Feeding adaptations of Melagraphia aethiops (Gmelin), an intertidal trochid mollusc

The feeding behaviour of the intertidal trochid Melagraphia aethiops (Gmelin) on a sheltered stony shore was investigated. The effects of snail size, season, and shore level on feeding were examined. Feeding was examined in the laboratory in aquaria fitted with glass plates coated with algal and detrital films. Rates of radula rasping and the frequency of feeding bouts (feeding activity) were measured.

Radula rate varied little with snail size and large snails were 35% more active feeders than small snails. Small snails, however, consumed three to four times as much food per radula rasp, on a unit weight basis, as large snails. Winter acclimatization involved an increased tolerance of low water temperatures and an overall 60% decrease in radula rasping rate compared with summer acclimatized animals. Concurrently, field crawling rates decreased 71% during the summer-winter transition suggesting that lower food intake was balanced by reduced locomotory activity. High and low shore inhabiting snails showed little difference in radula rasping rates. High shore snails, however, compensated for their shorter feeding periods by being 37% more active in feeding and crawling in the field 40% faster than low shore snails.     

Positive and negative effects of mesograzers on early‐colonizing species in an intertidal rocky‐shore community

The ecological consequences of human‐driven overexploitation and loss of keystone consumers are still unclear. In intertidal rocky shores over the world, the decrease of keystone macrograzers has resulted in an increase in the dominance of herbivores with smaller body (i.e., “mesograzers”), which could potentially alter community assembly and structure. Here, we experimentally tested whether mesograzers affect the structure of rocky intertidal communities during the period of early colonization after the occurrence of a disturbance. A manipulative field experiment was conducted to exclude mesograzers (i.e., juvenile chitons, small snails, amphipods, and juvenile limpets) from experimental areas in an ecosystem characterized by the overexploitation of keystone macrograzers and predators. The results of multivariate analyses suggest that mesograzers had significant effects on intertidal community structure through negative and positive effects on species abundances. Mesograzers had negative effects on filamentous algae, but positive effects on opportunistic foliose algae and barnacles. Probably, mesograzers indirectly favored the colonization of barnacles and foliose algae by removing preemptive competitors, as previously shown for other meso‐ and macrograzer species. These results strongly support the idea that small herbivores exert a firm controlling effect on the assembly process of natural communities. Therefore, changes in functional roles of top‐down controllers might have significant implications for the structure of intertidal communities.     

A test of the hypothesis that a pathway of intraguild predation limits densities of a wolf spider

Abstract.  1. The high number of potential predatory interactions among the many arthropod generalist predators in terrestrial food webs makes exhaustive testing of interaction strengths by field experiments unfeasible. Thus, correlative patterns and laboratory observations of behaviour often form the basis of inferences about the strength of interaction pathways involving generalist predators (intraguild predation).
2. Previous research has revealed a negative correlation between survival of juvenile wolf spiders of the genus Schizocosa (Lycosidae) and densities of another abundant spider family, the Gnaphosidae.
3. Feeding trials in laboratory microcosms with a leaf-litter substrate revealed that gnaphosids prey on juvenile Schizocosa in a structurally complex habitat.
4. Gnaphosid densities were manipulated in two different field experiments, each conducted in a different year, in order to test directly the hypothesis that intraguild predation by gnaphosids limits densities of juvenile Schizocosa .
5. Reducing numbers of gnaphosids, and doubling their numbers to two times the mean natural density, had no impact on the survival of juvenile Schizocosa in either field experiment. This finding suggests that correlative patterns in nature and feeding trials in the laboratory may at times provide deceptively simple and potentially misleading generalisations about the strengths of interaction pathways in complex networks of generalist predators.     

Enhanced leaf nitrogen status stabilizes omnivore population density

Plant traits can mediate the strength of interactions between omnivorous predators and their prey through density effects and changes in the omnivores’ trophic behavior. In this study, we explored the established assumption that enhanced nutrient status in host plants strengthens the buffering effect of plant feeding for omnivorous predators, i.e., prevents rapid negative population growth during prey density decline and thereby increases and stabilizes omnivore population density. We analyzed 13 years of field data on population densities of a heteropteran omnivore on Salix cinerea stands, arranged along a measured leaf nitrogen gradient and found a 195 % increase in omnivore population density and a 63 % decrease in population variability with an increase in leaf nitrogen status from 26 to 40 mgN × g^?1. We recreated the leaf nitrogen gradient in a greenhouse experiment and found, as expected, that increasing leaf nitrogen status enhanced omnivore performance but reduced per capita prey consumption. Feeding on high nitrogen status host plants can potentially decouple omnivore–prey population dynamics and allow omnivores to persist and function effectively at low prey densities to provide “background level” control of insect herbivores. This long-term effect is expected to outweigh the short-term effect on per capita prey consumption—resulting in a net increase in population predation rates with increasing leaf nitrogen status. Conservation biological control of insect pests that makes use of omnivore background control could, as a result, be manipulated via management of crop nitrogen status.     

The winter feeding ecology of Avocets Recurvirostra avosetta on intertidal areas. II. Diet and feeding mechanisms

The diet and feeding methods of Avocets Recurvirostra avosetta using the most common feeding strategy (Normal Feeding Strategy) on emerged intertidal areas in the Tagus estuary, Portugal, were studied during two winters.
Avocets using the Normal Feeding Strategy fed non-selectively in the top 20 mm of sediment, ingesting prey bigger than a minimum threshold dimension of around 1.5 mm. Some mud was also ingested whilst feeding. Prey were captured by the mechanical action of the bill, and the estimated number of items captured per sweep was quite high (mean = 10.9 worms). Most of the biomass ingested by Avocets consisted of oligochaetes, spionid worms, Capitella capitata and small ragworms Nereis diversicolor.
The estimated intake rate of Avocets suggested that the energy ingested during daylight hours did not represent a significant percentage of the daily requirements. Thus, feeding on emerged intertidal areas is probably a secondary component of their daily gross food intake, unless night feeding yields higher intake rates due to increased prey availability.     

Physical constraints on the foraging ecology of a predatory snail

We studied the effects of aerial exposure and high summer temperatures on the southern oyster drill ( Stramonita haemastoma ), feeding on the American oyster, Crassostrea virginica . In the laboratory, oyster drill feeding rates and growth were highest at 25 and 30°C, some mortality occurred at 35°C, all snails died at 40 and 45°C, and the 28-day LC 50 was 35.7°C. In a second experiment where both water temperature (25 vs . 33°C) and aerial exposure were varied, only simulated tidal exposure lowered oyster drill feeding and growth rates. In field cage experiments, oyster drills had reduced feeding rates and growth at intertidal sites, but snail growth rates increased in late summer with warmer water temperature. We therefore conclude that aerial exposure, not high temperature, is the major factor limiting oyster drill feeding and growth in intertidal oyster reefs. Field experiments with partial cages also suggested that ambient predation rates were much higher at a subtidal than at a nearby intertidal site. Because southern oyster drills have depressed feeding, growth, and possibly lower fitness in intertidal oyster reefs during the summer, this reduced predation risk may provide a refuge for intertidal oysters.     

Water temperature, predation, and the neglected role of physiological rate effects in rocky intertidal communities

Ecologists and physiologists working on rocky shores have emphasizedthe effects of environmental stress on the distribution of intertidalorganisms. Although consumer stress models suggest that physicalextremes may often reduce predation and herbivory through negativeimpacts on the physiological performance of consumers, few fieldstudies have rigorously tested how environmental variation affectsfeeding rates. I review and analyze field experiments that quantifiedper capita feeding rates of a keystone predator, the sea starPisaster ochraceus, in relation to aerial heat stress, waveforces, and water temperature at three rocky intertidal siteson the Oregon coast. Predation rates during 14-day periods wereunrelated to aerial temperature, but decreased significantlywith decreasing water temperature. There was suggestive butinconclusive evidence that predation rates also declined withincreasing wave forces. Data-logger records suggested that thermalstress was rare in the wave-exposed habitats that I studied;sea star body temperatures likely reached warm levels (>24°C)on only 9 dates in 3 yr. In contrast, wind-driven upwellingregularly generated 3 to 5°C fluctuations in water temperature,and field and laboratory results suggest that such changes significantlyalter feeding rates of Pisaster. These physiological rate effects,near the center of an organism's thermal range, may not reducegrowth or fitness, and thus are distinct from the effects ofenvironmental stress. This study underscores the need to considerorganismal responses both under "normal" conditions, as wellas under extreme conditions. Examining both kinds of responsesis necessary to understand how different components of environmentalvariation regulate physiological performance and the strengthof species interactions in intertidal communities.     

The winter feeding ecology of Avocets Recurvirostra avosetta on intertidal areas. I. Feeding strategies

The behaviour of Avocets Recurvirostra avosetta feeding on emerged intertidal areas in a major wintering area, the Tagus estuary (Portugal), was studied during 2 years.
Avocets used four different feeding strategies. By far the most common was the Normal Feeding Strategy, in which the median sweeping rate was 28 sweeps per min and the percentage of swallowing movements (ingestions) was very high (median = 90%). No prey were visible in the bill while birds were using this strategy. Another less common feeding strategy was the Worm Feeding Strategy, in which the sweeping rate was higher (median = 46 sweeps/min) and the percentage of swallowing movements was much lower (median = 6%). While using the Worm Feeding Strategy, Avocets preyed on ragworms Nereis diversicolor. The Mixed Feeding Strategy and Scrobicularia plana Siphons Feeding Strategy were rarely observed.
In a detailed study on the use of the mudflats by Avocets, it was found that the density of birds on the upper shore was much higher than on the lower shore. There were differences in feeding behaviour between these two areas, although the same feeding strategy was used. On the lower shore, where there was a lower biomass of prey available, sweeping rates were higher and aggressive behaviour did not occur. On the upper shore, sweeping rates were lower and aggressive behaviour was frequent, with some birds defending territories. Nevertheless, estimated food intake rates were 1.5 times higher on the upper shore.     

The role of flow velocity combined with habitat complexity as a top–down regulator in seagrass meadows

Large‐scale losses of seagrass areas have been associated with eutrophication events, which have led to an overproduction of photosynthetic organisms including epiphytes. Grazers that feed on epiphytes can exert a significant top–down control in the system, but the effects of physical factors on grazing activity and feeding behaviour have been rarely examined. We addressed the combination of hydrodynamic regime and seagrass shoot density can alter the feeding and foraging behaviours of mesograzers. A full factorial experiment, with flow velocity (high, medium and low) and shoot density (high versus low) as main factors, was conducted in a racetrack flume using artificial seagrass plots. The results showed that when high flow velocity conditions were combined with low shoot density, consumption of epiphytes by mesograzers was strongly reduced. In contrast, when flow velocity was low or shoot density was high, mesograzers exhibited high feeding rates and vigorous swimming behaviour. These results clearly indicate that hydrodynamic stress reduces the time that mesograzers can spend feeding, since it inhibits their swimming behaviour, and thus indirectly affecting to the density of epiphytes. Therefore, the triggering of trophic cascade effects in seagrass communities under these experimental conditions depended on the interrelationship and feedbacks among shoot density, abiotic (flow velocity) and biotic (epiphytes and mesograzers) compartments, with flow velocity exerting a top–down control on seagrass ecosystems.     

Effects of a Mycoinsecticide on Feeding and Fecundity of the Brown Locust Locustana pardalina

The entomopathogenic fungus Metarhizium anisopliae var. acridum is being developed for use as a mycoinsecticide against locusts and grasshoppers in Africa. In addition to causing significant mortality, studies have shown the pathogen to cause reductions in feeding of a number of target species during the disease incubation period. The present study extends this work by demonstrating significant reductions in feeding following infection in another important pest species, the brown locust Locustana pardalina (Walker). Insects collected from the field following a spray application and subsequently maintained in a field laboratory showed a significant reduction in per capita feeding, as indicated by faecal production, as early as 2 days after treatment; this was before any pathogen-induced mortality. This per capita reduction in feeding by infected individuals contributed to a total reduction in feeding of 65% relative to controls. In addition to this laboratory based assessment, experiments were conducted to examine the effects of infection on mortality, feeding and fecundity in the field. Mortality rates of infected individuals in the field were significantly slower than those in the laboratory (average survival time of approximately 39 days compared with 6 days), although total mortality and percentage mycosis were equivalent. Treated individuals showed a significant reduction in body fat accumulation at sexual maturity compared with controls. This may reduce dispersal ability and total reproductive fitness. However, treated individuals exhibited a reduced pre-oviposition period resulting in them laying more eggs than untreated individuals within the first weeks following fledging. Thus, no significant reductions in fecundity were recorded by the end of the assessment period when mortality of treated individuals exceeded 90%. Given that many untreated locusts were still alive and had the potential to continue oviposition for several more weeks, however, it is likely that differences in fecundity would have become apparent if the assessment had continued. These results provide important insights for evaluating overall impacts of spray applications of the mycoinsecticide.     

Large mobile versus small sedentary herbivores and their resistance to seaweed chemical defenses

Summary Small, relatively sedentary herbivores like amphipods and polychaetes (mesograzers) often live on the plants they consume and should therefore view plants as both foods and living sites. Large, relatively mobile herbivores like fishes commonly move among, and feed from, many plants; they should view plants primarily as foods and rarely as potential living sites. In marine communities, fishes that consume plants are also important predators on mesograzers. Since seaweeds avoided by fishes should represent safer living sites for small herbivores, mesograzers living on and consuming seaweeds that are not eaten by fishes should have higher fitness than mesograzers living on plants preferred by fishes. In previous work, we demonstrated that seaweed secondary metabolites that deterred feeding by a fish and sea urchin had no effect on feeding by a common amphipod (Hay et al. 1987a). We then hypothesized that mesograzers would, in general, be less affected by seaweed chemical defenses than larger, more mobile herbivores like fishes. In this investigation, we evaluate the generality of this hypothesis by comparing the feeding of an omnivorous fish (Lagodon rhomboides) with that of an omnivorous, tube-building polychaete (Platynereis dumerilii) to see if the mesograzer prefers seaweeds avoided by the fish and if it is less affected by seaweed chemical defense. Platynereis dumerilii fed almost exclusively on Dictyota dichotoma, the seaweed eaten least by Lagodon rhomboides. The diterpene alcohols (dictyol-E and pachydictyol-A) produced by Dictyota significantly deterred feeding by Lagodon but did not affect, or at one concentration stimulated, feeding by Platynereis. Our data support the hypothesis that small, relatively sedentary herbivores that live on plants are more resistant to chemical defenses than are large, relatively mobile herbivores that move among many plants.     

The effect of feeding by mud snails, Ilyanassa obsoleta (Say), on the structure and metabolism of a laboratory benthic algal community

Eastern mud snails, Ilyanassa obsoleta (Say), in densities of 0, 80, and 160 snails · m^?2 were placed in flow-through laboratory microcosms containing 5 cm of frozen and sieved sediments. Other microcosms were raked once daily to a depth of 10 mm. All these containers were incubated for 5 wk and regularly sampled for plant pigments and light and dark transfer of oxygen and carbon dioxide. Feeding at the low density significantly increased chlorophyll standing stock. Respiration and gross photosynthesis increased by an even greater percentage compared to ungrazed controls. Standing stocks of algal pigments, respiration, and photosynthesis were depressed in microcosms which received the 160-snail or raking treatments.The dominant benthic algae in the containers were pennate diatoms. Grazed containers contained a larger percentage of non-motile as compared to motile forms.Sediments fertilized with ammonium at a rate equivalent to excretion of six snails, showed increased chlorophyll content equal to the 80-snail treatment. Daily raking inhibited this effect.We conclude that low densities of Ilyanassa obsoleta stimulate algal growth by accelerating nitrogen cycling and selectively removing specific components of the diatom community. At high snail densities these effects are overwhelmed by overgrazing and sediment stirring.     

Effects of food density and temperature on feeding and growth of young inland silversides (Menidia beryllina)

Food consumption and growth rates of 7–28-day-old Menidia beryllina were measured in response to natural ranges of temperature and prey availability. Feeding level, temperature and age all had significant effects on growth rate, although the effect of feeding level explained most of the variance. Feeding level also had a significant effect on gross growth efficiency, but temperature and age did not. Absolute growth rates (mg per day) increased dramatically with temperature, feeding level, and age. Variability in growth was greatest for fish feeding at the lowest feeding level. For a given fish weight, temperature had a positive effect on consumption rate, and maximum consumption ( C _max) of any treatment combination reached 75% body weight per day. Maximum growth rate was estimated at 24.6% body weight per day, and gross growth efficiency reached an estimated maximum of 0.375 at an ingestion rate of 25% body weight per day. Starved larvae lost on average 5.4% body weight per day and larvae required 6.4% body weight food consumption per day for maintenance. Multiple regressions of feeding level, temperature, and age/size on instantaneous growth rates indicated that increases in temperature increased maintenance requirements and required that fish consume a greater proportion of C _max to attain maximum growth. Growth rates decreased with increases in temperature for fish eating a specific weight of food.     

Hunger-induced foraging behavior of two cyprinid fish: Pseudorasbora parva and Rasbora daniconius

The feeding and swimming behaviors of Pseudorasbora parva and Rasbora daniconius (Cyprinidae) with two different prey types (Daphnia pulex and Artemia salina) at different densities (0.5, 1, 2, 5, 10, or 25 per l) were studied after 36 h of food deprivation. Full satiation was defined as the cumulative number of attacks performed until fish attain a constant attack rate which for P. parva was 425 and R. daniconius was 390 attacks. Initial feeding rates showed marked variation with prey availability. Feeding rates of fish in high prey concentrations were higher at the beginning of the experiment and decreased faster than in low prey densities. Decreases in the feeding rate at high prey densities were due to faster attainment of satiation. Feeding rates of fish across high prey densities reached a steady level after satiation. Swimming speeds of fish were inversely proportional to prey density. Moreover, the change in swimming speeds was directly related to the level of satiation. The ratios of the attack rate and the encounter rate against prey density of both fish reveal that the search for prey triggered swimming and thereby feeding during the transition from hungry to satiation. The findings of this study demonstrate that satiation plays an important role in fish foraging that should be considered a significant factor in foraging analysis.     

The functional group approach to bioturbation: The effects of biodiffusers and gallery-diffusers of the Macoma balthica community on sediment oxygen uptake

The Macoma balthica community, which is widely distributed in intertidal soft sediments bordering the north Atlantic, is dominated by two functional groups with different sediment mixing modes: the biodiffusers M. balthica and Mya arenaria and the gallery-diffuser Nereis virens. To compare the effects of these two groups on sediment oxygen uptake rates, we used experimental microcosms with identical biovolumes to measure the influence of each species on oxygen uptake. The two biodiffusers had similar effects on oxygen uptake in spite of different space occupation and different feeding, ventilation and burrowing modes. Biodiffusers and gallery-diffusers had different effects on oxygen uptake. Periodic ventilation by the gallery-diffusers stimulated the oxygen uptake by the sediment more than the steady activities of the biodiffusers. Temporal variation in oxygen fluxes in bioturbated microcosms was linked to construction and maintenance of biogenic structures. The results confirm that the functional group approach to bioturbation is a useful tool for quantifying the effects of intertidal benthic communities on benthic fluxes.     

Testing the mechanisms by which source-sink dynamics alter competitive outcomes in a model system

Dispersal among sites can affect within-site competitive outcomes via source-sink dynamics. Source-sink dynamics are thought to affect competitive outcomes primarily via spatial subsidies: by redistributing individuals from sources to sinks, source-sink dynamics can alter competitive outcomes in both sources and sinks. However, dispersal also can affect competitive outcomes via demography modification, which occurs when dispersal alters the parameters governing species' per capita demographic rates. For instance, dispersal of exploitative competitors might cause extinction of some of the resources for which competition occurs, thereby altering the competition coefficients. I used protist microcosms as a model system to test whether spatial subsidies alone could explain the effects of source-sink dynamics on competitive outcomes. I examined the long-term outcome of exploitative competition among three bacterivorous ciliate protists in microcosms of high enrichment (sources) and low enrichment (sinks) in both the presence and the absence of dispersal. Dispersal altered competitive outcomes. Fitting mathematical models to the population dynamics revealed that spatial subsidies were insufficient to account for the effects of dispersal. Fitting alternative models strongly suggested that demography modification was an important determinant of competitive outcomes. These results provide the first evidence that dispersal does not simply redistribute competitors but can alter their per capita demographic rates.     

Reduction of consumption by grasshoppers (Orthoptera: Acrididae) infected with Nosema locustae canning (Microsporida: Nosematidae)

The effect of ingestion of Nosema locustae Canning spores on feeding by grasshoppers was measured in simultaneous laboratory and field experiments. After 21 days, fourth-instar Melanoplus sanguinipes (F.) nymphs, administered spores at the rates of 0, 2.0 × 10⁴, 2.0 × 10⁵, and 2.0 × 10⁶ per grasshopper, showed dry matter consumption of 102, 87, 64, and 26 mg in 48 hr, respectively. Rate of inoculation was a significant factor in suppression of feeding after correction for the effects of developmental stage, sex, and body weight. The quantity of dry matter consumed decreased linearly with increasing rate of spore ingestion. Experiments on 50 caged 1-m² plots on pasture grass yielded similar trends in per capita consumption independent of the effects of mortality. Field consumption per integrated grasshopper-day was 108, 77, 31, and 27 mg dry wt at the four inoculation rates, over 20 days.     

Effects of flooding on leaf litter decomposition in microcosms

Summary The effects of hydroperiod on decomposition rates of senescent Acer rubrum leaves were tested in microcosms in a controlled laboratory environment. Microcosm treatments included continuously flooded, continuously unflooded, and fluctuating hydroperiods. All flooding treatments promoted decomposition but variations in hydroperiod had no significant effects. A leaching experiment indicated the higher decay rates under flooded conditions were primarily due to high leaching losses from soaking. Unlike nutrient dynamics in the field, where net accumulation occurs, nitrogen and phosphorus in the litter in the microcosms exhibited net losses. The major external inputs which provide a source of nitrogen and phosphorus for immobilization in the field were lacking in the microcosms. Calcium, magnesium, and potassium exhibited net losses except for calcium in the unflooded microcosms. The microcosm results demonstrated the importance of external inputs to litter nutrient relations.     

Effects of leaf and sap feeding insects on photosynthetic rates of goldenrod

Summary Herbivory can alter the balance between sources and sinks within a plant, and changes in the source-sink ratio often lead to changes in plant photosynthetic rates. We investigated how feeding by three insect herbivores affected photosynthetic rates and growth of goldenrod (Solidago altissima). One, a phloem-sap feeding aphid (Uroleucon caligatum), creates an additional sink, and the other two, a leaf-chewing beetle (Trirhabda sp.) and a xylem-sap feeding spittlebug (Philaenus spumarius) both reduce source supply by decreasing leaf area. Plants were grown outside in large pots and insects were placed on them at predetermined densities, with undamaged plants included as controls. All insects were removed after a 12-day feeding period. We measured photosynthetic rates both of damaged leaves and of undamaged leaves that were produced after insect removal. Photosynthetic rates per unit area of damaged leaves were reduced by spittlebug feeding, but not by beetle or aphid feeding. Conductance of spittlebugdamaged leaves did not differ from controls, but internal carbon dioxide concentrations were increased. These results indicate that spittlebug feeding does not cause stomatal closure, but impairs fixation within the leaf. Effects of spittlebug feeding on photosynthetic rates persisted after the insects were removed from the plants. Photosynthetic rates per unit area of leaves produced after insect removal on spittlegug-damaged plants were lower than control levels, even though the measurements were taken 12 days after insect removal. The measurement leaf on spittlebugdamaged plants was reduced in area by 27% relative to the controls, but specific leaf area (leaf area/leaf weight) was increased by 18%. Because of the shift in specific leaf area, photosynthetic rates were also examined per unit leaf weight, and when this was done there were no significant differences between control and spittlebug-damaged plants. Beetle and aphid feeding had no effects on the photosynthetic rate of the leaves produced after insect removal. Plant relative growth rates (in terms of height) were reduced by spittlebugs during the period that the insects were feeding on the plants. Relative growth rates of spittlebug-damaged plants were increased above control levels after insect removal, but these plants were still shorter than controls 17 days after insect removal. Beetles and aphids did not affect plant relative growth rates or plant height. Feeding by both spittlebugs and beetles reduced leaf area, and the effect of the spittlebug was more severe than that of the beetle. These results show that effects of herbivory on photosynthetic rates cannot be predicted simply by considering changes in the source-sink ratio, and that spittlebug feeding is more damaging to the host plant than beetle or aphid feeding.     

Direct and indirect effects of ocean acidification and warming on a marine plant–herbivore interaction

The impacts of climatic change on organisms depend on the interaction of multiple stressors and how these may affect the interactions among species. Consumer–prey relationships may be altered by changes to the abundance of either species, or by changes to the per capita interaction strength among species. To examine the effects of multiple stressors on a species interaction, we test the direct, interactive effects of ocean warming and lowered pH on an abundant marine herbivore (the amphipod Peramphithoe parmerong), and whether this herbivore is affected indirectly by these stressors altering the palatability of its algal food (Sargassum linearifolium). Both increased temperature and lowered pH independently reduced amphipod survival and growth, with the impacts of temperature outweighing those associated with reduced pH. Amphipods were further affected indirectly by changes to the palatability of their food source. The temperature and pH conditions in which algae were grown interacted to affect algal palatability, with acidified conditions only affecting feeding rates when algae were also grown at elevated temperatures. Feeding rates were largely unaffected by the conditions faced by the herbivore while feeding. These results indicate that, in addition to the direct effects on herbivore abundance, climatic stressors will affect the strength of plant–herbivore interactions by changes to the susceptibility of plant tissues to herbivory.