Nicola Munro, David Lindenmayer and Joern Fischer are from the Fenner School of Environment and Society (Hancock Building (43), Biology Place, Australian National University, Canberra, ACT 0200, Australia; Tel. +61 2 61251495, Email: email@example.com or firstname.lastname@example.org). The research was undertaken in response to a need for collated and reviewed information on how fauna respond to revegetation and how we can best revegetate to maximize use by fauna.
Summary We reviewed the literature on fauna in revegetation in Australian agricultural areas. Of 27 studies, 22 examined birds, with few studies focusing on other faunal groups (four to six studies for each remaining group) and nine examined multiple groups. Existing evidence suggests that revegetation provides habitat for many species of bird and some arboreal marsupials. Species richness of birds was greater in revegetated areas that were large, wide, structurally complex, old and near remnant vegetation. Bats, small terrestrial mammals, reptiles and amphibians did not appear to benefit significantly from revegetation in the short term. Evidence to date suggests that revegetation is not a good replacement of remnant vegetation for many species. Key information gaps exist in the faunal response to (i) revegetation as it ages; (ii) different structural complexities of revegetation; (iii) revegetation that is composed of indigenous vs. non-indigenous plant species; and (iv) revegetation that is in riparian vs. non-riparian locations. In addition, little is known on the value of revegetation for declining or threatened fauna, or of the composition of fauna in revegetation. There is a need to better understand the balance between quantity of revegetation in the landscape, and the quality or complexity of revegetation at the patch scale. Based on current evidence, we recommend revegetation be conducted in patches that are large, wide and structurally complex to maximize the benefits to fauna.
The faunal response to revegetation in Australian agricultural areas has been reviewed by Ryan (1999) and Kimber et al. (1999). Both reports concluded (from the small number of studies then available) that revegetated sites provided habitat for a range of bird species (the only taxa studied) although the majority of these were generalist or edge species, and birds with specialized needs were not provided for by revegetation. Substantially, more research has been conducted since the earlier reviews providing the impetus for this paper.
We review the use by fauna of revegetation in Australian agricultural landscapes and the effectiveness of different revegetation strategies. We define ‘revegetation’ as an area where native plants have been actively introduced, but we do not stipulate by what method those plants were established. Our definition of ‘revegetation’ includes all plantings of woody vegetation (excluding grasslands) in an area where woody vegetation previously occurred, and where the planted vegetation is native to Australia (but not necessarily locally indigenous). This includes both single species and multispecies plantings (Fig. 1). We exclude plantations of exotic species (e.g. Pinus radiata), plantations dominated by tree crops (e.g. orchards) and industrial-scale plantations, to focus the review on small-scale farm and community plantings. We define two types of revegetation based on structural complexity (Fig. 1): ‘simple tree plantings’ include windbreaks, community plantings, woodlots and other farm plantings that are structurally simple; and ‘ecological restoration plantings’ which aim to re-create the vegetation communities present before land clearing and are usually structurally and floristically diverse. ‘Structural complexity’ is defined as the number of different attributes present and the relative abundance of these attributes (McElhinny et al. 2005). We explain how authors have measured structural complexity where possible and appropriate.
We summarize the responses of different taxa to revegetation, and discuss the faunal response to different attributes of revegetation, such as size and shape. We conclude by outlining priorities for future research and revegetation management.
We reviewed all known scientific literature on the faunal response to revegetation in Australian agricultural areas (29 articles describing 27 studies; Table 1). Literature was found by searches through databases and citation lists and interviews with experts. Anecdotal descriptions were not included. More than half the articles were published in peer-reviewed journals (18 of 29 articles). Five articles were theses, resulting in one journal publication. Where multiple publications were produced from the same study (e.g. a journal publication from a thesis), we used the journal article. Four articles were reports. The remaining two articles were one booklet and one book chapter.
Table 1. Studies of faunal response to revegetation in agricultural landscapes A tick indicates the vegetation types researched in each study/article
Different studies explored different combinations of site types. Site types examined in this review were remnant (woody) vegetation, ecological restoration planting (high plant species diversity), simple tree planting (low plant species diversity), and cleared farmland. Many studies compared plantings to reference sites such as remnants (22 studies) or cleared farmland (15 studies; Table 1). More studies examined simple tree plantings than ecological restoration plantings (19/27 vs. 11/27), although six compared these two revegetation types.
Most studies examined birds as a response variable (22 studies). There were four to six studies for each of the following groups: arboreal marsupials, small terrestrial mammals, bats, reptiles, amphibians, invertebrates (Table 2). Nine studies examined multiple taxa. Most studies were conducted in woodland (17 studies), and in areas with a temperate climate (22 studies). Three studies were conducted in tropical or subtropical rainforest (Table 1).
Table 2. The number of studies found on particular faunal groups The groupings used in this graph are those referred to throughout the review. Several studies researched more than one faunal group (see Table 1)
Number of studies
Information on site attributes was not always available in the reviewed articles. Frequently missing was information on the age, size, isolation and complexity of the revegetation. Missing information has hampered this review. Several studies had limited replication: four studies had <10 sites and only 11 studies had >50 sites (Table 1).
Recent revegetation guides suggest that planting local plant species should benefit local fauna (Bennett et al. 2000). This has been implicitly tested in only one study: the diversity of woodland birds was greater if local native plants were established, and conversely, exotic birds were more diverse if exotic trees were planted (Barrett 2000).
Bird abundance and species richness are relatively simple measures. Of perhaps more importance to restoration is the bird community composition in revegetation. Several studies found that the bird composition in revegetation as young as 5 years after mining resembled that in the surrounding forest, depending on the development of the vegetation, particularly the understorey (Nichols & Watkins 1984; Armstrong & Nichols 2000). Borsboom et al. (2002) found that largely undisturbed 40-year-old simple eucalypt plantings approached the plant species richness and abundance of selectively logged old-growth forest, and also approached the bird species richness and composition of the reference forest. This latter project, however, was unable to separate the effects of plantation age and structural complexity (because complexity increased with age). Catterall et al. (2004) separated these effects and compared ecological restoration plantings (high structural complexity) with simple tree plantings (low complexity) of the same age and found that bird composition in ecological restoration plantings was closer to that in remnant forest than in simple tree plantings.
Only two studies investigated the response of birds to planting area, with differing results. Borsboom et al. (2002) found no correlation between bird species richness and simple tree planting area. Kavanagh et al. (2005) found that bird species richness and abundance had a strong positive response to patch size. These studies differed in their ranges of patch sizes and complexity, with the former being small simple eucalypt plantings (1.5 to 10.5 ha), and the latter including large ecological restoration plantings (<5 to >1000 ha).
Several studies identified width of revegetation as being positively correlated with bird species richness (Taws et al. 2001; Merritt & Wallis 2004; Kavanagh et al. 2005) or richness of forest/woodland birds (Kinross 2000). The composition of birds in wider revegetation patches was no different to that in narrow revegetation patches (comparing <15 m with >19 m sites), although some small insectivorous species preferred wider sites to narrow (Kinross 2004).
Landscape-level attributes of revegetation have been little studied. Hobbs et al. (2003) found that adjacency to remnant vegetation increased the abundance of some birds in simple tree plantings, but overall differences between isolated plantings and those adjacent to remnant vegetation were relatively small. Kavanagh et al. (2005) compared birds in revegetation in two landscapes differing in vegetation cover – variegated and cleared – and found no difference in the total numbers of bird species in each landscape. Cunningham et al. (in press) demonstrated that bird richness was greater where the total area of both remnants and revegetation was greater. Also, the effect of plantings was greater on farms with little remnant vegetation, than on farms with more remnant vegetation (Cunningham et al. in press).
Older revegetation sites contain more arboreal marsupials than young sites (Kavanagh et al. 2005). The older areas of revegetation in that study were 20–25 years old, and so were unlikely to provide nesting hollows (Gibbons & Lindenmayer 2002) – hence it is unclear why these older sites contained more arboreal marsupials. Kavanagh et al. (2005) also found that arboreal marsupials were more abundant in relatively large revegetation sites (>5 ha), but did not respond to planting width (where a narrow site was <50 m wide).
A study by Cunningham et al. (2007) found that farms and landscapes with many revegetation plantings supported a lower abundance of arboreal marsupials. This was attributed to those farms supporting less remnant vegetation than farms and landscapes with few plantings.
Small native terrestrial mammals
Two of four studies examining small native terrestrial mammals had sufficient data to indicate the value of revegetation as habitat. In one study, two species were observed, and both occurred only in remnant vegetation and not in simple tree plantings (Hobbs 2003; Hobbs et al. 2003). In the other, one species was ubiquitous, and three were more abundant in remnant vegetation than simple tree plantings (Rossi 2003). Habitat complexity of plantings (as measured by the number of stratas including ground cover elements) explained most variability in native mammal richness (Rossi 2003).
Bats appeared to be insensitive to revegetation size and width as well as to the amount of vegetation cover in the landscape (Kavanagh et al. 2005; Law & Chidel 2006). Bat species richness and activity was negatively correlated with shrub cover, possibly because many bats experience ‘structural clutter’ which reduces foraging ability (Kavanagh et al. 2005).
Amphibians exhibited a mixed response to revegetation. Kavanagh et al. (2005) found that frogs were present in ponds with water regardless of vegetation type (remnant, revegetation or cleared farmland); Hobbs et al. (2003) found more frogs in remnants than in revegetation and cleared farmland, and no difference between the latter two. Frogs in western Victoria did not respond to planting width (Merritt & Wallis 2004).
Four studies on invertebrates found more taxa in remnant vegetation than in simple tree plantings (Green & Catterall 1998; Bonham et al. 2002; Schnell et al. 2003; Cunningham et al. 2005). However, the studies found different responses of invertebrates to revegetation compared with cleared farmland. One found more ant species in 6-year-old simple tree plantings than on cleared farmland (Schnell et al. 2003), whereas another study found no difference (Green & Catterall 1998). The latter study, plus another (Catterall et al. 2004) found highly variable responses by different invertebrate orders. Catterall et al. (2004) found that Orthoptera (grasshoppers) were much more abundant in cleared farmland than revegetation or remnants; Coleoptera (beetles) and Formicidae (ants) were reasonably abundant in all vegetation types (cleared farmland, revegetation, remnants); Amphipoda (litter hoppers) were abundant only in vegetation of high floristic diversity (remnant forest, regenerating forest and floristically rich ecological restoration plantings), with very low numbers in cleared farmland and monoculture revegetation. Cunningham et al. (2005) found the species richness of Coleoptera (beetles), Lepidoptera (moths) and Hymenoptera (ants, bees and wasps) did not differ between simple tree plantings, remnant vegetation and cleared farmland, but the community composition differed between site types for Coleoptera and Lepidoptera. They also found no differences in community composition of these insect groups between edge and interior habitats, or between isolated plantings and those adjacent to remnant vegetation. Bonham et al. (2002) found no difference in the number of native species of invertebrate with age of revegetation.
Majer and Nichols (1998) found that the composition of ants in an ecological restoration planting of a mined site approached that in a remnant forest sooner than that in a simple tree planting. Ant richness increased in both revegetation plots over a 14-year period, and the composition approached that of remnant forests in both revegetation types (Majer & Nichols 1998).
Revegetation attributes affecting fauna use
In fragmented landscapes, patch size of remnants tends to have a positive effect on birds (Loyn 1987; Lindenmayer et al. 2002; Seddon et al. 2003), arboreal marsupials (Pahl et al. 1988) and reptiles (MacNally & Brown 2001). The effect of patch size has been poorly researched in revegetation studies. Larger revegetation patches may benefit some faunal groups such as birds and bats (see sections above), whereas the effect of patch size on other faunal groups is largely unknown.
Width of revegetation
Bird species richness is generally higher in relatively wide plantings (see above), whereas frogs, bats, arboreal marsupials and reptiles appear to show no consistent response to revegetation width.
Age of revegetation
Birds and arboreal marsupials appear to increase in richness and abundance with increased revegetation age, but bats, reptiles and invertebrates do not. We found no studies with data on the response of small terrestrial mammals and amphibians to revegetation age. Most revegetation plantings examined in this review were young (mostly <30 years). Some key resources such as large logs, dead trees, tree hollows, or ground cover complexity may take longer than this to develop (McElhinny et al. 2006), whereas others may be independent of revegetation age (e.g. water availability, rocks).
Faunal composition also may change in revegetation over time. Young revegetated mine sites in south-west Western Australia contained competitive colonizing species or generalist species of mammal, bird and ant; then as the vegetation matured, a new suite of species took advantage of the changes in structure at the site (Majer & Nichols 1998; Nichols & Nichols 2003). In Queensland, bird guilds in simple tree plantings became more like those in selectively logged forest over time (Borsboom et al. 2002).
Structural complexity and floristic diversity
Structurally complex revegetation typically supports more fauna species and a different faunal composition than structurally simpler revegetation. Some attributes of complexity are particularly important to some faunal groups. For example, amphibians and reptiles respond predominantly to complexity in the ground layer, and small terrestrial mammals respond to complexity in the mid- and understorey layer (McElhinny et al. 2006). Similarly, the presence of old trees in a eucalypt plantation can significantly increase bird diversity and abundance (Grabham et al. 2002).
Adjacency to remnant vegetation can increase the use of revegetation by birds (Hobbs 2003; Hobbs et al. 2003). Less mobile species such as mammals are less likely to inhabit planted vegetation than highly mobile animals such as birds (Hobbs 2003; Hobbs et al. 2003). White et al. (2004) found that plantings close to remnants had higher numbers of rainforest plants dispersed by birds, small mammals and wind, than distant sites, indicating that adjacency may benefit plants as well as animals.
Revegetated mine sites provide an interesting parallel to revegetated areas in agricultural landscapes. However, the contextual position of revegetated mine sites, which are usually surrounded by remnant vegetation, is very different to revegetation in agricultural areas where issues of isolation and vegetation cover occur. Revegetated mine sites can therefore provide important information on the faunal use in the absence of isolation, landscape cover or gap-crossing issues.
Revegetated mine sites show successional trends in bird species, beginning with generalist taxa (Nichols & Nichols 2003). Recolonization of revegetated mine sites appears to be rapid: birds may recolonize within 6 years (Nichols & Watkins 1984); reptile species richness may resemble that of low quality remnant vegetation after 4 to 6 years (Nichols & Bamford 1985); many invertebrates orders had similar species richness to surrounding unmined forest within 7 years (Nichols et al. 1989); native small mammals recolonized sand-mined forests within 8 years (Fox & Fox 1984); and many birds were breeding in revegetated sites within 10 years (Curry & Nichols 1986). Birds that did not breed in the revegetated sites had requirements for features not yet available in the sites, such as tree hollows (Curry & Nichols 1986). The presence of lizards in rehabilitated sand-mining sites was predominantly explained by vegetation complexity (Twigg & Fox 1991). Bauxite mine sites in Western Australia have seen an evolving rehabilitation method (Collins et al. 1985; Armstrong & Nichols 2000). Older rehabilitation sites contained very little understorey vegetation, whereas more recent sites contained an understorey plant species richness and diversity comparable to unmined forests (Collins et al. 1985). The older sites contained very low bird species richness and densities, whereas the recent sites with understorey support bird species richness and densities similar to those in unmined forest (Nichols & Watkins 1984; Collins et al. 1985; Armstrong & Nichols 2000). Bird species composition was similar in the recently rehabilitated areas to that of forests (Collins et al. 1985). Similarly, ant species richness and composition was positively associated with plant species richness and diversity and age of the planting (Majer et al. 1984; Majer & Nichols 1998). These studies have emphasized the benefits of developing an understorey in the plantings (where an understorey originally occurred) (Armstrong & Nichols 2000).
Landscape-scale role of revegetation
Lindenmayer et al. (2002) suggested that remnant vegetation fragments of all sizes and shapes have significant conservation value, both as habitat and as stepping stones through the landscape (Fischer & Lindenmayer 2002). This notion may extend to revegetation, despite the lower faunal use compared to remnants. Revegetation may also help buffer adjacent remnants from climatic extremes and other degrading processes, and may stabilize key ecological processes in agricultural landscapes (e.g. by reducing water tables) (Hobbs 1993; Bennett et al. 2000; Kanowski et al. 2005). At a landscape scale, there may be negative consequences for fauna if remnant vegetation is replaced with revegetation (Cunningham et al. 2007), and positive consequences if revegetation is situated on already cleared farmland.
Progress to date
Much new research has been completed since previous reviews on revegetation in agricultural landscapes in Australia (Kimber et al. 1999; Ryan 1999) (Table 1). However, many knowledge gaps remain. Much research has focused on the value of revegetation for birds, but there is a paucity of information on other faunal groups and on threatened and declining taxa. Most research has focused on simple measures of species richness and abundance but faunal composition would provide valuable information on the benefits of revegetation to fauna.
Establishment of ecological restoration plantings is a relatively new practice. It is logical to study both ecological restoration plantings (as an example of the best revegetation currently conducted) and simple tree plantings (as the most common form of revegetation). Differences between these forms of revegetation can provide insights into the conservation capacity of revegetation under both a best-case scenario and the current scenario of mostly simple tree plantings.
The value of revegetation to fauna is rarely put into a landscape context. This context is important because patch-scale research provides information on the local faunal richness (alpha diversity), but it is the landscape faunal richness (beta diversity) that is often of greatest conservation concern (Tscharntke et al. 2005).
Most studies have not examined underlying processes involved in faunal use of revegetation. We found only one study which explored this issue – use of revegetation by birds for breeding (Bond 2004). To date, no research has been conducted on processes such as competition or predation in revegetation.
The faunal response to revegetation studied to date is mostly short-term because revegetation has become common only in recent decades. As revegetation ages, and incorporates more features such as logs and leaf litter, its value to wildlife may increase. Ongoing studies will be required to assess the long-term benefits of revegetation.
Many research projects are written as reports or unpublished theses that are not widely available. To maximize accessibility of findings to other researchers, we advocate publication in peer-reviewed journals. There is also a need for scientists to more clearly explain site attributes of revegetation – in particular age, size, isolation, and structural complexity and floristic diversity. Much of this basic information was unavailable in the reviewed articles. Clear and consistent information can provide future opportunities for systematic reviews or meta-analyses.
We suggest further research should target the following areas:
• long-term trends and successional changes in revegetation including the development of key structural features and their effect on fauna;
• comparisons of different types of revegetation including analyses of potential trade-offs between quantity and quality of revegetation at the landscape scale;
• the value of planting indigenous plant species for fauna;
• the faunal composition changes in revegetation over time and with different site attributes;
• the response by terrestrial mammals to revegetation;
• the resource needs of reptiles, amphibians and bats which could be provided by revegetation;
• the conservation value of revegetation for declining or threatened fauna;
• the value to wildlife of revegetation in riparian compared to non-riparian areas; and;
• the interaction of remnant vegetation and revegetation.
Sharon Rossi and Suzi Bond kindly provided copies of their theses for this review. This review is part of a PhD project on revegetation by the primary author. The primary author was in receipt of an ANU scholarship. Funding came also from the Fenner School of Environment and Society. Earlier versions of this review were greatly improved by Adam Felton, Suzi Bond, Carole Elliot, Geoff Barrett, and two anonymous referees.