BIODIVERSITY RESEARCH: 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.

Demography and population ecology of the Hadeda Ibis (Bostrychia hagedash) at its expanding range edge in South Africa

“…On the other hand, breeding success was on average high and variable for a bird of this size. This trait allows Hadedas to take advantage of favourable conditions, like irrigated fields and lawns throughout the Western Cape, which supply them with a year-round food source (Duckworth et al 2010). This may allow them to breed successfully, often more than once per year, which in turn allows them to persist and expand (Macdonald et al 1986).…”

“…Perhaps the ability to quickly take advantage of good conditions, a trait of a bet-hedging life strategy, makes the family prone to expanding their ranges, given the right conditions. For Hadedas, improved foraging success on irrigated land (Duckworth et al 2010) likely resulted in higher breeding success and population growth. Our study provides a baseline against which demographic studies at the core of the range can be compared.…”

“…Lack (1968 proposed that levels of food supply during the breeding season limit reproductive output. The many artificially irrigated fields and lawns throughout the study area may provide Hadedas with a constant source of food (Duckworth et al 2010), and, as a result, food appears not to be limiting reproduction, and hence Hadedas are able to reproduce year-round.…”

“…Artificial watering seems especially important, because this allows Hadedas to persist in areas where they otherwise would not be able to do so (Duckworth et al 2010).…”

Interrogating recent range changes in South African birds: confounding signals from land use and climate change present a challenge for attribution

“…, 1986; Okes et al. , 2008; Duckworth et al. , 2010) to afforestation and the spread of alien tree species (which has had the same effect –Macdonald, 1986; Curtis et al.…”