Limnephilus

Abstract: We conducted a series of field and laboratory experiments to determine the direct and indirect effects of a top predator, the tiger salamander (Ambystoma tigrinum nebulosum), on larvae of two species of limnephilid caddisflies (Limnephilus externus and Asynarchus nigriculus) in subalpine wetlands in central Colorado. Asynarchus larvae pre- dominate in temporary wetlands and are aggressive intraguild predators on Limnephilus larvae, which only predominate in permanent basins with salamanders. We first conducted a field experiment in mesocosms (cattle tanks) to quantify the predatory effects of different life stages of salamanders on the two caddisfly species. Two life stages of the salamanders (larvae and paedomorphs) preferentially preyed on Asynarchus relative to Limnephilus. Subsequent laboratory experiments revealed that high Asynarchus activity rates and rela- tively ineffective antipredatory behaviors led to higher salamander detection and attack rates compared to Limnephilus. In a second field experiment (full factorial for presence and absence of each of the three species), we found that salamander predation on Asynarchus had an indirect positive effect on Limnephilus: survival was higher in the presence of salamanders + Asynarchus than with just Asynarchus. In the laboratory we compared the predatory effects of salamanders with and without their mouths sewn shut and found the observed indirect positive effect on Limnephilus survival to be mainly the result of reduced numbers of Asynarchus rather than salamander-induced changes in Asynarchus behavior. We argue that indirect effects of predator-predator interactions on shared prey will be mainly density-mediated and not trait-mediated when one of the predators (in this case, Asynarchus) is under strong selection for rapid growth and therefore does not modify foraging behaviors in response to the other predator. The reciprocal dominance of Lim- nephilus and Asynarchus in habitats with and without salamanders probably reflects a trade- off between competitive superiority and vulnerability to predation. The high activity levels and aggressiveness that enable Asynarchus to complete development in temporary habitats result in strong asymmetric competition (via intraguild predation) with Limnephilus. In permanent habitats these same behaviors increase Asynarchus vulnerability to salamander predation, which indirectly benefits Limnephilus. This and previous work implicate sala- manders as keystone predators that exert a major influence on the composition of benthic and planktonic assemblages in subalpine wetlands.

Abstract: Watercress (Nasturtium officinale) is a spring-stream macrophyte that pos- sesses glucosinolates, which are hydrolyzed to feeding deterrent isothiocyanates when the enzyme myrosinase is released by tissue damage. Previous studies indicated that frequently associated aquatic shredders strongly prefer yellowed-senescent leaves over fresh-green foliage, because the latter releases much more isothiocyanate than senescent watercress. When the action of myrosinase was blocked by heating the tissue, the shredders' preference shifted to heated-green tissue, which contains much more nitrogen than that found in senescent tissue. Here we report a series of no-choice experiments on various tissue types designed to determine intermediate and long-term consumption rates and associated impacts on growth and survival of shredders associated with watercress. The amphipod Gammarus pseudo- limnaeus, the caddisflies Pycnopsyche sp., Hesperophylax designatus, and Limnephilus sp., and the snail Physella gyrina, all consumed much more senescent than fresh-green tissue. In long-term tests with G. pseudolimnaeus, H. designatus, and Limnephilus sp., growth was negative or zero on the defended fresh-green tissue and often highest on heated-green tissue. Reaction to glucosinolates differed among shredders. For the caddisflies, growth and survival were positively correlated with bulk tissue and nitrogen consumption, which were highest for heated-green tissue. However, amphipod survival was significantly lowered on heated- green watercress, although it was readily consumed. Therefore, consumption of high-glu- cosinolate tissue was detrimental to the amphipods but not to caddisflies. Our results indicate that the glucosinolate-myrosinase system defends live green wa- tercress against herbivory by an array of aquatic invertebrates. These generalist herbivores forgo eating high-quality fresh-green leaves because of plant defenses and instead settle for lower quality senescent leaves. When the defense system was experimentally shut down, these herbivores readily consumed young green tissue and frequently showed higher growth rates than those achieved on the senescent, but undefended, leaves they typically consume. Evidently, these shredders face a trade-off of high nitrogen and high defense vs. low nitrogen and low defense.

Abstract: Species replacements along freshwater permanence gradients are well documented, but underlying mechanisms are poorly understood for most taxa. In subalpine wetlands in Colorado, the relative abundance of caddisfly larvae shifts from temporary to permanent basins. Predators on caddisflies also shift along this gradient; salamanders (Ambystoma tigrinum nebulosum) in permanent ponds are replaced by predaceous diving beetles (Dytiscus dauricus) in temporary habitats. We conducted laboratory and field experiments to determine the effectiveness of caddisfly cases in reducing vulnerability to these predators. We found that larvae of a temporary-habitat caddisfly (Asynarchus nigriculus) were the most vulnerable to salamanders. Two relatively invulnerable species (Limnephilus externus, L. picturatus) exhibited behaviors that reduced the likelihood of detection and attack, whereas the least vulnerable species (Agrypnia deflata) was frequently detected and attacked, but rarely captured because cases provided an effective refuge. Vulnerability to beetle predation was also affected by cases. The stout cases of L. externus larvae frequently deterred beetle larvae, whereas the tubular cases of the other species were relatively ineffective. Two of these vulnerable species (A. nigriculus and L. picturatus) often co-occur with beetles; thus, case construction alone is insufficient to explain patterns of caddisfly coexistence along the permanence gradient. One explanation for the coexistence of these two species with beetles is that they develop rapidly during early summer and pupate before beetle larvae become abundant. One species (L. picturatus) pupates by burying into soft substrates that serve as a refuge. The other (A. nigriculus) builds stone pupal cases, which in field experiments, more than doubles survival compared to organic pupal cases. The combined results of these experiments suggest that caddisfly distributions along permanence gradients depend on a suite of primary and secondary predator defenses that include larval and pupal case structure, predator-specific escape behaviors, and the phenology of larval development.

Abstract: A checklist is provided for the 1653 species of caddisflies known from the United States (including Hawaii), Canada, Greenland, and Mexico. These species are classified in 164 genera in 24 families. The six families Hydroptilidae (308 spp.), Limnephilidae (294 spp.), Hydropsychidae (184 spp.), Rhyacophilidae (124 spp.), Glossosomatidae (113 spp.), and Polycentropodidae (112 spp.) include 69% of the species. Species of the six genera Rhyacophila (123 spp.), Limnephilus (112 spp.), Hydroptila (111 spp.), Ochrotrichia (73 spp.), Lepidostoma (69 spp.), and Polycentropus (61 spp.) comprise 33% of the North American +