From trees to fleas: masting indirectly affects flea abundance on a rodent host

Mast seeding causes strongfluctuations in populations of forest animals.Thus,this phenomenon can be used as a natural experiment to examine how variation in host abundance affects parasite loads.We investigatedfleas infesting yellow-necked mice in beech forest after 2 mast and 2 non-mast years.We tested 2 mutually exclusive scenarios:(1)as predicted by classical models of density-dependent transmission,an increase in host density will cause an increase in ectoparasite abundance(defined as the number of parasites per host),versus(2)an increase in host density will cause a decline inflea abundance(“dilution,”which is thought to occur when parasite population growth is slower than that of the host).In addition,we assessed whether masting alters the relationship between host traits(sex and body mass)andflea abundance.We found a hump-shaped relationship between host andflea abundance.Thus,the most basic predictions are too simple to describe ectoparasite dynamics in this system.In addition,masting modified seasonal dynamics offlea abundance,but did not affect the relationship between host traits andflea abundance(individuals with the highest body mass hosted the mostfleas;after controlling for body mass,parasite abundance did not vary between sexes).Our results demonstrate that pulses of tree reproduction can indirectly,through changes in host densities,drive patterns of ectoparasite infestation.