Extinction events are defined by periods of great loss of biodiversity. Fossil evidence has pointed to several mass extinctions from multiple causes including the great oxygenation event following the evolution of pant photosynthesis, environmental effects caused by forces outside the Earth, and indeed the expansion of human populations.
Depending on the data used and the thresholds set for what is considered a mass extinction, at least five and as many as a couple dozen extinctions events have been identified over the course of life on Earth (more than 4 billion years). Extinctions focus on multicellular life because it is readily observable compared to microbial life. The latter, however, accounts for most of the biodiversity on Earth. In addition to this, since the fossil record relies on fossilisation, it represents those specimens that are readily able to fossilise.
A theme of change in biodiversity surrounding extinction events is the opportunity provided by the extinction to displace the dominant species of large land animals, for example, and provide space for new species to take over. Ancient species resembling present crocodiles and birds (archosaurs) dominated the land before the Triassic-Jurassic extinction event which culled many of them, leaving space for dinosaurs to take over. Dinosaurs then went entirely extinct, bar for avian ones, during the next extinction event (Cretaceous-Paleogene), enabling the small land mammals to spread further and diversify.
Following each extinction, the few remainder species continued speciating out, regaining biodiversity over time. The extinction of species arising slowly over time as a reflection of changing environments is the background rate of extinction, and stands at a few taxonomic families of marine animals every million years. Extinction events far exceed the background rate of extinction in a short amount of time.
Human expansion has caused an uptick in the extinction rate of plants and animals, possibly initiating another extinction event. The extinction rate has recently increased by a factor of 1,000 up to 10,000.
There are difficulties with estimating current and past extinction rates. Models for working rates out have produced estimates that do not match up with the fossil record. When models are based on specific traits in a species, and attempt to show the potential for that species diversifying, they produce results that exceed what is actually possible.
In whale populations, the same data was used to produce extinction rate estimates that matched the fossil record, as well as estimates that did not. Statistical methods used to analyse data can determine the accuracy of the outcomes. It is very difficult to compute accurate extinction rates, especially for species that are already extinct and therefore no information is available on the timing of extinction.
Species diversity is the diversity of species in a community. Put simply, how many different species are there in a community? 5 or 5,000? Which has the higher diversity? Not rocket science I hope.
^That’s some rocket science, I don’t really know what it is, but I don’t wish to find out, and neither do you. Just a little motivator to not complain about biology.
What is a population? A population is all the individual organisms found in a given habitat, of one species. So you could talk about a population of wolves in the woods. If you want to talk about the wolves and rabbits in the woods, then you’d be referring to a community. A community is made up of the various populations in a habitat. So the summation of all the living things in a given area is called a community. What then is an ecosystem?
An ecosystem comprises the community of living organisms in a habitat, together with all the non-living components such as water, soil, temperature, etc. called abiotic factors.
Why are different organisms of different species able to coexist in the same habitat? How come they don’t directly compete with one another and drive others out? Have a watch…
So that’s the last and loveliest new term: niche. It rhymes with quiche. A niche is the interaction, or way of life, of a species, population or individual in relation to all others within an ecosystem. It’s how it behaves, what it eats, how it reproduces, where it sleeps, etc.; a species’ niche is determined by both biotic factors (such as competition and predation) and abiotic factors.
Species richness is defined by the number of different species in a habitat. However, in order to have biodiversity, the relative abundance of each species is also key. The more species, the higher the diversity. What if there are two separate communities like this:
Community #1 has 150 individuals per each of 20 different species (3000 individuals in total)
Community #2 has 10 individuals per each of 19 species, and 2990 individuals of the last species (3000 individuals in total)
It doesn’t take a complex formula to figure out that community #1 is far more diverse compared to community #2, despite them having the same number of species and individuals. The distribution of individuals to species is important in determining a community’s diversity.
Now for a little talk about deforestation and agriculture. Deforestation is the removal of trees in forests. and agriculture is the cultivation of useful plants to people which are often carefully selected for, and occupy a large area by themselves (like corn).
It’s not hard to figure out the impact both have on species diversity. Deforestation practically removes many, whole trees, and with them goes the shelter and food source of many other organisms. A great reduction in species diversity can be expected as a result.
Agriculture by humans results in a single dominant species which occupies vast land at the expense of others. Humans actively remove other species by the use of pesticides, insecticides and (indirectly) fertilisers. This, too, will lead to a great decrease in species diversity.
Other factors affecting species diversity include the degree of isolation, for example as seen on islands. The ability of individuals of certain species to move between different habitats can affect the biodiversity of different areas. When an area that is smaller harbours niches that can only cater to animals that can fly, or those that can eat fruit, the resulting community of populations of different species would not be as diverse as a larger, better connected area with more niches.
Alongside species diversity, other levels of diversity can be measured including genetic diversity at the level of different alleles, and ecosystem diversity at the level of different ecosystems in an area.
In the wild, each species may exist as one population or multiple populations. Different populations correspond to defined areas – habitats.
The sum of all present alleles for a given gene in a given population is known as the gene pool.
This is essentially a way of thinking about all the individuals in a population contributing their alleles towards the overall allele frequency. The extent of different alleles present gives the genetic diversity of a population.
The allele frequency in a population’s gene pool can change as a result of selection. The effectors of selection can be varied, yet the outcome is similar: advantageous or preferred alleles and the traits associated with them increase in frequency, while detrimental or disfavoured alleles and the traits associated with them decrease in frequency.
On the Earth as a whole, ecological diversity is overarching and includes within it both species diversity and genetic diversity.
It can be assessed in different ways, including geographically by ecosystem features e.g. deserts, oceans, forests, as well as biologically e.g. through the number of trophic levels in an ecosystem.