Understanding the role of plantations in the abundance of an arboreal folivore

Highlights

• We detected higher mean koala density in plantation than in native vegetation sites.

• Mean soil organic carbon was the strongest predictor of koala density.

• We provide an abundance map that identifies priority areas for population management.

• Overall landscape configuration should be considered for resource management actions.

Abstract

Forest cover is decreasing globally, chiefly due to the conversion of forest to agricultural landscapes. In contrast, the area under plantation forestry is increasing by up to 3 million hectares per annum. For wildlife occupying landscapes where native forest is the dominant land cover, plantations generally represent a lower value habitat; however, plantations established on land formerly used for pasture, may benefit wildlife by providing temporary forest habitat and increasing connectivity. This study investigates the influence of landscape, site and climatic factors on koala population density in far south-west Victoria where there has been extensive plantation establishment. We conducted koala surveys and habitat characteristic assessments at 72 sites across three habitat types: plantation, native vegetation blocks, and native vegetation strips. We employed a hierarchical modelling framework for estimating abundance, and constructed candidate multinomial N-mixture models to identify factors influencing the abundance of koalas. We detected higher mean koala density in plantation sites (0.85 per ha) than in either native block (0.68 per ha) or native strip sites (0.66 per ha). We found five covariates of koala density, and using these variables we spatially modelled koala abundance and discuss factors that are key in determining large-scale distribution and density of koala populations. We provide a distribution map that can be used to identify high priority areas for population management as well as habitat of high conservation significance for koalas. This information facilitates the linkage of ecological theory with the on-ground implementation of management actions, and may guide conservation planning and resource management actions to consider overall landscape configuration as well as the spatial arrangement of plantations adjacent to remnant forest.

Introduction

Deforestation is a major cause of biodiversity loss and a significant global concern as it is estimated that more than half of all terrestrial plant and animal species live in forests (Duraiappah et al., 2005). Globally, forest cover is decreasing by approximately 13 million hectares each year, chiefly due to the conversion of forest to agricultural landscapes (Butchart et al., 2013). In contrast, the area under plantation forest (currently representing 3.5% and 140 million ha of total global forest area) is increasing by up to 3 million hectares per annum (MacDicken et al., 2015). For wildlife occupying landscapes where native forest is the dominant land cover, plantations generally represent a lower value habitat (Lindenmayer et al., 2002, Lindenmayer and Hobbs, 2004). However, plantations established on land formerly used for agriculture or grazing may benefit wildlife by providing temporary forest habitat and increasing connectivity (Bennett and Saunders, 2011, Barbaro et al., 2012, Carnus et al., 2006). For example, in the core region of Brazilian cocoa production, primates such as the golden-headed lion tamarin (Leontopithecus chrysomelas) and the yellow-breasted capuchin (Cebus xanthosternos) both use cacao plantations as secondary or transitory habitats (Dietz, De Sousa, & Billerbeck, 1996); substantial populations of the endangered brown kiwi (Apteryx mantelli) exist in pine plantations in New Zealand (Kleinpaste, 1990), and the flightless cassowary (Casuarius casuarius) has frequently been recorded in hoop pine (Araucaria cunninghamii) plantations in Queensland (Keenan, Lamb, Woldring, Irvine, & Jensen, 1997).

Of the 125 million hectares of total forested area in Australia, 123 million hectares are native forests and approximately 2 million hectares are 'commercial plantations' comprising hardwood (eucalypt) and softwood (pine) tree species (Australian Bureau of Agricultural and Resource Economics and Sciences [ABARES], 2015). Animal diversity and composition varies between plantation types (Lindenmayer & Hobbs, 2004); however, the use of single-exotic tree species in plantations (e.g. Pinus radiata) leads to less favourable habitat than indigenous tree species (e.g. Eucalyptus globulus), as native-forest dwelling species have certain resource requirements that are not met by exotic tree species or the habitat they create (Carnus et al., 2006). In Australia, although Eucalyptus plantations may provide a temporary resource (e.g. food), species that are dependent on hollows may be slow to colonise plantations (Lindenmayer & Hobbs, 2004). For example, arboreal marsupials such as the common brushtail possum (Trichosurus vulpecula), sugar glider (Petaurus breviceps) and squirrel glider (Petaurus norfolcensis) are uncommon in eucalypt plantations, unless plantations are older than 25 years and occur in close proximity to remnant vegetation (Lindenmayer and Hobbs, 2004, Suckling, 1984, van der Ree, 2002). In contrast, the koala (Phascolarctos cinereus) does not require tree hollows for breeding or shelter, often forages in young eucalypt trees, and has been documented as moving over long distances despite their energetic constraints (Kavanagh et al., 2007, Woodward et al., 2008).

In a study by Kavanagh and Stanton (2012), young eucalypt plantations (of Eucalyptus camaldulensis, E. crebra, E. sideroxylon, E. pilligaensis and Corymbia maculata) were used more frequently than expected by koalas based on their availability in the landscape. Occupancy of eucalypt plantations and remnant forest patches was strongly influenced by the proximity of these sites to remnant vegetation (Kavanagh & Stanton, 2012). However, the Australian Koala Foundation (2008) concluded that although blue gum (Eucalyptus globulus) plantations are used by koalas and may provide corridors for movement, they are not preferred over native vegetation (Schlagloth, Mitchell, & Rhodes, 2008). The influence of plantations in a landscape for facilitating koala movements and population increase have not been examined but should be, given that timber production from blue gum plantations is set to become the main agricultural land use in Southern Australia (Lindenmayer & Hobbs, 2004). In far south-west Victoria, there are approximately 80,000 ha of commercial blue gum plantations which occur in the geographic range of koalas (DELWP, 2018b). Much of the remaining land in this region is cleared for pasture, which has reduced and fragmented koala habitat (Lindenmayer and Hobbs, 2004, Department of Environment, Land, Water Planning [DELWP], 2018b). The afforestation of landscapes through the establishment of plantations has the potential to increase habitat for koalas as well as a wide range of other fauna (Kavanagh and Stanton, 2012, Lindenmayer et al., 2002). Whilst an increase in koala populations can be viewed as a positive conservation outcome associated with blue gum plantations, it may also result in management challenges and koala welfare issues associated with plantation harvesting operations (Department of Environment, Land, Water Planning [DELWP], 2018b, Phillips et al., 2014).

Plantations may provide a valuable resource to wildlife by increasing the area of habitat and connectivity, thus facilitating an increase in abundance of a species. An understanding of how a species’ population density varies with site and landscape level characteristics can provide critical information for effective conservation planning and management. By integrating population density data with landscape and climatic information within a Geographic Information System, the abundance of a species can be predicted across large landscapes. These predictive maps may be used for resource management and conservation planning, and to test hypotheses on ecological processes. To this end, this study aimed to model the distribution of koalas in the plantation-dominated region of far south-west Victoria to provide a basis for managing koala populations and their habitat in this region. Our objective was to determine the influence of landscape scale factors (such as plantation cover), climatic variables, and site scale factors (such as habitat type and soil chemistry) on koala population density throughout the region.