Uninvited Guests: The Management of Invasive Species

AOf the plants and animals that people carry beyond their native ranges, only a small minority ever become troublesome, and ecologists have spent four decades trying to establish which minority that will be. The explanation long treated as correct locates the problem inside the organism: certain species are invasive by constitution, being quick to mature, tolerant of poor conditions, indiscriminate in diet and, in many cases, able to reproduce without a partner. Entire regulatory systems rest on that idea. Border agencies in more than forty countries now screen imports against checklists of such attributes, refusing entry to anything scoring above a stated threshold, and the arrangement has an obvious administrative appeal, since a list can be drawn up once and then applied to consignments that no inspector has time to examine individually. What the idea cannot easily accommodate is the observation that the same weedy species prospers in one region and fails, again and again, in another that appears to offer it everything it requires.

BHelena Brask, an ecologist at the Norrvik Institute for Applied Ecology, contends that the decisive variable has been sitting in the shipping records rather than in the organisms themselves. Working from customs manifests, angling club minutes and nursery catalogues, her group reconstructed 1,140 deliberate or accidental introductions into inland waters between 1994 and 2016, noting for each one how many individuals had arrived and how often such arrivals were repeated. Where a species entered a catchment on a single occasion, a population was still present two decades later in 8 per cent of cases; where entries had been repeated ten times or more, that figure rose to 61 per cent. Trait scores of the kind used by border agencies, calculated for the same species, accounted for under a fifth of the variation once the frequency of arrival had been allowed for. Repetition, on this reading, does what constitution was thought to do: it buys the newcomer enough attempts for one of them to coincide with a favourable season.

CBrask is careful not to state the claim too broadly. The advantage conferred by repeated arrival emerged clearly only in catchments whose banks had been dredged, drained or built upon within the previous decade; in the eighteen catchments classified as undisturbed, establishment stayed below 10 per cent however many times a species was delivered. Among the altered sites, moreover, the effect was strongest where the earthworks had been finished most recently. She attributes the difference to the observation that a settled community leaves few openings, so that arriving in bulk merely raises the number of individuals that die. The practical consequence is an awkward one for regulators, since it means that identical cargo carries very different risks according to where it happens to be unloaded. Disturbance, on this account, is not a background condition but part of the mechanism itself.

DNot everyone accepts the reconstruction. Tomás Iriarte, a professor of coastal ecology at the Cabrillo Marine Station, is sceptical of any dataset assembled from documents that were never compiled for the purpose, observing that unrecorded releases by aquarists and anglers are precisely the events least likely to reach a catalogue or a minute book. Under-recording of that kind, he maintains, would exaggerate the apparent gap between single and repeated introductions. His own screening trial, published in 2019, assessed ninety-six freshwater plants against a battery of physiological measurements taken in tanks, and correctly separated the species that had become nuisances from those that had not in 78 per cent of cases. Iriarte concedes, however, that the standing of every species in that trial was already known when the measurements were made, that the tanks held water at a constant temperature, and that no animal was examined at all.

EThe disagreement matters because the two accounts point towards different budgets. A regulator persuaded by Iriarte will invest in better lists and quicker identification at the frontier, whereas one persuaded by Brask will spend on the pathways themselves, licensing the online plant trade, reducing the frequency of movements and paying for the treatment of ballast water rather than judging each consignment on its merits. The two policies also differ in who pays, since pathway controls fall largely on carriers and traders while list-based screening is met from the public purse. Pathway measures have the advantage of catching organisms nobody has yet thought to assess, which is not a small category: rather more than half of the freshwater species now regarded as damaging in Europe were not on any watch list when they arrived. Such measures are also politically thankless, because their success consists of events that never happen, and a treatment plant that prevents an invasion leaves no evidence that it has done so.

FMost researchers now suspect that the dispute has been miscast as a choice, when the two processes may govern different stages of a single sequence. Becoming established appears to depend chiefly on arriving often enough, and in a place disordered enough, for one attempt in many to succeed; what follows afterwards, as a small population spreads across a region, may well depend on the physiological attributes that Iriarte measures. If that division of labour holds, screening lists are less wrong than misapplied, being used to predict entry when they are better suited to forecasting damage. Brask herself accepts that a species incapable of tolerating cold will not spread through a northern catchment however often it is delivered. The uncomfortable corollary is that neither approach can be judged quickly, because the interval between an arrival and a recognised invasion is commonly measured in decades.