Impacts of picnic areas on bird assemblages and nest predation activity within Australian eucalypt forests

Abstract

We assessed the impacts of picnic areas on birds within adjacent eucalypt forests in the Brisbane region, Australia. Within the picnic areas there was an assemblage of relatively large, aggressive birds, including several generalist predators (Torresian crow, grey and pied butcherbirds, Australian magpie) as well as the noisy miner, resembling that found in suburban areas within the study region. Eight transects abutting picnic areas (“picnic area edges”) were compared with two types of matched control transect within forest about 150 m away: “forest interiors”, which were also distant from any other edge and “context references”, which were similar to the picnic area edges in their proximity to the access roads and forest external edges. Bird assemblages at picnic area edges, which were dominated by the noisy miner and Australian magpie, clearly differed from those of forest interiors, which were characterised by a number of smaller bodied insectivorous species. Levels of artificial nest predation were several-fold greater adjacent to the picnic areas. Context reference transects were more often similar to forest interiors than to picnic area edges. We conclude that picnic areas exert strong localised edge effects on forest bird assemblages, and are likely to cause reduced reproductive success for small-bodied forest bird species which attempt to nest nearby.

Introduction

The development of day-use recreation areas (“picnic areas”) is a frequent part of conservation area management. However, their effects on wildlife have seldom been tested. In south east Qld, Australia, picnicking is the most common form of outdoor recreation, and a 1997 survey found that 65% of residents had picnicked outdoors each an average of seven times during the previous year (Queensland Government, 1998). Furthermore, there has been an overwhelming preference for picnicking in natural or near-natural areas (Queensland Government, 1998). Conservation managers and land use planners therefore often feel obliged to establish picnic areas within multiple-use forests and conservation reserves. Picnic areas may be relatively small, and thus assumed to have little ecological impact. There are, however, very few published studies which document their direct and indirect effects on wildlife (Cole and Landres, 1995).

Studies which have compared forest campgrounds, which have many similarities to picnic areas, to forest interiors have reported a range of effects of campgrounds on birds (Cole and Landres, 1995). Blakesley and Reese (1998) found seven bird species positively associated with campgrounds, and another seven species associated with non-campground sites (most of which nested on the ground or in shrubs or small trees). Foin et al. (1977) reported both positive and negative direct and secondary effects on birds and mammals in Yosemite campgrounds. However, these studies did not survey immediately adjacent to the campground edges, and the occurrence and extent of any edge effects into the forests were not quantitatively studied.

Beside the direct effects of human presence and recreational activities on wildlife distributions (van der Zande et al., 1984), there are two important mechanisms by which picnic areas may be expected to impact upon biota and ecological processes within adjacent forest. First, such areas cause increased availability of, or access to, certain resources, and provide novel resources for some species. Intentional and unintentional supplementary feeding of wildlife may lead to population increases or decreases (Green and Giese, 2004). Within Australian eucalypt forests, bird species which commonly benefit from these added resources, due to their relatively aggressive and inquisitive natures and generalist dietary preferences, include grey and pied butcherbirds (Cracticus torquatus and Cracticus nigrogularis), Australian magpie (Gymnorhina tibicen) and the pied currawong (Strepera graculina). These four larger artamid (family Artamidae) species are in many ways ecologically similar to corvids, and are well known as predators of birds’ eggs, nestlings and small vertebrates in general (Blakers et al., 1984, Major et al., 1996, Bayly and Blumstein, 2001). Other potential nest predators which may benefit from supplemental food resources within picnic areas are the brushtail possum (Trichosurus vulpecula), native and introduced species of Rattus and Varanid lizards.

Second, development of picnic areas leads to the creation of edge habitat. External edges of eastern Australian eucalypt forests cause strong edge effects on avifauna, including elevated abundances within a distinctive suite of larger, more aggressive birds, and the establishment of noisy miner (Manorina melanocephala) populations (Loyn, 1987, Catterall et al., 1991). Noisy miners are sedentary, colonial birds which flourish both in cleared, grassy areas which contain scattered eucalypts, and along eucalypt forest edges (Loyn, 1987, Sewell and Catterall, 1998, Piper and Catterall, 2003). They are intensely interspecifically aggressive, and cause large local reductions in the richness and abundance of smaller bodied birds (Dow, 1977, Piper and Catterall, 2003), as well as interfering with or depredating avian nests (Major et al., 1996, Piper and Catterall, 2004). Furthermore, eucalypt forest edges may have elevated levels of bird nest predation compared with forest interiors (Piper and Catterall, 2004), similar to the pattern reported from many parts of Europe and North America (Paton, 1994, Andrén, 1995). However, the effects of smaller scale edge-creation within these forests are not known.

The creation of all but the smallest, most carefully designed picnic areas is therefore likely to result in ecological changes within surrounding forests. However, we are not aware of any previously published Australian studies assessing the impacts of picnic areas on avian distributions or ecology. The aims of the present study are to assess the effects on birds of picnic areas within subtropical eucalypt forests. Two aspects of avian ecology are investigated: species composition and nest predation pressure. We compare assemblage structure and predation levels at artificial nests within forest transects at the edges of picnic areas with similar measurements from matched transects far from the edge, and others located similarly to the picnic edge with respect to all factors other than the picnic areas. We compare the effects of these edges with known effects of external edges, and consider implications for the management of remnant forests.