This week’s discussion focussed on mass extinctions, specifically asking whether or not we are experiencing a mass extinction right now.
Firstly, it became clear that the term ‘mass extinction’ is not easily defined. Barnosky et al. (2010) define a mass extinction as a loss of >75% of species over a ‘geologically short interval’ (<2 million years). In contrast to Barnosky et al.‘s rigid definition, Sepkoski (1986, p.278) defines a mass extinction merely as a substantial increase in the amount of extinction in more than one geographically wide-spread higher taxon during a relatively short interval of geologic time. Many other definitions exist, these two are merely two ends of a broad spectrum.
Previous mass extinctions occurred on a time-scale of many millennia. The most dramatic extinction, which marked the Permian-Triassic boundary, destroyed an estimated 90% of life on Earth but took 60,000±48,000 years to accomplish it. 60,000 years is far longer than our records of biodiversity change and so it is difficult to decide what the long-term trends of future biodiversity loss may be. Instead we rely on the fossil record, but fossils are rare and biased towards those species which possess hard fossilisable body parts and are found in environments where fossilisation is a possibility. Elevated trends seen by some researchers may merely be temporary blips that will resolve quickly, or they may indicate a trend that will continue.
Linking back to our earlier discussions on the theme of how biodiverse the Earth actually is and how much we know about those species, we talked about how measures of biodiversity loss are skewed towards charismatic terrestrial vertebrates. Understudied taxa such as bacteria or micro-arthropods may be going extinct faster than other taxa. Regnier et al. (2015) demonstrated from a random sample of terrestrial gastropods that we may have already lost 7% of known species, while only 0.04% of species have been officially recorded as extinct.
Barnosky et al. (2010) proposed six potential causes of the 6th mass extinction:
- Co-option of resources from other species
- Fragmentation of habitats
- Species invasions
- Pathogen spread
- Direct species extermination
- Global climate change
We concluded that (as always) a combination of factors was most likely to blame, unsurprising. When probed further about which cause had the biggest effect, habitat fragmentation and climate change were equally tied. To fix these two problems however, would require very different approaches.
We discussed the infamous Living Planet Index (LPI) Report and whether it can be used to infer biodiversity’s impending doom (as is the WWF’s wont). The main summary of the report is a graph showing how the index value (essentially an additive measure of population change since 1970) has steadily declined. However, population change ≠ extinction rate. Additionally, the index biases certain taxa (charismatic vertebrates), only 4.8% of the world’s vertebrate species have been analysed, it follows that those species which have been studied sufficiently to be included are also the ones most likely to be experiencing decline due to their contact with humans.
To round up our discussion we discussed whether any of us would be willing to say whether we are or aren’t currently in a mass extinction. The overwhelming decision was that we don’t have enough data and it’s too early to tell.
P.S. A tip for other discussion leaders, learn from me and don’t give your charges too much extra reading to do, keep the reading focussed to ensure a good discussion on a few topics rather than briefly touching on lots of points.