4.2 Biodiversity, species loss, and ecosystem function  (Page 4/9)

Tropical rainforests, which are the habitats for nearly half of the world's plant and animal species, covered about 4 billion acres in past centuries, but only 2.5 billion acres remain and nearly 1% is being lost annually. Losses have been especially severe in the "paleo" or old world tropics that include Africa and Southeast Asia.

The category "wetlands" includes many types of ecosystems, but current estimates indicate that about 50% of the world's wetland habitat has been lost. The former extent of wetland habitats worldwide (fresh, brackish and salt) is difficult to determine but certainly exceeded a billion acres.

Species and ecosystem loss in perspective

To understand why biologists talk about ongoing losses of species and ecosystems as the "biodiversity crisis," it is useful to put current and projected rates of species loss into historical perspective. Over the history of life on Earth—a span of 3.5 billion years—nearly all species that existed eventually became extinct. This, of course, is coupled with the processes of speciation and biological diversification. Rates of extinction and diversification have fluctuated significantly over geologic time. For extinction, paleontologists have detected five episodes of mass extinction over the last 540 million years. These periods contrast with the relatively constant "background rate" of extinction observed over the geologic record, and include the relatively well-known event 65 million years ago when most of the extant dinosaurs went extinct. By definition, these episodes are characterized by the comparatively rapid loss of at least three-fourths of the species thought to exist at the onset of the event.

Recently, the question has been posed whether present-day rates of species loss constitute a sixth episode of mass extinction ( Barnosky, et al., 2011 ). Even with caveats about uncertainty in how many species there are today (only a fraction of the estimated total have been described, especially for plants, invertebrates, and microbes) and about comparisons of the fossil record with modern data, it appears that estimated rates of loss in the near future could rival those of past mass extinctions. Some estimates indicate that we will see a 30% loss of species within decades. Put another way, forecasted rates of species loss could be as much as 1000 to 10,000 times higher than background rates.

How does loss of biodiversity concern people?

As we learn more about biodiversity, it is becoming clear that there is often a positive association between biodiversity and the integrity of biological systems. This is not to say more diverse systems are "better;" rather, this means that systems with a relatively pristine complement of biological and abiotic or physical components tend to be more resilient and robust. Whereas this is rather nebulous, there is little doubt that the integrity of ecosystems is of fundamental importance to nearly all phases of human life and culture.

Often called ecological services    , the products and processes associated with biological systems are of immense value to the well being of people. An incomplete list of these services and products includes the formation of soil and cycling of nutrients; provisioning of food, fresh water, fuel, fiber, and recreation opportunities; the regulation of climate, flooding, and disease. The value of these services is often overlooked or simply taken for granted, but one global estimate puts it somewhere between $16-64 trillion annually. From global food security, to a source of medicines, to even the oxygen in our air, we are dependent on biodiversity and the sustained integrity of ecological systems. Nature is also the basis for a significant part of aesthetic and spiritual values held by many cultures.

Given this dependence, it is astounding that many are unaware or—even worse—apathetic about what is occurring and what will likely happen in the near future to our biological resources. We do not contend that any loss of species will affect productivity or function at the ecosystem level. The function of one species can be redundant with others and its loss may not lead to a significant change at the ecosystem level. Whereas redundancy can contribute to the resiliency of natural systems, that should not be a source of comfort. Much ecological theory posits thresholds of species loss beyond which the integrity of ecosystems is threatened; unexpected and possibly permanent new "states" may result. Once a community or ecosystem reaches an alternative state, there may be little that can be done to restore or remediate the system. Therefore, even under optimistic scenarios for rates of species loss (from the local to global scale) we are facing an uncertain environment.

Review questions

References

Barnosky, A.D., Matzke, N., Tomiya, S., Wogan, G.O.U., Swartz, B., Quental, T.B., et al. (2011, March). Has the Earth's sixth mass extinction already arrived? Nature , 471 , 51-57. doi:10.1038/nature09678

Vie, J-C, Hilton-Taylor, C.&Stuart S.N. (Eds.). (2009). Wildlife in a Changing World: An Analysis of the 2008 IUCN Red List of Threatened Species ™. Gland, Switzerland: IUCN. Retrieved from (External Link) .