An invasive species, by today’s parlance, is a non-native that has harmful effects on environment, economy, and/or human health. These are short-term effects, measured over decades or, at most, centuries. But did species “invasions” occur before there were humans to measure them? Last week, Ithaca Darwin Days (sponsored by Paleontological Research Institution, Cornell University, The Kiplinger Foundation, and the Derek and Leora Kaufman Charitable Fund) had a lot to say about that. Friday Feb 15th’s panel specifically discussing invasive species in the fossil record consisted of Drs. Greg Dietl (PRI), Judith Nagel-Myers (PRI), Richard Kissel (PRI), and John Hermanson (CU College of Veterinary Medicine). Moderator/panelist Greg Dietl posed the question for all: What can we learn about species invasions from the fossil record?
Kissel began with the textbook example: the Great American Biotic Interchange. Beginning 3 million years ago, mass migrations across the newly formed Central American land bridge drastically changed continental vertebrate populations. Migrations in both directions brought cats, camels, bears, and rabbits to South America, and opossums and armadillos to North America. Driven to extinction by competition for resources (coupled with climate change and human hunters) were the South American saber-toothed marsupial, giant sloth, hoofed mammals, and many others. Hermanson continued by describing post-Pleistocene New Zealand – the perfect island “petri dish” for studying species invasions. Using zooarcheology (the study of remains in human kitchen middens), changes in biota can be learned from what people were eating – his most notable example was a decline in fur seals over the past 200 years. The “big baddies” in modern NZ history – mainly impacting native plant and bird populations – are rats, rabbits, stoats (introduced to kill the rabbits), pigs, goats, red deer, and the brushtail possum.
Nagel-Myers, a Paleozoic paleontologist, brought the discussion “home” to the Devonian (the predominant stratum in Ithaca), to an era unaffected by humans. The Upper Devonian (375 Ma) was a time of “biocrisis” – sea level changes opened up long-isolated ocean basins, resulting in species interchanges that had not occurred for over 5 million years. As a result, stable localized faunas were replaced by a homogenous, cosmopolitan fauna. Dietl concluded the presentations by asking “Are there any regions that produce (or receive) invaders more than others?” Using relatively recent examples from the Bering Strait and the Pacific rocky intertidal, he surmised that invasion is usually highly asymmetrical, from areas with larger, more diverse (more competitive) faunas to those with smaller less diverse ones. Invaders are by-in-large generalists, which gives them greater survival potential in their new habitat. Recipient biotas (the ones being invaded) are usually less competitive and thus more vulnerable to invasion. If so – and here comes the first provocative point of the discussion – invasion actually inserts “competitive vigor” into a biota, and can act to spur evolutionary change.
Ithaca Darwin Days panel members, (left to right) Greg Dietl, Richard
Kissel, Judith Nagel-Myers, and John Hermanson, discuss invasions
in the fossil record. Photo (c) Paleontological Research Institution.
Many questions then surfaced: Are human-facilitated invasions today the same kinds of events as Earth-facilitated changes in species distributions in the distant past? Are all species invasions “destructive”? Does invasion shut down speciation? Do we really know what the rate of invasion was in the past? Did any ancient invasions cause native populations to move (rather than go extinct) and thrive somewhere else? The human influence on what species invade is more selective than nature might have been – does it make any difference?
The panel members harkened back to Dr. Alycia Stigall (Ohio State University), who, in her keynote speech on Monday Feb. 11, touched on this subject. Stigall is specifically using changes in species distributions in the fossil record to understand how invasions function over the long term. In studying Devonian and Oligocene mass extinctions, which resulted in homogeneous faunas (similar to those after mass invasions), she used Ecological Niche Modeling to document that a decrease in speciation rate (= evolution) was as much or more to blame for a decrease in biodiversity as an increase in invasion.
In the end, although the discussion was lively, there were not a lot of definitive answers to these questions. In most instances, we simply don’t know. Time scales are radically different (millenia versus centuries), and we simply haven’t looked at our modern system long enough to predict whether the changes in the fossil record are good predictors for what could happen now. We certainly see changes in species distributions occurring much faster and over greater distances under our influence (think air travel!) than they would normally have happened (in close parallel with changes in global climate). Whether invasions are good or bad depends on when you measure the effect – invasion is “bad” when you lose diversity (the usual immediate result), but could be interpreted as “good” if such losses result in increased speciation. Certain topics (such as whether the rate of invasion is higher now than in the past, or if any fossil invasions caused native populations to move) simply haven’t been studied in this light, and might be hard to determine given the vastly different scales of the Recent versus the past. Long-necked dinosaurs left western North America and moved south during the Cretaceous – was this in response to an invader? Does more evolutionary change happen in such refugia than in the homogenous biota that results from an invasion? We just don’t know.
Is this merely semantics? Maybe. It was suggested – perhaps as a suitable end point to the evening’s discussion – that the value-laden term “invasion” be restricted to instances of human-mediated change, and that everything in the fossil record is simply “dispersal.” Maybe a good idea. But wait, Dr. Kissel posited one more: in their migrations from Africa to Asia to North America to South America, humans are perhaps the “worst” (or most successful) invasive species of all time. Good or bad is, as in many subjects, a matter of perspective.
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Stigall, Alycia L. 2010. Invasive species and biodiversity crises: testing the link in the Late Devonian. PLOS One, 5(12): e15584. doi:10.1371/journal.pone.0015584
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The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.