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Demography, dispersal and colonisation of larvae of Pacific giant salamanders (dicamptodon tenebrosus good) at the northern extent of their range Ferguson, Heather Margaret


The Pacific Giant Salamander, (Dicamptodon tenebrosus Good) is red-listed in British Columbia, the northern extent of the species' range. Little is known about the demography of these populations and their ability to recover from disturbance by recolonisation. I conducted a field experiment to measure the colonising ability of larval Pacific Giant Salamanders at 4 streams within the Chilliwack Valley of British Columbia. I also estimated basic survival, growth and dispersal rates for these larvae. These rates were compared to others from Oregon where this species is not considered threatened. Mark-recapture in 120 m reaches in four streams in 1996 and 1997 revealed (a) lower larval densities, (b) lower annual growth rates and (c) similar annual survival of these larvae in comparison to those in similar Oregon streams. Due to slower growth rates, I hypothesise the the larval period in British Columbia is 2-3 times longer than in Oregon. To study colonisation, larvae were removed from a 25-40 m section within each 120 m reach and the recolonisation of each section was monitored for 13 months. Depleted reaches were repopulated slowly by larval dispersal and more quickly by adult reproduction. Few larvae moved more than 4 m. Full recolonisation of these reaches was predicted to take 6-42 months. Provided terrestrial adults are available, local reproduction appears to be a more effective means of repopulating an area than larval immigration. Larval dispersal was not influenced by larval density, biomass, substrate, wetted width, depth, or pool-riffle composition. Logging-induced habitat shifts may thus have little consequence to larval dispersal as movement was uniformly low through a variety of microhabitats. Although logging and other disturbances may increase the rate of local extinction o f D. tenebrosus in British Columbia, these populations are not unusually susceptible to disturbance. Despite having lower density and growth rates than in other parts of their range, larvae in British Columbia exist within the survival and growth bounds of other non-threatened stream-dwelling salamanders. More importantly, recruitment can facilitate rapid recovery from small-scale disturbances. Conservation efforts should focus on terrestrial as well as aquatic habitat and dispersal routes.

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