Ofer Ovadia

Factors Influencing Site Abandonment and Site Selection in a Sit-and-Wait Predator: A Review of Pit-Building Antlion Larvae

There is a large body of evidence indicating that predator behavior may strongly influence patterns and processes at the population and community level. Site selection is a major component of fitness in sit-and-wait predators, especially when relocation is rare. Although several review articles dealt with these issues in web-building spiders, this is the first attempt to summarize the effects of biotic and abiotic factors on site selection and relocation in another group of sit-and-wait predators, the pit-building antlions (Neuroptera: Myrmeleontidae). Our synthesis shows that prey abundance may have relatively little effect on pit relocation and that physical properties of the habitat or competition often override its effect. We suggest that owing to a variety of constraints such as physiological constraints or difficulties in assessing site quality, site selection and relocation are not necessarily optimal and thus food intake rate is not maximized. We call for a multi-factorial study on a single species in order to pinpoint the dominant factors and to assess to what extent they influence site selection and relocation. We conclude by proposing new research directions, such as studying whether pit relocation is an adaptive response, when controlling for possible phylogenetic effects.

Foraging decisions and behavioural flexibility in trap-building predators: a review.

Foraging theory was first developed to predict the behaviour of widely‐foraging animals that actively search for prey. Although the behaviour of sit‐and‐wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap‐building predators (mainly pit‐building antlions and web‐building spiders), a specific group of sit‐and‐wait predators that construct traps as a foraging device, with those of widely‐foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap‐building and widely‐foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap‐building predators. In each chapter of this review, we summarize the response of trap‐building predators to a different factor, while contrasting it with the equivalent response characterizing widely‐foraging predators. We provide here evidence that the behaviour of trap‐building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individuals internal state and its interactions with external environmental factors.

Efficiency Evaluation of Two Competing Foraging Modes under Different Conditions

Various foraging modes are employed by predators in nature, ranging from ambush to active predation. Although the foraging mode may be limited by physiological constraints, other factors, such as prey behavior and distribution, may come into play. Using a simulation model, we tested to what extent the relative success of an ambush and an active predator changes as a function of the relative velocity and movement directionality of prey and active predator. In accordance with previous studies, we found that when both active predator and prey use nondirectional movement, the active mode is advantageous. However, as movement becomes more directional, this advantage diminishes gradually to 0. Previous theoretical studies assumed that animal movement is nondirectional; however, recent field observations show that in fact animal movement usually has some component of directionality. We therefore suggest that our simulation is a better predictor of encounter rates than previous studies. Furthermore, we show that as long as the active predator cannot move faster than its prey, it has little or no advantage over the ambush predator. However, as the active predator’s velocity increases, its advantage increases sharply.

Linking individuals with ecosystems: Experimentally identifying the relevant organizational scale for predicting trophic abundances

Ecosystems are complex owing to the fact that emergent properties like trophic structure and productivity depend on details related to lower-scale interactions among individuals. A key challenge is identifying how much individual-level detail is needed to predict patterns at the ecosystem level. We tested for the effect of individual herbivore body size on trophic interactions and consequent abundances of plant and herbivore trophic levels in a New England meadow ecosystem. Body size is an important determinant of vulnerability to predation and thus should influence the way individuals tradeoff time spent foraging against time spent avoiding contact with predators. Such tradeoffs can then influence the degree of damage herbivores inflict on their plant resources. We experimentally assigned field-caught grasshoppers to three distinct body size treatment groups (small, normal, and large) and crossed them with two spider predator treatments (spider present and absent) in a fully replicated design. We observed size-dependent differences in grasshopper survival and development. Moreover, predators caused grasshoppers to inflict greater damage to herbs and lesser damage to grasses relative to treatments without predators. However, there were no size-dependent differences in net damage level on grasses and herbs in either predator or no predator treatments owing to size-dependent compensation in grasshopper foraging effort. We thus conclude that in this ecosystem the foraging-predation risk tradeoff displayed by typical or average-sized herbivore is a sufficient amount of individual-level detail needed to explain ecosystem patterns.

Weather variation and trophic interaction strength: sorting the signal from the noise

Weather can have important consequences for the structure and function of ecological communities by substantially altering the nature and strength of species interactions. We examined the role of intra- and inter-annual weather variability on species interactions in a seasonal old-field community consisting of spider predators, grasshopper herbivores, and grass and herb plants. We experimentally varied the number of trophic levels for 2 consecutive years and tested for inter-annual variation in trophic abundances. Grasshopper emergence varied between years to the extent that the second growing season was 20% shorter than the first one. However, the damage grasshoppers inflicted on plants was greater in the second, shorter growing season. This inter-annual variation in plant abundance could be explained using the foraging-predation risk trade-off displayed by grasshoppers combined with their survival trajectory. Decreased grasshopper survival not only reduced the damage inflicted on plants, it weakened the strength of indirect effects of spiders on grass and herb plants. The most influential abiotic factor affecting grasshopper survival was precipitation. We found a negative association between grasshopper survival and the total yearly precipitation. A finer scale analysis, however, showed that different precipitation modalities, namely, number of rainy days and average precipitation per day, had opposing effects on grasshopper survival, which were inconsistent between years. Furthermore, our results suggest that small changes in these factors should result in changes of up to several orders of magnitude in the mortality rate of grasshoppers. We thus conclude that in this system the foraging-predation risk trade-off displayed by grasshoppers combined with their survival trajectory and relevant weather variability should be incorporated in analytical theory, whose goal is to predict community dynamics.

The Shape of a Gerbillus pyramidum (Rodentia: Gerbillinae) Isocline: An Experimental Field Study

Abramsky et al. developed a new technique to measure isoclines in the field. The method is based on the single-species habitat selection theory of Fretwell. Using short term (3-4 wk) and long term (1-yr) experiments, they tested it by measuring the isocline of Gerbillus allenbyi competing with G. pyramidum. In the present study, we measured the converse isocline of G. pyramidum competing with G. allenbyi. Like the isocline of G. allenbyi, the isocline of G. pyramidum is nonlinear and agrees with the predictions of the theory of optimal density-dependent habitat selection in a twospecies, shared-preference system. This is the first natural system in which both sets of isolegs and isoclines have been measured, in the field, for two competing species. A stability analysis suggested that the gerbil species can coexist under most observed density combinations

Density-Dependent Habitat Selection: Evaluation of the Isodar Method

We have tested the performance of the isodar method, in detecting density-dependent habitat selection, underlying modes of community organization, and competitive interaction. For that purpose, we used data obtained from intensive manipulation experiments conducted on two granivorous, desert gerbil species, Gerbillus allenbyi and G. pyramidum. The isodar method detected, in both species, density dependent habitat selection. The isodar method is a good tool to assess differences between habitats, shape and general location of fitness-density graphs. Using the isodar method we discovered new details about the two gerbil species. The G. pyramidum fitness-density graphs are two parallel straight lines. The G. allenbyi fitness-density graphs are two converging straight lines. Isodars estimated for mixed populations agree with the prediction that they should not differ from that calculated for each species in isolation. This point implies that isodars can be used to detect habitat preference of single species even though they exist in a community of competitors. The isodar method was also able to detect the shared preference community organization pattern of the two gerbil species. We concluded that the isodar method is a useful tool in detecting density-dependent habitat selection and community organization pattern. But we had to conclude that the isodar method is very limited in estimating interspecific competition. The method was not able to detect interference competition between the two gerbil species although results of manipulation experiments showed that it is the key process in the gerbil system. Also, the isodar method assumes that competition is fixed (density independent). This is a major flaw since all theories of optimal foraging, including density dependent habitat selection, predict that competition coefficients are density dependent. Thus, although the isodar method was able to estimate significant exploitation competition for the two species, we are not sure if it means anything. This is because the estimated isoclines of the two species, using field manipulations, revealed that the isoclines are highly nonlinear and that the magnitude of the interaction, for each density combination, depends on the degree of habitat overlap of the two species. Thus, habitat specific constant interaction coefficients estimated by the isodar method probably mean very little, if anything

Plastic bet-hedging in an amphicarpic annual: an integrated strategy under variable conditions

Amphicarpy is a form of diversified bet-hedging expressed mostly in annual plants, where two types of offspring are produced with two distinct ecological roles: long-range aerial dispersers and highly competitive subterranean, sedentary fruit. Emex spinosa is a semi-arid, amphicarpic annual, inhabiting habitats with different levels of environmental variation. We tested the hypothesis that, in E. spinosa, bet-hedging may be “fine-tuned” by plasticity in the phenotype ratio (aerial/subterranean fruit mass) as a function of environmental conditions. We conducted a greenhouse experiment, manipulating nutrient availability and intraspecific density, to determine the pattern of ratio shifts. In order to determine whether the integrated strategy is an adaptation to variable habitats, a similar common garden experiment was conducted, comparing two natural populations differing in environmental variability. The offspring ratio shifted in response to both nutrient availability and plant density. In pots containing single plants the ratio increased steeply with nutrient availability, while in pots containing eight plants a more moderate increase occurred. These shifts were the result of plasticity in allocation to both achene types, as well as ontogenetic effects on aerial achene production. The degree of response increased with the heterogeneity of the habitat of origin. We found evidence for an adaptive integrated strategy, with bet-hedging “fine-tuned” by phenotypic plasticity. Strenuous conditions tended to shift the offspring ratio towards securing subterranean reproductive success, while favorable conditions resulted in a shift towards dispersible achenes.

Mitochondrial DNA HV lineage increases the susceptibility to schizophrenia among Israeli Arabs

Haplotypes and haplogroups are linked sets of common DNA variants, acting as susceptibility or protective factors to complex disorders. Growing evidence suggests that dysfunction of mitochondrial bioenergetics contributes to the schizophrenia phenotype. We studied mitochondrial DNA haplogroups in schizophrenia patients. Since mitochondria are inherited from the mothers, we used healthy fathers as an ideal case-control group. Analysis of the distribution of mitochondrial haplogroups in schizophrenia patients compared to their healthy fathers (202 pairs) resulted in an over-representation of the mtDNA lineage cluster, HV, in the patients (p=0.01), with increased relative risk (odds ratio) of 1.8. Since mitochondrial DNA is small relative to nuclear DNA, a total mitochondrial genome analysis was possible in a hypothesis-free manner. However, mitochondrial DNA haplogroups are highly variable in human population and it will be necessary to replicate our results in other human ethnic groups.

Disrupting Mitochondrial–Nuclear Coevolution Affects OXPHOS Complex I Integrity and Impacts Human Health

The mutation rate of the mitochondrial DNA (mtDNA), which is higher by an order of magnitude as compared with the nuclear genome, enforces tight mitonuclear coevolution to maintain mitochondrial activities. Interruption of such coevolution plays a role in interpopulation hybrid breakdown, speciation events, and disease susceptibility. Previously, we found an elevated amino acid replacement rate and positive selection in the nuclear DNA-encoded oxidative phosphorylation (OXPHOS) complex I subunit NDUFC2, a phenomenon important for the direct interaction of NDUFC2 with the mtDNA-encoded complex I subunit ND4. This finding underlines the importance of mitonuclear coevolution to physical interactions between mtDNA and nuclear DNA-encoded factors. Nevertheless, it remains unclear whether this interaction is important for the stability and activity of complex I. Here, we show that siRNA silencing of NDUFC2 reduced growth of human D-407 retinal pigment epithelial cells, significantly diminished mitochondrial membrane potential, and interfered with complex I integrity. Moreover, site-directed mutagenesis of a positively selected amino acid in NDUFC2 significantly interfered with the interaction of NDUFC2 with its mtDNA-encoded partner ND4. Finally, we show that a genotype combination involving this amino acid (NDUFC2 residue 46) and the mtDNA haplogroup HV likely altered susceptibility to type 2 diabetes mellitus in Ashkenazi Jews. Therefore, mitonuclear coevolution is important for maintaining mitonuclear factor interactions, OXPHOS, and for human health.