Non-linear effects of landscape properties on mistletoe parasitism in fragmented agricultural landscapes |
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Authors: | Lindy M MacRaild James Q Radford Andrew F Bennett |
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Institution: | (1) Landscape Ecology Research Group, School of Life and Environmental Sciences, Deakin University, 221 Burwood Highway, Burwood, VIC, 3125, Australia;(2) Present address: Bush Heritage Australia, P.O. Box 329, Flinders Lane, Melbourne, VIC, 8009, Australia |
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Abstract: | Ecological processes such as plant–animal interactions have a critical role in shaping the structure and function of ecosystems,
but little is known of how such processes are modified by changes in landscape structure. We investigated the effect of landscape
change on mistletoe parasitism in fragmented agricultural environments by surveying mistletoes on eucalypt host trees in 24
landscapes, each 100 km2 in size, in south-eastern Australia. Landscapes were selected to represent a gradient in extent (from 60% to 2% cover) and
spatial pattern of remnant wooded vegetation. Mistletoes were surveyed at 15 sites in each landscape, stratified to sample
five types of wooded elements in proportion to their relative cover. The incidence per landscape of box mistletoe (Amyema miquelii), the most common species, was best explained by the extent of wooded cover (non-linear relationship) and mean annual rainfall.
Higher incidence occurred in landscapes with intermediate levels of cover (15–30%) and higher rainfall (>500 mm). Importantly,
a marked non-linear decline in the incidence of A. miquelii in low-cover landscapes implies a disproportionate loss of this species in remaining wooded vegetation, greater than that
attributable to decreasing forest cover. The most likely mechanism is the effect of landscape change on the mistletoebird
(Dicaeum hirundinaceum), the primary seed-dispersal vector for A. miquelii. Our results are consistent with observations that habitat fragmentation initially enhances mistletoe occurrence in agricultural
environments; but in this region, when wooded vegetation fell below a threshold of ~15% landscape cover, the incidence of
A. miquelii declined precipitously. Conservation management will benefit from greater understanding of the components of landscape structure
that most influence ecological processes, such as mistletoe parasitism and other plant–animal mutualisms, and the critical
stages in such relationships. This will facilitate action before critical thresholds are crossed and cascading effects extend
to other aspects of ecosystem function. |
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