Unit 3. Biological Diversity and Species Interactions

Currently, how many species are on Planet Earth?

A) The exact number of species on the planet is not known. Currently, there are about 1.5 million named species (excluding the bacteria and archaea).

1) Arthropods, nematodes, molluscs, protozoans, bacteria, fungi, flowering plants and fish are the most abundant major groups of organisms. The number of bird species (-9,600 species) and mammal species (-5400 species ) are actually very few relative to other taxa. In addition, most birds and mammals have been identified, whereas there are many invertebrate species left to be described and named.

2) The estimates over the years have varied greatly, from perhaps two million to one trillion species, depending on who you listen to. A novel concept by Camilo Mora (2011) suggests that there are about nine million eukaryotic species now on the planet. Mora used a concept from a paper by Carlo Ricotta (2002), where the authors estimated the number of species in a certain area, using Linnaean classification. Fine-scale data on the number of lower level taxa (species, genera, families) in a small area seemed to be predictable from the number of higher level taxa (orders, classes, phyla). Mora and his colleagues applied the same approach globally, and came up with an estimate of 8.7 ± 1.3 million eukaryote species currently present (and most have not been described).

B) Some species are more abundant or more widespread than others. We will talk about some of the reasons why this is so later. First, a few terms concerning biodiversity:

1) Exotic species: species that have been introduced to a new area. The zebra mussel and quagga mussel are two examples.

2) Endemic species: species that are present only in one particular native habitat, often in one location. The Galapagos tortoise is found only on seven islands in the Galapagos archipelago.

3) Endangered species: species known to be in a severe decline and facing a very high risk of going extinct in the wild throughout all (or a significant portion) of its geographic range. The tiger, lion, and the orangutan are examples of endangered species.

4) Threatened, near threatened, vulnerable species: there are slight differences in definitions of threatened or near threatened or vulnerable species, but in general, these are species that do not fit one of the criteria to be endangered, but are considered close to fitting one or more criteria. A threatened species likely can be declared endangered sometime in the near future.

5) Least concern species: a species that has been examined and is not close to fitting one of criteria to be listed as endangered or threatened. Blue jays, house sparrows, robins and cardinals would be examples.

6) Ubiquitous species: these species are found in a wide variety of ecosystems and habitats over a broad geographic area. The common house sparrow is one such example: it is found in cities and towns over the entire planet, other than Antarctica.

7) Extinct species: when there is no reasonable doubt that the last individual has died, based upon extensive surveying, the species in question is considered extinct.

C) The criteria to be considered endangered, according to the International Union for Conservation of Nature (IUCN):

1) reduction in population size over a period of time (10 years) or three successive generations,

2) geographic range constriction or loss of habitat area or quality,

3) population size has dropped below a certain point,

4) analysis suggests a >50% chance of extinction within the next decade or three generations.

Why preserve diversity? Think ‘A E I O U’

A) Why save endangered species? I have listed various reasons or justifications below, under five broad categories that overlap to some extent (I came up with this list). The five major reasons are as follows:

1) Aesthetic reasons,

2) Ecological reasons,

3) Intellectual reasons,

4) Obligatory reasons, and

5) Utilitarian reasons.

1) Aesthetic (cultural and emotive) reasons. We should preserve some species because of their beauty and the benefits we derive from observing them. Most people cannot look at many monumental vistas (such as the Grand Canyon) and various species (such as bald eagles, bison or elephants) and not feel awe and wonderment. Aesthetic justifications state that living things, as well as our natural ecosystems, appeal to our sense of natural heritage, and that these species add beauty that enhances the quality of our lives.

2) Ecological reasons. The health of the ecosystem depends on all species of the community. Natural communities help recycle nutrients and remove toxic materials, refreshing and replenishing air and water. An ecological justification centers on the crucial roles species fill in maintaining ecosystem functioning.

a) Are all species equally valuable? We do not know what the role of all species is in any ecosystem. Many ecologists assume each species is valuable to their community and ecosystem in some way.

i) Pollution control. Plants and bacteria remove toxic substances from the air, water, and soils. Because different species have different capabilities for this removal, a diversity of species can provide the best range of pollution control.

ii) Ecosystem services. Recently, ecologists have argued that natural ecosystems and the species that reside in them benefit humans and help sustain humanity, as well as our economies. In addition to pollution control (detoxifying pollutants), ecosystem services include purification of the air and water, a reduction of the severity of drought and floods, soil conservation and erosion control, nutrient cycling, natural pest control, food production, and other ‘services’, analogous to our power companies, our sewer companies, our water treatment facilities, our trash services, and our companies that generate wood, fiber and food.

b) Forests recycle many nutrients, and forests are important CO2 ‘sinks’. Forests (along with other aquatic and terrestrial plant communities) create the oxygen we breathe. Throughout its life, a single large tree is estimated to provide almost $200,000 worth of ecological services benefits, in terms of oxygen production, soil fertility, air purification and humidity control, in addition to providing wildlife a habitat. The same tree may be worth about $2,000 as lumber or pulp. Thus, any given tree is worth much more to all of us in the long term alive than its benefit to a few of us in the short term to be cut down and processed into wood products.

c) In 1997, Robert Costanza and his colleagues made an ‘estimate’ of the value of the world’s ecosystems – the value of everything. They came up with a total of 17 categories of ecosystem services (gas regulation, climate regulation, disturbance regulation, water regulation, water supply, erosion control, soil formation, nutrient recycling, waster treatment, pollination, biological control, refugia, food production, raw materials, genetic resources, recreation value, culture) and they classified the Earth’s total surface into 16 different biomes. They then estimated the value of each of 272 services/biome combinations at one time, and added it up. The total economic value of the Earth’s ecosystems (the total value of goods produced and services provided by the ecosystems) at the time was from US$16 to $54 trillion dollars annually, depending on various values and assumption (average of $33 trillion). This number is considered to be a very low minimum or conservative estimate by ecologists; the ‘real’ number may be considerably higher. Note that this estimate is larger than the global gross national product (global GNP) of about $18 trillion US dollars (current US dollars) per year at the time (GNP, the total value of goods produced and services provided by all countries combined). One way of looking at what this means is that in order to replace the world’s ecosystems’ services, we would need to increase the world’s GNP by US$33 trillion in order to replace the natural environment’s contribution. This does not lead to an increase in our standard of living, because the increase only replaces the existing ecosystem services’ contribution, which in many ways are irreplaceable. We simply do not have the machines, the infrastructure, nor the money that can do this for us. If the ecosystem services provided by the area that produced a product were actually ‘paid for’, the price of any commodity would be much more expensive. Ecosystem services are often ignored or undervalued in today’s economic market. Based on 2011 data, Costanza and his colleagues revised the estimated value of ecosystem services to be about 125 trillion dollars.

3) Intellectual reason: studying other organisms tells us about life in general and ourselves in particular (we are animals after all). This justification is one of the bases for various natural and social sciences, including comparative psychology, comparative anatomy, anthropology, medicine, animal behavior and the study of human behavior.

4) Obligatory reasons (religious/moral/ethical reasons): these arguments state we are to preserve nature because all species have an intrinsic value, independent of their impact or utility to humans. In other words, we are stewards for biodiversity.

a) For many people, in addition to (or instead of) following some command handed to us from a higher power (a deity or a secular authority), the responsibility we owe future generations (a sense of duty) is one important aspect of an obligatory justification. Our role as global stewards (as stated explicitly in many religions, including Christianity) is to protect and maintain biodiversity: it is a human obligation to assist the continued existence of species and to conserve biological diversity. This ‘obligation’ is stated in the UN General Assembly World Charter for Nature (1982) and the US Endangered Species Act – both include statements concerning the rights of organisms to exist.

b) Obligatory justifications have deep roots within human culture, religion, and society. The conviction that all creatures have the right to exist and that humans should not cause the extinction of other living things is sometimes referred to as deep ecology. Those who focus only on cost-benefit analyses tend to downplay an obligatory justification. However, although obligatory justification may not seem to have economic ramifications, in fact it does. Many international boycotts have economic impact: fur trade, teak wood for rainforests, ivory, and tuna fishing that kill dolphins that swim with the tuna – these industries have all been adversely affected by boycotts performed by people individually choosing to follow boycotts.

5) Utilitarian (economic/medical) reasons. These reasons include the direct ways other living things benefit humans, particularly for building materials, energy, medicine, and food. Utilitarian justifications are based on the potential benefits of species and species biodiversity for human benefit; there are many categories listed below.

a) Genetic characteristics. We need to continue searching for useful genetic material among wild plants to improve our crops, in terms of increased yields, increased cold or heat or drought hardiness, or increases disease resistance.

b) Chemical and medical uses. Even though big companies tout artificial / synthetic chemistry, pharmaceuticals rely heavily on chemical ingredients in wild plants. Approximately 25% of the 252 compounds identified by the WHO as basic and essential drugs contain active ingredients derived from natural wild angiosperms, other plants, and animals. About half of the drugs approved for use since the 1990s are directly from plants or derivatives of plant compounds. In 1997, of the top 150 prescription drugs used in the United States, 118 (78%) come from natural sources. Plants from the tropics contain many useful chemicals that we still do not now know about. Coral reefs are another source of potential natural chemicals for treating human disease. Coral reefs offer a particularly promising area of study for such compounds, because many coral reef species produce toxins to defend themselves. There are chemicals extracted from animals as well (for example, cone snails, horseshoe crabs, pit vipers). Unfortunately, some endangered species are under threat due to their perceived usefulness.

c) Some examples of medicinal importance of materials from wild plants:

i) The heart drug digitalis is from foxglove (small flowering plant in Europe and northern Africa).

ii) Aspirin (a common analgesic) is from willow bark extracts.

iii) Certain cancers are treated with chemicals from the rosy periwinkle. The rosy periwinkle, a flowering plant in the tropical forest of Madagascar, produces compounds that combat cancers such as Hodgkin’s disease. iv) Taxol, a powerful anti-cancer chemical, is from the Pacific yew tree. The tree is found in the same forests as the spotted owl.

v) Marine cone snails have powerful neurotoxins that can be of use for cancer and for many other uses, such as nicotine addiction.

vi) The blood of horseshoe crabs is crucially important to test for bacterial contamination of human blood samples.

d) In addition to the potential benefits from chemicals derived from other organisms, the anatomy/physiology or biochemistry of other organisms is of potential benefit to us. The armadillo is the only other species known to contract leprosy. Chimps are the only other species that can come down with certain human diseases. Pigs, guinea pigs and rats are useful for various research reasons, for example, guinea pigs have to obtain vitamin C from their diet, like humans, other primates, bats, along with a few fish and birds. We lack an enzyme (L-gulonolactone oxidase) that controls the last step in 65 converting glucose and galactose into ascorbic acid (vitamin C). (We have this enzyme in our genes, but apparently it has been inactivated.). Many species can use this enzyme to produce vitamin C in the liver or the kidneys. All of these species thus are important to us medically. Other animal species may also play an important role in medical research in the future.

e) New crops and products. Many commercial products come from natural plants. Other plants (some of which may yet to be discovered) may provide very important materials in the future. Biological diversity is important to indigenous people for food, fuel, and medicines.

f) Tourism. Ecotourism refers to the ‘shooting’ of big game in Africa with cameras, instead of elephant guns. This industry is worth many millions to cash-starved countries of Africa, South America and Asia, let alone to Europe and North America. Definitions of ecotourism involve ‘responsible travel to natural environments that conserves the environment while sustaining the economies of the local people’. Estimates of the value of ecotourism have ranged up to $210 billion dollars globally per year.

B) Should we conserve species or the total diversity of an ecosystem? Actually, we are going away from preserving a single species toward protecting whole habitats. After the Endangered Species Act was passed, it was argued that it is useless to attempt to protect the spotted owl, without simultaneously protecting the old growth forests in which it lives. Are there any species ‘more important’ than others?

Are there any redundant species? How many species are needed to maintain diversity?

A) At one time, most ecologists would say that the greater the number of species in the community, the greater chance that the community is stable (greater diversity leads to more stability). If one species is lost, then other species could take over its role, and thus no further extinctions may occur. The loss of one predator is compensated by other predators stepping in and preying upon the species the extinct predator used to feed upon. The same idea holds for prey species: if one prey species is lost, a predator could switch to other prey. The community that is more diverse was thought to be better able to respond to environmental stress than a community that is less species-rich.

B) In the 1970s, mathematical analyses suggested that less diverse communities were more stable. The species could multiple links with a number of species lower down on the food chain, and thus less chance for extinction to occur due to environmental stresses. This type of stability is called resistance stability: an analogous pattern would be the oak. Slight to moderate disturbances (winds, for example) do not cause the oak to move about, like a willow tree. A willow thrashes about even in a slight breeze. There must be a minimum number of species, and certain types of species are needed (for example, there has to be one or more plants, and one or more decomposers in order to have a functioning ecosystem.) However, it is hard to suggest what this number is.

C) New research suggests that more diverse communities are more stable in the sense of resilience. In a major study of grassland plots by David Tilman and colleagues (American), the species-rich grassland plots were more productive than species-poor fields and were more resilient (they showed smaller declines in productivity in response to drought, and returned to normal more quickly than species-poor plots). This type of stability is called resilience stability. A willow tree shows resilience: even after a hard windstorm, the willow returned to normal and is unharmed. The more resistant oak tree, however, was blown over by the winds. We’ll come back to this topic later. Part of the controversy surrounding stability and diversity depends on what is meant by stability.

D) Tropical rainforests are very resistant to change (resistance stability), but once they change, they recover very slowly (they so not exhibit resilience stability). Temperate zone grasslands burn easily and show species turnover (they have little resistance stability), yet they can maintain productivity in spite of disturbance, and bounce back quickly (they are resilient).

E) It is not clear if there are ‘redundant’ species. Part of the problem is defining what is meant by ‘redundant’. Some species may be quite rare, but still quite important in their communities. Some common species may be easily replaced by others.

1) In 1981, Paul and Anne Ehrlich (American) described the rivet hypothesis of species diversity. Each species plays a small role in the proper functioning of the community. Each extinction represents a loss of a ‘rivet’ popping out of a wing, weakening it. After a number of rivets have been lost, the wing breaks 66 away (the community collapses from a cascading effect of species loss). Under this hypothesis, it is important to protect and monitor essentially all species in order to protect the ecosystem and not continually force species towards extinction. Every species may be critical to protect.

2) A competing hypothesis to the rivet hypothesis was proposed by Brian Walker (Australian); this rival hypothesis is called the redundancy hypothesis of species diversity. In this hypothesis (first proposed in the early 1990s), most species are replaceable by others. If many species are considered to be members of guilds (we will go over the concept of a guild shortly), then the loss of one species could be partly compensated by the increase in a similar species, so that the community does not collapse (i.e., guilds allow for community resilience). This does not mean that the community is not stressed by the loss of species, however. The loss of an entire guild is an important consideration in the redundancy hypothesis, so that special attention should be given to guilds that may be represented by one or a few species in an ecosystem.

3) In 1998, both Ehrlich and Walker agreed in a co-authored article that both hypotheses are connected; they argued that we need to conserve as many species as possible. A loss of one species in the present, while perhaps not essential now, could become crucial for maintaining the ecosystem in the future, where the extinct species becomes important. That one extinct species may have been the only species that could adapt quickly to the new environmental conditions imposed by climate change.