OOS 5-10
Ecological succession as evidenced by colonization of Arctostaphylos host plants by Tamalia galling aphids
Fire is a major mode of disturbance maintaining chaparral ecosystems in California. The heat and smoke characterizing fires act to stimulate germination of manzanita (Arctostaphylos) shrubs, an important element of the chaparral. As succession proceeds in the wake of fire, young shrubs are colonized first by gall-inducing Tamalia aphids, followed closely by congeneric inquiline (gall-invading) aphids. Following a 1999 wildfire in the Cascade Range foothills, we have tracked the appearance of young plants and their colonization by Tamalia aphids beginning in 2003. We tested the hypothesis of minimum dispersal distance to predict relative rates of colonization by gall-inducers and inquilines. Our methods included mapping the spatial distribution of 500 host plants in a 1-hectare study population using a high-precision Trimble GPS instrument and the program arcGIS to process data. We surveyed all young plants and a representative proportion of mature plants to estimate the frequency of plants colonized by gall-inducers, and we sampled galls over the entire growing season to estimate the frequency of inquilines colonizing galls.
Results/Conclusions
Our results show that, starting in 2008, less than 4% of the 135 juvenile plants were colonized by Tamalia aphids as evidenced by the presence of fresh galls. The proportion of plants colonized has since then increased continuously: over 30% had been colonized by 2013. Dissections of sampled galls indicate a mean lag of less than one year between first colonization of young plants by gall-inducers and the appearance of inquilines, but in 18 cases, no lag was detected (gall-inducers and inquilines appeared together in the first samples collected). Assuming Tamalia aphids colonize young plants (sinks) from existing populations on mature plants (sources), our results are generally consistent with a minimum dispersal distance hypothesis, although spatial scale varies across one order of magnitude. Our data suggest inquilines can disperse and colonize new habitats efficiently, in synchronization with their gall-inducing hosts. Our results have implications for patterns of evolutionary diversification in both gall-inducer and inquiline lineages.