Population Biology of a Brine Fly (Diptera: Ephydridae) in the Presence of Abundant Algal Food

Collins, Nicholas

A filamentous, water-holding mat of a slow-growing green alga (genus Zygogonium) is the structural basis of a simple community of acid-tolerant algae and arthropods in Yellowstone National Park, USA. Populations of interstitial algae, mostly Euglena mutabilis and Chlamydomonas, infuse certain areas of the Zygogonium mat. The major herbivore, Ephydra thermophila, has a very high instrinsic rate of increase, but its population is relatively stable despite the apparent superabundance of algal food. However, only a small portion of this food is actually available to fly larvae during the summer months, and only a small fraction of that, the interstitial algae, provides positive nutrition for adult egg production and larval growth. Thus, nonobvious spatial heterogeneity and differences in food quality play major roles in limiting Ephydra's impact on the algal mat. Consistent with its importance for fly larval growth, the presence of interstitial algae is the primary criterion for selection of oviposition sites by adults. Use of this cue also serves to place larvae in higher temperature environments and areas most likely to be flushed free of larval toxins. Egg production of adult @V @V is severly limited by restricted access to interstitial algae, and the population of immatures is further reduced by the predatory larvae of a biting midge, Bezzia setulosa. The effect of this predator is probably density-independent. Despite these reductions, the Ephydra larvae are apparently abundant enough or food resources are diffuse enough so that adult size was reduced to 50%-65% of maximum throughout the study period. While the degree of resource limitation is relatively constant, large changes in the abundance of the flies result from seasonal alterations is flushing rates of the mat, which change the proportion of the mat environment suitable for larvae. Ephydra's inability to utilize effectively the superabundant Zygogonium may reflect anti-herbivore adaptations of the alga, but this hypothesis presumes strong herbivore impact on the alga's evolution. A less presumptuous explanation requires only that Zygognium have different proportions of fly nutrients than have interstitial algae. Nutritional adaptions for more efficient use of Zygogonium would then be selected against because they would require sacrifices in growth efficiency on the primary food, interstitial algae.

Population Biology of a Brine Fly (Diptera: Ephydridae) in the Presence of Abundant Algal Food

Series