Robert L. Baker

Predator‐‐Induced Life History Changes: Antipredator Behavior Costs or Facultative Life History Shifts?

We used laboratory experiments to show that the nonlethal presence of pumpkinseed sunfish (Lepomis gibbosus) results in smaller size at emergence, decreased growth and development rates, and lower fecundity of the dipteran Chironomus tentans. Smaller size at metamorphosis is often viewed as a cost of antipredator behavior. However, it may also partly result from a facultative change in life history in which prey increase their development rate to escape their risky larval habitat. To determine the mechanism responsible for this smaller size at emergence, we compared development rates of chironomid larvae raised in the absence and nonlethal presence of pumpkinseed sunfish. When we statistically controlled for effects of predators on larval growth rate, fish presence had no effect on development rate of female chironomids, but significantly slowed male development. These results clearly indicate that C. tentans does not increase its development rate as a means of escaping a risky habitat, and that smaller size at emergence is best viewed as a cost of larvae behaviorally avoiding fish predators. Observed sex differences in life history responses to predators may have important consequences for fitness.

Effects of size on predation risk, behavioural response to fish, and cost of reduced feeding in larval Ischnura verticalis (Coenagrionidae: Odonata)

SummaryWe used laboratory studies to examine the role of predation risk and cost of anti-predator behaviour in determining the behavioural response of several larval instars of Ischnura verticalis to a fish predator (Lepomis gibbosus). Smaller larvae were less susceptible to fish predation than larger larvae. Smaller larvae depressed movement to a greater degree in the presence of fish than did larger larvae; large larvae were generally less active than small larvae regardless of fish presence. Reduced feeding resulted in smaller larvae suffering more in terms of reduced growth than did large larvae. In general, our results tend to support the hypothesis that individuals that suffer high costs of anti-predator behaviour but little risk of predation may only exhibit anti-predator behaviours in the presence of predators, whereas individuals with a higher risk of predation and a lower cost of anti-predator behaviour may evolve anti-predator mechanisms that are in effect even in the absence of predators.

Condition and fecundity of the damselfly, Enallagma ebrium (Hagen): the importance of ectoparasites

SummaryThe extent, magnitude, and cause of natural covariation between degree of parasitism and other variables known or suspected of influencing host fitness (such as host age or body size) has been understudied. We demonstrate that degree of parasitism by larval water mites (Arrenurus spp.) was associated with reduced condition of males and with lowered fecundity of young females of the damselfly, Enallagma ebrium (Hagen) (Odonata: Coenagrionidae). We also demonstrate that degree of parasitism can covary with both age and size of host damselflies. We explain the putative causes of such natural covariation, and we suggest that degree of parasitism, host age, and host size can all interact to determine damselfly fitness. We expect that natural covariation between the hosts phenotype and degree of parasitism will be frequently observed. Studies of such natural covariation will help researchers to assess better the importance of several variables on host reproductive success and to understand better the dynamics of host-parasite interactions.

ENVIRONMENTAL FACTORS INFLUENCING TREMATODE PREVALENCE IN GREY TREE FROG (HYLA VERSICOLOR) TADPOLES IN SOUTHERN ONTARIO

The emergence or increased prevalence of various parasites may be linked to alterations in host—parasite interactions caused by environmental changes. We investigated prevalence of trematode infections in grey tree frog (Hyla versicolor) tadpoles from ponds in nonagricultural settings versus ponds adjacent to cornfields in southern Ontario. We found that agricultural activity was a significant factor in determining the percentage of tadpoles infected by 1 or more trematodes from 1 or more species (combined trematode infection). However, we found no associations between combined trematode infection and forest cover; pond size; road density; and measures of water quality, such as nitrate level and the presence of the herbicide atrazine. Although combined trematode prevalence was associated only with agricultural activity, prevalence of Alaria species showed a positive association with forest cover. This latter result probably reflects the importance of habitat suitability for the canid definitive hosts of this trematode species.

Conflict between antipredator and antiparasite behaviour in larval damselflies

Abstract Larval damselflies resist infestation by parasitic larval mites by exhibiting behaviours such as grooming, crawling, swimming, and striking at host-seeking mites. Larval damselflies are known to increase time spent in these behaviours in the presence of mites but reduce time spent in these behaviours in the presence of fish predators. The presence of both fish and larval mites presents an obvious conflict: a larval damselfly may actively avoid parasitism by mites, thus increasing its risk of predation, or it may reduce its activity when fish are present, thus increasing its risk of parasitism. We analysed the behaviour of larval Ischnura verticalis in an experiment where we crossed presence and absence of fish with presence and absence of larval mites. Presence of mites induced a large increase in activity of larval I. verticalis but fish had no effect and there were no interpretable interactions between effects of mites and fish. Subsequent experiments indicated that larval I. verticalis in the presence of both mites and fish were more likely to be attacked and killed by fish than those exposed only to fish. The high activity level of I. verticalis larvae in the presence of both fish and mites may suggest that costs of parasitism are high, or that under field conditions it is rare for larvae to be in the immediate presence of both fish predators and potentially parasitic mites.

Frequency of female morphs is related to an index of male density in the damselfly, Nehalennia irene (Hagen)

Researchers have questioned whether variation in population density influences the maintenance of two or more morphs within and between insect populations. Two distinct morphs of female Nehalennia ...

Effect of body size and feeding on fecundity in the damselfly Ischnura verticalis (Odonata: Coenagrionidae)

We looked for relationships between number of eggs and the independent factors of food availability and body size in lab-reared females of the damselfly Ischnura verticalis. Multiple regression analysis revealed a strong positive relationship between number of eggs in the abdomen and both short term and long term food intake. There was no relationship between body size (wing length or head width) at maturity and number of eggs in the abdomen. Larval diet during the final instar affected development rate and adult size but not fecundity. Food consumed after emergence was negatively correlated with time to sexual maturity. Significantly more animals given low food diets died before maturity. Field data of gut weights suggest that animals in the field frequently have empty guts and we infer that food availability may be an important determinant of realized fecundity. Failure to find food on any one day will have consequences, not only for clutches laid the next day, but also for subsequent clutches, and finally for probability of surviving to the next day.

Contaminant effects on host‐parasite interactions: atrazine, frogs, and trematodes

The effects of contaminants on multispecies interactions can be difficult to predict. The herbicide atrazine is commonly used in North America for corn crops, runs off into wetlands, and has been implicated in the increasing susceptibility of larval frogs to trematode parasites. Using experimental challenges with free-living stages of trematodes (cercariae), it was found that Rana sylvatica tadpoles exposed to 30 microg/L of atrazine had significantly higher intensity of parasitism than did larval frogs either not exposed or exposed to 3 microg/L of atrazine. This result could not be explained by high concentrations of atrazine diminishing antiparasite behavior of tadpoles. Furthermore, when tadpoles and cercariae both were exposed to the same concentration of atrazine, either 3 or 30 microg/L, the abundance of formed cysts was not different from the condition in which both were housed at 0 microg/L of atrazine. Atrazine appears to be debilitating to both free-living cercariae and tadpoles. Studies examining relations between parasitism and contaminant levels must account for such combined effects as well as influences on other interacting species (e.g., first intermediate snail hosts).

EFFECTS OF ATRAZINE ON CERCARIAL LONGEVITY, ACTIVITY, AND INFECTIVITY

Susceptibility of free-living infective stages of parasites to contaminants is relatively understudied compared with independent effects on measures of host health or immunity, but may be important in affecting prevalence and intensity of parasite infections. We investigated whether atrazine, an herbicide commonly used in North America, affected the cercariae of 4 different species of digenetic trematodes, and found that effects of atrazine concentration on mortality and activity of cercariae varied among species. Mortality of Echinostoma trivolvis increased in a 200 μg/L atrazine solution, and a species of Alaria showed both decreased activity and increased mortality. We also examined whether the ability of E. trivolvis to infect the second intermediate host, larval amphibians, was compromised by atrazine exposure. Longevity and prevalence of E. trivolvis cercariae was affected at 200 μg/L atrazine, whereas intensity of infection in Rana clamitans tadpoles was reduced at both 20 μg/L and 200 μg/L atrazine. Our results indicate that the viability of cercariae of some species is compromised by exposure to atrazine, emphasizing the importance of considering the influence of contaminants on free-living stages of parasites in addressing how environmental degradation may relate to host parasitism.

Susceptibility to parasitism: experiments with the damselfly Enallagma ebrium (Odonata: Coenagrionidae) and larval water mites, Arrenurus spp. (Acari: Arrenuridae).

The objectives of this study were to determine whether damselfly larvae actively defend against colonization by larval water mites and to assess what effects developmental stage mass and condtion of damselflies have on their likelihood of being heavily parasitized