Vertebrate Responses to Spatiotemporal Variation in Seed Production of Mast-Fruiting Dipterocarpaceae

Abstract

To examine the interspecific reproductive synchrony of Dipterocarpaceae with vertebrate responses to seed availablity, we monitored the spatiotemporal distribution and phenology of more than 2367 adult dipterocarp individuals of 54 species from March 1985 to January 1993 in the Gunung Palung National Park, West Kalimantan, Indonesia. Seven vegetational formations were sampled along an altitudinal gradient from peat swamp forest (5 m a.s.l. [above sea level]) across lowlands to upper montane zones (1100 m a.s.l.) that encompassed two upland valley complexes in a 15-km2 area. Four significant reproductive events were documented: (1) a common lowland species reproduced outside of mast events in 1986 and in 1990; (2) a localized lowland “minor” mast event in 1986 in which 24.3% of the adult trees (21 spp.) participated; (3) a major community-wide mast event (92.8%, 48 spp.) in 1987, just 6 mo after the minor event; and (4) another major community-wide mast fruiting event after a 4-yr intermast interval (88%, 48 spp.) in 1991. West Kalimantan export records of illipe nut (Dipterocarpaceae: Shorea section Pachycarpae) from 1968 to 1997 were compiled as a baseline measure of the frequency and relative intensity of dipterocarp mast-fruiting events in the region (cv = 152%). A “bumper crop” occurred about every 5 ± 2.6 yr (mean ± 1 sd; range 3–9 yr). Fruit production was significantly associated with El Niño–Southern Oscillation (ENSO) events. The 1987 and 1991 mast events monitored were the third and fourth largest export years in the province since 1968. Because of the disputed role of vertebrate seed predators in causing and maintaining mast-fruiting behavior, the response of seed-eating vertebrates to this spatiotemporal variation in dipterocarp seed production was examined. Seed of a common, but asynchronous, lowland species was largely consumed by vertebrates. In the 1986 minor mast, 21 dipterocarp species that produced 60 000 seeds/ha (dry mass 46 kg/ha) lost all monitored viable seed to a diversity of resident and nomadic vertebrate seed predators. Timed with dipterocarp seed production in all mast events, nomadic vertebrates increased their populations through both reproduction and regional movement (numerical response). However, in both the 1987 and 1991 mast events, resident vertebrates destroyed only 1.5% and 2.6% of community seed production, and predation was recorded only in the tails of the fruit-fall distribution. During these community mast events, resident vertebrates switched from dipterocarp seed to feed on fruit and seed from other available species. Nomadic vertebrates arrived late in the fruit-fall period during both major mast events and, thus, were able to destroy only seed dispersed in the final 1–3 wk of fruit-fall. Seed escape, and thus regeneration, only occurred in major mast events when all dipterocarp species across large areas participated. Considerable seedling recruitment was recorded in both the 1987 (∼95 000 seedlings/ha) and 1991 (155 824 ± 36 764 seedlings/ha) mast events. Results from this natural experiment indicate that resident and nomadic vertebrates cause stabilizing selection for tight interspecific synchrony of dipterocarp seedfall over vast regions. Although the pattern generally conformed to the predator-satiation hypothesis, the observed mechanisms for seed escape within a mast-fruiting event did not. Resident and nomadic vertebrate foraging and ranging patterns resulted in dipterocarp seed “escape” rather than local “swamping” with copious seed production per se. To account for these observations, two hypotheses, “interfamilial satiation” and “regional escape,” were developed. The period of seed availability and length of the regional intermast interval is examined with seed-predator reproduction and behavior to assess the impact of seed availability on seed-predator populations. The importance of rare events and appropriate spatial scale for investigating evolutionary and ecological processes such as mast fruiting are addressed.