Size-biased dispersal prior to breeding in a damselfly

Dispersal is notoriously difficult to measure, so its potential population consequences are often unknown. If dispersal is density-dependent, it can act in population regulation. Adult damselflies Enallagma boreale (Zygoptera: Coenagrionidae) raised as larvae under a range of competitive regimes were individually measured and marked. Individuals that survived to reproductive maturity were either recovered at the natal pond or had dispersed to nearby water bodies. Dispersing individuals were heavier at emergence than those returning to the natal pond to breed. Therefore, an increased probability of dispersal does not appear to be a response to poor conditions in this species.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47786/1/442_2004_Article_BF00317564.pd

Predictable changes in predation mortality as a consequence of changes in food availability and predation risk

Theory predicts that animals will have lower activity levels when either the risk of predation is high or the availability of resources in the environment is high. If encounter rates with predators are proportional to activity level, then we might expect predation mortality to be affected by resource availability and predator density independent of the number of effective predators. In a factorial experiment, we tested whether predation mortality of larval wood frogs, Rana sylvatica, caused by a single larval dragonfly, Anax junius, was affected by the presence of additional caged predators and elevated resource levels. Observations were consistent with predictions. The survival rate of the tadpoles increased when additional caged predators were present and when additional resources were provided. There was no significant interaction term between predator density and food concentration. Lower predation rates at higher predator density is a form of interference competition. Reduced activity of prey at higher predator density is a potential general mechanism for this widespread phenomenon. Higher predation rates at low food levels provides an indirect mechanism for density-dependent predation. When resources are depressed by elevated consumer densities, then the higher activity levels associated with low resource levels can lead to a positive association between consumer density and consumer mortality due to predation. These linkages between variation in behaviour and density-dependent processes argue that variation in behaviour may contribute to the dynamics of the populations. Because the capture rate of predators depends on the resources available to prey, the results also argue that models of food-web dynamics will have to incorporate adaptive variation in behaviour to make accurate predictions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42739/1/10682_2004_Article_176894.pd

Rodent malaria-resistant strains of the mosquito, Anopheles gambiae, have slower population growth than -susceptible strains

<p>Abstract</p> <p>Background</p> <p>Trade-offs between anti-parasite defence mechanisms and other life history traits limit the evolution of host resistance to parasites and have important implications for understanding diseases such as malaria. Mosquitoes have not evolved complete resistance to malaria parasites and one hypothesis is that anti-malaria defence mechanisms are costly.</p> <p>Results</p> <p>We used matrix population models to compare the population growth rates among lines of <it>Anopheles gambiae </it>that had been selected for resistance or high susceptibility to the rodent malaria parasite, <it>Plasmodium yoelii nigeriensis</it>. The population growth rate of the resistant line was significantly lower than that of the highly susceptible and the unselected control lines, regardless of whether mosquitoes were infected with <it>Plasmodium </it>or not. The lower population growth of malaria-resistant mosquitoes was caused by reduced post blood-feeding survival of females and poor egg hatching.</p> <p>Conclusion</p> <p>With respect to eradicating malaria, the strategy of releasing <it>Plasmodium</it>-resistant <it>Anopheles </it>mosquitoes is unlikely to be successful if the costs of <it>Plasmodium</it>-resistance in the field are as great as the ones measured in this study. High densities of malaria-resistant mosquitoes would have to be maintained by continuous release from captive breeding facilities.</p

Mark-recapture Estimates of Survival in Populations of the Asp Viper, Vipera aspis aspis

We estimated th eannual survivorship of two populations of the asp viper, Vipera aspis aspis, by recapturing known dult individuals in the field over six and nine years respectively. The snakes at the two study sites in th eJura mountains of northwestern Switzerland were active between mid-March and mid-October. Vispera a. aspis is easily captured by hand, has individually recognizable marks and is, therefore, well suited for long-term mark-recapture studies. The maximum likelihood estimates of annual survivorship are about 0.75 in both populations. The probability that a snake will be recaptured in any given year is about 0.4 (habita A) and about 0.33 (habitat B). Sampling effort within a year had little effect on the probability of recapture. Our best estimate of annual survibval of 0.75 for the two habitats combined compares well with other previous estimates in viperids and in V. aspis in particular. We found no detectable differences in the survival rates between sexes even when the two habitats were combined. We found weak evidence that there was a difference in the recapture probability between the sexes. The minimal adequate model for the combined data set suggested that there is a higher probability of recapturing surviving females (0.545) than males (0.331)

Food web persistence is enhanced by non-trophic interactions.

The strength of interspecific interactions is often proposed to affect food web stability, with weaker interactions increasing the persistence of species, and food webs as a whole. However, the mechanisms that modify interaction strengths, and their effects on food web persistence are not fully understood. Using food webs containing different combinations of predator, prey, and nonprey species, we investigated how predation risk of susceptible prey is affected by the presence of species not directly trophically linked to either predators or prey. We predicted that indirect alterations to the strength of trophic interactions translate to changes in persistence time of extinction-prone species. We assembled interaction webs of protist consumers and turbellarian predators with eight different combinations of prey, predators and nonprey species, and recorded abundances for over 130 prey generations. Persistence of predation-susceptible species was increased by the presence of nonprey. Furthermore, multiple nonprey species acted synergistically to increase prey persistence, such that persistence was greater than would be predicted from the dynamics of simpler food webs. We also found evidence suggesting increased food web complexity may weaken interspecific competition, increasing persistence of poorer competitors. Our results demonstrate that persistence times in complex food webs cannot be predicted from the dynamics of simplified systems, and that species not directly involved in consumptive interactions likely play key roles in maintaining persistence. Global species diversity is currently declining at an unprecedented rate and our findings reveal that concurrent loss of species that modify trophic interactions may have unpredictable consequences for food web stability