Evidence that predator satiation drives reproductive synchrony in the desert masting grass, soft spinifex (Triodia pungens)

Abstract

<p>The predator satiation (PS) hypothesis of masting holds that seeding periodicity among plant populations enhances seed survival by starving granivores during low-output periods and satiating reduced consumer populations during mast (high-output) years. Although well-studied in the northern hemisphere and in New Zealand, the PS hypothesis has received scant research attention in Australia. This is despite vegetation communities across nearly half the continent being dominated by masting species subject to high levels of seed predation (e.g. mulga [<i>Acacia aneura</i>] shrublands and spinifex [<i>Triodia</i> spp.] grasslands). Here, phenological monitoring and experimental testing of the PS hypothesis in the desert masting grass <i>Triodia pungens</i> (soft spinifex) occurred over 10 years at Deep Well station, central Australia. Field evidence supported the hypothesis that masting enhances seed escape from predators, as mean proportion seed survival at experimental bait stations was significantly greater during mast years (0.38 in 2012 and 0.33 in 2017) than during inter-mast years (0.01 in 2016 and 2018). The possibility that seed decay as opposed to consumption affected results was controlled for by a deterioration experiment that indicated no significant loss of seed viability after 6-months burial. The results support the hypothesis that masting in <i>T. pungens</i> is an adaptive trait related to the satiation of seed predators. However, whether masting enhances seed escape because predators are satiated at higher seed densities (i.e. masting causes a predator functional response) and/or because frequent low-output periods reduce predator populations and make satiation more likely (i.e. a numerical predator masting response), remains unresolved. Aside from <i>T. pungens</i>, there are numerous other plants of the Australian arid zone that experience high levels of granivory and display masting. Seed predation may be a more important selective force acting on arid Australian plant reproduction than previously thought.</p>