The big picture

Biodiversity is short for biological diversity and it refers to the degree of variation of all life on earth, both within and between species and habitats. It is a complex concept incorporating three components, species diversity, habitat diversity and genetic diversity.

Biodiversity is not constant over space or time, different areas of the world have an inherently higher biodiversity than others. Tropical rainforest biomes are some of the most diverse terrestrial ecosystems on the planet whilst hot and cold deserts are some of the most sparse. The tropical rainforest has around 1,500 species of flowering plants, over 700 species of trees and 400 species of birds. This compares to the Sahara desert with 500 species of plants, 90 species of birds but more than 100 species of reptile. Figures 1a and 1b give some impression of the stark differences in biodiversity of these two terrestrial biomes.

Similarly, the aquatic realm shows great variation in diversity. Tropical coral reefs displaying a wide variety of life due to the favourable conditions - warm shallow waters with plenty of sunlight (Figure 2a). On the other hand oceanic abyssal zone (Figure 2b) has very low diversity due to the cold dark conditions present at 4,000-6,000 meters deep.

Biodiversity is essential for planet Earth to function effectively but we are currently losing species and habitat at an alarming rate and in order to conserve biodiversity we must be able to both understand and quantify it.

What and where is biodiversity?

“This is the assembly of life that took a billion years to evolve. It has eaten the storms-folded them into its genes-and created the world that created us. It holds the world steady.” - Edward O. Wilson (Biologist and researcher)

The quote by Edward O. Wilson evokes a holistic, almost simplistic view of biodiversity - the assembly of life. However, within that singularity, biodiversity is a highly complex concept that covers three different types of diversity.

Here is a short video on what biodiversity is.

Definition

Biodiversity is the variety of all life one earth. It includes genetic diversity, species diversity and habitat diversity.

Biodiversity is not constant, it changes over space and time. Figure 1 shows the spatial variation of terrestrial biodiversity, and it is apparent that the highest biodiversity is around the equator and the tropics whilst there is much lower diversity towards the poles. So why is this so?

Theory of Knowledge

What role do two of the ways of knowing play in establishing a price for biodiversity?

Species diversity

Some facts and figures

Figure 1. Distribution of species.

• Scientists generally agree that there are 8.7 million species on earth plus or minus 1.3 million. Previous estimates ranged between 10 and 100 million. This is discussed in more detail in previous sections. This video is a reminder

• Three-quarters of the known species are on land.

• 14% of land species have been logged and only 9% of aquatic species.

• 50% - 90% of the plant and animals species are in the tropical rainforests.

• More than half of the animals species are invertebrates.

What is species diversity?

At first glance species diversity seems straightforward – the number of different species in a given area. It is however more complicated than that and when measuring diversity two factors must be considered; richness and evenness.

Theory of Knowledge

Is it possible to be certain the richness and evenness are the most important aspects of species diversity?

Richness is a measure of the number of different species in an area; more species means a richer environment. However no account is taken of how common or rare each species is. Using richness alone to assess diversity can be inaccurate as species need to reproduce to maintain their presence in the ecosystem. For instance, in Figure 2, the lone daffodil in the field of hyacinths is unlikely to reproduce so the long-term presence of daffodils in the environment may be questionable. The species richness could fall very quickly.

Figure 2. Species richness - the lone daffodil.

​Evenness looks at the relative abundance of the species, it therefore takes into account the abundance or scarcity of each species and eliminates the problems of just analysing species richness in an area.

Definition

Species diversity is the number of different species in a given area taking into account the richness and evenness of the species.

Measuring species diversity

Figure 3a and b shows images of two different areas with the same colour flowers (reds, whites and blues). They are taken from different angles but they illustrate the principles being discussed here. Which one looks more diverse to you?

Figure 3a. The Meadow.

Figure 3b. White Haven.

Hopefully you said The Meadow. Although both areas have the same richness of species colour (three) the relative number of the species is very different. In The Meadow there is a more or less even number of each species of flower whereas White Haven is dominated by the white flowers. Richness is the same but evenness is not. The Meadow would be considered more diverse.

Richness and evenness are combined in Simpson Diversity Index.

D=N(N−1)/∑n(n−1)

Where:

• D is the Simpson diversity index.
• N is the total number of organisms of all species found.
• n is the number of individuals of a particular species.

Simpson Diversity Index for The Meadow:

D = 150(149) / 50(49) + 50(49) + 50(49)

D = 22,350 / 2,450 + 2,450 + 2,450

D = 22,350 / 7,350

D = 3.04

Try for yourself!

Try for yourself!

Calculate the Simpson Diversity Index for White Haven.

The higher the value of D the higher the diversity of the area.

Examiner Tip

You may be asked to calculate the Simpson Diversity Index in the exam. You will be given the formulae so don't worry about memorising it. You do however have to remember what N and n represent.

A diversity index allows us to put a mathematical value on the diversity of a community, which in turn allows us to compare communities over time or space. It is not realistic to compare a rainforest with grassland as that is like comparing oranges with grapes; but is can be very useful to compare communities that should have similar diversity e.g. two tropical grassland areas.

For example the pictures in Figure 3a and b are both from similar meadow areas, why then does one have a lower diversity than another? The area with lower diversity may:

1. Be close to a source of pollution.
2. Be affected by agricultural spraying of herbicides.
3. Have been subject to harvesting of some of the species.
4. Be closer to human habitation and so subject to degradation.

It is also possible to use diversity indices to assess the state of an ecosystem over time. For example a new development project close to an area of natural beauty would raise concerns amongst conservationists. A baseline diversity could be established before the development starts and then closely monitored as it progresses. Any changes in diversity could then be documented in an Environmental Impact Assessment (EIA).

Theory of Knowledge

How do we know that we are measuring the right factors when assessing changes in species diversity over time?

Why is species diversity important?

Species diversity in any ecosystem is the culmination of millions of years of evolution. This evolution happens, not in isolation but in association with other living organisms in the same area. The result is an ecosystem in balance; species interact with each other and the abiotic environment and perform various functions within the system. Thus, the loss of just one species affects many others and causes the system to go out of balance. For instance insects (bees), mammals (bats), and birds pollinate over 90% of all plants.

In addition to the environmental value of species diversity there is the fact that humans rely on many species. A large percentage of our modern pharmaceuticals are derived from plants - aspirin, penicillin and many painkillers. And the majority of the people living in the developing world rely plants for their medication.

International-mindedness

Different areas of the world have very different levels of species diversity. Consider the impact of different EVS's on the maintenance of the levels of diversity.

Genetic diversity

Figure 1. The DNA question.

Genetic diversity refers to the variation of genes within the genetic pool of a population of a species; it is the means by which that population can adapt to change. If there is a high level of genetic diversity there is a higher probability that some individuals within the population possess the genes that are best suited to any changes in the environment. A smaller population of organisms is likely to have smaller gene pool, lower genetic diversity and thus less adaptability.

Within a species there are individuals and populations. Every individual will have a slightly different genetic make up from every other member of the species. However, individuals within a population will be more similar to each other than individuals from another population.

Definition

Genetic diversity is the variation of genes within the genetic pool of a population of a species.

This can be seen in the range of domestic cats. Cats all belong to the same species (and in theory can interbreed to produce fertile offspring) but there is a huge variation between the different breeds. Figure 2a and b show two breeds of cat, the overall genetic make up of which will be very similar. However specific traits have been selected by breeders to give the distinctive short legs of the Munchkin and the signature coat of the Russian Blue. Breeders understand the important of genetic diversity within a breed. So when they breed the cats they will use individuals from different “populations”, maybe from another area of the country or different part of the world.

Genetic diversity in wild animal populations is crucial for their survival. Animals that are widespread and found in many different areas have a higher chance of survival. This is because:

• If one population is wiped out by a disaster then there are other populations that will be unaffected and the species carries on.
• Each population will have different genetic make up and so each will have different strengths and adaptability.

One of the most widespread species on the planet is Vulpes vulpes or the red fox (distribution shown in red on Figure 3). It is a highly successful species with the ability to adapt to and thrive in urban environments. If one population of red foxes were to fall to disease then it would not cause the extinction of the species.

Figure 3. Global distribution of red foxes and lemurs.

International-mindedness

If a species has a global distribution does it matter if they go extinct locally?

In contrast to the red fox are the 99 species of lemurs. Native to the island of Madagascar (black and white striped in Figure 3) lemurs are and are found nowhere else in the wild. This situation makes lemurs far more susceptible to extinction than the red fox.

• If disaster struck Madagascar (natural or human induced) and the lemurs were wiped out, their only chance of recovery is found in zoos.
• With such a small population of individuals the genetic diversity is low and in-breeding may also weaken the species.

Be Aware

If you study biology as well as Environmental Systems and Societies you must be able to distinguish between genetic diversity and genetic variability. Genetic variability describes the variation of genetic characteristics.

Why is genetic diversity important?

1. There is a better chance that some members of the species will be resilient and survive environmental change caused by things such as disease, climate change and pollution.
2. Low genetic diversity (often caused by small populations) causes inbreeding which makes the genetic make-up of the population more uniform. This means that any flaws or disabilities within that population will become more common. E.g. the Florida Panther population has very low genetic diversity and a high incidence of heart defects and weak immune systems.
3. Extinction is frequently preceded by a drop in genetic diversity.
4. Once lost genetic diversity is almost impossible to regain.
5. Genes are the instructions on how to make organisms; so our food, medicine and biofuels are all coded by genes. If something we rely (e.g. wheat) is stuck by disease then genetic diversity increases the chances of us finding alternatives to that are disease free.

Theory of Knowledge

Consider the reliability of the various ways of knowing that would give us the data to say why genetic diversity is important.

Habitat diversity

"The current massive degradation of habitat and extinction of species is taking place on a catastrophically short timescale, and their effects will fundamentally reset the future evolution of the planet's biota." - National Academy of Sciences

International-mindedness

Stop and consider this quote, what does it mean on a local scale and on an international scale?

Definition

Habitat diversity is the range of different habitats in an area.​

Note: Habitat diversity is referred to as ecosystem diversity by some people.

Habitat is the abiotic and biotic environment that a species usually lives in, so habitat diversity is the range of places where plants and animals can live.

Terrestrial biomes

In terrestrial biomes it is the abiotic environment that is most important for plants; temperature range, soil type, precipitation and light all determine what species of plants can grow in an area. The plants then act as habitat for animals so a variety of vegetation communities in an area will mean a larger habitat range for animals. Type of vegetation is also important, woodland vegetation has many layers and thus creates a multi-storied apartment block for animals (Figure 1a). In contrast to that are the grasslands, with single story living there is less diversity of accommodation.

Stop and think for a minute about the habitat diversity in the two pictures above.

Marine Habitats

Marine habitats are different to terrestrial ones in that they are transient and changeable and it is not just the vegetation that provides habitats for animals. The suitability of habitats in the marine environment is mostly dependent on the abiotic factors themselves and not so much the plants that are determined by the abiotic factors. These factors include, dissolved gases, land run-off, marine topography, nutrients, ocean gyres and currents, pH, salinity, sunlight, temperature (linked to latitude, ocean currents and hydrothermal vents) and turbulence.

The links

Figure 3. The links between the three types of diversity.

Notes to Figure 3 above:

1. High habitat diversity gives different areas for populations of species to spread into. Separation prompts greater variations in the gene pools.
2. High habitat diversity gives a wide range of spaces for animals to adapt and/or move in to so high species diversity.
3. High genetic diversity increases species adaptability and can lead to speciation and thus higher species diversity.
4. High species diversity of plants so higher habitat diversity.