How community ecology links natural mortality, growth, and production of fish populations

Size-spectrum theory is used to show that (i) predation mortality is a decreasing function of individual size and proportional to the consumption rate of predators; (ii) adult natural mortality M is proportional to the von Bertalanffy growth constant K; and (iii) productivity rate P/B is proportional to the asymptotic weight W8 -1/3. The constants of proportionality are specified using individual level parameters related to physiology or prey encounter. The derivations demonstrate how traditional fisheries theory can be connected to community ecology. Implications for the use of models for ecosystem-based fisheries management are discussed.

Stable breeding despite variable feeding in two sympatric auk species

Capsule Despite substantial inter-annual and inter-specific variance in the composition of chick diet, the breeding success of Guillemots (Common Murres) Uria aalge and Razorbills Alca torda remained constant from 2008 to 2010.
Aims To examine inter-specific and inter-annual differences in breeding success, chick provisioning behaviour and predation between two sympatric auk species.
Methods Focal observations of breeding auks at Rathlin Island, Northern Ireland, during 2008, 2009 and 2010 recorded reproductive success, reasons for breeding failure, prey composition and quality and chick provisioning rates.
Results Breeding success of both species was stable over the three years, despite significant variance in the composition and quality of the diet provided to chicks. Razorbills experienced greater rates of failure than Guillemots owing to chick loss and had lower overall breeding success.
Conclusion Guillemot and Razorbill breeding success was independent of the composition and quality of prey items delivered to chicks. Inter-specific differences in reproductive success may have been attributed to greater rates of predation at Razorbill rather than Guillemot nests.

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).

Importance of consumers on exposed and sheltered rocky shores.

It is often suggested that the relative importance of biotic processes, such as recruitment, competition and predation of marine benthic species, varies predictably along a gradient of exposure to wave action. Several established models of community dynamics on rocky shores predict that top-down processes are more important for structuring communities on sheltered than on exposed shores. To test the relative dominance of top-down processes, we first measured the establishment of key benthic species (mussels, barnacles and algae) on 3 sheltered and 3 exposed rocky shores in southwest Ireland over two 6 mo periods. We then manipulated the presence of consumers (e.g. grazing gastropods, crabs, whelks), using caged exclosures, on 2 sheltered and 2 exposed shores to test for an interaction between effects of consumers and shore exposure on the establishment of benthic species. In contrast to predictions, we found that consumers strongly affected establishment of all species regardless of shore exposure. We also found that shore exposure was not a reliable predictor for spatial and temporal variation in rates of establishment of sessile benthic species. Our findings provide experimental evidence which demonstrates the importance of consumers in early post-settlement stages of benthic species-essential for the development of benthic-pelagic models. © 2011 Inter-Research.


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THE IMPACT OF BURNING AND GRAZING OF HEATHLAND PLANTS AND INVERTEBRATES IN COUNTRY ANTRIM

The impact of burning and grazing on plant, ground beetle and spider species was investigated experimentally in stands of varying ages (burnt in 1982 and 1988 and unburnt plots) on an area of heather moorland in County Antrim, north-east Ireland. Burning initiated complex succession pathways which appear to have characteristic plant and invertebrate species associations. Removal of Calluna dominance initiated a period of high plant species diversity. Investigation of initial post-fire regeneration suggested that the frequency of occurrence of plant species changed over time and was affected by grazing. Grouping of species by the position of their renewal bud, i.e. their life-form, did not account for all observed interspecific variation. The dominant species after burning were Eriophorum vaginatum, E. angustifolium and Vaccinium myrtillus. Studies of vegetation canopy structure showed that, even with the exclusion of the main grazing herbivores, Calluna will not re-establish itself as the dominant species until several years after burning. The ground beetle Nebria salina was trapped more often on plots burnt in 1988 than on unburnt plots or those burnt in 1982. In comparison, Pterostichus niger and Carabus granulatus were trapped in greater numbers on plots burnt in 1982 than on unburnt plots and plots burnt in 1988. The large species Carabus problematicus and Carabus glabratus were trapped in greater numbers on unburnt plots. Similarly, more of the spiders Ceratinella brevipes and Centromerita concinna were trapped on the plots burnt in 1982. In comparison, Lepthyphantes zimmermanni and Robertus lividus were trapped more often on unburnt plots than on plots burnt in 1982 and 1988. Results are discussed with respect to the importance of the continuation of traditional heathland management practices.

THE IMPACT OF GRAZING ON COMMUNITIES OF GROUND-DWELLING SPIDERS (ARANEAE) IN UPLAND VEGETATION TYPES

Adult spider communities were sampled by pitfall trapping over a 24-month period in plots subjected to a range of grazing regimes on five vegetation types on a hill farm in County Antrim, north-east Ireland. Spider community composition was influenced by vegetation type and grazing regime. Variation in the number of individuals and species diversity was also apparent between vegetation types and grazing regime. Plots grazed by all herbivores were characterised by the predominance of species characteristic of disturbed land. Inbye land and areas where grazing had ceased had characteristic coloniser species. The spiders Erigone dentipalpis, Allomengea scopigera and Centromerita bicolor were trapped with greater success in vegetation types where grass species dominated.

ASSESSMENTS AND DECISIONS IN METELLINA-SEGMENTATA (ARANEAE, METIDAE) - EVIDENCE OF A PHEROMONE INVOLVED IN MATE GUARDING

Assessments and decision-making underlying the initiation of mate guarding in a common web-building spider, Metellina segmentata, are examined in a series of field and laboratory studies. Adult males do not build webs but wander in search of females and mating opportunities. Adult males then wait at the edge of the webs of females and guard them prior to courtship and mating. Guarded females were heavier, larger and carried more mature eggs than solitary females. An active process of information gathering is apparent from introductions of males to the webs of females. Males make accurate assessments about female quality, even in the absence of the resident female. Cues involving web architecture are not used. Males may assess pheromonal cues on the web of the female in deciding whether to guard or abandon a female.

Marine reserve designation, trophic cascades and altered community dynamics

Marine ecosystems and their associated populations are increasingly at risk from the cumulative impacts of many anthropogenic threats that increase the likelihood of species extinction and altered community dynamics. In response, marine reserves can be used to protect exploited species and conserve biodiversity. The increased abundance of predatory species in marine reserves may cause indirect effects along chains of multi-trophic interactions. These trophic cascades can arise through direct predation, density-mediated indirect interactions (DMIIs), or indirect behavioural effects, termed trait-mediated indirect interactions (TMIIs). The extent of algal cover and the abundance of 4 primary consumers were determined in Lough Hyne, which was designated Europe's first marine nature reserve in 1981. The primary consumers were the sea urchin Paracentrotus lividus, the topshell Gibbula cineraria, the oyster Anomia ephippium, and the scallop Chlamys varia. The abundances of 3 starfish species (Marthasterias glacialis, Asterias rubens, and Asterina gibbosa) were also determined, as were 2 potential crustacean predators, Necora puber and Carcinus maenas. These data were compared with historical data from a 1962 (prey) and a 1963 (predator) survey to determine the nature of community interactions over adjacent trophic levels. The present study reveals a breakdown in population structure of the 4 surveyed prey species. Marine reserve designation has led to an increase in predatory crabs and M. glacialis, a subsequent decrease in primary consumers, especially the herbivore P. lividus, and an increase in macroalgal cover which is indicative of a trophic cascade. The study shows that establishing a Marine Reserve does not guarantee that conservation benefits will be distributed equally.

Functional responses of the intertidal amphipod Echinogammarus marinus: effects of prey supply, model selection and habitat complexity.

The influence of predation in structuring ecological communities can be informed by examining the shape and magnitude of the functional response of predators towards prey. We derived functional responses of the ubiquitous intertidal amphipod Echinogammarus marinus towards one of its preferred prey species, the isopod Jaera nordmanni. First, we examined the form of the functional response where prey were replaced following consumption, as compared to the usual experimental design where prey density in each replicate is allowed to deplete. E. marinus exhibited Type II functional responses, i.e. inversely density-dependent predation of J. nordmanni that increased linearly with prey availability at low densities, but decreased with further prey supply. In both prey replacement and non-replacement experiments, handling times and maximum feeding rates were similar. The non-replacement design underestimated attack rates compared to when prey were replaced. We then compared the use of Holling’s disc equation (assuming constant prey density) with the more appropriate Rogers’ random predator equation (accounting for prey depletion) using the prey non-replacement data. Rogers’ equation returned significantly greater attack rates but lower maximum feeding rates, indicating that model choice has significant implications for parameter estimates. We then manipulated habitat complexity and found significantly reduced predation by the amphipod in complex as opposed to simple habitat structure. Further, the functional response changed from a Type II in simple habitats to a sigmoidal, density-dependent Type III response in complex habitats, which may impart stability on the predator−prey interaction. Enhanced habitat complexity returned significantly lower attack rates, higher handling times and lower maximum feeding rates. These findings illustrate the sensitivity of the functional response to variations in prey supply, model selection and habitat complexity and, further, that E. marinus could potentially determine the local exclusion and persistence of prey through habitat-mediated changes in its predatory functional responses.

A novel sensor kinase-response regulator hybrid controls biofilm formation and type VI secretion system activity in Burkholderia cenocepacia

Burkholderia cenocepacia is an important opportunistic pathogen causing serious chronic infections in patients with cystic fibrosis (CF). Adaptation of B. cenocepacia to the CF airways may play an important role in the persistence of the infection. We have identified a sensor kinase-response regulator (BCAM0379) named AtsR in B. cenocepacia K56-2 that shares 19% amino acid identity with RetS from Pseudomonas aeruginosa. atsR inactivation led to increased biofilm production and a hyperadherent phenotype in both abiotic surfaces and lung epithelial cells. Also, the atsR mutant overexpressed and hypersecreted an Hcp-like protein known to be specifically secreted by the type VI secretion system (T6SS) in other gram-negative bacteria. Amoeba plaque assays demonstrated that the atsR mutant was more resistant to Dictyostelium predation than the wild-type strain and that this phenomenon was T6SS dependent. Macrophage infection assays also demonstrated that the atsR mutant induces the formation of actin-mediated protrusions from macrophages that require a functional Hcp-like protein, suggesting that the T6SS is involved in actin rearrangements. Three B. cenocepacia transposon mutants that were found in a previous study to be impaired for survival in chronic lung infection model were mapped to the T6SS gene cluster, indicating that the T6SS is required for infection in vivo. Together, our data show that AtsR is involved in the regulation of genes required for virulence in B. cenocepacia K56-2, including genes encoding a T6SS.

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>

Emergence time and foraging activity in Pallas' mastiff bat, Molossus molossus (Chiroptera: Molossidae) in relation to sunset/sunrise and phase of the moon

The decision on when to emerge from the safety of a roost and forage for prey is thought to be a result of the trade off between peak insect abundance and predation pressure for bats. In this study we show that the velvety free-tailed bat Molossus molossus emerges just after sunset and just before sunrise for very short foraging bouts (average 82.2 min foraging per night). Contrary to previous studies, bats remain inactive in their roost between activity patterns. Activity was measured over two complete lunar cycles and there was no indication that phase of the moon had an influence on emergence time or the numbers of bats that emerged from the roost. This data suggests that M. molossus represents an example of an aerial hawking bat whose foraging behaviour is in fact adapted to the compromise between the need to exploit highest prey availability and the need to avoid predation.

Hierarchical Bayesian models in ecology: Reconstructing species interaction networks from non-homogeneous species abundance data

The relationships among organisms and their surroundings can be of immense complexity. To describe and understand an ecosystem as a tangled bank, multiple ways of interaction and their effects have to be considered, such as predation, competition, mutualism and facilitation. Understanding the resulting interaction networks is a challenge in changing environments, e.g. to predict knock-on effects of invasive species and to understand how climate change impacts biodiversity. The elucidation of complex ecological systems with their interactions will benefit enormously from the development of new machine learning tools that aim to infer the structure of interaction networks from field data. In the present study, we propose a novel Bayesian regression and multiple changepoint model (BRAM) for reconstructing species interaction networks from observed species distributions. The model has been devised to allow robust inference in the presence of spatial autocorrelation and distributional heterogeneity. We have evaluated the model on simulated data that combines a trophic niche model with a stochastic population model on a 2-dimensional lattice, and we have compared the performance of our model with L1-penalized sparse regression (LASSO) and non-linear Bayesian networks with the BDe scoring scheme. In addition, we have applied our method to plant ground coverage data from the western shore of the Outer Hebrides with the objective to infer the ecological interactions. (C) 2012 Elsevier B.V. All rights reserved.

A complete analytic theory for structure and dynamics of populations and communities spanning wide ranges in body size

The prediction and management of ecosystem responses to global environmental change would profit from a clearer understanding of the mechanisms determining the structure and dynamics of ecological communities. The analytic theory presented here develops a causally closed picture for the mechanisms controlling community and population size structure, in particular community size spectra, and their dynamic responses to perturbations, with emphasis on marine ecosystems. Important implications are summarised in non-technical form. These include the identification of three different responses of community size spectra to size-specific pressures (of which one is the classical trophic cascade), an explanation for the observed slow recovery of fish communities from exploitation, and clarification of the mechanism controlling predation mortality rates. The theory builds on a community model that describes trophic interactions among size-structured populations and explicitly represents the full life cycles of species. An approximate time-dependent analytic solution of the model is obtained by coarse graining over maturation body sizes to obtain a simple description of the model steady state, linearising near the steady state, and then eliminating intraspecific size structure by means of the quasi-neutral approximation. The result is a convolution equation for trophic interactions among species of different maturation body sizes, which is solved analytically using a novel technique based on a multiscale expansion.