James R. Ott

A trade-off between flight capability and reproduction in males of a wing-dimorphic insect.

The widespread occurrence of dispersal polymorphisms among insects is consistent with the hypothesis that fitness costs are associated with flight capability. Al- though trade-offs between flight capability and reproduction are well documented in the females of many wing-polymorphic insect species, the relationship between dispersal ca- pability and reproductive success in males is poorly established. Here we examine the potential cost of flight capability in males of the salt-marsh-inhabiting planthopper Pro- kelisia dolus (Hemiptera: Delphacidae). This species exhibits a dispersal polymorphism with both flightless adults (brachypters) and migratory adults (macropters) occurring in populations. In a competitive setting in the laboratory, brachypterous males exhibited a threefold mating advantage over macropterous males; they obtained 77% of the matings with bra- chypterous females. This mating advantage resulted, in part, from brachypterous males aggressively displacing rival macropters during courtship. There was also a nonsignificant tendency for brachypterous males to arrive before macropterous males to court a calling female. Neither female rejection behavior nor male body size appeared to contribute to the mating advantage of brachypterous males. When macropterous females were contested, the mating advantage of the male wing forms changed and macropterous males obtained the majority of matings. Thus, there was evidence for assortative mating based on wing form. When placed with ten brachypterous females, brachypterous males sired twice as many offspring as did macropterous males. This advantage was due to brachypters siring more offspring per female and not from inseminating more females; hence differences in sperm load between the male wing forms are implicated in the siring advantage of brachypters. There was, however, no tendency for brachypterous males to survive longer than their macropterous counterparts; thus, differences in longevity did not contribute to enhanced siring ability. Overall, these results provide support for a trade-off between dispersal ca- pability and reproductive success in males.

HOST PLANT QUALITY AND LOCAL ADAPTATION DETERMINE THE DISTRIBUTION OF A GALL-FORMING HERBIVORE

Herein we report results of transplant experiments that link variation in host plant quality to herbivore fitness at the local scale (among adjacent plants) with the process of local (demic) adaptation at the landscape scale to explain the observed distribution of the specialist gall former Belonocnema treatae (Hymenoptera: Cynipidae) within populations of its host plant, Quercus fusiformis. Field surveys show that leaf gall densities vary by orders of magnitude among adjacent trees and that high-gall-density trees are both rare (< 5%) and patchily distributed. B. treatae from each of five high-gall-density trees were reared on (1) the four nearest low-gall-density trees, (2) the four alternative high-gall-density trees, and (3) their natal trees (control). Each treatment (source X rearing site) was replicated three times. Nine components of performance that sequentially contribute to fitness were evaluated with over 21000 galls censused across the 25 experimental trees. When reared on their natal trees and compared with low-gall-density neighbors, transplanted gall formers had higher gall initiation success (P < 0.05), produced more (P < 0.001) and larger galls (P < 0.001), and produced a higher proportion of galls that exceeded the threshold size for natural enemy avoidance (P < 0.05). Comparison of gall-former performance on natal vs. alternative high-gall-density trees demonstrated significant (P < 0.001) differences in six performance measures with five differing in the direction predicted by the hypothesis of local adaptation. Overall, these linked experiments document direct and indirect effects of host plant variation on gall-former performance and demonstrate convincingly that (1) high-gall-density trees equate to high-quality trees that are surrounded by trees of relatively lower quality to the herbivore and (2) gall-former populations have become locally adapted to individual trees.

The effect of nectar variance on bumblebee patterns of movement and potential gene dispersal

Ott, J. R., Real, L. A. and Silverfine, B. M. 1985. The effect of nectar variance on bumblebee patterns of movement and potential gene dispersal. - Oikos 45: 333-340. The effect of variation in nectar reward on flight patterns, and correlatively potential gene flow, was studied using an enclosed colony of Bombus pennsylvanicus foraging on an artificial flower system. At low nectar variance foragers primarily visited adjacent (near neighbor) plants. Foragers responded to increasing variation in nectar reward by deviating from near neighbor visitation. Correspondingly, significant increases in the mean and variance of interplant flight distance were observed. By applying the isolation-by-distance model of population structure and equating interplant flight distance variance with gene dispersal variance, neighborhood size, area and diameter were estimated. Neighborhood size and area increased approximately threefold and neighborhood diameter increased 65% over the range of nectar variances tested. These results suggest that variation in nectar reward encountered by foraging bumblebees could play a role in mediating pollen dispersal in natural systems.

RELIABILITY OF OCCUPANCY AND BINOMIAL MIXTURE MODELS FOR ESTIMATING ABUNDANCE OF GOLDEN-CHEEKED WARBLERS (SETOPHAGA CHRYSOPARIA)

ABSTRACT. Reliable estimates of population parameters derived from survey methods are essential for decision making in management of endangered species. We evaluated whether point-count surveys used in conjunction with occupancy and binomial mixture models (BMMs) constituted a reliable approach for monitoring the federally endangered Golden-cheeked Warbler (Setophaga chrysoparia) on a preserve in central Texas. Occupancy and abundance were estimated using point-count surveys conducted on each of five 113-ha detection grids in 2008 and seven grids in 2009. Single-season occupancy models and BMMs were used to estimate occupancy and abundance, respectively. Occupancy estimates per grid ranged from 0.48 to 1.0 in 2008 and from 0.52 to 1.0 in 2009. Estimates of abundance were compared with territory densities independently estimated using spot mapping, the standard by which all other avian survey methods are often compared. Abundance estimates produced by BMMs were significantly higher than territory density estimates at all but one site in 2008 and two sites in 2009. While estimation techniques incorporating detection probabilities should be considered in monitoring programs, our results suggest that BMMs deserve careful scrutiny before being used to estimate abundance or to monitor population trends.

Developmental plasticity and reduced susceptibility to natural enemies following host plant defoliation in a specialized herbivore

Host-specific phytophagous insects that are short lived and reliant on ephemeral plant tissues provide an excellent system in which to investigate the consequences of disruption in the timing of resource availability on consumer populations and their subsequent interactions with higher tropic levels. The specialist herbivore, Belonocnema treatae (Hymenoptera: Cynipidae) induces galls on only newly flushed leaves of live oak, Quercus fusiformis. In central Texas (USA) episodic defoliation of the host creates variation in the timing of resource availability and results in heterogeneous populations of B. treatae that initiate development at different times. We manipulated the timing of leaf flush in live oak via artificial defoliation to test the hypothesis that a 6- to 8-week delay in the availability of resources alters the timing of this gall former’s life cycle events, performance and survivorship on its host, and susceptibility to natural enemies. B. treatae exhibits plasticity in development time, as the interval from egg to emergence was significantly reduced when gallers oviposited into the delayed leaf flush. As a consequence, the phenologies of gall maturation and adult emergence remain synchronized in spite of variation in the timing of resource availability. Per capita gall production and gall-former performance are not significantly affected by the timing of resource availability. The timing of resource availability and natural enemies interact, however, to produce strong effects on survivorship: when exposed to natural enemies, B. treatae developing in galls initiated by delayed oviposition exhibited an order-of-magnitude increase in survivorship. Developmental plasticity allows this gall former to circumvent disruptions in resource availability, maintain synchrony of life cycle events, and results in reduced vulnerability to natural enemies following defoliation of the host plant.

Geographic and genetic isolation in spring-associated Eurycea salamanders endemic to the Edwards Plateau region of Texas

Populations of neotenic, spring-associated salamanders of the genus Eurycea occupy discontinuous sites throughout the Edwards Plateau of central Texas and many warrant conservation attention. Here we used DNA sequence data from a nuclear (rag1) and a mitochondrial (ND4) gene to determine (1) the extent of genetic isolation among seven Edwards Plateau Eurycea populations and (2) the relationship between genetic distance and both geographic distance and hydrogeological features. Coalescent-based methods detected little gene flow among the sampled Eurycea populations, and we were unable to reject a model of complete isolation for any pair of populations. These findings were consistent with the relatively high genetic distances we detected among the sampled Eurycea populations (pairwise ϕST ranged from 0.249 to 0.924). We detected a positive correlation between genetic distance and geographic distance, which is consistent with a pattern of isolation by distance. However, while controlling for geographic distance, we did not detect a positive relationship between genetic distance and aquifer or river distance. Thus, we found no evidence that aquifers and/or rivers serve as dispersal corridors among isolated Eurycea populations. Based on these results, we have no evidence that re-colonization of spring sites by migrant salamanders following local extirpation would be likely. Our findings indicate that spring-associated Eurycea salamander populations occupying the Edwards Plateau region are genetically isolated, and that each of these populations should be considered a distinct management unit.

Divergent host-plant use promotes reproductive isolation among cynipid gall wasp populations

Ecological speciation occurs when reproductive isolation evolves as a consequence of divergent natural selection among environments. A direct prediction of this process is that ecologically divergent pairs of populations will exhibit greater reproductive isolation than ecologically similar pairs of populations. By comparing allopatric populations of the cynipid gall wasp Belonocnema treatae infesting Quercus virginiana and Quercus geminata, we tested the role that divergent host use plays in generating ecological divergence and sexual isolation. We found differences in body size and gall structure associated with divergent host use, but no difference in neutral genetic divergence between populations on the same or different host plant. We observed significant assortative mating between populations from alternative host plants but not between allopatric populations on the same host plant. Thus, we provide evidence that divergent host use promotes speciation among gall wasp populations.

Parallel Patterns of Morphological and Behavioral Variation among Host-Associated Populations of Two Gall Wasp Species

A powerful approach to address the general factors contributing to ecological speciation is to compare distantly related taxa that inhabit the same selective environments. In this design, similarities among taxa can elucidate general mechanisms of the process whereas differences may uncover specific factors important to the process for individual taxa. Herein, we present evidence of parallel patterns of morphological and behavioral variation among host-associated populations of two species of cynipid gall wasps, Belonocnema treatae and Disholcaspis quercusvirens, that each exhibit a life cycle intimately tied to the same two host plant environments, Quercus geminata and Q. virginiana. Across both gall-former species we find consistent differences in body size and gall morphology associated with host plant use, as well as strong differences in host plant preference, a measure of habitat isolation among populations. These consistent differences among taxa highlight the important role of host plant use in promoting reproductive isolation and morphological variation among herbivorous insect populations–a prerequisite for ecological speciation.

An Ecological Framework for the Study of Planthopper Mating Systems

Mating system studies seek to elucidate the selective regimes responsible for the evolution of observed reproductive strategies (Emlen and Oring 1977; Halliday 1978, 1983; Vehrencamp and Bradbury 1984; Davies 1991). Investigations of mating systems, therefore, attempt to (1) quantify variation in male and female reproductive success and evaluate the extent and form of sexual selection, (2) understand the role of the environment in shaping the mating system by examining spatial and temporal variation in ecological parameters thought to be important in structuring mating systems, (3) determine the adaptive significance of behaviors and morphologies involved in mate search, courtship, competition, and choice, (4) elucidate constraints acting on behaviors and morphologies associated with reproduction, and (5) explain among-species variation in reproductive strategies. Two central premises underlie this research. First, the analysis of species’ mating systems in their current ecological context can be used to infer the environmental conditions and selection regimes responsible for the evolution of observed reproductive behaviors and morphologies. Second, the reproductive strategies of the sexes and species’ mating systems reflect the outcome of selection to maximize reproductive success.

Detection Heterogeneity and Abundance Estimation in Populations of Golden-Cheeked Warblers (Setophaga chrysoparia)

ABSTRACT. Abundance estimators that account for imperfect detection, such as N-mixture models, assume that detection of individuals is independent of abundance. Using spot-mapping and N-mixture models applied to point-count data, we estimated abundance of Goldencheeked Warblers (Setophaga chrysoparia) in two years at six study sites at the Balcones Canyonlands Preserve, Austin, Texas. N-mixture model estimates deviated from spot-mapping estimates at the site level by overestimating at low abundances, and at the survey-station level by underestimating at high abundance, which suggests that model assumptions may have been violated. We tested whether detection of individuals is influenced by abundance by assessing per capita song rate in relation to abundance. Per capita song rate increased with abundance, illustrating how the behavior of a territorial passerine may violate the independent-detectability assumption. We next explored violation of this assumption at the survey-station level by applying N-mixture models to simulated data exhibiting heterogeneity in detection. This exercise revealed a slight but increasingly negative bias (underestimation of abundance) in the estimator as the actual abundance increased, given positive density-dependent detection. The simulations also revealed a potentiel effect of sampling variation on misestimation by N-mixture model estimators. Assessing the strength, basis, and prevalence of density-dependent detection; further analyzing the effects of nonrandom heterogeneity in producing estimator bias; and accounting for nonrandom detection heterogeneity in abundance estimators are fruitful areas for further study.