Todd P. Livdahl

Non-Linear Interactions of Populations and the Importance of Estimating Per Capita Rates of Change

On teste la valeur de modeles simples lineaires de productivite de la population (application a Aedes triseriatus)

The community ecology of Aedes egg hatching: implications for a mosquito invasion

Abstract. 1. A recently introduced treehole mosquito from Asia, Aedes albopictus, is spreading throughout eastern North America, especially in tyre‐refuse piles. Previous studies have identified inhibitory effects of larvae on egg hatch as a potential population regulatory mechanism within Aedes. Larva‐egg interactions may also occur between species. This experiment assesses the ability of larvae of A. albopictus and two possible competitors in North America, A.triseriatus and A.aegypti, to suppress hatching of conspecific and congeneric eggs.

Aedes aegypti and Aedes albopictus in Bermuda: extinction, invasion, invasion and extinction

We provide an analysis of the invasion and spread of the container inhabiting mosquitoes Aedes aegypti and Aedes albopictus in the Bermuda Islands. Considered eradicated in the mid-1960s, A. aegypti was redetected in 1997, and A. albopictus was first detected in 2000. Based on weekly ovitrap data collected during the early stages of the invasion, we mapped the spread of Aedes throughout the islands. We analyzed the effects of buildings and roads on mosquito density and found a significant association between density and distance to roads, but not to buildings. We discuss the potential role of human transport in the rapid spread in the islands. The temporal correlation in ovitrap collection values decreased progressively, suggesting that habitat degradation due to control efforts were responsible for local shifts in mosquito densities. We report a sharp decrease in A. aegypti presence and abundance after the arrival of A. albopictus in the year 2000. Possible mechanisms for this rapid decline at relatively low density of the second invader are discussed in the context of classical competition theory and earlier experimental results from Florida, as well as alternative explanations. We suggest that support for the competition hypothesis to account for the decline of A. aegypti is ambiguous and likely to be an incomplete explanation.

The complex hatching response of Aedes eggs to larval density

Abstract. 1. We investigated the effects of hatching medium, larval density and larval instar on egg hatching in Aedes triseriutus (Say) (Diptera: Cuficidae).

Evaluation of species distribution model algorithms for fine-scale container-breeding mosquito risk prediction

The present work evaluates the use of species distribution model (SDM) algorithms to classify high densities of small container‐breeding Aedes mosquitoes (Diptera: Culicidae) on a fine scale in the Bermuda Islands. Weekly ovitrap data collected by the Department of Health, Bermuda for the years 2006 and 2007 were used for the models. The models evaluated included the algorithms Bioclim, Domain, GARP (genetic algorithm for rule‐set prediction), logistic regression and MaxEnt (maximum entropy). Models were evaluated according to performance and robustness. The area under the receiver operating characteristic curve was used to evaluate each models performance, and robustness was assessed according to the spatial correlation between classification risks for the two datasets. Relative to the other algorithms, logistic regression was the best and MaxEnt the second best model for classifying high‐risk areas. We describe the importance of covariables for these two models and discuss the utility of SDMs in vector control efforts and the potential for the development of scripts that automate the task of creating risk assessment maps.

Egg hatching inhibition: field evidence for population regulation in a treehole mosquito

ABSTRACT. 1. We investigated the influence of larval density on egg hatching in the treehole mosquito, Aedes triseriatus Say (Diptera: Culicidae) by submerging laboratory‐produced egg batches in natural habitats stocked with fixed densities of larvae.

EVOLUTION OF HANDLING TIME: THE FUNCTIONAL RESPONSE OF A PREDATOR TO THE DENSITY OF SYMPATRIC AND ALLOPATRIC STRAINS OF PREY

A large body of theory regarding the evolution of life history parameters in populations has been developed with emphasis on the importance of interspecific interactions and environmental uncertainty (see Stearns, 1976, for a recent review of the ideas, and Gill, 1974). Strangely, although the development of theoretical models of predator and prey relationships has been equally elaborate, predator-related parameters as evolved characteristics of prey life history have yet to be included into the array of hypotheses concerning life history evolution. This omission becomes more glaring when the documentation of the evolutionary importance of predation is noted in comparison with evidence concerning resource competition as a selective agent. Adaptation to predation may occur in numerous ways (e.g., cryptic coloration, mimicry, unpalatability, evasiveness, indigestibility, defensive tactics, structures or chemicals), and the evolution of a particular tactic will ultimately influence either the rate at which the prey are captured or the time required for ingestion. This paper describes an experiment designed to assess the evolutionary impact, if any, of a predator on its functional response relationship with its prey. To do this, I present estimates of the functional response parameters for predation on sympatric and allopatric strains of prey, i.e., prey which have evolved with and without the predator. The most frequently used description of experimental functional response relationships is the disc equation of HoIling (1959):

Cannibalism among Aedes triseriatus larvae

Interactions within populations of Aedes triseriatus (Say), a treehole mosquito, are confounded by a phenomenon known as ‘erratic or instalrnent hatching’. in which only a fraction of the eggs of the same age hatches in response to a given stimulus (Breeland & Pickard, 1967; Wilson & Horsfall, 1970; Gillett, 19S5a, b , 1972; Gillett et d., 1977). Such temporal staggering makes interactions likely between hatching cohorts of different developmental stages. Livdah (1Y82) inspected the intraspecific interactions between two larval cohorts with a field container experiment performed in natural treehole fluid. H e showed that the earlier larvae impose a strong adverse effect on a later cohort, which was hatched and added 7 days after the establishment of the experiment. This adverse effect was not reciprocated by the late cohort. O n the contrary. the addition of a second cohort buffered the first cohort‘s female development time against increasing density and relative shortages of food. In addition, at the intermediate density and low food level (forty larvae per 250ml diluted fluid) the survivorship to adulthood of the earlier larvae was enhanced by the addition of the new cohort. H e suggested that facultative cannibalism might provide a simple mechanism for these higher order interactions. This paper provides evidence that cannibalism is possible in Aedes triseriatus. Our work-

Environmental effects on bet hedging in Aedes mosquito egg hatch

We used a path analysis procedure to examine the influence of environmental effects on the egg hatching response of the container breeding mosquito, Aedes triseriatus. A. triseriatus eggs were collected from 22 different sites across the eastern US, and exposed to repeated hatch stimuli in the laboratory. The resulting data were used to construct hatch indexes for each site. Structural equation modeling was used to discriminate among hypotheses relating to the functional relationships between population hatch trait and local climatic conditions. The results suggest that the delayed hatch pattern is an adaptive bet-hedging strategy that allows the species to manage desiccation risks. The selected model indicates that environmental variables differentially affect the immediate and the delayed, long term hatch patterns. High temperatures directly reduce the number of egg hatching on the first stimulus, but only indirectly affect the delayed hatch pattern. Low precipitation and high variability in precipitation directly increase the delaying pattern. The hatch trait appears to be a critical adaptation that allows the species to occupy a broad range in east North America.

Environmental Uncertainty and Selection for Life Cycle Delays in Opportunistic Species

A model is presented which considers genetic variation in the duration of egg dormancy in populations occupying larval habitats that occasionally become entirely unsuitable. The probability of such a catastrophe and the net reproductive rate in the absence of catastrophes (and in density independent conditions) determine the conditions required for fixation of either the early- or late-hatching allele. The model shows that a more catastrophic environment will favor the late-hatching allele, presenting the possibility for a decline in rm with an increase in environmental uncertainty. The implications of this result to the concept of r-selection are discussed, and examples of systems where the model could apply are presented.