Homing pigeons use olfactory cues for navigation in England

Although the use of olfactory cues in pigeon navigation is well established, the generality of olfactory navigation remains uncertain because of apparent variability in results gained by different researchers in different regions. We report the results of the first experiments investigating the effect of anosmia on homing pigeons reared in a previously uninvestigated region, southern England. In series 1, experienced birds showed little effect of anosmia induced with zinc sulphate at unfamiliar sites 30 km and 39 km from the loft, but treated birds were significantly poorer than controls at homing from an unfamiliar site 66 km distant (and in pooled results). In series 2, naive (untrained) birds, both control and zinc-sulphate-treated, showed poor homing abilities and initial orientation from sites 25 km, 36 km and 39 km from the loft. Nevertheless, in pooled results, controls showed significantly better homeward orientation than anosmic birds and were significantly more likely to home on the day of release. The most likely explanation for our results is that pigeons are able to use olfactory navigation in southern England, but that for some reason the olfactory map is relatively weak.

Temporal Variability in Predator-Prey Relationships of a Forest Floor Food Web

Connectance webs represent the standard data description in food web ecology, but their usefulness is often limited in understanding the patterns and processes within ecosystems. Increasingly, efforts have been made to incorporate additional, biologically meaningful, data into food web descriptions, including the construction of food webs using data describing the body size and abundance of each species. Here, data from a terrestrial forest floor food web, sampled seasonally over a 1-year period, were analysed to investigate (i) how stable the body size abundance and predator prey relationships of an ecosystem are through time and (ii) whether there are system-specific differences in body size abundance and predator prey relationships between ecosystem types.

Temperature, predator-prey interaction strength and population stability

Warming could strongly stabilize or destabilize populations and food webs by changing the interaction strengths between predators and their prey. Predicting the consequences of warming requires understanding how temperature affects ingestion (energy gain) and metabolism (energy loss). Here, we studied the temperature dependence of metabolism and ingestion in laboratory experiments with terrestrial arthropods (beetles and spiders). From this data, we calculated ingestion efficiencies (ingestion/metabolism) and per capita interaction strengths in the short and long term. Additionally, we investigated if and how body mass changes these temperature dependencies. For both predator groups, warming increased metabolic rates substantially, whereas temperature effects on ingestion rates were weak. Accordingly, the ingestion efficiency (the ratio of ingestion to metabolism) decreased in all treatments. This result has two possible consequences: on the one hand, it suggests that warming of natural ecosystems could increase intrinsic population stability, meaning less fluctuations in population density; on the other hand, decreasing ingestion efficiencies may also lead to higher extinction risks because of starvation. Additionally, predicted long-term per capita interaction strengths decreased with warming, which suggests an increase in perturbation stability of populations, i.e., a higher probability of returning to the same equilibrium density after a small perturbation. Together, these results suggest that warming has complex and potentially profound effects on predator-prey interactions and food-web stability.

Predator diversity enhances secondary production and decreases the likelihood of trophic cascades

We manipulated the diversity of top predators in a three trophic level marine food web. The food web included four top benthic marine fish predators (black goby, rock goby, sea scorpion and shore rockling), an intermediate trophic level of small fish, and a lower trophic level of benthic invertebrates. We kept predator density constant and monitored the response of the lower trophic levels. As top predator diversity increased, secondary production increased. We also observed that in the presence of the manipulated fish predators, the density of small gobiid fish (intermediate consumers) was suppressed, releasing certain groups of benthic invertebrates (caprellid amphipods, copepods, nematodes and spirorbid worms) from heavy intermediate predation pressure. We attribute the mechanism responsible for this trophic cascade to a trait-mediated indirect interaction, with the small gobiid fish changing their use of space in response to altered predator diversity. In the absence of top fish predators, a full-blown trophic cascade occurs. Therefore the diversity of predators reduces the likelihood of trophic cascades occurring and hence provides insurance against the loss of an important ecosystem function (i.e. secondary production).

Predator-prey body size, interaction strength and the stability of a real food web

1. We examined the empirical relationship between predator-prey body size ratio and interaction strength in the Ythan Estuary food web.

Trait-mediated indirect interactions in a marine intertidal system as quantified by functional responses

Studies of trait-mediated indirect interactions (TMIIs) typically focus on effects higher predators have on per capita consumption by intermediate consumers of a third, basal prey resource. TMIIs are usually evidenced by changes in feeding rates of intermediate consumers and/or differences in densities of this third species. However, understanding and predicting effects of TMIIs on population stability of such basal species requires examination of the type and magnitude of the functional responses exhibited towards them. Here, in a marine intertidal system consisting of a higher-order fish predator, the shanny Lipophrys pholis, an intermediate predator, the amphipod Echinogammarus marinus, and a basal prey resource, the isopod Jaera nordmanni, we detected TMIIs, demonstrating the importance of habitat complexity in such interactions, by deriving functional responses and exploring consequences for prey population stability. Echinogammarus marinus reacted to fish predator diet cues by reducing activity, a typical anti-predator response, but did not alter habitat use. Basal prey, Jaera nordmanni, did not respond to fish diet cues with respect to activity, distribution or aggregation behaviour. Echinogammarus marinus exhibited type II functional responses towards J. nordmanni in simple habitat, but type III functional responses in complex habitat. However, while predator cue decreased the magnitude of the type II functional response in simple habitat, it increased the magnitude of the type III functional response in complex habitat. These findings indicate that, in simple habitats, TMIIs may drive down consumption rates within type II responses, however, this interaction may remain de-stabilising for prey populations. Conversely, in complex habitats, TMIIs may strengthen regulatory influences of intermediate consumers on prey populations, whilst potentially maintaining prey population stability. We thus highlight that TMIIs can have unexpected and complex ramifications throughout communities, but can be unravelled by considering effects on intermediate predator functional response types and magnitudes.

Ecological impacts of an invasive predator explained and predicted by comparative functional responses

Forecasting the ecological impacts of invasive species is a major challenge that has seen little progress, yet the development of robust predictive approaches is essential as new invasion threats continue to emerge. A common feature of ecologically damaging invaders is their ability to rapidly exploit and deplete resources. We thus hypothesized that the 'functional response' (the relationship between resource density and consumption rate) of such invasive species might be of consistently greater magnitude than those of taxonomically and/or trophically similar native species. Here, we derived functional responses of the predatory Ponto-Caspian freshwater 'bloody red' shrimp, Hemimysis anomala, a recent and ecologically damaging invader in Europe and N. America, in comparison to the local native analogues Mysis salemaai and Mysis diluviana in Ireland and Canada, respectively. This was conducted in a novel set of experiments involving multiple prey species in each geographic location and a prey species that occurs in both regions. The predatory functional responses of the invader were generally higher than those of the comparator native species and this difference was consistent across invaded regions. Moreover, those prey species characterized by the strongest and potentially de-stabilizing Type II functional responses in our laboratory experiments were the same prey species found to be most impacted by H. anomala in the field. The impact potential of H. anomala was further indicated when it exhibited similar or higher attack rates, consistently lower prey handling times and higher maximum feeding rates compared to those of the two Mysis species, formerly known as 'Mysis relicta', which itself has an extensive history of foodweb disruption in lakes to which it has been introduced. Comparative functional responses thus merit further exploration as a methodology for predicting severe community-level impacts of current and future invasive species and could be entered into risk assessment protocols.

Downregulation of the motA gene delays the escape of the obligate predator Bdellovibrio bacteriovorus 109J from bdelloplasts of bacterial prey cells

Bdellovibrio bacteriovorus is a Gram-negative bacterium that preys on other Gram-negative bacteria. The lifecycle of B. bacteriovorus alternates between an extracellular flagellated and highly motile non-replicative attack-phase cell and a periplasmic non-flagellated growth-phase cell. The prey bacterium containing periplasmic bdellovibrios becomes spherical but osmotically stable, forming a structure known as the bdelloplast. After completing the growth phase, newly formed bdellovibrios regain their flagellum and escape the bdelloplast into the environment, where they encounter more prey bacteria. The obligate predatory nature of B. bacteriovorus imposes a major difficulty to introducing mutations in genes directly involved in predation, since these mutants could be non-viable. This work reports the cloning of the B. bacteriovorus 109J motAB operon, encoding proteins from the flagellar motor complex, and a genetic approach based on the expression of a motA antisense RNA fragment to downregulate motility. Periplasmic bdellovibrios carrying the plasmid expressing antisense RNA displayed a marked delay in escaping from bdelloplasts, while the released attack-phase cells showed altered motility. These observations suggest that a functionally intact flagellar motor is required for the predatory lifecycle of B. bacteriovorus. Also, the use of antisense RNA expression may be a useful genetic tool to study the Bdellovibrio developmental cycle.

Animal group forces resulting from predator avoidance and competition minimization

A new model to explain animal spacing, based on a trade-off between foraging efficiency and predation risk, is derived from biological principles. The model is able to explain not only the general tendency for animal groups to form, but some of the attributes of real groups. These include the independence of mean animal spacing from group population, the observed variation of animal spacing with resource availability and also with the probability of predation, and the decline in group stability with group size. The appearance of "neutral zones" within which animals are not motivated to adjust their relative positions is also explained. The model assumes that animals try to minimize a cost potential combining the loss of intake rate due to foraging interference and the risk from exposure to predators. The cost potential describes a hypothetical field giving rise to apparent attractive and repulsive forces between animals. Biologically based functions are given for the decline in interference cost and increase in the cost of predation risk with increasing animal separation. Predation risk is calculated from the probabilities of predator attack and predator detection as they vary with distance. Using example functions for these probabilities and foraging interference, we calculate the minimum cost potential for regular lattice arrangements of animals before generalizing to finite-sized groups and random arrangements of animals, showing optimal geometries in each case and describing how potentials vary with animal spacing. (C) 1999 Academic Press.</p>

Beyond the ideal free distribution: More general models of predator distribution

The ideal free distribution model which relates the spatial distribution of mobile consumers to that of their resource is shown to be a limiting case of a more general model which we develop using simple concepts of diffusion. We show how the ideal free distribution model can be derived from a more general model and extended by incorporating simple models of social influences on predator spacing. First, a free distribution model based on patch switching rules, with a power-law interference term, which represents instantaneous biased diffusion is derived. A social bias term is then introduced to represent the effect of predator aggregation on predator fitness, separate from any effects which act through intake rate. The social bias term is expanded to express an optimum spacing for predators and example solutions of the resulting biased diffusion models are shown. The model demonstrates how an empirical interference coefficient, derived from measurements of predator and prey densities, may include factors expressing the impact of social spacing behaviour on fitness. We conclude that empirical values of log predator/log prey ratio may contain information about more than the relationship between consumer and resource densities. Unlike many previous models, the model shown here applies to conditions without continual input. (C) 1997 Academic Press Limited.</p>

Emerging asymmetric interactions between forage and predator fisheries impose management trade-offs

A size and trait-based marine community model was used to investigate interactions, with potential implications for yields, when a fishery targeting forage fish species (whose main adult diet is zooplankton) co-occurs with a fishery targeting larger-sized predator species. Predicted effects on the size structure of the fish community, growth and recruitment of fishes, and yield from the fisheries were used to identify management trade-offs among the different fisheries. Results showed that moderate fishing on forage fishes imposed only small effects on predator fisheries, whereas predator fisheries could enhance yield from forage fisheries under some circumstances.

Fortune favours the bold: a higher predator reduces the impact of a native but not an invasive intermediate predator

Emergent multiple predator effects (MPEs) might radically alter predictions of predatory impact that are based solely on the impact of individuals. In the context of biological invasions, determining if and how the individual-level impacts of invasive predators relates to their impacts in multiple-individual situations will inform understanding of how such impacts might propagate through recipient communities. Here, we use functional responses (the relationship between prey consumption rate and prey density) to compare the impacts of the invasive freshwater mysid crustacean Hemimysis anomala with a native counterpart Mysis salemaai when feeding on basal cladoceran prey (i) as individuals, (ii) in conspecific groups and (iii) in conspecific groups in the presence of a higher fish predator, Gasterosteus aculeatus. In the absence of the higher predator, the invader consumed significantly more basal prey than the native, and consumption was additive for both mysid species - that is, group consumption was predictable from individual-level consumption. Invaders and natives were themselves equally susceptible to predation when feeding with the higher fish predator, but an MPE occurred only between the natives and higher predator, where consumption of basal prey was significantly reduced. In contrast, consumption by the invaders and higher predator remained additive. The presence of a higher predator serves to exacerbate the existing difference in individual-level consumption between invasive and native mysids. We attribute the mechanism responsible for the MPE associated with the native to a trait-mediated indirect interaction, and further suggest that the relative indifference to predator threat on the part of the invader contributes to its success and impacts within invaded communities.