No evidence of nonlinear effects of predator density,refuge availability,or body size of prey on prey mortality rates

Predator density, refuge availability, and body size of prey can all affect the mortality rate of prey. We assume that more predators will lead to an increase in prey mortality rate, but behavioral interactions between predators and prey, and availability of refuge, may lead to nonlinear effects of increased number of predators on prey mortality rates. We tested for nonlinear effects in prey mortality rates in a mesocosm experiment with different size classes of western mosquitofish (Gambusia affinis) as the prey, different numbers of green sunfish (Lepomis cyanellus) as the predators, and different levels of refuge. Predator number and size class of prey, but not refuge availability, had significant effects on the mortality rate of prey. Change in mortality rate of prey was linear and equal across the range of predator numbers. Each new predator increased the mortality rate by about 10% overall, and mortality rates were higher for smaller size classes. Predator–prey interactions at the individual level may not scale up to create nonlinearity in prey mortality rates with increasing predator density at the population level.     

Individual interaction data are required in community ecology: a conceptual review of the predator–prey mass ratio and more

Community ecology is traditionally species-based and assumes that species comprise identical individuals. However, intraspecific variation is ubiquitous in nature because of ontogenetic growth and critical in food-we dynamics. To understand individual interaction-mediated food webs, researchers have recently focused on body size as the most fundamental biological aspect and assessed a parameter called the predator–prey mass ratio (PPMR). Herein, I review the conceptual development of the PPMR and suggest four major concerns regarding its measurement: (1) PPMR should be measured at the individual level because species-averaged values distort actual feeding relationships, (2) individual-level PPMR data on gape-unconstrained predators (e.g., terrestrial carnivores) are limited because previous studies have targeted gape-limited fish predators, (3) predators’ prey size selectivity (preferred PPRM) is conceptually different from dietary prey size (realized PPMR) and should be distinguished by incorporating environmental prey abundance information, and (4) determinants of preferred PPMR, rather than those of realized PPMR, should be identified to describe size-dependent predation. Future studies are encouraged to explore not only predation but also other interaction types (e.g., competition, mutualism, and herbivory) at the individual level. However, this is not likely to occur while ecological communities are still considered to be interspecific interaction networks. To resolve this situation and more comprehensively understand biodiversity and ecosystem functioning, I suggest that community ecology requires a paradigm shift in the unit of interaction from species to individuals, similar to evolutionary biology, which revolutionized the unit of selection, because interactions occur between individuals.     

Abundance–body mass relationships in size-structured food webs

In communities sharing a common energy source, the energetic equivalence hypothesis predicts that numerical abundance (N) scales with body mass (M) as M^?0.75. However, in size‐structured food webs all individuals do not share a common energy source, and the energy available (E) to larger individuals is constrained by inefficient energy transfer through the food chains that support them. This is expected to lead to steeper scalings of N with M. Here, we formalize and test an existing model for predicting abundance–body mass scaling, where the decline in E with M is calculated from the mean predator–prey body mass ratio (from size‐based nitrogen stable isotope analysis) and trophic transfer efficiency. We show that the steep predicted scalings of abundance and body mass (N scales as M^?1.2, B scales as M^?0.2) in a marine food web are consistent with empirical estimates and can be attributed to the small predator–prey body mass ratio (106 : 1). As a previous study has shown that environmental stability may favour low predator–prey mass ratios and long food chains, we predict that steeper abundance–body mass relationships will be found in more stable environments.     

Influence of intra‐ and interspecific variation in predator–prey body size ratios on trophic interaction strengths

1. Predation is a pervasive force that structures food webs and directly influences ecosystem functioning. The relative body sizes of predators and prey may be an important determinant of interaction strengths. However, studies quantifying the combined influence of intra‐ and interspecific variation in predator–prey body size ratios are lacking.
2. We use a comparative functional response approach to examine interaction strengths between three size classes of invasive bluegill and largemouth bass toward three scaled size classes of their tilapia prey. We then quantify the influence of intra‐ and interspecific predator–prey body mass ratios on the scaling of attack rates and handling times.
3. Type II functional responses were displayed by both predators across all predator and prey size classes. Largemouth bass consumed more than bluegill at small and intermediate predator size classes, while large predators of both species were more similar. Small prey were most vulnerable overall; however, differential attack rates among prey were emergent across predator sizes. For both bluegill and largemouth bass, small predators exhibited higher attack rates toward small and intermediate prey sizes, while larger predators exhibited greater attack rates toward large prey. Conversely, handling times increased with prey size, with small bluegill exhibiting particularly low feeding rates toward medium–large prey types. Attack rates for both predators peaked unimodally at intermediate predator–prey body mass ratios, while handling times generally shortened across increasing body mass ratios.
4. We thus demonstrate effects of body size ratios on predator–prey interaction strengths between key fish species, with attack rates and handling times dependent on the relative sizes of predator–prey participants.
5. Considerations for intra‐ and interspecific body size ratio effects are critical for predicting the strengths of interactions within ecosystems and may drive differential ecological impacts among invasive species as size ratios shift.

     

Predator size and phenology shape prey survival in temporary ponds

Theoretical efforts suggest that the relative sizes of predators and their prey can shape community dynamics, the structure of food webs, and the evolution of life histories. However, much of this work has assumed static predator and prey body sizes. The timing of recruitment and the growth patterns of both predator and prey have the potential to modify the strength of predator–prey interactions. In this study, I examined how predator size dynamics in 40 temporary ponds over a 3-year period affected the survival of spotted salamander (Ambystoma maculatum) larvae. Across communities, gape-limited predator richness, but not size, was correlated with habitat duration (pond permanence). Within communities, mean gape-limited predator size diminished as the growing season progressed. This size reduction occurred because prey individuals grew into a body size refuge and because the largest of the predators left ponds by mid-season. Elevated gape-limited predation risk across time and space was predicted by the occurrence of two large predatory salamanders: marbled salamander larvae (Ambystoma opacum) and red-spotted newt adults (Notophthalmus viridescens). The presence of the largest gape-limited predator, A. opacum, predicted A. maculatum larval survival in the field. The distribution of large predatory salamanders among ponds and across time is expected to lead to differing community dynamics and to generate divergent natural selection on early growth and body size in A. maculatum. In general, a dynamic perspective on predator size often will be necessary to understand the ecology and evolution of species interactions. This will be especially true in frequently disturbed or seasonal habitats where phenology and ontogeny interact to determine body size asymmetries. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predator–prey mass ratio drives microbial activity under dry conditions in Sphagnum peatlands

Mid‐ to high‐latitude peatlands are a major terrestrial carbon stock but become carbon sources during droughts, which are increasingly frequent as a result of climate warming. A critical question within this context is the sensitivity to drought of peatland microbial food webs. Microbiota drive key ecological and biogeochemical processes, but their response to drought is likely to impact these processes. Peatland food webs have, however, been little studied, especially the response of microbial predators. We studied the response of microbial predators (testate amoebae, ciliates, rotifers, and nematodes) living in Sphagnum moss carpet to droughts, and their influence on lower trophic levels and on related microbial enzyme activity. We assessed the impact of reduced water availability on microbial predators in two peatlands using experimental (Linje mire, Poland) and natural (Forbonnet mire, France) water level gradients, reflecting a sudden change in moisture regime (Linje), and a typically drier environment (Forbonnet). The sensitivity of different microbial groups to drought was size dependent; large sized microbiota such as testate amoebae declined most under dry conditions (?41% in Forbonnet and ?80% in Linje). These shifts caused a decrease in the predator–prey mass ratio (PPMR). We related microbial enzymatic activity to PPMR; we found that a decrease in PPMR can have divergent effects on microbial enzymatic activity. In a community adapted to drier conditions, decreasing PPMR stimulated microbial enzyme activity, while in extreme drought experiment, it reduced microbial activity. These results suggest that microbial enzymatic activity resulting from food web structure is optimal only within a certain range of PPMR, and that different trophic mechanisms are involved in the response of peatlands to droughts. Our findings confirm the importance of large microbial consumers living at the surface of peatlands on the functioning of peatlands, and illustrate their value as early warning indicators of change.     

The role of size and colour pattern in protection of developmental stages of the red firebug (Pyrrhocoris apterus) against avian predators

We investigated how predator/prey body‐size ratio and prey colour pattern affected efficacy of prey warning signals. We used great and blue tits (Parus major and Cyanistes caeruleus), comprising closely related and ecologically similar bird species differing in body size, as experimental predators. Two larval instars and adults of the unpalatable red firebug (Pyrrhocoris apterus), differing in body size and/or coloration, were used as prey. We showed that prey body size did not influence whether a predator attacked the prey or not during the first encounter. However, smaller prey were attacked, killed, and eaten more frequently in repetitive encounters. We assumed that body size influences the predator through the amount of repellent chemicals better than through the amount of optical warning signal. The larger predator attacked, killed and ate all forms of firebug more often than the smaller one. The difference between both predators was more pronounced in less protected forms of firebug (chemically as well as optically). Colour pattern also substantially affected the willingness of predators to attack the prey. Larval red–black coloration did not provide a full‐value warning signal, although a similarly conspicuous red‐black coloration of the adults reliably protected them. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 890–898.     

Feedback between size balance and consumption strongly affects the consequences of hatching phenology in size‐dependent predator–prey interactions

In many size‐dependent predator–prey systems, hatching phenology strongly affects predator–prey interaction outcomes. Early‐hatched predators can easily consume prey when they first interact because they encounter smaller prey. However, this process by itself may be insufficient to explain all predator–prey interaction outcomes over the whole interaction period because the predator–prey size balance changes dynamically throughout their ontogeny. We hypothesized that hatching phenology influences predator–prey interactions via a feedback mechanism between the predator–prey size balance and prey consumption by predators. We experimentally tested this hypothesis in an amphibian predator–prey model system. Frog tadpoles Rana pirica were exposed to a predatory salamander larva Hynobius retardatus that had hatched 5, 12, 19 or 26 days after the frog tadpoles hatched. We investigated how the salamander hatch timing affected the dynamics of prey mortality, size changes of both predator and prey, and their subsequent life history (larval period and size at metamorphosis). The predator–prey size balance favoured earlier hatched salamanders, which just after hatching could successfully consume more frog tadpoles than later hatched salamanders. The early‐hatched salamanders grew rapidly and their accelerated growth enabled them to maintain the predator‐superior size balance; thus, they continued to exert strong predation pressure on the frog tadpoles in the subsequent period. Furthermore, frog tadpoles exposed to the early‐hatched salamanders were larger at metamorphosis and had a longer larval period than other frog tadpoles. These results suggest that feedback between the predator‐superior size balance and prey consumption is a critical mechanism that strongly affects the impacts of early hatching of predators in the short‐term population dynamics and life history of the prey. Because consumption of large nutrient‐rich prey items supports the growth of predators, a similar feedback mechanism may be common and have strong impacts on phenological shifts in size‐dependent trophic relationships.     

Flow‐mediated predator–prey dynamics influence fish populations in a tropical river

相似文献   

Rethinking trophic niches: Speed and body mass colimit prey space of mammalian predators

1. Realized trophic niches of predators are often characterized along a one‐dimensional range in predator–prey body mass ratios. This prey range is constrained by an “energy limit” and a “subdue limit” toward small and large prey, respectively. Besides these body mass ratios, maximum speed is an additional key component in most predator–prey interactions.
2. Here, we extend the concept of a one‐dimensional prey range to a two‐dimensional prey space by incorporating a hump‐shaped speed‐body mass relation. This new “speed limit” additionally constrains trophic niches of predators toward fast prey.
3. To test this concept of two‐dimensional prey spaces for different hunting strategies (pursuit, group, and ambush predation), we synthesized data on 63 terrestrial mammalian predator–prey interactions, their body masses, and maximum speeds.
4. We found that pursuit predators hunt smaller and slower prey, whereas group hunters focus on larger but mostly slower prey and ambushers are more flexible. Group hunters and ambushers have evolved different strategies to occupy a similar trophic niche that avoids competition with pursuit predators. Moreover, our concept suggests energetic optima of these hunting strategies along a body mass axis and thereby provides mechanistic explanations for why there are no small group hunters (referred to as “micro‐lions”) or mega‐carnivores (referred to as “mega‐cheetahs”).
5. Our results demonstrate that advancing the concept of prey ranges to prey spaces by adding the new dimension of speed will foster a new and mechanistic understanding of predator trophic niches and improve our predictions of predator–prey interactions, food web structure, and ecosystem functions.

     

Multi-predator effects across life-history stages: non-additivity of egg- and larval-stage predation in an African treefrog

The effects of multiple predators on their prey are frequently non‐additive because of interactions among predators. When prey shift habitats through ontogeny, many of their predators cannot interact directly. However, predators that occur in different habitats or feed on different prey stages may still interact through indirect effects mediated by prey traits and density. We conducted an experiment to evaluate the combined effects of arboreal egg‐stage and aquatic larval‐stage predators of the African treefrog, Hyperolius spinigularis. Egg and larval predator effects were non‐additive – more Hyperolius survived both predators than predicted from their independent effects. Egg‐stage predator effects on aquatic larval density and size and age at hatching reduced the effectiveness of larval‐stage predators by 70%. Our results indicate that density‐ and trait‐mediated indirect interactions can act across life‐stages and habitats, resulting in non‐additive multi‐predator effects.     

Can size distributions of European lake fish communities be predicted by trophic positions of their fish species?

An organism''s body size plays an important role in ecological interactions such as predator–prey relationships. As predators are typically larger than their prey, this often leads to a strong positive relationship between body size and trophic position in aquatic ecosystems. The distribution of body sizes in a community can thus be an indicator of the strengths of predator–prey interactions. The aim of this study was to gain more insight into the relationship between fish body size distribution and trophic position in a wide range of European lakes. We used quantile regression to examine the relationship between fish species'' trophic position and their log‐transformed maximum body mass for 48 fish species found in 235 European lakes. Subsequently, we examined whether the slopes of the continuous community size distributions, estimated by maximum likelihood, were predicted by trophic position, predator–prey mass ratio (PPMR), or abundance (number per unit effort) of fish communities in these lakes. We found a positive linear relationship between species'' maximum body mass and average trophic position in fishes only for the 75% quantile, contrasting our expectation that species'' trophic position systematically increases with maximum body mass for fish species in European lakes. Consequently, the size spectrum slope was not related to the average community trophic position, but there were negative effects of community PPMR and total fish abundance on the size spectrum slope. We conclude that predator–prey interactions likely do not contribute strongly to shaping community size distributions in these lakes.     

Optimal foraging constrains macroecological patterns: body size and dietary niche breadth in lizards

Aim  To explore and identify probable mechanisms contributing to the relationships among body size, dietary niche breadth and mean, minimum, maximum and range of prey size in predaceous lizards.
Location  Our data set includes species from tropical rainforests, semi-arid regions of Brazil, and from deserts of the south-western United States, Australia and the Kalahari of Africa.
Methods  We calculated phylogenetic and non-phylogenetic regressions among predator body size, dietary breath and various prey size measures.
Results  We found a negative association between body size and dietary niche breadth in 159 lizard species sampled across most evolutionary lineages of squamate reptiles and across major continents and habitats. We also show that mean, minimum, maximum and range of prey size were positively associated with body size.
Main conclusions  Our results suggest not only that larger lizards tend to eat larger prey, but in doing so offset their use of smaller prey. Reduction of dietary niche breadth with increased body size in these lizards suggests that large predators target large and more profitable prey. Consequently, the negative association between body size and niche breadth in predators most likely results from optimal foraging. Though this result may appear paradoxical and runs counter to previous studies, resources for predators may be predictably more limited than resources for herbivores, thus driving selection for more profitable prey.     

Invasion of a naticid predator and associated changes in death assemblages of bivalve prey in northern Japan: implications for palaeoecological studies

The recent invasion of a naticid predator (Laguncula pulchella) and associated changes in the death assemblages of bivalve prey (Ruditapes philippinarum) provide a baseline for interpreting predator–prey interactions in the fossil record. This article presents quantitative data on size‐frequency distributions (SFDs) of living and death assemblages, prey size selectivity and drillhole site selectivity from the Tona Coast, northern Japan. Before the appearance of the predator, the SFD of the death assemblage exhibited a right‐skewed platykurtic distribution, and there were very few predatory drillholes. Once the predator appeared, frequencies of predatory drillholes increased, particularly in the smallest size class (2–10 mm shell length). Furthermore, juvenile peaks in the SFDs of death assemblages sharpened, and thus, SFDs exhibited strongly right‐skewed leptokurtic distributions. These changes suggest that intense naticid predation precluded juvenile clams from growing to adulthood, and thus, many dead shells of juvenile clams were introduced into the sediment. The changes in SFDs may also indicate intensification of predation pressure in the fossil record. No temporal shifts in prey size selectivity and drillhole site selectivity were noted, despite substantial changes in the demographics of Ruditapes philippinarum. This suggests that lack of specific size classes of preferred prey species is unlikely to be a primary factor accounting for size mismatches between predator and prey, because, in such situations, naticid predators probably attack other prey species. Therefore, such a factor is unlikely to primarily explain the less stereotypical predatory behaviour (i.e. low prey size selectivity and low drillhole site selectivity), which has been frequently recognized in fossil assemblages. Such less stereotypical predatory behaviour in fossil assemblages is likely to be explained by other factors, such as the existence of multiple predator taxa and lack of specific size classes of all available prey.     

Predator-prey ratios in communities of freshwater invertebrates: the role of enemy free space

SUMMARY.

• 1 The ratio: number of predator species/number of prey species is reviewed using comprehensive faunal lists for a range of freshwater habitats in Britain and North America. Prey species are defined as detritivores, herbivores and fungivores; predators eat metazoan animals as the main component of their diet. Our data refer only to invertebrates.
• 2 The numbers of predators and prey species are apparently very closely correlated in freshwater communities (r=0.84, In transformed data), with an average ratio of predators to prey of 0.36. The average ratio of predators to prey changes from 0.48 in small (species-poor) collections to 0.29 in large (species-rich) collections.
• 3 We suggest that an approximately constant ratio of predators to prey may be generated by: (a) the number of predator species being a function of the number of broad classes or kinds of prey; and (b) the number of prey species being constrained by competition between prey for ‘enemy free space’, i.e. species that are too similar are unable to coexist with shared predators.

     

Growth,Grazing, and Starvation Survival in Three Heterotrophic Dinoflagellate Species

To assess the effects of fluctuating prey availability on predator population dynamics and grazing impact on phytoplankton, we measured growth and grazing rates of three heterotrophic dinoflagellate species—Oxyrrhis marina, Gyrodinium dominans and Gyrodinium spirale—before and after depriving them of phytoplankton prey. All three dinoflagellate species survived long periods (> 10 d) without algal prey, coincident with decreases in predator abundance and cell size. After 1–3 wks, starvation led to a 17–57% decrease in predator cell volume and some cells became deformed and transparent. When re‐exposed to phytoplankton prey, heterotrophs ingested prey within minutes and increased cell volumes by 4–17%. At an equivalent prey concentration, continuously fed predators had ~2‐fold higher specific growth rates (0.18 to 0.55 d^?1) than after starvation (?0.16 to 0.25 d^?1). Maximum specific predator growth rates would be achievable only after a time lag of at least 3 d. A delay in predator growth poststarvation delays predator‐induced phytoplankton mortality when prey re‐emerges at the onset of a bloom event or in patchy prey distributions. These altered predator‐prey population dynamics have implications for the formation of phytoplankton blooms, trophic transfer rates, and potential export of carbon.     

Predation Risk Avoidance by Terrestrial Amphibians: The Role of Prey Experience and Vulnerability to Native and Exotic Predators

We studied avoidance, by four amphibian prey species (Rana luteiventris, Ambystoma macrodactylum, Pseudacris regilla, Tarichia granulosa), of chemical cues associated with native garter snake (Thamnophis elegans) or exotic bullfrog (R. catesbeiana) predators. We predicted that avoidance of native predators would be most pronounced, and that prey species would differ in the intensity of their avoidance based on relative levels of vulnerability to predators in the wild. Adult R. luteiventris (presumably high vulnerability to predation) showed significant avoidance of chemical cues from both predators, A. macrodactylum (intermediate vulnerability to predation) avoided T. elegans only, while P. regilla (intermediate vulnerability to predation) and T. granulosa (low vulnerability to predation) showed no avoidance of either predator. We assessed if predator avoidance was innate and/or learned by testing responses of prey having disparate levels of prior exposure to predators. Wild‐caught (presumably predator‐exposed) post‐metamorphic juvenile R. luteiventris and P. regilla avoided T. elegans cues, while laboratory‐reared (predator‐naive) conspecifics did not; prior exposure to R. catesbeiana was not related to behavioural avoidance among adult or post‐metamorphic juvenile wild‐reared A. macrodactylum and P. regilla. These results imply that (i) some but not all species of amphibian prey avoid perceived risk from garter snake and bullfrog predators, (ii) the magnitude of this response probably differs according to prey vulnerability to predation in the wild, and (iii) avoidance tends to be largely learned rather than innate. Yet, the limited prevalence and intensity of amphibian responses to predation risk observed herein may be indicative of either a relatively weak predator–prey relationship and/or the limited importance of predator chemical cues in this particular system.     

The larval diet of three anisopteran (Odonata) species

SUMMARY.

• 1 Comparisons between larval diets of three anisopteran species (Anax imperator, Aeshna cyanea and Libellula depressa) showed that their food intake changed depending on: (1) predator species, (2) time of year, and (3) developmental stage.
• 2 Although this last factor is not so important, the mean size of prey items and the range of prey species eaten by Auax imperator and Aeshna cyanea larvae increased with predator size.
• 3 Comparisons between prey availability and diets indicated differential selectivity by these predators.

     

Effects of size structure and habitat complexity on predator–prey interactions

1. A predator's ability to suppress its prey depends on the level of interference among predators. While interference typically decreases with increasing habitat complexity, it often increases with increasing size differences among individuals. However, little is known about how variation in intrinsic factors such as population size structure alters predator–prey interactions and how this intrinsic variation interacts with extrinsic variation. 2. By experimentally varying the level of vegetation cover and the size structure of the predatory damselfly Ischnura posita Hagen, we examined the individual and interactive effects of variation in habitat complexity and predator size structure on prey mortality. 3. Copepod prey survival linearly increased as the I. posita size ratio decreased and differed by up to 31% among different predator size structures. Size classes had an additive effect on prey survival, most likely because intraspecific aggression appeared size‐independent and size classes differed in microhabitat preference: large I. posita spent 14% more time foraging on the floor than small larvae and spent more time in the vegetation with increasing habitat complexity. Despite this difference in microhabitat use among size classes, habitat structure did not influence predation rates or interference among size classes. 4. In general, results suggest that seasonal and spatial variation in the size structure of populations could drive some of the discrepancies in predator‐mediated prey suppression observed in nature, and this variation could exceed the effects of variation in habitat structure.     

Foraging habitat determines predator–prey size relationships in marine fishes

Predator–prey size (PPS) relationships are determined by predator behaviour, with the likelihood of prey being eaten dependent on their size relative to that of the consumer. Published PPS relationships for 30 pelagic or benthic marine fish species were analysed using quantile regression to determine how median, lower and upper prey sizes varied with predator size and habitat. Habitat effects on predator foraging activity/mode, morphology, growth and natural mortality are quantified and the effects on PPS relationships explored. Pelagic species are more active, more likely to move by caudal fin propulsion and grow more rapidly but have higher mortality rates than benthic species, where the need for greater manoeuvrability when foraging in more physically complex habitats favours ambush predators using pectoral fin propulsion. Prey size increased with predator size in most species, but pelagic species ate relatively smaller prey than benthic predators. As pelagic predators grew, lower prey size limits changed little, and prey size range increased but median relative prey size declined, whereas the lower limit increased and median relative prey size was constant or increased in benthic species.