A 12-year phenological record of fruiting: Implications for frugivore populations and indicators of climate change

Answering the fundamental ecological question of what determines animal abundance has become critical with the accelerating need for informed management plans for endangered species. A major difficulty in testing general hypotheses to account for variation in abundance is that periods of food scarcity, which may be responsible for limiting population size, occur on a superannual basis. Research on folivorous primates suggests that periods of food scarcity are critical in determining regional biomass; however, studies of frugivores have found no single fallback food generally used by all species. In this study we quantify fruit availability during a 12-year period in Kibale National Park, Uganda to determine patterns of fruit scarcity. Over these 12 years, temporal variability in fruit availability was high; the proportion of trees per month with ripe fruit varied from 0.14 to 15.93%. In addition, there was dramatic interannual variation in fruit availability: in 1990, on average only 1.09% of trees bore ripe fruit each month, while in 1999 an average of 6.67% of trees bore fruit each month. Over the past 12 years, fruit has become more available, fruit-scarce months have declined in frequency, and the duration of periods of fruit scarcity has decreased. If figs (Ficus spp.) served as a fallback food resource over these 12 years, they would have had to be available during months when few trees were fruiting. Over this 149-month period, there were 34 months when less than 1% of monitored trees fruited. Figs were not fruiting in 17 of these months, and, in only 11 of the 34 months were more than 1% of the fig trees fruiting. Rainfall data collected since 1903 indicates that the region is becoming moister, and droughts are less frequent. There has also been a significant increase in the maximum mean monthly temperature and a decrease in the minimum mean monthly temperature since we started recording these data in 1990.