Close Encounters

How mammals and molluscs attack oaks

Foraging decisions of molluscs and mammals repulse seedlings from old adults.

While there is plenty of research on how insects target trees as herbivores, there is not so much on mammals or molluscs. In a new study, Maud Deniau and colleagues argue that mammals and mollusc foraging is distinctly different. Their results explain why a poor place for a seedling is where an adult is already thriving.

“Mammals and molluscs differ from insect herbivores by the fact that they travel across dozens to thousands of host seeds/plants while feeding, whereas most insect herbivores stay on a single or a few host plants during their feeding stage,” write Deniau and colleagues.

This need for quantity means mammals and molluscs face two problems when feeding. First, they need to find the most nutritious locations. “The numbers of seeds and seedlings within a patch or matrix may be difficult to perceive while being inside the matrix or the patch,” write the authors. “But adult trees might be used as proxy: as seed production of adult trees increases with adult age, the patch surrounding an old adult might be richer in seeds and seedlings than a patch surrounding a young adult. Adult age, in turn, can be assessed from trunk size. Even terrestrial molluscs might be able to perceive trunk size as they orient themselves towards larger dark zones.”

The other major problem is travel. Molluscs and mammals need to move when they exhaust a patch. When they do, they’re a target for predators. To hide, small mammals need cover. Aerial predators like owls could be foiled by coverage from undergrowth. “Even trunks of standing trees may provide partial shelter to small mammals up to a few metres, leading to higher seed-removal activity of small mammals close to the trunks. In a ‘landscape of fear’, such small-scale shelters are essential.”

For molluscs, they have an extra problem. Mammals can be omnivorous, and a seed-eating mammal could become a slug-eating mammal if it’s hungry enough. So mollusc foraging might be where the small mammals are not. Or they could stay close to the tree trunks themselves.

To find out how herbivores could target plants, the botanists visited the local forest. Here, they examined oaks, Quercus petraeaQ. robur or their hybrids. Close to the trees, they exposed acorns. They then excluded herbivores in a series of experiments.

Design of the study of acorn removal and seedling herbivory, established for 20 focal oaks. Source: Deniau et al. 2021.

The results were a bit of a surprise, with no effect of spatial distance to adult conspecifics on the removal of acorns or leaf area. “We found, however, that acorn removal by rodents was faster around conspecific adults surrounded by phylogenetically distant neighbours, where the focal adult tree might appear comparatively more attractive for enemies,” write Deniau and colleagues. “Moreover, acorn removal by rodents was faster around adult trees with a high ground cover where acorn predators are sheltered from their own enemies.”

This cover would make acorns lying beneath plants such as ferns attractive to foragers. As Deniau and colleagues point out, these sheltered locations are where seedlings would struggle the most. Out in exposed territory, open to the sunlight, is a much better site for a germinating acorn.

The attraction of large adult trees as a food source would add to pressures close to a parent tree. As a result, Deniau and colleagues argue that predator pressure shapes the fine structure of a forest when the parent is isolated.