IB Biology Sub-topic A4.2 Notes

Biodiversity

Biodiversity is a commonly discussed concept in the media, but few know what it actually means. By definition, biodiversity refers to the variety of life on Earth and is divided into three subtypes:

1. Genetic diversity - the number of different genetic characteristics within a species. Species with a high genetic biodiversity tend to survive better and be less susceptible to disease.

2. Species diversity (species richness) - the number of different species existing in a given community and their species awareness or their relative abundance. Stable ecosystems usually possess a high species diversity.

3. Ecosystem diversity - the number of different habitats, communities, and ecological niches within an ecosystem at a given location. This typically includes the ecosystem's impact on humans and the environment.

Since biodiversity is commonly used to track the status of an ecosystem or community, it must have some importance. It is estimated that 8 million species currently exist on earth, the highest number of species alive on Earth than at any previous time. However, this number is likely inaccurate as less than 20% of organisms have been identified and most of these unknown species are most likely invertebrates that are difficult to find.

On the other hand, it is also possible that historical biodiversity is underestimated. We know that there is much phenotypic variation between individuals of the same species within the same population. Such differences are even more apparent between males and females, and in individuals of different ages. Use of equipment that permits genetic analysis allows for accurate classification confirmation of live species, but this is more difficult in extinct species. Thus, classification in such cases is reliant exclusively on morphology, which can occur in two ways:

1. Splitting - each new fossil showing any difference from those already classified is designated as a new species.

2. Lumping - each fossil with a slight difference from one already classified is grouped with the existing classification. 

For example, when five hominid skulls were found in a cave in the Republic of Georgia, all with slight differences, it was initially deemed to be evidence of multiple species having occupied the same location, employing the splitting approach. Another group of scientists, bearing the concept of niche exclusivity in mind hypothesised that the five skulls where actually all of the same species, and lumped them together. Their explanation for the minor differences between the skulls was that a similar range of variation is apparent in today’s modern day ape species and humans. Thus, lumping of fossils may result in a lower biodiversity than was actually present.

Mass extinction events

On the other hand, it has been calculated 99% of all species to have ever existed on this planet are already extinct. The majority of these are thought to have died during mass extinction events. There have been five such mass extinction events since the first organisms developed around 3.5 million years ago, with the sixth said to be occurring right now.

This current sixth mass extinction is attributed to anthropogenic causes, such as unsustainable use of energy, land, and water, as well as climate change. As a result, it is estimated that one species per million per year becomes extinct. These extinction rates are estimated using background extinction rates as a reference comparison. You are expected to remember the extinction of three species:

1. The North Island Moa

   In New Zealand, the North Island Moa (Dinornis novae-zelandiae) is a large flightless herbivore that went extinct 600 years ago. The extinction occurred within 150 years of the arrival of human settlers to New Zealand from Polynesia, referred to as the Moa Hunter period. The activities of the settlers, hunting, gathering eggs, burning down areas to repurpose the land and the introduction of predators underpin the Moa's extinction.

2. The Caribbean Monk Seal

   Once widely distributed across the Caribbean, the Caribbean Monk Seal populations were in the hundreds of thousands. European immigration to the area brought seal hunting with it. Thus, between the 17th and 19^th centuries, huge numbers of monk seal were killed for their meat and oil, leading to the species' extinction by the 1950’s.

3. The last example will be of your teacher's choice. 

However, rapid extinctions such as above also provide an insight into the Quaternary megafauna extinction 10,000-50,000 years ago. During this time, 178 species of mammals, including giant kangaroos, mammoth and ground sloth vanished. It is believed that the arrival of humans in certain regions correlate with these events. Thus, any rapid extinction event, such as the dodo, will suffice.