The Potential of Revegetation Programs to Encourage Invertebrates and Insectivorous Birds

Abstract

There are extensive revegetation programs in the wheatbelt of Western Australia. Revegetation has many objectives including lowering water tables to combat water logging and soil salinisation, improving agricultural productivity, and producing a commercial crop of trees for harvesting.Trees are planted by farmers, conservation groups and Government authorities to rehabilitate, beautify and manage degraded agricultural land, parks and road verges.In addition to improving plant diversity and restoring ecosystem functions, revegetation is an opportunity to provide food and habitat for wildlife and to conserve regional biodiversity. The objective of the study reported here was to investigate whether the tree species planted in the wheatbelt are colonized by invertebrates (e.g.insects and spiders) and whether the abundance and variety of invertebrates on planted trees differs between tree species and between revegetation and remnant native vegetation. The study also investigated the use of revegetation by birds and compared this to bird communities in remnant vegetation.Invertebrates were sampled on trees planted along the Great Eastern Highway as part of the Main Roads Department 'Ribbons of Green' program, as well as trees planted by community groups and Greening Western Australia. We asked whether the best species of trees were being planted to restore and enhance regional biodiversity. The canopy invertebrate fauna of 10 trees of each of eight species of Eucalyptus and jam wattle (Acacia acuminata) was sampled by chemical knockdown. Jam wattle and three of the eucalypts, including wandoo (E. wandoo), are indigenous to the Northam District. Three of the eucalypts are indigenous to the south coast of Western Australia, one to northwestern Western Australia, and the eighth is indigenous to coastal South Australia. Wandoo was sampled in both revegetation and remnant natural vegetation. In addition to sampling invertebrates, leaf toughness and levels of foliar nutrients (NPK) were sampled for all tree species. Leaf toughness and foliar nutrients were measured as other studies had found relationships between toughness and nutrients, with the abundance and variety of canopy invertebrates.Moderate to high invertebrate densities were found on all tree species. Indigenous trees tended to support the most diverse and abundant invertebrate faunas: species originating from southern coastal regions and northwestern Western Australia supported the least. Wandoo trees in revegetation tended to have higher populations of some insects than wandoo growing in remnant vegetation. Leaf toughness appeared to affect the size of invertebrate populations on some eucalypt species, but the effects of foliar nutrients were inconsistent, possibly because nutrient levels were elevated as a result of fertiliser applications. During winter (June),three patches of remnant vegetation and seven replanted areas were surveyed for birds. Twenty-five species of birds were recorded of which three were found only in remnant vegetation and six were found only on the replanted areas. However, all species recorded are widely distributed throughout the Western Australian wheatbelt and, with the possible exception of the White-browed Babbler (Pomatostomus supercilosus), no significance can be attributed to the differences in bird species composition between remnant and replanted areas: at least in winter, birds are as likely to use revegetated areas as remnant vegetation. The absence of the babbler from revegetated areas is possibly due to the lack of logs and woody debris on the planted sites. Sixteen of the 25 bird species are predominantly insectivorous, four are nectarivores, four are seed-eaters, and one is a frugivore. This suggests that a similar range of foraging resources are available in both remnant vegetation and revegetation.To restore and enhance regional biodiversity, we recommend that revegetation programs, including commercial plantings, should use a variety of tree species and emphasise regional species. Where this is not possible, species from nearby regions should be used. Planted areas should also be diversified by using a variety of indigenous shrubs and herbs, as well as trees, and by adding logs and coarse woody debris to the area planted. Provision of nest boxes would accelerate the colonization of revegetated areas by hole-nesting birds.