In response to Richard Plate’s question about neutrality: first, I suggest that you have a look at Colautti and MacIsaac’s “neutral terminology” proposal in Diversity and Distributions 10:135-141 (2004). I think their attempt was commendable, but it ultimately failed for the same reasons the hodge-podge they were critiquing failed. You could also look at my own “Rise and Fall of Biotic Nativeness…” and “Anekeitaxonomy…”, both available via academia.edu (last link below) to get a further sense of how the current situation developed and some of its inherent weaknesses.
The primary difficulty with all categorization attempts to date has been their anthropocentricity. It’s clear that many families and genera of plants and animals are represented by species on multiple continents and islands, or in multiple, recently disjunct watersheds or distantly separated seas. Things got well around long before human agency provided means of transport. Descriptions of that process tend to default to a gradualist approach, giving some number of arrivals per unit time as if that represented either a typical, normal, or good state of affairs. It’s more reasonable to assume that dispersal events happened in clusters when conditions favored certain kinds of transport. It’s also clear that whole assemblages or communities are not equally transportable; some taxa are just more so than others under a given set of conditions. Our traditional reaction has been to sort such events into two categories: natural and unnatural, with unnatural being synonymous with human facilitation. Coastal marine organisms, domesticated plants and animals and perhaps pests of stored foodstuffs were certainly being transported quite early on, but dependable accounts begin appearing around 1600. I haven’t found any clear evidence of the distinction in ancient texts, but that may be as much a matter of limited taxonomic capability at the time as anything else. Unfortunately it is now practically impossible to recognize a “natural” long range transport event. Even the few celebrated cases (like the historically documented arrival of cattle egrets in South America) cannot be reliably disentangled from human agency (in that case, the inception of forest clearing and cattle ranching in Atlantic coastal colonies). We tend to assume, these days, that the sudden appearance of locally unfamiliar organisms anywhere in the world is human induced, therefore unnatural (and by implication “wrong” as an affront to either god or nature)—unless it was done on purpose and produced a more or less intended benefit. That distinction had been codified by 1855. Every scheme envisioned from the first to the most recent has used human history or human experience as a basis for sorting biota into nominalist (i.e., convenient) categories like natives, aliens and invasives. That might be fine if everyone understood nominalism and adhered to its limitations. However, we are more readily inclined to view the world in essentialist terms--as if apparent categories automatically correspond to natural kinds. Any biologist who has studied or practiced taxonomy without being made aware of this issue has been done a disservice. I have never encountered a successful essentialist approach to the kind of sorting we’re discussing. There is, for example, no objectively defensible threshold rate of spread separating normal from abnormal dispersal, in other words, distinguishing (by any terminology) establishment from invasion. Richardson, et al (Diversity and Distributions 6:93-107, 2000) tried to do just that, but their standard was arbitrary. Invading is called invading because somebody feels the rate at which a population is growing or diffusing is uncomfortably rapid. How rapidly should a population increase or spread? Your answer will depend on your personal basis of comparison. If we choose the rate at which poplar seedlings colonize a damp sandbar, it’s pretty quick. If we choose pond eutrophication or deciduous forest succession it’s slower. But no standard is ever neutral, because the cases that attract attention and study are those already labeled “problems” according to some human assessment of value. The change is altogether unwanted, or happening at a (mentally) disturbing rate. So not only am I not denying that people are having problems as a result of some species introductions, I’m explaining that our concepts are entirely based on human judgment that a problem exists. It is a taxonomy of problems, period. Colautti and MacIsaac (2004) decided that a history of coevolution was more valuable than the absence of such a history. That is a way of “smuggling” in the natural/unnatural distinction. The absence of such a history, being unnatural, constituted a problem for them. Richardson, et al (2000) actually put a single number to the threshold rate at which any plant population diffusion becomes a problem, to similar effect. Neither engaged with the ecological fact that populations growing and/or spreading rapidly are undeniably exhibiting fitness under prevailing conditions, no matter how or where that fitness developed. We could develop standards for every species based on their rates of spread in their “native” ranges, except that practically by definition species aren’t spreading in their native ranges. A similar problem arises if we compare rates of spread of adventive populations to those of the local “natives”, because natives aren’t spreading there, either. Those, briefly, are the major parameters of our problem. They tend to become more complicated rather than less so with greater detail. I only touched on the additional, indirect effects of habitat conversion in the context of the cattle egret. In one case that offers a lot of potential for gaining useful insights, Reclamation-era river damming and diversion rendered some native tree species unfit but resulted in some Tamarix spp. being very fit indeed (Chew 2009 and Stromberg, et al 2009, both via the academia link below). Further complications attend the fact that native ranges are mostly understood only at very coarse scales based on historical anecdote. It’s all but impossible to precisely describe the current native range of any species or population. By the time you publish a result, the range will have changed. Look into the range status of the once aptly-named Quiscalus mexicanus or the never aptly named Zenaida asiatica in North America. Neither species is thought to have been actively introduced via human agency (except possibly the latter into southern Florida and Puerto Rico), but both their ranges have expanded significantly over the past century. How do they fit into the native/alien/invasive picture? My solution, as I have stated, is to call the whole conception a problem of inappropriate categorization. Is there an ecology-based way out, short of treating each instance as a unique event? Should there be? Or should ecologists be content with wholly anthropocentric, unscientific anekeitaxonomies? Give it a try and see what happens. Matthew K Chew Assistant Research Professor Arizona State University School of Life Sciences ASU Center for Biology & Society PO Box 873301 Tempe, AZ 85287-3301 USA Tel 480.965.8422 Fax 480.965.8330 [email protected] or [email protected] http://cbs.asu.edu/people/profiles/chew.php http://asu.academia.edu/MattChew
