Enhancing food supplies for waders: inconsistent effects of substratum manipulations on aquatic invertebrate biomass.
The potential of substratum manipulations as a technique for enhancing food supplies for waders was investigated in 98 constructed ponds at 6 sites in the Upper Waitaki Basin, New Zealand. Substrata were manipulated by using different construction methods, and by adding barley straw or stones to some ponds. Total invertebrate biomass and chironomid biomass were measured 3 and 15 months after ponds were constructed. Food supplies in many newly constructed ponds contained large proportions of chironomid larvae, an attractive food for waders. However, this pattern was not universal; food supplies in many other newly constructed ponds were dominated by various nektonic invertebrates, or, at one site, by molluscs. Food supplies developed rapidly; 3 months after construction most ponds contained similar or greater total invertebrate biomass (mean, range: 1.20, 0.10-7.66 g m-2 dry mass) and greater chironomid biomass (0.66, 0-5.41 g m-2) than nearby 'pre-existing' wetlands (1.79, 0.39-6.18 g m-2 total invertebrate dry mass; 0.01, 0-0.08 g m-2 chironomid dry mass). However, ponds with stony substrata contained relatively low total invertebrate and chironomid biomass. 4. Substratum manipulations affected chironomid or total invertebrate biomass at all sites. Differences in biomass between substrata in constructed ponds were large at some sites (up to 2.7 g m-2 dry mass for chironomids, and up to 3.39 g m-2 dry mass for total invertebrates), indicating that substratum manipulations have good potential as a tool for increasing food supplies for wetland birds. Despite the sometimes large effects of substratum manipulations, no technique achieved consistently positive results. The absolute and relative effects of any given substratum manipulation varied greatly among sites. Although substratum manipulations can be worthwhile, they need to be evaluated on a site by site basis.