Inter-annual variability influences the eco-evolutionary dynamics of range-shifting

Peer reviewedPublisher PD

Social and physical environment independently affect oviposition decisions in Drosophila

In response to environmental stimuli, including variation in the presence of conspecifics, genotypes show highly plastic responses in behavioral and physiological traits influencing reproduction. Although extensively documented in males, such female responses are rather less studied. We expect females to be highly responsive to environmental variation and to differentially allocate resources to increase offspring fitness, given the major contribution of mothers to offspring number, size, and developmental conditions. Using Drosophila melanogaster, we (a) manipulate exposure to conspecific females, which mothers could use to anticipate the number of potential mates and larval density, and; (b) test how this interacts with the spatial distribution of potential oviposition sites, with females from higher densities expected to prefer clustered resources that can support a larger number of larvae. We found that high density females were slower to start copulating and reduced their copulation duration, the opposite effect to that observed in males. There was a parallel, perhaps related, effect on egg production: females previously housed in groups laid fewer eggs than those housed in solitude. Resource patchiness also influenced oviposition behavior: females preferred aggregated substrate, which attracted more females to lay eggs. However, we found no interaction between prior housing conditions and resource patchiness, indicating that females did not perceive the value of different resource distributions differently when exposed to environments that could signal expected levels of larval competition. We show that, although exposure to consexual competition changes copulatory behaviors of females, the distribution of oviposition resources has a greater effect on oviposition decisions

Elevated mortality of fish larvae on coral reefs drives the evolution of larval movement patterns

Journal ArticleCoral reef fishes typically undergo a pelagic larval phase prior to recruitment to reef habitat. This is potentially risky, but likely to be important in connecting populations and avoiding local crowding. Predation pressures on larvae on and off the reef are likely to differ both in origin and intensity. In this study, we used individual-based models to explore in isolation the effect of elevated levels of larval mortality on the reef in 22 different landscapes. We allowed the movement of highly simplified larvae to evolve through selection and mutation events and tracked emerging strategies over 180 generations. The pressure of increased larval mortality on the reef affected overall population sizes and selected for higher levels of larval movement. However, the evolution of movement was constrained, and self-recruitment back to the reef of origin was prevalent, with levels rarely dropping below 50%. The evolved strategies were highly landscape specific, suggesting that movement in contiguous reefs is more readily evolved than at isolated reefs. Future development of this simulation approach will provide a valuable research tool for exploring important evolutionary, ecological, and management-based questions. © Inter-Research 2007.NER

Landscape as a Model: The Importance of Geometry

In all models, but especially in those used to predict uncertain processes (e.g., climate change and nonnative species establishment), it is important to identify and remove any sources of bias that may confound results. This is critical in models designed to help support decisionmaking. The geometry used to represent virtual landscapes in spatially explicit models is a potential source of bias. The majority of spatial models use regular square geometry, although regular hexagonal landscapes have also been used. However, there are other ways in which space can be represented in spatially explicit models. For the first time, we explicitly compare the range of alternative geometries available to the modeller, and present a mechanism by which uncertainty in the representation of landscapes can be incorporated. We test how geometry can affect cell-to-cell movement across homogeneous virtual landscapes and compare regular geometries with a suite of irregular mosaics. We show that regular geometries have the potential to systematically bias the direction and distance of movement, whereas even individual instances of landscapes with irregular geometry do not. We also examine how geometry can affect the gross representation of real-world landscapes, and again show that individual instances of regular geometries will always create qualitative and quantitative errors. These can be reduced by the use of multiple randomized instances, though this still creates scale-dependent biases. In contrast, virtual landscapes formed using irregular geometries can represent complex real-world landscapes without error. We found that the potential for bias caused by regular geometries can be effectively eliminated by subdividing virtual landscapes using irregular geometry. The use of irregular geometry appears to offer spatial modellers other potential advantages, which are as yet underdeveloped. We recommend their use in all spatially explicit models, but especially for predictive models that are used in decisionmaking

Population fragmentation drives up genetic diversity in signals of individual identity

Many species advertise their unique identity to conspecifics using dedicated individuality signals: one familiar example is human faces. But how unique in the global population do these signals need to be? While human faces are highly variable, each person interacts with many fewer individuals than are found in the total population. This raises the question of how evolutionary mechanisms drive up population-wide diversity when selection occurs at such a local level. We use an individual-based model in which individuals broadcast their identity and quality in separate, genetically-coded signals. Mimicking, for example, scent marking mammal species, females in the model assess males using the quality signal, then attempt to relocate the highest quality male using his identity signal. We ask how population fragmentation affects genetic diversity in the individual identity-signalling region under sexual selection, predicting one of two opposing outcomes: 1) divided populations evolve fewer signal variants globally, since repetition of signals is not costly when individuals interact only with local conspecifics, or 2) stochasticity in mutation and selection cause divergence among subpopulations, increasing the global number of signal variants. We show that local selection drives up global genetic diversity substantially in fragmented populations, even with extremely low rates of dispersal. Because new signal variants arise by mutation and then sweep through their subpopulation, a fragmented population has more global signal variation. This result furthers our understanding of how high levels of diversity in individuality signals are maintained

The evolution of plasmid stability: Are infectious transmission and compensatory evolution competing evolutionary trajectories?

Conjugative plasmids are widespread and play an important role in bacterial evolution by accelerating adaptation through horizontal gene transfer. However, explaining the long-term stability of plasmids remains challenging because segregational loss and the costs of plasmid carriage should drive the loss of plasmids though purifying selection. Theoretical and experimental studies suggest two key evolutionary routes to plasmid stability: First, the evolution of high conjugation rates would allow plasmids to survive through horizontal transmission as infectious agents, and second, compensatory evolution to ameliorate the cost of plasmid carriage can weaken purifying selection against plasmids. How these two evolutionary strategies for plasmid stability interact is unclear. Here, we summarise the literature on the evolution of plasmid stability and then use individual based modelling to investigate the evolutionary interplay between the evolution of plasmid conjugation rate and cost amelioration. We find that, individually, both strategies promote plasmid stability, and that they act together to increase the likelihood of plasmid survival. However, due to the inherent costs of increasing conjugation rate, particularly where conjugation is unlikely to be successful, our model predicts that amelioration is the more likely long-term solution to evolving stable bacteria-plasmid associations. Our model therefore suggests that bacteria-plasmid relationships should evolve towards lower plasmid costs that may forestall the evolution of highly conjugative, 'infectious' plasmids

Reliable microsatellite genotyping of the Eurasian badger (Meles meles) using faecal DNA

The potential link between badgers and bovine tuberculosis has made it vital to develop accurate techniques to census badgers. Here we investigate the potential of using genetic profiles obtained from faecal DNA as a basis for population size estimation. After trialling several methods we obtained a high amplification success rate (89%) by storing faeces in 70% ethanol and using the guanidine thiocyanate/silica method for extraction. Using 70% ethanol as a storage agent had the advantage of it being an antiseptic. In order to obtain reliable genotypes with fewer amplification reactions than the standard multiple-tubes approach, we devised a comparative approach in which genetic profiles were compared and replication directed at similar, but not identical, genotypes. This modified method achieved a reduction in polymerase chain reactions comparable with the maximumlikelihood model when just using reliability criteria, and was slightly better when using reliability criteria with the additional proviso that alleles must be observed twice to be considered reliable. Our comparative approach would be best suited for studies that include multiple faeces from each individual. We utilized our approach in a well-studied population of badgers from which individuals had been sampled and reliable genotypes obtained. In a study of 53 faeces sampled from three social groups over 10 days, we found that direct enumeration could not be used to estimate population size, but that the application of mark–recapture models has the potential to provide more accurate results

Climate change, climatic variation and extreme biological responses

Extreme climatic events could be major drivers of biodiversity change, but it is unclear whether extreme biological changes are (i) individualistic (species- or group-specific), (ii) commonly associated with unusual climatic events and/or (iii) important determinants of long-term population trends. Using population time series for 238 widespread species (207 Lepidoptera and 31 birds) in England since 1968, we found that population 'crashes' (outliers in terms of species' year-to-year population changes) were 46% more frequent than population 'explosions'. (i) Every year, at least three species experienced extreme changes in population size, and in 41 of the 44 years considered, some species experienced population crashes while others simultaneously experienced population explosions. This suggests that, even within the same broad taxonomic groups, species are exhibiting individualistic dynamics, most probably driven by their responses to different, short-term events associated with climatic variability. (ii) Six out of 44 years showed a significant excess of species experiencing extreme population changes (5 years for Lepidoptera, 1 for birds). These 'consensus years' were associated with climatically extreme years, consistent with a link between extreme population responses and climatic variability, although not all climatically extreme years generated excess numbers of extreme population responses. (iii) Links between extreme population changes and long-term population trends were absent in Lepidoptera and modest (but significant) in birds. We conclude that extreme biological responses are individualistic, in the sense that the extreme population changes of most species are taking place in different years, and that long-term trends of widespread species have not, to date, been dominated by these extreme changes.This article is part of the themed issue 'Behavioural, ecological and evolutionary responses to extreme climatic events'

Evolution of predator dispersal in relation to spatio-temporal prey dynamics : how not to get stuck in the wrong place!

Peer reviewedPublisher PD

Prioritising surveillance for alien organisms transported as stowaways on ships travelling to South Africa

The global shipping network facilitates the transportation and introduction of marine and terrestrial organisms to regions where they are not native, and some of these organisms become invasive. South Africa was used as a case study to evaluate the potential for shipping to contribute to the introduction and establishment of marine and terrestrial alien species (i.e. establishment debt) and to assess how this varies across shipping routes and seasons. As a proxy for the number of species introduced (i.e. 'colonisation pressure') shipping movement data were used to determine, for each season, the number of ships that visited South African ports from foreign ports and the number of days travelled between ports. Seasonal marine and terrestrial environmental similarity between South African and foreign ports was then used to estimate the likelihood that introduced species would establish. These data were used to determine the seasonal relative contribution of shipping routes to South Africa's marine and terrestrial establishment debt. Additionally, distribution data were used to identify marine and terrestrial species that are known to be invasive elsewhere and which might be introduced to each South African port through shipping routes that have a high relative contribution to establishment debt. Shipping routes from Asian ports, especially Singapore, have a particularly high relative contribution to South Africa's establishment debt, while among South African ports, Durban has the highest risk of being invaded. There was seasonal variation in the shipping routes that have a high relative contribution to the establishment debt of the South African ports. The presented method provides a simple way to prioritise surveillance effort and our results indicate that, for South Africa, port-specific prevention strategies should be developed, a large portion of the available resources should be allocated to Durban, and seasonal variations and their consequences for prevention strategies should be explored further. (Résumé d'auteur