Species invasions and replacements in a native crayfish community.

The introduction of non-native signal crayfish, Pacifastacus leniusculus, into the Pit River drainage in northeastern California has led to invasions into previously allopatric populations of the endangered Shasta crayfish, Pacifastacus fortis. This research examined the effects of this invasion and the potential for the non-native crayfish to replace P. fortis. Field studies determined the range and abundance of both native and invading crayfish, species attributes, and the species composition of commensal crayfish worms (Branchiobdellida) on each crayfish. The distribution of P. fortis within its range was extremely fragmented, whereas P. leniusculus were found throughout most of the area. Although these species exhibit very different behavior, they overlap along several important niche dimensions. In addition, differences in size, activity, aggression, reproduction, diet, and environmental tolerances support the hypothesis that P. leniusculus will ultimately replace P. fortis. The growth rate of P. fortis during a field enclosure experiment was significantly reduced in treatments with P. leniusculus , particularly with large P. leniusculus. A laboratory experiment showed that P. fortis initiated fewer non-aggressive contacts and spent significantly more time being passive and burrowing when P. leniusculus were present. Pacifastacus leniusculus were more aggressive and initiated most agonistic behavior. Intraguild predation of P. fortis by large P. leniusculus was also shown in the laboratory. Interspecific interactions of these congeneric crayfish were strongly asymmetric and size dependent. Pacifastacus fortis sympatric with P. leniusculus would have reduced activity and feeding that would result in a lower growth rate. A secondary invasion of non-native branchiobdellidans is replacing a native branchiobdellidan species on P. fortis in populations sympatric with P. leniusculus. The proportion of the native branchiobdellidan, Magmatodrilus obscurus, replaced by non-native Xironogiton victoriensis in the field was found to be a function of the time since invasion of P. leniusculus. A laboratory experiment showed that predation by the non-native resulted in lower survivorship of M. obscurus. The P. leniusculus invasion has resulted in rapid, drastic declines in the abundance of the native crayfish and one of its native commensals. The invasions of the non-native crayfish and its commensal into this native crayfish community were highly successful and appear likely to result in the replacement and extinction of two native endemics.Ph.D.Aquatic sciencesBiological SciencesEcologyForestryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131885/2/9938432.pd