Rephotography, permanent plots and remote sensing data provide varying insights on vegetation change on subantarctic Macquarie Island, 1980–2015

Abstract

Despite its remoteness, the vegetation of Subantarctic Macquarie Island is amongst the most thoroughly surveyed and monitored in Australia. Eradication of invasive European rabbits was achieved in 2011, after a decline in introduced rabbit populations in the eighties and nineties and a resurgence in the early twentyfirst century. The impacts of introduced rabbits and climate change on the vegetation of Macquarie Island are well-documented from long-term plots situated in some, but not all, of the vegetation types. Satellite data are sporadic, with substantial could-free coverage rare. The island has been well-photographed over many years, and several series of long-term photo-monitoring sites have been established over the past few decades. We compared insights on vegetation change derived from repeat landscape photography three years after myxoma virus was established, rabbit population resurgence and rabbits eradication, with plant species cover data from a set of long-term monitoring sites, and with spectral vegetation indices derived from satellite imagery covering the transition from high rabbit numbers to no rabbits. The repeat photography confirmed many well-documented vegetation trends, as well as providing an indication of previously unreported trends: stability of Acaena herbfield despite reduced disturbance; increase in cover of Pleurophyllum hookeri; increase in short grassland vegetation; and constancy in cover of Azorella macquariensis. Although subject to errors related to differences in sample size and resolution, the rephotography data gave the best information on spatial and temporal patterns of overall vegetation change. The plot data, where comparable, were not fully consistent with the rephotography data and are expensive to collect, requiring scientific expertise in the field and much time, while the sporadically available remote sensing data were effective in monitoring the revegetation/devegetation of ground and the recovery of Poa foliosa but we were unable to detect other changes.

Long-term spatially-replicated data show no physical cost to a benefactor species in a facilitative plant-plant interaction

“…However, these types of responses are not captured by the snapshot approach because snapshot approaches show the potential influence of beneficiaries on their benefactors at a specific point in time. In contrast, a repeated-measures approach is able to demonstrate the long-term effects of prevailing plant-plant interaction accumulated over time and provide insights into vegetation dynamics (Schöb et al 2012;Fitzgerald et al 2021). However, a greater understanding of the context-dependency of the long-term impacts of beneficiary feedback effects, as well as the generality of our findings, is needed (see also Schöb et al 2014a).…”

“…Therefore, studies describing the long-term impact of biotic interactions in systems where facilitation is prominent are lacking (but see Armas and Pugnaire 2005, Miriti 2006, García et al 2016). The approach of monitoring the same species and/or individuals using long-term datasets (i.e., through repeated measurements of the same interacting individuals, e.g., repeat photography) is an effective method for examining changes in community structure and composition (Magurran et al 2010, Fitzgerald et al 2021). However, there are limitations associated with these types of correlative methods, including their inability to demonstrate causality and to provide a mechanistic understanding of the link between abiotic conditions and changes in the outcome of plant-plant interactions (Metz and Tielbörger 2016).…”

High canopy cover of invasive Acer negundo L. affects ground vegetation taxonomic richness

“…We used the term ‘species richness’ when evaluating alpha diversity. The community composition is sometimes analyzed from photos to solve particular problems 59 , 60 .…”

Determining the distributions of plant communities in subantarctic vegetation using species distribution models

“…Feral rabbits (Oryctolagus cuniculus) have had a profound effect on the vegetation of Macquarie Island by intense grazing of select plant species, promoting vegetation dominated by less palatable or more resilient plants (Scott and Kirkpatrick 2013;Whinam et al 2014). There are signs of vegetation changes following eradication of rabbits in 2011 (Shaw et al 2011;Williams et al 2016;Fitzgerald et al 2021).…”