Deer Indirectly Alter the Reproductive Strategy and Operational Sex Ratio of an Unpalatable Forest Perennial.

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 understory perennials.

Pollination ecology and reproductive success in Jack-in-the-pulpit (Arisaema triphyllum) in Québec (Canada).

Pollination ecology and reproductive success of Jack-in-the-pulpit (Arisaema triphyllum) were studied in two natural populations in Québec, Canada. Individual A. triphyllum plants can be of three types: male, female or bisexual. In both populations studied, the presence of bisexual inflorescences was not negligible (13%), where 'female' and 'male' bisexual plants were categorised according to the relative number of stamens and ovaries. 'Male bisexual' plants produce only pollen and 'female bisexual' plants produce only fruit. Hence, A. triphyllum is a true dioecious plant, as each plant only reproduces through either the male or the female function. 'Female bisexual' plants were equivalent to female plants in terms of visitation rate by insects, fructification rate and production of berries and seeds. Neither agamospermy in female plants nor self-pollination in 'female bisexual' plants was found, thus A. triphyllum relies on insects for cross-pollination. Despite the long flowering cycle, a low visitation rate was documented: only 20-40% of inflorescences were visited, according to gender, by a mean of 1.5 insects. In this study, Mycetophilidae represented the most generically diversified and abundant family, as well as the most efficient insect pollinator, especially the genera Docosia and Mycetophila.

Nonconsumptive effects of a generalist ungulate herbivore drive decline of unpalatable forest herbs.

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.