More on “What is natural habitat?”

Picking up where we left off in last month’s Research Blog, in considering so-called natural habitats, I thought I would revisit the topic from a slightly different perspective. What really happens following removal of native vegetation and alteration of habitats? Whether it is desert, rainforest, prairie, Piedmont mixed woodlands, or some other natural habitat, removal or heavy disturbance of the natural landscape (see June 2020 blog: https://zooatlanta.org/what-is-natural-habitat/) fundamentally changes the local ecology and biodiversity. But we often hear that clearing native habitats leads to barren, virtually lifeless landscapes. This is not always true, as the real effects of landscape clearing are more nuanced.

Lifeless dirt-scapes can result in the most extreme cases of land-use abuse, such as strip-mining, where local recoveries may not even be possible without significant restoration efforts. Other areas that may have already been marginal in terms of their vegetation, biodiversity, and rainfall may not recover without human help. Examples here would be deserts or the driest of grasslands. But a totally barren landscape is unlikely to be the long-term result in most cases because of the abundance of so-called weedy species in most areas. These species of plants and animals behave ecologically quite like invasive species but are distinguished because they are actually native to the region. It is an interesting phenomenon, so let’s consider examples from my field sites. The regular reader may remember that I also touched on this topic about the local ecological effects of coffee production in converted rainforests: https://zooatlanta.org/for-the-love-of-coffee/.

Field site: Amazonian Peru, lowland tropical rainforest. We were performing biodiversity surveys at this remote site in the early 90s. The team included entomologists, botanists, ornithologists, mammalogists, and herpetologists like me. Most everyone would consider this site to be pure untouched native rainforest. Virtually no people lived in the region at that time, there were no trails and only one dirt road that ran between two such camps; we reached the site by helicopter. But see last month’s blog again, to be reminded that such “untouched” places, even in Amazonia, may well have an old pre-Columbian history of human influence. 

We set up base-camp in little trailers in a small recently cleared area near a medium-sized river. We lived there with some oil exploration crews that had built the camp. The cleared areas around the trailers had already been colonized by a non-native crab-grass type groundcover. In the clearings cane toads (Rhinella marina) and ameiva lizards (Ameiva spp.; larger versions of our local six-lined racerunner lizard Aspidoscelis sexlineatus, black vultures Coragyps atratus, among other species were common—one could see many each day without trying. But those species were very rare in the adjacent rainforest. Unlike the invasive crab-grass (whose identity I do not know), the toad, the vulture, and the lizards were all native species to that region of Peru. Here we are going to consider the differences between native “weedy” species and non-native invasive species. They have some things in common, but the distinction is important.

What we are considering here are a whole host of species that live in a region and specifically exploit habitats that naturally have a mostly open vegetation structure, such as tropical savannas that may occur in pockets or vast expanses in and around rainforests in some regions and/or (and these are not mutually exclusive) regions that are naturally disturbed with some frequency, such as the banks of rivers that get re-shuffled every year when the river floods such that large trees can never get established there. This sets up an interesting local dichotomy in which native, but weedy, species like cane toads are very rare in deep forest but thrive along its edges and interspersed open areas. Hence, when humans come along and clear forests, they simply are creating more habitat for the weedy subset of the native fauna. Following human developments, the weedy species become more common locally and the non-weedy forest-specialists become more rare. It is a local re-shuffling, really, which is very different from a wholescale loss of local biodiversity following human settlement. This begs a perspective-check from us, don’t you think?  If there was no recent deforestation in Amazonia, cane toads, ameivas, black vultures, and many other species would be considered rare there.  We might even grow concerned about them and list them as endangered species!  How to conserve the endangered charismatic cane toads of Amazonia: cut down the trees!           

Thanks to the ameiva lizards (and my thesis advisor) I was able to see firsthand how this plays out on a different and very small-scale level. Like the toads, these lizards are common in disturbed fields, around our trailers, along the rivers, etc. but rare in deep forest. Tree-fall gaps are an interesting local phenomenon. When a big canopy tree goes down in the rainforest, it takes down a few nearby smaller trees with it, and this creates a novel sunny patch on a now-exposed forest floor. The more recent treefalls are obvious when you come across them, because whereas you’ve been in the shade of the forest canopy for hours, suddenly you are in a patch of blazing sunlight. In very recent falls, there is nothing but destruction from the event. As the patch ages, however, all the local weedy species of plants and animals seem to find it and suddenly you can find four ameiva lizards in 10 minutes, versus essentially never in the rainforest. Eventually, over the course of years, the tree-fall site recovers and becomes indistinguishable to most of us; botanists can spot old now-revegetated treefalls easily because of the plants growing there—they are patches of very large weedy trees concentrated in a spot surrounded by non-weedy vegetation. I don’t know the plants of most areas well enough to spot the older re-vegetated sites.

If you visit ecolodges in rainforests, watch for this phenomenon. It is quite easy to observe, even if you don’t really know the local species well. Regardless of their proper identifications, you will easily notice that the abundant biodiversity near your lodge (and in a treefall you might find) is different from what is in the forest. All may be local, but they live very different lifestyles, with the weedy species benefitting greatly from most human activities.

Invasive species, then, really are interesting because the locally weedy species often turn out to be great invasive colonizers of new non-native areas. Now that we understand the natural history of the cane toad in its native range, it is altogether unsurprising that they do very well if they get transported to new areas of the world that are either naturally or unnaturally open or disturbed. Like the sugarcane fields that became their namesake, and beyond. With this knowledge in hand, the dramatic success of cane toads invading southern Florida or Australia was completely predictable. 

Much of the U.S. also serves as a great example, as raccoons, bullfrogs, opossums, coyotes, black vultures, etc., are all far more common than they were centuries ago. These are native weedy species that benefit from human activities. However, in exchange for the increased abundance of these wonderful native species, we lost or reduced other native non-weedy with species like red-cockaded woodpeckers, indigo snakes, gray wolves, and passenger pigeons. Again, with these perspectives in mind, which North American species do you think would do better if introduced to a deforested area of Amazonia, the northern mockingbird or the red-cockaded woodpecker? I would argue that the mockingbird would be the kudzu or carp of Amazonia, being a relatively weedy species in the U.S. and a candidate for an invasive in Peru if it ever got there.  Other naturally weedy U.S. species that are terrible invasives on other continents include the red fox and the red-eared slider turtle.

Thanks for reading, my friends!  I hope to see you sometime soon at the Zoo, and I hope I’ve given you some food for thought the next time you hear about habitat loss or, even better, you get to visit an area that has disturbed and non-disturbed in close associations so you can make your own observations of the plants and animals and how they respond to human influence.

Joe Mendelson, PhD
Director of Research