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

Defensive synergisms?: Reply to Pennings

DOI: 10.2307/2265799©1996 Ecological Society of Americ

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

Cascading predator effects in a Fijian coral reef ecosystem

Coral reefs are among Earth's best-studied ecosystems, yet the degree to which large predators influence the ecology of coral reefs remains an open and contentious question. Recent studies indicate the consumptive effects of large reef predators are too diffuse to elicit trophic cascades. Here, we provide evidence that such predators can produce non-consumptive (fear) effects that flow through herbivores to shape the distribution of seaweed on a coral reef. This trophic cascade emerged because reef topography, tidal oscillations, and shark hunting behaviour interact to create predictable "hot spots" of fear on the reef where herbivores withhold feeding and seaweeds gain a spatial refuge. Thus, in risky habitats, sharks can exert strong ecological impacts even though they are trophic generalists that rarely feed. These findings contextualize the debate over whether predators influence coral reef structure and function and move us to ask not if, but under what specific conditions, they generate trophic cascades

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

Herbivore vs. nutrient control of marine primary producers: context-dependent effects

DOI:10.1890/0012-9658(2006)87[3128:HVNCOM]2.0.CO;2© Ecological Society of AmericaPervasive overharvesting of consumers and anthropogenic nutrient loading are changing the strengths of top-down and bottom-up forces in ecosystems worldwide. Thus, identifying the relative and synergistic roles of these forces and how they differ across habitats, ecosystems, or primary-producer types is increasingly important for understanding how communities are structured. We used factorial meta-analysis of 54 field experiments that orthogonally manipulated herbivore pressure and nutrient loading to quantify consumer and nutrient effects on primary producers in benthic marine habitats. Across all experiments and producer types, herbivory and nutrient enrichment both significantly affected primary-producer abundance. They also interacted to create greater nutrient enrichment effects in the absence of herbivores, suggesting that loss of herbivores produces more dramatic effects of nutrient loading. Herbivores consistently had stronger effects than did nutrient enrichment for both tropical macroalgae and seagrasses. The strong effects of herbivory but limited effects of nutrient enrichment on tropical macroalgae suggest that suppression of herbivore populations has played a larger role than eutrophication in driving the phase shift from coral- to macroalgal-dominated reefs in many areas, especially the Caribbean. For temperate macroalgae and benthic microalgae, the effects of top-down and bottom-up forces varied as a function of the inherent productivity of the ecosystem. For these algal groups, nutrient enrichment appeared to have stronger effects in high- vs. low-productivity systems, while herbivores exerted a stronger top-down effect in low-productivity systems. Effects of herbivores vs. nutrients also varied among algal functional groups (crustose algae, upright macroalgae, and filamentous algae), within a functional group between temperate and tropical systems, and according to the metric used to measure producer abundance. These analyses suggest that human alteration of food webs and nutrient availability have significant effects on primary producers but that the effects vary among latitudes and primary producers, and with the inherent productivity of ecosystems

Palatability and Chemical Defense of Marine Invertebrate Larvae

Risk of larval mortality is a critical component of models and debates concerning the ecology and evolution of the differing reproductive characteristics exhibited by marine invertebrates. In these discussions, predation often is assumed to be a major source of larval mortality. Despite limited empirical support, most marine larvae are thought to be palatable and broadly susceptible to generalist predators. Previous studies of larval-planktivore interactions have focused primarily on larvae that typically feed, grow, and develop for weeks to months in the plankton. Such planktotrophic species commonly produce large numbers of small larvae that disperse over vast distances. In contrast, the nonfeeding lecithotrophic larvae from sessile invertebrates that brood are often large and conspicuous, lack morphological defenses, and have limited dispersal distances because they typically are competent to settle minutes to hours after spawning. Interactions between lecithotrophic larvae and consumers are not well studied. This has limited the ability of previous authors to test broad generalities about marine larvae. We show that brooded larvae of Caribbean sponges (11 species) and gorgonians (three species) as well as brooded larvae of temperate hydroids (two species) and a bryozoan are unpalatable to co-occurring fishes. In contrast, brooded larvae of temperate ascidians (three species), a temperate sponge, and Caribbean hard corals (three species) are readily consumed by fishes, as are larvae from four of six species of synchronous broadcast-spawning gorgonians from the Florida Keys. Frequencies of survivorship for larvae attacked and rejected by fishes were high and statistically indistinguishable from frequencies for unattacked control larvae. Frequency of metamorphosis (when it occurred) of rejected larvae never differed significantly from that of unattacked control larvae. Assays testing for larval vs. adult chemical defenses for five species with distasteful larvae showed that larvae of all five species were chemically distasteful to fishes, whereas only three of five adult extracts deterred fish feeding. A comparison of larval palatability among chemically rich taxa showed that brooded larvae were significantly more likely to be unpalatable (86% of the species tested) than larvae of broadcasters (33%), and that palatable larvae were rarely released during the day (23%) while unpalatable larvae usually were (89%). Additionally, the frequency of bright coloration was high (60%) for unpalatable larvae and low (0%) for palatable larvae, suggesting that unpalatable larvae often may be aposematically colored. Results of this broad survey cast doubt on the widely accepted notion that virtually all marine larvae are suitable prey for most generalized planktivores. Among species that do not chemically or physically protect larvae against fishes, selection appears to favor the release of larvae at night, or the production of smaller more numerous offspring that grow and develop at sea as a way of escaping consumer-rich benthic habitats. Because distasteful larvae are not similarly constrained, distasteful species should exhibit reproductive and larval characteristics selected more by the fitness-related consequences of larval development mode and dispersal distance than by the necessity of avoiding benthic predators