M. Henry H. Stevens

THE GROWTH-DEFENSE TRADE-OFF AND HABITAT SPECIALIZATION BY PLANTS IN AMAZONIAN FORESTS

Tropical forests include a diversity of habitats, which has led to specialization in plants. Near Iquitos, in the Peruvian Amazon, nutrient-rich clay forests surround nutrient-poor white-sand forests, each harboring a unique composition of habitat specialist trees. We tested the hypothesis that the combination of impoverished soils and herbivory creates strong natural selection for plant defenses in white-sand forest, while rapid growth is favored in clay forests. Recently, we reported evidence from a reciprocal-transplant experiment that manipulated the presence of herbivores and involved 20 species from six genera, including phylogenetically independent pairs of closely related white-sand and clay specialists. When protected from herbivores, clay specialists exhibited faster growth rates than white-sand specialists in both habitats. But, when unprotected, white-sand specialists outperformed clay specialists in white-sand habitat, and clay specialists outperformed white-sand specialists in clay habitat. Here we test further the hypothesis that the growth defense trade-off contributes to habitat specialization by comparing patterns of growth, herbivory, and defensive traits in these same six genera of white-sand and clay specialists. While the probability of herbivore attack did not differ between the two habitats, an artificial defoliation experiment showed that the impact of herbivory on plant mortality was significantly greater in white-sand forests. We quantified the amount of terpenes, phenolics, leaf toughness, and available foliar protein for the plants in the experiment. Different genera invested in different defensive strategies, and we found strong evidence for phylogenetic constraint in defense type. Overall, however, we found significantly higher total defense investment for white-sand specialists, relative to their clay specialist congeners. Furthermore, herbivore resistance consistently exhibited a significant trade-off against growth rate in each of the six phylogenetically independent species-pairs. These results confirm theoretical predictions that a trade-off exists between growth rate and defense investment, causing white-sand and clay specialists to evolve divergent strategies. We propose that the growth-defense trade-off is universal and provides an important mechanism by which herbivores govern plant distribution patterns across resource gradients.

PLANT DENSITY DETERMINES SPECIES RICHNESS ALONG AN EXPERIMENTAL FERTILITY GRADIENT

A number of authors have suggested that, within areas a few square meters to many square kilometers in size, species diversity appears to peak at moderate levels of productivity, and this pattern is currently unexplained. Among the best examples of this pattern have been descriptions of vegetation in which species richness declines as soil fertility increases. We tested two hypotheses that have been proposed to explain this pattern. The interspecific competitive exclusion hypothesis proposes that dominant species suppress the growth of competitively subordinate species and exclude subordinate species as fertility rises. In contrast, the assemblage-level thinning hypothesis proposes that individuals of all species tend to become larger as fertility rises, and individuals of all species tend to exclude subordinate individuals of each species. Because total density declines, samples of finite numbers of individuals will result in fewer species by chance alone. To test these hypotheses, we established an experimental productivity gradient in a first- year old field using four levels of slow-release NPK fertilizer (0, 8, 16, and 32 g N/M2). At the end of the growing season, we sampled aboveground biomass and numbers of stems for each species in 72 20 X 20 cm subplots (18 reps X 4 levels), with an average sample size of 260 individual stems per plot. We observed an 80% decline in stem density with increasing fertility, and a 50% decline in species richness along this fertility gradient. A simulation of random thinning along a fertility gradient showed a nearly identical decline in species richness, supporting the assemblage-level thinning hypothesis. We also found that responses of individual species to the soil fertility gradient showed virtually no support for interspecific competitive exclusion. The overwhelming influence of density found in this study suggests that plant species richness along many productivity gradients may be strongly influenced by total stem density, and that differences in competitive ability among species, although generally important, are not necessary to create dramatic changes in species richness along fertility gradients.

ALLOCHTHONOUS SUBSIDY OF PERIODICAL CICADAS AFFECTS THE DYNAMICS AND STABILITY OF POND COMMUNITIES

Periodical cicadas emerge from below ground every 13 or 17 years in North American forests, with individual broods representing the synchronous movement of trillions of individuals across geographic regions. Due to predator satiation, most individuals escape predation, die, and become deposited as detritus. Some of this emergent biomass falls into woodland aquatic habitats (small streams and woodland ponds) and serves as a high-quality allochthonous detritus pulse in early summer. We present results of a two-part study in which we (1) quantified deposition of Brood X periodical cicada detritus into woodland ponds and low-order streams in southwestern Ohio, and (2) conducted an outdoor mesocosm experiment in which we examined the effects of deposition of different amounts of cicada detritus on food webs characteristic of forest ponds. In the mesocosm experiment, we manipulated the amount of cicada detritus input to examine if food web dynamics and stability varied with the magnitude of this allochthonous resource subsidy, as predicted by numerous theoretical models. Deposition data indicate that, during years of periodical cicada emergence, cicada carcasses can represent a sizable pulse of allochthonous detritus to forest aquatic ecosystems. In the mesocosm experiment, cicada carcass deposition rapidly affected food webs, leading to substantial increases in nutrients and organism biomass, with the magnitude of increase dependent upon the amount of cicada detritus. Deposition of cicada detritus impacted the stability of organism functional groups and populations by affecting the temporal variability and biomass minima. However, contrary to theory, stability measures were not consistently related to the size of the allochthonous pulse (i.e., the amount of cicada detritus). Our study underscores the need for theory to further explore consequences of pulsed allochthonous subsidies for food web stability.

Effects of Long-Term Disturbance on Riparian Vegetation and In-Stream Characteristics

Abstract We assessed the influence of riparian disturbance on 26 stream variables in Linesville Creek and six tributaries, in northwestern Pennsylvania, USA. Redundancy analysis, a canonical ordination technique, was used in three separate analyses to test for significant relationships between riparian characteristics and the multivariate stream data (aspects of geomorphology, substrate composition, leaf litter processing and macroinvertebrate shredder species composition). Ordination (PCA) showed riparian vegetation to be strongly related to land use. Pasture sites, characterized by grasses and Eupatorium spp., had the most distinctive plant assemblages, while species composition was most variable among woodland sites. In-stream variables were significantly related to both land use and riparian vegetation. Generally, disturbed sites tended to have more silt and higher populations of Gammarus sp. Land use, in particular, had a great effect on detritus processing and storage. Stream reaches in pastures ten...

On Distance-Based Permutation Tests for Between-Group Comparisons

Permutation tests based on distances among multivariate observations have found many applications in the biological sciences. Two major testing frameworks of this kind are multiresponse permutation procedures and pseudo-F tests arising from a distance-based extension of multivariate analysis of variance. In this article, we derive conditions under which these two frameworks are equivalent. The methods and equivalence results are illustrated by reanalyzing an ecological data set and by a novel application to functional magnetic resonance imaging data.

Adaptation of the Spore Discharge Mechanism in the Basidiomycota

Background Spore discharge in the majority of the 30,000 described species of Basidiomycota is powered by the rapid motion of a fluid droplet, called Bullers drop, over the spore surface. In basidiomycete yeasts, and phytopathogenic rusts and smuts, spores are discharged directly into the airflow around the fungal colony. Maximum discharge distances of 1–2 mm have been reported for these fungi. In mushroom-forming species, however, spores are propelled over much shorter ranges. In gilled mushrooms, for example, discharge distances of <0.1 mm ensure that spores do not collide with opposing gill surfaces. The way in which the range of the mechanism is controlled has not been studied previously. Methodology/Principal Findings In this study, we report high-speed video analysis of spore discharge in selected basidiomycetes ranging from yeasts to wood-decay fungi with poroid fruiting bodies. Analysis of these video data and mathematical modeling show that discharge distance is determined by both spore size and the size of the Bullers drop. Furthermore, because the size of Bullers drop is controlled by spore shape, these experiments suggest that seemingly minor changes in spore morphology exert major effects upon discharge distance. Conclusions/Significance This biomechanical analysis of spore discharge mechanisms in mushroom-forming fungi and their relatives is the first of its kind and provides a novel view of the incredible variety of spore morphology that has been catalogued by traditional taxonomists for more than 200 years. Rather than representing non-selected variations in micromorphology, the new experiments show that changes in spore architecture have adaptive significance because they control the distance that the spores are shot through air. For this reason, evolutionary modifications to fruiting body architecture, including changes in gill separation and tube diameter in mushrooms, must be tightly linked to alterations in spore morphology.

Density-dependent Growth

Lets go back to our Song Sparrow (Melospiza melodia) data from Chapter 1 on density-independent growth — now we look at all the data.

Local Patterns of Nucleotide Polymorphism Are Highly Variable in the Selfing Species Arabidopsis thaliana

Neighboring genes predictably share similar evolutionary histories to an extent delineated by recombination. This correlation should extend across multiple linked genes in a selfing species such as Arabidopsis thaliana due to its low effective recombination rate. To test this prediction, we performed a molecular population genetics analysis of nucleotide polymorphism and divergence in chromosomal regions surrounding four low-diversity loci. Three of these loci, At1g67140, At3g03700, and TERMINALFLOWER1 (TFL1), have been previously implicated as targets of selection and we would predict stronger correlations in polymorphism between neighboring loci due to genetic hitchhiking around these loci. The remaining locus, At1g04300, was identified in a study of linkage disequilibrium surrounding the CRYPTOCHROME2 (CRY2) locus. Although we found broad valleys of reduced nucleotide variation around two of our focal genes, At1g67140 and At3g03700, all chromosomal regions exhibited extreme variation in the patterns of polymorphism and evolution between neighboring loci. Although three of our four regions contained potential targets of selection, application of the composite-likelihood-ratio test of selection in conjunction with a goodness-of-fit test supports the selection hypothesis only for the region containing At3g03700. The degree of discordance in evolutionary histories between linked loci within each region generally correlated with estimates of recombination and linkage disequilibrium for that region, with the exception of the region containing At1g04300. We discuss the implications of these data for future population genetics analyses and genomics studies in A. thaliana.

Thermoregulatory performance and habitat selection of the eastern box turtle (Terrapene carolina carolina)

Conservation requires knowledge of the interaction between an organism and its environment. Thermal conditions have an important role in habitat selection, movement, and maintenance of physiological processes for ectotherms. We found that box turtles chose cooler temperatures than expected and allowed their body temperatures to match the environment, but that this had little effect on their movement.

The ecological side of an ethnobotanical coin: legacies in historically managed trees.

UNLABELLED • PREMISE OF THE STUDY A growing body of literature now documents how ancient human management of the landscape echoes through to extant environments in eastern North America. Plant domestication is a major theme in the study of human-nature interactions. Long-term ecological impacts of human selection may last for centuries after management ends, yet little work has focused on legacies in the evolution of historically used trees. Ecological data will be valuable in teasing apart myriad variables that confound questions of land-use legacies. We discuss the potential for legacies of ancient human selection and present a preliminary case study for the approach of integrating ecological and historical data for Diospyros virginiana, the American persimmon.• METHODS Herbarium samples of D. virginiana (28 male and 40 female) from across the species range provided specimen localities for edaphic analysis. Soil and environmental data were analyzed using nonparametric ordination, Wilcoxon summed rank test, and permutational MANOVA.• KEY RESULTS Edaphic data demonstrated substantial variation among sites, but revealed no significant differences between sexes. Permutational MANOVA showed no difference in environmental preferences for the tested variables between male and female trees (R(2) < 0.01, P = 0.8).• CONCLUSIONS Extending our understanding of landscape history to the long-term impacts of artificial selection at the species or population level would be valuable in both theoretical and applied botanical research. Multidisciplinary approaches integrating ecological data will be essential for investigation of the evolutionary implications of historical human selection in economic species and the potential for adaptive flexibility in reproductive systems of long-lived perennials.