Seed production in Festuca novae-zelanidiae: the effect of altitude and pre-dispersal predation

Festuca novae-zelandiae is a New Zealand endemic bunchgrass that occurs fro m near sealevel to 1400 m. In a study of seed production at a range of alti tudes, flowering plants were found to produce between 0 and 967 seeds. On a verage, 28.3% of florets were attacked by invertebrate seed predators and a further 46.2% of ovules failed to produce a mature seed for reasons other than predation. The most common, identifiable, predators were Dipterid larv ae, including the flightless Diplotoxa moorei. Seed set was most strongly r elated to the number of florets initiated, but was also significantly relat ed to rates of ovule failure and pre-dispersal predation. Seed set did not vary systematically with altitude, but ovule failure rate showed a signific ant increase and predation rate a significant decrease with increasing alti tude. Predation rate was also positively related to plant density; altitude and plant density together accounted for 56% of variation in predation rat e. Path analysis showed that the near-zero direct correlation between seed set and altitude was the product of an indirect positive effect, via a stro ng negative effect on predation rate, being cancelled out by indirect negat ive effects via reduced floret initiation and increased ovule failure rates with increasing altitude.