Sessions, L. A.; Kelly, D. 2001. Heterogeneity in vertebrate and invertebrate herbivory and its consequences for New Zealand mistletoes. Austral Ecology 26: 571-581.
Abstract
Vertebrate herbivores generally have greater effects
than invertebrates on plants. However, few studies have investigated
the effects of both invertebrate and vertebrate herbivores on a single
plant species. In New Zealand, nationwide declines in mistletoe populations
have often been attributed to possum herbivory, but never to insect
herbivory. The main goal of this study was to document levels of vertebrate
and invertebrate herbivory on endemic New Zealand mistletoe plants to
suggest whether herbivory is leading to mistletoe decline. In this study
we measured the annual amount of leaf loss due to herbivory by the brushtail
possum (Trichosurus vulpecula), insect herbivory, and leaf
abscission on two populations each of three mistletoe species (Alepis
flavida, Peraxilla colensoi, and P. tetrapetala, Loranthaceae).
. In two populations of each species from February 1997 to February
1998, abscission accounted for by far the most leaf loss (range 10-84%
of total mean leaf area, mean 33%), while insects and possums usually
removed small and similar amounts (less than 3%). Possum browse caused
large amounts of abscission in only one population (A. flavida
at Eglinton). Observed possum browse was more heterogeneous than insect
browse among branches within a plant (possum coefficient of variation
= 2.63, insect CV = 1.98, P < 0.001), among plants in a population
(possum CV = 2.15, insect CV = 0.69, P < 0.001), and between populations
(possum CV = 1.36, insect CV = 1.09). Moreover, insects damaged 100%
of the study plants but never removed more than 16% of leaf area on
a single plant, while possums only browsed 32% of the study plants but
severely defoliated some plants. Thus, while the mean amount of biomass
removed across a population may have important consequences for mistletoe
survival, the effect of possums on mistletoe populations may also depend
on the heterogeneity of browse among individuals in the population.
Keywords:
Alepis; Loranthaceae; patchiness; possum; Peraxilla; spatial heterogeneity; Trichosurus vulpecula