Plant Cuttings

So that’s what bears do in the woods!

Through plant–animal interaction, black bears have been shown to help growth and reproduction of yellow rabbitbrush, by feeding on ants.
Image: Mike Bender/US Fish & Wildlife Service.
Image: Mike Bender/US Fish & Wildlife Service.

We probably all know what bears are supposed to do in the woods. And there, I suspect, most of us would be happy to leave it (the notion and the subject matter…) not thinking about it further. But scatological considerations notwithstanding, that’s not all that bears do – either in the woods or anywhere else, as Joshua Grinath et al. have shown. In fact they have unearthed another intriguing example of the interconnectedness of living things that we call ecology (‘the scientific analysis and study of interactions among organisms and their environment’), centred around the feeding habits of black bears (Ursus americanus) and its effect on yellow rabbitbrush (Chrysothamnus viscidiflorus), a member of the Asteraceae (the daisy family). Although black bears are omnivorous, they don’t eat the rabbitbrush but they do eat ants. Now, the ants, Formica obscuripes (the western thatching ant), have a mutualistic relationship with a plant-sucking insect known as a treehopper, (Publilia modesta). In that intimate association, honeydew (‘a sugar-rich sticky liquid, secreted by insects as they feed on plant sap’), excreted by the latter, is used by the former as payment for their protection of the latter from its own predators such as lady beetles (known as ladybirds in the UK). Still with it? The treehopper’s sap-sucking activities siphon energy-rich photosynthetically-derived resources from the rabbitbrush, all of which should be used for the plant’s own growth and development, but which isn’t, and plant reproduction is consequently lowered. However, where bear-feeding pressure on the ants is high (evidenced by damage to the ants’ nests), Grinath et al. found that the rabbitbrush grew better and was more reproductive. This is attributed to enhanced activities of the treehopper’s predators, a consequence of reduced ant-protection, with consequent reduced ‘diversion’ of plant resources. Nobody said ecology was straightforward(!), but what a fascinatingly entangled web of feeding interactions! And there’s more (potentially…). Although the group only examined rabbitbrush (and that seems complicated enough!), the treehopper–ant mutualism investigated is found in at least 15 plant species. Thus, the bear’s ant-eating antics ‘could have indirect effects on the majority of plant species in this ecosystem’. So, it’s ultimately all about the bear’s necessities of life (in this mountain meadow in this part of Colorado, USA, anyway…). [Or is it just that black bears are bolder, in Colorado…? – Ed.] And by way of redressing the balance somewhat, we are happy to report that the mounds and activities of another ant-like social insect, the termite, are implicated in enhancing plant growth and diversity by altering soil properties. Furthermore, by examining spatial patterning of termite mounds in arid ecosystems of Africa, Juan Bonachela et al. conclude that such mound-field landscapes are ‘more robust to aridity, suggesting that termites may help stabilize ecosystems under global change’. So, little things really do matter, a lot!

[For more on the nutrient-island role of termite mounds, see Elizabeth Pennisi’s In Depth item that accompanies the Bonachekla et al. article. And, in the absence of bears to come to the rescue, Arjen vanDoorn and Martin de Vos consider ‘Resistance to sap-sucking insects in modern-day agriculture’ in their mini-review – Ed.]