Learning from the past: why do we go wrong?

A few weeks ago, I ran a discussion group hoping to discuss the primary cause of the Pleistocene megafaunal extinction event. Archaeological evidence suggests that up to 85% of large fauna weighing over 44kg went extinct on several continents and many islands towards the end of the epoch. Given the timings of these events, two major, competing hypotheses still rage on in the literature today, almost with ideological fervour. The first hypothesis is that the extinction of these fauna coincide with the retreat of the Last Glacial Maximum (LGM), and that many of these larger, already food-limited species were simply unable to adapt to the warming climate. The second hypothesis places us at the centre of the scene: the bloody culprit, Homo sapiens and predecessors thereof. Proponents of this “overkill hypothesis” argue that the last known fossils of these extinct genera and the first known evidence of human appearance coincide in the fossil record in each of these locations too often for it to be a coincidence. One of the reasons why the argument has become controversial is that some people have begun to question our moral requirement to pay ecological reparations of kind. If humans were indeed the primary cause behind such a large extinction event, do we feel an obligation to make amends?

So we mused and puzzled over these lines of evidence, trying to determine within the short space of an hour whether one was more convincing than the other. However, shortly into the discussion, there came one point in which we became quite unstuck. Instead of weighing up the data as if they were equal, we started wondering; how can anyone be sure of causation using archaeological evidence at all? It was quite unlike the data sets that we are used to. So, say you are digging in the Yukon Territory in Canada and you come across several sets of equid fossil species spanning several thousand years, clearly these horses are getting smaller over time you observe, clearly humans weren’t in this region at the time you think, clearly indicative of environmental pressure of the glacial retreat you surmise, clearly reflective of the climate change hypothesis you conclude. Excellent. A nature paper awaits! But wait. It is likely that you may have committed quite a few of the major issues associated with extrapolating information from fossil data in your haste.

Firstly, you are likely suffering the plague of all paleo-scientists: taphonomic bias. Taphonomic bias is the process by which we are only privy to the type of data that can inherently survive throughout geological time. Soft materials, smaller fauna and events that took place in raging rivers are less likely to be preserved than their counterparts, leading to the under- or over-representation of certain taxa or events. Whole species, whole ecosystems could have occurred during this time, ones that we may never even know they are missing. A potentially very significant source of taphonomic bias that we came across during our discussion was the timing of the first hominid arrival into North America. The current general consensus is that this fell somewhere between 13,000 and 20,000 years ago. However, there are some pieces of evidence that suggest it could even be as far back as 50,000. The difference in the upper and lower ends of the estimate could have significant consequences for how much blame we can attribute to humans for the extinction event. Humans that arrived only 13,000 years ago were indeed quite pushed for time when trying to condemn equid species to extinction. Of course you only have to look back to see that the presence of a rather large ice sheet over North America until about 20,000 years ago could have had significant impacts on whether we would be able to detect their presence beforehand.

Another pitfall when trying to make conclusions based on this type of data is that it is impossible to conduct experiments or make observations because there are no factors to manipulate and no events to watch. Determining causation becomes, at best, an educated guess and using contemporary situations as a stop-gap can be no more illuminating. In his paper, Guthrie (2003) states that past equids were obligatory grazers based on the assumption that modern-day horses can be used as a proxy measure for their predecessors feeding habitats. However, using modern-day analogies to try to extrapolate into the past can also be very misleading. Contemporary equids are adapted to a different climate, different predators and by now a range of anthropogenic factors.


Figure 3 This data set examines the change in body size of equid species over time. The equid species shows an Alaskan extinction event at 12,510Ka. The significance is this decline is supposed to be indicative of environmental pressures coinciding with the end of the LGM but not with hominid arrival on the continent.

The one thing that could help to mitigate these issues – a plethora of data – is also seldom available when using taphonomic data. Given how difficult it is for data to make it through the battering of time, it is not all that surprising either. In the figure above, the sparsity of measurements has meant that the author has made statistical inference about this relationship based on one data point over a 10,000 year time period. Without this solitary data point, would the relationship still hold true? Would the timings of events still line up in a way that supports his argument?

So assault after assault, how do these questions ever get answered? It seems almost overwhelming and we certainly weren’t going to solve it in the hour that we had. What we did get, however, was an insight into a surprisingly dynamic field for an event which may or may not have even occurred when humans were around to witness it.

By Josie


Dale Guthrie, R. (2003). Rapid body size decline in Alaskan Pleistocene horses before extinctionNature, 426(6963), pp.169-171.

Ripple, W. J., & Van Valkenburgh, B. (2010). Linking top-down forces to the Pleistocene megafaunal extinctionsBioScience60(7), 516-526.

Soligo, C. and Andrews, P. (2005). Taphonomic bias, taxonomic bias and historical non-equivalence of faunal structure in early hominin localitiesJournal of Human Evolution, 49(2), pp.206-229.

Perceptions of rarity and commonness – a quick experiment in our Critical Thinking group

From a Critical Thinking 2015-16 alumna, Gergana – an update on last year’s end of course project:

We, Isla’s 2015-2016 Critical Thinking group, have been working on a research project investigating how species’ rarity or commonness influenced rates of population change. We sought out to address the question of whether rare species (those with smaller geographic range and lower local abundance) are more likely to be experiencing population declines. We have already shared how we developed our project, and what our key results are, but we haven’t written about what prompted us to think in this direction – the very essence of this course, critical thinking!

Critical thinking is many things, in this occasion, questioning a very common assumption – rare species are more likely to go extinct/be declining. Reading this statement will hardly surprise anyone, but now, putting our critical thinking skills into practice, we can go beyond reading it and nodding in agreement. Are rare species really more likely to be declining? More likely compared to what? Common species? But what is a common species, and what is a rare one? Can a species be rare, but not threatened? How do we draw the line between naturally rare species, and those that have earned a rarity status due to human impacts on ecosystems?

An interesting fact to ponder is that rare species are common, and common species are rare. We confirmed this in our own analysis as well – a histogram of the population size of 211 UK species showed that the majority of species have very small population sizes (Figure 1).

Figure 1. A histogram of the population sizes of the 211 UK animals included in the the Living Planet Index (LPI) dataset with greater than a decade of data and more than five monitoring years.

At the start of our tutorial, we did a quick experiment that we can repeat here: take a look at these photos and without thinking about it too much, decide whether the species is rare or common. If you could distance yourself from all your scientific knowledge, do it – this is about what first comes to your mind, how you innately perceive species’ rarity or commonness.


Figure 2. Birds of many colours and shapes – photos by Gergana Daskalova.

Of course, rarity is not an absolute variable of rare or not, it’s a continuum and quite a complicated one. Nevertheless, having to decide rare or not on the spot prompted us to think about our own biases – how colourful or exotic a species looks tells us little about whether it is rare or not, and yet those species are considered ‘rare’ more often. In reality, Hall’s babbler, third image on the left row, and the juvenile South Island robin, second image bottom row, are the rarest of the birds shown, but they are also the dullest looking ones. The Sulphur-crested cockatoo, third in the top row, looks like it’s sporting a crown, an exciting sight compared to the pigeons we are used to seeing, but it’s also very common in Australia.

We discussed how “rare” and “threatened” are often used interchangeably, especially in conservation. Threatened species tend to be rare, but as we found out in our analysis, rare species are not always declining (and therefore threatened with extinction). We talked about how people perceive rarity, based on photos, now, to take it a bit further, think of a rare species, what is the first one that comes to mind? I thought of a yellow-footed rock wallaby – they live on rocky outcrops in Outback Australia and are one of those species that is both rare and threatened. However, yellow-footed rock wallabies are also cute and fluffy – most of all, to me they are very exotic – aside from one kangaroo that escaped from a private collection, there were no jumping marsupials out and about in Bulgaria.

So what do we perceive rare species to be? From a European perspective, we often think of rare species as colourful, exotic, beautiful, and exciting, a tick on someone’s bucket list. It often comes as a surprise to people that there are rare species where they live as well – if we have grown up with something, it’s hard for us to think of it as rare. Even more, what is rare in one place, is not rare in another – in the UK, for example, tree sparrows are a much rarer sight than they are in Bulgaria. Rare species come in all colours, including dull grey and brown. In summary, the term ‘rare species’ ought to be used with caution and with more context provided – at what scale is the species rare, why is it rare, is it also threatened?

By Gergana

Commonness, rarity and conservation – research findings and future steps

From the 2015/16 Critical Thinking Alumni Gergana and John and a year in the making, here are the final results of our exciting research project, are rare species really more likely to be declining in the UK?

Around this time last year, we, Isla’s 2015/2016 Critical Thinking group, started a research project – it was a great experience for all of us and it was fun to go through the different stages of a research project together. We have since graduated, but our project, and in particular the results, are still on our minds. We are preparing a manuscript for submission to a peer-review journal, so hopefully you’ll be able to read all about the outcomes of our research soon. Until then, here is a summary of our project’s goals, outcomes, as well as the key things we learned in the process.

Research Question:

Are rare species (those with smaller geographic ranges and lower local abundance) more likely to have declining population trends than common species?


H1: As geographic range increases populations will have a lower rate of population change.

H2: As geographic range increases populations will have a higher rate of population change.

H0: The rate of population change will not vary with geographic range size.

To address out research question, we compiled population trends data for 211 UK vertebrate species from the Living Planet Index (LPI) with occurrence data for the same species from the Global Biodiversity Information Facility (GBIF). We thought about how we will define ‘rare’ and ‘common’, and decided on using a combination of geographic range extent and local abundance. Habitat specificity is another possible measure of rarity and commonness, which we could include in the analysis as we take it further. Nevertheless, we already have exciting results from our analysis using geographic range and local abundance.

We extracted the slopes of experienced population change for our species during the time period for which LPI data were available (at least 5 years), with positive slopes indicating a population increase, and negative slopes – a decline.  We then plotted slopes against geographic range and local abundance. We estimated species’ geographic range in two ways: firstly, we looked at occurrence maps and categorised the range of each species as UK, partEurope, Europe, partWorld, and World. Secondly, we quantified range as the maximum differences in latitude and longitude of occurrences in the GBIF dataset.


We found that the species with the smallest ranges, those in the UK and partEurope categories, are actually the ones that are experiencing the smallest amount of population change, and most of them have positive slopes, indicating population growth. There is a big spread in the slopes of species with a partWorld – World distribution, so this is geographic range at which both most declining and most increasing populations are found. Comparing levels of population change across all the range categories did not reveal clear evidence in support of one of our hypotheses. We did not find strong evidence that species with more local ranges are showing declines across the time period of their respective LPI data (H1). There is a weak trend of rates of population change becoming higher as geographic range increases (H2), but overall, we didn’t detect much variation in population change based on our geographic range categories (H0).


Figure 1. Rate of population change versus extent of geographic range for 211 UK species populations. Data obtained from LPI and GBIF.

Looking at how our second measure of geographic extent, maximum differences in latitude and longitude of occurrences in the GBIF dataset, influences rates of population change provided support for our null hypothesis – rate of population change does not appear to vary significantly with range size (Figure 2). We find no evidence that population declines or increases are occurring more frequently for UK species with a smaller geographic range.


Figure 2. The geographic range (maximum differences in latitude and longitude of occurrences in the GBIF dataset) versus the population change overtime for the 211 LPI species monitored in the UK.

Investigating rates of population change versus local abundance also showed that population declines or increases are not occurring more frequently for UK species with a smaller population size (Figure 3). These results are part of an ongoing investigation, but for now, we conclude that for the UK species populations we studied, rate of population change does not vary with geographic range size (H0).


Figure 3. The log population size (number of individuals) versus the population change overtime for the 211 LPI species monitored in the UK.

Future steps:

With the data present unwrapped, the next steps of our research project are to ponder what are the implications of our findings, and write a manuscript to communicate our key findings. Questions we will consider include why are patterns of rarity and commonness poor predictors of rates of population change for our UK species, what does this mean for conservation, and since we know that geographic range doesn’t appear to be influencing population change in those species, we could hypothesise what other factors are affecting species’ population trends.

By Gergana and John

Science is in the Air

Chrissy’s Tutorial Group
Student lead discussion session 2, led by Antonia

Land-atmosphere interactions are a dynamic and exciting area of research – starting from the 1970s with papers such as the one by Idso et al., 1974 discussing the most basic of physical processes, up to now, when we have gained an in-depth knowledge on these same processes and many more.


Figure 1. Remnants of the Tokage Tropical Cyclone visible by AQUA MODIS. Source: NASA

Because of the above-mentioned reasons, for my tutorial this week (15/11/16), I chose two papers which framed the topic – one from the early days of the field by Charney et al., 1975, and another by the team of modellers of Wang et al., published in 2015.


The first paper proposed a theory that changes in surface albedo due to human intervention were the sole reason for the Sahelian drought in the 1970s. Charney et al., 1975 suggested that the system was locked in a positive feedback loop. An increase in surface albedo, due to a reduced plant cover, decreased net incoming radiation. This led to an increased cooling of the air and consequential rainfall reduction. This whole process is suggested to have been initiated by overgrazing of cattle. In an earlier work, the same author briefly mentions the same theory, giving as a reason for vegetation loss the livestock trampling of vegetation while they are taken to the local water wells. The only parameter which was manipulated in the study, was the surface albedo.

Our tutorial group agreed that the simulations were very simplistic. Although the feedback loop mentioned above does sound straightforward and is an established physical pathway, the simulations did not take neither external influences nor global atmosphere circulation into account. The group agreed that a greater system complexity should be added to account for the overall micrometeorology changes. Another shortcoming of the paper was that the net effect was not considered. In order to do that, both biogeophysical and biogeochemical effects should be taken into account. A model representing the biosphere’s characteristics should also be incorporated within the simulations before any conclusions can be drawn.

Although the paper has many flaws, as seen from the above, it was one of the pioneers in the area, and the first one to tackle the effect albedo has on the microclimate of an area. If examined in that light, it is indeed a valuable paper because it provoked a lot of thought. In fact, it was disproved the very next year (1976) by Ripley et al., who discovered that the draught has less to do with anthropogenic influences prompting a change in albedo and more to do with Atlantic circulation.


Figure 2. Figure taken from Wang et al., 2015 showcasing the fine scale that we are able to track atmospheric changes over. Displayed here are the observed temperature differences between 1980s and 1950s.


This leads us to the second paper by Wang et al. (2015) which demonstrates a far more sophisticated understanding of the matter. It includes several different models assessing the impact of land use land cover changes on the Sahelian microclimate. Some of the group members pointed out that the paper was very technical, and we were in agreement that some of the model descriptions were hard to understand without a solid understanding of modelling, which none of us have (yet!). This paper showed how much our understanding has changed since 1975. The main drivers of the drought were pointed out as the state of the boundary layer conditions, and evapotranspiration (ET). ET is shown to be a key element, which was not previously considered. The group also though that the paper dealt better with external influences such as the African Easterly Jet.

Although the two papers were not part of our expertise, we thought that this particular area of research has come a long way, as showcased above. Future research is heavily dependent on satellite observations, and more sophisticated models, incorporating local, regional and global conditions altogether. At the end of the tutorial, I briefly mentioned the challenge that the role of clouds presents in the area, as there has not yet been developed an approach for their incorporation into models. The 2.6 billion dollar U.S. Global Change Research Program for Fiscal Year 2016 puts “the role of clouds” as their most important and costly area of research within their Climate and Hydrological Systems research.

I personally find climate tracking and model predictions extremely interesting, so if you share that curiosity, take a peek at the following websites:

http://cola.gmu.edu/ – The Center for Ocean-Land-Atmosphere Studies

http://www.noaa.gov/ – National Oceanic and Atmospheric Association

http://www.metoffice.gov.uk/public/weather/storm-tracker/#?tab=map – Met Office storm tracker

https://www.nasa.gov/mission_pages/hurricanes/main/ – NASA Hurricanes and Tropical Storms




Charney, J., Stone, P. H., Quirk, W. J. (1975) Drought in the Sahara: A Biogeophysical Feedback Mechanism. Science 187: 434-435


Idso, S. B., R. D. Jackson, R. J. Reginato, B. A. Kimball, and F. S. Nakayama (1975) The dependence of bare soil albedo on soil water content. Journal of Applied Meteorology 14, 109–113.

Wang, G., Yu, M., Xue, Y. (2015) Modelling the potential contribution of land cover changes to the late twentieth century Sahel drought using a regional climate model: impact of lateral boundary conditions. Climate Dynamics 382: 1-21

“Let’s not jump to conclusions here.”

Isla’s Tutorial Group
Student lead discussion session 1, led by Georgie

This is what I should have said before embarking on the task of leading a discussion addressing relationships between ecosystem services and trophic interactions. The theme of my discussion connects the impacts of trophic levels on ecosystem services. Reading material was selected to be compelling, challenging, and provocative to inspire discussion and debate. The first paper, investigated the relationships between habitat loss, trophic collapse, and ecosystem services through a “phenomenological” model of biodiversity loss (Dobson et al 2006). The second paper, furthers the discussion by challenging a general assumption that one species has similar effects on ecosystem services across habitat types. It does this through an investigation of the effect the presence of gray wolves on trophic patterns and further on carbon cycling in two tundra habitats (Wilmers and Schmitz, 2016). What became clear from the tutorial group, was the difficulty in understanding how the methodologies and the assumptions in each of the papers led to the results. Alas, our discussion hit a critical turning point revealing the importance of having a clear understanding of methodologies before assessing the conclusions.

To whet the appetites of my classmates, I sent around a link to a YouTube video titled How Wolves Change Rivers.  The video illustrated that the reintroduction of wolves to Yellowstone has spurred an increase in biodiversity thereby increasing functions of many ecosystem components such as water cycling. It breaks a very complex issue into bite size pieces which is good for communication to the general public. However, the video over-simplifies many of the underlying processes leading to miscommunication of the scale of which wolves have an impact. This is dangerous as not all the information is present in the video, introducing biased conclusions about the impacts of wolves in reality. So we ask, how are the conclusions made?

In discussing the papers, it was apparent from the group that the methods were not easily understood, notably in Dobson et al (2006). A question asked was “would you be able to replicate these methods?” There was clear use of modelling in Dobson’s paper – agreed upon throughout the group. There are distinct stages of developing the model; however, we found it hard to interpret the model itself from the equation alone. Figure 1 is a simple graphical representation of the modelled relationship. The main conclusion was there is a sequential loss of ecosystem services as there is a hierarchical loss of species from different trophic levels. But, the simplicity in the assumptions ignored important factors such as species interactions between trophic levels and basic community dynamics which undermine the confidence of the conclusions. Overall, we agreed that the paper is useful as it opens a door for further research; but, although the results are intriguing broad assumptions and lack of clarity in the model make it difficult to be confident in the applicability of the findings to real world scenarios.


From Dobson et al (2006) paper showing functional forms for the relationship between loss of biodiversity and loss of ecosystem service function for each trophic type. Predators, showed the most sensitivity and decomposers the most resilience.

The Wilmers and Schmitz paper challenges the assumption that species interactions don’t matter in tropic collapses and even cite Dobson et al (2006). They used data from various studies to investigate whether there was a significant difference between the indirect effects from trophic interactions of gray wolves on carbon sequestration between two different tundra habitats, grasslands and boreal forests. The findings revealed that through trophic interaction, wolves negatively impact carbon sequestration in grasslands and positively impact the process in boreal forests. Their paper has more defined assumptions relevant to the tundra biome, but was still a bit challenging to fully understand. In review, there was no explicit mathematical model presented in the study, so we discussed whether it really was a modelling paper or rather more of a data synthesis. I learned here it is important understand the type of paper under assessment as a means of deciphering a studies’ conclusions.


by Gabe Ginsberg, National Geographic. Accessed: http://www.nationalgeographic.com/animals/mammals/g/gray-wolf/

All in all, the tutorial group found discussion of the papers a bit challenging as we had some difficulties in understanding of the methodologies. Nonetheless, the findings and conclusions for both papers are compelling – it’s no wonder I jumped. They say, curiosity killed the cat – well, enthusiasm killed my inner critic. Taking a more analytical approach to future readings focusing on methodologies, assumptions in context and ability to replicate these studies will enable a more enlightened conversation of the conclusions and potential for the application of the findings.

by Georgie


Wilmers, C. and Schmitz, O (2016). Effects of gray wolf-induced trophic cascade on ecosystem carbon cycling. Ecosphere, 7(10). http://onlinelibrary.wiley.com/doi/10.1002/ecs2.1501/full

Dobson, A., Lodge, D., Alder, J., Cumming, G. S., Keymer, J., McGlade, J., Mooney, H., Rusak, J. A., Sala, O., Wolters, V., Wall, D., Winfree, R. and Xenopoulos, M. A. (2006). Habitat loss, trophic collapse, and the decline of ecosystem services. Ecology, 87: 1915–1924. http://onlinelibrary.wiley.com/doi/10.1890/0012-9658(2006)87[1915:HLTCAT]2.0.CO;2/full

Food for Thought

Chrissy’s Tutorial Group
Student lead discussion session 1, lead by Amy.  

For my tutorial I led a discussion with Antonia, Ariana, Emma, Rob and Chrissy on two papers regarding the environmental impacts of food. One paper  compared the greenhouse gases associated with the average UK diet to various scenarios of vegetarian and vegan diets. The other study looked at GHG emissions and blue water scarcity index  of three carbohydrates commonly eaten in the UK – potato, pasta and basmati rice.

The first paper made the conclusion that the vegan diet has the potential to reduce an individual’s greenhouse gas footprint to the equivalent of a 50% reduction in exhaust pipe emissions from the entire UK passenger fleet. The study on carbohydrates found that whilst potato consumption in the UK has fallen since the 80s and pasta or rice are now preferred, potatoes have the least environmental impact because of their lower water footprint and locality.

The group thought that both papers provided valuable information but there were doubts as to how widespread the studies can be applied. The paper based on different diets used data from the US and UK, but the environmental footprint of foods may vary from country to country, depending on factors such as food miles.  We thought the papers were good at taking a variety of aspects into account, such as food waste and packaging, however the results could have been presented more efficiently and concisely.  The conclusions were somewhat 1-dimensional as there are other ways that food can damage the environment – for example pollution from pesticide run-off, land-use change such as deforestation causing loss of species, and erosion of land from poor management practices.  However, it would be difficult to take everything into account and both studies gave valuable ideas of ways that we can reduce our impact on the environment three times (or more!) a day.  More plant foods and local foods!

We weren’t sure how much of a difference the findings would make to people’s eating habits but Rob came up with an interesting idea – supermarkets could provide footprint information on the packaging of food. This would at least make people more aware of their impact, and particularly of the large differences there can be between foods.

Most members of the group disagreed that a tax on the highest GHG producers  (meat and dairy) is a fair way to deal with things as it could impact local farmers – but thought that tax could be higher on items that have travelled far to reach the consumer country. Increased awareness of food footprints, encouragement of local food and less food waste were some of the main areas that the group thought can decrease the environmental impact associated with food consumption.

By Amy

Constructive criticism on citation classics

If we, one of the University of Edinburgh Critical Thinking tutorial groups, were on the Editorial board at Am. Nat. in 1959 (but with our modern take on ecology and scientific writing):

Dear Prof. Hutchinson and Dr. MacArthur,

Thank you for submitting your work to The American Naturalist.  Before we can accept these manuscripts for publication we have some constructive criticism for you to incorporate.

Prof. Hutchinson, in the written form of your address to the American Society of Naturalists entitled “Homage to Santa Rosalia or Why are there so many kinds of animals?”, though we very much appreciate the personal detail and wandering nature of your narrative, we wonder if your manuscript would benefit from greater structure including an abstract summarizing the results and clearer linkages between the different sections and less deviation from the primary message of the paper.


The official logo of the American Society of Naturalists now and in 1959 (presumably).

Think on the future students reading this address, which may for all we know now go on to become a citation classic.  You might improve your communication of your synthesis of ecology – including food webs and niche theory as it stands in 1959, which is later called limiting similarity – to that future audience by being more clear and structured in your delivery.

In regards to your co-authored manuscript, Prof. Hutchinson and Dr. MacArthur, entitled “A theoretical ecological model of size distributions among species of animals”, we have additional and more detailed feedback.  Though we very much appreciate the impressive novelty of the ideas and theoretical concepts with tests from real world data presented in this manuscript, we also feel that the clarity of writing and specific details presented in the tables and figures are somewhat lacking.

We feel that this manuscript, like your other submission could use better structure and greater attention to scientific writing conventions, some of which you may not be familiar since they haven’t yet been formulated (see The 5 Pivotal Paragraphs in a Paper).

In particular, we would like to draw your attention to the fact that Table 1 does not include an informative table caption explaining the variables summarized by the letters x, n and r.  Though these variables are somewhat explained in the text, a lot of the understanding is left up to the readers’ own intuition.  In figure 4, and some of the other figures, we notice that axis labels are missing.  This obscured the message being conveyed by these figures and puts the burden on your readers to figure out what each axis might represent.


Table 1. from Hutchinson and MacArthur 1959 – note the lack of table caption. This creates much more work and additional confusion for the reader!

We also feel that it is perhaps unnecessary for you to resort to the following statement in the caption to Figure 4 “The straight line is fitted by eye with a slope of 1:2”, even in 1959 you would have had access to graphing paper and a ruler so as to be more precise. Perhaps you would appreciate knowing that one day such a linear regression can be statistically calculated using something called a ‘laptop computer’ and the statistical software known as ‘R’, meaning that you will no longer have to resort to the eyeball approach in future.


Figure 4. from Hutchinson and MacArthur 1959 – note the lack of axis labels. This is unacceptable for figure presentation even in 1959!

We think perhaps that the statement: “In a certain sense the application of the theory to mammals tells us little that we did not know before, and is therefore trivial”, may not be required to make the point that you are making.  We believe that this study may have major influence on the field of ecology as a whole, perhaps opening up entire new sub-disciplines such as one we might call ‘macroecology’ and future concepts that one might term ‘metabolic theory’ – and we do not necessarily think it is in any way trivial.

With the successful revision of these two manuscripts, we would be happy to accept them for publication in The American Naturalist.  We appreciate the great insight and potential future impact of these studies, but feel that the syntheses and findings could be a bit better communicated to your future audience of fourth year ecology and environmental science students in 2016.

By Isla and her Critical Thinking Tutorial Group

More trees – dark…

Jakub: The notion of more trees as a measure to address climate change has been around for a while. This idea stems from the fact that trees readily absorb CO2 from the air and effectively fix it in biomass. I have thought that trees were crucial in keeping global temperatures to acceptable levels. Until I came across an article by Bala et al. (2007). ..

The authors of this study modelled the overall effects (not only photosynthesis, but also physical and other chemical properties) trees have on temperature. They found that the impact of forest on air temperature varies across latitudinal bands. Their results show that trees in high latitudes actually exacerbate rather than ameliorate climate change because of physical properties of trees. Lower albedo of trees compared to bare soil causes more solar radiation to be absorbed. This results in significant warming which is of a higher magnitude than cooling effect of photosynthesis by those trees.  They propose that further afforestation of high and mid-latitudinal bands will bring no additional benefit in terms of climate change mitigation with some high-latitude locations in Russia experiencing up to 6oC decline after the place has been deforested. Conversely, the impact of trees in tropical regions is opposite. Although deforestation would bring some benefit in terms of decreased air temperature, trees in tropics transpire to a high degree and are thus responsible for cloud cover, which has high albedo and therefore reflect large proportion of incoming insolation. This ultimately results in net cooling effect of trees in tropics.

This article was an interesting read because it confronted my previous belief that we should plant more trees wherever is possible. It showed me that it is a much more complicated topic and many more factors will therefore have to be considered when determining the role of trees in climate change mitigation, especially in specific regions.

Bala, G., Caldeira, K., Wickett, M., Phillips, T. J., Lobell, D. B., Delire, C., & Mirin, A. (2007). Combined climate and carbon-cycle effects of large-scale deforestation. Proceedings of the National Academy of Sciences, 104(16), 6550-6555.


Hannah’s selection was inspired by a talk given at the University of Edinburgh by George Monbiot – which is itself documented online.

Rewilding is a highly debated topic that has convincing arguments both for and against implementation. When deciding whether rewilding will occur, one of the most important things to consider is the motive behind it. In this talk, George pushes the idea that we should rewild the world for humanity, in order to ‘grant us freedom’. I believe this motive is questionable, and perhaps biased towards the limited members of the population who regularly enjoy the outdoors and wild places. Dramatically altering a landscape should be done for a better reason than to grant freedom to humans who have destroyed the wild in the first place. We should be rewilding, for example, to restore the natural system to a landscape to relieve the need for human intervention, such as deer culls.

While we broadly agreed with George’s interesting view on how we regularly destroy in order to conserve, such as through burning, and how odd this idea is. This followed onto a discussion about the word ‘conservation’ and how to conserve an ecosystem or a species implies that we are attempting to keep it the same. In reality we are not able to conserve things in the natural world because change is too rapid, and perhaps the field of conservation science needs a new name which incorporates the idea of adaptation.

We identified one of the major flaws in George’s talk to be ignorance to the value of certain ecosystems and small species, claiming that conservation of which ‘shows how limited out aspirations have become’. George displays a strong dislike of heather moorland ecosystems, while seemingly ignoring the important ecosystem services such as carbon storage, water filtration and flooding reduction in addition to the many species that heather moorland provides a home for, including black grouse, field voles, merlins and adders. The lack of large mammals and top predators in a heather moorland ecosystem appear to mean it is deemed useless and unenjoyable to George.

Based on this talk we decided that George’s opinion on rewilding appears unscientific, biased and idealistic. His dream of extreme ecosystems and top predators all over the world is not realistic and raises the debate on the importance of conserving multiple species, small and large, even if they are not considered exciting and wild.

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Plastic aphids

Christie Paterson reported on recent study of aphid plasticity, with respect to their host plant species. It was published by Hong Lu et al. in Nature Scientific Reports:
Aphids become specialised to plant hosts as they evolve to overcome plant defences. In this paper by Hong Lu et al., the ability of pea aphids to adapt to other plants was examined. This was done by examining the demography, feeding behaviour, and gene expression in the salivary glands of the aphids, after short term and long term acclimation.

It was found that when moved to non host plant species, aphids grew to a smaller size, had a lower reproductive rate, slower population growth, slower individual development, but longer life span. Aphids on non host plant spent less time consuming phloem from the plant than they did on their host plant. However, it was found that after a period of long acclimation, feeding success increased in two out of three of the plants. It was also found that aphids expressed different salivary gland genes when on different plants, and that the number of these host specific genes expressed negatively correlated with the success of the aphid on that plant.

Together these results how that, although the pea aphid has evolved to feed from just one host plant, it still exhibits plasticity to other, similar plants though with reduced success. This could have implications for pest management and provides important information on host plant specialisation in aphids generally.

Although this paper brought some important and interesting information to light, there are some aspects which could be improved upon. Firstly, winged aphids were excluded from the population demography analysis. Neglecting a sub set of the population means that the results are not conclusive of the population as a whole.  Secondly, despite the inferred importance of it, leaf characteristics were not well controlled for or tested in the experiment.

In conclusion, this paper found that despite high levels of selection, pea aphids have retained an element of plasticity and can adapt to some other plant species. Further research should investigate the effect of plant leaf characteristics, and determine whether this is observed in other plant and aphid species.