What makes a non-native species invasive?

I frequent very few other blogs these days, and I will soon have a list up of the ones I think are the best. However, one stands out as a well-written blog that has offered a number of great thoughts (some of which I've shamelessly stolen and written about here). It's the Hog Blog, by Phillip Loughlin, an excellent writer and thinker.

A few posts back, Mr. Loughlin brought up the question as to whether or not wild hogs are "invasive." This article (read it here) brought back some thoughts I've had in the past over the idea surrounding the term, a controversy that brings ethical questions to the fore.

Throughout the country, people encourage non-native species. The gardener who buys a pack of chilly ladybugs or some tablets of Bacillus thuriengiensis (Bti) to kill mosquitos and other pests may very well be distributing non-native species. Birdfeeders often attract and feed many nonnatives, from house sparrows to Eastern tree squirrels. State conservation agencies like my California Dept. of Fish & Game introduce non-natives, like McCloud river rainbow trout and wild turkeys, with myriad effects.

Of course, the contrary is also true: Many folks in California obtain depredation permits to kill wild turkeys, due to the agricultural damage they may cause. Mr. Loughlin's blog mentioned the Missouri Dept. of Conservation requesting hunters kill wild hogs on sight, fearing damage they may wreak upon the ag. industry.

In my various careers & hobbies, I've come across some interesting fights over non-native species. Feral cats come to mind. Cats, just about the most common pet in the US, have a special place in the hearts of many. And yet, their ability to breed quickly and tolerate people and each other in very small, overlapping territories, added to their nearly perfect bird-killing design, mean that feral cats have a tremendously negative impact. Since they are not a game animal (and who would eat one, anyway?), and since they are so close to humans, emotionally, the fight over controlling them is tough.

Eucalyptus trees in Santa Cruz also help to illustrate the complexities of non-native species and the designation of "invasive." Two groups fight over eucalyptus in Santa Cruz, a group that is pro-native plant and completely against non-natives, and a pro-monarch group that sees the eucalyptus as providing needed winter fuel for the butterflies.

Other, bigger fights exist, too, like non-native deer species in National Parks, giant river reed as a biofuel, and the wild pigs. Taken together, however, one can develop some general ideas to help direct management decisions.

I start with a tangential thought related to Aldo Leopold's quotation that, "to keep every cog and wheel is the first precaution of intelligent tinkering." What is the role, in its new environment, of the particular species in question? Does it replace, or more efficiently inhabit, the niche of an existing entity? Does it take on the role of an extirpated species? Does it create negative space or null space, effectively crowding out species and taking up habitat, without contributing to the system? I ask the ecosystem questions first, and leave out economic and other considerations until these are generally understood, as these relationships necessarily have economic consequences, as well, and their foundation should first be sound. If the species in question has a negative or null effect on the system, I'd advise against introduction, and for efficient removal.

Applying these questions, I think that pigs may be helpful to the California ecosystem. I don't think pigs create 'negative' space in the way that, say, giant river reed does (sucking up water, providing no canopy for birds, killing neighboring plants). Pigs harbor native bugs, and can be eaten by apex predators like coyotes, bobcats and mountain lions. I don't know if pigs are more efficiently replacing any native species. The local blacktail deer seem to be doing very well. In fact, California is experiencing a dearth of large animals, an ecologically recent phenomenon, especially in regions now inhabited by wild pigs, though not of their doing. And, we had a large animal that behaved very much like wild hogs, up until just over one hundred years ago: The California Golden bear. Grizzlies, though major predators, are also scavenging omnivores, like pigs. They turn over logs, uproot and dig, & eat all kinds of stuff. They basically disturb the earth wherever they go, an important component to many a healthy, functioning ecosystem, and one which only wild pigs, controlled fires, and black bears do now. Unfortunately, pigs don't bring in the needed energy and nutrients from the ocean like grizzlies did through the salmon, but what they are doing a keeping habitats in flux, turning over, aerating, fixing nitrogen, etc.

There may, of course, be downsides to pigs in the ecosystem, possibly as vectors of new diseases or pests, or the destruction of threatened species that bears would have left alone, but I don't have that information. Should those be the case, I'm prepared to change my mind. California has many ecosystems, so there is of course no 'one-size-fits-all', but for the most part, I think wild pigs here look to be alright. Turkeys, though, are a different story.

Please tell me what you think! What am I missing from my suggestions? What kind of scientific or ethical decisions have I lacked here? Comment and let me know.