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Strangler figs (Ficus spp.) are amazing plants that provide excellent examples of all manner of ecological relationships. They are considered keystone species in rainforest ecosystems, because they are a vital food source for a large number of animals (blueplanetbiomes.org). They also affect the other species around them in several other ways, which will be dicussed throughout this paper. They are important to humans in that understanding them can help us figure out how to best protect them, and the rainforests of which they are such an intricate part.
The Growth Process
Stranglers begin life in the forest canopy. When animals drop or excrete the seeds of a strangler fig into the branches of another tree, the seeds germinate and grow into an epiphyte, or plant that lives on other plants. At this stage of its life, the fig does not hurt its host tree. It merely uses the host tree to get closer to the sunlight and rain, and finds nutrients in decaying leaves and soil caught in nooks of the host tree's branches (blueplanetbiomes.org). This is a case of commensalism: one organism benefits while the other is unaffected.
Once the fig has established itself on the host tree, it begins to send down root tendrils, which can either dangle freely or wrap around the host tree's trunk. These roots grow at an average pace of 5 meters per year, and can take several years to reach the ground, depending upon the height of the host tree. When the roots reach the soil and begin taking in nutrients from it, the plant becomes known as a hemi-epiphyte (cloudbridge.org).
When two root tendrils from a strangler fig touch, they fuse together. Because the roots wrap around the trunk of the host tree, they overlap a lot and eventually form a mesh that completely encircles the host. Sometimes, multiple fig plants will grow on the same host tree, and will fuse with eachother in a process called allofusion. This creates a compound organism that is structurally one plant, but which has genetically different branches (Thomson et al, 1991).
As the strangler fig grows big enough to be a tree in its own right, it begins to harm its host. The roots thicken and tighten enough to prevent the host tree from growing wider - literally, strangling it. Worse, the strangler competes with the host for water, nutrients, and sunlight. Typically, the host tree dies, and eventually decomposes, leaving the fig tree standing independently on its now strong and woody, but hollow, root mesh (blueplanetbiomes.org). This is a case of parasitism: one organism benefits from an interaction that harms the other organism.
The flower of the strangler fig is a bulbous green object that looks like a fruit rather than a flower. This "fruit" is called a synconium, and is really a carpet of tiny flowers attached to a green gourdlike structure. Oddly, these little flowers are on the inside of the structure, completely surrounded by it, with no opening to the outside world. The carpet of tiny flowers in the interior of the synconium consists of both male and female flowers, which cannot pollinate eachother as they mature at different times, and also sterile "gall flowers" (Kricher 1997).
The strangler figs have a symbiotic relationship with their sole pollinator: the fig wasps (Family Agaonidae). This is an advanced form of mutualism (where both organisms benefit from interacting): the strangler figs cannot possibly live without the fig wasps, and vice versa. Each has very specific needs that the other is precisely adapted to fill. This is the result of a long path of co-evolution, which has made the lives of these organisms inextricable; to explain the pollination of the fig's strange flower is to explain the life cycle of the fig wasp (Kricher 1997).
The gall flowers in the fig synconium contain fig wasp eggs, laid by the previous generation. The male wasps hatch first, and inseminate the female wasps while they are still unborn. The females then hatch, already pregnant, at the exact same time that the male flowers mature (Kricher 1997). How this timing is managed, especially given that different fig trees flower at different times (Thomson et al, 1991), has not been explained by any source that I have found.
The female wasps get covered with pollen from the male flowers before digging their way out of the synconium, and flying off in search of other fig trees. When a female wasp finds a synconium on another tree, she burrows in and lays her eggs, then dies. In the process, the pollen she has carried with her is brushed off onto female fig flowers, which are thus pollinated (Kricher 1997). How the wasp locates a synconium in the right stage of development, with mature female flowers, is another question that my research has not answered. She certainly cannot try multiple flowers; the female wasp is often injured by the process of burrowing into the synconium (Kricher 1997).
The females only live for about a day outside the synconiums (Kricher 1997). Yet a genetic analysis study of fig paternity (ie, which trees donated pollen for which fruits) has shown that the wasps, despite their extremely short lives, are "efficient agents of long-distance dispersal, routinely moving up to 10 km between flowering trees" (Nason et al, 1996)!
Implications for Conservation
Numerous studies have been done regarding various aspects of strangler fig ecology. Some have focused on the conditions for strangler fig seed germination: Putz and Holbrook (1989) showed that humus (from decaying leaves) available to epiphytic figs in palm tree canopies was higher in nitrogen, magnesium, and potassium than soil on the ground, suggesting that nutient levels may be a factor in the germination and growth of figs, and possibly other epiphytes as well. Swagel et al (1997) demonstrated that Ficus aurea seeds require high substrate water levels for germination, and suggested this as another reason that stranglers typically germinate only in the canopy, and, in drier areas, mostly on palm trees.
Other studies have focused on factors affecting genetic diversity among seemingly small populations of strangler fig trees. Thomson et al (1997) examined cases of allofusion (fusion of multiple individuals into one tree, as discussed above) and found that while the trees produced flowers of multiple genotypes, all of the flowers in a given "mosaic" tree flowered in synchrony, preventing those separate genotypes from pollinating eachother (since the male and female flowers do not mature at the same time on a given tree). This means that the gene pool may be larger than the number of trees (since there can be multiple genotypes per tree), but a certain population size of whole trees is still needed to meet pollination requirements and sustain wasp populations.
Nason and Hamrick (1997) asserted that the wide dispersal range of fig wasps allows strangler figs to be pollinated even in otherwise isolated forest fragments. Moreover, conservation of strangler figs, and by implication all of the species that the figs support, "may be dependent on the preservation of forest elements in a surrounding fragmented landscape."
Swagel, E N; Bernhard, A Van H; Ellmore, George S (1997). "Substrate water potential constraints on germination of the strangler fig Ficus aurea (Moraceae)." American Journal of Botany, v. 84 issue 5, p. 716-722.
Laman, Timothy G (1995). "The ecology of strangler fig seedling establishment." Selbyana, v. 16 issue 2, p. 223-229.
Nason, John D.; Herre, E. Allen; Hamrick, James L (1996). "Paternity analysis of the breeding structure of strangler fig populations: Evidence for substantial long-distance wasp dispersal." Journal of Biogeography, v. 23 issue 4, p. 501-512.
Nason, J. D.; Hamrick, J. L (1997). "Reproductive and genetic consequences of forest fragmentation: Two case studies of Neotropical canopy trees." Journal of Heredity, v. 88 issue 4, p. 264-276.
THOMSON J, D; HERRE E, A; HAMRICK J, L; STONE J L (1991). "GENETIC MOSAICS IN STRANGLER FIG TREES IMPLICATIONS FOR TROPICAL CONSERVATION." Science (Washington D C), v. 254 issue 5035, p. 1214-1216.
Thomson, James D.; Dent-Acousta, Sara; Escobar-Paramo, Patricia; Nason, John D (1997). "Within-crown flowering synchrony in strangler figs, and its relationship to allofusion." Biotropica, v. 29 issue 3, p. 291-297.
PUTZ F, E; HOLBROOK N M (1989). "STRANGLER FIG ROOTING HABITS AND NUTRIENT RELATIONS IN THE LLANOS OF VENEZUELA." American Journal of Botany, v. 76 issue 6, p. 781-788.
Kricher, John (1997). A Neotropical Companion. Princeton University Press, p. 30, 128-129.
Benders-Hyde, E (2002). "Strangler Figs." Website. Accessed May 12, 2004. Available: http://www.blueplanetbiomes.org/strangler_figs.htm
Giddy, Ian (2004). "Cloudbridge Project in Costa Rica: Strangler Fig." Website. Accessed May 12, 2004. Available: http://cloudbridge.org/strangler.htm
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