The susceptibility of 82 species of tropical seaweeds to grazing by herbivorous fishes was assessed on 8 different coral reefs in the Florida Keys, USA. Most species were simultaneously assayed for the presence or absence of unusual secondary metabolites and recorded as having either calcified or uncalcified thalli. Both production of secondary metabolites and of a calcified thallus were associated with low susceptibility to herbivory. However, the relative importance of calcification versus chemical deterrents cannot be assessed for the calcified species since almost all these also contained secondary metabolites. Eighty-five % of calcified species, but only 39 % of uncalcified species, produced secondary metabolites. Secondary metabolites were produced by 71 % of the species least susceptible to herbivory (75 % eaten). Calcified thalli were produced by 50 % of the lowest preference species but by only 9 % of the highest preference species. Thus, several common reef seaweeds appear to resist herbivory by relying primarily on chemical deterrents (genera Dictyota, Dilophus, Stypopodium, Lobophora, Avrainvillea, and some Caulerpa species) but many appear to combine both chemical and morphological defenses (genera Penicillus, Halimeda, Rhipocephalus, Udotea, Amphiroa, and Galaxaura). We suggest that multiple defenses will be common among seaweeds on coral reefs since herbivore diversity is high and it is unlikely that any single defense will be effective against the many types of herbivores that encounter these plants. This may account, in part, for the diversity of secondary metabolites produced by some tropical seaweeds.
Coral reefs are in precipitous global decline. In the last 3 – 4 decades, coral cover has declined by 80% throughout the Caribbean and 50% throughout the tropical Pacific, with seaweeds commonly replacing corals. Much of the decline and lack of recovery can be attributed to alterations in fundamental biotic interactions that are mediated via bioactive secondary metabolites. Experiments on Fijian reefs demonstrate that herbivory by specific mixes of herbivorous fishes is critical for suppressing chemically-rich seaweeds that damage corals on contact via allelopathic lipids on their surfaces. Of equal importance is how coral and fish larvae respond to chemical cues from overfished areas dominated by seaweeds versus no-take marine protected areas (MPAs) dominated by corals. Recruiting fishes and corals chemically sense and are attracted to coral dominated areas protected from fishing while being chemically repulsed by seaweed dominated areas that are overfished. Attraction and repulsion are cued by odors from specific corals and seaweeds that best predict reef quality. Both recruiting fishes and coral larvae refused to settle in overfished, seaweed dominated areas, but recruited readily to immediately adjacent reefs where fishing was banned and corals dominated. These chemically-cued behaviors can close the open nature of marine populations, suppress larval export from coral dominated marine protected areas to degraded reefs, and prevent recovery of coral and fish populations once reefs degrade and become dominated by seaweeds.
Abstract Bioprospecting is the exploration of biodiversity for new resources of social and commercial value. It is carried out by a wide range of established industries such as pharmaceuticals, manufacturing and agriculture as well as a wide range of comparatively new ones such as aquaculture, bioremediation, biomining, biomimetic engineering and nanotechnology. The benefits of bioprospecting have emerged from such a wide range of organisms and environments worldwide that it is not possible to predict what species or habitats will be critical to society, or industry, in the future. The benefits include an unexpected variety of products that include chemicals, genes, metabolic pathways, structures, materials and behaviours. These may provide physical blueprints or inspiration for new designs. Criticism aimed at bioprospecting has been addressed, in part, by international treaties and legal agreements aimed at stopping biopiracy and many activities are now funded by agencies that require capacity‐building and economic benefits in host countries. Thus, much contemporary bioprospecting has multiple goals, including the conservation of biodiversity, the sustainable management of natural resources and economic development. Ecologists are involved in three vital ways: first, applying ecological principles to the discovery of new resources. In this context, natural history becomes a vast economic database. Second, carrying out field studies, most of them demographic, to help regulate the harvest of wild species. Third, emphasizing the profound importance of millions of mostly microscopic species to the global economy.
The separate and interactive effects of light intensity and nutrient availability on the growth, protein content, concentration of C-based secondary metabolites, and susceptibility to herbivory of two brown seaweeds, Dictyota ciliolata (Dictyotales) and Sargassum filipendula (Fucales), were assessed in the context of the carbon/nutrient balance (CNB) hypothesis. Responses of Dictyota and its terpenoid secondary metabolites were often at variance with predictions of the CNB hypothesis while several responses of Sargassum and its phenolic secondary metabolites were often predicted by the CNB hypothesis. Findings for these, and other, seaweeds parallel recent terrestrial studies suggesting that the CNB hypothesis rarely predicts responses of terpene producing plants, but more commonly predicts responses of phenolic producing plants. In a microcosm experiment, nutrient addition significantly increased Dictyota growth but had no effect on concentrations of sterols, proteins, terpenes, or susceptibility to herbivores. In a field experiment, nutrient addition did not affect growth or protein levels, generally increased terpenoid metabolites, but did not affect the susceptibility of Dictyota to herbivores. Increased light in outdoor microcosms resulted in more growth, reduced terpene concentrations, and increased protein content in Dictyota. The high-light plants with more protein and less terpenes were preferentially consumed by the sea urchin Arhacia punctulata, but were eaten significantly less readily by the amphipod Ampithoe longimana. In a field experiment, higher light intensity increased growth but had no effect on secondary metabolites, protein content, or the alga's susceptibility to herbivores. For Sargassum filipendula, plants that settled from spores and grew naturally in an experimentally shaded environment had lower concentrations of phlorotannins than less-shaded plants, but this difference did not affect the plants' susceptibility to amphipod grazing. Branches of S. filipendula transplanted to a heavily shaded environment grew less than branches transplanted to a less shaded environment (i.e., light limited growth) ; however, the concentration of protein and phlorotannins, and the susceptibility of plants to amphipod grazing did not differ. Nutrient addition had no effect on protein or phlorotannin concentrations, but significantly increased the palatability of plants in the higher light treatment. Although the CNB hypothesis has been influential and provides a theoretical template for predicting how various stresses will affect plant defenses and susceptibility to herbivory, ecologists need to consider additional factors in predicting the responses of plants to environmental variation.
Article Chemical Defenses, Protein Content, and Susceptibility to Herbivory of Diploid vs. Haploid Stages of the Isomorphic Brown Alga Dictyota ciliolata (Phaeophyta) was published on January 1, 1996 in the journal Botanica Marina (volume 39, issue 1-6).
This study further highlights the importance of biodiversity in affecting ecosystem function in that different species of herbivorous fishes had very different impacts on reef communities depending on the developmental stage of the community. The species-specific effects of herbivorous fishes suggest that a species-rich herbivore fauna can be critical in providing the resilience that reefs need for recovery from common disturbances such as coral bleaching and storm damage.
Although the South Atlantic Bight has experienced few invasions of invertebrate macro-fauna, the green porcelain crab, Petrolisthes armatus, recently invaded oyster reefs at densities of thousands m. We assessed the spatial and temporal dynamics of P. armatus densities, sex ratios, sizes, and reproductive patterns via a large-scale monitoring program in two estuaries of coastal Georgia, U.S.A. Exotic crab densities generally were higher in the low versus the high intertidal and in the lower versus upper regions of estuaries. In warmer months, 20 to 90% of adult females were gravid, recruits were dense, and population levels were high. In colder months, densities dropped by 64 to >99%. Male:female ratios were near 1:1 across times and locations. Native mud crab densities were unrelated to, or positively correlated with, densities of exotic crabs; correlations were never significantly negative. These correlations did not lend support to the hypothesis that the invasive crab is negatively affecting native crabs with which it most closely associates. Maximum mean densities of P. armatus in Georgia were 37 times the highest densities recorded in the historic range, crabs in the new range reproduced at a smaller size, and the percentage of gravid females was similar between the old and new range. Thus, population fecundity in the new range exceeds that of the native range by more than an order of magnitude. The effects of high density, rapidly
Numerous studies have assessed the individual effects of prey nutritional quality or chemical defenses on consumer feeding behavior. However, little is known about how these traits interact to affect consumer feeding and performance. We tested the separate and interactive effects of prey chemical defenses and nutritional quality on the feeding behavior and fitness of six sympatric crustacean mesograzers. Natural concentrations of diterpene alcohols (dictyols) from the brown alga Dictyota menstrualis were incorporated, or not incorporated, into lower quality and higher quality foods to create artificial diets mimicking prey of variable value and defense. Five amphipods (Ampithoe longimana, A. valida, Cymadusa compta, Gammarus mucronatus, and Elasmopus levis) and one isopod (Paracerceis caudata), representing a continuum of closely to distantly related organisms, were fed intact algae or lower and higher quality diets containing or lacking dictyols. All six mesograzers preferred the green alga Enteromorpha intestinalis to the dictyol producing alga Dictyota menstrualis. In assays allowing consumers to choose between simultaneously available foods, dictyols deterred feeding by all five amphipods, but not the isopod; this occurred for both lower and higher quality foods. In no-choice assays, where consumers were confined with only one of our four treatment diets, effects on feeding became more complex. Nutritional quality alone affected feeding by five of the six species. These grazers compensated for lower quality by increasing consumption. Dictyols suppressed feeding for four of the six species. More interestingly, there were significant dictyol x quality interactions for three species. Dictyols decreased feeding more when placed in lower quality foods than higher quality foods. Two amphipods deterred by dictyols in the choice assays readily consumed dictyol-containing foods in no-choice situations and suffered few negative effects of doing so. Although all amphipods were deterred by dictyols in choice assays, dictyols decreased fitness (survivorship, growth, or reproduction) for only four of the five species. These effects included large and immediate decreases in survivorship, dramatic effects on reproduction, and modest effects on female growth. Dictyols enhanced survivorship of the isopod. Thus, the effects of secondary metabolites on feeding in choice situations vs. fitness in long-term assays were inconsistent. For three amphipods, certain effects of food quality, dictyols, or their interaction were detected only for females. In general, negative effects of dictyols on fitness were greater in lower than in higher quality foods, suggesting that prey nutritional value may counteract the effects of defensive metabolites. For example, when G. mucronatus consumed dictyols in lower quality foods, mortality was >80% by day 5; for dictyols in higher quality foods, 80% mortality took 28 days to develop. Lower quality foods alone significantly decreased growth for the isopod, three of the amphipods, and the females of a fourth amphipod, concomitantly reducing fecundity for four of the five amphipods studied. The effects of both chemical defenses and nutritional quality were unrelated to consumer phylogeny; responses of congeners varied as much, or more, than responses of more distantly related consumers. Understanding mechanisms and consequences of food selection requires that the interactive effects of both chemical defenses and prey nutritional characteristics be considered explicitly.
