Small Prey Species Research Articles

Intra-clade predation facilitates the evolution of larger body size

A computer simulation based on a food web model was used to examine whether predator–prey interactions facilitate the evolution of larger body size, and if so, what conditions accelerate the body size increase. If predator species were assumed to feed only on smaller prey species, the evolution of larger body size was driven in the food web model. Intra-clade predation (predator–prey relationship within a same clade) accelerated the body size increase in the simulation. When predators and prey belong to the same clade, they tend to have similar body sizes; even a slight increase in prey body size reduces predation. Moreover, in such a case, the former prey readily becomes a predator of its former predator, because their feeding preferences and characters are similar to each other. Such a reversal of predator–prey relationship forces a stepwise increase in the body size of both species in a positive feed back process. The conditions needed for the positive feed back process may exist just after the K/T mass extinction. Thus, the process suggested in this study might explain the rapid evolution of larger body size in mammals at the beginning of the Paleocene.

Prey selection by the Indian tiger (Panthera tigris tigris) in Nagarjunasagar Srisailam Tiger Reserve, India

Summary Very little is known about prey selection by the Indian tiger Panthera tigris tigris in tropical dry deciduous forests or in wild herbivore-depleted habitats with high livestock pressures. We undertook a short-term study in a large south Indian tiger reserve and examined two intensive study areas (SA's) from October 1998 – June 1999. In each area, herbivore sightings were recorded, scats were collected, and wild prey and livestock kills documented. Chital, wild boar and sambar were the most abundant among the wild herbivores in the study area. Scat analysis revealed wild boar ( Sus scrofa ) being the most common prey followed by chital ( Axis axis ) and sambar ( Rusa unicolor ). Livestock comprised less than 7% of diet intake. Here, the tigers consumed a lower mean prey mass (56.3 kg) than in other reserves. Our study suggests that in tropical dry deciduous forests with low natural prey density, smaller prey species, and high livestock biomass, tigers preferentially kill smaller prey and generally avoid livestock predation.

Evolution of conspicuous colouration, body size and gregariousness: a comparative analysis of lepidopteran larvae

Protective colouration in animals includes camouflage (i.e., crypsis), that decreases the risk of detection, and conspicuous colouration, which is often used in combination with chemical defences to deter predators from attacking. Experiments have shown that the efficacy of conspicuous colouration increases with increasing size of pattern elements and larger body size. Prey species that have acquired avoidance inducing colouration therefore may be exposed to selection for larger body size, and such colouration may more easily evolve in large than in small prey species. Here we test for a difference in body size between species with different colouration modes and perform a comparative analysis based on phylogenetically independent contrasts to examine if evolutionary shifts in colour pattern have been associated with evolutionary changes in body size, using data for 578 species of moths. Larval body size did not differ between species with signalling and non-signalling larvae, and results from the comparative analysis suggest that these two traits have not evolved in parallel. The lack of association between evolutionary changes in colouration and body size may reflect a confounding influence of lifestyle, because evolutionary shifts from solitary to group-living larvae were associated with decreased larval body length and adult wing span. Because evolutionary changes in larval body size were associated with evolutionary changes in adult wing span the predicted association between colouration and size may have been confounded also by conflicting selection on body size in larvae and adults.

Assessing Raptor Diet: Comparing Pellets, Prey Remains, and Observational Data at Hen Harrier Nests

AbstractWe compared techniques to assess diet at 17 Hen Harrier (Circus cyaneus) nests. Diet was measured directly from observations and compared to estimates from pellets, prey remains and a combination of pellets and remains. For data pooled across nests, pellets over-represented mammalian prey and under-represented avian prey. Prey remains over-represented large prey and under-represented small prey. Combining pellet and remains data did not eliminate these biases. Pellets gave higher diversity values than direct observations and detected more small prey species. For data analyzed on a nest by nest basis, estimates from pellets were significantly related to estimates from direct observations for three prey types by frequency and all types by biomass. These linear relationships were used to predict frequencies and biomass of prey types in the observed diet at five new nests. Our findings suggest that pellets are useful for estimates of prey diversity and as an index of the frequency of certain prey types in the diet, but direct observations are necessary to help quantify the biases inherent in diet estimates.

ASSESSING RAPTOR DIET: COMPARING PELLETS, PREY REMAINS, AND OBSERVATIONAL DATA AT HEN HARRIER NESTS1

We compared techniques to assess diet at 17 Hen Harrier (Circus cyaneus) nests. Diet was measured directly from observations and compared to estimates from pellets, prey remains and a combination of pellets and remains. For data pooled across nests, pellets over-represented mammalian prey and under-represented avian prey. Prey remains over-represented large prey and under-represented small prey. Combining pellet and remains data did not eliminate these biases. Pellets gave higher diversity values than direct observations and detected more small prey species. For data analyzed on a nest by nest basis, estimates from pellets were significantly related to estimates from direct observations for three prey types by frequency and all types by biomass. These linear relationships were used to predict frequencies and biomass of prey types in the observed diet at five new nests. Our findings suggest that pellets are useful for estimates of prey diversity and as an index of the frequency of certain prey types in the diet, but direct observations are necessary to help quantify the biases inherent in diet estimates.

Winter lynx Lynx lynx predation on semi‐domestic reindeer Rangifer tarandus in northern Sweden

The predation behaviour of six lynx Lynx lynx family groups, i.e. adult females with dependent kittens, was studied using radio‐tracking and snow‐tracking in the Sarek area of northern Sweden during winter 1995/96 and 1996/97. One hundred and six daily radio‐locations were obtained, and 340 km of intervening tracks were followed in the snow. Forty‐one scats were collected, and 57 hunting attempts, 37 of which were successful, were recorded. Semi‐domestic reindeer Rangifer tarandus contributed over 90% to lynx ingested meat calculated from both scats and kills. Eighty‐three percent of hunting attempts on reindeer, and 53% of attempts on small prey species, mainly Lagopus sp. and mountain hares Lepus timidus, were successful. Four incidents of multiple‐killing of reindeer were documented. Reindeer were generally in poor body condition, with an average femur marrow fat content of 27%. Lynx spent an average of three nights at each reindeer kill‐site. Reindeer were less completely consumed than small prey (61% vs 99%). The amount of meat eaten from a reindeer was proportional to the number of lynx‐nights on the kill. An overall kill rate of one reindeer per five days was calculated. We concluded that reindeer are a very important food source for lynx in winter, which potentially could lead to problems in resolving the camivore‐livestock conflicts in the region.

Predatory Promesostoma species (Plathelminthes, Rhabdocoela) in the Wadden Sea

Laboratory experiments were performed on the food ecology of four congeneric species of free-living plathelminths, Promesostoma caligulatum, P. marmoratum, P. rostratum, and P. meixneri, all inhabiting an intertidal sandflat near the island of Sylt (North Sea). Their prey spectrum is within the microcrustaceans: P. caligulatum preferred ostracods, while the other three species favoured copepods, with species-specific differences for copepod species and size classes. Daily consumption of prey species varied with the size of both the predator and the prey. On average, P. marmoratum consumed 0.76 Harpacticus flexus per day while this rate decreased to 0.06 in P. meixneri, the smallest predator. When these Promesostoma species were fed with Tachidius discipes, a smaller prey species, their predation rates were about 25% higher. While the larger predators preferred the larger harpacticoids as prey, the small P. meixneri preferred small cyclopoids over larger harpacticoids. In terms of biomass, P. marmoratum's mean consumption of T. discipes per day was about half the predator's own weight. This average varied with prey density and temperature. A comparison of these consumption rates with the field densities of the predators and their prey shows that the plathelminth predators may consume as much as 10% per day of their copepod prey populations, thus strongly influencing these prey populations on these sandflats. The predation pressure of P. caligulatum on ostracods was about 1% per day of the prey population. Since ostracods usually have fewer generations per year, the total effect on the population dynamics may be similar to that on copepods. Therefore, nocturnal swimming of copepods in the water column may be interpreted as an attempt to escape plathelminth predators.

Feeding habits of John Dory, Zeus faber, off the Portuguese continental coast

The feeding habits of John Dory (Zeus faber) were studied, based on the analysis of stomach contents from fish sampled in five groundfish surveys. These surveys were carried out off the Portuguese coast during different seasons between 1990 and 1992. The main aspects of feeding biology analysed in this paper are: ontogenetic diet changes, temporal variations in food composition and feeding intensity. Multivariate methods were used to investigate ontogenetic diet shifts. Two main length groups were identified: 8.0–24.9 cm fish, feeding mainly on dragonets and silvery pout, and 25.0–55.9 cm fish whose diet was mainly composed of blue whiting and snipefish. A transitional phase (24.0–30.9 cm fish) with a mixed food composition was observed. This ontogenetic diet shift does not seem to correspond to any important change in body morphology but it does coincide with the onset of sexual maturity in the species.John Dory switched from a diet of small prey species with more pronounced benthic behaviour to a diet of larger schooling pelagic species. This suggests parallel evolution to more pelagic foraging behaviour. However, John Dory feeding habits appear to be largely controlled by the availability and accessibility of prey species: (i) the diet of adult John Dory is dominated by very abundant species; (ii) shifts in the main prey items between different times of the year and between different areas seem to be related both with their absolute and relative abundance in the environment and with the overlap between the depth distribution of predator and prey.

Assessment of the Diet of the Fox, Vulpes vulpes, in Habitats of the Eastern Otway Ranges, Victoria.

The Eastern Otway Ranges, Victoria have highly diverse native mammal communities. Although the Red Fox (Vulpes vulpes) has been recorded in the area the extent of its distribution and effects on native mammals are unclear. The aims of this study were to analyse the diet of the Red Fox in the study area, to compare the diet between seasons and habitats (woodland, forest, heathland) and to assess the diet in the habitat of the New Holland Mouse (Pseudomys novaehollandiae) which is endangered in Victoria. Fox scats collected monthly in each habitat were analysed to determine the composition of the diet and small mammal trapping was carried out in each habitat to determine potential small mammal prey species. Overall, mammals (42%) constituted the highest proportion in the fox scats and sixteen species, including nine native species were recorded. Large-sized native mammals including Swamp Wallaby (Wallabia bicolor) and the Eastern Grey Kangaroo (Macropus giganteus) constituted 60% of the mammal diet category. The occurrence of plant material was significantly different between seasons, being more abundant in winter. There was a significant difference in the frequency of large introduced mammals in the diet between seasons, with consumption being higher in winter. The occurrence of large, native mammalian prey increased from 15% in winter to 47% in autumn. The frequency of occurrence of the major dietary categories (plant material, invertebrates, reptiles, birds, mammals) was similar across habitats. Large introduced mammals occurred in higher proportions in the diet from woodland habitat. Small mammals were more numerous in scats of heathland and scats from the forest contained the highest abundance of medium-sized mammals. There was no evidence of the endangered New Holland Mouse in scats collected from habitat where a population of the species was present.

Unique Structural Properties of Pelagic Food Webs

Two large food web data sets, each characterized by comprehensive sampling and high taxonomic resolution, display major differences in their structural properties. In 60 food webs compiled by Sugihara et al., representing such diverse habitats as springs, creeks, rotting logs, treeholes, and forest canopies, there is no significant increase in web complexity (SC) with species number (S), and predator/prey (PP) ratios average 2.2. The fractions of top (TF), intermediate (IF) and basal (BF) species average 0.47, 0.35, and 0.18, respectively. Along a gradient of increasing S, IF increases and BF declines. Maximal food chain length (MAX) also increases with S in the Sugihara webs, and averages 2.7. In 50 food webs compiled by Havens, representing the pelagic communities of small lakes, SC increases rapidly with S, and PP averages only 0.6. Species fractions average 0.06 (TF), 0.44 (IF), and 0.50 (BF), and only IF is significantly correlated with S. MAX increases more rapidly with S than in the Sugihara webs, and it averages 4.7. Distinct properties of pelagic food webs, including high SC that increases rapidly with S, low PP, and high BF, are evidenced by the results of other studies, and may be due to (1) the predominance of filter-feeding predators that consume numerous prey taxa; and (2) to the very high diversity of small particulate autotrophic prey species found in pelagic communities.

Feeding preference and population growth of Asplanchna brightwelli (Rotifera) offered two non-evasive prey rotifers

Population growth rates of the predatory rotifer Asplanchna brightwelli were determined at 25 °C using a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer prey species in three concentrations (0.5, 0.1 and 2.0 μg dry weight ml-1) and in five combinations. The prey ingestion time by the predator was also measured. For B. calyciflorus the ingestion time (22.97–8.95 s) was more than six times that of A. fissa (3.68 ± 0.93). Regardless of prey type, the population growth of Asplanchna increased with increasing food density. There was a direct correlation between densities of amictic and mictic fernales. The maximum rate of population growth (1.01 ± 0.10 d-1) was higher at high density of A. fissa prey than that at the same density of B. calyciflorus. Progressive increase of A. fissa density in the offered food combination resulted in a corresponding increase of the predator's number. Gut content analysis of A. brightwelli revealed that the number of prey ingested increased with increasing prey densities.

Retarded growth of cladoceran zooplankton in the presence of a copepod predator.

Various instars of four different cladoceran species representing a wide spectrum of body size were grown at high food availability in the presence and in the absence of natural densities of an invertebrate predator, a cyclopoid copepod Acanthocyclops robustus (G.O. Sars). Daily weight increments calculated from individual weights at the end and at the beginning of each 1, 2 or 4 day experiment, showed that individual growth was more or less drastically retarded in the presence of the predator as well as when exposed to water in which the predator had been feeding. The data also showed that the effect of this invertebrate predator was more pronounced in small prey instars and small prey species that were more vulnerable to predation than large prey.

Temporal, spatial, sex-ratio and body-size heterogeneity of prey species taken by the beewolfPhilanthus sanbornii(Hymenoptera: Sphecidae)

More than 3000 prey representing 108 species of bees and wasps were identified from exoskeletal remains taken from nests of the beewolf Philanthus sanbornii at a site in eastern Massachusetts over a period of five years. Quantitative reference samples totalling more than 4000 items were collected from flowers at the same site over a period of four years. These data give a uniquely detailed view of the way in which a generalist predator exploits a diverse prey community. Most species show striking year-to-year variation in relative abundance, in both the prey and reference collections, but the overall abundances of species tend to be similar in the two collections, as do their sex ratios. This shows: (i) that P. sanbornii takes virtually every bee and wasp species found at flowers during its flight season (except for the relatively small number of species too large to handle); (ii) that prey are taken at rates roughly proportional to their local abundances (with a few exceptions); and (iii) that the local bee and wasp communities have lively dynamics (at least on spatial scales equivalent to the flight ranges of P. sanbornii females). Prey species are non-randomly distributed among nests of individual females within years, and among cells within nests, in a pattern suggesting that females often return repeatedly to hunting sites at which they have had success; the pattern does not suggest that individual females develop preferences for particular prey taxa. The size-abundance distributions of female bees appear to be trimodal at both the individual and species levels, most strongly so when cleptoparasitic species are removed from the sample. Such patterns are seen weakly or not at all in the size-abundance distributions of male bees, male wasps, and female wasps. Bees and wasps of both sexes visit flowers for nectar, but only non-parasitic female bees harvest pollen; this suggests that the multimodality may be caused by aspects of pollen collection that tend to scale with size. The seven Philanthus species of eastern North America vary greatly in size, but they share a common set of relatively small prey species. As in other parts of North America, larger species of Philanthus tend to have relatively broad diets because they also take larger prey that are not available to their smaller congeners. However, long-tongued bees appear to be under-represented in the diets of most North American Philanthus outside the zebratus species group.

Intercontinental Comparison of Small-Lake Fish Assemblages: The Balance between Local and Regional Processes

We compared characteristics of fish assemblages from Finland and northern Wisconsin on several scales to investigate community-level similarity in environmentally similar but faunistically different small lakes. Although Finland has only half as many fishes in its regional species pool, local species richness did not differ between the two regions. Variability in species composition among assemblages was lower in Finland, suggesting that a large proportion of the fauna could maintain populations across a broad range of environmental conditions, whereas the Wisconsin fauna was composed of more specialized species. Three types of fish assemblages were identified in Wisconsin based on species' presence or absence, whereas on the same scale, Finnish assemblages presented a hierarchical continuum based on species addition. These and other patterns suggest that habitat specialization and the exclusion of small prey species from lakes with piscivores contribute to the occurrence of presence-absence assemblage types in Wisconsin. Biotic interactions can affect population densities of Finnish species without exclusion, and assemblage types could be identified in Finland only on the basis of species' relative abundance. Finnish assemblages dominated by perch, roach, and crucian carp occurred in lakes that were environmentally similar to lakes in Wisconsin with bass, pike, and mudminnow assemblages, respectively. Because of these similar relations, discriminant functions from one region could successfully identify the assemblage types that occurred in lakes from the other region; this community-level predictability suggests that fish assemblage structures in northern forest lakes are influenced by common sets of environmental factors. Similarities and differences on different scales reflect the balance between local and regional processes; although local characteristics are well matched between regions, many regional and historical phenomena are dissimilar. Because of influences operating on several scales, viewed as a series of filters, community ecologists should not be content with simple acceptance or rejection of the convergence hypothesis. Greater understanding of communities will come from comprehensive studies that encompass a hierarchy of causes and effects.

Prey choice by marten during a decline in prey abundance.

We examined variation in diet choice by marten (Martes americana) among seasons and between sexes and ages from 1980-1985. During this period prey populations crashed simultaneously, except for ruffed grouse (Bonasa umbellus) which was common at the beginning and end of the study, and masked shrews (Sorex cinereus) which were abundant in 1983. Marten were catholic in selection of prey and made use of most available mammalian prey, ruffed grouse, passerine birds, berries, and insects. Diet niche was widest during the latter three years when prey was scare, particularly in late winter. Diet niche breadth was negatively correlated with abundance of all common prey species. Proportion of small prey species in the diet was correlated with absolute abundance of those species, but proportion of some large prey was related to their relative abundance. Diet choice varied among years and among seasons. Berries and insects were common in summer diets while large prey, particularly varying hare (Lepus americanus), were more frequent in winter diet than in summer diet. We found little evidence that any small mammal species was a preferred prey. Sexual size dimorphism between the sexes did not affect prey choice, nor did age. Reduced foraging effort in winter resulted in a wider diet niche only when prey was scarce. The only prediction of optimal foraging models fully supported by our data was a wider diet niche with reduced prey abundance. However, among the three most profitable prey species choice was dependent on the absolute abundance of the most profitable type (varying hare). We suggest that marten primarily forage for large prey but employ a strategy which results in encounters with small prey as well. These small prey are eaten as they provide energy at minimal cost, between captures of large prey.

Stomach evacuation rate in the planktivorous Antarctic fish Pagothenia borchgrevinki

Specimens of the Antarctic fish Pagothenia borchgrevinki were collected from just below the sea-ice in McMurdo Sound over the period 5–16 November 1987. The stomach contents of 54 freshly captured fish were analysed and showed very strong similarities with an earlier, more extensive, dietary analysis conducted at the same location (Foster et al. 1987). There was as much variation in stomach contents from fish within a school, as there was between schools, contrary to the expectation that fish within a school would have shared a more common feeding history. Fish were maintained in holding tanks for up to 48 h after capture; groups were sacrificed at 6 h intervals and the stomach contents analysed to determine evacuation rates. Evacuation rates of total stomach contents and individual prey species were fitted with an exponential model of decay. The exponential time constant for decay of stomach contents was 32 h (Table 2), while time constants for individual prey species ranged from 12 h for small prey species to 49 h for the larger crustacea. The feeding rate of P. borchgrevinki on important prey species was calculated from the above data (Table 2). Low feeding rates, and the low numbers of P. borchgrevinki argue against removal by predation as being the explanation for the observed reduction in plankton numbers seen in shallow waters (Foster 1989).

Predation by Didinium Nasutum: Effects of Predator and Prey Size

Experimental manipulation of the size of Didinium nasutum, a protozoan predator, feeding on large and small Paramecium species was used to examine the effects of predator and prey size on predator behavior and capture success. Didinum size is a function of prey size; large and small Didinium were obtained by conditioning predators on large or small prey species prior to experimentation. The predatory behavior of large and small predators feeding on large and small prey was monitored through an entire division cycle. Large Didinium had shorter handling times and encounter times on both sizes of prey than small Didinium. Large Didinium made more captures per division on either size of prey. Large prey required longer handling times, longer engulfing times, and were encountered more frequently than small prey for both sizes of predator. First captures were more difficult than subsequent captures for all predators. Small predators were more successful initially in capturing small prey than were large predators; first captures by large predators on small prey were particularly difficult. Encounter times, the number of encounters per capture, capture times, and handling times varied with the size of the predator and also varied within the division cycle of the predator. The effect of time within the division cycle may be due to increasing size as prey are captured and/or to adaptations to the specific prey species following initial captures. Didinium may adapt to new prey species through morphological changes in the oral apparatus, chemoreceptors, or extrusive organelles, in addition to shifts in size. That each size of Didinium had more successes per encounter on the size of prey to which it had been conditioned suggests that the size shift improves that aspect of the predatory behavior. However, other aspects (such as handling time and time to each encounter) show a clear advantage to large size. Large size also improves survival ability of Didinium under conditions of starvation (Hewett 1987). For large predators to maintain a large size on small prey would require a substantial increase in the division time. Selection forces favoring fast division rates, may impose restraints, making the size of Didinium on any prey species a compromise.

Three-dimensional predator–prey interactions: a computer simulation of bird flocks and aircraft

Three-dimensional interactions between grouped aerial predators (frontal discs of aircraft engines), either linearly arrayed or clustered, and flocks of small birds were studied using interactive computer simulation techniques. Each predator modelled was orders of magnitude larger than an individual prey, but the prey flock was larger than each predator. Expected numbers of individual prey captured from flocks were determined for various predator speeds and trajectories, flock–predator initial distances and angles, and flock sizes, shapes, densities, trajectories, and speeds. Generally, larger predators and clustered predators caught more prey. The simulation techniques employed in this study may also prove useful in studies of predator–prey interactions between schools or swarms of small aquatic prey species and their much larger vertebrate predators, such as mysticete cetaceans.The study also provides a method to study problems associated with turbine aircraft engine damage caused by the ingestion of small flocking birds, as well as net sampling of organisms in open aquatic environments.

Prey profitability for adult Grey Herons Ardea cinerea and the constraints on prey size when feeding young nestlings

Patterns of handling time and profitability are examined for adult Grey Herons feeding on carp, eels and catfish. Handling times generally increased with prey size but were influenced markedly by the morphology and behaviour of the prey. Profitability was highest for carp (max. 0.9 g/s for 15–20 cm fish), lowest for catfish (max. 0.05 g/s) and intermediate for eels (max. 0.1 g/s). Nestlings were unable to ingest the sizes of fish most profitable for the adults to consume until aged 20 days; by the age of 30 days, they could consume the full size‐range of prey taken by the adults. In order to feed their young chicks, adults must therefore either select smaller prey, or break their large prey into smaller pieces. The diet of nestling Grey Herons in the Camargue is examined for evidence to support or refute the former hypothesis.Young nestlings (≤20 days) regurgitated smaller carp than old nestlings (> 20 days). Comparison of prey types in the diet of the two groups showed that small prey species occurred significantly more often in the diet of young chicks, while the converse was true for larger prey species. The occurrence of particular prey types in the diet only of young chicks suggests that adults may forage in more marginal, shallower water (where small prey are probably more abundant) to meet the requirements of their brood during the early part of the nestling phase. The second hypothesis, that the adults break down large prey into smaller pieces, was not examined, although evidence from other studies suggests that this does occur; both mechanisms may therefore be important.

Tree Use and Predator Vulnerability of Wintering Passerines

Pygmy Owls in winter took a range of small avian and mammalian prey up to a weight of around 40 g. The owls showed a functional response and concentrated on bird prey at high bird densities, whereas diet composition appeared unrelated to rodent abundance. Small and less profitable avian prey were taken disproportionately often, counter to the expectation from models of an energetically optimized diet. A correlation between prey habitat use and relative predation risk suggests a niche specific difference in vulnerability between prey species as an explanation for the preponderance for small prey species in the diet of the Pygmy Owl. Niche differentiation may not only require adaptations for foraging efficiency but also to different predator regimes.