversity.
What factors influence biodiversity?
Biodiversity is heavily influenced by both human and non-human factors. Throughout the module, we will spend a significant amount of time studying the negative impacts of humans on biodiversity. However, it’s important to remember that humans have a very complicated relationship with biodiversity. In some cases, human activities enhance biodiversity through habitat modification or periodic disturbance. In others, certain types of biodiversity are favored over others because of human influences. For these and other reasons, it is helpful to always think of biodiversity as part of a human-environment coupled system.
Biodiversity varies significantly among different regions. Polar icecaps and tropical deserts are almost devoid of life, while tropical rainforests and coral reefs are extremely biodiverse. A forest in the mid-latitudes, in places like Pennsylvania, might have 30-40 tree species per square kilometer, whereas a square kilometer of tropical rainforest in Borneo or Ecuador might have 300-400 species.
Geographers, ecologists, and conservation biologists have learned a great deal about the patterns of biodiversity around the planet. One pattern that’s important to recognize for our purposes is that the number of species is much higher near the equator and decreases as you move toward the poles. This is known as the latitudinal gradient of species richness and is largely shaped by the availability of energy and water in each respective region. This general pattern is apparent on every continent of the world. It may also exist in the oceans, although researchers have not yet collected enough data about oceanic biodiversity.
On a smaller scale, other factors have a significant influence as well. Factors that seem to foster an increase in biodiversity include:
Physically diverse habitats. If a region has a variety of different “microclimates” caused by variability in topography, water availability, and sunlight, it’s likely to have more biodiversity than a more uniform landscape.
Moderate disturbance. Disturbances include weather or geological events, fires, or insect outbreaks. If disturbances are too extreme, such as a volcanic eruption that kills all the vegetation in a region, then biodiversity is reduced, but a moderate amount of disturbance helps create a variety of habitats and fosters evolution. Humans practicing slash-and-burn agriculture in a tropical rainforest can create this type of moderate disturbance in some cases.
Large area. Regions that are a part of a large, connected land mass are likely to have higher biodiversity than those that are geographically isolated. Small islands that are far away from the mainland of a continent will have fewer species than large islands that are near the coast.
Longevity of system. If a particular region has been spared from extreme disturbance events like being covered by glaciers or volcanic ash, or being clear-cut or plowed by humans, it is likely to have a higher level of biodiversity. This is true even on a very long time-scale. For example, there are 85% more coral species in the Pacific Ocean than in the Atlantic Ocean, because the Pacific is a much older ocean basin.
So, if you are in a continental region of the world that receives a lot of sunlight and rainfall and is buffered from extreme disturbance events, you can expect it to be highly biodiverse. On the other hand, in isolated regions or those with low water or sunlight availability, or those that are subject to frequent extreme disturbance, you’ll likely find quite a bit fewer species.
One goal of this overview is to emphasize how important the concept of scale is to understanding and studying biodiversity. It’s important to both think about biodiversity on a very large scale, such as a biome or continent, and on a very small scale, such as a farmer’s field or a particular section of a stream. Understanding the factors that shape biodiversity on these different scales is quite challenging but also incredibly interesting and important.