Are we Mass Murderers?: Earth's 6th Mass Extinction - Part 2: The Debate

Apologies for the delay between posts, but welcome to the highly anticipated Part 2 of the 6th Mass Extinction debate. In this post I will be discussing whether the current rates of extinction warrant being promoted to an Anthropogenic 6th MEE. Hopefully I will be able to give you a flavour of both sides of the debate and would be really interested to hear your thoughts and opinions in the comments! Don't forget to vote in the poll (right hand side), especially if your opinion has changed.

If you missed Part 1, catch it here.

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The question of whether this is the Anthropogenic 6th Mass extinction event is a contentious topic. If scientists decide that we are indeed in a 6th mass extinction, it would be a profound social statement and thus requires careful consideration before a decision is reached. As you will see, there is not quite a clear answer yet. Humans have been responsible for extinctions though activity such as habitat destruction, pollution, hunting and simple carelessness for many centuries, and some scientists argue that the 6th Mass Extinction encompasses the last 10,000 years and even as far back the Late Quaternary megafaunal extinctions (but whether or not those are anthropogenic is a debate for another post).

As we saw previously, a mass extinction is typically characterized as a relatively short period of time (geologically speaking) when the Earth loses a significant percentage of species, some suggest the threshold as 75%. This was certainly the case for the 'Big Five' extinctions that were discussed in the previous post, with the End-Permian event, as the biggest MEE in known history, reaching 96% loss of species. In a review paper published in Nature, Barnosky et al. compiled the below graph assessing modern extinctions in relation to the proposed 75% benchmark. It is clear that for most orders, modern extinctions do not reach anywhere near the required 75%. That does not, however, mean that the answer to our question is a 'no'. There are a plethora of data issues associated with fossil species estimates (Sampling bias, preservation bias, etc.), notably that most assessments of fossil diversity are done from genus level and calculated using species-to-genus ratios, extrapolating in potentially dodgy territory. There are also many modern taxa that are highly understudied, leaving modern assessments far from adequate for many groups (remember the snails?) - Barnosky et al. point out that only 1% of today's bivalves have been assessed, despite these making up a significant part of most of the 'Big Five' species counts. There also also other issues, such as the use of different species concepts inflating numbers of modern species. Due to this, magnitude estimates rely on theoretical predictions as opposed to empirical data for many species. Equations such as species-area relationships (SARs - have linked for Wikipedia for anyone feeling curious and mathematical) and other ecological models suggest extinctions could be between 21-52% at current. SARs have a whole host of problems associated with them in terms of validity and overestimation (Seriously, I'm not even going to go there), so this is still not terribly useful.

Percentage species loss recorded (white) and threatened (black) for modern orders in comparison to other MEEs; Source.

So really, we cannot reliably use percentage species loss to definitively validate or reject an Anthropogenic mass extinction. The other defining character of a MEE is that extinction rate reaches in excess of 1000x the natural background rate. This is typically expressed as E/MSY or Extinctions per Million Species-years, and the agreed background rate varies between groups of species and through time but on average tends to be 1 E/MSY. Recent and presumably anthropogenicly caused extinction rates have been estimated in various studies but are roughly between 100-1000 pre-human levels or background levels, and has the potential to increase tenfold if we lost all currently threatened species. These studies, in comparison to those only considering magnitude, tend to point towards a 6th Mass extinction event, or at least that we are approaching one. However, just as with the magnitude studies, this does not mean the answer to our question is a simple 'yes'. The major problem with the E/MSY metric is that fossil background rates are calculated on geological time scales, of a million year plus, whereas modern rates are calculated on centennial scales. Extinction rates have the potential, and do, vary immensely dependent on the length of time over which they are measured. We are extrapolating, again, to compare the rates from short timescales to rates from geological timescales and, as Barnosky et al. highlight, the rate is likely either much faster or much slower. Whilst we can definitely say that current rates are definitely higher than average fossil rates, and even above those from the late Quaternary megafaunal extinctions, there is considerable uncertainty within calculated rates. Some argue that they are likely serious underestimates of current E/MSY, but the elevated extinction rates have been unable to solve the debate.

Barnosky et al. were able to look at both of these variables together in a combined rate-magnitude comparison, shown below. The two, they argue, are intimately linked and therefore should be considered together in order to answer the question of whether the the 6th Mass Extinction is comparable to the Big Five. They were able to estimate the extinction rate of each Big Five event if it had hypothetically happened over 500 years, as is time scale of the current data (coloured dots below), which all sit roughly at 1000 E/MSY as expected. They found that the current extinction rates over the last 500 years for all vertebrate groups are at least as fast as and in some cases faster than rates that would have produced the Big Five over millions of years (Vertical lines below). From these results they were able to estimate that current rates may be sever enough to carry extinction magnitude to the 75% threshold within a few centuries. Again there are major unknowns within this data, as to whether the critically endangered and threatened species will go extinct and within calculation of the rates themselves. There could be a significantly different picture shown if invertebrate groups were included, but as previously mentioned, they are heavily understudied so any estimates for them have little meaning.

Complicated diagram but the message is extinction rates are high and on par with estimated precursors rates to the Big Five; Source.

Whilst there is not yet a stratigraphic signal for it, the 6th Mass Extinction or at least an elevated extinction rate is a clear part of the recent history of life on Earth. Numerically speaking, the evidence we have, when considered in the correct way, seems to suggest that although we are not quite at the charateristic 75% and 1000 E/MSY we are far from what should be though of as normal, and possibly not that far from reaching these thresholds. It is up to you to decide whether we have entered the 6th Mass Extinction or not, but it is for all of us to consider the message we need to take away from this: the world's biodiversity faces escalating threats and there is an urgent need for effective conservation measures. This will be expanded upon in part 3, but for now, I'll let you have a breather!