Prevalence of Chemical Defenses among Freshwater Plants

Abstract

Although macrophyte herbivore interactions in freshwater systems were generally disregarded for many years, recent data suggest that herbivory can be intense and important in structuring freshwater communities. This has led to the hypothesis that chemical defenses should be common among freshwater plants, but few studies have reported such chemical defenses, and no previous studies have assessed the frequency of chemical defenses among a substantial number of freshwater plant species. In a study of 21 macrophyte species co-occurring with the omnivorous crayfish Procambarus acutus in a southeastern USA wetland environment, we found that extracts of 11 species (52%) deterred feeding by P. acutus when tested in artificial foods at natural concentrations. Of these 11 chemically defended species, one species, Eupatorium capillifolium, consistently had a more unpalatable extract following mechanical damage to plant tissue, indicative of an activated chemical defense. Because herbivores are commonly nitrogen-limited and select food based on several plant traits, including plant nutritional value, it might be expected that chemical defenses would be especially important for protein-rich plants. However, we found no relationship between soluble protein concentration and deterrence of plant extracts.

Chemical Defense in Elodea nuttallii Reduces Feeding and Growth of Aquatic Herbivorous Lepidoptera

“…Not many studies have yet investigated chemical defenses in submerged freshwater angiosperms. Many studies have focused on marine macroalgae (Hay 1996;Paul et al 2006) and emergent or floating-leaved freshwater angiosperms Dorn et al 2001;Kubanek et al 2001;Prusak et al 2005). These studies often used omnivorous crustaceans as grazers, against which extracts of submersed species have turned out to be inactive Prusak et al 2005).…”

“…Many studies have focused on marine macroalgae (Hay 1996;Paul et al 2006) and emergent or floating-leaved freshwater angiosperms Dorn et al 2001;Kubanek et al 2001;Prusak et al 2005). These studies often used omnivorous crustaceans as grazers, against which extracts of submersed species have turned out to be inactive Prusak et al 2005). In contrast, other authors have suggested chemical defenses in submerged freshwater macrophytes against planktivorous Daphnia, lepidoptera, and gastropod herbivores based on observed effects and correlations between secondary plant metabolites and feeding activity or changes in behavior (Pennak 1973;Burks et al 2000;Choi et al 2002;Cohen et al 2002;Elger and Lemoine 2005).…”

Plant Antiherbivore Defense in Diverse Environments

“…Defense and resistance mechanisms against herbivores have been poorly understood in freshwater; even so we now know that freshwater plants are frequently chemically or structurally defended from consumers [29][30][31]. Structural defenses are more commonly found among upland plants than wetland plants [22]; in some cases, we can find thorns or tough leaves [32].…”

“…Structural defenses are more commonly found among upland plants than wetland plants [22]; in some cases, we can find thorns or tough leaves [32]. Chemical defenses are more widespread in macrophytes [23,31] as well as in various algae, cyanobacteria [22].…”

“…The different types of chemical defenses can vary between species, localities, time, and environmental conditions [31]. Many of them have not been identified; some studies have found multiple dissuasive components in the chemical extracts analyzed, but the low concentrations or their unstable state makes their identification difficult and therefore their correlation with the dynamics of the aquatic community [35].…”