Stream mosses as chemically-defended refugia for freshwater macroinvertebrates

Abstract

Marine and terrestrial studies show that small, sedentary herbivores that utilize plants as both food and habitat can gain enemy-free space by living on hosts that are chemically defended from larger, generalist consumers. Although large herbivores are increasingly recognized as important consumers of macrophytes in freshwater communities, the potential indirect effects of herbivory on plant-associated macroinvertebrates have rarely been studied. Here, we show that the large, generalist consumers in a riverine system, Canada geese, Branta canadensis , and crayfish, Procambarus spiculifer , both selectively consumed riverweed, Podostemum ceratophyllum , over an aquatic moss, Fontinalis novae-angliae, even though moss comprised 89% of the total plant biomass on riverine rocky shoals. Moss supported twice as many plant-associated macroinvertebrates as riverweed, suggesting that it might provide a spatial refuge from consumption by these larger consumers. Bioassay-guided fractionation of moss extracts led to the isolation of a C 18 acetylenic acid, octadeca-9,12-dien-6-ynoic acid, that deterred crayfish feeding. In contrast to results with Canada geese and crayfish, both the amphipod Crangonyx gracilis and the isopod Asellus aquaticus consumed significant amounts of moss but rejected riverweed in laboratory feeding assays. Moreover, neither amphipod nor isopod feeding was deterred by the crude organic extract of Fontinalis , suggesting that these mesograzers tolerate or circumvent the chemical defenses that deterred larger consumers. Thus, herbivory by large, generalist herbivores may drive freshwater plant community structure towards chemically defended plants and favor the ecological specialization of smaller, less mobile herbivores on unpalatable hosts that represent enemy-free space.

Factors shaping submerged bryophyte communities: A conceptual model for small mountain streams in Germany

“…They also offer macroinvertebrates shelter against physically and chemically related impacts (e.g. Glime, 1994;Parker et al, 2007). Aquatic bryophytes have rarely been used for classification purposes (e.g.…”

Detecting herbivory in two mosses from an Atlantic Forest, Brazil

“…According to the 'chemically-defended refuges' hypothesis, smaller and less mobile herbivores are favoured by grazing on unpalatable hosts and enemy-free space because large and generalist herbivores avoid chemically-defended (Parker et al, 2007) or nutrient-poor plants. This hypothesis could explain the associations between micro-herbivores and mosses verified in this study, but in this case the plants are probably a valuable alternative food source for small animals, with more proteins than phenols and no larger herbivores to compete with.…”

Induced chemical defenses in a freshwater macrophyte suppress herbivore fitness and the growth of associated microbes

“…To date, 22 species of freshwater macrophytes have been demonstrated to produce chemical defenses against herbivores (Newman et al 1996;Cronin 1998;Kubanek et al 2001;Cronin et al 2002;Prusak et al 2005;Parker et al 2006Parker et al , 2007Miller and Provenza 2007;Erhard et al 2007), and 3 species have been shown to induce either structural defenses (Center and Van 1989;Kouki 1993) or an unidentiWed defense (JeVries 1990), but only 1 species (Nuphar luteum) is known to induce a chemical defense . The chemicals responsible for this induced defense were not identiWed and the physiological eVects on herbivores were not investigated .…”