What do you think about emotive language in science?
Are foxes invasive feral vermin, or simply introduced? When shrub cover increases, should we describe the process as shrub encroachment, invasion, expansion or woody thickening? Does it make a difference if the shrub species is indigenous?
In a paper now available in Austral Ecology early view, we looked at associations between native mammal occurrence and Yarra Burgan (Kunzea leptospermoides) canopy cover in a eucalypt forest. Yarra Burgan is native to the Yarra Valley, but has spread rapidly within Coranderrk Bushland Reserve over the past 20 years. Burgan grows to more than 10 m tall, shades out understorey plants and doesn’t appear to be eaten by anything much.
Our vegetation surveys showed that sites with high Burgan cover had a sparser understorey, fewer grasses and higher moss cover than sites without Burgan. More of the eucalypts at Burgan-encroached sites were dead, and there were greater volumes of logs and fallen branches. So Burgan expansion is definitely associated with changes in vegetation. In some cases, though, it’s a chicken-and-egg problem: did the eucalypt trees die from competition with Burgan or did Burgan colonise the areas where eucalypts had died?
In contrast, native mammals didn’t seem to care either way. Echidnas, antechinus, long-nosed bandicoots and swamp rats all preferred sites with dense understorey, and eastern grey kangaroos were associated with high grass cover; but each species was just as likely to be found at sites where Burgan cover was high as sites where it was low. Swamp wallabies occurred everywhere and showed a temporal pattern in habitat selection, spending their days at sites with dense Burgan cover and their nights in open areas.
Shrub expansion (perhaps the most neutral term, although it doesn’t work well as an adjective – shrub-expanded forest sounds rather peculiar whereas shrub-encroached works fine, and we used both in the paper) is widely perceived as a negative process. Most previous studies have focused on grasslands (where shrub expansion reduces grazing for livestock) or examined the effects on flora: plant diversity is generally lower in areas with dense shrub cover.
There is no doubt that shrubs need to be managed at some locations (now is that neutral language, or just a euphemistic way of saying chopped down?) Nonetheless, it is important that the language we use doesn’t bias our decisions: Burgan might be invasive but it also supports high invertebrate abundance and richness, provides nesting habitat for woodland birds, shelter for macropods, roost sites for powerful owls (a family bred in the Coranderrk while we were conducting this research), and the rotting eucalypt trees and logs in areas with dense Burgan presumably provide good foraging and shelter resources for insectivorous mammals and birds. How we manage shrub-encroached forests will depend on which aspects of the biota we value most.
Emotive language can be great for attracting public attention, but we should take care that it doesn’t shape our science.