Effects of experimental acidification and liming on terrestrial invertebrates: implications for calcium availability to vertebrates

Recent indications are that acidification might affect terrestrial vertebrates by reducing the availability of calcium-rich invertebrate prey, but experimental data are scarce. We therefore compared the abundances and chemical composition of invertebrates in replicate catchments in base-poor areas of Norway and Wales that have been experimentally acidified (n=2), deacidified by liming (n=3), and kept as references (n=2+3). Six and five years after acidification and liming, significant treatment effects were confined to reduced numbers of spiders and Hemiptera in limed catchments. Calcium-rich invertebrates such as Isopoda were scarce at all sites, and unaffected by either treatment. There were no differences between reference and treatment catchments in calcium (0.7–1.7 mg Ca g−1 dry mass), phosphorus (6–9.4 mg P g−1), aluminium (0.1–0.35 mg Al g−1) or magnesium (0.9–2.4 mg Mg g−1) content in bulked invertebrate samples, nor in ants, spiders or larval Lepidoptera. In the acidification experiments, none of these elements differed between the vegetation of treated and reference areas. These data provide no support for effects by upland acidification on calcium availability to insectivorous terrestrial vertebrates, indicating that calcium might be as scarce in arthropods in naturally base-poor locations as in some areas affected by acid deposition. Our data on prey chemistry, coupled with calculated mineral budgets for a typical bird, suggest that any effects by acid deposition are liable to be greater if they involve losses of calcium-rich prey rather than change in prey chemistry; better data are therefore needed on the effects of acidification on groups such as isopods and molluscs. Vertebrate calcium budgets, calcium sources and breeding performance in both acidified and naturally base-poor locations require much further investigation.