Landscape Ecol (2007) 22:731–745 DOI 10.1007/s10980-006-9068-0 RESEARCH ARTICLE Effect of fragmentation, habitat loss and within-patch habitat characteristics on ant assemblages in semi-arid woodlands of eastern Australia Valerie J. Debuse Æ Judith King Æ Alan P. N. House Received: 27 June 2006 / Accepted: 21 November 2006 / Published online: 28 February 2007 Ó Retained by the State of Queensland 2007 Abstract The reliability of ants as bioindicators patches in landscapes that exhibited a range of of ecosystem condition is dependent on the consis- fragmentation states. Of fragmentation measures, tency of their response to localised habitat charac- changes in patch area and patch edge contrast teristics, which may be modified by larger-scale exerted the greatest influence on species assem- effects of habitat fragmentation and loss. We blages, after accounting for differences in habitat assessed the relative contribution of habitat frag- loss. However, 35% of fragmentation effects on mentation, habitat loss and within-patch habitat species were confounded by the effects of habitat characteristics in determining ant assemblages in characteristics and habitat loss. Within-patch hab- semi-arid woodland in Queensland, Australia. itat characteristics explained more than twice the Species and functional group abundance were amount of species variation attributable to frag- recorded using pitfall traps across 20 woodland mentation and four times the variation explained by habitat loss. The study indicates that within- patch habitat characteristics are the predominant The State of Queensland’s right to retain a non-exclusive, royalty free license in and to any copyright is drivers of ant composition. We suggest that caution acknowledged. should be exercised in interpreting the independent effects of habitat fragmentation and loss on ant & V. J. Debuse ( ) assemblages without jointly considering localised Department of Primary Industries and Fisheries, Horticulture and Forestry Science, Locked Bag 16, habitat attributes and associated joint effects. Gympie, QLD 4570, Australia e-mail: [email protected] Keywords Landscape Á Functional groups Á Variance partitioning V. J. Debuse The Centre for Innovative Conservation Strategies, Griffith University, PMB 50, Gold Coast MC, QLD 9726, Australia Introduction J. King Department of Primary Industries and Fisheries, Habitat fragmentation and loss are critical Horticulture and Forestry Science, 80, Meiers Road, processes influencing species distribution across Indooroopilly, QLD 4068, Australia a landscape. The commonly recognised conse- quences of increased fragmentation on land- A. P. N.House CSIRO Sustainable Ecosystems, 306, Carmody Road, scape structure are increased isolation among St Lucia, QLD 4067, Australia similar patch types and reduction in patch size 123 732 Landscape Ecol (2007) 22:731–745 (MacArthur and Wilson 1967; Andre´n 1994). habitat loss impacts are not misinterpreted as Classically, patch area reduction and increased fragmentation effects (Fahrig 2003). isolation should have detrimental effects on The consistency of species responses to frag- species abundance and richness since patches mentation may be further modified by within- are less likely to be colonised after a local patch habitat conditions. Indeed, the degree of extinction (Brown and Kodric-Brown 1977; natural and anthropogenic disturbance may influ- Hanski 1999), and should hold smaller popula- ence species dynamics to an equal or greater tions that are more vulnerable to environmental extent than larger scale landscape structure (Ross and demographic stochasticity (Fahrig 1997; et al. 2002; Jellinek et al. 2004; Pharo et al. 2004). Foley 1997). These deleterious effects may be Disturbances such as grazing and fire in woodland exacerbated by an associated increase in edge habitats have major impacts on vegetation struc- environments that influences species movement ture (Russell-Smith and Stanton 2002; Bowman patterns across patch boundaries (Bhar and and Prior 2004), plant cover (Hobbs 2001), soil Fahrig 1998). nutrition (Guinto et al. 1999) and soil–water While patch area and isolation are the most infiltration (Hobbs 2001), thereby altering habitat commonly tested measures of fragmentation suitability for a range of fauna (Martin and Green (Laurance and Bierregaard 1997; Fahrig 2003), 2002). Localised habitat effects should be partic- they are unable to consistently predict species ularly influential on populations of smaller, less richness patterns in manipulative fragmenta- vagile species such as many ground-dwelling tion studies (Debinski and Holt 2000). The insects, for which dispersal distances are too low conceptual shift from island biogeography the- for larger-scale landscape structure to exert a ory (MacArthur and Wilson 1967) to models strong influence (Abensperg-Traun et al. 1996). that consider the landscape matrix to be heter- Ants are the most commonly used insect ogeneous and not necessarily hostile to native bioindicators of habitat condition in Australia, species (e.g. habitat variegation concept, owing to their abundance, diversity, sensitivity to McIntyre and Barrett 1992), has led to alterna- disturbance and close relationship with soil tive landscape features such as landscape heter- structure and nutrient cycling (Hoffmann and ogeneity and contrast between adjacent land Andersen 2003; Andersen and Majer 2004). types being increasingly incorporated into frag- Functional groupings of ants that are based mentation studies (McGarigal and Marks 1995; primarily on competitive dynamics and inter-taxa Wiens 1997; Fahrig 2003). Such features are differences in habitat requirements (Andersen important drivers of population dynamics in 1995) have been applied successfully to assess some species (Niemela¨ 2001; Jules and Shahani post-disturbance recovery from fire, mining and 2003), which may override the effects of area grazing (Vanderwoude et al. 1997; Hoffmann and and isolation. Inconsistent effects of area and Andersen 2003). However, the importance of isolation among fragmentation studies also result habitat differences as determinants of ant com- from confounding effects of habitat loss (Andre´n position, and thus the reliability of ants as habitat 1994; Fahrig 2003); once the effects of fragmen- bioindicators, has not been quantified against the tation and loss are separated, habitat loss may independent (pure) effects of habitat loss and exert a stronger and, in some cases, opposite fragmentation and joint (confounded) effects that effect on species dynamics to fragmentation are simultaneously explained by aspects of (Fahrig 2003). Differences in patch isolation habitat fragmentation, loss and within-patch have been shown to reflect changes in habitat characteristics (Cushman and McGarigal 2002). loss more closely than changes in fragmentation To determine this, we devised a simple concep- per se (defined as the breaking apart of habitat tual model that emphasised three major influ- independent of habitat loss; Fahrig 2003). Thus, ences (habitat fragmentation, habitat loss, within- it is important to account for the relationship patch habitat characteristics) and associated joint between the amount of habitat in the landscape effects on ant species assemblages in fragmented and fragmentation characteristics to ensure that landscapes (Fig. 1). 123 Landscape Ecol (2007) 22:731–745 733 Unexplained bioregion has been extensively modified for agricultural development since approximately Habitat VR 1850. In the western region, where we conducted characteristics our study, approximately 36% of native vegeta- VS tion remained in 2003 (Queensland Department of Natural Resources and Water 2003). Vegeta- tion clearing has had a major impact on distribu- V V HF+S HA+S tion and abundance of poplar box woodlands, and VHF+HA+S the condition of remaining poplar box remnants is VHF VHA largely unknown. By understanding the relative Landscape VHF+HA Habitat cover importance of habitat loss, fragmentation and fragmentation within-patch habitat characteristics, we can deter- mine whether ants are viable bioindicators of poplar box woodland condition, or whether ant communities are being driven more strongly by Fig. 1 Conceptual model of major determinants that landscape structure. influence ant assemblages in fragmented landscapes divided into the independent effects of landscape frag- mentation (VHF), habitat cover (VHA) and within–patch Methods habitat characteristics (VS), corresponding joint effects (VHF+HA, VHF+S, VHA+S, VHF+HA+S) and unexplained effects (VR) The study was conducted from March–August 2003 in southern Queensland, Australia. Ants We formulated two hypotheses. First, we were sampled in poplar box remnants across an predicted that differences in habitat loss would area of approximately 4.5 million ha, extending account for a greater proportion of variation in ant west from Miles (150.11 E 26.39 S) to Morven species assemblages among patches than the (147.6 E 26.25 S) and north from Surat (149.4 E effects of habitat fragmentation. This hypothesis 27.9 S) to Injune (148.33 E 25.50 S). Using tested Fahrig’s (2003) theory that habitat loss 1:100,000 ecosystem mapping, 20 patches were exerts a greater influence on species dynamics than selected that were mapped either as poplar box fragmentation once both effects are separated. woodland on alluvial plains or as poplar box Second, we predicted that ant composition should dominant or co-dominant ecosystems. Within the be more strongly influenced by within-patch