(Editor's Note: This article was originally published on March 2, 2011. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
California is under a constant threat of invasive species, thanks to its location at the coast as well as its favorable temperate climate. My yard is no exception, though it's not near the coast and often it is difficult to imagine this climate as favorable (I write this on a day the wind is blowing nearly sixty miles per hour and the temps hover just above freezing... but much of the country is far more miserable than we are).
What is an invasive species? The definitions are rarely complete but most say it is a new species to a geographic area that has potential to upset the local ecology and economy. What it always leaves out is ‘how new' the species has to be. If you take that literally, nearly all life on earth is invasive. All California's native flora and fauna invaded at one point or another, and few could be considered more upsetting to the local ecology and economy than us humans. We are by far the most dangerous and destructive species of them all, there is no doubt. So there must be some time at which point an invasive species becomes a native species. Is it 100 years? More? I don't know if that time has been incorporated into any working definitions of invasive species, but for our current situation, it seems invasive species are those new ones that have either been introduced, or are becoming locally upsetting over the last 50 or more years (sort of a wishy washy cut off I know). Notice that not all invasive species have to be even new by that definition. Sometimes an established species can become invasive if something else has changed in their environment that allows for that to happen. For example, coyotes and deer have become locally invasive here in California thanks to changing food supply (coyotes- mostly our fault thanks to our overpopulation and negative effect upon their local food supply) or changing predator status (deer losing most of their natural predators- again, our fault). In almost all cases of invasive species occurrences, one can trace the ‘blame' back to people, by their either intentionally or accidentally importing something, or by our negative impact upon the local environment. And then we complain that invasive species are something that has happened to us and our poor native fauna and flora.
When one Googles ‘invasive species' one will get millions of hits. If I use ‘Bing' I still get nearly five million hits. Even if I Google invasive animal species in California I get nearly 200,000 hits. One of the things I discovered during my searches of invasive species was how many of them I actually get in my back yard. And I have a fenced in back yard. The list of invasive species I count may not be impressive, but what I find most distressing is how mundane and ‘ordinary' they are. Nothing like the exotic Brown Snake that has nearly decimated the avian population of Guam, or the sneaky predators that have overrun many areas of the rural and forested country side, the Fox. No, mine are a common lot. But still, quite impressive in their invasive qualities as far as invasive species go.
The fox squirrels (Sciurus niger) I am constantly yelling at (and my always helpful dogs are barking at, and helpful cats pretending to chase down) are (despite my having seen them ever since I myself became an invasive presence in California) not a native species and are considered an invasive species. I am assuming at some point in the future, they will become adopted into the ‘native' category. I just don't know when.
Fox Squirrel in my yard
They are perfectly adapted to this rural environment, easily traveling from neighborhood to neighborhood, finding ample food (since they eat just about anything) and plenty of breeding locations. Like many new invasive species, they fill a niche by creating one (rural life), and forcing local species out (forest life) of theirs (that would be the Grey Squirrel, Sciurus griseus). They certainly have an impact on the local ecology, constantly digging up gardens and moving plant material about (they plant at least one fig tree in my garden each year, and remove all my bulbs). They also reportedly like feasting on eggs and baby birds which can't be good for the native bird populations. I do not know what impact they have on the economy, however. This is a species we love to hate... until a baby falls into our hands and we bend over backwards to save its life, rehabilitate it, and happily send it back out into the ‘wilds' (our back yards, usually).
Another frequenter of my yard is the Opossum, Didelphis virginiana. This is an east coast native, the only marsupial native to North America, but it was introduced (purposely?) ‘many years ago' to California. Now it is found throughout California, and probably the entire US. Eventually, I suppose, it will ‘become' a native at some point in the distant future. But for now it is an invasive species, excellently adapted to the rural and suburban lifestyle (as well as the forests) in which most find it in nowadays. Unlike the squirrel, this one is usually only around at night, so it is less obviously a pest. But it is a major carrier of cat fleas, a avid predator of birds, reptiles and small mammals (as well as insects, carrion, fruit and nuts), and has a unique but obviously effective reproductive strategy. I see these in my yard whenever I have to round up the dogs in the middle of the night from one of their rabid barking fests they happily partake in whenever they discover one of these on the fence or elsewhere in the property.
Opossum (photo Wikipedia)
I do not have bird feeders, but my yard is still host to a variety of birds that like to aggravate the dogs and feed on berries, flowers and look out over the landscape. Sternus vulgaris (common European Starlilng) is an infrequent visitor, but definitely fits the definition of invader. I know what damage they can do economically (crop destruction), but it is the damage they do by outcompeting the native birds that is the most upsetting. They do not live in Los Angeles that I know of, but seem to just pass through as they migrate north and south. They are extremely adaptable birds and it may only be a matter of time before they call Los Angeles their home, too.
European Starling (photo Wikipedia)
Another invasive avian species that I occasionally see in the garden, but usually on the surrounding telephone wires and poles, is the pigeon (Columba livia). This European native made its way to the US so long ago I cannot find a reference for its introduction. When will this be considered a native itself? Pigeons are likely the only invasive avian species more invasive and successful than the Starling (possibly the Purple House Finch is a competitor, too... but it is actually a California native), though they do not appear to be quite as economically destructive. It is definitely a disease carrier, spreading parasites, viruses and even a fungus (I see this fungus show up in cats now and then- nasty and difficult disease to treat). And it is ubiquitous. It is hard to imagine an urban scene in the US without pigeons in it. Los Angeles and the rest of southern California are certainly no exception.
Pigeon (photo Wikipedia)
One of the most destructive invasive species I see frequently in my yard is Felis catus, the common house cat. I have my own cats, but they stray a bit, as do all the neighbor's house cats, and their loose social network spans nearly the entire inhabited geography of the United States. The damage done to native species by feral and roaming pet cats is hard to estimate, but it is likely quite massive. They prey on numerous native species, spread viruses and parasites and just the disposal of cat litter alone is causing serious problems for native wildlife along the coastal regions of California. There was an estimate that every domestic cat catches and kills about 5 birds a year. The number killed by feral cats must be much higher than that. There are an estimated 180,000,000 cats in the US, only half which are owned by anyone. So you can imagine the impact on just the bird populations by these cats. Fortunately Mr. Smith, my lazy pet cat, is terrified of birds, and most anything that moves (save a rolled up wad of paper) and is unlikely to be guilty of adding to this problem.
Sad to say, but another invasive species we find far too often (once a twice a year is too often for me, and ever is too often for my wife) in the yard (and sometimes the house) is the Norway or Brown Rat, Rattus norvegicus. So far, I have yet to see the Black Rat, Rattus rattus, but I would not be surprised it has made its way into the yard now and then, too. These are the penultimate invasive species and their invasion is pandemic. Even the human race is neck and neck with these creatures for the prize of worst invasive species on the planet. They are ideally suited for this purpose eating almost anything they come across (even humans do not match this species omnivorous palate), breeding like mad, and able to live and hide almost anywhere. Yet amazingly we are not (here in my yard) overrun with Brown Rats, though I am sure I underestimate the number of rats my yard has seen. The economic and ecologic damage the rat has caused almost everywhere it goes is inestimable. They are very important parasite and disease vectors, at one time killing off one third of the population of the ‘modern world' with their spread of the black plague. It is no wonder my wife freaks out when she discovers one of these is about (though honestly plague is really not her primary objection- they're just creepy).
Brown or Norway Rat (photo Wikipedia)
Well, that pretty much sums up the invasive list of animals that frequent my back yard. Sad as it is to say, if it weren't for them, I would hardly ever see an animal in the back yard, save the rare migrating bird or my ever present insect pests. So I guess I have to be thankful for what ‘wildlife' there is out there, in the middle of my sprawling, grid work, concrete neighborhood.