by: Nicole Kitner, Ella Martin, Mikayla Salmon-Beitel, Carole-Anne Williams
In the depths of the Costa Rican rainforest, one can find many species of animals, insects, and plants. The interactions of these organisms vary, from the symbiotic relationship between hummingbirds and bromeliads, to the incredibly organized nest of leaf cutter ants, where they tend to their mutualistic relationship with fungi. Out of all the incredible wonders you may find on your journey through the rainforest, there is one species that seems to, quite literally, overtake the rest. The strangler fig trees are unlike most other trees in their environment. Their adaptations to utilize another tree’s resources, their unique relationship with wasps, and interactions with their ecosystem, make them an exceptionally fascinating species that have important impacts on their environment.
The strangler fig tree has certain adaptations to better suit its typical tropical rainforest environment. They are semi-deciduous, and only lose their foliage for short periods of time, usually during the dry season. They have a high drought tolerance, which is an adaptation that proves to be very important, particularly during the summer months when humidity and access to water is decreased (Brown). This tolerance enables survival, despite the less-than-optimal conditions during this period. The strangler fig’s foliage is entire, alternate, dark green, and somewhat leathery, and its leaf’s shape is elliptical to oblong, with cordate, acute, or obtuse bases, and a pointed apex (Brown). The pointed apex of the leaf is optimal in the tropical rainforest, allowing for easy runoff of water. Although the tree has no visible flowers, they are actually hidden within the hollow receptacle, which is called the fig. The pollination of these flowers is dependent on fig wasps, which have a symbiotic relationship with the tree (Prasertong).
The fig-fig wasp relationship is one of the key examples of coevolution and mutualistic symbiosis: each species of fig has evolved alongside a unique species of wasp that is small enough to crawl inside the fig through a tiny opening called the ostiole to lay its eggs inside, although they often may lose their wings or antennae in the process. In exchange, the wasp brings pollen into the fig, which is actually a cluster of seeds and flowers called an inflorescence (not a fruit), fertilizing the ovaries (Kline). Due to competition for a limited number of oviposition sites, the queen may visit three or four different figs to increase reproductive chances, but because she lost her wings they must be within close proximity to each other (Suleman, Nazia, et al.). Once her mission is complete, the queen then dies and is digested by the fig, providing it with nutrients. Once her eggs hatch, the males and females first mate with each other, then the females collect pollen to continue the process, while the males carve a path out of the fig to allow the females’ escape. The males are wingless and will spend their entire life within the fig (Kline).
Although it seems simple, several factors can arise to complicate this relationship. One is the possibility of uncooperative symbionts. In this case, one of the organisms (generally the wasp), fails to uphold its end of the deal (pollinating the fig), and becomes parasitic. If such a situation arises, the host fig tree can provide sanctions that limit the reproductive success of the cheating wasps. However, the tree is only able to target the entire fruit that are not performing, as opposed to individual wasps. Thus, pollen-free wasps are persistent in fig trees, albeit at low levels, as long as there are others within the same fig that can do their job for them (Jandér, et al.).
Another complication is specialized parasitic wasps that can lay their eggs within the fig from the outside using a long ovipositor, and that do not pollinate the fig in return. This leads to increased competition for oviposition sites, and possibly a lack of sites for the wasps that are pollinating. Something that reduces this effect is the presence of some ant species, which live on the fig tree and are able to move rapidly to catch flying insects, and actually patrol the fig to prey on the parasitic wasps. They have been shown to significantly reduce the wasps’ negative effect on the mutualism, and allow this relationship to continue (Schatz, Bertrand, et al.).
Strangler figs also participate in another form of symbiosis. This is their most notable, as well as arguably most interesting adaptation: it has the ability to steal another tree’s access to sunlight and nutrients by essentially killing the host tree. The strangler fig can grow from seeds, but in tropical rainforests, it has adapted to start as an epiphyte on another tree instead.
At the epiphytic stage, the fig tree’s relationship with its host is seemingly innocent. Birds eat the fig’s fruit and drop its seeds between the leaf bases or in the nooks between branches of the host tree – usually palm and oak trees (Brown). The seedling grows there, finding nutrients in decaying leaves and soil, and growing towards the sunlight. This relationship is referred to as commensalism: the fig tree is benefitting from the host’s resources, while the host remains unaffected (Sack). As the epiphyte grows and sends its roots down along the trunk of the host, their relationship begins to change. When two root tendrils touch, they fuse together in a process called anastomose, eventually forming a massive woody mesh that completely encircles the trunk of the host. As the host continues to grow, the roots’ grip gets tighter, crushing its bark and constricting vital phloem and cambial layers (Armstrong). If the grip is tight enough, the host may no longer be able to transport nutrients throughout its system, dying of strangulation. Though the fig is not directly feeding on its host’s resources, it can be argued that their relationship at this point is parasitic, as the fig is harming the host tree in the hopes of eliminating competition for rain and sunlight (Sack). In most cases though, the host is sooner affected by the shading effects of the fully grown fig tree than by strangulation. This is especially true when the host is a palm tree, as palms lack an outer cambial and phloem layer (Armstrong).
The ficus family of trees, vines and shrubs are absolutely vital to many tropical ecosystems. One study recorded a total of 990 bird and 284 mammal species who eat figs as a part of their diet at one point during the year (Shanahan, Mike et al). These numbers include frugivores such as bats, monkeys, hornbills, toucans and parrots in addition to some invertebrates, that have been known to eat figs and aid in seed dispersal like the dung beetle, ants and, of course, fig wasps (Shanahan, Mike et al). Some of these animals rely completely on figs; they are called specialists (hornbills, some parrots and some toucans) and they would die out if figs disappeared. The majority of the animals that fall into the category of fig-eaters only do so at certain periods of the year when the main fruits of their diet are unavailable, or in times of food shortage. Despite the fact that figs are only eaten for a short period of time, in these cases they are vital: without them, these animals could starve (Kinnaird, M. ). In the food shortage of 1971 when only 50% of fruit trees in Barro Colorado Panama were producing fruit, the rates of starvation were better than they would have been due to fig trees in the area. In this case figs were a fallback food source for many species (Leigh, Egbert Giles).
Ficus tree shading out its cabbage palm host in Fort Lauderdale, Florida
There are several reasons why figs sustain so many animals in tropical rainforests. A big part is due to their fruiting patterns: most have very short periods between fruiting episodes, so there is fruit practically all year round. Also, they produce in the hundreds of thousands of figs that ripen at the same time, which increases the incentive of animals to feed there (Shanahan, Mike et al). The fruit itself comes in different colours and scents which caters to the variety of animals foraging for food. For example, the figs that ripen to be a deep red colour are easily spotted by birds whereas cauliflorous figs, that remain green but smell sweet, are scented at night by bats (Shanahan, Mike et al).
Figs are also very easy for animals to eat. They do not have any protective coat ( no thorns or hard outer layer to dissuade hungry animals) and they are very nutritious. Although they do not contain high levels of carbohydrates or lipids like other fruits, they are much higher in protein, calcium and magnesium. Fig fruits contain 3.2 times more calcium than other tropical fruits, which is important for strong eggshells and increasing bone density (Kinnaird, M.). The protein often comes from the fig wasps inside the fruits, dead male wasps or eggs. Also, the sheer amount of available fruit per tree reduces competition between animals because there is more than enough for all (Shanahan, Mike et al). All in all, it is undeniable to say that if the ficus family disappears, many other animals would disappear with them.
The strangler fig is a unique species of tree that holds an important role in its tropical rainforest ecosystem. Over time, it has evolved a slew of adaptations that make it competitive to the many other species in its surroundings. Not only is the strangler fig physically well adapted to its environment, but it has also developed both mutualistic and parasitic relationships with other species in order to ensure its survival. Its fruit also serve as the main food source for many species of mammals and aves. The strangler fig is a perfect example of the complex adaptations necessary for survival in the tropical rainforest ecosystem. Due to its warm climate and fertile land, the tropical rainforest is a very productive environment, where plant species evolve twice as quickly as those in Antarctica (“Hotspots for Evolution”)! Because of this speedy rate of change, plant species have needed to develop complex interrelationships with other species in order to maintain a competitive advantage. Though strangulation may seem like a violent means by our standards, it is not in fact one of the most intense adaptations. The Acacia tree, for example, is covered with spikes housing colonies of poisonous ants that will not hesitate to attack the unsuspecting passerby. So unless you are Tarzan, we suggest being very careful when entering the rainforest jungle, as you never know what sneaky adaptation evolution will surprise you with next!
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Picture 1: Ella Martin, 2016
Picture 2: Brown, Stephen H. “Ficus Aurea.” (2011): n. pag. University of Florida IFAS Extension. University of Florida, 2011. Web. 2 Apr. 2016.
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Picture 4: Wilson, Dede. “Comparing Fresh Fig Varieties.” Bakepedia. Bakepedia, 24 Aug. 2013. Web. 08 Apr. 2016. (Link: http://www.bakepedia.com/tipsandtricks/comparing-fresh-fig-varieties/)