Friday, August 7, 2009

PS 81-60: Extinction-colonization dynamics of a spore-feeding beetle community on long-lived bracket fungi Ganoderma in New Zealand

Kohmei Kadowaki, The University of Auckland


In patch dynamics models, competition-colonization trade-offs have been a central but controversial coexistence mechanism of competitors exploiting the same resource in a spatially homogenous environment.  Many empirical studies have focused on correlation between life-history traits, local coexistence patterns in annual plants, plant-ant mutualist guilds, and host-parasitoid systems.  However, they often suffered from mismatches between theory and study systems and practical difficulties in observing extinction-colonization dynamics.  Spore-feeding beetle communities on long-lived bracket fungi offer an excellent solution.  Such spore-feeding beetles can compete within patches (i.e. sporocarps) during their larval stage, and each patch is connected only by the adult-stage dispersal.  Also, measuring local resource dynamics and extinction-colonization dynamics of competitors is feasible.  We undertook an observational study of a three-species beetle community on Ganoderma spp. in northern New Zealand. 


Our longitudinal results over 25 patches showed that spore production did not depend on environmental variables, but was controlled by spatial asynchronous fluctuations.  The extinction-colonization dynamics of the beetle community was explained primarily by competitive interaction between spore-feeders, as well as demographic stochasticity at the initial stage of population establishment. We suggest that the superior competitor Holopsis sp.1 (Corylophidae) could outcompete the inferior competitor Zearagytodes maculifer Brown (Leiodidae).  Such displacement might occur because the long rostrum of Holopsis sp.1 allowed it to consume immature spores which would be not available to other competitors.  The inferior competitor Z. maculifer had a refuge due to seasonality and/or high dispersal ability, but the competition-colonization trade-off model failed to give a unified explanation to the community-level coexistence.