Sky Islands: Isolated Mountaintops Teem With Unique Insect Communities

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For a study of the communities of parasitic wasps on mountains in the Interior Highlands of Arkansas, one of the sites chosen was Mount Magazine State Park in Arkansas, which rises 709 meters (2,326 feet) above sea level. With cooler, wetter climates than lowlands nearby, such each feature their own communities of parasitic wasps—and likely other insects—that differ from the insect fauna found on other mountains and in the surrounding valleys, according to a new study published in August in Environmental Entomology. (Photo courtesy of Allison Monroe)

By Ed Ricciuti

Ed Ricciuti

It’s not quite Sir Arthur Conan-Doyle’s Lost World of dinosaurs, but the insect life found by scientists atop so-called “sky islands” in Arkansas ranks as truly unique.

“Sky island” is a term popularized in the 1960s to describe isolated mountains with environments markedly different than that of surrounding lowlands. Conan-Doyle prefigured such environments in his story about an expedition that explored a plateau rising above jungle, where prehistoric dinosaurs, reptiles and “ape men” had survived the ages.

Although not as dramatic as dinosaurs, isolated endemic populations of animals of any size excite scientists. According to a study published in August in Environmental Entomology, such distinct assemblages of insects in the order Hymenoptera (sawflies, bees, wasps, and ants) live atop uplands in Oklahoma, Arkansas, Missouri, and Illinois called the Interior Highlands.

The study, by student researchers at Hendrix College in Conway, Arkansas, focused on parasitic wasps inhabiting three mountains, but the results can be extrapolated to other sky islands in the region and their insects in general, the researchers say.

“Given that each sky island in our study showed unique community characteristics of Hymenoptera, it is reasonable to predict that other insects follow the same pattern,” the authors write. Mountains studied were Petit Jean Mountain at 253 meters (830 feet) in elevation, Mount Magazine at 709 meters (2,326 feet) and Rich Mountain at 747 meters (2,451 feet).

Parasitic Hymenoptera are a multitudinous group, with 50,000 or so identified species and perhaps millions in all. Typically, they parasitize other insects by laying their eggs in host eggs, larvae, or pupae. They are of immense ecological importance because they are fine-tuned to specific hosts, including many pest species, which they can regulate, like natural pest control managers. “We chose parasitic Hymenoptera as our focal group because they are considered bioindicators of broader diversity patterns, especially those of other insects,” the authors write.

The Interior Highlands, centered in Missouri and Arkansas and including the Ouachita Mountains and Ozark Plateau, were chosen as a study site because they have been above sea level for 320 million years, likely serving as a refuge for ecological communities avoiding the impact of the Pleistocene glaciers. The region is the only major mountainous area between the Appalachians and the Rockies, covering much more area than the Black Hills of South Dakota. Typical of the Interior Highlands, Mount Magazine is 10 degrees Fahrenheit cooler than normal temperatures in the landscape down below and wet, with an annual rainfall of 54 inches. Crowned with upland hardwood and upland pine-hardwood forests, these mountains rise from grasslands, with vegetation ranging from tallgrass prairie to lowland pine-hardwood and bottomland hardwood forests.

Much of the area where the research was conducted lies in state and federal lands. Sweating in the hot summer sun, the research team trekked along hiking trails from grasslands into woodlands. They set up traps, then collected insects from them.

“Though evidence is accumulating that the Interior Highlands host unique species relative to other areas of the North American continent, there is less known about how mountaintops within the region compare in terms of biodiversity,” the researchers write. “We used parasitic Hymenoptera to explore biodiversity patterns across high elevation areas in Arkansas to determine whether these patterns are similar to those exhibited by other sky island regions.”

Each mountaintop had its distinct community of parasitoid species, indicating that the same applies to Hymenoptera in general and even to other groups of insects. On a given mountaintop, communities differed stratigraphically, with those on the ground distinct from those in the forest canopy.

The results of the study suggest the need for additional research. “Our study suggests that these highland areas are important regions of North American biodiversity and that they should be evaluated individually for conservation efforts in order to preserve their distinctive community structure,” the authors write.

Elaborating on the study, lead author Allison Monroe, says, “This study is important for a variety of reasons. Parasitic wasps are deeply important to our environment but are often overlooked if not deeply hated.”

Monroe, now a Ph.D. candidate at the Oregon State University College of Forestry, says, “Arkansas is an incredibly biodiverse state with high rates of agricultural production, yet little research exists on insect biodiversity trends and their applied impacts on diverse land management strategies within this system. We hope that this paper brings to light the extraordinary diversity housed in Arkansas, the importance of insect biodiversity more broadly, and the significance of parasites in our pursuits of nature conservation.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

  

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