Coral reefs in crisis: reversing the biotic death spiral

Coral reefs are disappearing due to global warming, overfishing, ocean acidification, pollution, and interactions of these and other stresses. Ecologically informed management of fishes that facilitate corals by suppressing seaweeds may be our best bet for bringing reefs back from the brink of extinction

Herbivory, algal distribution, and the maintenance of between-habitat diversity on a tropical fringing reef

© 1981 The University of ChicagoThe bases of coral reefs in the Caribbean often abut sandy plains covered by sea grasses (Randall 1965; Ogden et al. 1973b) or algae (Earle 1972; Dahl 1973). Interactions occurring at the border of reefs and sea grass beds have been studied on several occasions (Randall 1965; Ogden et al. 1973b; Ogden 1976; Parrish and Zimmerman 1977; Ogden and Lobel 1978), but little is known about those which occur between reefs and sandy plains dominated by algae. Unlike sea grasses, which root into the sand, many of the algal species that occur on sand plains require hard substrates (Dahl 1973) such as shells and coral fragments. Suitable attachment sites are uncommon on the sand plain at Galeta Point, Panama, and many are periodically buried or turned over during heavy seas. Paradoxically algal species that predominate on the sand plain tend to be rare or absent from the shallower reef slope where stable, hard substrate is abundant. The maintenance of such distinct distributional boundaries is often attributed to differential competitive abilities (Connell 1961; Holmes 1961) or to restrictive specialization to particular physical regimes (Doty 1946; Terborgh 1971). These factors have been hypothesized to be especially important (Dobzhansky 1950; Janzen 1967; Ashton 1969; Diamond 1975) in diverse tropical communities where specialization to narrow niches is thought to promote resource partitioning and allow increased coexistence. While such explanations are often consistent with observed patterns, they are seldom tested using controlled field experiments. Without such field manipulations it is impossible to adequately assess the relative importance of competition, predation, and physical stress in determining the distribution and abundance of species or the intensity of interactions that occur between them. In this paper I examine experimentally the factors maintaining these separate algal assemblages and contend that the sand-plain species (algae that are almost never found on the reef) would competitively exclude other species from the reef slope if they were not selectively removed by reef-associated grazers. Competition and physiological specialization appear to have no effect on excluding sandplain species from the reef slope. In the remainder of the paper I test the following hypotheses. (1) Low light and scarcity of attachment sites severely limit the growth of algae on the sand plain. (2) Sand-plain algae are not physiologically restricted to deep waters; they are most fit in the physical regime typical of shallower reef-slope habitats. (3) Reef-associated grazers are responsible for excluding sand-plain species from the reef slope. (4) In the absence of herbivores, sand-plain species competitively dominate reef-slope species. (5) Because of differential competitive ability, sand-plain genera are better adapted for temperate areas than are reef-slope genera

Chemically mediated behavior of recruiting corals and fishes: A tipping point that may limit reef recovery

Coral reefs are in global decline, converting from dominance by coral to dominance by seaweed. Once seaweeds become abundant, coral recovery is suppressed unless herbivores return to remove seaweeds, and corals then recruit. Variance in the recovery of fishes and corals is not well understood.We show that juveniles of both corals and fishes are repelled by chemical cues from fished, seaweed-dominated reefs but attracted to cues from coral-dominated areas where fishing is prohibited. Chemical cues of specific seaweeds from degraded reefs repulsed recruits, and cues from specific corals that are typical of healthy reefs attracted recruits. Juveniles were present at but behaviorally avoided recruiting to degraded reefs dominated by seaweeds. For recovery, degraded reefs may need to be managed to produce cues that attract, rather than repel, recruiting corals and fishes

Chemical Defense of the Eastern Newt (Notophthalmus viridescens): Variation in Efficiency against Different Consumers and in Different Habitats

Amphibian secondary metabolites are well known chemically, but their ecological functions are poorly understood—even for well-studied species. For example, the eastern newt (Notophthalmus viridescens) is a well known secretor of tetrodotoxin (TTX), with this compound hypothesized to facilitate this salamander's coexistence with a variety of aquatic consumers across the eastern United States. However, this assumption of chemical defense is primarily based on observational data with low replication against only a few predator types. Therefore, we tested the hypothesis that N. viridescens is chemically defended against co-occurring fishes, invertebrates, and amphibian generalist predators and that this defense confers high survivorship when newts are transplanted into both fish-containing and fishless habitats. We found that adult eastern newts were unpalatable to predatory fishes (Micropterus salmoides, Lepomis macrochirus) and a crayfish (Procambarus clarkii), but were readily consumed by bullfrogs (Lithobates catesbeianus). The eggs and neonate larvae were also unpalatable to fish (L. macrochirus). Bioassay-guided fractionation confirmed that deterrence is chemical and that ecologically relevant concentrations of TTX would deter feeding. Despite predatory fishes rejecting eastern newts in laboratory assays, field experiments demonstrated that tethered newts suffered high rates of predation in fish-containing ponds. We suggest that this may be due to predation by amphibians (frogs) and reptiles (turtles) that co-occur with fishes rather than from fishes directly. Fishes suppress invertebrate consumers that prey on bullfrog larvae, leading to higher bullfrog densities in fish containing ponds and thus considerable consumption of newts due to bullfrog tolerance of newt chemical defenses. Amphibian chemical defenses, and consumer responses to them, may be more complex and indirect than previously appreciated

GEOGRAPHIC VARIATION AMONG HERBIVORE POPULATIONS IN TOLERANCE FOR A CHEMICALLY RICH SEAWEED

Previous investigations have shown that the sedentary amphipod Ampithoe longimana escapes consumers by selectively living on and eating chemically defended seaweeds in the genus Dictyota. However, A. longimana and Dictyota overlap only in the southern portion of the amphipod's range; Dictyota is not available to amphipods from more northerly regions. We used this disjunction in distribution to test the hypothesis that A. longimana populations co-occurring with Dictyota would have greater tolerance for the seaweed's chemical defenses and would display higher feeding preference for, and fitness on, the seaweed than would more northerly populations. We also evaluated the genetic vs. phenotypic basis of these patterns and attempted to detect trade-offs between tolerance for Dictyota and ability to use other plants as foods. Such geographic studies of herbivory have been conducted using terrestrial insects, but few studies have focused on other herbivores, and this is especially true for marine systems. In multiple-choice feeding assays with both field-collected and laboratory-reared adults, a North Carolina population of A. longimana sympatric with Dictyota more readily fed on Dictyota and was more resistant to Dictyota's deterrent chemistry than was a Connecticut population from outside of Dictyota's geographic range. When raised on Dictyota menstrualis and D. ciliolata, A. longimana juveniles from North Carolina grew faster, matured faster, and produced more reproductive females than did Connecticut juveniles. The differential tolerance for Dictyota has a genetic basis, as it was maintained through two generations grown to maturity in a common environment. When several northern and southern populations were assayed, they displayed similar regional differences in feeding preferences. Though southern juveniles had higher fitness on Dictyota than northern juveniles, southern juveniles performed as well as northern juveniles when raised on seven other seaweeds, including seaweeds (e.g., Fucus vesiculosus and Sargassum filipendula) that produce secondary metabolites in a different class from those found in Dictyota. Thus, tolerating Dictyota did not incur detectable performance trade-offs when juveniles were confined to feeding on alternative seaweeds. Our results suggest that the evolution of host preferences may depend more on the host value as a refuge from enemies than on minimizing the costs of tolerating plant secondary metabolites

Susceptibility to herbivores depends on recent history of both the plant and animal

DOI: 10.2307/2265549©1996 Ecological Society of AmericaPhysical stress to seaweeds and hunger stress of herbivores can influence the outcome of chemically mediated seaweed—herbivore interactions. The unpalatable brown seaweed Dictyota ciliolata produces the diterpenoid secondary metabolites pachydictyol A, dictyol B acetate, and dictyodial. At natural concentrations, pachydictyol A deterred the sea urchin Arbacia punctulata but did not inhibit feeding by the pinfish Lagodon rhomboides or the amphipod Ampithoe longimana until concentrations were 2.5—5 times natural levels. Dictyol B acetate deterred the urchin, amphipod, and pinfish at, or far below, natural concentrations. Dictyodial was too unstable to assay directly, but indirect experiments suggested that natural concentrations deterred the urchin, but not the pinfish or amphipod. Mild desiccation of D. ciliolata reduced concentrations of the different secondary metabolites by 7—38% and plants became 2.6—3.4 times more susceptible to urchin and amphipod grazing. The combined concentrations of pachydictyol A and dictyol B acetate found in undesiccated Dictyota ciliolata deterred feeding by urchins, but this deterrent effect was lost at concentrations found in the desiccated plants. Desiccated and undesiccated plants did not differ in nutritive value (as measured by protein and total N content) or toughness. Thus, desiccated plants became more palatable because chemical defenses were lost, not because nutritive value was increased. The stress of near—surface ultraviolet radiation also caused significant physiological changes in Dictyota ciliolata. UV—exposed blades bleached, senesced, and grew 84% less than blades protected from UV radiation. Tissue loss and minimal growth of UV—stressed plants constrained our sample sizes, but the limited assays that could be run suggested that UV stress may lower chemical defenses and increase plant susceptibility to herbivores. Because many previous investigations of herbivore feeding patterns used animals that had been starved for days before an assay, we tested the effects of this commonly used procedure on feeding discrimination. Recently fed urchins always avoided food containing natural concentrations of pachydictyol A during separate feeding trials performed on each of four consecutive days. In contrast, urchins deprived of food for 3 d before this assay did not avoid the treated food on days 1 and 2 of feeding trials, but did avoid it on days 3 and 4 after their hunger was reduced by feeding during days 1 and 2. If we had used only starved urchins (a common procedure in previous investigations), we could have concluded, with apparent justification, that urchins were unaffected by pachydictyol A (if the assays were run for only 1—2 d) or that they needed 2 d of exposure to the compound in order to learn to avoid it. Both of these conclusions would have been incorrect