Soil moisture limits foraging: A possible mechanism for the range dynamics of the hadeda ibis in southern Africa

Abstract

Aim: Climate limits the ranges of many animals, but the mechanism whereby it does so remains poorly understood. One explanation is that climate (e.g. temperature or rainfall) affects energy expenditure, eventually limiting where a species can occur. We propose that climate can also affect energy uptake through its effect on foraging efficiency. We examined this idea for the case of the hadeda ibis (Bostrychia hagedash) which has considerably expanded its range in southern Africa over the past 80 years. Hadedas forage mainly by extracting earthworms and other invertebrates from soft soil. Soil moisture, in the absence of irrigation largely determined by climate and soil composition, may therefore be a factor limiting feeding efficiency in hadedas. Location: We tested this hypothesis by observing foraging hadedas in Cape Town, South Africa. Results: We found that soil moisture limited the rate at which hadedas caught prey items, with an optimum on relatively moist ground. We further measured the energy content of the hadedas' main prey, earthworms. Using published physiological relationships, we estimated that hadedas need to forage for about 6.3 h to meet their daily energy requirements under optimal soil moisture conditions. On dry soils, they need to forage for >12 h, thus showing that soil moisture has the potential to limit the range of this species. Main conclusions: Hadedas originally only occurred in the wettest parts of South Africa, but gradually colonized drier areas, and are now absent only from the driest parts of the country. Our results support the view that climate (determining soil moisture) originally limited the hadedas range and that irrigation has been an important factor facilitating their range expansion. The hadeda is an example for a species whose range expansion is driven by interactions between climate and land-use change. © 2010 Blackwell Publishing Ltd.