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For Insect Ecologists, Some Rare Species Interactions Are a Sign of Things to Come

Bombus rupestris

The cuckoo bumble bee Bombus rupestris is one of several species examined in a study on cuckoo bumble bees that appears in a new special collection in Annals of the Entomological Society of America on rare interactions among insects and between insects and plants. Such bumble bees, which parasitize other social bumble bee colonies, are understudied, but researchers suggest they have “unrealized potential to reveal processes of coevolution between social parasites and hosts.” (Photo credit: Flickr/S. Rae, CC BY 2.0)

By Elsa Youngsteadt, Ph.D.

An ecosystem is much more than an inventory of its species. It’s a web of interactions. This insect nibbles the leaves of a plant; that one visits its flowers; both are potential prey for other insects, lizards, or bats. But this web is fraying.

Elsa Youngsteadt, Ph.D.

Elsa Youngsteadt, Ph.D.

The “insect apocalypse” has become a household (if hyperbolic) phrase, as more studies point to loss of insect species and populations, alongside the ongoing, less newsy, threats to plants and vertebrates. But interactions often disappear before their participating species do—an advance wave of extinctions that weakens the stability and function of an ecosystem even while the once-interacting species are still around to be counted.

This was the context I stepped into in 2016 when I took on a new project examining the reproductive ecology of three endangered plants, none of whose pollinators or seed dispersers were known. One of them in particular—a slender, elegant inhabitant of wet meadows—seemed to be growing in isolation, failing to interact. Its little star-shaped yellow flowers would open, looking fresh and optimistic, then wither days later without having seen a single visitor. Was this normal? Had a pollinator gone extinct? Had the plant become so rare that it was no longer able to attract the species that should have visited it? Interactions are far less inventoried than species themselves, and there was little evidence of what used to be.

In an era of insect decline, I wanted to know how other insect ecologists were grappling with rare interactions: What clever methods were they using to detect these uncommon or fleeting events? How should we go about conserving interactions—or using our knowledge of interactions to conserve the participating species? These questions were central to a symposium in the 2017 annual meeting of the Entomological Society of America, and now a special collection in the Annals of the ESA representing six of those presentations.

Ecology in the Sixth Mass Extinction ad

Take a look at rare biotic interactions and find some fresh perspectives on the study of species interactions in the new special collection, “Ecology in the Sixth Mass Extinction,” in Annals of the Entomological Society of America.

Two of the studies in this collection add to the very basic reasons that rare interactions are worth the trouble it takes to study them. Cuckoo bumble bees are social parasites that never produce worker bees; they infiltrate colonies of other species and rely on their workforce. One paper in the collection reviews what is known of these rarely encountered insects, arguing that they have much unrealized potential to reveal processes of coevolution between social parasites and hosts.

Another paper works with a rare pollination interaction in which flowers produce oils instead of the usual nectar, thereby attracting a small set of specialized, oil-collecting bees. Only about 1 percent of flowering plants have made this switch, but the resulting specialized interactions make a nice, self-contained study system to ask some basic questions—in this case, how overlap in climatic requirements of plants and their pollinators drives the geographic distribution of the interaction itself.

If these kinds of interactions are to persist, conservation management is likely required. Biotic interactions are rarely incorporated into conservation plans, but a third paper makes a strong argument that they should be. Using the American burying beetle and prairie mole cricket as case studies, the authors show that understanding a species’ competition, predation, and mating interactions can help us understand why it is rare and, potentially, contribute to its restoration.

Three papers take on pollination questions. One uses video surveillance to assemble remarkably complete interaction networks and show how plants just a few hundred meters apart can be visited by markedly different insects—emphasizing that it takes a lot of space to support the full range of interactions in which a species may participate. Another takes on the pollination interactions of the Venus flytrap, a rare, fire-dependent species with a tiny home range in the Carolinas. And my own contribution to the collection looks at the pollination interactions of an endangered sumac—which, it turns out, has no trouble attracting insects; but, because the plant grows in isolated, single-sex patches, those pollinators often fail to deliver the goods to female flowers. (The accompanying photo gallery offers some images from these latter two studies.)

My little wet-meadow plant with the lonely yellow flowers remains a mystery, but working with the presenters and authors in this symposium and collection has been both sobering and a source of hope. Despite widespread changes in our global ecosystem, the majority of species do have the potential to persist if the worst threats are mitigated. As technological advances improve our ability to detect rare interactions and we accumulate more evidence of their ecological and evolutionary significance, we will be better equipped to conserve even the rarest links in the rich web of interactions all around us.

Elsa Youngsteadt, Ph.D., is an assistant professor of applied ecology at North Carolina State University in Raleigh. Email: ekyoungs@ncsu.edu.

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