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Sources of variation in larval survival, growth and development rates and their consequences for adult survival and reproductive success in Enallagma boreale Selys (Odonata: Coenagrionidae) Anholt, Bradley Ralph


The sources of variation in larval survival, growth, and development rates and their consequences for adult survival and reproductive success of the coenagrionid damselfly Enallagma boreale Selys were examined in two years with contrasting weather. The effect of larval density, food availability, and interference competition was studied in field enclosures. Habitat complexity was manipulated using artificial macrophytes to decouple exploitative and interference competition. At high larval densities or low habitat complexity, larvae were more evenly spaced among the artificial macrophytes than expected if they were distributed independently of each other. Lower survival, growth and development rates were exhibited by larvae that experienced high density and low food availability. There was no effect of the habitat complexity manipulation on these rates. Interference competition therefore has a low cost. Adults that had been experimentally manipulated as larvae were individually marked to assess whether larval conditions affected adult survival and reproductive success. Females had lower survival to sexual maturity than males. Females increased their mass by nearly 30 percent independent of their mass at emergence. Small males increased their mass more than large males but the mean mass gain was less than five percent of mass at emergence. Female survival to reproductive maturity was independent of size at emergence in both years. Large males had better survival in 1985, a warm dry year, but there was no relationship in 1986, a cool wet year. Male survival was independent of date of emergence in 1985, but late emerging males had better survival in 1986 (after the weather had improved). Early emerging females survived better in 1985, but late emerging females did better in 1986. Lower survival of females and small males suggests that mortality is correlated with mass gain in the pre-reproductive period, possibly due to increased risk of predation while foraging. The target of selection may not be body size but behaviour which is correlated with body size. Increased mass due to foraging almost certainly contributes to reproductive success of females by providing resources for eggs. Ignoring the mortality associated with this increase in mass underestimates the variance in reproductive success of females. While large males may have had better survival, of those males that did survive, small males had higher reproductive success. Reproductive success was independent of size at or date of emergence for females in both 1985 and 1986.

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