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

Authors

  • Alison M. Miller,

    Corresponding author
    1. CRC Forestry Ltd., Private Bag 12, Hobart, Tasmania 7001, Australia
    2. Schools of Zoology and Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia
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  • Clare McArthur,

    1. School of Biological Sciences, Building A08, University of Sydney, NSW 2006, Australia
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  • Philip J. Smethurst

    1. CRC Forestry Ltd., Private Bag 12, Hobart, Tasmania 7001, Australia
    2. CSIRO, Private Bag 12, Hobart, Tasmania 7001, Australia
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Correspondence author. E-mail: Alison.Miller@utas.edu.au

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.

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