VARIABLE SELECTION ON EUROSTA' S GALL SIZE. II. A PATH ANALYSIS OF THE ECOLOGICAL FACTORS BEHIND SELECTION

Urban socioeconomic variation influences the ecology and evolution of trophic interactions

As urbanization expands, it is becoming increasingly important to understand how anthropogenic activity is affecting ecological and evolutionary processes. Few studies have examined how human social patterns within cities can modify eco-evolutionary dynamics. We tested how socioeconomic variation corresponds with changes in trophic interactions and natural selection on prey phenotypes using the classic interaction between goldenrod gall flies (Eurosta solidaginis) and their natural enemies: birds, beetles, and parasitoid wasps. We sampled galls from 84 sites across neighbourhoods with varying socioeconomic levels, and quantified the frequency of predation/parasitism on flies and natural selection by each enemy. We found that bird predation was higher in the highest income neighbourhoods, increasing the strength of selection for smaller galls. Wasp and beetle attack, but not their strength of selection, increased in lower income neighbourhoods. We show that socioeconomic variation in cities can have strong unintended consequences for the ecology and evolution of trophic interactions.

Trade-off between fecundity and survival generates stabilizing selection on gall size

Complex interactions within multitrophic communities are fundamental to the evolution of individual species that reside within them. One common outcome of species interactions are fitness trade-offs, where traits adaptive in some circumstances are maladaptive in others. Here, we identify a fitness trade-off between fecundity and survival in the cynipid wasp Callirhytis quercusbatatoides that induces multichambered galls on the stem of its host plant Quercus virginiana. We first quantified this trade-off in natural populations by documenting two relationships: a positive association between the trait gall size and fecundity, as larger galls contain more offspring, and a negative association between gall size and survival, as larger galls are attacked by birds at a higher rate. Next, we performed a field-based experimental evolution study where birds were excluded from the entire canopy of 11 large host trees for five years. As a result of the five-year release from avian predators, we observed a significant shift to larger galls per tree. Overall, our study demonstrates how two opposing forces of selection can generate stabilizing selection on a critical phenotypic trait in wild populations, and how traits can evolve rapidly in the predicted direction when conditions change.

Ecological rigidity and the hardness of selection in the wild

Hutchinson's ecological theater and evolutionary play is a classical view of evolutionary ecology-ecology provides a template in which evolution occurs. An opposing view is that ecological and evolutionary changes are like two actors on a stage, intertwined by density and frequency dependence. These opposing views correspond to hard and soft selection, respectively. Although often presented as diametrically opposed, both types of selection can occur simultaneously, yet we largely lack knowledge of the relative importance of hard versus soft selection in the wild. I use a dataset of 3000 individual gall makers from 15 wild local populations over 5 years to investigate the hardness of selection. I show that enemy attack consistently favors some gall sizes over others (hard selection) but that these biases can be fine-tuned by density and frequency dependence (soft selection). As a result, selection is hard and soft in roughly equal measures, but the importance of each type varies as species interactions shift. I conclude that eco-evolutionary dynamics should occur when a mix of hard and soft selection acts on a population. This work contributes to the rapprochement of disparate views of evolutionary ecology-ecology is neither a rigid theater nor a flexible actor, but instead embodies components of both.

Individual and Population Differences Shape Species Interactions and Natural Selection

Trait variation is central to our understanding of species interactions, and trait variation arising within species is increasingly recognized as an important component of community ecology. Ecologists generally consider intraspecific variation either among or within populations, yet these differences can interact to create patterns of species interactions. These differences can also affect species interactions by altering processes occurring at distinct scales. Specifically, intraspecific variation may shape species interactions simply by shifting a population's position along a trait-function map or by shifting the relationship between traits and their ecological function. I test these ideas by manipulating within- and among-population intraspecific variation in wild populations of a gall-forming insect before quantifying species interactions and phenotypic selection. Within- and among-population differences in gall size interact to affect attack rates by an enemy community, but among-population differences were far more consequential. Intraspecific differences shaped species interactions by both shifting the position of populations along the trait-function map and altering the relationship between traits and their function, with ultimate consequences for patterns of natural selection. I suggest that intraspecific variation can affect communities and natural selection by acting through individual- and population-level mechanisms.

Indirect Interactions Shape Selection in a Multispecies Food Web

Species do not live, interact, or evolve in isolation but are instead members of complex ecological communities. In ecological terms, complex multispecies interactions can be understood by considering indirect effects that are mediated by changes in traits and abundances of intermediate species. Interestingly, traits and abundances are also central to our understanding of phenotypic selection, suggesting that indirect effects may be extended to understand evolution in complex communities. Here we explore indirect ecological effects and their evolutionary corollary in a well-understood food web comprising a plant, its herbivores, and enemies that select for opposite defensive phenotypes in one of the herbivores. We show that ecological indirect interactions are mediated by changes to both the traits and the abundances of intermediate species and that these changes ultimately reduce enemy attack and weaken selection. We discuss the generality of the link between indirect effects and selection. We go on to argue that local adaptation and eco-evolutionary feedback may be less likely in complex multispecies food webs than in simpler food chains (e.g., coevolution). Overall, considering selection in complex interaction networks can facilitate the rapprochement of community ecology and evolution.

Predation on rose galls: parasitoids and predators determine gall size through directional selection

Both predators and parasitoids can have significant effects on species’ life history traits, such as longevity or clutch size. In the case of gall inducers, sporadically there is evidence to suggest that both vertebrate predation and insect parasitoid attack may shape the optimal gall size. While the effects of parasitoids have been studied in detail, the influence of vertebrate predation is less well-investigated. To better understand this aspect of gall size evolution, we studied vertebrate predation on galls of Diplolepis rosae on rose (Rosa canina) shrubs. We measured predation frequency, predation incidence, and predation rate in a large-scale observational field study, as well as an experimental field study. Our combined results suggest that, similarly to parasitoids, vertebrate predation makes a considerable contribution to mortality of gall inducer larvae. On the other hand, its influence on gall size is in direct contrast to the effect of parasitoids, as frequency of vertebrate predation increases with gall size. This suggests that the balance between predation and parasitoid attack shapes the optimal size of D. rosae galls.