Frank Johansson

Life history plasticity in a damselfly : Effects of combined time and biotic constraints

Optimal values for life history traits are expected to depend upon environ- mental conditions during development and the period within which development is con- strained (e.g., biotic factors and time constraints, respectively). Theory predicts that life history responses to both biotic factors and time constraints may be both direct and be- haviorally mediated. Few experimental studies of life histories have considered the joint effects of biotic factors and time constraints, and fewer still have been able to disentangle direct from behaviorally mediated effects. We studied such interactions by manipulating the perceived time to the onset of winter, predation risk, and food resources level in larvae of the damselfly Lestes sponsa. In the first experiment (predation 3 time constraint), the presence of a predator caused an overall reduction in foraging activity, development rate, and mass at emergence. However, larvae that had less time available before the end of the season, increased foraging activity and development rate, while mass at emergence de- creased. These results suggest that the observed changes in life history characters were behaviorally mediated in the presence of predators. In contrast, life history responses of time-constrained larvae occurred independently of the behavioral changes and, therefore, were direct. In the second experiment (food level 3 time constraint) larvae under high food levels had a higher foraging activity, increased development rate, and higher growth rates, compared to low food-level treatments. Time-constrained larvae accelerated devel- opment and had a smaller mass at emergence at high food levels than larvae that were not time constrained. In contrast, and opposite to predictions, time-constrained larvae at low food levels had the slowest development rate and the largest mass at emergence. We suggest that larvae in the latter group were aiming to delay emergence to the next season (cohort splitting). Our results suggest that both behaviorally mediated and direct responses to biotic factors and time constraints are a feature of the life history of this damselfly.

The degree of adaptive phenotypic plasticity is correlated with the spatial environmental heterogeneity experienced by island populations of Rana temporaria.

Although theoretical models have identified environmental heterogeneity as a prerequisite for the evolution of adaptive plasticity, this relationship has not yet been demonstrated experimentally. Because of pool desiccation risk, adaptation of development rate is important for many amphibians. In a simulated pool‐drying experiment, we compared the development time and phenotypic plasticity in development time of populations of the common frog Rana temporaria, originating from 14 neighbouring islands off the coast of northern Sweden. Drying regime of pools used by frogs for breeding differed within and among the islands. We found that the degree of phenotypic plasticity in development time was positively correlated with the spatial variation in the pool‐drying regimes present on each island. In addition, local adaptation in development time to the mean drying rate of the pools on each island was found. Hence, our study demonstrates the connection between environmental heterogeneity and developmental plasticity at the island population level, and also highlights the importance of the interplay between local specialization and phenotypic plasticity depending on the local selection pressures.

CONFLICTING SELECTION PRESSURES ON THE GROWTH/PREDATION‐RISK TRADE‐OFF IN A DAMSELFLY

Activity is an important behavioral trait that in most animals mediates a trade-off between obtaining food for growth and avoiding predation. Active individuals usually experience a higher encounte ...

ONTOGENETIC REACTION NORMS OF PREDATOR-INDUCED DEFENSIVE MORPHOLOGY IN DRAGONFLY LARVAE

The study of phenotypic plasticity, one of the most important mechanisms of phenotypic adaptation, is by tradition focused on differences in ontogenetically static phenotypic expression in different environments. Ontogenetic reaction norms, in contrast, describe how phenotypes unfold during growth in different environments. We studied the ontogenetic reaction norms of the morphological shape of a series of defensive abdominal spines in dragonfly larvae, both in the laboratory and in a number of natural populations. In a laboratory rearing experiment, we demonstrated that these spines grew more solid and elongated when waterborne environmental cues of fish predators were present: this is evidence of phenotypic plasticity in defensive spine morphology. The ontogenetic reaction norms of defensive spines were also found to differ in natural populations with and without fish. A detailed analysis of the growth trajectories showed that this differentiation was primarily due to ontogenetic acceleration in environments with fish, leading to relatively exaggerated spine shape in these environments. However, while the ontogenetic trajectories of shape in some spines diverged at the onset of ontogeny in the two environments, those of others remained parallel until a given phase of ontogeny. Hence, the timing of the developmental divergence of these phenotypically integrated traits differed, suggesting differences in the underlying regulatory mechanisms. Our results illustrate that a conceptual integration of environmental and ontogenetic approaches to the study of phenotypic differentiation can significantly promote our understanding of the ecology and evolution of adaptive phenotypic plasticity.

GENE FLOW AND SELECTION ON PHENOTYPIC PLASTICITY IN AN ISLAND SYSTEM OF RANA TEMPORARIA

Gene flow is often considered to be one of the main factors that constrains local adaptation in a heterogeneous environment. However, gene flow may also lead to the evolution of phenotypic plasticity. We investigated the effect of gene flow on local adaptation and phenotypic plasticity in development time in island populations of the common frog Rana temporaria which breed in pools that differ in drying regimes. This was done by investigating associations between traits (measured in a common garden experiment) and selective factors (pool drying regimes and gene flow from other populations inhabiting different environments) by regression analyses and by comparing pairwise FST values (obtained from microsatellite analyses) with pairwise QST values. We found that the degree of phenotypic plasticity was positively correlated with gene flow from other populations inhabiting different environments (among‐island environmental heterogeneity), as well as with local environmental heterogeneity within each population. Furthermore, local adaptation, manifested in the correlation between development time and the degree of pool drying on the islands, appears to have been caused by divergent selection pressures. The local adaptation in development time and phenotypic plasticity is quite remarkable, because the populations are young (less than 300 generations) and substantial gene flow is present among islands.

Behaviour and growth of dragonfly larvae along a permanent to temporary water habitat gradient

Abstract.  1. Freshwaters form a gradient from small temporary waters to large permanent waters. Identifying and examining traits that restrict the distribution of species along this gradient are crucial to the understanding of community structure in these habitats.

Generalists and specialists along a latitudinal transect: patterns of thermal adaptation in six species of damselflies

Tropical organisms colonizing temperate environments face reduced average temperatures and dramatic thermal fluctuations. Theoretical models postulate that thermal specialization should be favored either when little environmental variation is experienced within generations or when among-generation variation is small relative to within-generation variation. To test these predictions, we studied six temperate species of damselflies differing in latitudinal distribution. We developed a computer model simulating how organisms experience environmental variation (accounting for diapause and voltinism) and performed a laboratory experiment assaying thermal sensitivities of growth rates. The computer model showed opposing latitudinal trends in among- and within-generation thermal variability: within-generation thermal variability decreased toward higher latitudes, whereas relative levels of among-generation thermal variability peaked at midlatitudes (where a shift in voltinism occurred). The growth experiment showed that low-latitude species were more thermally generalized than mid- and high-latitude species, supporting the prediction that generalists are favored under high levels of within-generation variation. Northern species had steeper, near-exponential reaction norms suggestive of thermal specialization. However, they had strikingly high thermal optima and grew very slowly over most of the thermal range they are expected to experience in the field. This observation is at present difficult to explain. These results highlight the importance of considering interactions between life history and environmental variation when deriving expectations of thermal adaptation.

Interactions between predator- and diet-induced phenotypic changes in body shape of crucian carp

Predator cues and diet, when studied separately, have been shown to affect body shape of organisms. Previous studies show that the morphological responses to predator absence/presence and diet may be similar, and hence could confound the interpretation of the causes of morphological differences found between groups of individuals. In this study, we simultaneously examined the effect of these two factors on body shape and performance in crucian carp in a laboratory experiment. Crucian carp (Carassius carassius) developed a shallow body shape when feeding on zooplankton prey and a deep body shape when feeding on benthic chironomids. In addition, the presence of chemical cues from a pike predator affected body shape, where a shallow body shape was developed in the absence of pike and a deep body shape was developed in the presence of pike. Foraging activity was low in the presence of pike cues and when chironomids were given as prey. Our results thereby suggest that the change in body shape could be indirectly mediated through differences in foraging activity. Finally, the induced body shape changes affected the foraging efficiency, where crucians raised on a zooplankton diet or in the absence of pike cues had a higher foraging success on zooplankton compared to crucian raised on a chironomid diet or in the presence of pike. These results suggest that body changes in response to predators can be associated with a cost, in terms of competition for resources.

Fish-induced variation in abdominal spine length of Leucorrhinia dubia (Odonata) larvae?

The lengths of dorsal and lateral spines on abdominal segments 4, 6 and 9 were measured on last-instar larvae of Leucorrhinia dubia (Odonata) from seven natural lentic systems containing fish and nine systems lacking fish. Larvae from systems with fish had significantly longer spines than larvae from systems without fish. In contrast, lake/pond area and pH had no effect on the length of spines. The length of the spines was not correlated with larval size, but there was a high correlation between the length of the three spines measured. Also, abundances of L. dubia larvae differed between systems, being significantly lower in systems with fish. Laboratory experiments showed that perch (Perca fluviatilis) handled long-spined larvae for significantly longer times than short-spined larvae. The results suggest that fish predators may induce morphological defences in insects.

INVERTEBRATE PREDATION SELECTS FOR THE LOSS OF A MORPHOLOGICAL ANTIPREDATOR TRAIT

Abstract Antagonistic selection by different predators has been suggested to underlie variation in morphological antipredator traits among and within species. Direct empirical proof is equivocal, however, given the potential interrelationships of morphological and behavioral traits. Here, we tested whether spines in larvae of the dragonfly Leucorrhinia caudalis, which are selected for by fish predators, are selected against by invertebrate aeshnid predators. Using a manipulative approach by cutting spines instead of making comparisons among species or inducing spines, we were able to decouple the presence of spines from other potentially covarying morphological antipredator traits. Results showed survival selection for the loss of spines imposed by invertebrate predation. Moreover, spined and nonspined larval L. caudalis did not differ in the key antipredator behaviors, activity level, and escape burst swimming speed. The observed higher mortality of spined larvae can therefore be directly linked to selection by aeshnid predation against spines.