Capita Feeding Rates Research Articles

Handling- or digestion-limited predators: the role of body mass and habitat complexity in predator functional response

The predator functional response quantifies the per capita feeding rate of predators as a function of prey density and is a key element of feeding interactions. Variations in its parameters are strongly associated with interaction strength and population dynamics. We examined 18 functional responses within marine whelk-bivalve systems, varying predator body size, prey species, and habitat structure. Our findings suggest that the marine whelk Rapana venosa is handling-limited, a predator type that has received less attention in previous research. We propose further categorizing handling-limited predators into 2 types: pursuit-limited (where maximum feeding rate could be influenced by habitat complexity) and ingestion-limited (where maximum feeding rate is impacted not by habitat complexity, but by predator-prey body mass ratios and prey defense strategy). We found that handling time scales negatively with predator-prey body mass ratios, but this trend exhibits layers of complexity. We propose that the transition from handling to digestion limitation with increasing predator-prey body mass ratios underlies this trend. Our study also confirms the importance of prey types, in addition to known effects of body mass ratios and habitat structure. In summary, our study reveals that simple assumptions about body masses and prey defense strategy may usefully refine estimates of feeding interactions in complex food webs.

Threats at home? Assessing the potential ecological impacts and risks of commonly traded pet fishes

Invasive alien species (IAS) are major drivers of global biodiversity loss, and the poorly regulated international pet trade is a source of emerging and future invaders. Predictions of the likely ecological impacts and risks of such IAS have been significantly enhanced in recent years with new metrics, which require application to many more actual and potential IAS. Hence, this study assesses the potential ecological impacts and risks of two readily available pet trade species: goldfish, Carassius auratus, a species with non-native populations worldwide; and white cloud mountain minnow, Tanichthys albonubes, a species with a limited invasion history to date. First, we compared the per capita feeding rates of these non-native species with two European trophically analogous natives – the stone loach, Barbatula barbatula, and the common minnow, Phoxinus phoxinus – using the Comparative Functional Response method. Second, we used foraging experiments in conspecific pairs to determine synergistic, neutral or antagonistic intraspecific interactions. Third, we performed novel object experiments using the two pet trade species to assess boldness, a known “dispersal enhancing trait”. Goldfish had the highest maximum feeding rates of the four species, while white cloud mountain minnows had the lowest. Neutral interactions were observed for all four species in the paired foraging experiments, with goldfish having the highest consumption and white cloud mountain minnows having the lowest. Goldfish demonstrated greater boldness, being more active during the experimental trials and more likely to approach a novel object than white cloud mountain minnows. Further, combining maximum feeding rates, boldness and species availabilities from our survey of pet shops, we assessed the relative invasion risks (RIR) of the two non-natives. This highlighted goldfish as the higher risk and most worthy of management prioritisation, mirroring its more extensive invasion history. We propose that such metrics have potential to direct future IAS policy decisions and management towards the ever-increasing rates of biological invasions worldwide.

Night warming alters mean warming effects on predator–prey interactions by modifying predator demographics and interaction strengths

Abstract Temperatures are increasing globally, but this increase is not symmetrical; instead, night‐time minimum air temperatures increase faster than daytime maximum temperatures. However, we still know little about when and how this differential increase in maximum versus minimum air temperatures affects ecological interactions between species. Understanding the connection between different types (night vs. day) of warming and interspecific interactions is essential to predict the ecological consequences of climate change for natural ecosystems. Here we used experiments in a common predator (lady beetle)–prey (aphid) system to examine how night warming, thermal regime (constant vs. fluctuating) and prey density influence demographic rates and per capita feeding rate and thus predation pressure. We mainly focused on the demographic variables and prey consumption as well as energy‐related variables of the predators to understand how changes in night‐time temperature, temperature regime and prey density affect predator performances and to differentiate these effects between constant and fluctuating thermal conditions. We found that warming effects on most of the individual demographic rates and per capita consumption rates of the predator differed between constant and fluctuating regimes. These differences scaled up to alter population stage structure and population growth rate of the predator and ultimately the predation pressure in our system. Furthermore, warming had stronger and negative consequences on predator population growth and thus predation pressure in constant regime, but little or even positive effects under fluctuating conditions. Importantly, these results are consistent across prey densities. Our results suggest that compared to night warming, predictions based on mean warming can under‐ or overestimate species demographic rates and per capita predation effects, causing inaccurate or even misleading expectations on population structure, long‐term population growth and interspecific interactions. These results emphasize the importance of accounting for both ecologically relevant diurnal temperature cycle and lifelong demographics and per capita consumption when assessing how warming affects interspecific interactions. Our findings thus provide better understanding of how night warming will affect predator–prey interactions and energy flux within trophic cascades and also have implications for predicting the consequences of top‐down control in natural and agricultural ecosystems under climate change. A free Plain Language Summary can be found within the Supporting Information of this article.

Breathing space: deoxygenation of aquatic environments can drive differential ecological impacts across biological invasion stages

The influence of climate change on the ecological impacts of invasive alien species (IAS) remains understudied, with deoxygenation of aquatic environments often-overlooked as a consequence of climate change. Here, we therefore assessed how oxygen saturation affects the ecological impact of a predatory invasive fish, the Ponto-Caspian round goby (Neogobius melanostomus), relative to a co-occurring endangered European native analogue, the bullhead (Cottus gobio) experiencing decline in the presence of the IAS. In individual trials and mesocosms, we assessed the effect of high, medium and low (90%, 60% and 30%) oxygen saturation on: (1) functional responses (FRs) of the IAS and native, i.e. per capita feeding rates; (2) the impact on prey populations exerted; and (3) how combined impacts of both fishes change over invasion stages (Pre-invasion, Arrival, Replacement, Proliferation). Both species showed Type II potentially destabilising FRs, but at low oxygen saturation, the invader had a significantly higher feeding rate than the native. Relative Impact Potential, combining fish per capita effects and population abundances, revealed that low oxygen saturation exacerbates the high relative impact of the invader. The Relative Total Impact Potential (RTIP), modelling both consumer species’ impacts on prey populations in a system, was consistently higher at low oxygen saturation and especially high during invader Proliferation. In the mesocosm experiment, low oxygen lowered RTIP where both species were present, but again the IAS retained high relative impact during Replacement and Proliferation stages at low oxygen. We also found evidence of multiple predator effects, principally antagonism. We highlight the threat posed to native communities by IAS alongside climate-related stressors, but note that solutions may be available to remedy hypoxia and potentially mitigate impacts across invasion stages.

Small burrowing amphipods cause major damage in a large kelp.

Large herbivores such as sea urchins and fish consume a high proportion of benthic primary production and frequently control the biomass of marine macrophytes. By contrast, small mesograzers, including gastropods and peracarid crustaceans, are abundant on seaweeds but have low per capita feeding rates and their impacts on marine macrophytes are difficult to predict. To quantify how mesograzers can affect macrophytes, we examined feeding damage by the herbivorous amphipods Sunamphitoe lessoniophila and Bircenna sp., which construct burrows in the stipes of subtidal individuals of the kelp Lessonia berteroana in northern-central Chile, southeast Pacific. Infested stipes showed a characteristic sequence of progressive tissue degeneration. The composition of the amphipod assemblages inside the burrows varied between the different stages of infestation of the burrows. Aggregations of grazers within burrows and microhabitat preference of the amphipods result in localized feeding, leading to stipe breakage and loss of substantial algal biomass. The estimated loss of biomass of single stipes varied between 1 and 77%. For the local kelp population, the amphipods caused an estimated loss of biomass of 24–44%. Consequently, small herbivores can cause considerable damage to large kelp species if their feeding activity is concentrated on structurally valuable algal tissue.

Relative impacts of the invasive Pacific oyster, Crassostrea gigas, over the native blue mussel, Mytilus edulis, are mediated by flow velocity and food concentration

The ecological impacts of invasive species can be severe, but are generally viewed as highly unpredictable. Recent methods combiningper capitafeeding rates, population abundances and environmental contexts have shown great utility in predicting invader impacts. Here, clearance rates of the invasive Pacific oyster,Crassostreagigas, and native mussel,Mytilusedulis, were investigated in a laboratory experiment where oscillatory water flow and algal food concentrations were manipulated. Invasive oysters had lower clearance rates than native mussels in all experimental groups and did not differ among flow velocities or food concentrations. Native mussel clearance rates were higher at 5 cm s-1compared to 0 and 15 cm s-1flow velocities and increased with increasing food concentration. The Relative Impact Potential (RIP) metric was used to assess (i) the influence of flow velocity and food concentration on potential impacts ofC.gigason plankton resources and, (ii) the impacts of coexisting reefs, containing both species, on resources compared to monospecific native mussel beds. Greatest Relative Impact Potential of invasive oysters was seen at the lowest flow velocity, but became reduced with increasing flow velocity and food concentration. Relative Impact Potentials of coexisting reefs were generally greater than monospecific native mussel beds, with greatest impacts predicted at lowest flow velocity. We suggest that the greatest ecological impacts and competition potential ofC.gigaswill occur in areas with low flow velocity, but that increased flow will mediate co-existence between the two species.

High abundance of an invasive species gives it an outsized ecological role

Abstract Many communities are shifting composition, with losses of native species and increases of non‐indigenous species (NIS). At its extreme, such alteration of ecological guilds can result in simplification with a single NIS performing an ecological role once carried out by a suite of natives. This alteration has occurred in many rivers of the south‐eastern U.S.A., where the invasive filter‐feeding freshwater clam Corbicula fluminea has proliferated following the nearly complete extirpation of native mussels. We investigated the factors controlling the distribution and abundance of Corbicula, as well as estimated the ecological service it provides via water filtration. With a nested design, we surveyed multiple transects within four to six sites within each of four rivers that spanned three large catchments in the Georgia piedmont, collecting data on Corbicula density and physical habitat characteristics associated with its presence. We found Corbicula present in over half of the 1,536 sampled 0.044 m2 sampled plots, 90 of the 93 transects that spanned the width of the river, and all 1–2 km sample sites, underscoring the clam's ubiquity in the study region. At the river scale, Corbicula densities ranged from 50–212 Corbicula m−2, although individual sites ranged from 7–483 Corbicula m−2. Corbicula was more abundant in areas with higher proportions of gravel, and less abundant with higher proportions of bedrock. A hierarchical model with river, site, and these two substrate variables explained 32% of the variation in Corbicula density. Using observed densities and published per capita feeding rates, we calculated system‐wide collective filtration rates provided by Corbicula. In the four rivers surveyed and based on estimated residence times for median flows for the summer of 2012, Corbicula is estimated to filter water as many as seven times during median flows and 18 times during minimum flows before water flows out of a 10‐km reach. Due to high abundances and per biomass filtration rates, Corbicula plays an important role in these rivers. Invasive species, biotic homogenisation, and the loss of functional redundancy may mean that many more rivers are similar to our studied rivers, with a single, often invasive, species dominating ecosystem function. Understanding the influence of biotic homogenisation on ecosystem function is of foremost importance to evaluate the resilience of natural systems.

Size-dependent predation and intraspecific inhibition of an estuarine snail feeding on oysters

Predator outbreaks have increased in the past two decades in many ecosystems and are predicted to become more common with climate change. During these outbreaks, predator densities increase rapidly, and can cause large reductions in prey populations or shifts in prey size structure. However, unexpected interactions may occur at high predator densities, necessitating a mechanistic understanding of how increased predator density affects predator-prey dynamics. In the northern Gulf of Mexico, outbreaks of southern oyster drill, Stramonita haemastoma (Linnaeus, 1767), occur during high salinity events, and can greatly reduce eastern oyster, Crassostrea virginica (Gmelin, 1791), populations. A large outbreak of drills occurred from 2013 to 2015 in Apalachicola Bay, FL which corresponded with an oyster fishery collapse in that bay. To improve our mechanistic understanding of predation during such a high-density outbreak, laboratory experiments based on field observations were used to quantify the prey size selection by drills and the drill functional response, as a function of drill abundance. Drills fed on medium-sized oysters (50–75 mm) more often than small and larger-sized oysters, and often formed aggregations during feeding events. However, despite this aggregative response, there was a negative relationship between per capita feeding rates and drill abundance. Indeed, the Crowley-Martin functional response model had the most parsimonious fit to the data, suggesting that predator-predator inhibition reduced attack rates and increased handling times. Due to an increase in regional drought conditions and water usage in the southeastern United States, drill outbreaks will likely increase in frequency and duration. A greater understanding of how predation rates change with predator densities during outbreaks will improve predictions of oyster mortality, and strengthen the scientific framework for oyster fishery decisions.

Predation risk between cannibalistic aeshnid dragonflies influences their functional response on a larval salamander prey

AbstractIn size‐structured populations of predators, the threat of cannibalism can influence behavior of prey that are attempting to avoid predation by larger conspecifics, including reducing their foraging rates. Such behavioral responses subsequently release basal prey of both cannibal and non‐cannibal predators from predation risk. However, whether the non‐consumptive effects of cannibalism on conspecific predators varies with basal heterospecific prey density is relatively unexplored. I conducted a laboratory study in plastic containers to test whether cannibalism risk influenced the functional response of dragonflies while foraging on three different densities of larval salamander prey. Dragonfly predators foraged with a Type II functional response, and when exposed to cannibalism cues, per capita feeding rates were lower at high prey densities. Salamander prey mortality rates declined with increasing prey densities, but the presence of cannibalism risk did not influence this pattern. Overall, this study shows that functional response curves of predators can vary in response to whether cannibalism risk is present in their environment across a small range of prey densities. Incorporating synergistic effects of multiple processes, such as non‐consumptive risk factors and prey density, may help further elucidate the processes that structure both predator and prey population dynamics.

Soil nutrient additions increase invertebrate herbivore abundances, but not herbivory, across three grassland systems.

Resource availability may influence invertebrate communities, with important consequences for ecosystem function, such as biomass production. We assessed: (1) the effects of experimental soil nutrient additions on invertebrate abundances and feeding rates and (2) the resultant changes in the effects of invertebrates on aboveground plant biomass at three grassland sites spanning the North American Central Plains, across which plant tissue chemistry and biomass vary. Invertebrate communities and rates of herbivory were sampled within a long-term nutrient-addition experiment established at each site along the broad Central Plains precipitation gradient. Additionally, the effects of invertebrates on aboveground plant biomass were determined under ambient and elevated nutrient conditions. At the more mesic sites, invertebrate herbivore abundances increased and their per capita rate of herbivory decreased with nutrient additions. In contrast, at the semi-arid site where plant biomass is low and plant nutrient concentrations are high, invertebrate herbivore abundances did not vary and per capita rates of herbivory increased with nutrient additions. No change in the effect of invertebrate herbivores on aboveground plant biomass was observed at any of the sites. In sum, nutrient additions induced shifts in both plant biomass and leaf nutrient content, which altered invertebrate abundances and feeding rate. However, due to the inverse relationship between changes in herbivore abundance and per capita rates of herbivory, nutrient additions did not alter the effect of invertebrates on aboveground biomass. Overall, we suggest that this inverse response of herbivore abundance and per capita feeding rate may buffer ecosystems against changes in invertebrate damage in response to fluctuations in nutrient levels.

Prey density affects strengths of density- and trait-mediated indirect interactions of predators on an algal community

In trophic cascades, predators can indirectly affect resources by reducing prey density (density-mediated indirect interactions; DMIIs) or by changing prey traits, such as their behavior, morphology, physiology, or life history (trait-mediated indirect interactions; TMIIs). Although the importance of predator (e.g., foraging strategy) and resource characteristics (e.g., quantity) in these indirect interactions is well recognized, little attention has been paid to prey characteristics (e.g., density), especially in field studies. We focused on a marine trophic cascade involving the carnivorous snail Thais clavigera, its limpet prey Siphonaria sirius, and the algae Lithoderma sp. and Ulva sp. Using intertidal rocks as natural replicates, we experimentally evaluated the in situ effects of the DMIIs and TMIIs on the algal community under two density ranges of prey. The strengths of consumptive effects (CEs) and non-consumptive effects (NCEs) of the predator on the prey limpet were also monitored to elucidate the mechanisms of the indirect interactions. At high densities, CEs decreased the percentage of individual limpets that disappeared (that likely died); however, neither DMIIs nor TMIIs were detected. At low densities, both CEs and NCEs reduced the per capita feeding rate and CEs reduced the growth rate of limpets. Moreover, Lithoderma sp. was replaced by Ulva sp. through both DMIIs and TMIIs. These results suggest that prey density is a key determinant of the strengths of CEs and NCEs, and notably, of DMIIs and TMIIs.

Eating between the lines: functional feeding response of bonnetheads (Sphyrna tiburo)

Mobile mesopredators can have strong influences in the structuring of marine food webs in shallow coastal ecosystems. Few quantitative estimates of the effects of predation on prey populations by highly mobile mesopredators exist, yet they are necessary to evaluate the impact mesopredators have on prey resources. Quantifying a predator’s functional response provides valuable information on how a predator’s per capita consumption rate of prey can influence community structure and prey populations. We examined the functional response of bonnetheads (Sphyrna tiburo) to one of its few natural prey items brown shrimp (Farfantepenaeus aztecus) to determine if per capita feeding rates would increase with increases in prey density. We simulated natural conditions in outdoor mesocosms and offered live prey at varying densities. The functional response of a predator can take one of three forms: linear (type I), rise to an asymptote (type II), and a sigmoid shape (type III). Bonnetheads consumed prey proportional to prey density and demonstrated a response that was in between a type I (R2 = 0.847, p < 0.01) and type II (R2 = 0.877, p < 0.01) functional response. Bonnetheads showed an increased rate of consumption with initial low prey densities but high densities of available prey did not consistently result in increased consumption rates. At the maximum prey density offered, bonnetheads consumed less prey than at intermediate densities. Bonnetheads may not have as strong of an influence on prey populations at high prey densities compared to predation impacts at low densities. Bonnethead per capita feeding rates did not conform to a type I, type II, or type III functional response and our results suggest that bonnetheads follow a functional response continuum more closely. This study highlights the importance and difficulty in understanding the feeding ecology of highly mobile mesopredators.

Responses by Central‐Place Foragers to Manipulations of Brood Size: Parent Flickers Respond to Proximate Cues but do not Increase Work Rate

AbstractManipulations of brood size measure the willingness or ability of parents to invest in offspring and different reproductive roles may lead to differences in feeding effort between the sexes. Parental investment in birds is usually assessed by quantifying feeding rates, but this provides an incomplete picture of parental effort because it fails to account for how parents collect food on the landscape. We studied northern flickers (Colaptes auratus), a woodpecker in which males provide the majority of parental care and used a repeated measures design and short‐term (24 h) brood enlargements (N = 35) and reductions (N = 27) to assess effects of treatment on feeding rates to nestlings and parental foraging behaviour. Parents of enlarged broods did not significantly increase feeding rate, resulting in a decline in nestling mass. Parents of reduced broods decreased their feeding rates by 84%, but increased per capita feeding rates, resulting in nestling mass gain. The variation in feeding rates to enlarged broods was not influenced by feather corticosterone, body condition, feather re‐growth rate or mass change between the incubation and nestling periods. Foraging pattern on the landscape remained the same during the enlarged treatment for both sexes. We conclude that flickers respond to proximate cues in brood demands, but do not increase feeding rates to enlarged broods, at least in the short term. A literature review suggested that this lack of response is atypical for short‐lived species. We hypothesize that parents in species with large home ranges and long nestling periods face energetic limitations that constrain their ability to respond to enlarged broods. We encourage future studies to assess foraging behaviour on the landscape to document important trade‐offs for parents such as predation risk and energy expenditure while feeding offspring.

Predator ontogeny determines trophic cascade strength in freshwater rock pools

Ontogenetic changes in consumers can influence the magnitude and outcome of direct and indirect ecological interactions. Although most research has focused on the consequences of qualitative changes in diet (i.e., shifts in trophic guild between life‐history stages), quantitative effects of ontogeny, such as size‐dependent per capita consumption rates, likely also influence food webs by modulating the relative importance of top‐down control. I examined the effect of predator ontogeny on per capita consumption rates, selectivity between prey species, and the resulting food‐web consequences using a system of freshwater rock pools on Appledore Island, Maine, USA. The rock pools house a simple tri‐trophic food chain consisting of chlorophyte algae consumed by cladoceran grazers (Moina macrocopa and Daphnia pulex), which are then preyed upon by the aquatic insect Trichocorixa verticalis (Corixidae). Laboratory studies showed that Trichocorixa grows substantially during its life history, with size‐dependent (i.e., allometric) increases in per capita predation rates (consuming both Moina and Daphnia). Predation rates were significantly higher on Moina than Daphnia in single‐prey experiments and all instars of Trichocorixa significantly preferred Moina in choice experiments. In a mesocosm experiment, predation by Trichocorixa on zooplankters created a top‐down trophic cascade by releasing phytoplankton from grazing and the strength of the cascade increased significantly with Trichocorixa life‐history stage. A yearlong observational study of three rock pools in the field indicated that top‐down interactions are present and strongly affect food‐web dynamics in situ, as well. In particular, time‐series modeling showed that increases in Trichocorixa biomass (due to ontogenetic growth and hatching) led to decreases in Moina population growth rates, which caused increases in phytoplankton population growth rates. Taken together, these results indicate that consumer ontogeny can affect food‐web and ecosystem dynamics without qualitative niche shifts if per capita feeding rates change substantially over the consumer's life history. Given the prevalence of both allometrically scaling consumption rates and dynamically structured predator populations, top‐predator demography may be an important driver of the trophic structure and dynamics of food webs.

Response of the benthic food web to short- and long-term nutrient enrichment in saltmarsh mudflats

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 474:27-41 (2013) - DOI: https://doi.org/10.3354/meps10090 Response of the benthic food web to short- and long-term nutrient enrichment in saltmarsh mudflats Pierre-Yves Pascal1,2,*, John W. Fleeger2, Henricus T. S. Boschker3, Hanan M. Mitwally4, David S. Johnson5 1UMR 7138, Equipe Biologie de la Mangrove, Laboratoire de Biologie Marine, Université des Antilles et de la Guyane, 97159 Pointe-à-Pitre, Guadeloupe, France 2Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, USA 3Center for Estuarine and Coastal Ecology, Netherlands Institute of Ecology, 4400 AC Yerseke, The Netherlands 4Oceanography Department, Faculty of Science, Alexandria University, 21511 Alexandria, Egypt 5The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA *Email: pypascal@univ-ag.fr ABSTRACT: We examined the responses of biota at or near the base of the benthic food web to nutrient enrichment in salt marsh mudflats in Plum Island estuary (Massachusetts, USA). To simulate eutrophication, nitrate and phosphate loading rates were increased 10- to 15-fold in creeks fertilized for 2 mo (i.e. short-term enrichment) or 6 yr (chronic enrichment). We found that benthic invertebrate community structure was not altered by nutrient enrichment, although the abundance of epifaunal, but not infaunal, grazers increased. Short-term enrichment had no effect on the food web, but significant changes were detected with chronic enrichment. Grazing experiments with 15N-enriched bacteria and 13C-enriched benthic algae revealed higher per capita ingestion rates of benthic microalgae by nematodes, copepods and hydrobiid snails in the creek with chronic nutrient enrichment where isotope composition also indicated that algae increased in dietary importance. The fraction of bacterial biomass grazed was not affected by nutrient enrichment; however, the fraction of benthic algal biomass grazed increased by 235% with chronic enrichment. This higher grazing pressure was partly the result of dietary changes (increases in per capita feeding rate or a change in selection) but was mostly due to an increased abundance of the grazing consumer with the highest biomass, the snail Nassarius obsoletus. This increased top-down control partially masked the bottom-up effects of nutrient enrichment on algal biomass and helps explain the slow and inconsistent response of microalgal biomass to chronic nutrient enrichment previously observed in this estuary. Our research shows that eutrophication may subtly affect benthic food webs before large, sustained increases in algal biomass are observed. KEY WORDS: Nutrient enrichment · Benthic food web · Bacteria · Benthic microalgae · Meiofauna · Nassarius obsoletus · Grazing rates Full text in pdf format PreviousNextCite this article as: Pascal PY, Fleeger JW, Boschker HTS, Mitwally HM, Johnson DS (2013) Response of the benthic food web to short- and long-term nutrient enrichment in saltmarsh mudflats. Mar Ecol Prog Ser 474:27-41. https://doi.org/10.3354/meps10090 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 474. Online publication date: January 31, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Using functional response modeling to investigate the effect of temperature on predator feeding rate and energetic efficiency

Temperature is one of the most important environmental parameters influencing all the biological processes and functions of poikilothermic organisms. Although extensive research has been carried out to evaluate the effects of temperature on animal life histories and to determine the upper and lower temperature thresholds as well as the optimal temperatures for survival, development, and reproduction, few studies have investigated links between thermal window, metabolism, and trophic interactions such as predation. We developed models and conducted laboratory experiments to investigate how temperature influences predator-prey interaction strengths (i.e., functional response) using a ladybeetle larva feeding on aphid prey. As predicted by the metabolic theory of ecology, we found that handling time exponentially decreases with warming, but--in contrast with this theory--search rate follows a hump-shaped relationship with temperature. An examination of the model reveals that temperature thresholds for predation depend mainly on search rate, suggesting that predation rate is primarily determined by searching activities and secondly by prey handling. In contrast with prior studies, our model shows that per capita short-term predator-prey interaction strengths and predator energetic efficiency (per capita feeding rate relative to metabolism) generally increase with temperature, reach an optimum, and then decrease at higher temperatures. We conclude that integrating the concept of thermal windows in short- and long-term ecological studies would lead to a better understanding of predator-prey population dynamics at thermal limits and allow better predictions of global warming effects on natural ecosystems.

Warming up the system: higher predator feeding rates but lower energetic efficiencies

Predictions on the consequences of the rapidly increasing atmospheric CO2 levels and associated climate warming for population dynamics, ecological community structure and ecosystem functioning depend on mechanistic energetic models of temperature effects on populations and their interactions. However, such mechanistic approaches combining warming effects on metabolic (energy loss of organisms) and feeding rates (energy gain by organisms) remain a key, yet elusive, goal. Aiming to fill this void, we studied the metabolic rates and functional responses of three differently sized, predatory ground beetles on one mobile and one more resident prey species across a temperature gradient (5, 10, 15, 20, 25 and 30 °C). Synthesizing metabolic and functional-response theory, we develop novel mechanistic predictions how predator–prey interaction strengths (i.e., functional responses) should respond to warming. Corroborating prior theory, warming caused strong increases in metabolism and decreases in handling time. Consistent with our novel model, we found increases in predator attack rates on a mobile prey, whereas attack rates on a mostly resident prey remained constant across the temperature gradient. Together, these results provide critically important information that environmental warming generally increases the direct short-term per capita interaction strengths between predators and their prey as described by functional-response models. Nevertheless, the several fold stronger increase in metabolism with warming caused decreases in energetic efficiencies (ratio of per capita feeding rate to metabolic rate) for all predator–prey interactions. This implies that warming of natural ecosystems may dampen predator–prey oscillations thus stabilizing their dynamics. The severe long-term implications; however, include predator starvation due to energetic inefficiency despite abundant resources.

Indirect interactions in a rice ecosystem: density dependence and the interplay between consumptive and non-consumptive effects of predators

Summary 1. Density- and trait-mediated indirect interactions (DMIIs and TMIIs, respectively) in food chains play crucial roles in community structure and processes. However, factors affecting the relative strength of these interactions are poorly understood, including in widespread and important freshwater rice ecosystems. 2. We studied the strength of DMIIs and TMIIs in a food chain involving a predator (the Reeve’s turtle Chinemys reevesii), its herbivorous prey (the apple snail Pomacea canaliculata) and a plant (rice Oryza sativa) in outdoor containers simulating rice fields. We also evaluated consumptive and non-consumptive effects of the predator on the snail. We removed a fixed proportion of snails every 2 days to simulate prey consumption and introduced a caged turtle that was fed daily with snails to simulate non-consumptive effects. 3. Direct consumptive effects increased growth of the remaining snails and their per capita feeding rate. Moreover, consumptive and non-consumptive effects, and their interaction, affected the proportion of snails buried in the soil. This interaction was presumably because increasing food availability per snail induced their self-burying behaviour. 4. Both DMIIs and TMIIs affected the number of rice plants remaining, whereas their interaction term was not significant. 5. In summary, density dependence and interactions between consumptive and non-consumptive effects influenced snail growth and behaviour, respectively. However, no cascading effects of these complicated interactions on rice plants were detected.

Behavioral correlations provide a mechanism for explaining high invader densities and increased impacts on native prey

The fact that superabundant invasive pests are also sometimes highly aggressive represents an interesting paradox. Strong intraspecific aggression should result in high intraspecific competition and limit the densities reached by exotic species. One mechanism that can allow invaders to attain high densities despite high intraspecific aggression, involves positive correlations between aggression and other behaviors such as foraging activity. We conducted a mesocosm experiment to quantify the ecological implications of correlations between aggressiveness and foraging activity among groups of exotic signal crayfish (Pacifastacus leniusculus) at low and high densities. Our results showed that high invader densities increased intraspecific aggression and per capita interactions between crayfish, but also increased foraging activity and impacts on preferred prey. As a result, exotic crayfish did not show density-dependent reductions in per capita feeding or growth rates. We suggest that the positive correlation between aggression and activity is part of an aggression syndrome whereby some individuals are generally more aggressive/active than others across situations. An aggression syndrome can couple aggressive behaviors important to population establishment of invasive species with foraging activity that enhances the ability of invaders to attain high densities and have large impacts on invaded communities.

LARVAL AGGREGATION AFFECTS FEEDING RATE IN CHLOSYNE POECILE (LEPIDOPTERA: NYMPHALIDAE)

Larvae of Chlosyne poecile (Felder) (Lepidoptera: Nymphalidae: Melitaeini) on Razisea sp. (Acanthaceae) feed in large aggregations as early instars but disperse and feed in small groups or as solitary caterpillars in later instars. The effect of group size on per capita feeding rate was tested by manipulating the number of larvae on a leaf and measuring the leaf area eaten in short-term feeding trials. Feeding rate increased significantly with group size for first instars but decreased with group size for all larger instars. Although feeding rate decreased significantly with group size for second instars, second instars in the field were usually found in large groups and did not begin to disperse until the third instar. Variance-to-mean ratios indicate that larval aggregation was lower in later instars, slowly approaching a random distribution. The distributions of larger instars may therefore be the result of random dispersal to food and not the active avoidance of other larvae. If the observed lag between the predicted optimal time to disperse and the observed pattern is adaptive, then it may be due to unmeasured benefits of aggregation, such as lower predation rates and unmeasured costs of dispersal. Egg clustering and aggregation of larvae may be more common for butterflies in the Neotropics than in other areas.