An investigation of gardening in the sedentary caddisfly Tinodes waeneri across a nutrient gradient

Abstract

Sedentary species face a trade-off between the advantages of exploiting food close to their homes and the cost of defending it. Consequently, the net benefit of this lifestyle may be greatest at intermediate productivity. In aquatic systems, it has been suggested that some sedentary grazers can increase the range of circumstances under which they are able to compete with mobile grazers by enhancing food resources within their feeding territories through ‘gardening’. This was examined for the retreat-building sedentary larvae of the caddis Tinodes waeneri, which are often dominant in the littoral of lakes. The hypotheses tested were 1) T. waeneri gardens by fertilising its retreat (a fixed ‘gallery’ on which periphyton grows), and 2) gardening will be more important in lower productivity lakes. Detailed field sampling across a lake productivity gradient was coupled with a laboratory mesocosm study. A natural abundance stable isotope technique was developed to identify gardening. A survey of six populations in the English Lake District indicated that larvae garden as they fertilise gallery biofilm with excreted nitrogen and feed on their galleries. Galleries also contained more food than the epilithon and larval assimilation of galleries was related to food availability. Galleries contained a higher proportion of diatoms than the epilithon, and gallery diatom communities were associated with higher nutrient levels, especially in the lower productivity lakes. Gardening also occurred in the experimental mesocosms. Furthermore, the amount of gardening was related to nutrient levels; more gardening occurred at low nutrients than at high nutrients. Thus, ‘gardening’ is widespread in T. waeneri populations and may allow this species to be successful in low resource environments. It may also substantially affect ecosystem processes within the littoral of lakes by influencing patterns of nitrogen retention and enhancing overall productivity.