Invasive plants can lead to significant changes in the abundance and diversity of the existing flora. Restoration programs, therefore, largely focus on the recovery of the vegetation. Faunal responses have received less attention. Here we examined whether or not bird communities recovered following removal of a native, invasive tree in South Eastern Australia with a view to evaluating whether this could be used as a tool for assessing the effectiveness of the remediation programs. Pittosporum undulatum is an Australian native tree that has become highly invasive in areas well outside its original range within Australia and in many other regions of the world. In the Azores, for example, it is associated with changes in bird communities. In Australia, high density P. undulatum is the cause of major declines in biodiversity of flora and the total suppression of regeneration of Eucalyptus. Its removal from nature reserves across temperate south eastern Australia has been effective in allowing many plant communities to recover, but the impact on birds is unknown. We compared the species richness, density and functionality of local resident bird communities across original remnant vegetation and invaded areas with those that had been cleared of invasive P. undulatum populations at different times. Areas infested with P. undulatum had fewer carnivorous birds but overall there did not appear to be any strong influence on species richness, or density. However, when invaded areas were examined at a finer scale by partitioning the observations into ‘above’ or ‘within’ and ‘below’ the dense P. undulatum canopy, strong differences were detected with fewer birds and fewer species relative to the diversity and abundance of birds in the Eucalyptus overstory. Our work demonstrates that while P. undulatum provides habitat for birds, there is a difference in the relative proportions of different functional groups. This is important, particularly in light of the long-term decline in ground dwelling bird communities across the temperate regions of the continent. We conclude that birds are a useful bioindicator of habitat health and that the inclusion of avian monitoring programs could improve the evaluation of the efficacy of restoration projects.

Site selection and bird surveys

Ten sites across peri-urban areas of Melbourne, in south-eastern Australia, were selected to evaluate their bird communities (Table 1). The vegetation at these sites has been surveyed and analysed in a previous study (O’Leary et al. 2018 Forest Ecology and Management 408: 112-120). Sites were selected to contain (1) an area of uninvaded, remnant vegetation supporting a Eucalyptus overstorey of varying species composition consistent with local conditions (“reference control”); (2) an area currently infested by P. undulatum (“invaded”); and (3) a formerly invaded area where P. undulatum had been removed (“cleared”). Two sites (Ferntree Gully and Sherbrooke Forest) lacked invaded patches but are included here for analyses that do not depend on a sequence from control to invaded to cleared patches. We used Ecological Vegetation Class (EVC) mapping supported by on-ground observations to ensure that the vegetation patches within a site supported similar vegetation (DELWP 2017). Sites ranged in size from 1–12 ha. The management area at each site was characterised as having a severe P. undulatum infestation (30–70% canopy cover) prior to removal work.

Bird surveys were conducted on three separate mornings at each site from mid-May to late June of 2017. Surveys were conducted within the first three hours after sunrise. Following a modified version of the process established by Loyn (1986) and outlined in Loyn et al. (2007), i.e. 10 minutes surveying time was implemented for each hectare of sampling area at each site, to a maximum of 20 minutes. This timeframe has been observed as appropriate to survey bird communities within south eastern Australian forests, whilst reducing the risk of bias towards conspicuous species with distinctive and/or frequent calls. 

All birds observed by sight and call within and below the vegetation canopy were identified to species level. Birds flying overhead were not included in the study. Bird species relative abundance was determined by dividing the total number of birds observed over the three surveys at each site. Surveys distinguished bird use of the habitat within or below the P. undulatum canopy (PU) from use of the overstorey (NPU) due to the predicted large effects of dense P. undulatum canopies. Density was determined by dividing the numbers of birds by the area surveyed.

Values of five traits reflecting ecological functionality (life history, habitat preference, and feeding guild) were extracted for each bird species identified (Higgins et al. 2006 Handbook of Australian, New Zealand and Antarctic Birds, Vol 1-7, Oxford University Press, Melbourne, Australia.). 

Statistical analysis

Differences in bird species richness and individual abundance among reference, invaded, and cleared vegetation types and between the P. undulatum  understory and overstorey in invaded sites were tested with one-way ANOVAs. To compare species richness in individual feeding guilds across the three vegetation types, we used Kruskal-Wallace tests, due to the smaller numbers of species involved in each guild. The effect of time since P. undulatum removal was tested with linear regression on relativized measures of species richness and individual abundance (richness or abundance in cleared patch divided by richness or abundance in the corresponding reference control). To summarize the differences and similarities among bird communities in the three vegetation types we employed principal components analysis (PCA) of bird species presence/absence data using the prcomp function in R (R Core Team 2017). Loadings on the first two principal components were examined to account for the contribution of individual species to the community configuration. We repeated the PCA using bird abundance data using nonmetric multi-dimensional scaling (NMDS) and checked that the results were biologically meaningful as suggested by Björklund (201, Evolution, 73(10), 2151-2158). Since all results were in qualitative agreement, we present PCA of the presence-absence data here. Scores on the first two principal components were compared among reference, invaded, and cleared vegetation types using Wilcoxon signed-rank tests.

PCA was also used to investigate the functional response of bird communities to P. undulatum infestation and removal. Mean functional trait values in the species assemblage were calculated from the traits of individual species weighted by the abundance of individuals of the species. All data were centred and scaled to unit variance prior to analysis. All analyses were conducted using the R statistical platform (R Core Team 2017).

Processing

The data was originally assembled in csv files for analysis in R and they are provided as supplementary data in that form. Here we have collected the files into a single Excel file for ease of upload.

The data is presented as four sheets in a single Excel file. Each sheet corresponds to a figure in the paper and is clearly labelled as such.  The file is called O'Leary Data - birds in Pittosporum undulatum sites.xls