Will tropical food webs collapse without the survival of the fig wasps?

Author: Birgitt Keinanen, Edited by: Dr. Priya Roy

Fig trees are a keystone species in many tropical forest ecosystems. There are 755 species of figs (Ficus) identified to be in existence. Although we think of figs as a food source for humans, dating back to biblical times, they play a bigger role in the functioning of the tropical forest ecosystems. More than 1200 species of vertebrates eat figs, and for many types of frugivores (fruit eaters), it is their staple food. These animals include fruit bats, monkeys, langurs, pigeons, hornbills, fig parrots and many invertebrates like the citrus long-horned beetle and an assortment of caterpillars [4]. The significance of figs as a major food source for so many tropical organisms make them an ecologically importance food resource [2,5]. Like the earlier flowering of plants in the spring that has caused a misalignment of wild flowers to their pollinator insects, global warming temperatures may also have an effect on the pollination of fig trees and on the life of fig wasps (see Figure 1).

Learn more about the cool life cycle of Fig trees here.

Figure 1. Fig Wasp (Blastophaga psenes) credit: Alchetron.com

Important Terms:

• Keystone Species: A species that has a disproportionate effect on its habitats relative to its abundance. More on the role of Keystone species here.

• Pollinator: Any agent that carries the pollen and help in plant reproduction. More on the types of pollinator here.

• Coevolution: Closely associated species influencing each other’s evolution. Recent papers on coevolution can be found here.

• Mutualism: Synonym to Symbiosis where the relationship is beneficial to both organisms involved. Examples of mutualism can be found here.

• Co-extinction: Loss of one species leads to the loss or decline of any and all dependent species. More on coextinction here.

• Specialist & Generalist Species: Generalist organisms can feed on a variety of food items whereas specialist are limited to a narrow choice of food. Find examples of each here.

The uniqueness of the Fig Fruit

Figure 2. Wild Fig fruit credit: Singapore Plants

It’s not unusual for a flowering plant to require a pollinator. However, in the case of the fig fruit (see Figure 2.), the pollination of the fig flower is very specialized. Fig fruits are actually an outgrowth of the stem inside of which the flowers face inward and are thus not readily accessible to pollinators. In the case of nearly all species of figs (with the exception of the domestic fig, which forms fruit without fertilization) the critical pollinator is the fig wasp [1].

The fig wasp

All fig wasps go from egg to adult inside the fig fruit. The fig wasp larvae feed on the ripening fruit’s seeds. The mature male, which has no wings, fertilizes the mature female wasp within the fig fruit and dies young. The female wasp then leaves the fruit through a tiny hole at the tip of the fig known as an ostiole (see figure 3.). She carries with her the pollen of the male flower on her body. She is then attracted to a young fig fruit by the fragrance of female flowers within, which only female fig wasps can sense. As she enters through the ostiole in the new fig she sheds her wings and antennae. Inside she inadvertently spreads the pollen that she has on her body among the female flowers. She then lays her eggs and dies. Another generation of fig wasps matures over a few weeks and the cycle begins again.

Figure 3. Fig Inside-out Inflorescence Credit: www2.palomar.edu

Learn more about the Fig wasps and its life-cycle here.

Bound together…

The relationship between the fig tree and the fig wasp is considered obligate mutualism that derived from coevolution [1]. This means that the pollination of the fig tree depends solely on the fig wasp and the fig wasp depends absolutely on the fig tree. There would be no fig fruit without fig wasps to pollinate the flowers. Without fruit, there would be no more fig trees. Without trees, there would be no more fig seeds for the fig wasps to feed on. The two species are absolutely dependent on each other for existence.

What’s the problem?

The fig wasps are extremely tiny insects (1–2 mm) with a short life cycle (1–2 days) which makes them more susceptible to climate change than their larger and longer- lived host trees. With the onset of warming global temperatures, scientists are interested in what will happen to the fig wasps. A study by Jevanandam et al. (2013) (3) using wasps from Singapore, examined the thermal tolerances of four different unrelated species of adult female fig wasps. They found that in all four species, survival rates decreased consistently with increasing temperatures. This study also found that with an increase of 3°C above the normal current temperatures there would be a dramatic decrease in the adult lifespan of the female. The lifespan of female fig wasps was only two hours in all four species, compared to their current range of 10-24 hours (see Figure 4). This shortened life span could decrease pollination rates of the fig trees because the female wasp would not be able to travel as far and might have a smaller window of time to find a new fruit home. This would decrease the pollination of the fig flower and egg deposit sites. 

Another study by Patiño et al. (1994) (5) revealed that the varied fruit sizes (5mm to 50 mm in diameter) in 11 different species of Panamanian fig trees affects the transpiration. The larger the fruit the hotter the internal temperature. When the fruit were placed in conditions of 3°C above ambient the larger fruit were lethal to its pollinators inside. A temperature rise of this magnitude may negatively affect the larger fruited species of fig trees.

Figure 4. Life Span of Fig Wasps duration verses Temperature (Jevanandum et al., 2013)

Figure 5. Grey hornbill feeding on figs in India. Image by Anil Mahajan via Wikimedia Commons

Double Whammy!

This could be very bad news. This specialized pollination relationship between the fig tree and fig wasp is projected to be even more vulnerable with climate change. Fig trees (see figure 5.) unlike fig wasps are long lived. The different physiologies of the fig tree and the wasp will make them respond very differently to the warming temperatures. While fig trees can survive for years without reproduction fig wasps cannot. Rising temperatures could eventually disrupt their mutualistic relationship to the point that it might result in co-extinction [4]. Fig trees in the tropics are the center of a complex food web of a large range of specialist and generalist animals. Unless fig wasps adapt to the climate change there could be an upset in the multiple trophic levels in the tropical regions of the world.

Citations:

1. Anstett, M. C., Hossaert-McKey, M., & Kjellberg, F. (1997). Figs and fig pollinators: evolutionary conflicts in a coevoled mutualism. Trends in Ecology & Evolution, 12(3), 94-99.

2. Cheek M., & Jebb, M. (2001). Flora Malesiana. Series I, Seed plants. Volume 15: Nepenthaceae. Flora Malesiana. Series I, Seed plants. Volume 15: Nepenthaceae.

3. Jevanandam, N., Goh, A. G., & Corlett, R. T. (2013). Climate warming and the potential extinction of fig wasps, the obligate pollinators of figs. Biology Letters, 9(3), 20130041.

4. Koh, L. P., Dunn, R. R., Sodhi, N. S., Colwell, R. K., Proctor, H. C., & Smith, V. S. (2004). Species coextinctions and the biodiversity crisis. science, 305(5690), 1632-1634.

5. Patino, S., Herre, E. A., & Tyree, M. T. (1994). Physiological determinants of Ficus fruit temperature and implications for survival of pollinator wasp species: comparative physiology through an energy budget approach. Oecologia, 100(1-2), 13-20.

6. Shanahan, M., So, S., Gompton, S. G., & Gorlett, R. (2001). Fig‐eating by vertebrate frugivores: a global review. Biological Reviews, 76(4), 529-572.