Abstract
The tick Ixodes ricinus finds its hosts by climbing vegetation and adopting a sit-and-wait tactic. This “questing” behaviour is known to be temperature-dependent, such that questing increases with temperature up to a point where the vapor pressure deficit (drying effect) forces ticks down to rehydrate in the soil or mat layer. Little if any attention has been paid to understanding the questing of ticks from an evolutionary perspective. Here we ask whether populations from colder climatic conditions respond differently in terms of the threshold temperature for questing and the rate of response to a fixed temperature. We find significant variation between populations in the temperature sensitivity of questing, with populations from cooler climates starting questing at lower temperatures than populations from warmer temperatures. Cool climate populations also quest sooner when the temperature is held constant. These patterns are consistent with local adaptation to temperature either through direct selection or acclimation and challenge the use of fixed thresholds for questing in modeling the spread of tick populations. Our results also show how both time and temperature play a role in questing, but we are unable to explain the relationship in terms of degree-time used to model Arthropod development. We find that questing in response to temperature fits well with a quantitative genetic model of the conditional strategy, which reveals how selection on questing may operate and hence may be of value in understanding the evolutionary ecology of questing.
Highlights
While understanding the ecology of pathogen vectors is vital to increasing our ability to forecast changes in disease risk, changes in range and prevalence in response to changes in climate will involve evolution [1]
Vector-borne diseases are likely to be impacted by climate change, since the Arthropod vectors are strongly affected by temperature and climatic conditions [2,3]
We investigate the hypothesis that time, as well as the temperature, above a metabolically significant threshold temperature combine to explain variation in questing, with the prediction that patterns of questing in constant temperatures and those in increasing temperatures can be reconciled by combining time and temperature with the ‘degree time’ formula used to model Arthropod development
Summary
While understanding the ecology of pathogen vectors is vital to increasing our ability to forecast changes in disease risk, changes in range and prevalence in response to changes in climate will involve evolution [1]. The effects of changing climate on the distribution and prevalence of disease is a major concern both for the health of human populations and the health and persistence of populations of domestic and wild animals and plants [4,5,6,7,8]. The expansion of I. ricinus’ European range in recent decades to higher latitudes and altitudes, concomitant with increases in its population density [9,10], and increases in reported cases of tick-borne diseases in some areas [11], have been partly attributed to changes in climate. There is speculation and some concern that climate change already has influenced, and will continue to influence, I. ricinus and the epidemiology of the pathogens it transmits [2,3,12]
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