Wednesday, February 27, 2008

Oh, the slime! Slugs a threat to native ecosystems

Gardeners have long known the threat of garden slugs to their seedlings, but natural resources managers are only now recognizing that the slimy critters are also significant threats to rare native plants.

(Images of slugs in the Hawaiian environment courtesy Stephanie Joe. The spotted character at the top is a large European native named Limax maximus. She's a pet, and Joe calls her Destiny.)

“Slug herbivory may be skewing species abundance in favor of certain non-native and native plants,” wrote University of Hawai'i botanist Stephanie Joe and UH botany professor Curtis Daehler, in an article in the journal Biological Invasions.

Their article is entitled “Invasive slugs as under-appreciated obstacles to rare plant restoration: evidence from the Hawaiian Islands.”

Garden stores sell slug bait, which home gardeners use to keep slugs from munching their new seedlings right down to the ground. But this is not something the first residents of the Islands had to face.

There were no native slugs in Hawai'i before humans arrived. The slugs' relatives, tree snails like the famed and generally endangered Achatinella, were animals that did not appear to eat plants. They crawl on leaves of forest plants but don't eat the leaves themselves. Rather, they feed on the algae and fungi that grow on plants.

But a dozen or more species of slugs have appeared in the Islands over time. They are not well studied, Daehler said, and it's not really clear how many species there are, or exactly where they are.

However, “they are pretty widely distributed. Everywhere we've looked, we find evidence of them,” he said.

There are warm-weather slugs in the lowlands, and slugs from temperate regions in the cooler high forests.

To determine what their impact on native plants might be, Joe and Daehler developed experiments that involved growing seedings of two endangered native plants, Cyanea superba and Schiedea obovata, a non-endangered native, Nestegis sandwicensis (Olopua), and two weeds, Psidium cattleianum (strawberry guava) and Clidemia hirta (Coster's curse).

They planted the seedings out in the forest in the Waianae mountains, and watched what happened.

It wasn't pretty.

“In our field study, both of the critically endangered species had 50 percent higher mortality when exposed to slugs,” they wrote. The greehouse-grown plants were comparatively large when they were planted in the forest, and the authors feel the loss could be higher for tiny naturally germinating seedings.

The slugs did not appear to similarly damage the weedy alien species, or even the non-endangered Olopua, which may give these plants a competitive advantage at the same time the rarest Hawaiian natives are being knocked down by slugs.

And it's not just seedings that are affected.

“We've seen that they climb up on larger plants and burrow into the plant. A lot of them can climb up defoliate a bush-sized plant,” Daehler said.

The research has serious implications for the protection of native plant communities. For a time, it seemed enough to fence out deer, goats and pigs. Then scientists found that rats were a major problem, and they used both traps and poisons to control them. Now it appears slugs will also need to be controlled.

“Slug and snail control measures have generally not been used in the management of rare plants in Hawaii, nor have published studies documented their use to facilitate rare plant restoration on other islands around the world, where endemic plants might be expected to be highly susceptible to introduced herbivores,” The Joe-Daehler paper says.

“The implications are especially relevant for rare plant outplantings and population restoration efforts... Outplanted seedings that are unprotected from slug predation may suffer from high mortality.”

Hawai'i's native tree snails are not known to be herbivores. They crawl on leaves but don't eat the leaves themselves. Rather, they feed on the algae and fungi that grow on plants.

© 2007 Jan W. TenBruggencate