Solving some statistical problems in analyses of experiments on choices of food and on associations with habitat

Statistical tests for non-random associations with components of habitat or different kinds of prey require information about the availability of sub-habitats or types of prey. The data are obtained from sampling (Stage 1 samples). Tests are then constructed using this information to predict what will be the occupancy of habitats or composition of diet under the null hypothesis of random association. Estimates of actual occupancy of habitats or composition of diet are then obtained from Stage 2 sampling and tests are done to compare the observed data from Stage 2 with what was predicted from Stage 1.Estimates from each stage of sampling are subject to sampling error, particularly where small samples are involved. The errors involved in Stage 1 sampling are often ignored, resulting in biases in tests and excessive rejection of null hypotheses (i.e. non-random patterns are claimed when they are not present). Here, accurate tests are developed which take into account both types of error.For animals in patchy habitats, with two or more types of patch, the data from Stages 1 and 2 are used to derive maximal likelihood estimators for the proportions of area occupied by the sub-habitats and the proportions of animals in each sub-habitat. These are then used in χ² tests.For composition of diets, data are more complex, because the consumption of food of each type (on its own) must be estimated in separate experiments or sampling. So, Stage 1 sampling is more difficult and the maximal likelihood estimators described here are more complex. The accurate tests described here give much more realistic answers in that they properly control rates of Type I error, particularly with small samples. The effects of errors in Stage 1 sampling are, however, shown to be important, even for quite large samples. The tests can and should be used in any analyses of non-random association or preference among sub-habitats or types of prey.     

Identifying and understanding ecological preferences for habitat or prey

Patchy, non-random associations of species with habitats and of consumers with particular types of food are commonly found in the ecological literature. In many cases, these patterns are reported to show some preference by an animal making choices about its environment. Generally, however, what is reported is simply the pattern of association and the process that gives rise to this pattern is not further examined. Nevertheless, there are numerous concepts that need to be considered simply to demonstrate the pattern, including the spatial and temporal scales at which the observations are made. When animals make choices between two objects, it is difficult to separate out potential negative, neutral or positive responses to either or both of the objects, without well thought-out manipulative experiments. Apparent preference for food may be influenced by “catchability” or “acceptability” of the prey and/or the past history of the consumer and the experiments to separate these effects are naturally complex. Many experiments examining preferences are beset by problems of non-independence and lack of appropriate controls, which makes them difficult to interpret. This review introduces some of the logical, conceptual, experimental and statistical problems that beset many studies of preference and proposes important steps that must be considered in further studies to unravel this fascinating topic.     

Toxicity of organophosphorus systemic pesticides to predator mites and prey

Three systemic OP insecticides, Di-Syston^®, Meta-Systox-R^® and Systox^® were added at six concentrations (0.002–0.032% active ingredient) to nutrient solutions in which bean plants were cultured. On all concentrations the materials were less toxic to the two-spotted spider mite,Tetranychus urticae Koch feeding on the plant than toNeoseiulus fallacis (Garman) preying on theT. urticae. Of the three, Di-Syston was the least toxic to both mite species. WhenT. urticae were cultured on detached bean leaves, placed on cellucotton in nutrient solutions containing the same pesticide concentrations, their eggs were also toxic toN. fallacis. Longevity studies in conjunction with³²P labelled foliage showed thatN. fallacis are incapable of feeding directly on bean leaves.

---

Zusammenfassung Drei systemische Phosphorinsektizide, Di-Syston^®, Meta-Systox^® und Systox^®, wurden, in sechs Konzentrationen (0,002–0,032% Wirkstoff) zu Nährstofflösungen hinzugefügt, in denen Bohnenpflanzen kultiviert wurden. Für die SpinnmilbeTetranychus urticae, die an den Bohnenblättern saugte, waren die drei Insektizide in sämtlichen Konzentrationen weniger toxisch als für die RaubmilbeNeoseiulus fallacis, die sich vonT. urticae ernährte. Es ergab sich demnach keine ökologische Selektivität. Von den drei geprüften Mitteln war Di-Syston für die beiden Milben am wenigsten toxisch. Wenn manT. urticae auf abgetrennten Bohnenblättern, die sich auf Cellucotton in Nährstofflösungen mit den genannten Insektizidkonzentrationen befanden, züchtete, wurden ihre Eier fürN. fallacis ebenfalls toxisch. Lebensdauer-Versuche mit Blättern, die³²P enthielten, zeigten, daßN. fallacis nicht fähig ist, sich direkt von Bohnenblättern zu ernähren.

     

Multiple predator effects result in risk reduction for prey across multiple prey densities

Investigating how prey density influences a prey’s combined predation risk from multiple predator species is critical for understanding the widespread importance of multiple predator effects. We conducted experiments that crossed six treatments consisting of zero, one, or two predator species (hellgrammites, greenside darters, and creek chubs) with three treatments in which we varied the density of mayfly prey. None of the multiple predator effects in our system were independent, and instead, the presence of multiple predator species resulted in risk reduction for the prey across both multiple predator combinations and all three levels of prey density. Risk reduction is likely to have population-level consequences for the prey, resulting in larger prey populations than would be predicted if the effects of multiple predator species were independent. For one of the two multiple predator combinations, the magnitude of risk reduction marginally increased with prey density. As a result, models predicting the combined risk from multiple predator species in this system will sometimes need to account for prey density as a factor influencing per-capita prey death rates.     

Density-dependent effects of prey defenses and predator offenses

Defenses protect prey, while offenses arm predators. Some defenses and offenses are constitutive (e.g. tortoise shells), while others are phenotypically plastic and not always expressed (e.g. neckteeth in water fleas). All of them are costly and only adaptive at certain prey densities. Here, I analyse such density-dependent effects, applying a functional response model to categorize defenses and offenses and qualitatively predict at which prey densities each category should evolve (if it is constitutive) or be expressed (if it is phenotypically plastic). The categories refer to the step of the predation cycle that a defense or offense affects: (1) search, (2) encounter, (3) detection, (4) attack, or (5) meal. For example, prey warning signals such as red coloration prevent predator attacks and are hence step 4 defenses, while sharp predator eyes enhance detection and are step 3 offenses. My theoretical analyses predict that step 1 defenses, which prevent predators from searching for their next meal (e.g. toxic substances), evolve or are expressed at intermediate prey densities. Other defenses, however, should be most beneficial at low prey densities. Regarding predators, step 1 offenses (e.g. immunity against prey toxins) are predicted to evolve or be expressed at high prey densities, other offenses at intermediate densities. I provide evidence from the literature that supports these predictions.     

Sustainability and optimal control of an exploited prey predator system through provision of alternative food to predator

In the present paper, we develop a simple two species prey-predator model in which the predator is partially coupled with alternative prey. The aim is to study the consequences of providing additional food to the predator as well as the effects of harvesting efforts applied to both the species. It is observed that the provision of alternative food to predator is not always beneficial to the system. A complete picture of the long run dynamics of the system is discussed based on the effort pair as control parameters. Optimal augmentations of prey and predator biomass at final time have been investigated by optimal control theory. Also the short and large time effects of the application of optimal control have been discussed. Finally, some numerical illustrations are given to verify our analytical results with the help of different sets of parameters.     

Laboratory observations of prey behaviour for prey acquisition by a predator among three larval mosquitoes

The effect of prey behaviour on prey acquisition by a predator was examined in the laboratory, using three larval mosquitoes.Aedes albopictus was acquired by the predator,Toxorhynchites towadensis, at a higher rate thanOrthopodomiya anopheloides. Toxorphynchites towadensis was a sit-and-wait predator.Aedes albopictus was more active thanO. anopheloides. Orthopodomiya anopheloides ran away before the predator attacked, butA. albopictus did not. The escape ratio inO. anopheloides was higher than that inA. albopictus. These results suggest that the difference in the prey acquisition ratio by the predator between prey species is caused by different behavioural patterns of the prey to the predator.     

Experimental evidence for emergent facilitation: promoting the existence of an invertebrate predator by killing its prey

1. Recent theoretical insights have shown that predator species may help each other to persist by size-selective foraging on a shared prey. By feeding on a certain prey stage, a predator may induce a compensatory response in another stage of the same prey species, thereby favouring other predators; a phenomenon referred to as emergent facilitation. 2. To test whether emergent facilitation may occur in a natural system, we performed an enclosure experiment where we mimicked fish predation by selectively removing large zooplankton and subsequently following the response of the invertebrate predator Bythotrephes longimanus. 3. Positive responses to harvest were observed in the biomass of juvenile individuals of the dominant zooplankton Holopedium gibberum and in Bythotrephes densities. Hence, by removing large prey, we increased the biomass of small prey, i.e. stage-specific biomass overcompensation was present in the juvenile stage of Holopedium. This favoured Bythotrephes, which preferentially feed on small Holopedium. 4. We argue that the stage-specific overcompensation occurred as a result of increased per capita fecundity of adult Holopedium and as a result of competitive release following harvest. If shown to be common, emergent facilitation may be a major mechanism behind observed predator extinctions and patterns of predator invasions.     

Predator prey interactions: predator choice for prey group size and composition in Crenicichla alta, a natural predator of the Trinidadian guppy, Poecilia reticulata

Young salmonids may use substratum as hiding stations and/or shelter and they depend on invertebrates, which develop on substratum, for their feeding. For several decades, human activities have contributed to increase siltation in streams, and negative consequences on trout production have sometimes been highlighted. In the research devoted to the understanding of that negative effect, most studies have focused on embryo‐larval survival, and consequences of substrate embeddedness on later stages have rarely been investigated. In the present work we attempt at studying the impact of embeddedness on brown trout juveniles. In an experimental channel, trout growth was compared in embedded and non‐embedded sections. Growth was reduced with embeddedness due to change in trophic conditions and/or in habitat. To investigate the direct role of substratum for fish, trouts behaviour was observed from an under water observation room in two cages offering embedded and non‐embedded substrate conditions but similar trophic conditions. Competition appeared heavier in the embedded cage where dominated fishes stayed almost motionless. The effect of substratum quality on intra‐specific competition is discussed in relation with visual isolation and territory size.     

Movements of wintering surf scoters: predator responses to different prey landscapes

The distribution of predators is widely recognized to be intimately linked to the distribution of their prey. Foraging theory suggests that predators will modify their behaviors, including movements, to optimize net energy intake when faced with variation in prey attributes or abundance. While many studies have documented changes in movement patterns of animals in response to temporal changes in food, very few have contrasted movements of a single predator species naturally occurring in dramatically different prey landscapes. We documented variation in the winter movements, foraging range size, site fidelity, and distribution patterns of a molluscivorous sea duck, the surf scoter (Melanitta perspicillata), in two areas of coastal British Columbia with very different shellfish prey features. Baynes Sound has extensive tidal flats with abundant clams, which are high-quality and temporally stable prey for scoters. Malaspina Inlet is a rocky fjord-like inlet where scoters consume mussels that are superabundant and easily accessible in some patches but are heavily depleted over the course of winter. We used radio telemetry to track surf scoter movements in both areas and found that in the clam habitats of Baynes Sound, surf scoters exhibited limited movement, small winter ranges, strong foraging site fidelity, and very consistent distribution patterns. By contrast, in mussel habitats in the Malaspina Inlet, surf scoters displayed more movement, larger ranges, little fidelity to specific foraging sites, and more variable distribution patterns. We conclude that features associated with the different prey types, particularly the higher depletion rates of mussels, strongly influenced seasonal space use patterns. These findings are consistent with foraging theory and confirm that predator behavior, specifically movements, is environmentally mediated.     

Effects of predator removal on vertebrate prey populations: birds of prey and small mammals

We studied the effects of removal of breeding nomadic avian predators (the kestrel, Falco tinnunculus and Tengmalm's owl, Aegolius funereus) on small mammals (voles of the genera Microtus and Clethrionomys and the common shrew, Sorex araneus) during 1989–1992 in western Finland to find out if these predators have a regulating or limiting impact on their prey populations. We removed potential breeding sites of raptors from five manipulation areas (c. 3 km² each), whereas control areas had nest-boxes in addition to natural cavities and stick-nests. Densities of small mammals were monitored by snap-trapping in April, June, and August, and densities of mammalian predators (the least weasel, Mustela nivalis nivalis, the stoat, M. erminea and the red fox, Vulpes vulpes) by snow tracking in early spring and late autumn. The yearly mean number of raptor breeding territories was 0.2–1.0 in reduction areas and 3.0–8.2 in control areas. Breeding raptors alone did not regulate prey populations in the long term, but probably caused short-term changes in the population dynamics of both the main prey, the sibling vole (Microtus rossiaemeridionalis) and an alternative prey (the common shrew). The densities of an alternative prey, the bank vole (Clethrionomys glareolus) decreased in raptor reduction areas, most likely due to increased least weasel predation pressure in the absence of breeding avian predators.     

Responses of a top and a meso predator and their prey to moon phases

We compared movement patterns and rhythms of activity of a top predator, the Iberian lynx Lynx pardinus, a mesopredator, the red fox Vulpes vulpes, and their shared principal prey, the rabbit Oryctolagus cuniculus, in relation to moon phases. Because the three species are mostly nocturnal and crepuscular, we hypothesized that the shared prey would reduce its activity at most risky moon phases (i.e. during the brightest nights), but that fox, an intraguild prey of lynx, would avoid lynx activity peaks at the same time. Rabbits generally moved further from their core areas on darkest nights (i.e. new moon), using direct movements which minimize predation risk. Though rabbits responded to the increased predation risk by reducing their activity during the full moon, this response may require several days, and the moon effect we observed on the rabbits had, therefore, a temporal gap. Lynx activity patterns may be at least partially mirroring rabbit activity: around new moons, when rabbits moved furthest and were more active, lynxes reduced their travelling distances and their movements were concentrated in the core areas of their home ranges, which generally correspond to areas of high density of rabbits. Red foxes were more active during the darkest nights, when both the conditions for rabbit hunting were the best and lynxes moved less. On the one hand, foxes increased their activity when rabbits were further from their core areas and moved with more discrete displacements; on the other hand, fox activity in relation to the moon seemed to reduce dangerous encounters with its intraguild predator.     

Influence of prey body characteristics and performance on predator selection

At the time of settlement to the reef environment, coral reef fishes differ in a number of characteristics that may influence their survival during a predatory encounter. This study investigated the selective nature of predation by both a multi-species predator pool, and a single common predator (Pseudochromis fuscus), on the reef fish, Pomacentrus amboinensis. The study focused on the early post-settlement period of P. amboinensis, when mortality, and hence selection, is known to be highest. Correlations between nine different measures of body condition/performance were examined at the time of settlement, in order to elucidate the relationships between different traits. Single-predator (P. fuscus) choice trials were conducted in 57.4-l aquaria with respect to three different prey characteristics [standard length (SL), body weight and burst swimming speed], whilst multi-species trials were conducted on open patch reefs, manipulating prey body weight only. Relationships between the nine measures of condition/performance were generally poor, with the strongest correlations occurring between the morphological measures and within the performance measures. During aquaria trials, P. fuscus was found to be selective with respect to prey SL only, with larger individuals being selected significantly more often. Multi-species predator communities, however, were selective with respect to prey body weight, with heavier individuals being selected significantly more often than their lighter counterparts. Our results suggest that under controlled conditions, body length may be the most important prey characteristic influencing prey survival during predatory encounters with P. fuscus. In such cases, larger prey size may actually be a distinct disadvantage to survival. However, these relationships appear to be more complex under natural conditions, where the expression of prey characteristics, the selectivity fields of a number of different predators, their relative abundance, and the action of external environmental characteristics, may all influence which individuals survive.     

Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis

1. Individual variability in prey preferences can have marked effects on many demographic parameters from individual survival and fecundity to the vital rates of entire populations. A population level response is ultimately determined by individual prey choices; however, the effect of individual dietary choice is often overlooked. 2. We determined prey choice by individual consumers, light-bellied Brent geese Branta bernicla, during the overwintering period. Two hundred and eighty-one individuals were sampled at distinct temporal points over two winters. Stable isotopic ratios of carbon and nitrogen for blood cells and blood plasma, from each sampled individual were measured. Isotopic ratios for potential prey items were also measured. 3. Delta15N and delta13C for blood samples were both significantly different between sample months. Generally we found a decrease in both isotopic ratios during the course of the winter. All potential prey items were also isotopically distinct. Multisource mixing models (isosource) were used to determine the range of possible contribution to the diet of individuals. 4. During early winter, diet consisted almost exclusively of sea grass Zostera spp. The level of Zostera spp. in the diet dropped until mid-winter, and was supplemented by the utilization of green algae Ulva lactuca, and Enteromorpha spp., and terrestrial grasses. Terrestrial grass comprised an increasing proportion of the diet in late winter, representing virtually the exclusive food source by April. 5. By examining intrapopulation variability in resource utilization we highlight a number of ecologically important factors not addressed by previous population level studies.