In nature there exists many delicate relationships that are crucial to a species’ survival. Particularly exceptional is the essential bond between African fig trees (Ficus sycomorus L.) and a very small fig wasp (Ceratosolen arabicus). Of the roughly 800 unique species of fig trees alive today, each relies, exclusively, on just one or a few host specific wasps for pollination (Heere et al. 2008).
In order to understand just how delicate this relationship is, it is helpful to examine the unique life cycle of the fig wasp. These small, slow moving wasps have only 48 hours of life to carry out their inherited duties. As explained by DeHaan, wasps are born within a cavity of the fig tree’s fruit where they must find a mate and tunnel out of the fig. Upon the female’s escape, she collects pollen from the fig, which she then transports with her to another fig tree of the same species. Once a tree is found, she burrows into another fig cavity, thereby pollinating the fruit, and lays her eggs. This fruit will be the female’s final resting place as the process of entering the fig usually removes her wings, rendering her unable to escape. This process would appear to be a daunting task in African landscapes plagued, by deforestation where fig trees are often highly isolated and appear in very low densities.
This interesting relationship sparked a question in PhD student Sofia Ahmed from the University of Leeds, United Kingdom. In African areas, with a fair number of very isolated trees just, how far are these wasps able to travel in order to reach new host trees? And perhaps more importantly, do factors contributing to low population densities, like deforestation, pose a significant threat to a fig tree’s ability to pollinate?
Ahmed and her team mapped the fig tree population starting at the base of the Ugab River (Namibia) going 240 km inland. 79 different trees in the area were mapped by GPS and leaf and seed samples were collected from each. The collected seeds were germinated in a lab and subjected to paternity analysis. These results were then compared to DNA extracted from the leaf samples in order to determine the father trees.
As stated by Ahmed et al. (2009) tracking the movement of fig wasps via their pollen cargo would reveal both the distance and direction of travel. The results found were very intriguing. The distances between assigned parents ranged from 14.2 km to 164.7 km with a mean separation of 88.6 km. Similarly, all fathers were located east of the study area. Both of these observations lead to the conclusion that the wasps were riding the westerly prevailing wind currents; currents that were capable of helping wasps travel well over 100 km in a single night.
An additional 14 unknown fathers were needed to account for all of the fig tree offspring. These fathers were trees outside of the tested area and therefore, were potentially, located even farther away than the farthest trees in the test area; 164.7 km inland.
Interestingly, it was also noted that crop size was independent of a tree’s isolation. The first tree tested was perhaps the most isolated, with the closest tree being 81.6 km away, however it had a large, healthy crop of mature figs. The large magnitudes of both the tree separation and the average distance traveled by wasps (88.6 km) suggest that pollination does not usually occur between close proximity trees. This increases genetic diversity within populations and gene flow between populations.
The Ugab region experiences a change in direction of prevailing winds during the summer season. During these months, predominately easterly currents exist during the nighttime. This change led Ahmed et al. (2009) to predict “a seasonal reversal in the direction of pollen flow between trees.” Therefore, trees isolated east of the majority of the population would be pollinated as well.
The results of this study provide great insight into the endurance of African fig trees. The trees’ unique pollination system has rendered them largely resistant to many threats that can cripple species, not only Namibia but all over the world. It can be inferred that habitat loss and population defragmentation due to deforestation and other human activities do not pose a significant threat to fig tree populations. The African fig tree and its wasp have become very accustomed to a long-distance relationship.
Based on article
Ahmed, Sofia, et al. "Wind-borne insects mediate directional pollen transfer between desert fig trees 160 kilometers apart." Proceeding of the National Academy of Sciences 106.48 (2009): 20342-20347.
Other Resources
Dehaan, Mike. The life cycle of the fig wasp. 13 October 2010
Heere, Edward Allen, K. Charlotte Jander and Carlos Alberto Machado. "Evolutionary ecology of figs and their associates: Recent progress and outstanding puzzles." The Annual Review of Ecology, Evolution, and Systematics (2008): 439-458.
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