Graduate Thesis Or Dissertation
 

Population and production ecology of Lipsothrix spp. (Diptera:Tipulidae)

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  • Craneflies of the genus Lipsothrix inhabit decomposing wood in streams. The life cycles and ecological relationships of L. nigrilinea (Doane) and L. fenderi Alexander were investigated to determine how they exploit and respond to the wood habitat and their role in degradation of woody debris in western Oregon. Surveys of the western states and provinces provided data on geographical distribution of the four western species, and a key is given to the five species known from America north of Mexico. The non-adult stages are spent within single logs, primarily red alder (Alnus rubra), in headwater streams where disturbance by abrasion is minimized. The two species are sympatric in soft logs in constant contact with water. L. fenderi larvae are found in a wider variety of wood types, including harder wood, other species of wood, and in marginal habitats in which they are more susceptible to desiccation and interactions with the semi-terrestrial community. Habitat selection may relate to the specificity of ovipositing females. Biological and behavioral characteristics of the life stages are described. Both species have a basic biennial life cycle with an ephemeral (1-8 days) adult stage. This long cycle may be possible because because the habitat is relatively predictable and constant. L. fenderi is smaller than L. nigrilinea, and some individuals emerge as annuals, probably in response to an indirect thermal cue in the fall. L. nigrilinea uses combined cues of temperature and descending water level, which more directly preclict suitable conditions for emergence in spring and summer. Some L. nigrilinea take three years or longer if they do not receive the water level cue in a portion of the log. A large degree of variability exists in size of both species, and especially L. nigrilinea, within a single log. Though growth rate differences are correlated with temperature and food quality, variability in timing of oviposition probably accounts for the majority of the difference. The life history traits of L. fenderi are more conducive to increasing reproductive quantity when compared to L. nigrilinea, which may be related to the association of L. fenderi with relatively less predictable habitats. The mean biomass of L. nigrilinea was near 12.0 mg/100 cm², with low variation, while L. fenderi had low biomass in the fall (1.40 mg/100 cm²) and peaked prior to emergence (12.65 mg/100 cm²). Wood hardness was the major determinant of larval densities and feeding rates of both species. Egestion rates were estimated at 88% of dry weight per day in firm wood and 223% per day in highly decayed material. Egestion is increased by a factor of two between low (3°C) and high (15°C) temperatures encountered in the field. The total ecological impact will be the greatest where the highest abundance of the most suitable habitat is found, as in small streams during the middle stages of forest succession.
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