Size Classes Of Predators Research Articles

Size-dependent functional response ofXenopus laevisfeeding on mosquito larvae

Predators can play an important role in regulating prey abundance and diversity, determining food web structure and function, and contributing to important ecosystem services, including the regulation of agricultural pests and disease vectors. Thus, the ability to predict predator impact on prey is an important goal in ecology. Often, predators of the same species are assumed to be functionally equivalent, despite considerable individual variation in predator traits known to be important for shaping predator–prey interactions, like body size. This assumption may greatly oversimplify our understanding of within-species functional diversity and undermine our ability to predict predator effects on prey. Here, we examine the degree to which predator–prey interactions are functionally homogenous across a natural range of predator body sizes. Specifically, we quantify the size-dependence of the functional response of African clawed frogs (Xenopus laevis) preying on mosquito larvae (Culex pipiens). Three size classes of predators, small (15–30 mm snout-vent length), medium (50–60 mm) and large (105–120 mm), were presented with five densities of prey to determine functional response type and to estimate search efficiency and handling time parameters generated from the models. The results of mesocosm experiments showed that type of functional response of X. laevis changed with size: small predators exhibited a Type II response, while medium and large predators exhibited Type III responses. Functional response data showed an inversely proportional relationship between predator attack rate and predator size. Small and medium predators had highest and lowest handling time, respectively. The change in functional response with the size of predator suggests that predators with overlapping cohorts may have a dynamic impact on prey populations. Therefore, predicting the functional response of a single size-matched predator in an experiment may misrepresent the predator’s potential impact on a prey population.

Hypotheses and trends on how body size affects trophic interactions in a guild of South American killifishes

AbstractA chief structuring force in food webs is the hierarchy of trophic interactions, where bigger animals feed on smaller ones. The anatomic and physiological explanations of why body size determines this hierarchy are embodied within the concept of gape limitation. The relaxation of gape limitation and an increase in energetic demands due to predators' larger body size determine the size and diversity of prey species. However, these patterns may be related to further trends in trophic interactions with body size, which have been less considered. Specifically, the passive incorporation of prey should involve a nested distribution of prey among predator size classes. However, predators avoid smaller resources because of their low energy return, with a clumped distribution of prey potentially generating modular organization with qualitative changes in prey identity (e.g. zooplankton, macroinvertebrates and fishes). Finally, size‐mediated interactions (such as direct and indirect competition) may cause predators of similar body size to differentiate among prey organisms, resulting in a checkerboard distribution of prey identity. Consequently, nestedness, modularity and checkerboard distributions of prey among predators of different size classes should form emergent network structures that are directly related to clear ecological mechanisms. We analyse these predictions in a killifish guild, where trends in trophic positions, prey richness, evenness and the number of energy sources systematically scale with body size. We found significant nestedness and segregation in diet among different size classes, supporting the progressive incorporation of prey items coupled with prey differentiation among similar classes. However, we also detected an ‘anti‐modular’ trend, which contradicts theoretical expectations and previous results. We hypothesize that this anti‐modularity is determined by the high biodiversity of the system and the continuous representation of prey size classes. These results reinforce the concept of size‐mediated interactions and its connection with community biodiversity as a main structuring force of food webs.

Size Selectivity by Atlantic Mud CrabsPanopeus herbstii(Milne Edwards) Feeding on Ivory BarnaclesBalanus eburneus(Gould)

Many crabs preferentially consume prey items that are relatively small to optimize energy gain and to minimize the risk of claw damage. The Atlantic mud crab Panopeus herbstii is an abundant predator on benthic epifauna, including oysters (Crassostrea virginica) and barnacles (e.g., Balanus eburneus) in southeastern U.S. saltmarsh ecosystems. Size selectivity by mud crab predators feeding on barnacle prey was examined using laboratory experiments in which individual mud crabs were offered a size range of ivory barnacles. Small (carapace width, <30 mm) and large (carapace width, >30 mm) crabs were offered small (test width, <7 mm), medium (test width, 7–8.9 mm), large (test width, 9–10.9 mm), and extra large (test width, >11 mm) ivory barnacles. Small crabs selectively consumed small barnacles at greater frequencies than other barnacles. Large crabs consumed proportionately smaller barnacles, relative to their carapace width, than small crabs. The relationship between predator crusher height and prey test width indicates that larger mud crabs reduce their risk of claw damage when attacking barnacles relative to smaller conspecifics. The observed barnacle critical size varied across predator size classes and was not strongly related to crab carapace width or crusher morphology. The median barnacle size of vulnerability increased with increasing predator carapace width. The observed ontogenetic size selectivity by mud crabs feeding on barnacles may be a more relevant predictor of prey vulnerability than barnacle critical size.

Preliminary estimates of consumption rates of Rapana venosa (Gastropoda, Muricidae); a new threat to mollusk biodiversity in the Río de la Plata

The veined whelk Rapana venosa (Valenciennes, 1846) is an active, Asian, invasive mollusk predator that has been present along the coastal zone of Maldonado, Uruguay since 2004. The feeding rate of R. venosa on native mussels was estimated in a laboratory experiment. A flume with a constant water flow was used, in which the mean (± SD) temperature was 25.4 ± 1.8 °C, the mean (± SD) salinity was 18.5 ± 1.6, and a 15:9 light-dark schedule was followed. Each experimental trial lasted 9 days and a constant prey supply was maintained. Three size classes of predators and three size classes of prey (Brachidontes rodriguezii and Mytella charruana), were used. For all sizes of predator combined, the average (± SD) absolute consumption rate was 0.88 ± 0.3 g day -1 , and the relative consumption rate was 0.057 ± 0.034 g g -1 day -1 . No significant differences were found between the number of prey and total grams consumed among the three size categories of snails. However, for the relative consumption rate, small snails consumed significantly greater (0.10 g g -1 day -1 ) amounts than intermediate and large snails (0.04 and 0.02 g g -1 day -1 respectively). All sizes of snails consumed significantly more intermediate- than small-sized mussels. This study provides important knowledge for future research to assess the impact of this invasive species on native bivalve resources.

Seasonal shifts in predator body size diversity and trophic interactions in size‐structured predator–prey systems

1. Theory suggests that the relationship between predator diversity and prey suppression should depend on variation in predator traits such as body size, which strongly influences the type and strength of species interactions. Prey species often face a range of different sized predators, and the composition of body sizes of predators can vary between communities and within communities across seasons. 2. Here, I test how variation in size structure of predator communities influences prey survival using seasonal changes in the size structure of a cannibalistic population as a model system. Laboratory and field experiments showed that although the per-capita consumption rates increased at higher predator-prey size ratios, mortality rates did not consistently increase with average size of cannibalistic predators. Instead, prey mortality peaked at the highest level of predator body size diversity. 3. Furthermore, observed prey mortality was significantly higher than predictions from the null model that assumed no indirect interactions between predator size classes, indicating that different sized predators were not substitutable but had more than additive effects. Higher predator body size diversity therefore increased prey mortality, despite the increased potential for behavioural interference and predation among predators demonstrated in additional laboratory experiments. 4. Thus, seasonal changes in the distribution of predator body sizes altered the strength of prey suppression not only through changes in mean predator size but also through changes in the size distribution of predators. In general, this indicates that variation (i.e. diversity) within a single trait, body size, can influence the strength of trophic interactions and emphasizes the importance of seasonal shifts in size structure of natural food webs for community dynamics.

Predation of shore crabs ( Carcinus maenas (L.)) and starfish ( Asterias rubens L.) on blue mussel ( Mytilus edulis L.) seed from wild sources and spat collectors

In The Netherlands, several pilot projects are carried out on the use of spat collectors as an additional supply of seed for bottom culture of mussels ( Mytilus edulis). The method proves to be successful in yielding substantial amounts of seed. One of the conditions for successful application of collector seed on bottom plots is a good yield of the seed on bottom plots. Mussel seed of different origin (from wild littoral and sublittoral beds or from collectors) was offered to predators (crabs Carcinus maenas and starfish Asterias rubens) and seed survival was monitored. In addition, the effect of density and size of collector seed on predation was studied. Circular cages containing predators and seed were placed in a basin with running seawater, or suspended from a jetty in a harbour. Two size classes of predators and three size classes of seed were used. Survival was monitored. Consumption of mussel seed by starfish was much lower than by crabs. Maximum observed consumption rates were 23 seed/day/crab and 1 seed/day/starfish. Consumption rates increased significantly with decreasing seed size. Seed larger than 20 mm were consumed at a significantly lower rate. Seed density did not affect survival. Collector seed was not consumed at higher rates than wild littoral or sublittoral seed. In conclusion, collector seed can be a promising additional source of seed for bottom culture of mussels in The Netherlands.

Predation during early post-settlement varies in importance for shaping marine sessile communities

Predation on newly settled sessile invertebrates is an important process shaping the structure of benthic marine communities in localised areas on the northeastern coast of North Amer- ica. There are no studies that have tested whether predation acts similarly in other locations, so it is not clear whether generalisations can be made about the effects of predation on different sessile communities. In this study we determined whether predation on newly settled recruits altered the structure of 2 different sessile communities in Port Phillip Bay, Australia. We conducted a series of predator exclusion experiments using full cage, partial cage and no cage treatments at 2 study sites, Williamstown and Queenscliff. Full cage and partial cage treatments of either 1 cm or 2 mm mesh sizes were used to separate the effects of different size classes of predators. At both sites, a variety of colonial and solitary ascidians, bryozoans, sponges and polychaetes settled onto experimental sur- faces. Predation had little impact on the recruitment success of taxa present at Williamstown and did not alter community structure. At Queenscliff, didemnid ascidians had higher abundances on com- pletely caged plates (2 mm mesh) after 40 d, suggesting that they may have been preyed upon in treatments exposed to carnivores. However, predation had no effect on the densities of other taxa found on experimental surfaces, and there were no differences in overall community structure between treatments. Recruitment rates were low and predators were never observed on experimen- tal surfaces at Queenscliff, so predation on newly settled recruits may be an uncommon occurrence for most taxa. In contrast, predators were commonly found on experimental surfaces at Williamstown, but recruitment rates were high and predators had little effect on the abundance of newly settled prey. The results of this study show that predators can have weak effects on recent recruits and that predation during early post-settlement is not a major process shaping the structure of all marine sessile communities.

Size‐ and density‐dependent habitat use in predators: consequences for habitat shifts in young fish

Summary When different size classes of predators use different habitats, the habitat‐specific predation risk for prey may change as prey increase in size due to the size‐dependency in predator–prey interactions. To study the effects of predation risk on habitat shifts in growing prey, we used a combination of whole‐lake manipulations and field data, covering 5 years from three lakes on (1) body sizes at habitat shifts in young‐of‐the‐year perch (YOY) perch (Perca fluviatilis) and (2) habitat use and densities of two size classes of cannibals (1 year old and ≥ 2 years old). YOY perch shifted habitat to the pelagic immediately after hatching and exhibited a second shift back to the littoral zone at a size range of 11–30 mm. Cannibalism from 1‐year‐old perch decreased over time, whereas cannibalism from perch ≥ 2 years old increased over time but did not include early larval stages. The habitat use of 1‐year‐old perch was restricted to the littoral habitat, whereas perch ≥ 2 years old used both the pelagic and the littoral habitats. Habitat shifts in YOY perch were unrelated to differences in resource levels between the two habitats as YOY perch were either yolk sac feeding (first habitat shift) or had similar foraging gains in both habitats (second habitat shift). The size when YOY perch shifted back to the littoral zone was related positively to the densities of 1‐year‐old perch, whereas no clear relationship was found between shift size and the density of perch 2 ≥ years old. The latter was suggested to be due to the higher density and the larger variation in densities of 1‐year‐old perch and that the predation risk from perch ≥ 2 years old was less habitat‐specific. Our results suggest that size‐dependent habitat use and size specific predation risk evoked by cannibals affect the size when YOY perch shift back to the littoral zone.

SIZE AND CARBON ACQUISITION IN LIZARDS FROM AMAZONIAN SAVANNA: EVIDENCE FROM ISOTOPE ANALYSIS

We studied the effects of lizard size, and availability of C3 and C4 plants, on stable carbon-isotope (δ13C) ratios of lizards in an Amazonian savanna. Availability of C3 and C4 plants was estimated from 500-point quadrats in each of 38 plots of 3.75 ha. Ratios of δ13C in muscle tissue were used to estimate the contributions of C3- and C4-based food chains to the diets of four species of lizards: Cnemidophorus lemniscatus, Kentropyx striata, Ameiva ameiva, and Anolis auratus. The relative contributions of C3 and C4 plants to the diets of the lizards were affected by the extent of shrub cover in C. lemniscatus, A. auratus, and possibly in K. striata. However, there was a strong effect of lizard size among and within lizard species. These results are at variance with a previous evaluation of diet overlap among size classes of these species that was based on overlap in prey taxonomic categories. This indicates that species and/or size classes of predators on arthropods may obtain their energy from food chains based on different autotrophic sources, despite high overlap in the taxonomic categories of prey.

Size and Carbon Acquisition in Lizards from Amazonian Savanna: Evidence from Isotope Analysis

We studied the effects of lizard size, and availability of C3 and C4 plants, on stable carbon-isotope (δ13C) ratios of lizards in an Amazonian savanna. Availability of C3 and C4 plants was estimated from 500-point quadrats in each of 38 plots of 3.75 ha. Ratios of δ13C in muscle tissue were used to estimate the contributions of C3- and C4-based food chains to the diets of four species of lizards: Cnemidophorus lemniscatus, Kentropyx striata, Ameiva ameiva, and Anolis auratus. The relative contributions of C3 and C4 plants to the diets of the lizards were affected by the extent of shrub cover in C. lemniscatus, A. auratus, and possibly in K. striata. However, there was a strong effect of lizard size among and within lizard species. These results are at variance with a previous evaluation of diet overlap among size classes of these species that was based on overlap in prey taxonomic categories. This indicates that species and/or size classes of predators on arthropods may obtain their energy from food chains based on different autotrophic sources, despite high overlap in the taxonomic categories of prey.