Biodiversity, Species Loss, and Ecosystem Function

OpenStax-CNX module: m41619 1

Biodiversity, Species Loss, and

Ecosystem Function*

Je�rey Brawn

Michael Ward

Angela Kent

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 3.0�

Abstract

In this module, the following topics are covered: 1) biodiversity, 2) trends in biodiversity loss withreference to species and ecosystems, 3) ways human activity a�ects biodiversity, and 4) biodiversity losse�ects on people

1 Learning Objectives

After reading this module, students should be able to

• de�ne biodiversity• articulate current trends in biodiversity loss with reference to species and ecosystems• explain some of the ways human activity a�ects biodiversity• explain how biodiversity loss concerns people

2 What is Biodiversity?

You're probably familiar with the word, biodiversity, whether or not you can give an exact de�nition ofit. It's common on the signs at zoos, parks, and nature centers, and it's often used without explanationor de�nition. Most people understand biodiversity in general terms as the number and mix of plant andanimal species that occurs in a given place. Scientists are more precise and include more in their de�nition.The International Union for the Conservation of Nature (IUCN)1 , which coordinates e�orts to catalogueand preserve biodiversity worldwide, de�nes biodiversity as "the variability among living organisms from allsources including terrestrial, marine and other aquatic ecosystems, and the ecological complexes of whichthey are part; this includes diversity within species, between species, and of ecosystems." Rather than justspecies, biodiversity therefore includes variation from the level of genes and genomes to that of ecosystemsto biomes.

*Version 1.6: Mar 9, 2012 12:45 am +0000�http://creativecommons.org/licenses/by/3.0/1http://www.iucn.org/

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Even within a single ecosystem, the numbers of species can be impressive. For example, there is a largeregion of dry forest and savanna in Brazil known as the Cerrado (see Figure Cerrado Forest (Figure 1)).This ecosystem alone hosts over 10,000 species of plants, almost 200 species of mammals, over 600 speciesof birds, and about 800 species of �sh.

Figure 1: Cerrado Forest. Photograph of the Cerrado Forest. Source: C2rik via Wikimedia Com-mons2.

Generally, biodiversity is greatest in tropical areas�especially "rainforests"�but there are terrestrialbiodiversity "hotspots" on all the major continents. (View an interactive map of hotspots3 .)

3 Current Trends: Species Loss and Decline

One way scientists gauge trends in biodiversity is by monitoring the fate of individual species of animalsand plants. For more than 40 years, the IUCN has compiled information in the "Red List of ThreatenedSpecies," which "provides a snapshot of what is happening to species around the world." Updates to theRed List are released every four years. Here is how the authors of the most recent one, released in 2008,characterize the news it holds: "The overwhelming message" from the 2008 Red List, they write, "is that

2http://en.wikipedia.org/wiki/File:Cerrado.jpg3http://www.biodiversityhotspots.org/xp/hotspots/Pages/default.aspx



the world is losing species and that the rate of loss appears to be accelerating in many taxonomic groups"(Vie, Hilton-Taylor, & Stuart, 2008, p. 38 (p. 10)).

Figure 2: Summary of Threatened Species. Table lists the numbers and proportions of speciesassessed as threatened on the 2008 IUCN Red List by major taxonomic group. Source: IUCN Red List,Wildlife in a Changing World 20084, p. 17. Please see IUCN Terms of Use5 for copyright restrictions.

4http://iucn.org/about/work/programmes/species/red_list/review/5http://www.iucnredlist.org/info/terms-of-use



3.1 Vertebrates

Scientists know much more about the state of vertebrates�especially mammals, birds, and amphibians�than they do about other forms of animal life. Every one of the 5,488 species of mammals that have beendescribed, for example, has been evaluated for purposes of the Red List. Of them, 76 species have becomeextinct since 1500, and two, Pere David's deer, which is native to China, and the scimitar oryx from Africasurvive only in managed facilities. Another 29 of the mammal species listed as critically endangered are alsotagged as "possibly extinct;" they are very likely gone, but the sort of exhaustive surveys required to con�rmthat fact have not been conducted. Overall, approximately 22% of mammal species worldwide are known tobe threatened or extinct. (In the terms of the Red List, the broad designation "threatened" includes threelevels of risk for extinction in the wild: Vulnerable [high], Endangered [higher], and Critically Endangered[highest].)

The Red List categorizes a smaller proportion of the world's 9,990 described bird species�14%�asthreatened or extinct. But the raw number of species lost since 1500 is at least 134, and four more speciespersist only in zoos. Another 15 species of birds are considered possibly extinct. The fact that 86% of birdspecies are categorized as "not threatened" is good news in the context of the Red List.

Figure 3: Passenger Pigeons. North American passenger pigeons lived in enormous �ocks andwere once the most numerous birds on earth. Market hunting on a massive scale and habitat destructioncombined to extinguish them as a species in the early twentieth century. Source: Ltshears via WikimediaCommons6



Among the well-studied vertebrates, amphibians are faring especially poorly. Of the more than 6,000known species of amphibians, 38 have become extinct worldwide since 1500, and another one, the Wyomingtoad, survives only in captivity. Another 120 species are considered possibly extinct. Overall, 2,030, orone-third of the world's amphibian species are known to be threatened or extinct. More troubling still, manyamphibian species�42.5%�are reported to be declining, and that number is probably low, since trendinformation is unavailable for 30.4% of species.

Figure 4: Monteverde Golden Toad. The golden toad of Monteverde, Costa Rica, is one of 11species of amphibians to become extinct since 1980. Habitat loss and chytrid fungus. Source: U.S. Fishand Wildlife Service via Wikimedia Commons7.

Only small proportions of the world's species of reptiles and �sh have been evaluated for purposes of theRed List. Among those, the numbers of species that fall into the threatened category are very high: 1,275of the 3,481 evaluated species, or 37%, for �sh; and 423 of 1,385 evaluated species, or 31%, for reptiles. Itshould be noted, however, that these percentages are likely overestimates, since species of concern are morelikely to be selected for evaluation than others.

3.2 Invertebrates

The category "invertebrates" lumps together the vast majority of multi-cellular animals, an estimated 97%of all species. It includes everything from insects and arachnids, to mollusks, crustaceans, corals, and more.

6http://commons.wikimedia.org/wiki/File:Passenger_Pigeon_065.jpg7http://en.wikipedia.org/wiki/File:Bufo_periglenes2.jpg



Few of these groups have been assessed in a comprehensive way, and so as with �sh and reptiles, the RedList percentages of threatened species are skewed high. But assessments within some groups call attentionto disturbing, large-scale trends. For example, 27% of the world's reef-building corals are already consideredthreatened, and many more of them are experiencing rates of decline that move them toward threatenedstatus. The demise of reef-building corals has magni�ed ecological impacts, since so much other marine lifedepends on them.



Figure 5: Pink Soft Coral with Reef Fish. Photograph shows some pink, soft coral with reef �shnearby. Source: Linda Wade via National Oceanic & Atmospheric Administration (NOAA)8.



It should be understood that information about familiar creatures such as amphibians, mammals, andbirds is just a beginning, and that even with the inclusion of some invertebrates the Red List does notprovide a comprehensive picture of life on Earth. Scientists have described fewer than 2 million of the 8-9million species of organisms thought to exist, most of which are insects. And of those 2 million, the statusof only 44,838 has been assessed by IUCN.

In addition, it should be understood that among the species that have been assessed so far, there is astrong bias toward terrestrial vertebrates and plants, especially the ones that occur where biologists havevisited frequently. Red List assessments also tend to focus on species that are likely to be threatened, sincethe e�ort also has the aim of enabling people to conserve species.

Whereas extinction is the global loss of a species, the elimination of species at a local level�known asextirpation � also poses threats to the integrity and sustainability of ecosystems. Widespread extirpationobviously leads to threatened or endangered status, but absence of species, even at a local scale, can a�ectecosystem function. For example, by the mid-1920s wolves had been extirpated from Yellowstone NationalPark, although they continued to thrive elsewhere. When wolves were reintroduced to the park in the mid-1990s, numbers of elk (a main prey item) decreased signi�cantly. This, in turn, reduced browsing pressureand had a signi�cant e�ect on the vegetation and plant communities. What mattered for ecosystem functionin Yellowstone was whether wolves were present there, not just whether the species survived somewhere.

The human activities that account for extinction and extirpation vary considerably from one speciesto another, but they fall into a few broad categories: habitat destruction and fragmentation; intentionaland unintentional movement of species that become invasive (including disease-causing organisms); over-exploitation (unsustainable hunting, logging, etc.); habitat/ecosystem degradation (e.g. pollution, climatechange).

4 Current Trends: Ecosystem Loss and Alteration

Another way of gauging biodiversity involves assessment on the scale of ecosystems. The causes of wholesalelosses of ecosystems are much the same as those driving extinction or endangerment of species, with habitatloss and fragmentation being the primary agent. Worldwide, for example, the conversion of land to agricultureand cultivation have led to signi�cant losses in grassland ecosystems. In North America, nearly 70% of thetallgrass prairie ecosystem (which once covered 142 million acres) has been converted to agriculture, andlosses from other causes, such as urban development, have brought the total to about 90%. Current estimatesindicate that agricultural activity and cultivation systems now cover nearly 25% of the Earth's surface.

Tropical rainforests, which are the habitats for nearly half of the world's plant and animal species, coveredabout 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 about50% of the world's wetland habitat has been lost. The former extent of wetland habitats worldwide (fresh,brackish and salt) is di�cult to determine but certainly exceeded a billion acres.

5 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 oflife 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 diversi�cation. Rates of extinction anddiversi�cation have �uctuated signi�cantly over geologic time. For extinction, paleontologists have detected�ve episodes of mass extinction over the last 540 million years. These periods contrast with the relativelyconstant "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 de�nition, these

8http://www.photolib.noaa.gov/htmls/reef0484.htm



episodes are characterized by the comparatively rapid loss of at least three-fourths of the species thought toexist at the onset of the event.

Recently, the question has been posed whether present-day rates of species loss constitute a sixth episodeof mass extinction (Barnosky, et al., 2011 (p. 10)). Even with caveats about uncertainty in how manyspecies 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 thatestimated rates of loss in the near future could rival those of past mass extinctions. Some estimates indicatethat we will see a 30% loss of species within decades. Put another way, forecasted rates of species loss couldbe as much as 1000 to 10,000 times higher than background rates.

6 How Does Loss of Biodiversity Concern People?

As we learn more about biodiversity, it is becoming clear that there is often a positive association betweenbiodiversity 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 physicalcomponents tend to be more resilient and robust. Whereas this is rather nebulous, there is little doubt thatthe 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 areof immense value to the well being of people. An incomplete list of these services and products includesthe formation of soil and cycling of nutrients; provisioning of food, fresh water, fuel, �ber, and recreationopportunities; the regulation of climate, �ooding, and disease. The value of these services is often overlookedor 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 onbiodiversity and the sustained integrity of ecological systems. Nature is also the basis for a signi�cant partof aesthetic and spiritual values held by many cultures.

Given this dependence, it is astounding that many are unaware or�even worse�apathetic about what isoccurring and what will likely happen in the near future to our biological resources. We do not contend thatany loss of species will a�ect productivity or function at the ecosystem level. The function of one species canbe redundant with others and its loss may not lead to a signi�cant change at the ecosystem level. Whereasredundancy can contribute to the resiliency of natural systems, that should not be a source of comfort. Muchecological 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 analternative state, there may be little that can be done to restore or remediate the system. Therefore, evenunder optimistic scenarios for rates of species loss (from the local to global scale) we are facing an uncertainenvironment.

7 Review Questions

Question 1

What is the di�erence between extinction and extirpation?

Question 2

What are some human activities that impact species diversity and ecosystem function?

Question 3

Does the loss of one species lead to loss of ecosystem function? Why or why not?

Question 4

How does biodiversity promote sustainability?



8 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 Anal-

ysis of the 2008 IUCN Red List of Threatened Species�. Gland, Switzerland: IUCN. Retrieved fromhttp://data.iucn.org/dbtw-wpd/edocs/RL-2009-001.pdf9 .

Glossary

De�nition 5: biodiversity

The number of di�erent species within an ecosystem (or globally). Biodiversity is also considereda metric of ecosystem health.

De�nition 5: ecosystem function

Processes such as decomposition, production, nutrient cycling, and �uxes of nutrients and energythat allow an ecosystem to maintain its integrity as a habitat.

De�nition 5: extinction

The death of all individuals within a species. A species may be functionally extinct when a lownumber of surviving individuals are unable to reproduce.

De�nition 5: extirpation

Local extinction of a species; elimination or removal of a species from the area of observation.

De�nition 5: ecological services

Ecosystem functions that are essential to sustaining human health and well-being. Examples includeprovisioning services such as food, �ber and water; regulating services such as climate, �ood, anddisease control; cultural services such as spiritual and recreational bene�ts, and supporting servicessuch as nutrient cycling. Also called ecosystem services.

9http://data.iucn.org/dbtw-wpd/edocs/RL-2009-001.pdf


Biodiversity, Species Loss, and Ecosystem Function

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