Empirical evidence is not needed when observational evidence shows severe and widespread adverse effects of invasive species on local systems. Examples I know of include: Reed Canary Grass (Phalaris arundinacea) monocultures replacing diverse native sedge and forb wetland species mixes and reducing wildlife habitat productivity. Himalyan (Armenian) Blackberry (Rubus discolor) monocultures taking over meadows, pastures and field edges and reducing native wildlife use. Feral Horse (Equus cabalus) overuse of steppe grasslands and damage to streambanks, increasing soil erosion and stream degradation. Feral Pig (Suus scrufa) soil disturbance, vegetation removal and disease transmission. Knapweeds (Centaurea and Acroptilon spp.) taking over both disturbed and undisturbed rangelands, replacing native grasses and forbs, and reducing herbivory; some species also supress native grasses and forbs through allelopathy. Garlic mustard (Alliaria petiolata) takeover of forest floor systems by outcompeting native species and effective allelopathy. English and Irish Ivy (Hedera spp.) monocultural takeover of forest floor and shrub systems, excluding native forbs and shrubs, adversely affecting tree survival and severely reducing native wildlife use. Cheatgrass (Bromus tectorum) invasion of rangelands outcompeting native grasses by usurping soil water and promoting early wildfire (after which it quickly reseeds).
As is the case with many ecological concepts, "invasive" is a subjective and relative term and not an absolute categorization. Some may consider dandelions to be invasive and others may say they're just a weed (another subjective term) adding diversity to lawn monocultures -- they are seldom a "takeover" species causing localized extirpation of native species as is the case with too many other prolific exotics. And not all "invasives" are non-native to a particular continent or region -- but they are usually and typically non-native to a particular adversely affected ecosystem. My conclusion: There have been successes in invasive control (e.g., tansy ragwort), and there are numerous cases where lack of control efforts will seriously deplete natural system diversity and value (both ecological and economic). All of the species I've listed above, and many more, are worth controlling or eliminating, and not all of this effort will make Monsanto richer. Warren W. Aney Senior Wildlife Ecologist Tigard, Oregon -----Original Message----- From: Ecological Society of America: grants, jobs, news [mailto:ECOLOG-L@LISTSERV.UMD.EDU] On Behalf Of Jeff Houlahan Sent: Saturday, 11 June, 2011 16:19 To: ECOLOG-L@LISTSERV.UMD.EDU Subject: Re: [ECOLOG-L] EcoTone: Speaking of species and their origins Hi all, not that Esat needs me to defend him but the list of species that can be 'googled' and identified as invasive scourges is, I suspect, longer than the list that actually are scourges. One of the species that was identified in Amyarta's list, purple loosestrife, is a classic example. You can go to hundreds of websites that will identify it as a species that competitively excludes native plant species and causes local extirpations. The empirical evidence to support this claim is almost non-existent (or was a couple of years ago when I checked last). There have been several reviews done on the topic and most conclude that there is little evidence that loosestrife causes extinctions at almost any scale. This isn't to suggest that invasives are never a problem but my understanding of the literature is that there is lots of evidence of extinctions caused by invasive predators and relatively little evidence of extinctions caused by competitive exclusion (zebra mussels are probably an exception to that general statement). I don't think it's a bad idea to actually step back and see if the investment in controlling invasive species is warranted. Jeff Houlahan
