Ecology of the Sycamore Fig Tree [Species Interactions]

Ingrid Schoonover

February 1, 2021

The Sycamore Fig Tree is quite the “Queen of Trees”, found in tropical Africa along riverbanks this tree produces several tons of fruit each year and supports a diversity of species. The Sycamore Fig Tree does not flower, instead they contain microscopic flowers inside the young figs, which either develop into seeds or a nursery for their pollinator, the Fig Wasp. This unique dynamic means that the Sycamore Fig Tree and the Fig Wasp are completely dependent on a mutualistic relationship for their reproduction. There are many other species that fit into this ecological puzzle, including ants, hornbills, bees, geckos, elephants, bats, and parasites. In this essay I will describe the complexity of species interactions that surround the ecology of the Sycamore Fig Tree. I will start by describing the mutualistic (+/+) interactions, and then I will introduce three types of exploitative (+/-) interactions (herbivory, parasitism, and predation), after this I will discuss commensalism (+, 0) and amensalism (-,0), and finally I will conclude with competitive (-,-) interactions. Some of these species’ interactions are straight-forward while others are more ambiguous and blur the line between “friend” and “foe”.

The most obvious species interaction depicted in the “Queen of Trees” is mutualism (+/+); in this type of interaction both species gain something and are better off because of their relationship. An extreme example of mutualism is highlighted with the relationship between the Fig Wasp and the Sycamore Fig Tree. The Sycamore Fig Tree provides a nursery for developing Fig Wasps inside the young figs, and in return the wasps pollinate the flowers inside the figs with pollen from a different tree. Impregnated female Fig Wasps are attracted to the smell of the figs and will travel great distances to find a receptive Sycamore Fig Tree. The Fig Wasps are tiny, only a millimeter in length, so they can squeeze inside the fig through a one-way gate at the top of the fig. They have a wedge-shaped head that aids in the process of prying open the fig gate, but it is still a tight squeeze, so their wings and antenna are usually ripped off in the process. Once inside, the female wasp needs to travel down a spiral valve until she reaches the microscopic flowers. The female wasp lays her eggs here and unpacks pollen from a specialized breast pocket to fertilize the flowers. The female wasp dies very quickly, but the Sycamore Fig Tree surrounds each wasp egg with a gall capsule that nourishes and protects the eggs. The entrance to the fig is also sealed off with an antiseptic liquid that deters some predators and parasites. The Sycamore Fig Tree spends the next two months making sugar to feed the developing wasps and fig seeds. The male Fig Wasps are the first to hatch, they are wingless with tiny antennae and eyes, but what they do have to their advantage is powerful jaws that they use to cut out of their gall and release the females. They also have a long and flexible reproductive organ that they use to impregnate the female wasps before they even hatch. They release the females from their galls and chop down the white anthers of the internal flowers, these flowers contain pollen that the female will use to fertilize the flowers at her destination. The male wasps will spend the few remaining hours of their lives tunneling out of the fig to release the female wasps. Then all at once, millions of female fig wasps emerge from the figs into the world to repeat the process, they will only survive for a few hours at this point so they move quickly to find another fig tree which might be nearby or as far as 100 kilometers away if they get picked up by wind. These anatomical and behavioral adaptations result from millions of years of coevolution between the two species, and now neither species would be able to exist independently from the other, which is why the Sycamore Fig Tree must fruit multiple times annually.

The Sycamore Fig Tree and Fig Wasp also have mutualistic relationships with several other species that make the tree their home. While the figs are developing the ant plays an important role in protecting the fig seeds and developing wasp, this is because they will kill or drive away anything that comes in their path. So, the ants provide the developing wasps and tree with protection from herbivores and parasites, and in return the tree provides the ants with a home and hunting ground. The ants also have a mutualistic relationship with another species called the Hilda, they look after the Hilda eggs and protect the nymphs that hatch, and in return the Hilda Nymphs are farmed by the ants for the sweet honeydew food. The nymphs have hollow piercing mouth parts that tap into the figs sweet sugary sap, and then the ants consume the sugary liquid is vented by the nymphs. Similarly, the Grey Hornbill plays a role in protecting the figs in exchange for a place to raise their offspring. The Grey Hornbill will build a nest in a hollow of the tree and the female will temporarily confine herself inside the tree to lay her eggs before the rains come. The timing of this process is important because they are insectivores and the rain season will trigger a proliferation of insects that will be used to feed their offspring. Many insect species such as the cicada will exploit and damage the tree, but the hornbills keep some of these numbers in check by eating them, they also discourage bees from nesting in the tree by eating scouts in search of a new nesting site.

Additionally, seed-dispensing species such as bats, birds, elephants, and catfish play an important role in proliferating the germination of Sycamore Fig Tree seeds once the figs have ripened. The figs ripen when the seeds are ready to be dispersed and the wasps’ have hatched, the bright color and smell encourages animals to feed on the figs which are high in sugar and contain many nutrients. Fruit bats for example, which eat half of their weight in fig pulp each night, are of critical importance because they carry the figs a short distance away to their perch where they suck the juice out of the fig and discard the seeds below them. This promotes the growth of tiny fig gardens below their perches. Fruit bats are not as agile as birds so the tree accommodates them when the fruit is ripe by dropping their leaves at night so that the figs hang off the branches where bats can reach them. Over 100 kinds of birds will also eat the ripe figs containing seeds and fly off with the seeds thus spreading them. Elephants can smell ripe figs from miles away and will travel to eat them, they will then disperse the seeds while traveling. Surprisingly migrating catfish also play a role in transporting figs, because they will consume figs that fall into the water and occasionally will carry some of them upstream where they will be planted and fertilized if the catfish dies. These four examples with the fruit bats, birds, elephants, and catfish are mutualistic interactions because the seed-disperser species benefits from eating the nourishing ripe figs and the fig trees benefit because the seed-dispersers are needed to perpetuate new generations of fig trees.

The most common type of interaction in the Sycamore Fig Tree ecosystem is exploitation (+/-); in this type of interaction one species benefits at the expense of the other species’ suffering or loss. The three types of exploitative interactions include: herbivore/plant, parasite/host, and predator/prey.

Herbivory (+/-) occurs when one species consumes the plant material from another species, this benefits the herbivore because they will use the energy to grow and reproduce, and this is bad for the plant species which loses energy or could die. During the early stages of fig development, the biggest threat to the Sycamore Fig Tree and the Fig Wasp are herbivorous species that would eat the leaves or figs. Giraffes feed on young fig tree leaves, this is a problem for the Fig Tree because the leaves are required to make the sugar to grow the energy-intensive fig seeds. If the tree loses too many leaves then they will abort the figs. This is also problematic for the Fig Wasps because if the Giraffes eat the leaves then the tree will have less energy available to nourish the wasps. Feeding on the fig nursery is bad for the wasps inside for obvious reasons and means that the tree has less seeds to disperse. The Green Pigeon is a fig specialist that travels miles to feed on figs, if it eats them before they are ripe then the wasps inside will die, and the reproductive potential of the tree is diminished. The Fig Tree has evolved a chemical defense and pumps a foul-tasting and sticky latex into the fig leaves and figs that deters some herbivores. However, there are some insects that have coevolved adaptations to overcome this obstacle. The first example of such is the Fig Longhorn Beetle which bites the veins of the leaves to disrupt the flow of latex, and then feed on the leaf downstream of the cuts. Similarly, the Fig Katydid stops the flow of the latex by entirely cutting the leaves off. These insects are problematic for the same reason as giraffes and other animals that consume the leaves. Another threat to the Sycamore Fig Tree includes species that tap into the sweet sap, such as the Hilda Nymph and to a greater extent the Fig Cicadas, another specialist that emerges after the rain once a year and taps into the sap with their mouthparts. At peak infestation this is a major threat to the tree and sap will run down the trunk of the trees and cover the leaves and branches, this attracts other opportunistic herbivores that will feed on the sap and cicada excretions. The opportunistic herbivores are a threat to the Sycamore Tree as well and include monkeys that will eat the sap, leaves, and figs, as well as ants and bees that will eat the sap. Finally, the last herbivore/plant interaction to mention is that between the tiny seed bugs and figs from the Sycamore, these tiny bugs are fig specialists that steal the fig seeds out of ripe figs that have fallen to the ground below the tree. These bugs benefit from the tasty meal, but the fig tree suffers because no fig trees will ever sprout in the area around them. The Sycamore Fig Tree plays such an important ecological role because it fruits several times per year which provides a year-round food source for many herbivorous animals, in fact, across Africa more animals eat figs than any other fruit.

Parasitism is another exploitative (+/-) interaction, in which the parasite will live in the host species and steal energy from it. There are two types of parasites that benefit at the expense of the Fig Wasp’s suffering: Nematodes and Parasitic Wasps. Nematodes are a type of parasite that burrow into the body of the Fig Wasps and then eat them alive from the inside. They are a big threat to the Fig Wasps because the antiseptic fluid that fills the fig is not affective against them. Nematodes enter the fig nursery stowed away in the body of the female wasp and then burst out to infect the next generation of wasps. The population size of Nematodes is kept in check simply by the fact that if they kill their host too efficiently then the parasite will die too because they need some male wasps to survive for the purpose of digging an escape hole from the fig. Parasitic wasps are another type of parasite that targets the gall capsule of developing Fig Wasps, they drill into the figs and then with their long ovipositor lay eggs onto the young wasp eggs. Their offspring will take over the capsule, consume the fig wasps, and then hatch out 2 months later. Interestingly, the parasitic wasp is the host of the parasitic banded wasp, which also has a long but flexible ovipositor which they use to navigate the hole drilled by the parasitic wasp and then lay their eggs on top of the parasitic wasp eggs.

Predation is the last type of exploitative (+/-) interaction, in which one species kills and consumes another species, for the predator the benefit is energy in the form of food, but for the prey the consequence of this interaction is death. There are several predators that prey upon the Fig Wasp, such as spiders which trap recently emerged Fig Wasps in their webs and then wrap them in silk to trap them before eating them. The Tiger Beetle is another predator of the Fig Wasp, they will feed on the fig wasps inside figs that monkeys have discarded onto the ground. Previously, the mutualistic relationship between developing Fig Wasps and ants was discussed, but once the Fig Wasps hatch then this dynamic flips in favor of the ants. The ants do not distinguish between parasites and pollinators so they will kill the Fig Wasps, and anything else that they can capture. Also, day geckos feed on Fig Wasps because they are nourishing and do not sting, but they need to be careful or they themselves will become prey to the snakes that hunt on the fig trees. In addition to snakes there are several other species that use the fig tree or figs as their hunting ground, including the Grey Hornbill, humans, the Pink Mantis, and crocodiles. The Grey Hornbill feed on insects such as cicadas and butterflies, the Pink Mantis will feed on larger insects like the Fig Katydid as adults, but they feed on smaller insects such as vinegar flies while they are young. Humans will sometimes smoke bees out of their tree nests and take their honey. Finally, there are crocodiles that will hunt the groups of fish attracted to fallen figs in the water, they lie in wait with open mouths in areas of fast-moving water because the fish cannot see where they are going and will swim right into their jaws.

Another somewhat common type of species-interaction that is worth mentioning is called commensalism (+/0), this is where one species gains something or is better off as a result of the relationship and the other species is unaffected, neither gaining or losing anything. Such an example is seen with bees/ants and cicadas, where the bees/ants get the benefit of a meal and the cicada is unaffected. When the fig cicadas feed on the sap from the tree they vent the excess sugary liquid and leave holes that leak sap. The ants are bribed by the sweet sap, so they feed on it instead of attacking the cicadas. Bees also like to consume the sap, so they will swarm to the tree to feed but do not disturb the cicadas. A different example of commensalism is observed during the fermentation process of rotting figs. When figs fall to the ground or are discarded as waste by other animals they begin to rot and ferment on the hot forest floor, the fig juice ferments into alcohol and the smell attracts vinegar flies and butterflies that will consume the sugary drink. This benefits the vinegar flies and butterflies because they have an easy meal, and this does not harm the tree because the figs were already on the ground. However, it is worth mentioning that example with the butterflies can also turn quickly into amensalism (-/0), another type of species interaction in which one species suffers and the other species is unaffected. This can happen if the butterflies become too inebriated from the alcohol and are put at risk of being consumed by birds or monkeys, still this does not harm the fig tree because the figs were already on the ground.

The last type of species interaction to mention is competition (-/-), where both species are worse off and suffer from the interaction. A good example of competition is seen with the frenzy that results from the fig cicada invasion, when ants, bees, and monkeys will all fight for the chance to feed on the sap that runs from the tree. The ants will bite and kill the bees, the bees will sting the monkeys and the ants will bite the monkeys, and the monkey will hit and catch the biting/stinging insects in defense. All of these species will suffer as a result of the presence of the other because there is limited resources and sharing with another species means that there is less available for their own species, which translates to less energy being available for growth and reproduction. Similarly, there is competition for other resources that the tree has to offer (leaves, figs, nesting sites). This competition will take place between different species, or between different individuals of the same species as was seen with the hornbill chick that died due to competition with the larger sibling.

To conclude, the Sycamore Fig Tree and Fig Wasp support a wide variety of species interactions in several direct and indirect ways, and the health of this ecosystem is interconnected with all the species involved. These species interactions result from millions of years of coevolution and can be of different magnitudes, with some species interactions being more impactful or important than others. What further complicates the matter is that the dynamics of these interactions are not static, for example: ants protect the developing fig wasps but will kill adults and its harmful to the tree if birds eat premature figs but beneficial to the tree if they eat mature figs and disperse the seeds.

Table of Species Interactions

• (+/+) Interactions:
  1. Mutualism – both species better off/both species gain something.
     1. Example: Sycamore Fig Tree (+) and Fig Wasp (+).
        • The Sycamore Fig Tree provides a nursery for Fig Wasps which nourishes and protects the developing wasps. In return, the wasps pollinate the flowers inside the figs with pollen from a different tree. Thus, both species rely on each other for reproduction.
        • Fig Wasps have a special breast pocket to carry pollen from one fig tree to another.
     2. Example: Ants (+) and Sycamore Fig Tree (+) and Fig Wasp (+).
        • The ants will kill or drive away anything that comes in their path. This protects the fig tree from herbivores and protects the fig wasp from parasites. In return for this protection the ants receive a home and food.

• Example: Ants (+) and Hilda Nymph (+).
  • The ants look after Hilda eggs and herd up the Hilda nymphs that hatch from the egg. The nymphs are provided with protection and in return they are farmed by the ants for food (sweet honeydew). This is because the nymphs have hollow piercing mouth parts that can tap into the figs sweet sap. The sap has so much sugar that when it is excreted it still has energy that the ants can use as food.

1. Example: Seed Dispersing Birds (+) and Sycamore Fig Tree (+).
   • The figs of the Sycamore Fig Tree ripen after the seeds are ready to be dispersed and the wasps’ have hatched, they contain nutrients and are high in sugar, their bright color encourages animals to feed on them. Over 100 kinds of birds will eat the ripe figs containing seeds and fly off with the seeds thus spreading them.
2. Example: Bats (+) and Sycamore Fig Tree (+).
   • Fruit bats eat half of their weight in fig pulp each night. Fruit bats are the most important seed dispersers because they carry the figs a short distance away to a perch where they will suck the juice out of the fig and discard the seeds below. This promotes the growth of tiny fig gardens below their perch. This benefits the fig trees because seed dispersers are needed to perpetuate new generations of fig trees. Fruit bats are not as agile as birds so the tree accommodates them when the fruit is ripe by dropping their leaves at night so that the figs hang off the branches where bats can reach them.
3. Example: Elephants (+) and Sycamore Fig Tree (+).
   • Elephants can smell ripe figs from miles away and will travel to eat them. The elephants will then disperse the seeds while traveling.

• Example: Catfish (+) and Sycamore Fig Tree (+).
  • Fish feed on the figs that fall into the water. When migrating catfish eat the figs they will carry some of them upstream, some of these figs will be planted and fertilized if the catfish dies.
• Example: Grey Hornbills (+) and Sycamore Fig Tree (+).
  • The Sycamore Fig Tree provides a nest for Hornbills to lay their eggs in the tree and raise their young, female hornbills are temporarily confined to tree inside hollow. In return for this home they feed on insects (such as cicadas) that would exploit the fig tree and discourage bees from nesting in the tree.
• (+/-) Interactions: Exploitation, with one species benefiting/gaining and the other species losing/suffering.
  1. Predation or Predator/Prey Interactions.
     1. Example: Spiders (+) and Fig Wasp (-).
        • Spider traps recently emerged Fig Wasps in their webs and then wrap them in silk to trap them before eating them.
     2. Example: Ants (+) and Fig Wasp (-).
        • Ants do not distinguish between parasites and pollinators so will kill the Fig Wasps.

• Example: Day Gecko (+) and Fig Wasp (-).
  • Day geckos feed on the Fig Wasps because they are very nourishing and do not sting.

1. Example: Snake (+) and day gecko (-).
   • Snakes hunt in the fig trees and feed on the day geckos.
2. Example: Grey Hornbill (+) and insects such as cicadas/butterflies (-).
   • Grey Hornbills time their nests so that babies hatch after the rain when insects are most plentiful, they feed on insects.
3. Example: Crocodile (+) and Fish (-).
   • Fallen figs in the water attract groups of fish and then crocodiles wait in fast moving water to catch a fish and eat it.

• Example: Pink Mantis (+) and Fig Katydid (-).
  • Pink mantis will feed on Fig Katydids.
• Example: Pink Mantis (+) and Vinegar Flies (-).
  • Pink mantis will feed on vinegar flies when they are young.

1. Example: Tiger Beetles (+) and Fig Wasp (-).
   • When monkeys feed on the figs they will discard some onto the ground, on the ground tiger beetles will feed on the fig wasps.
2. Example: Ants (+) and parasites/herbivores (-).
   • The ants will kill everything that they are able to capture and sting/bite larger herbivores to drive them away.
3. Example: Humans (+) and Bees (-).
   • The Masai people will smoke bees out of the tree and take their honey.
4. Parasitism or Parasite/Host Interactions.
   1. Example: Nematode (+) and Fig Wasp (-).
      • Nematodes are parasites that burrow into the body of the Fig Wasps and then eat them alive from the inside.
   2. Example: Parasitic Wasp (+) and Fig Wasp (-).
      • The parasitic wasp can drill into the figs and then with its long ovipositor it lays eggs into the gall capsule of the developing fig wasps.

• Example: Banded Wasp (+) and Parasitic Wasp (-).
  • The banded wasp has a long flexible ovipositor which they enter into the hole of the fig that was drilled by the parasitic wasp, then they lay their eggs on the egg of the parasitic wasp.

1. Herbivory or Herbivore/Plant Interactions.
   1. Example: Monkey (+) and Figs from Fig Tree (-).
      • Monkeys will feed on the sap released by cicadas, consume leaves, and consume figs from the fig tree.
   2. Example: Green Pigeon (+) and Figs from Fig Tree (-).
      • Green pigeons are fig specialists that will travel miles to feed on figs, then they return to their nests to feed their chicks a milk made from liquid fig.
      • Over 100 kinds of birds will eat the figs, if the figs are eaten before they are ripe then the wasps inside will die, and the reproductive ability of the tree is reduced.

• Example: Giraffe (+) and Figs/Leaves from Fig Tree (-) and Fig Wasps (-).
  • Giraffes feed on young fig tree leaves, this is a problem for the Fig Tree because the leaves are required to make the sugar to grow the energy-intensive fig seeds. If the tree loses too many leaves then they will abort the figs. This is also problematic for the Fig Wasps because if the Giraffes eat the leaves then the tree will have less energy available to nourish the wasps. Feeding on the fig nursery is bad for the wasps inside and means that the tree has less seeds.

1. Example: Fig Longhorn Beetle/Fig Katydid (+) and leaves from Leaves from Fig Tree (-).
   • As a chemical defense against predators the Fig Tree pumps a foul-tasting and sticky latex into the fig leaves and figs. There are some insects that have coevolved adaptations to overcome this obstacle.
   • The Fig Longhorn Beetle bites the veins of the leaves to disrupt the flow of latex, this cuts off flow of latex downstream, of the bites where they can feed.
   • The Fig Katydid stops the flow of the latex by entirely cutting the leaves off.
2. Example: Hilda Nymph (+) and Sycamore Fig Tree (-).
   • Hilda nymphs have hollow piercing mouth parts that can tap into the figs sweet sap.
3. Example: Fig Cicadas (+) and Sycamore Fig Tree (-).
   • Cicadas emerge after the rain and tap into the sap with their mouthparts. This is a major assault to the tree and this attracts other opportunistic herbivores that will also feed on the sap and cicada excretions.

• Example: Tiny Seed bugs (+) and Figs from Fig Tree (-).
  • Tiny Seed bugs are fig specialists that will steal the fig seeds out of ripe figs that have fallen to the ground below the tree, and because of this no fig trees will sprout below the tree.
• (-/-) Interactions:
  1. Competition - both species are worse off from the interaction, both suffer.
     1. Example: Grey Hornbills (-) and Grey Hornbills (-).
        • The developing hornbill chicks have a pecking-order that decides which chick gets to eat first, the smallest chicks do not eat until the largest one is full. This resulted in one chick dying from starvation and another one not developing fast enough.
     2. Example: Ants (-) and Bees (-) and Monkeys (-).
        • When the cicadas invade the fig trees then the trunk will run with sap and the branches and leaves will have sap as well. Then there is competition between the ants, bees, and monkeys to feed on the sap.

• (+/0) Interactions:
  1. Commensalism – one species gain/better off, and the other species is unaffected (neither gains nor loses).
     1. Example: Vinegar flies (+) and rotting figs from Fig Tree (0).
        • Rotting figs on the ground will begin to ferment which attracts vinegar flies because it is a tasty meal.
     2. Example: Ants (+) and Cicadas (0).
        • The ants do not attack the cicadas because when they feed on the tree they will leave a drop of sweet fig sap that the ants will feed on.

• Example: Bees (+) and Cicadas (0).
  • The bees do not disturb the cicadas but swarm to the trees to feed on the sap released by cicadas.

1. Example: Butterflies (+) and rotting figs from Fig Tree (0).
   • The fig juice in rotting figs will ferment into alcohol, this smell attracts butterflies who drink the juice.

• (-,0) Interactions:
  1. Amensalism – one species suffers/loses and the other species is unaffected/neutral (neither gains nor loses). In class we gave the example of humans wearing fur/feathers for beauty and not gaining anything while the bird suffers.
     1. Example: Inebriated Butterflies (+) and rotting figs from Fig Tree (0).
        • The fig juice in rotting figs will ferment into alcohol, this smell attracts butterflies who drink the juice. If they become too inebriated from the alcohol, then they are at risk for being consumed by birds and monkeys.