A Second Opinon on Invasive Species

I recently read a confounding article on Garden Rant (a gardening blog like no other). The author was reporting on some new research suggesting that not all invasive species should be eradicated.

lonicera-honeysuckle-dbg-19sept05-lah-250-1Specifically, a Penn State biologist named Tomas Carlo studied Japanese Honeysuckle (considered a noxious weed in much of the eastern US) and found that its presence increased the biodiversity of bird species as well as the number of individual birds. In fact, the fall berries are a major source of food for fruit-eating birds.

It seems that the balance of nature has shifted to include Japanese Honeysuckle, and removing it now would actually cause harm to a number of native species. Who would have thought?

russian-olive-swa-along-i-76-to-brush-lah-026This reminded me of Russian Olives here in Colorado. With their Siberian heritage and pretty silvery leaves, Russian Olives seemed like perfect landscape plants for the high, dry plains. They were—too perfect. With their seeds dispersed by birds, they quickly escaped cultivation. You can find Russian Olives growing wild all along the Front Range, but you won’t find them for sale in nurseries any more—they’ve been banned.

elaeagnus-angustifolia-russian-olive-cc-2003jul06-lah-002I’ve always been taught that Russian Olives were bad. The USDA’s National Invasive Species Information Center clearly considers them “planta” non grata, explaining that they crowd out native species. As a result, considerable time and effort—and money—has gone into removing them from natural areas such as my local regional park and the extraordinary bird habitat of Colorado’s Chico Basin Ranch.

However, after reading Penn State’s article on Japanese Honeysuckle, I got curious. Are Russian Olives really the good-for-nothing invaders I was led to believe? Or do they have some redeeming qualities?

Turns out they do, but these assets are far outweighed by the problems associated with their presence.

Russian Olives were widely planted in shelter belts following the Dust Bowl. In that role, they have effectively blocked the wind and reduced soil erosion. They also provide excellent cover and an abundant food source for game birds such as Ring-necked Pheasants (another imported species).

cedar-waxwing_chicobasinranchco_20100501_lah_4191In fact, an article by Montana State University states, “More than 50 kinds of birds and mammals eat the fruit of Russian olive. The berries are a choice of food of robins, cedar waxwings and sharp-tailed grouse. Plant cover is used by partridge, chukar, pheasants, sharp-tailed grouse and mourning doves.” Western Tanagers and Cedar Waxwings (right) are also among the olive-eating junkies.

That sounds wonderful until you learn that Russian Olives don’t stay where they’re wanted. They actually prefer to grow in riparian sites and quickly colonize near-by riverbanks. Compare those 50 species with the number of species that use cottonwoods and willows, the two plants most likely to be replaced if the spread of Russian Olives is allowed to continue unabated.

More than 150 North American birds (not including mammals and other animals) depend heavily on cottonwoods for nest sites, food and shelter.

Russian Olives just don’t measure up from the birds’ point of view. For example, cavity nesters such as Flickers are able to excavate nest holes in cottonwoods, but will not do so in Russian Olives. Insect-eating birds find the olives to be bare of bugs.

To make matters worse, both cattle and wild mammals (such as beaver) prefer browsing on cottonwoods and willows, giving the olives an unfair advantage. As the olives squeeze out native vegetation, they form dense thickets along the stream banks, providing excellent cover for hawks, skunks and raccoons—all of which feed on duck and grouse eggs. The result is an imbalance in the relationship between predators and prey.

The downsides continue to pile up. (You can read more in Montana Audubon’s excellent fact sheet on Russian Olives.)

populus_cottonwoods_buena-vista-co_lah_3318There’s one more reason to prefer cottonwoods over Russian olives… one that I’ve never seen mentioned. Aspen aren’t the only Colorado trees to turn gold in the fall. A stream lined with glowing yellow cottonwoods takes my breath away. Let’s see those gray-leafed olives out-perform that show!

So, it appears that we’re justified in our hostility toward Russian Olives after all. We’ll be keeping them on the list of plants we’d love to send back to Asia—right there after bindweed.

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