Nocturnal Pollinators: A forgotten ecology?

In the post below I wanted to share what I have been working on in my independent study project. The project was about trying to establish a baseline for the diversity and importance of nocturnal pollinators, as they are a hugely under-researched group. Much of the focus of pollination research and media representation in recent years has been firmly on bees, and to a lesser extent butterflies, but the nocturnal component of the system remains unknown to many. I'll be giving a brief summary of the answers I found to my main research questions, and then talking about the relevance of this work in the context of this blog. I hope you find it as interesting as I did and, as always, please let me know if you have any questions in the comments!

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In the same train of thought as my earlier soil biota post, I feel that nocturnal pollination is another forgotten ecology - something critically important to both the ecosystem itself and to provision of ecosystem services but unappreciated as it isn't something we often see. The diversity of nocturnal pollinators and their importance within ecosystems are two questions to which current research does not provide a simple answer. Little is known about the scale and importance of nocturnal pollination services, largely due to the impracticalities of studying pollination in the dark. However, with the documented decline of known nocturnal pollinators throughout Europe and other continents (in particular, moths and bats), it is crucial that we reach a better understanding of their role in respective ecosystems. As some put it, we remain rather ‘in the dark’ about what happens after dusk.

Manduca sexta feeding from a Datura flower; Source. 

How many nocturnal pollinators are there and which taxonomic groups are they in?

Any number I gave you would be wildly innacurate. We can try to make estimations based on those species which have been observed, but a recurring issue throughout the research is the impracticality of observing things at night. Also there are taxonomic issues, particularly for invertebrates as I have discussed in previous posts. We don’t know what percentage of species we have described and many families, such as the Noctuidae moth family, are paraphyletic and contain genera not robustly assigned to subfamilies.

As for the taxonomic groups the pollinators belong to...

Lepidoptera: We know of 21 families of moths involved in nocturnal pollination. but from these some of the most important are the Noctuidae and Geometridae families which land on the flowers the same was as butterflies do but also the Sphingidae or 'hawk moths' which hover and reach the nectar with their extremely long tongues. Moths and the other insects I will discuss typically pick up pollen on their legs and wings when they visit flowers by accident and deposit it on subsequent floral visits. A few species of moth, however, are the only known insects to do this purposefully.

Hymenoptera: The family containing bees, wasps and ants. Bees are commonly thought of as diurnal pollinators but there are nocturnal bees which play an important role in desert environments. Wasps and ants are not well studied as pollinators, but we know that some species are involved in nocturnal pollination.

Coleoptera: Beetles are one of the most neglected groups in literature despite being among the first animals involved in pollination. There are several families of small beetles which are fairly well studied but we also know that large beetles, particularly scarabs, act as pollinators. Most beetle pollination is found in tropics and linked with commercially important palm trees.

Diptera: We know that flies are important pollinators and often considered second only to bees, but again they are neglected in a nocturnal context. They are particularly important in regions where bees aren't as capable, such as high altitude areas and alpine environments. The Syrphid family is considered most important among diurnal pollinators and likely there are members of this which act as nocturnal pollinators as well. Mosquitoes are actually important nocturnal pollinators, and are well studied in desert environments.

A cross section of  a cactus flower, showing how the bat pollinates it; Source.

Chiroptera: Although well known as nocturnal mammals, it is not often known that bats act as crucial pollinators. Over 500 plant species, including many tropical fruits, rely on bats. Similar to hawk moths, they hover in front of the flower, and stick their head and long tongues into the flower to reach the nectar reward – their heads get covered in pollen and they look very cute but are then effective vectors to carry this pollen to the next plant they visit.

Non-Flying Mammals: There are also mammals other than bats involved. Mostly within marsupial, rodent and primate families, these mammals make big contributions to pollination in Australia and South Africa. These nectarivorous mammals are very cute as well, with obvious nocturnal adaptations in terms of big eyes and ears. Similarly to bats, pollen gets stuck in their fur and they transfer it between flowers they feed on.

Squamata: : Lizards! There are 3 known nocturnal pollinators in this group, and all of them are geckos. Nectivory is quite well established among geckos so there is a huge potential for nocturnal that may have been missed. Research also suggests that nocturnality is ancestral state for geckos, so this further hints that these guys could be important nocturnal pollinators.

How effective are nocturnal pollinators in comparison to diurnal?

This graph is from data I collected and shows the number of studies which considered a certain pollinator more or less effective than the diurnal counterparts. The data was sorted into three categories: more effective, less effective and those which were unclear or considered equal. There isn't a straightforward answer – there is a fairly even spread between the three columns, and no one category is significantly larger than the other two. There is a potential for bats to be strong pollinators, as they have larger amounts of effective studies, but moths are seem equal in all categories despite some papers being written about biological reasons for their effectiveness. Other groups did not yield not enough data to make any sensible inferences, again highlighting bias issues within nocturnal research. What is clear, however, is that in many instances nocturnal pollinators are highly important components of the system.

How many plants are pollinated by nocturnal pollinators? Are they commercially important?

Again, this is a very difficult question. Perhaps the most sensible approach would be to consider pollination syndromes and make the assumption that if a plant displays adaptations for nocturnal pollination then it is nocturnally pollinated. However, the validity of pollination syndromes has been in question a lot incurrent literature, and is largely seen as outdated, so perhaps this isn't best approach. Alternatively, we could make the assumption that all plants with nocturnal anthesis are nocturnally pollinated, but again we know this isn't true, as some of these plants will self pollinate. In addition, there are some plants which have nocturnal-diurnal anthesis and are pollinated by a mix of daytime and night-time pollinators, so for most of we don’t know which of these are primarily reliant on nocturnal pollinators, if at all.

There is potential for nocturnal pollinators to make significant contributions to a limited set of agricultural plants. Many essential staple food crops do not rely on biological pollination whatsoever, such as corn, wheat, and rice which are all wind pollinated and  need no insect help at all. Other staple food crops, such as bananas and plantains, are propagated from cuttings meaning that they require no pollination of any form. Where we see nocturnal pollinations evidenced is mostly in tropical fruit crops including species of mango, banana, cocoa, palm, durian, guava and agave (used to make tequila). Bats and beetles are, of all groups, the most likely to be responsible for pollinating these crops. So, next time you eat some chocolate, say thanks to the bats!

Bats making their home in palm tree, which they also pollinate; Source.

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So, we've established that this is an extremely diverse group that is likely highly important commercially and ecologically. Therefore, the threats that they are currently under should be taken seriously. Unfortunately, due to the lack of research in general about nocturnal pollinators, there is an equal lack of literature considering threats and appropriate conservation measures. Whilst there are likely many more threats than this, and indeed many we are unaware of, I'll discuss one of the main, unrecognised, threats to nocturnal pollinators.

Light pollution
Artificial lighting has become a huge component of many urban areas, and has established effects on a lot of nocturnal wildlife. Moths are famously attracted to bright lighting, and whilst we use this to our advantage when using light-trapping to survey them, there is also evidence that urban lighting is effecting moths negatively. MacGregor et al. suggested that artificial night lighting could potentially limit reproduction and make moths more vulnerable to predation. Both of these impacts are quite well established with evidential backing, but there may also be impacts of their ability to see properly. Moth population declines are likely linked to artificial lighting in some capacity, though MacGregor et al. suggest that the risk goes beyond decline, but to species loss and changes in community assemblages. Other research has linked artificial lighting to declines in bat populations and disruption to ecosystem services.

Alongside other threats such as habitat loss, global warming and invasive species - many species of nocturnal pollinator, both in the UK and around the world are at risk. In the tropics, where bats and beetles provide crucial pollination services, the impacts deforestation and climate change have been felt severely and I don't doubt we will have already seen extirpations of pollinating species. It is an unfortunate situation where a combination of under-researched fields meet at a crossroads and we are probably losing pollinators before we have identified them, let alone studied their importance.

Moths below a streetlight; Source.

Look Deeper: Extinction Risks to Soil Biota

Earlier this week, we celebrated World Soil Day (4th Dec). One of the key themes in this years festivities was to highlight the need to increase research into soil ecology and underground ecosystems. In particular, to warn of the potential underground extinction risks that we have an very limited understanding of. A new collection of papers has been published in Nature as part of World Soil Day to highlight these issues among other soil related discussions. So, in accordance, I will be bringing these issues to my blog to give us all an education about soil ecology, as despite the relative neglect compared other areas of ecology, the soil and its biota are fundamental components of all terrestrial ecosystems.

In order to form conservation approaches for underground ecosystems, we need a clearer understanding of the diversity and functionality of them. We know that soils provide vital ecosystem services, particularly for agriculture, so there is clear motive for conservation action - regardless of a lack of moral motive, as concern for things small and dirty tends to be less. The extent of biodiversity below ground is hugely unknown, partially due to the impracticalities of studying these organisms due to their size and inaccessible habitat.  As shown in the graph below, taken from one of the new papers, the vast majority of what lives in the soil is under 1mm in size. What the graphs also shows, however, is the extreme abundance of these organisms - over 1kg of bacteria per meter squared is an insane amount of individuals! Aside from our poor understanding of the diversity, other challenges for understanding potential extinction risks include the lack of suitable models, the complexity and density of microhabitats within the soil and uncertainty about temporal and spatial scales. I'm not going into these here, but they are all detailed here if you're interested!

Soil biota, shown by coloured crosses, are generally very small and very abundant ; Source. 

The most important extinction risk factors for soil organisms are not dissimilar to those faced by their terrestrial counterparts. Habitat loss, a concept we are all familiar with in relation to aboveground organisms, can cause equal disruption for soil ecosystems, for example through the fragmentation of the soil surface via urbanisation. This has been shown to link to declines in abundance of nematodes and other soil biota. Equally, climate change and global warming pose a threat just as they do to terrestrial and marine biomes. Extreme drought events can cause devastation of soil based habitats, for example. Climate change is also thought to have some more complex impacts on soils which can be equated to habitat loss, due to the degree to which the soils are changed. It has been shown that the increased atmospheric CO2 concentrations lead to a reduction of nematode diversity due to the loss of pore spaces in the soil which are required by the organisms. Soil habitats are also impacted by agricultural practices, such as tilling, irrigation and fertilization which impact both soil structure and composition. It is thought that these factors drive constant habitat loss for components of the subterranean community. Although it is difficult, as mentioned above, to know the extent of these impacts, we do know that many of the susceptible species, such as earthworms and mycorrhizas, are of major functional importance to their own ecosystems and ours. There are also potential risks from invasive species, but at a microbial level more than invertebrate, which have likely been spread as a result of travel and tourism. Many of the soil biota are highly specialized in their niche, which is commonly considered to make them more vulnerable to both invasions and extinctions.

So there are considerable risk factors stacked up against soil ecosystems, much as there are against the terrestrial. There has, unfortunately, been very little conservation action in comparison. Some studies have already documented local extinctions of earthworms and various fungi due to the factors discussed above. So perhaps we should start taking some action? This, if any, is a good time to start. 2015 is the International Year of Soils, as declared by the UN General Assembly, after-all. This declaration was made with the intent to raise awareness of the life-supporting functions of soils and to promote the importance of soils in achieving a number of the new 17 Sustainable Development Goals. I hope that you've learned something about the oft-neglected but important goings on beneath our feet, and that in future you might give them a thought in conservation discussions.

Happy World Soil Day! 

The Mystery (or not) of the Megafauna

Between 50,000 - 10,000 years ago over 90 genera and many hundreds of species of megafauna went extinct in what have become known as the Late Quaternary Megafaunal extinctions. Megafauna, or for our purposes large mammals weighing over 44 kg, lived around the globe on all continents except Antarctica but today they persist only in Africa. The causes for this dramatic decline in species over a relatively short window of time is a contentious topic which has two polar ends: humans and climate. Some papers argue that humans were the overriding factor through a variety of impacts such as hunting and habitat destruction, whilst others maintain that climate change was the real killer.

Some of the Quaternary Megafauna; Source.

The megafauna were more vulnerable to extinction than other animals, which is in part why they suffered such extreme losses, whether they be from humans or climate. In general, animals of a large size are more vulnerable to extinctions due to larger ranges and lower population densities. Johnson argued, however, that is was not size but a slow life history (slow reproductive rates and few young) that made the megafauna more susceptible to extinctions. Due to the fact that they have long lives (and therefore later sexual maturity) and relatively few young, they are more vulnerable to sudden changes in climate and habitat because it takes longer for generations to pass and therefore longer for the species to adapt to these changes. This supports climate causes over human, as hunting hypotheses focus on body-size. Johnson also found that those alpine, arboreal and nocturnal species with lower reproductive rates experienced less extinctions, likely due to their reduced exposure to humans but there is no clear link to climate.

Barnosky et al. conducted a review of the evidence from a variety of fields. They found that in some areas, such as Eurasia and North America there was strong evidence that humans contributed to the extinctions but also that pronounced climate change had a significant role. In Australia, humans were almost exclusively responsible for the loss of 21 genera whilst there was scarce evidence for the influence of climate. In other areas of the world, South America and Africa, there was not sufficient evidence to make a judgement on the causes of the extinctions, despite the fact that South America experienced the greatest species loss of any continent. In Africa, there is a very low number of genera lost in comparison to other continents. Why? This is yet another unanswered question but we think it is due to coevolution of humans alongside the megafauna.

Assessment of causes of extinction on each continent; Source.

There are various hypotheses for human caused extinctions, but I will briefly discuss the most common ideas. 

1. Blitzkrieg

Whilst this is now considered somewhat outdated by most scientists, one of the first ideas about the loss of megafauna was that humans had hunted them to extinction at such a rapid rate that their populations did not have a chance to recover. First proposed by Martin, the hypothesis assumes that as the animals had not encountered humans before, they would naively not be afraid of them and therefore easily hunted. Martin sought to provide explanation for the fact that there were so few archaeological sites containing extinct megafaunal remains, as the hypothesis put the extinctions within the space of 500-1000 years so interactions between humans and extinct species would have been brief. There are, however, a number of issues with this hypothesis. There is a severe lack of archaeological sites where large mammals are associated with stone tools and Barnosky et al state that when the entire breadth of climatic and archaeological evidence is considered, that blitzkrieg scenarios can be firmly rejected in western Europe, Siberia, Alaska, and probably Australia and central North America. Additionally, Wroe et al. among others argued that prey naivete would not have been sufficient for blitzkreig, as this is really not how animals work - prey quickly learn to flee from new predators. Whilst we know that hunting and predation by humans did have an impact on populations, and of certain species in particular, we know fairly certainly that it was not a blitzkrieg.

2.Sitzkrieg 

The wittily named Sitzkrieg hypothesis refers to the slower, sometimes indirect impacts humans would have had on megafaunal populations other than directly hunting them. This includes habitat loss and fragmentation, use of fire, and the introduction of foreign species and diseases. This has proven difficult to quantify as, for example, we cannot pinpoint whether charcoal comes from human or natural fire events and it is also difficult to know the extent of habitat loss. We have seen in island communities that 'sitzkrieg' style events have caused extinctions through multiple effects and synergy with hunting. For example, black rats as a human introduced species have caused extirpations in island communities, but this has not been replicated on mainland. Some suggest a hyper-disease hypothesis in which extinction is a result of hyper-virulent diseases to which the native species have no resistance. Again, there is a lack of evidence to support this - we do not actually know of any such extremely lethal cross species pathogens, for example. In their review, Koch and Barnosky find that none of the indirect sitzkrieg models make strong predictions regarding the megafaunal extinctions. Of all, the factors, habitat alteration seems the most likely culprit but it is unlikely any sitzkrieg parameters were dominant causes of the extinctions.

Hyper-disease, plausible or silly?; Source.

As for environmental hypotheses, climate change is the prevailing theory but there are also some other weird ideas out there.

1. Climate change

Throughout the last 50kyrs, there has been a lot of fluctuation between warm and cold events in the climate, known as Dansgaard–Oeschger (D-O) events (rapid warming events) and Heinrich events (cold events).  These events are likely to have caused abrupt shifts in temperature and precipitation which would destabilise habitats and species ranges. In a recent paper, Cooper et al identified that many extinctions seem to coincide with D-O events, shown in the graph below. Additionally, Gill et al found that Sporormiella (a dung fungus associated with large herbivores) declines in correlation with D-O events and overall gradually throughout the period, contrary to the aforementioned blitzkrieg ideas. The other trend we see is a gradual warming from the last glacial maximum (26.5kya) to the Holocene (11 kya), after which temperatures remain relatively stable. This warming would have caused major ecological changes, in particular the reduction of tundra biome and taiga forests reaching further north. Some species, such as mammoths, were not adapted to these warmer, wetter forests and would have suffered from a decreased range and were likely unable to keep up with a shifting habitat due to their large size and slow reproductive cycles.

Light grey bars represent interstadial warming events and have a marked association with extinctions; Source

2. A comet?

The Younger Dryas impact hypothesis suggests that a barrage of meteorites hit the ice sheet in North America and created instability and caused the onset of the Younger Dryas. However, this is widely discredited as we have evidence (such as Gill's sporormiella) that tells us the megafaunal extinctions were not clustered around or after 12.9kya when the impact occurred. The event itself is disputed due to lack of typical bolide indicators found in sediments from that time. So, not really anything to write home about, but just letting you know what's out there. ;)

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In summary, as shown in the original graph by Barnosky et al, the evidence points towards a mix of human impacts with a background of climatic events as the drivers of megafaunal extinctions across the Northern Hemisphere Whilst the jury is still out for most of the Southern Hemisphere, Australian extinctions appear to be very closely related to human impacts, with little climatic correlation. We have a good understanding of what happened to many of these genera, as shown by the number of green circles representing 'robust evidence' in Barnosky's graph, but there is still room for improvement, particularly for understanding what happened in South America and Africa. It would be interesting to hear your opinions on all this, particularly if you disagree with anything I've said! As always, questions and debate and welcomed in the comments section.

Are We Mass Murderers? Earth's 6th Mass Extinction - Part 3: Implications

The third and final entry in my discussion on whether or not we are entering a 6th mass extinction event. In this post, having got all the complicated stuff out the way, I will be discussing the societal implications of the 6th Mass Extinction as a concept and also the message we need to be taking away from this research, regardless of a definitive 'yes' or 'no' answer to the question. I hope you've enjoyed the journey and I would be really interested to know if your opinion on the 6th MEE has changed throughout it, let me know in the comments :)

If you missed them, check out Part 1 and Part 2!

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The results of the poll that you, my lovely readers, have been partaking in over the last few weeks show that really, we're all a bit confused about what's going on but we know it's something significant. To be fair, I think that quite adequately sums up how the academic community feel as well. There are a lot of big unknowns and uncertainties in the data, as discussed in the previous post, which have led to a lack of definite answers about extinction rates and extinction magnitude. Some of this can be improved by collecting more data on currently understudied groups, especially invertebrates - reminder that only 1% of bivalves have been assessed by the IUCN! However, there are many, many unsolvable issues with fossil data as every good palaeontologist knows, we can extrapolate and model and adjust for bias to our hearts content, but there are some species we will never know about and we will never know exactly what happened to them and their relatives. There are inherent problems with comparing data gathered in completely different ways and from animals in different states of existence, so perhaps the question is simply one we cannot easily answer. I pose a new question to you - does this actually matter? Does it matter that current extinction rates don't quite fit with the technical definitions of a mass extinction? We are well aware from the data of the abnormality of current extinction rates in comparison to natural variability through time, and well aware that we hold the majority of the blame for this. Do we need a dramatic title for it?

Your votes! Homegrown data.

In some ways, the answer is yes. It's a kind of depressing yes. If you Google "6th Mass Extinction", you will be greeted by a plethora of sensationalist news articles claiming that the 6th MEE has begun , and more importantly, will humans survive it?! Humans tend to care more about things when humans are involved, as we have demonstrated time and time again throughout history. If we look in the comments on some of these articles, readers actually don't seem to be getting the message that we need to change our collective behaviours. See the enlightened comment below from a Daily Mail reader, sporting the 2nd highest amount of up votes on the article:

"Whatever changes the climate may experience, the consequences will pale into insignificance compared to the catastrophe resulting from massive unrestrained population growth in the developing world. Open borders will make this our problem too"

Bringing it back to humans, with a hint of UKIP. Daily Mail readers aside, if you ask anyone in the street, they would probably say that of course they are aware of humans causing extinctions, but do they understand the extent of the problem? Probably not. Education, if you can call it that, through sensationalism is one way to get the message to the wider public. However, as the above shows, we need to be doing this in a more effective way. If scientists did agree on a technically defined or supported 6th MEE, then perhaps it could be something that got more serious journalistic attention and much needed political attention. Social scientists have suggested that "inadvertant envrionmentalism" is the way forward, as often there is a gap between our values, however sincere, and our actions. They advocate that government policies, such as limiting deforestation in this instance, as they have the power to change behaviours without the need for values. This is one way in which mainstream attention to a dramatically definite 6th MEE has the power to help. If we want to be more optimistic and think about changing values via awareness, Racing Extinction and the #StartWith1Thing campaign is great example that again relies on the sensationalism associated with the 6th MME (The working title of the film was 6!).

Sensationalist and melodramatic representation of the 6th MEE, could it be useful?; Source.

In others, the answer is no. In light of the sheer numbers of species lost, and the potential there is for that to increase in coming years, what we need to be thinking about are urgent conservation measures. As multiple papers have highlighted, the extinction rates and magnitude is likely to increase as years, and harmful anthropogenic practices, continue. In particular, Barnosky et al. highlight the destructive potential  for synergy between co-occurring environmental factors such as atmospheric composition and unusual climate dynamics, similar to how co-occurrences of bolide impacts and flood basalt volcanism have interacted dangerously in the past. Without mitigation of these unnatural factors, particuarly pollution and CO2 levels, extreme ecological stresses will be placed upon most living species, especially given the feedback of individual stressors. Ceballos et al. point out that, in human timescales, the loss of these species and the ecosystems services they provide would be effectively permanent. They argue, as do Barnosky et al, that avoiding a biodiversity crisis (regardless of what it may be called) requires rapid and greatly intensified efforts to alleviate pressures on endangered species and protect them from exploitation. Many of the pressures are intrinsicly related to population growth and economic inequalities, and have the potential to threaten humans alongside other species.

Are we going to run ourselves into the ground? Let me know your thoughts in the comments!; Source.

In summary, it is debatable whether or not the 6th Mass Extinction being confirmed matters, but there is one thing that isn't debatable, and I will leave this as a closing thought for you:

The window of opportunity to act is rapidly closing and with the COP21 talks starting in under a week, it is imperative that we are mindful of the extent to which we have affected global biodiversity and will continue do whilst CO2 levels continue to rise and corporate greed is placed above moral concerns.

Racing Extinction

Exciting news and a date for your calenders - On the 2nd of a documentary called Racing Extinction will première on the Discovery Channel, which had a première in New York on Friday. The documentary is focused on the anthropogenic impacts to biodiversity and the potential of a man-made extinction event, (6th Mass Extinction, anyone?). It takes a hard look at the international wildlife trade and shows covert footage of illegal trade in endangered species. The documentary aims to educate and inspire people to act and help prevent further species loss by embracing the solutions that will ensure a thriving planet for future generations - and the trailer will get you pretty excited for it!

Their website is a great interactive resource to educate yourself about your impact and what you can to do "Challenge Yourself" and lead a more environmentally friendly lifestyle. I highly encourage you to check it out and sign up for some of their challenges. Their tag - #StartWith1Thing - is "a call-to-action to each and every one of us across the globe to make small changes in our lives that will have a huge impact on the world". They want people to make a start with 1 thing, no matter how small, to make a difference to species loss and use the hashtag on social media to encourage their family and friends to do the same. I think it's a great idea and I will be 'blasting' it out, as recommended, on the 2nd December when the documentary airs - I hope to see some of you doing the same!

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!