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Friedenberg, N.A., S.
Sarkar, N. Kouchoukos, R.F. Billings, M.P. Ayres. 2008. Temperature
extremes, density dependence, and southern pine beetle (Coleoptera:
Curculionidae) population dynamics in east Texas. Environmental
Entomlogy 37: 650-659 Previous studies of the southern pine beetle, Dendroctonus frontalis Zimm., established that its population in east Texas responds to a delayed density-dependent process, whereas no clear role of climate has been determined.We tested two biological hypotheses for the influence of extreme temperatures on annual southern pine beetle population growth in the context of four alternative hypotheses for density-dependent population regulation. The significance of climate variables and their interaction with population regulation depended on the model of density dependence. The best model included both direct and delayed density dependence of a cubic rather than linear form. Population growth declined with the number of days exceeding 32 C, temperatures previously reported to reduce brood survival. Growth was highest in years with average minimum winter temperatures. Severely cold winters may reduce survival, whereas warm winters may reduce the efficiency of spring infestation formation. Whereas most previous studies have incorporated climate as an additive effect on growth, we found that the form of delayed density dependence changed with the number of days over 32 C. The interaction between temperature and regulation, a potentially common phenomenon in ecology, may explain why southern pine beetle outbreaks do not occur at perfectly regular intervals. Factors other than climate, such as forest management and direct suppression, may have contributed significantly to the timing, severity, and eventual cessation of outbreaks since the mid-1950s. |
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Friedenberg, N.A., B.M.
Whited, D.H. Slone, S.J. Martinson, M.P. Ayres. 2007. Differential
impacts of the southern pine beetle, Dendroctonus frontalis,
on Pinus palustris and Pinus taeda. Canadian Journal
of Forest Research 37: 1427-1437 Patterns of host use by herbivore pests can have serious consequences for natural and managed ecosystems, but are often poorly understood. Here, we provide the first quantification of large differential impacts of the southern pine beetle, Dendroctonus frontalis Zimmermann, on loblolly pine, Pinus taeda, and longleaf pine, P. palustris, and evaluate putative mechanisms for the disparity. Spatially extensive survey data from recent epidemics indicate that, per km2, stands of loblolly vs. longleaf pine in four forests (380-1273 km2) sustained 3-18 times more local infestations and 3-116 times more tree mortality. Differences were not attributable to size or age structure of pine stands. Using pheromone-baited traps, we found no differences in the abundance of dispersing D. frontalis or its predator, Thanasimus dubius Fabricius, between loblolly and longleaf stands. Trapping triggered numerous attacks on trees, but the pine species did not differ in the probability of attack initiation, nor in the surface area of bark attacked by growing aggregations. We found no evidence for post-aggregation mechanisms of discrimination or differential success on the two hosts, suggesting that early colonizers discriminate between host species before a pheromone plume is present. |
Friedenberg, N.A.,
J.A. Powell, M.P. Ayres. 2007. Synchrony's double edge: transient
dynamics and the Allee effect in stage structured populations. Ecology
Letters 10: 564-573 In populations subject to positive density dependence, individuals can increase their fitness by synchronizing the timing of key life history events. However, phenological synchrony represents a perturbation from a population’s stable stage structure and the ensuing transient dynamics create troughs of low abundance that can promote extinction. Using an ecophysiological model of a mass-attacking pest insect, we show that the effect of synchrony on local population persistence depends on population size and adult lifespan. Results are consistent with a strong empirical pattern of increased extinction risk with decreasing initial population size. Mortality factors such as predation on adults can also affect transient dynamics. Throughout the species range, the seasonal niche for persistence increases with the asynchrony of oviposition. Exposure to the Allee effect after establishment may be most likely at northern range limits, where cold winters tend to synchronize spring colonization, suggesting a role for transient dynamics in the determination of species distributions. |
Dennehy, J.J., N.A.
Friedenberg, Y. Yang, P.E. Turner. 2007. Virus population
extinction via ecological traps. Ecology Letters 10: 230-240 Populations are at risk of extinction when unsuitable or when sink habitat exceeds a threshold frequency in the environment. Sinks that present cues associated with highquality habitats, termed ecological traps, have especially detrimental effects on net population growth at metapopulation scales. Ecological traps for viruses arise naturally, or can be engineered, via the expression of viral-binding sites on cells that preclude viral reproduction. We present a model for virus population growth in a heterogeneous host community, parameterized with data from populations of the RNA bacteriophage U6 presented with mixtures of suitable host bacteria and either neutral or trap cells. We demonstrate that viruses can sustain high rates of population growth in the presence of neutral non-hosts as long as some host cells are present, whereas trap cells dramatically reduce viral fitness. In addition, we demonstrate that the efficacy of traps for viral elimination is frequency dependent in spatially structured environments such that population viability is a nonlinear function of habitat loss in dispersal-limited virus populations. We conclude that the ecological concepts applied to species conservation in altered landscapes can also contribute to the development of trap cell therapies for infectious human viruses. |
Dennehy, J.J., N.A.
Friedenberg, Y. Yang, P.E. Turner. 2006. Bacteriophage
migration via nematode vectors: host-parasite-consumer interactions
in laboratory microcosms. Applied and Environmental Microbiology
72: 1974-1979 Pathogens vectored by nematodes pose serious agricultural, economic, and health threats; however, little is known of the ecological and evolutionary aspects of pathogen transmission by nematodes. Here we describe a novel model system with two trophic levels, bacteriophages and nematodes, each of which competes for bacteria. We demonstrate for the first time that nematodes are capable of transmitting phages between spatially distinct patches of bacteria. This model system has considerable advantages, including the ease of maintenance and manipulation at the laboratory bench, the ability to observe many generations in short periods, and the capacity to freeze evolved strains for later comparison to their ancestors. More generally, experimental studies of complex multispecies interactions, host-pathogen coevolution, disease dynamics, and the evolution of virulence may benefit from this model system because current models (e.g., chickens, mosquitoes, and malaria parasites) are costly to maintain, are difficult to manipulate, and require considerable space. Our initial explorations centered on independently assessing the impacts of nematode, bacterium, and phage population densities on virus migration between host patches. Our results indicated that virus transmission increases with worm density and host bacterial abundance; however, transmission decreases with initial phage abundance, perhaps because viruses eliminate available hosts before migration can occur. We discuss the microbial growth dynamics that underlie these results, suggest mechanistic explanations for nematode transmission of phages, and propose intriguing possibilities for future research. |
Dennehy, J.J., N.A. Friedenberg,
R.D. Holt, P.E. Turner. 2006. Viral ecology and the maintenance
of novel host use. American Naturalist 167: 429-439 Viruses can occasionally emerge by infecting new host species. However, the early phases of emergence can hinge upon ecological sustainability of the virus population, which is a product of both within-host population growth and between-host transmission. Insufficient growth or transmission can force virus extinction before the latter phases of emergence, where genetic adaptations that improve host use may occur. We examined the early phase of emergence by studying the population dynamics of RNA phages in replicated laboratory environments containing native and novel host bacteria. To predict the breadth of transmission rates allowing viral persistence on each species, we developed a simple model based on in vitro data for phage growth rate over a range of initial population densities on both hosts. Validation of these predictions using serial passage experiments revealed a range of transmission rates for which the native host was a source and the novel host was a sink. In this critical range of transmission rates, periodic exposure to the native host was sufficient for the maintenance of the viral population on the novel host. We argue that this effect should facilitate adaptation by the virus to utilize the novel host—often crucial in subsequent phases of emergence. |
Friedenberg, N.A.
2003. Determinism in a transient assemblage: the roles of dispersal
and local competition. American Naturalist 162: 586-596 Both dispersal and local competitive ability may determine the outcome of competition among species that cannot coexist locally. I develop a spatially implicit model of two-species competition at a small spatial scale. The model predicts the relative fitness of two competitors based on local reproductive rates and regional dispersal rates in the context of the number, size, and extinction probability of habitat patches in the landscape. I test the predictions of this model experimentally using two genotypes of the bacteriophagous soil nematode Caenorhabditis elegans in patchy microcosms. One genotype has higher fecundity while the other is a better disperser. With such a fecundity-dispersal trade-off between competitors, the model predicts that relative fitness will be affected most by local population size when patches do not go extinct and by the number of patches when there is a high probability of patch extinction. The microcosm experiments support the model predictions. Both approaches suggest that competitive dominance in a patchily distributed transient assemblage will depend upon the architecture and predictability of the environment. These mechanisms, operating at a small scale with high spatial admixture, may be embedded in a larger metacommunity process. |
Friedenberg, N.A.
2003. Experimental evolution of dispersal in spatiotemporally variable
microcosms. Ecology Letters 6: 937 The world is an uncertain place. Individuals' fates vary from place to place and from time to time. Natural selection in unpredictable environments should favour individuals that hedge their bets by dispersing offspring. I confirm this basic prediction using Caenorhabditis elegans in experimental microcosms. My results agree with evolutionary models and correlations found previously between habitat stability and individual dispersal propensity in nature. However, I also find that environmental variation that triggers conditional dispersal behaviour may not impose selection on baseline dispersal rates. These findings imply that an increased rate of disturbance in natural systems has the potential to cause an evolutionary response in the life history of impacted organisms. |
Hampton, S.E., and N.A.
Friedenberg. 2002. Nocturnal increases in the use of near-surface
water by pond animals. Hydrobiologia 477: 171-179 We assessed diel animal habitat use in three shallow ponds, using unbaited funnel traps, a large column sampler, and sweep net collections in the upper stratum (0–0.3 m) of littoral and open habitats. In all three ponds, more animals were caught at night than during the day, indicating that use of near-surface waters was greatest at night, particularly in the fishless ponds. All methods yielded similar patterns. Our results demonstrate that nocturnal observations of pond animals are necessary to describe their ecology, even in fishless ponds where diel differences in habitat use or behavior might not be anticipated. |