26 February 2007

Let them disperse

Some noteworthy ideas derived from a paper by Bilton et al.1 on the dispersal of freshwater invertebrates.

The authors define dispersal as "the movement of individuals or propagules between spatially (or temporally) discrete localities or populations". It is somewhat redundant in its wording, for an individual that disperses is a propagule and there is no need to specify that dispersal is to be between "populations".

What are the advantages of dispersal? It reduces competition, which matters if resources are limited, lowers the chances of inbreeding (in sexually reproducing animals) and of encountering predators and parasites. In other words, dispersal provides an escape mechanism from density-dependent adverse events. One other advantage of dispersal that the paper neglects to emphasize is that for those organisms that live in ephemeral habitats, dispersal may provide a significant long-term survival mechanism and the only route to the colonization of new ephemeral habitats.

Dispersal results in gene flow and, consequently, reduced genetic differentiation between colonies.

Dispersal can be active or passive. Active dispersal requires the ability to fly or crawl between habitats and, of course, means to detect a suitable habitat from a distance. Wandering around blindly would most likely end in the death of the wanderer. Passive dispersal may be done while being attached to the body of a larger and more mobile animal. The technical term for this type of hitchhiking is phoresy.

Freshwater larvae in the act of phoresy while attached to a water beetle. When the beetles move from one habitat to another, they inadvertently disperse the mites, which are actually parasitic. Photos from Bilton et al.

Blowing around in the wind is another potential mechanism of passive dispersal. But, its outcome may not be too different than that of wandering around blindly. The authors question "the reliability of dispersing by wind to suitable freshwater habitats owing to the high likelihood of terrestrial deposition".

Dormant stages of passive dispersers usually have reduced metabolic rates and increased tolerance to desiccation and temperature extremes. These traits enable them to survive in the absence of water. Did such traits originally evolve as adaptations for dispersal? I suspect that the dormant stages may have first appeared as adaptations for in situ survival of temporary dry conditions.

The paper also discusses some apparent cases of evolutionary trade-offs between dispersal and reproductive performance in aquatic insects. Among the several examples presented, one is about water boatmen (Sigara spp.), in which individuals without flight muscles (those who, therefore, cannot disperse) have higher fecundity (more egg production). Likewise, in some water beetles the flight muscles autolyse after reproduction starts. Despite, or perhaps, because of such trade-offs, populations are variable in their dispersal ability.

In addition to spatial dispersal, some freshwater invertebrates have dispersal in time, or temporal dispersal. Temporal dispersal takes place when dormant propagules of a species accumulate in its habitat, forming "propagule banks", and are activated at different times in the future. Some dormant zooplankton eggs have been claimed to remain alive for more than 200 years. According to the paper, temporal dispersal may benefit predominantly clonal or inbred organisms by introducing genes from the past, so to speak. I suspect that propagules also function as reservoirs of individuals to recolonize their habitat if and after an environmental catastrophe destroys all the active individuals.

The authors note that "despite our longstanding appreciation that freshwater organisms achieve dispersal, the extent and mode of dispersal remain poorly understood."

Some of my past posts on dispersal of snails:

Darwin’s snails

Flying mollusks

Flying clausiliids

It was raining mussels and clams

1Bilton, Freeland & Okamura. 2001. Dispersal in freshwater invertebrates. Ann Rev. Ecol. Syst. 32:159-81.

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