Non-random patterns of vegetation clearing and potential biases in studies of habitat area effects

Abstract

Context Native vegetation extent is often a proxy for habitat area in studies of human-modified landscapes. However, the loss and retention of native vegetation is rarely random among landscapes. Instead, the extent of native vegetation in landscapes may be correlated with abiotic factors, thereby obscuring or distorting relationships between ecological phenomena and area. Objectives We asked: (1) how has the potential for non-random vegetation loss to confound area effects been addressed in the landscape ecology literature? (2) Are consistent patterns of non-random vegetation loss and retention evident from modified regions of two countries? Methods We reviewed 118 papers that related area to an ecological response, to determine whether potential biases associated with non-random vegetation loss and retention were considered. We then analysed *18,000 100 km 2 landscape units in Australia and South Africa to identify how different abiotic factors correlate with the extent of native vegetation retained in those landscapes. Results Only 21% of the studies we reviewed explicitly or implicitly considered spatial biases in vegetation clearing. Yet, across modified regions of Australia and South Africa, landscape-scale native vegetation extent was consistently and often strongly related to abiotic factors, particularly soil properties and topographic variability. Conclusion Patterns of vegetation clearing and retention commonly reflect underlying abiotic heterogeneity. These biases, which are infrequently highlighted in studies focussing on area effects, have implications for how we assess the importance of vegetation extent for species and assemblages. Failure to account for correlates of vegetation extent risks erroneous area-based conservation prescriptions in human-modified environments.

The vegetation structure and condition of contracting lowland habitat for Leadbeater's possum (Gymnobelideus leadbeateri)

“…Our case study of Leadbeater's possum at Yellingbo exemplifies limitations that often beset research on critically endangered wildlife species, including (1) restricted sample sizes and study locations (in this case, there is only one study location available given a single extant lowland population), (2) data collection that commenced after the population decline commenced, and (3) vegetation data from a single point in time. The last remaining occupied habitat may not be a good guide to a species' realizable niche (Kerley et al 2012;Simmonds et al 2017;Lentini et al 2018), and it is possible that Leadbeater's possums occupied a broader suite of lowland vegetation associations previously, however, appropriate historic baseline data are lacking (e.g. McClenachan et al 2012).…”

All threatened species habitat is important

“…For example, what does the ubiquitous appropriation of productive land for agricultural and urban land use mean for animal populations? For species that show a strong preference for fertile, and thus often highly fragmented environments, small patches (Tulloch et al, 2016) and even scattered trees (Fischer, Stott & Law, 2010) are likely increasingly important (Simmonds, van Rensburg & Maron, 2017). Alternatively, are animals that persist in degraded or sub-optimal habitat the last vestiges of populations suffering extinction debt (McAlpine et al, 2015); such as those with long generation times?…”

Identifying opportunities for private land conservation

“…The historical way in which land has been allocated in Australia in part drives some of the similarities and differences among protected area governance types. A major factor contributing to the diversity and abundance of biodiversity today is the legacy of historical land-use allocation and practices (Foster et al 2003;Simmonds et al 2017). The general similarities we identified between private and government protected areas and differences between Indigenous protect areas may also be attributed to the constraints and opportunities for reservation.…”