Higher-taxon and functional group responses of ant and bird assemblages to livestock grazing: A test of an explicit surrogate concept

Highlights

  • Surrogates of biodiversity responses to ecological treatments require explicit validation.
  • Different sub-components of biotic assemblages can respond to treatments in ways that differ from overall species richness.
  • Simple correlation between surrogate and target variables might not provide inference of matched response to a treatment.

Biodiversity monitoring programs are routinely established to quantify changes in biotic communities in response to land management. Surrogacy is implicitly used in many such monitoring programs whereby the measurement of a component of biodiversity is used to infer responses of broader biodiversity. Yet rarely is this surrogacy validated by demonstrating that measured variables and the target variable of interest have matching responses to management treatments. Here we examined the responses of higher-taxon and functional groupings of ants and birds (our surrogate variables) two years after the implementation of experimental livestock grazing treatments, and compared these with the responses of total ant and bird species richness (our target variables) to the same treatments. We found significant and strong correlations between surrogate and target variables, but this did not predict corresponding similar response to treatments. For ants, we found that the genus Monomorium had a negative response to the grazing exclusion treatment, but there was no matching response of species richness, and so no surrogacy was identified. For birds, total species richness had a weak positive response to spring/summer grazing exclusion, and the abundance of honeyeaters (Meliphagidae) showed a similar positive response, suggesting surrogacy. Our study highlights that correlations among variables do not necessarily lead to surrogacy, and indeed that different sub-components of biotic assemblages can respond in ways that contrast with overall species richness. Careful assessment of the matched responses of surrogate and target variables to management can provide a simple and robust way to critically assess biodiversity surrogacy.

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Fig. 1. A surrogate concept that incorporates the relationships between treatment, surrogate, and target, as well as covariates. (a) We quantified the effects of the grazing treatments on a range of candidate surrogate variables, as well as (b) treatment effects of the target for both bird and ant assemblages. Environmental covariates were also considered in separate models. (c) We then examined the correlations between surrogate and target variables to see if this gave any insight into which surrogate and target responses to the treatment were similar.

Barton, P.S., Evans, M.J., Sato, C.F., O’Loughlin, L.S., Foster, C.N., Florance, D. & Lindenmayer, D.B. (2019) Higher-taxon and functional group responses of ant and bird assemblages to livestock grazing: A test of an explicit surrogate concept. Ecological Indicators 96, 458-465 // PDF DOI

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