Des murs (Walls)

In recent years, much attention has been devoted to understanding how human impacts on the environment affect its flora and fauna. There simply can be no denying that humans are responsible for dramatic and unprecedented changes, if not outright transformations, to habitats across the globe. From the increasing frequency of extreme weather events to landscape-scale conversion of natural habitat to cities and agricultural lands, such impacts are widely reported in the media and are topics for much scientific investigation. For example, there is now an entire journal devoted to understanding the impacts of humans on biological systems, Global Change Biology. From the perspective of reptiles and amphibians (which is generally the perspective I take on such matters), such shifts in land use and habitat structure present a challenge to some species while presenting opportunity for others. A very nice volume containing a variety of studies examining the impacts of urbanization on reptiles and amphibians was published a few years back, Urban Herpetology (unfortunately now out of print). Recent studies have documented interesting changes in behaviour and physiology in Anolis lizards in urban environments (for example, work by the lab group of Jason Kolbe at the University of Rhode Island). Other studies demonstrate the effects on gene flow and population genetics (in a shameless bit of self-promotion, see our paper recently published in Herpetological Conservation and Biology on garter snakes living in urban Denver).

While recent extreme environmental changes have garnered much attention (deservedly), it is also interesting to ponder how smaller, long-term changes affect these organisms. The Common Wall Lizard is so named because of its affinity for the stone walls that transect much of southern Europe. From a lizard-eye view, it seems that human impacts on the environment in some areas, such as southern France, were conducted explicitly and intentionally to construct favorable habitat. These stone walls, bridges, and other structures date back centuries, if not millennia, in many areas. For example, just outside of Moulis there is a pile romaine dating back to the fourth century. Many lizards call this ancient rock pile their home; we caught several lizards here for our experiments.

Mahaut scouts for lizards on the pile romaine in Luzenac. We spotted a couple basking on the very top of the structure!

We also collected lizards on a Roman bridge in Aubert and on a variety of stone walls in other villages. In fact, when identifying a new site the first thing we look for is a stone wall: beside a river, along a property line, or surrounding a cemetery. These structures provide the ideal habitat for Podarcis: plenty of cracks and crevices for shelter, stable thermal environments, easy access to sunshine, and generally lots of little bugs to eat. Humans have worked diligently to provide exactly the right habitat for these lizards on a continental scale and indeed the lizards have benefited.

A stone wall along the road in Astien. This wall is home to approximately one million lizards (rough estimate).

The Romans (and later French) dedicated much time and energy to building ideal habitat for Podarcis lizards. For example, at this ancient stone bridge in Aubert we found many lizards.

We humans have been changing landscapes for a long, long time – some plants and animals have benefited, while others have not. While true that wall lizards have been extremely successful, it is vital not to hold them up as an example of whatever-we-do-is-alright-because-nature-finds-a-way-to-survive. Rather, we can study Podarcis as the cosmopolitan minority to understand how they survive and by contrast why the vast majority of plants and animals do not benefit from anthropogenic habitat changes. This is true for the centuries-long process of building stone walls but even more relevant in the context of the rapid and extreme alterations happening today. Not everyone can be a wall lizard.