Bj. Downes et Mj. Keough, SCALING OF COLONIZATION PROCESSES IN STREAMS - PARALLELS AND LESSONS FROM MARINE HARD SUBSTRATA, Australian journal of ecology, 23(1), 1998, pp. 8-26
Understanding colonization processes specifically requires a comprehen
sive understanding of the frequency and scales over which dispersal oc
curs, which in turn determines the degree of linkage within and betwee
n populations and possibly results in metapopulation structure. What i
s our level of such knowledge for stream systems! We examine colonizat
ion processes in streams using a framework produced by analogous liter
ature on sessile, marine species, which are comparatively well-known a
nd may provide insights for stream biota. For simplicity, we separate
dispersal into two, somewhat artificial scales: microscale (between ha
bitat patches - cm to m) and mesoscale (between groups of patches - te
ns to thousands of metres), and consider dispersal links between them.
Traditional views on dispersal developed similarly in both systems, w
ith a strong initial focus on mesoscale dispersal followed by an aware
ness that some species do not disperse usually over these scales and t
hat there are a wide range of dispersal profiles. In both habitats, th
ere are few data on actual dispersal distances, and longevities are so
metimes used to infer them instead. Organisms can be transported by wa
ter currents (marine larvae, stream larvae) or wind (adult insects), t
he directions and strengths of which are relatively predictable at mes
oscales. However, behavioural choices of organisms during dispersal ca
n change their potential dispersal distances and directions markedly.
Additionally, predictability of transport processes at microscales is
very poor. As a consequence, simple, lone biological measures (like lo
ngevity) or simple, lone physical measures (like discharge) are useles
s for predicting dispersal frequencies and scales in most cases. Morta
lity during dispersal is also extremely important but there are few da
ta; this represents a major information gap in both sets of literature
. Finally, if organisms end dispersal by searching for and being able
to respond to specific cues, then we may be able to predict colonizati
on by looking at the distribution of such cues; different cues are lik
ely though to vary greatly in space and time. There is potential for d
ispersal 'structure' to develop in rivers of different hydrology, and
for sets of correlated life-history characters to result in dispersal
at particular scales, but there are very few stream studies that bear
on these issues unambiguously. Progress in understanding the scales of
dispersal, in both habitats, will require a lot more studies designed
formally to test clear hypotheses about the scales of dispersal, rath
er than continuing a status quo of generating essentially anecdotal in
formation.