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


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