Sunday, June 1, 2008

Talk: Vida Kenk

Vida Kenk: Fauna of epiphytic bromeliads

An epiphyte is a plant that grows on another plant for support (i.e. not in a parasitic sense). Big trees often collect a good deal of dirt, leaf litter, water, and other good stuff in the crotches between branches, and even in depressions along the tops of branches. These spots provide everything a plant needs to grow, and are even, by virtue of being up in the canopy, better illuminated than spots on the forest floor. So there are plants, epiphytes, that take advantage of this opportunity.
Bromeliads are a type of flowering plants, members of the pineapple family. There are more than 2600 species of bromeliad in 56 genera; they diverged from a common ancestor about 20 million years ago (mya). They seem to have met with a great deal of success due to their use of CAM photosynthesis (a special, efficient type of photosynthesis) and their evolution of the ability to live as epiphytes. They are nearly exclusively neotropical (i.e. New World tropical); there is one species in West Africa that may have rafted over on a floating mat of vegetation. Bromeliads now occur in many habitats: granitic outcrops, coastal dune fields, high altitude cloud forests, and rain forests. 26 of the 56 genera of bromeliads include epiphytes as over half of their species, so it is a very common habit among the bromeliads.
Bromeliads grow with a ring or "whorl" of leaves that enclose a central "tank" where water is stored (think of a pineapple's leaves). Water and organic debris from above accumulate in the tank, creating a microhabitat suitable for all sorts of life. Bromeliads dominate the epiphytic vascular flora of the neotropics; their biomass exceeds that of all the other angiosperm families combined (such as orchids). In a Colombian cloud forest, over 175,000 mature bromeliads may occur in a single hectare, resulting in the storage of perhaps 50,000 liters of water per hectare in bromeliad tanks (the largest bromeliads can hold 45 liters each, although most are much smaller). This means that bromeliads create an ephemeral island-like freshwater habitat for other species. It also means that countless millions of semi-isolated habitats exist simultaneously; this may have caused rapid evolutionary radiation among species that use bromeliad tanks, since each tank is a sort of independent experiment.
Bromeliads possess these tanks for a reason: they are "animal-assisted saprophytes". As organic debris such as leaf litter falls into the tank, it is decomposed by microorganisms, and the various decomposition products are eventually absorbed by the bromeliad through specialized trichomes. Due to this evolution towards absorption of nutrients from decomposition products in their tanks, it is thought that the bromeliads are currently evolving towards carnivory, along a path similar to that taken by the pitcher plants one often sees in bogs.
More than 500 animal species have been described within bromeliad tanks, about 470 of which are insects. Protozoa, bacteria and fungi are also commonly seen. A wide variety of species are found in bromeliad tanks; they include a great many ants, somewhat less beetle larvae and fly larvae, and occasional observations of taxa as diverse as amphibians, annelids, all sorts of arachnids (more pseudoscorpions than spiders, interestingly), isopod crustaceans, butterflies, molluscs, nematodes, and even one onychophoran worm. Some of these organisms use bromeliad tanks facultatively, meaning that the tanks are not essential to their life cycle: mosquito larvae can breed in bromeliad tanks, for example, but there are many other places with stagnant pools of water where they can also breed. Other species appear to be obligate users of bromeliad tanks, meaning that they rely exclusively upon them in some way, and cannot survive or reproduce without them: three clades of diving beetles have been found, for example, that appear to have evolved in close association with the original evolutionary radiation of the bromeliads 20 mya, and now live exclusively in the tanks. Some species spend only part of their life cycle in bromeliad tanks, usually in a larval phase; the family of tree frogs that includes the poison dart frogs, Hylidae, sometimes lays its eggs in tanks, or transports its eggs to tanks after laying them elsewhere, and then feeds their tadpoles with additional eggs (fertilized or unfertilized) as they grow in the tanks.
An interesting question that remains largely unanswered is how these species (particularly those living in bromeliads obligately) disperse to new sites. Bromeliads die, like any plant, and their tank then disappears; so all of the organisms that use or rely upon them must have a means of finding and dispersing to new bromeliad tanks. Some organisms, such as most insects, are motile: the adults can simply explore and find new tanks in which to lay eggs, for example. The non-motile, obligately aquatic animals, from ostracods to annelids, may be carried on the skin of motile animals such as frogs, or may be carried by being eaten and emerging unharmed in fecal material. Other organisms may disperse by being windblown as cysts.
Bromeliad tanks are related to human health, interestingly, because they provide a breeding ground for mosquitoes that act as vectors for important diseases such as dengue and yellow fever. Even when extensive urban control measures are taken, the bromeliads in nearby forest can act as a reservoir for mosquito breeding populations.

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