(This is the first in a series of posts about invasive species on the Georgia barrier islands, their traces, the ecological impacts of these traces, and why people should be aware of both their traces and impacts.)
Paleontologists like me face a challenge whenever we study modern environments while trying to learn how parts of these environments might translate into the geologic record. Sure, we always have to take into account taphonomy (fossil preservation), through which we acknowledge that nearly none of the living and dead bodies we see in a given environment will become fossilized; relatively few of their tracks, trails, burrows, or other traces are likely to become trace fossils, either.
Because of this pessimistic (but realistic) outlook, paleontologists often rub a big eraser onto whatever we draw from a modern ecosystem, telling ourselves what will not be there millions of years from now. We then retroactively apply this concept – a part of actualism or, more polysyllabically, uniformitarianism – to what happened thousands or millions of years ago. When paleontologists do this, they assume that today’s processes are a small window through which we can peer, giving insights into processes of the pre-human past.
Feral horse (Equus caballus) tracks crossing coastal dunes on Cumberland Island, Georgia. During their evolutionary history, horses originated in North America and populations migrated to Asia, but populations in North America went extinct during the Pleistocene Epoch about 10,000 years ago. Using the perspective of geologic time, then, could someone argue that horses are actually “native,” and these feral populations are restoring a key part of a pre-human Pleistocene landscape? (Photograph by Anthony Martin.)
However, a huge complication in our quest for actualism is this reality: nearly every ecosystem we can visit on this planet is a hybrid of native and alien species, the latter introduced – intentionally or not – by us. Thus when we watch modern species behaving in the context of their environments, we always need to always ask ourselves how non-native species have cracked the window through which we squint, through the past darkly.
This theme is considered in Charles C. Mann’s most recent book, 1493: Uncovering the New World Columbus Created, in which he argues how nearly all terrestrial ecosystems occupied by people were permanently altered by the rapid introduction of exotic species worldwide following Columbus’s landfall in the Western Hemisphere. Going even further back, though, the introduction of wild dogs (dingoes) into mainland Australia by humans about 5,000 years ago irrevocably changed the environments of an entire continent. Examples like these show that European colonization and its aftermath in human history during the last 500 years was not the sole factor in the spread of non-native species, and hints at how species invasions have been an integral part of humanity and its movement throughout the world.
Something tells me we’re not in Georgia any more. A male-female pair of dingoes (Canis lupus dingo) pose for a picture in Kakadu National Park, Northern Territory, Australia. Although now considered “native,” dingoes are an example of an invasive species that had a huge impact once brought over by people from southeast Asia about 5,000 years ago. For one, its arrival is linked to the extinction of native carnivorous mammals in the mainland Australia, such as thylacines (Thylacinus cynocephalus) and Tasmanian devils (Sarcophilus harrisii). (Photograph by Anthony Martin.)
Well-meaning (but deluded) designations of “pristine,” “untouched,”and “unspoilt” aside, the Georgia barrier islands are no exception to alien invaders. Moreover, like many barrier-islands systems worldwide, they differ greatly from island to island in: which species of invaders are there; numbers of individuals of each species; and the degree of how these organisms impact island ecosystems and even their geological processes.
Feral cat tracks in back-dune meadows of Jekyll Island, Georgia. Jekyll is one of the few Georgia barrier islands with a significant human presence year-round, hence these cats are descended from domestic cats that were either purposefully or accidentally let loose by residents. What impact do these cats have on native species of animals and ecosystems, and are these effects comparable to those of other invasive species on other islands? Scale = 15 cm (6 in). (Photograph by Anthony Martin.)
This is one of the reasons why I devoted several pages of my upcoming book, Life Traces of the Georgia Coast, to the traces of invasive species – tracks, trails, burrows, and so on – despite their failing an “ecological purity test” for anyone who might prefer to focus on native species and their traces. With regard to invasive species, the genie is out of the bottle, so we might as well study what is there, rather than apply yet another metaphorical eraser to species that are drastically shaping modern ecosystems and affecting the behavior of native species, thus likewise altering their traces.
A large pit of disturbed sand in a back-dune meadow caused by feral hogs (Sus crofa) on St. Catherines Island, Georgia. Because feral hogs are wide-ranging omnivores with voracious appetites, they cause considerable alterations to island habitats, from maritime forests to intertidal beaches. How do these traces affect the behavior and ecology of other species, especially native ones, in such a broad range of environments on the Georgia barrier islands? Can their traces actually alter the geological character of the islands? (Photograph by Anthony Martin.)
What are some of these invasive species? What makes for an “invasive species” versus a mere “exotic species”? How do the traces of invasive species affect native species on the Georgia barrier islands, and the ecology and geology of the islands themselves? And how do paleontologists and geologists figure into the study of invasive species?
These are all questions that I hope to explore in upcoming weeks here, and for the sake of simplicity, I will showcase an invasive species of mammal and its traces each week. Some of the photos shown here serve as a visual teaser of the invasive species and their traces that will be covered: feral horses (Equus caballus), cattle (Bos taurus), hogs (Sus crofa), and cats (Felis domestica). Yes, I know, there are many others, but these four are among the most ecologically significant species, they consist of animals that nearly everyone knows, and – best of all – they make easily identifiable traces. So these fours species will provide a starting point in our learning how the Georgia barrier islands can be used as case studies in the traces and ecological effects of traces made by invasive species.
Trail made by feral cattle (Bos taurus) cutting through a salt marsh and extending to the horizon, providing a clue of how this forest-dwelling animal can travel deeply into and affect marginal-marine environments. How might such traces show up in the geologic record, and was there a species that might have made similar traces on the islands in the recent past? (Photograph by Anthony Martin.)