Climate warming and the potential extinction of fig wasps, the obligate pollinators of figs

4. Discussion

Much of lowland southeast Asia currently experiences mean daily temperatures of around 27°C and daily maxima in the 30–36°C range [11]. The IPCC's report [12] projects a regional warming of 2.2–3.0°C (25–75% quartile values from 21 models) for the moderate A1B greenhouse gas scenario by 2080–2099 and approximately 2.6–3.5°C for the A2 scenario. An increase in mean temperature of 3.0°C would result in a decrease in fig wasp LD in three of the four species with C. c. hewitti displaying the largest decrease at 25 per cent (figure 1a). However, mean daily temperatures are averaged over 24 h and daytime temperatures are more relevant for day-dispersing wasps that live less than 24 h. The mean daytime temperature (7.00–19.00) of Singapore for 2010 was 29°C [13] and an increase in 3.0°C, which is expected by 2100 [12], would reduce the LD of day-dispersing V. malayana, C. c. hewitti, C. appendiculatus and E. verticillata by 33–60% (figure 1a). Night-dispersing species will experience lower absolute temperatures, but a similar degree of warming [14].

Reduced LD and increased MR for fig wasps, some of which disperse tens of kilometres to receptive host figs [15], would mean that wasps would have less time to locate a receptive host syconium and pollinate and/or oviposit in the hundreds of florets within it [16]. The potential consequences include: (i) declines in the populations of fig wasps; (ii) reduced pollination services leading to fig population declines; (iii) reduced fig populations reducing food abundance for frugivores, resulting in secondary population declines. Besides supporting a huge range of vertebrates, a single host fig can support up to 30, more or less host-specific, non-pollinating fig wasp species [17]. Sustained collapses in the fig–fig wasp mutualism could thus affect multiple trophic levels.

Pollinator species with a wide latitudinal range will have been exposed to higher thermal extremes over their evolutionary history than those that occur only in the equatorial lowlands, and might be expected to have a higher degree of thermal tolerance [10]. Indeed, the thermal tolerances of the four species of fig wasp studied here appear to correspond to the ranges of their host figs, with the least thermally tolerant, V. malayana (F. grossularioides), restricted to the aseasonal tropics, and the most thermally tolerant, E. verticillata (F. microcarpa) and C. appendiculatus (F. variegata), ranging from Queensland, Australia to south Japan [4]. Ceratosolen c. hewitti (F. fistulosa) has an intermediate range. Rainfall and humidity may also influence wasp survival ([16,18] but see [19]), but the humidity treatments in this study did not affect LD and MR significantly.

Developing fig syconia are able to use evaporative cooling to avoid temperatures that would be lethal to fig wasp larvae [20]. Even within the same tree, however, syconia are exposed to different microclimates, whereas wasps from a single syconium may have several mothers. Individual wasps may, therefore, vary in both their opportunities for temperature acclimation during development and in the genetic component of thermal tolerance. We did not assess either in this study, but the significant variation in thermal tolerance among wasp broods from the same host tree suggests that one or both may be significant. The literature suggests that the ability of insects to acclimate to higher temperatures may be limited [21], but the short generation times of fig wasps (approx. one month) would favour rapid evolution if heritable genetic variation in heat tolerance exists. It is also important to note that we assessed only the ability of wasps to move. Both pollination and oviposition are complex behaviours in fig wasps and their success may be more sensitive to warming. Moreover, in a Neotropical pseudoscorpion a 3.5°C warming had only a small impact on survival but rendered the females sterile [22].

The three genera of fig wasps we tested represent three widely divergent clades [23], suggesting that our results can be generalized to other fig wasps. This implies that fig populations, not just in Singapore but in the entire aseasonal tropics, could be extremely vulnerable to anthropogenic global warming. Because of their ecological importance, any loss of fig species or reduction in their abundance would be of major conservation concern. We hope this study will encourage more research.