What is natural habitat?

Habitat conversion is, inarguably, the most devastating impact that humans have upon the natural world. Directly witnessing the effects of large-scale projects such as damming rivers, draining wetlands, busting sod on a prairie, mining, or clear-cutting a woodland of any type is simultaneously humbling and depressing. The effects on local biodiversity seem so clear and predictable. No ponds = no fish, no trees = no treefrogs or monkeys, etc. What I want to discuss in this column is the pernicious reality of what conservationists call “shifting baselines” and our general perceptions of what constitutes original, native habitat. I will use Georgia, and my study sites in the Neotropics, to bring some perspective.

A broadscale review of Georgia’s current habitat types would identify huge areas of agriculture and areas similarly totally modified into urban and suburban developments. Pine plantations may appear to the highway traveler to be natural forested lands, but in fact they are biological deserts. The remainder of our state then exists as various forms of natural areas that support our considerable biodiversity. We are lucky, here in Georgia, that we do have considerable areas of habitat that still can support most of our spectacular native fauna and flora. But we risk falling for a great deception—the reality of shifting baselines in our ecological perceptions.

Very little of Georgia’s habitats represent truly native historical conditions. The entire eastern United States was essentially clear-cut in the 1800s. The lumber built the great cities, heated buildings, cooked the food, and helped power the great steamboats and locomotives. The easiest way to realize this is to look at historical photographs. The images were invariably taken for different reasons but, regardless of the purpose of the image or the actions or figures in the foreground, the backgrounds tell the story.

For example, consider Georgia’s Kennesaw Mountain historic site. When we visit Kennesaw Mountain, we may think to ourselves what a wonderful example of native forest that the site preserves, outside of its historical significance. But again, the reality is that Kennesaw Mountain was completely deforested, and what we see today is secondary growth that we know is not the same as the original forest. It is hard to find original, untouched native habitats in the eastern United States. The best examples are places like Joyce Kilmer Memorial Forest in North Carolina, where for various reasons, the forests simply were not cleared. My visit there really impressed upon me how large our native trees can get and how scrawny are 99% of the forest trees across our region.

Human influences on habitat, however, do not start in the post-Columbian era. In his wonderful book Forgotten Grasslands of the South: Natural History and Conservation, author Reed Noss discusses the likelihood that many areas that we consider to be natural grasslands (vs. woodlands) in our region may be the legacy of purposefully set fires by indigenous peoples intended to reduce woodlands and encourage grasslands. Payne’s Prairie, a popular reserve near Gainesville, Florida, would be a possible example. Relatedly, fire suppression and control by well-intentioned humans has directly contributed to the invasion of native woody shrubs into areas that historically were grasslands—grasslands that historically were maintained free of shrubs by intermittent naturally induced wildfires.

These themes expand to even older time scales. While we all lobby to save rainforests worldwide, we may be doing so in ignorance that many of those same rainforests are in the same situation as Georgia. A landmark paper by Heckenberger et al. (2007; doi: 10.1098/rstb.2006.1979) forever changed my view of Amazonia. The verbiage in the Abstract tells the whole story:

“For centuries Amazonia has held the Western scientific and popular imagination as a primordial forest, only minimally impacted by small, simple and dispersed groups that inhabit the region. Studies in historical ecology refute this view. Rather than pristine tropical forest, some areas are better viewed as constructed or ‘domesticated’ landscapes, dramatically altered by indigenous groups in the past.”

They found huge areas in which the soils were Terra Preta, or Black Earth, which, in contrast to the typical red-colored clay soils of Amazonia, represent areas of intense human habitation and fires. They also, ingeniously, mapped the distributions of native trees that bear fruits that humans enjoy. Clusters of food-bearing trees and human-modified black earth identified long-forgotten human plantations in communities that greatly modified their regions. Other studies have found clear evidence of massive systems of roads and waterworks, indicating large-scale human habitations that since have been covered by rainforest. The profound implication here is that Amazonia, as we mostly know it, is a spectacularly re-vegetated vacant lot that hosts an amazing biodiversity. One can only wonder what that biodiversity may have been prior to the arrival of humans. European colonizers are kind of an ecological afterthought, I feel.

The same is true for the areas of the Yucatan Peninsula of Mexico and Guatemala and nearby areas. This was the stronghold of the Mayan empires, and it is now clear that those cultures virtually denuded the entire region of its forests. So, the native rainforests in that area are technically native, but decidedly “un-natural” in the sense the that they merely represent the results of vacant-lot vegetative succession. The remaining forests of much of Amazonia and the Yucatan region are just older versions of Kennesaw Mountain.

What was the habitat originally in these areas? Would we recognize truly native habitat if we saw it? What was the original biodiversity? Those questions seem to slip into the realm almost of paleontology, as our poor understanding of how and when the ecological baselines shifted does not allow us to visualize, or maybe even imagine, such.

Interestingly, these human-induced changes wrought by Amazonian civilizations were only about 500 years ago. How were tropical ecologists so unaware of such relatively recent events of such importance to biodiversity? That story delves quickly into human history, rather than natural history. In his best-selling book 1491: New Revelations of the Americas before Columbus, author Charles Mann provides a nice summary of the very-much ignored human history of the New World. The short and very blunt version of the history is that pre-Columbian populations in the Americas were very much larger than we generally consider (e.g., the massive infrastructures and secondary forests of Amazonia and Yucatan). This was true across the Americas, and those civilizations had major impacts on the regional habitats.

The grand illusion here begins with the earliest European colonizers who found continents with relatively few people and boasting what they could only assume to be truly primeval un-touched habitats. This is the illusion that most of us maintain today. The history that author Mann reveals is that non-native human pathogens entered the Americas via the few Europeans that made contact here well prior to Columbus. Those pandemics effectively de-populated the hemisphere. This allowed formerly deforested areas to recover, for fire-maintained grasslands to again host shrubs, or more generally for the flora to reconsider how to reconvene on landscapes modified by humans. Colonists of the New World, and we conservationists, sometimes do not consider any of these realities. This is not our fault, I suggest, because our perception of “natural, native, and untouched” is rather unfounded because of human history.

None of this is to say that conservation of native forests is a foolish ruse. I would never say such a thing! My point is that we should base our orientation of what is normal and native with respect to history. The Earth has a dynamic history that includes substantial global changes and extinctions. Humans are a recent part of that dynamic story, in many areas, and we need to temper our perceptions of natural, native normalcy with the realities of historical human interventions.

I love to discuss big-picture stuff like this, so if you see me on your next visit to the Zoo, grab me and let’s deep-dive together!

References:
Heckenberger MJ, Christian Russell J, Toney JR, Schmidt MJ. The legacy of cultural landscapes in the Brazilian Amazon: implications for biodiversity. Philosophical Transactions of the Royal Society B: Biological Sciences. 2007 Feb 28;362(1478):197-208. doi: 10.1098/rstb.2006.1979

Joe Mendelson, PhD
Director of Research