Spatial scale and opportunities for choice influence browsing and associational refuges of focal plants

Abstract

Summary 1.Foraging decisions by herbivores depend on variation in food types, the scale(s) at which this variation occurs and the opportunity and capacity for herbivores to respond to such variation. These decisions affect not only the herbivores themselves, but also the vulnerability of individual plants to being eaten. Associational plant refuges, in which neighbouring plants alter focal plant vulnerability, are an emergent property of foraging decisions. 2. Using the red-bellied pademelon (Thylogale billardierii) as a model generalist mammalian herbivore, we investigated the spatial scale(s) at which animals made foraging decisions and the resultant effect on focal plant vulnerability. In a replicated design, we varied vegetation at the individual plant scale, generating intraspecific differences in Eucalyptus nitens seedlings by altering their nutrient status (high, low). We varied vegetation at the patch scale, in which seedlings were planted, using high-(grass) and low-(herbicided) quality patches. Animals were allowed to choose where they fed and what they ate. Animal behaviour was recorded and intake of seedlings measured. 3. We found that animals made foraging decisions first at the patch scale then at the scale of individual plants; both patch and focal seedling characteristics influenced browsing. Pademelons spent most of their time in high-quality patches, and seedlings were consequently more vulnerable there than in low-quality patches. Pademelons also ate more foliage from high-than from low-nutrient status seedlings. This behaviour concentrated resources, increasing foraging efficiency and making focal plants more vulnerable to browsing. 4. The opportunity and capacity to choose at both plant and patch scales resulted in a pattern of focal plant vulnerability consistent with the repellent-plant hypothesis. This contrasts with our previous study, in which animals were only provided with choice at the plant level and plant vulnerability followed the attractant-decoy hypothesis. These combined results demonstrate that the influence of neighbouring vegetation on consumption of a focal plant depends on the spatial scale of selection and on opportunities (and capacity) for herbivores to choose.

Non-lethal strategies to reduce browse damage in eucalypt plantations

“…bracken, Pteridium esculentum) in place on plantations due to positive effects on seedlings. This is consistent with previous research at the plantation scale (Bulinski and McArthur, 2003;Miller et al, 2006;Ward and Mervosh, 2008) and indicates that, when offered the choice, the browsing marsupial herbivores select at the patch scale first followed by the plant, consistent with studies with captive animals (Miller et al, 2009).…”

Spatially complex neighboring relationships among grassland plant species as an effective mechanism of defense against herbivory

“…The magnitude of herbivore foraging selectivity has been frequently used in explaining plant associational defense, but rarely quantified to explain impacts on plant population and community vulnerability on the basis of neighborhood (Pfister and Hay 1988;Milchunas and NoyMeir 2002;Miller et al 2009). We advocate greater emphasis in understanding plant associational defense and spatial plant-herbivore interactions from the view of an overall plant defense-guild that comprises the whole plant community as proposed by Atsatt and O'Dowd (1976).…”

“…Although such neighborhood effects have been well documented, the studies were limited in that only an individual plant's risk of herbivory was investigated and the consequence of neighbor effects at different patches was studied in isolation. Most previous studies on associational defense focused on how characteristics of neighboring vegetation influence the consumption of a focal plant within a given vegetation patch, or on comparing consumption of the focal plant between different patches with different neighbors (Pietrzykowski et al 2003;Baraza et al 2006;Miller et al 2009). A few studies, however, considered the herbivory risk of the whole plant population and the overall neighborhood effects.…”

Mixed effects of ant–aphid mutualism on plants across different spatial scales

“…Resources are often heterogeneously distributed at different scales for plants and animals (Miller, McArthur, & Smethurst 2009;Ostoja, Schupp, & Klinger 2013;Wilby & Shachak 2000). Larger scales often mean more heterogeneous environments for organisms, and varied environments can lead to the change of biotic interactions (Griffin, Byrnes, & Cardinale 2013).…”