Browsing by Author "Ohm, Johanna R."

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    A comparative approach to testing hypotheses for the evolution of sex-biased dispersal in bean beetles
    (Wiley, 2015) Downey, Michelle H.; Searle, Rebecca; Bellur, Sunil; Geiger, Adam; Maitner, Brian S.; Ohm, Johanna R.; Tuda, Midori; Miller, Tom E.X.
    Understanding the selective forces that shape dispersal strategies is a fundamental goal of evolutionary ecology and is increasingly important in changing, human-altered environments. Sex-biased dispersal (SBD) is common in dioecious taxa, and understanding variation in the direction and magnitude of SBD across taxa has been a persistent challenge. We took a comparative, laboratory-based approach using 16 groups (species or strains) of bean beetles (generaﾠAcanthoscelides,ﾠCallosobruchus, andﾠZabrotes, including 10 strains of one species) to test two predictions that emerge from dominant hypotheses for the evolution of SBD: (1) groups that suffer greater costs of inbreeding should exhibit greater SBD in favor of either sex (inbreeding avoidance hypothesis) and (2) groups with stronger local mate competition should exhibit greater male bias in dispersal (kin competition avoidance hypothesis). We used laboratory experiments to quantify SBD in crawling dispersal, the fitness effects of inbreeding, and the degree of polygyny (number of female mates per male), a proxy for local mate competition. While we found that both polygyny and male-biased dispersal were common across bean beetle groups, consistent with the kin competition avoidance hypothesis, quantitative relationships between trait values did not support the predictions. Across groups, there was no significant association between SBD and effects of inbreeding nor SBD and degree of polygyny, using either raw values or phylogenetically independent contrasts. We discuss possible limitations of our experimental approach for detecting the predicted relationships, as well as reasons why single-factor hypotheses may be too simplistic to explain the evolution of SBD.
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    Balancing anti-herbivore benefits and anti-pollinator costs of defensive mutualists
    (Ecological Society of America, 2014) Ohm, Johanna R.; Miller, Tom E.X.
    Quantifying costs and benefits of ostensibly mutualistic interactions is an important step toward understanding their evolutionary trajectories. In food-for-protection interactions between ants and extrafloral nectar (EFN)-bearing plants, tending by aggressive ants may deter herbivores, but it may also deter pollinators. The fitness costs of pollinator deterrence are not straightforward for long-lived iteroparous plants, because reproductive vital rates often contribute weakly to fitness relative to growth and survival (vital rates that may be enhanced by ant defense). We used field manipulations of ant and pollinator activity and demographic modeling to examine how pollination costs of ant defense translate to plant fitness, given the benefits of ant defense elsewhere in the plant life cycle. We contrasted the net fitness effects of alternative ant partner species. Our work focused on the tree cholla cactus, Opuntia imbricata, an EFN-bearing plant associated with two ant species (Crematogaster opuntiae and Liometopum apiculatum) that differ in quality of defense against insect herbivores. We found that ant defense imposed pollination costs, despite evidence for ant-repellent floral volatiles and temporal partitioning of ant and pollinator activity. The two partner species similarly reduced pollinator visitation and seed mass, and one (C. opuntiae) additionally reduced seed number. We used the experimental data and other long-term demographic data to parameterize an integral projection model that integrated costs and benefits of ant defense over the complete plant life cycle. Model results indicated that the pollination costs of L. apiculatum were balanced by beneficial effects on growth, leading to a net fitness effect that was neutral to positive. By contrast, pollination costs outweighed benefits for C. opuntiae, the weaker defender, rendering this species a reproductive parasite. Thus, we infer that pollination costs destabilize mutualism with one partner species, but are offset by strong defensive benefits provided by the other, leading to contrasting selective pressures imposed by alternative associations. Accounting for ontogenetic turnover in ant partner identity indicated that most plants avoid the parasitic effects of C. opuntiae by associating nonrandomly with L. apiculatum at reproductive life stages. Our results highlight the value of a demographic approach to quantifying the costs and benefits of mutualism.