Christopher J. Dibble

Does phylogeny matter? Assessing the impact of phylogenetic information in ecological meta‐analysis

Meta-analysis is increasingly used in ecology and evolutionary biology. Yet, in these fields this technique has an important limitation: phylogenetic non-independence exists among taxa, violating the statistical assumptions underlying traditional meta-analytic models. Recently, meta-analytical techniques incorporating phylogenetic information have been developed to address this issue. However, no syntheses have evaluated how often including phylogenetic information changes meta-analytic results. To address this gap, we built phylogenies for and re-analysed 30 published meta-analyses, comparing results for traditional vs. phylogenetic approaches and assessing which characteristics of phylogenies best explained changes in meta-analytic results and relative model fit. Accounting for phylogeny significantly changed estimates of the overall pooled effect size in 47% of datasets for fixed-effects analyses and 7% of datasets for random-effects analyses. Accounting for phylogeny also changed whether those effect sizes were significantly different from zero in 23 and 40% of our datasets (for fixed- and random-effects models, respectively). Across datasets, decreases in pooled effect size magnitudes after incorporating phylogenetic information were associated with larger phylogenies and those with stronger phylogenetic signal. We conclude that incorporating phylogenetic information in ecological meta-analyses is important, and we provide practical recommendations for doing so.

Resolving the roles of body size and species identity in driving functional diversity

Efforts to characterize food webs have generated two influential approaches that reduce the complexity of natural communities. The traditional approach groups individuals based on their species identity, while recently developed approaches group individuals based on their body size. While each approach has provided important insights, they have largely been used in parallel in different systems. Consequently, it remains unclear how body size and species identity interact, hampering our ability to develop a more holistic framework that integrates both approaches. We address this conceptual gap by developing a framework which describes how both approaches are related to each other, revealing that both approaches share common but untested assumptions about how variation across size classes or species influences differences in ecological interactions among consumers. Using freshwater mesocosms with dragonfly larvae as predators, we then experimentally demonstrate that while body size strongly determined how predators affected communities, these size effects were species specific and frequently nonlinear, violating a key assumption underlying both size- and species-based approaches. Consequently, neither purely species- nor size-based approaches were adequate to predict functional differences among predators. Instead, functional differences emerged from the synergistic effects of body size and species identity. This clearly demonstrates the need to integrate size- and species-based approaches to predict functional diversity within communities.

Waiting time to infectious disease emergence

Emerging diseases must make a transition from stuttering chains of transmission to sustained chains of transmission, but this critical transition need not coincide with the system becoming supercritical. That is, the introduction of infection to a supercritical system results in a significant fraction of the population becoming infected only with a certain probability. Understanding the waiting time to the first major outbreak of an emerging disease is then more complicated than determining when the system becomes supercritical. We treat emergence as a dynamic bifurcation, and use the concept of bifurcation delay to understand the time to emergence after a system becomes supercritical. Specifically, we consider an SIR model with a time-varying transmission term and random infections originating from outside the population. We derive an analytic density function for the delay times and find it to be, in general, in agreement with stochastic simulations. We find the key parameters to be the rate of introduction of infection and the rate of change of the basic reproductive ratio. These findings aid our understanding of real emergence events, and can be incorporated into early-warning systems aimed at forecasting disease risk.

Effects of potential predator and competitor cues and sibship on wood frog (Rana sylvatica) embryos

Chemical cues emitted from predators or competitors are often important for animals living in aquatic ecosystems as they allow potential prey to assess predation risk and make appropriate risk-sensitive responses. In our experiment, we examined if wood frog (Rana sylvatica) embryos exposed to potential predator and competitor cues would alter their time to hatching, size at hatching, or survivorship. Eggs from four different sibships were subjected to a variety of cues including dragonfly larvae (potential tadpole predator), mosquitofish (Gambusia affinis; a non-native potential egg and tadpole predator), and overwintered tadpoles of Rana sp. (potential competitors). We found no significant effects of any of the cues. However, we did find significant variation in mean time to hatching and mean hatchling size among sibships. Our results suggest that wood frog embryos may have limited ability to respond to some cues at the embryonic stage, at least for the concentrations and conditions used in this experiment. Our results do indicate genetic or parental effects can affect embryonic wood frog development rate and hatchling size.

Intraspecific priority effects and disease interact to alter population growth

Intraspecific variation may shape colonization of new habitat patches through a variety of mechanisms. In particular, trait variation among colonizing individuals can produce intraspecific priority effects (IPEs), where early arrivers of a single species affect the establishment or growth of later conspecifics. While we have some evidence for the importance of IPEs, we lack a general understanding of factors affecting their presence or magnitude across a landscape. Specifically, IPEs should depend strongly on success of colonizers in the new habitat patch. This success hinges on interactions between colonizer traits and local selective pressures, but such context dependence remains unexplored experimentally. We addressed this gap by looking for the dynamical signature of IPEs in environments with and without a selective (parasite) pressure. We tested whether IPEs affected the population dynamics of a zooplankton host species (Daphnia dentifera) collected from two populations showing a tradeoff between growth rate and resistance to a fungal parasite (Metschnikowia bicuspidata). Differences in arrival order significantly altered population growth during a period of rapid resource depletion, driving large (up to 65%) differences in population abundance. Furthermore, the presence of IPEs was context dependent, as parasites reduced the impact of early arrivers on later arrivers. Such context-dependent IPEs, mediated by colonizer traits, colonization order, and selective pressures, may play an unanticipated role in the ecological and evolutionary dynamics of natural metapopulations. This mechanism highlights the overall importance of intraspecific variation for understanding ecological patterns.

Effects of mosquitofish and ammonium nitrate on activity of green frog (Lithobates clamitans) tadpoles: a mesocosm experiment

We conducted a mesocosm experiment that allowed us to examine the effects of mosquitofish (Gambusia affinis) and ammonium nitrate on the activity of green frog (Lithobates clamitans) tadpoles under the more natural conditions of a mesocosm as opposed to a laboratory experiment. The proportion of active tadpoles was lower in the presence of mosquitofish than when mosquitofish were not present. There was no effect of ammonium nitrate or interaction between mosquitofish and ammonium nitrate on tadpole activity. These results suggest that green frog tadpoles reduce their activity levels in the presence of mosquitofish, but the ability of ammonium nitrate to mediate the outcome of potential predator–prey interactions between green frog tadpoles and mosquitofish appears to be limited, at least at the ecologically relevant concentration we studied.

Effects of Invasive Western Mosquitofish and Ammonium Nitrate on Green Frog Tadpoles

Invasive fish frequently negatively affect amphibian populations around the world. In agricultural regions of the world, the effects of invasive fish on amphibians may coincide with pollution by agricultural fertilizers. We conducted a mesocosm experiment to examine the potential independent and interactive effects of introduced Western Mosquitofish (Gambusia affinis) and ammonium nitrate, an agricultural fertilizer, on Green Frog (Lithobates clamitans) tadpoles. Mosquitofish had a negative effect on the survivorship of Green Frog tadpoles. Green Frog tadpoles from mesocosms with mosquitofish were larger than tadpoles from mesocosms without mosquitofish, possibly due to a thinning effect that reduced intraspecific competition, as well as increased primary productivity. Ammonium nitrate additions did not affect survivorship in Green Frogs. However, Green Frog tadpoles in ammonium nitrate addition treatments were larger than those from treatments without ammonium nitrate addition, possibly due to increased phytoplankton abundance. In conclusion, mosquitofish and ammonium nitrate addition each had independent and additive effects on Green Frog tadpoles, but there was no evidence for significant interactions between these two stressors. Our results suggest that environmental stressors can have additive effects in some systems and that introduced fish predators may have greater impacts on amphibian populations than low level chemical contamination.

Top predators determine how biodiversity is partitioned across time and space

Natural ecosystems are shaped along two fundamental axes, space and time, but how biodiversity is partitioned along both axes is not well understood. Here, we show that the relationship between temporal and spatial biodiversity patterns can vary predictably according to habitat characteristics. By quantifying seasonal and annual changes in larval dragonfly communities across a natural predation gradient we demonstrate that variation in the identity of top predator species is associated with systematic differences in spatio-temporal β-diversity patterns, leading to consistent differences in relative partitioning of biodiversity between time and space across habitats. As the size of top predators increased (from invertebrates to fish) habitats showed lower species turnover across sites and years, but relatively larger seasonal turnover within a site, which ultimately shifted the relative partitioning of biodiversity across time and space. These results extend community assembly theory by identifying common mechanisms that link spatial and temporal patterns of β-diversity.

Diet and sexual dimorphism of the desert iguana, Dipsosaurus dorsalis, from Sonora, Mexico

Abstract We examined the diet and sexual dimorphism of the desert iguana, Dipsosaurus dorsalis, from Sonora, Mexico. The diet consisted primarily of vegetation, with insect material contributing little to the overall volume of the diet. Plant matter was restricted to leaves and seeds. Animal prey consisted mostly of ants, termites, and beetles. Males and females did not differ significantly in snout–vent length, head length, or head width. These results confirm that in general D. dorsalis is herbivorous and not sexually dimorphic through much of its range.

Natural History of Procinura aemula (Serpentes: Colubridae) From Chínipas, Chihuahua, Mexico

Abstract We examined several aspects of the biology of Procinura aemula, including diet, sexual dimorphism, and clutch size, from Chínipas, Chihuahua, Mexico. Diet of P. aemula was composed entirely of invertebrates. Orthopterans and crickets were the majority of the diet, both numerically and volumetrically. Prey size was significantly related to snake size (snout–vent length, head size). There was no sexual dimorphism in snout–vent length or head size. Mean clutch size was 4.6 eggs. Clutch size tended to increase with snout–vent length of females, but the trend was not statistically significant.