Guest post by Dr. Ester Polaina
Here we are, immersed in the Anthropocene, witnessing how days go by amidst fabric tote bags, flygskam and demonstrations against impassible governments that don’t seem to care about the air we breathe, the land we grow our food, and the non-human living beings that regulate our very own (and unique) Earth. Not surprisingly, teenagers lead the ecologist movements of our days. It makes sense. They will be alive in 50-70 years. Most political leaders and big companies owners will not. Understanding the temporal component, and associated lag, between human actions and nature responses is crucial to fully comprehend the global environmental problems we are starting to observe and future generations will entirely suffer.
Whereas predicting the future is still impossible (although we scientists try very hard using powerful computers), showing that past human activities had an influence on the biodiversity we see today is an interesting finding that questions the way ecology has traditionally tackled questions related to species distributions. Additionally, it is a step forward to argue that our actions of today will have an impact on the humans, animals and plants of tomorrow.
In our study, we focused on one facet of human past impact, land-use change, i.e. the estimated degree of transformation of natural habitats for any kind of human activity, such as urbanization or agriculture, during the period c.a. BC6000-AD2000. And how it may have impacted mammals that inhabit the world today.
Low-used doesn’t equal unused
According to the data used in our study, more than 50% of global land surface remained little used by 2000 (the situation by 2019 surely looks much worse), which translates into relatively abundant available natural habitat for mammal species. Not only are these areas characterized by presenting low levels of human activities on land at present (10% of the area used on average), but also by a slow pace of land-use change throughout the whole period we studied (BC6000-AD2000). We call these areas low-used and they are mainly located in regions of low primary productivity (e.g. Tundra or deserts) or relatively remote locations with limited accessibility until recent times (e.g. the Amazon or Borneo). Despite the relatively low human pressure on land and, consequently, limited habitat loss within these areas, we still found a signal of positive correlation between rates of pre-industrial transformation of land for human activities and total numbers of threatened mammals. This finding suggests that numerous mammal species did not have time to recover from past human pressures and this is reflected in their current poor conservation status. In ecology, this is called extinction debt.
Recently-transformed areas present traces of extinction debt, as low-used areas do
An important portion of land surface, around 30%, started to be considerably transformed only around AD1750. That’s to say, when transformations started in productive areas of Australia, North America or South Africa (just to mention some examples), they happened quickly, reaching the highest levels of transformation across the Globe nowadays. Today, the average level of land intended for human use in these areas exceeds 60%. Therefore, available habitat for mammal species is much reduced. Contrary to expectations, these areas do not present a net lower total mammalian richness or higher proportions of them under threat (after controlling for other factors known to influence these biodiversity metrics). We find slightly more evidence of correlation between past human transformation and number/proportion of threatened mammals, compared to low-used areas; but in general, we can extract the same conclusion: the higher or quicker the transformation in the past, namely during the pre-industrial period, the more threatened species we find today.
The old cradles of civilization present less signs of extinction debt
The remaining 16% of the land surface is composed by territories where human presence has been noticeable for a long time, since the beginning of the first human settlements (in our case, from c.a. BC6000). These areas include parts of the Middle East, Europe, India, eastern China, the Sahel and Central America. Since then, human appropriation of land has grown at a relatively constant pace. In these areas, the positive correlation between past transformation of land and current terrestrial mammalian diversity is not so clear, pointing to the fact that these steadily-used areas are more heterogeneous in terms of human development and gradients in mammals richness/threatened richness can be better explained by environmental factors.
Everything is not lost… yet
In conclusion, our results show traces of extinction debts on more than 80% of the land surface, which means that many terrestrial mammals may be doomed to extinction in the forthcoming years. Even more if human pressure on land continues, as it is expected to do, in the following centuries. Additionally, comparing the magnitude of impacts of the 18th century with the present ones, one could expect that current impacts may have an even greater imprint on species than the one we observed from past changes. Therefore, conservation status of species may be deteriorated to the maximum, not just worsening conservation status, but directly driving species to extinction.
A little bit of hope?
By definition, extinction debts are collected even if human actions halt, which would leave us with little room for hope. Still, the signal of extinction debt we show is relatively low compared to traditional environmental factors that determine species distributions, meaning that natural factors are still the ones predominantly ruling macroecological patterns. Thus, stopping, reducing or even reverting human land-use change may help to keep the situation as we depict it in our study and not going any further. The day we find studies like ours showing macroecological patterns not mainly driven by environmental factors, but predominantly by human modification indicators, then the days of optimism are counted.
*This research was the final chapter of Ester’s PhD research, which I co-supervised with Eloy Revilla. If you want to read more about her work, you check the publications on her website or mine, and also on this blog post.
Full citation: Polaina, E; González-Suárez, M; Revilla, E (2019). The legacy of past human land use in current patterns of mammal distribution. Ecography. DOI: 10.1111/ecog.04406
Ramblings of an ecóloga 
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