Quantifying the Benefits of Mast Seeding on Predator Satiation and Wind Pollination in Chionochloa pallens (Poaceae)

Abstract

Mast seeding could be favoured by a number of factors, including predator satiation and wind pollination. When both these factors could be involved, determining which is more important has been very difficult. In this paper we present 10 years of data on flowering intensity, seed predation and estimated pollination success for Chionochloa pallens (Poaceae), an alpine tussock grass, at Mt Hutt, South Island, New Zealand. C. pallens suffers from high levels of pre-dispersal seed predation (up to 94%) and is wind pollinated, so provides an ideal experimental system. C. pallens had very variable flowering intensity between years (coefficient of variation of floret production (CV)= 1.80). A significant predator satiation effect was found. Predation was not affected by the absolute level of flowering intensity in the current year, but did vary significantly with the change in flowering intensity between the current and previous years. Therefore, predator satiation seemed to involve a numerical response (starving the predator in low seed years). There was a significant wind pollination benefit from larger flowering efforts, which depended only on the size of the current flowering effort. Therefore, for benefits via pollination, C. pallens populations with high levels of resources can satisfactorily produce a large flowering effort every year; in contrast, to gain benefits via predator satiation, fluctuating flowering intensity is required regardless of mean levels. The relative benefits of masting versus constant reproduction were modelled; benefits from wind pollination increased very little with increased CVs, to give a benefit of 2.3% fewer florets with unexpanded ovaries at the field CV of 1.8 than at CV = 0. The overall mean flowering effort is apparently high enough in the study population for efficient pollination even if masting did not occur. Predator satiation gave large benefits from masting, from 8% of all florets at Cv = 1.25 increasing rapidly to 42% at CV = 1.8 and 62% at CV = 2.25. Therefore predator satiation is vastly more important than wind pollination in favouring mast seeding in C. pallens, providing 92-95% of the benefits at CVs>1.5. The rapid increase in benefits from predator satiation at CV > 1.5 may help explain why Chionochloa species have such high CVs.