Praying mantises (known to entomologists as “mantids”) have been frequent topics on forums and blogs this week. Hummingbird enthusiasts are often alarmed to find a mantis/mantid lurking in their flowers or perched brazenly on their feeders, and rightly so. Big mantises can overpower, kill, and eat small vertebrates such as hummingbirds (grisly photo documentation of a mantis eating a Ruby-throated Hummingbird here).
Normally I’d be tempted to shrug and say, “That’s nature,” but among the twenty-odd mantids in the United States the only widespread species that’s large enough to catch hummingbirds is one that doesn’t belong here: The Chinese Mantid, Tenodera aridifolia sinensis. At an adult length of at least four inches, close to twice the size of most common native species, this is one formidable predator. It’s probably the most widespread species in North America thanks to mail-order sales of egg cases (oothecae) to gardeners and insect enthusiasts.
I’m very fond of mantids, and I remember being deeply disappointed when I first learned that the biggest, most dramatic ones are aliens. A little Web research confirms that Chinese Mantids are the largest of their kind in North America, though estimates of maximum size vary from 85 to 150 mm (3.3-6 inches) [Wikipedia][Enature]. Having seen big females that were at least four inches long, I tend to trust the higher figure. I’m not sure how big one would have to be to catch and hold a hummingbird long enough to kill it, but somewhere we’ve got a slide showing a concerned Black-chinned Hummingbird hovering a discreet distance from a feeder occupied by a Chinese Mantid less than four inches long.
One of the less helpful pages I came across while researching the relative sizes of native and exotic mantids was an information sheet on praying mantids at the University of Arizona’s Center for Insect Science Education Outreach (CISEO). Each of the site’s information sheets has a section on positive and negative impacts on the ecosystem of the critters in question—a great idea in theory. The mantid page mentions under positive impacts that they are indiscriminate predators but lists no known negative impacts on the ecosystem. This is certainly true for our smaller native species, but what about the humongous Chinese Mantid?
In the interests of accuracy and balanced reporting, I shot off a quick email to the webmaster suggesting that the page be revised to mention the Chinese Mantid and its predation on vertebrates. This was his reply:
Interesting that you consider a negative impact only for large species. What about all of the small insects that all species of mantids eat? All species consume a variety of things in order to survive. Plants included. The only species I know of that has a truly negative impact because of what it eats is us. So, if I had written the text I don’t think I would have included the negative impact. Unfortunately the concept of negative impact is almost entirely anthropocentric. I like hummingbirds too, and the fact that some species find them tasty is just a part of the natural world.
The CISEO webmaster is more than entitled to an entomocentric view of the world, but the insinuation that my view is anthropocentric, or even avicentric, is way off base. The issue is not so much size or phylum as how alien species effect natives and their ecosystems. I’d take an equally dim view of a fellow ornithologist championing House Sparrows, European Starlings, Eurasian Collared-Doves, or Ring-necked Pheasants in North America.
The predator-hater implication also misses the mark by miles. Growing up with hunters and ranchers on one side and Disney on the other, I found myself drawn to predators as fascinating underdogs. Hawks and snakes are still among my favorite animals, and I certainly don’t begrudge native predators the occasional meal of a hummingbird or any other species of wildlife, native or exotic.
My concern is not that mantids are eating hummingbirds but that the Chinese Mantid is sufficiently different from our native mantids for its predation to have an unnatural impact on native wildlife and ecosystems. Its far greater size allows it to kill much larger prey than its native counterparts, and the vertebrates it kills have far greater potential longevity and far lower reproduction rates than its invertebrate prey, magnifying the ecological consequences of such depredations. In this broader view of the issues it seems disingenuous and anti-educational for the information sheet to claim no known negative ecological impact and to fail to acknowledge that not all mantids in Arizona actually belong here.
The point that we tend to view the value of wildlife, exotic or otherwise, through an anthropocentric lens is well taken, and the reputation of mantids as beneficial insects, whether native or not, is a sterling example of how this lens can distort your thinking. It brings to mind a call to Ramsey Canyon Preserve from a man in Phoenix who apparently took the “tree-hugger”epithet a bit too literally. He wanted The Nature Conservancy’s help to stop his neighbor from cutting down some eucalyptus trees. I explained that TNC was about protecting native species and ecosystems, and that exotic suburban ornamental trees didn’t qualify. “But I thought you people saved trees!” he protested.
One of the biggest challenges for environmental educators is that most people’s opinions of other species depend on what’s in it for them. Mantids are promoted and sold as “friends” of gardeners and farmers. Shady eucalyptus trees comfort people who haven’t embraced desert life. Voracious aquatic predators are stocked in lakes and streams far and wide to thrill fishermen who enjoy a good fight. Ungulates with ornamental headgear that looks good on a wall are imported and bred on ranches for the shooting pleasure of trophy hunters. Problem is, many of the anthropocentric efforts to promote “desirable” species threaten the integrity of native ecosystems.
Environmental education can help create a better-informed and broader-minded constituency for conservation, but is this really the goal of CISEO? The webmaster’s e-mail address is at the University of Arizona’s College of Agriculture, and the project is funded by grants from the National Institute of Health, so it’s tempting to see a thinly veiled agenda. The mission statement on the home page says:
Our objectives are to develop new integrated education materials that foster the use of live insects as teaching models and to offer teacher training in background information about arthropods and how to use them in the classroom.
Perusing other information pages turns up the positive contributions of mosquitoes to ecosystems as food for fish, birds, bats, and other arthropods, so it’s not all about direct economic and/or health impacts on humans.
One would also hope that the webmaster, a retired entomology professor, might be aware that exotic insects endanger native insects. One prominent victim is the Xerces Blue (Glaucopsyche [lygdamus] xerces), a butterfly whose slide to extinction coincided with the arrival in its California home of the Argentine Ant, Linepithema humile, which displaced the native ant species that cared for the tiny butterfly’s caterpillars. The Xerces Blue may not be significant in terms of agriculture or public health, but the Argentine Ant’s ecological and economic impact has been far reaching.
The Argentine Ant is just one example that refutes the CISEO webmaster’s notion that Homo sapiens is the only species whose feeding habits have negative ecological impacts. We’ve contributed to the decline of many species and extinction of a few in the course of feeding ourselves through hunting, fishing, and agriculture (whose impacts include habitat destruction, pesticides, predator control, water diversion, and diseases introduced by livestock), but alien wildlife can also eat native animals and plants out of existence. Just ask Arizona’s own Santa Cruz (Monkey Spring) Pupfish, Cyprinodon arcuatus…oh, wait, you can’t—they’ve been extinct for decades, gobbled up by exotic Largemouth Bass. Looking for a hummingbird example? How about the critically endangered Juan Fernández Firecrown, found only on the remote islands off the coast of Chile from which it gets its name? One subspecies is already extinct, and the few hundred remaining individuals are threatened by the feeding habits of non-native rats, cats, coatis, and rabbits as well as loss of native vegetation to alien plants and logging (which ironically seems to have benefited its less specialized cousin and competitor, the Green-backed Firecrown).
These are problems of our own making. We may not be able to bring back what we’ve lost, but the very least we can do is have the intellectual honesty to take responsibility in hopes that some of the damage can be reversed and that future ecological meddling can be averted. It’s never too early to encourage responsible stewardship of biological diversity, and I hope CISEO will consider incorporating this subject into its lesson plans.