Abstract
Organisms can have large effects on the physical properties of the habitats where they live. For example, measurements in laboratory stream microcosms have shown that the presence of silk net-spinning insect larvae (Trichoptera: Hydropsychidae) can increase the shear force required to initiate movement of riverbed sediments. Few studies, however, have moved beyond laboratory settings to quantify the engineering impacts of aquatic insects under more complex field conditions. To bridge the gap between small-scale laboratory experiments and natural stream ecosystems, we conducted experiments in large (50 m2) outdoor river channels where net-spinning aquatic insects were manipulated in sediment patches that were 5 to 25 times larger than in previous studies. We tested whether larvae of two caddisfly species (Arctopsyche californica and Ceratopsyche oslari) influenced the stability of gravel during simulated floods when alone in monoculture and together in polyculture. On average, populations of caddisflies increased the critical shear stress required to initiate sediment movement by 20% compared to treatments without caddisflies. Per capita effects of caddisflies on sediment stability were similar between previous laboratory studies and this field experiment, and Arctopsyche had a larger per capita effect than Ceratopsyche, perhaps because of its larger size and stronger silk. Contrary to prior laboratory flume results, the effects of the two species on critical shear stress when together were similar to the additive expectation of both species when alone, but effects of the two species together were higher than the additive expectation when we accounted for density. Comparisons of total population and per capita effects suggest that caddisfly density, identity, and coexisting species likely have effects on the magnitude of caddisfly impacts on critical shear stress. Our findings imply that consideration of both the abundances and traits of ecosystem engineers is needed to describe and model their effects on sediment mobility.
Highlights
The influence of organisms on their abiotic environment has been the focus of a long history of research [1,2,3]
Values reported are the means of the three riffles across the three sampling dates for each creek. In this semi-natural experiment, we found that critical shear stress was significantly higher in all caddisfly larvae treatments than it was in controls without caddisflies
This result was qualitatively consistent with previous laboratory experiments, differences in critical shear stress among caddisfly treatments, and between observed and expected values of critical shear stress for polycultures, were not consistent between the semi-natural and laboratory experiments
Summary
The influence of organisms on their abiotic environment has been the focus of a long history of research [1,2,3]. The short colonization time of working patches (three days) probably resulted in biofilm, detrital, and invertebrate assemblages different from those in the surrounding channels, previous studies have reported that the shear stress needed to mobilize sediments did not differ between gravel substrate without hydropsychids that were and were not pre-conditioned for three to eight weeks in streams, indicating that these biological conditions did not affect sediment movement [27,29]. Because most previous studies have examined the effects of single or amalgams of hydropsychid species on substrate movement [30,38], this study represents an advance in examining the individual and combined effects of different hydropsychid species on sediment mobilization under semi-natural conditions, allowing us to link the impacts of diversity and of different ecosystem engineers on physical processes. We addressed the following questions in this study: Are the effects of caddisflies on the critical shear stress required for sediment movement detectable in sediment patches an order of magnitude greater, and under more complex outdoor conditions, than those used in previous experiments? If so, are the effects of caddisflies on incipient sediment motion in more realistic, semi-natural stream channel patches similar in magnitude to the effects detected in the laboratory? Do species identity and interspecific interactions influence the engineering effects of caddisfly larvae on sediment movement under these semi-natural conditions? We hypothesized that caddisflies would increase the critical shear stress necessary for sediment movement, that the per capita effect of the larger-bodied Arctopsyche would be greater than that of the smaller Ceratopsyche, and that the effects of both species when together would be greater than the additive effects predicted from the effects of each species when alone; we expected the magnitude of these effects to be smaller in the semi-natural channels than in the laboratory because of the high caddisfly densities and restricted caddisfly dispersal typical of laboratory microcosm experiments [30,38]
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