Christopher D Heckel

Environmental conditions impose restrictions and costs on reproduction. Multiple reproductive options exist when increased reproductive costs drive plant populations toward alternative reproductive strategies. Using 4 years of demographic data across a deer impact gradient, where deer alter the abiotic environment, we parameterize a size-dependent integral projection model for a sexually labile and unpalatable forest perennial to investigate the demographic processes driving differentiation in the operational sex ratio (OSR) of local populations. In addition to a relative increase in asexual reproduction, our results illustrate that nontrophic indirect effects by overabundant deer on this perennial result in delayed female sex expression to unsustainably large plant sizes and lead to more pronounced plant shrinkage following female sex expression, effectively increasing the cost of reproduction. Among plants of reproductive age, increased deer impact decreases the size-dependent probability of flowering and reduces reproductive consistency over time. This pattern in sex expression skews populations toward female-biased OSRs at low deer impact sites and male-biased OSRs at intermediate and high deer impact sites. While this shift toward a male-biased OSR may ameliorate pollen limitation, it also decreases the effective population size when coupled with increased asexual reproduction. The divergence of reproductive strategies and reduced lifetime fitness in response to indirect deer impacts illustrate the persistent long-term effects of overabundant herbivores on unpalatable under-story perennials.

When large herbivores exert selection on their prey plant species, co-occurring, non-prey species may experience selection through non-trophic indirect effects. Such selection is likely common where herbivores are overabundant. Yet, empirical studies of non-trophic indirect effects as drivers of non-prey trait evolution are lacking. Here we test for adaptive shifts in life history traits in an unpalatable species, Arisaema triphyllum, a common forest perennial that is unique because it exhibits size-dependent sex switching. We collected A. triphyllum from six sites that experience a gradient in abiotic stress caused by deer browse pressure on prey plant species that generate indirect effects. We grew A. triphyllum from these sites in a common garden for five years to evaluate life history predictions linking strong indirect effects and abiotic stress to changes in life history traits: flowering onset size threshold, female flowering size threshold, relative growth rate (RGR), biomass allocation, and asexual reproduction. Despite observed differences among phenotypes in the field, expression of flowering onset size threshold, biomass allocation, and asexual reproduction did not differ among the six populations in the garden, indicating common plastic responses. In contrast, A. triphyllum collected from sites experiencing the two highest deer impacts exhibited smaller female flowering size thresholds and the highest RGR. Responses in these traits support the predictions of adaptive divergence in response to indirect effects. Our results reinforce the idea that non-trophic indirect effects of large herbivores can elicit evolutionary responses in some traits of non-prey species. In general, life history traits of unpalatable species may be cryptically adapting to stressful indirect effects where large herbivores are overabundant.

High herbivore pressure is expected to benefit unpalatable species that co-occur with palatable browsed species. However, for five unpalatable understory species we found no evidence of benefit from deer browse. Detailed Studies of one species in natural populations, Arisaema triphyllum, revealed surprising changes in its Population structure and demography: deer browse level on a palatable species significantly correlates with reduced plant size and seed rain and male-biased sex ratios of co-occurring Arisaema populations. Analyses of individual size in five unpalatable forest plant species in long-term experimental paired deer exclosure/deer access plots corroborate the natural site results; all five species were smaller in deer access plots. Analyses of abiotic variables in natural and experimental sites suggest one potential mechanism for indirect effects of deer. Deer-mediated soil quality declines included increased soil penetration resistance and decreased leaf litter depth, which are known to hinder plant growth. Our results are likely applicable to other unpalatable forest species and have clear consequences for understory biodiversity. Unpalatable plant species in forests experiencing high deer numbers may be in decline along with their palatable neighbors. Our study implicates deer overabundance in the cascade of forest species decline and the urgency of this conservation issue in North America.

Knowledge of ecological characteristics of rare species is important in assessing their threat status. Trillium reliquum is one of four globally vulnerable North American Trillium species. Yet little is known about the ecology and reproductive biology of T. reliquum. Here we present the first description of the natural history and reproductive biology of T. reliquum. We observed the timing of emergence, flowering, fruit maturation, and seed dehiscence of a population in south-central Georgia (USA). In addition, we conducted a series of pollen manipulation experiments to assess the breeding system and pollen limitation. We found the life history of this species to be similar to other Trillium species; individuals progressed through four morphologically distinct life stages with increases in size and age. However, T. reliquum exhibited an earlier and shorter germination period than other Trillium species and many plants exist in a dormant state during the growing season. This species matured fruit equally well with either cross- or self-pollen and by apomixis. Seeds produced per fruit did not differ among reproductive treatments. The lack of differences in fruit and seed production among pollination treatments suggested there was no pollen limitation in 2004 for this population. The data we report here provide a baseline that can be used to generate assessments of the population stability of T. reliquum.

In order to study the effects of EtOH and/or nicotine on brain membrane fatty acid composition, various concentrations of EtOH and/or nicotine were injected into the air sac of chicken eggs at 0 days of incubation. Controls were injected with saline. Experimental groups were injected with either 200 μmol EtOH/kg egg, 100 μmol nicotine/kg egg, 200 μmol nicotine/kg egg, 200 μmol EtOH/kg and 100 μmol nicotine/kg egg, or 200 μmol EtOH/kg and 200 μmol nicotine/kg egg. In all experimental groups, EtOH- and nicotine-induced decreases in brain long-chain polyunsaturated membrane fatty acids were observed in stage 44 embryos, stage 45 embryos, and neonatal chicks. These EtOH- and nicotine-induced decreases in brain membrane polyunsaturated fatty acids correlated with elevated levels of brain lipid hydroperoxides and reduced brain acetylcholinesterase (AChE; EC. 3.1.1.7) activities.