Effectiveness of vegetation‐based biodiversity offset metrics as surrogates for ants

Abstract

Biodiversity offset schemes are globally popular policy tools for balancing the competing demands of conservation and development. Trading currencies for losses and gains in biodiversity value at development and credit sites are usually based on several vegetation attributes combined to yield a simple score (multimetric), but the score is rarely validated prior to implementation. Inaccurate biodiversity trading currencies are likely to accelerate global biodiversity loss through unrepresentative trades of losses and gains. We tested a model vegetation multimetric (i.e., vegetation structural and compositional attributes) typical of offset trading currencies to determine whether it represented measurable components of compositional and functional biodiversity. Study sites were located in remnant patches of a critically endangered ecological community in western Sydney, Australia, an area representative of global conflicts between conservation and expanding urban development. We sampled ant fauna composition with pitfall traps and enumerated removal by ants of native plant seeds from artificial seed containers (seed depots). Ants are an excellent model taxon because they are strongly associated with habitat complexity, respond rapidly to environmental change, and are functionally important at many trophic levels. The vegetation multimetric did not predict differences in ant community composition or seed removal, despite underlying assumptions that biodiversity trading currencies used in offset schemes represent all components of a site's biodiversity value. This suggests that vegetation multimetrics are inadequate surrogates for total biodiversity value. These findings highlight the urgent need to refine existing offsetting multimetrics to ensure they meet underlying assumptions of surrogacy. Despite the best intentions, offset schemes will never achieve their goal of no net loss of biodiversity values if trades are based on metrics unrepresentative of total biodiversity.

Quantifying the impact of vegetation‐based metrics on species persistence when choosing offsets for habitat destruction

“…Reliance on habitat-and vegetation-based offsetting metrics (Gibbons et al 2018) can be problematic when such metrics do not strongly correlate with the ecological features that an offsetting program seeks to conserve (Kujala et al 2015). Often, habitat attributes and vegetation-based surrogates fail to capture the extent of biodiversity that is claimed (Cristescu et al 2013;Hanford et al 2016). Moreover, current offsetting metrics are likely to result in undervaluation of degraded or smaller patches, even when these are of high ecological importance (Wintle et al 2019).…”

Vascular plant species richness and bioindication predict multi‐taxon species richness

“…Despite their wide use, the evidence for using plants as surrogates for multi‐taxon biodiversity is equivocal (Myšák & Horsák, ; Sætersdal et al., ; Westgate, Barton, Lane, & Lindenmayer, ; Wolters et al., ). Complex metrics representing habitat quality based on weighted measures of vegetation structure (e.g., native plant species richness, number of trees with hollows, and total length of fallen logs), plant species richness and functional diversity, have also been suggested to work as surrogates for overall biodiversity, but with limited success (Hanford, Crowther, & Hochuli, ; Kwok, Eldridge, & Oliver, ). Despite the moderate support, plant‐based monitoring programmes and conservation guidelines remain a common practice, even at supranational levels.…”

Planning biodiversity offsets - Twelve operationally important decisions

“…Specialist species are also more often threatened than common species (Kotiaho et al 2005;Mattila et al 2006;. Hanford et al (2017) studied how well a simplified (one-dimensional) biodiversity metric covered species occurring in the area. They arrived at the conclusion that the metric did not work.…”