Food Limitation Affects Algivory and Grazer Performance for New Zealand Stream Macroinvertebrates

Abstract

We studied herbivory and grazer performance (i.e., fitness correlates) for the hydrobiid snail Potamopyrgus antipodarum, the leptophlebiid mayfly Deleatidium spp., and the conoesucid caddisfly Pycnocentrodes aeris, common, co-occurring algivores in many New Zealand streams. Grazing effects and costs of coexisting differed among these taxa reared at ambient densities in different combinations in microcosms with algal food conditions (on clay tiles) characteristic of heavily grazed streams. The prostrate diatoms Staurosirella leptostauron, Cymbella novazealandia, and Achnanthidium minutissimum were the dominant algal species on pre-and post-grazed tiles. The relative abundance of erect physiognomic forms, dominated by Synedra ulna and Fragilaria vaucheriae, were 2-3Â higher in ungrazed controls and in snail alone treatments than in other grazer treatments. The green filamentous algae Mougeotia sp. and Stigeoclonium lubricum, and the cyanophyte Merismopedia glauca were present only in ungrazed controls. Grazers significantly reduced algal community biomass in treatments by 26-52% relative to controls, except snails alone. Snails (15-30%) burrowed into surrounding sand substrates, dampening their grazing impact on tiles. Caddisflies were more effective than mayflies or snails at removing algae because of higher foraging rates, a larger body size, and an abrasive sand-grained case. Algal biomass reductions did not affect grazer growth. However, pre-pupation rates of caddisflies and emergence rates of subimago mayflies were significantly higher in caddisfly-alone and mayfly-alone treatments, respectively, than in combined-species treatments. These results imply that a limited periphytic food supply (<0.3 mg AFDM cm )2 ) even over a relatively brief period ( £ 16 d) may have population-scale consequences for co-existing P. aeris and Deleatidium spp.

The invasive New Zealand mudsnail, Potamopyrgus antipodarum, is an effective grazer of algae and altered the assemblage of diatoms more than native grazers

“…Also, P. antipodarum can remove more algal biomass than mayflies (Deleatidium spp.) and caddisflies (Pycnocentrodes aeris) ; but see Holomuzki & Biggs, 2006). Although Brachycentrus sp.…”

How do stream grazers partition their benthic habitat?

“…Baetis showed the opposite pattern (in terms of proportional representation in samples), and Epeorus showed no response to algal quantity. Although these patterns could indicate competitive displacement, it should be noted that more algae does not necessarily mean more food, since periphytic algae are not all equally palatable (Holomuzki & Biggs, 2006). For example, thin periphytic films are more readily consumed by some species (Wellnitz & Poff, 2012), and thicker mats may hinder grazer movement across substrates (Hoffman et al, 2006).…”

Dietary variation and food selection by mayfly grazers in a subtropical mountain stream

“…Epilithic algal species were categorized into five physiognomic groups, including adnate, stalked, erect, motile, and filamentous, using a modified version of the methods of Wellnitz and Ward (1998) and Holomuzki and Biggs (2006). To assess seasonal changes in the diets of the mayflies, variations in the algal compositions of their gut contents were examined using multivariate analyses in PRIMER v6 (Clarke and Gorley 2006, PRIMER-E Ltd, Plymouth, UK).…”

“…A mat of benthic algae generally consists of a variety of algal physiognomic forms (Wellnitz and Ward 1998;Holomuzki and Biggs 2006). Algae in the upper layer of the mat or the overstory are expected to be easily accessible to most aquatic insects, whereas prostrate forms in the understory can only be ingested by aquatic insects possessing mouthparts specialized for detaching these algae from the substratum (Steinman 1996).…”