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In this experiment we investigate how chemical and physical attributes affect the grazing of 14 species of Hakea (Proteaceae) seedlings by western grey kangaroos (Macropus fuliginosus). Needle-leaved Hakeas were preferred over broad-leaved congeners, although differences between species within these groups were sometimes large. Needle-leaved species had smaller, thicker and spinier leaves with lower phenolic contents, but similar nitrogen contents (except for high levels in H. platysperma). The three tallest needle-leaved species, with longest, most upright and thickest leaves (H. adnata, H. obliqua and H. platysperma) were most preferred. Principal components analysis on attributes of the 14 species showed that the relative volume of shoot material consumed was significantly related (inversely) to phenolic content. Chemical defences therefore appear to have a significant deterrent role at this stage in the plant?s life history. Physical defences however were ineffective deterrents against kangaroo herbivory raising the possibility that other ecophysiological factors contribute to the possession of structures such as spines and sclerophylly in young seedlings.

Preliminary studies indicate that western grey kangaroos browse seedlings of non-Myrtaceae species rather than Myrtaceae. Seven morphologically-matched species pairs of Myrtaceae/non-Myrtaceae placed at three field sites showed that kangaroos avoided the essential-oil-containing Myrtaceae, but readily consumed the matched essential-oil-lacking non-Myrtaceae. The one exception (Pittosporaceae) had limited herbivory and was later found to possess two essential oils in its leaves. Gas chromatography and mass-spectra showed the seven Myrtaceae plants contain between 2 and 9 essential oils in their leaves, particularly the highly volatile monoterpene, 2,5-dimethyl-3-methylene-1,5-heptadine. Three of the above species pairs were used to gauge their effectiveness as nurse plants for a highly palatable legume. Plants placed beside Myrtaceae nurse plants were less browsed than those placed beside non-Myrtaceae nurse plants. We conclude that western grey kangaroos use olfactory cues to avoid foliage containing potentially toxic essential oils, and that this also has implications for seedling recruitment patterns in regenerating communities.

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Investigations into the mechanisms underpinning plant selection by herbivores have often yielded conflicting results. Such inconsistency might stem from whether selection experiments are conducted with captive or wild populations, and upon the different measures of plant selection used to determine herbivore preference.Here we compared the feeding preferences of captive and wild kangaroos (Macropus fuliginosus) using a standard set of plant species (14 Hakea spp., Proteaceae) and several measures of herbivore selection to examine how environment influenced relative consumption. Three indices of herbivore consumption were measured: number of plants (NP), total shoot volume per plant (TV) and percentage available shoot volume (PV) consumed. NP and TV were closely correlated in the wild and captive populations and consistently correlated with six morphological and chemical plant attributes examined, the most notable being a strong negative correlation with shoot phenolic content. This uniformity suggests that plant selection by captive kangaroos is broadly consistent with that observed in field trials, and consequently that for macropods, at least, captive trials offer a valid way to determine the relative acceptability of different plant species. However, the fact that our third measure of herbivore selection PV was weakly correlated in captive and wild populations and showed no relationship with shoot phenolic content highlights the importance of which measure of plant selection is applied. We suggest that, while NP and TV are potentially confounded by plant size and availability, they offer the clearest insight into plant selection from the point of view of the herbivore, while PV is more suitable for plant-centred studies.