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

Abstract

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.

Size-dependent sex allocation and the expression of andromonoecy in a protogynous perennial herb: both size and timing matter

“…Our results are also consistent with the notion that flowering through the female function (in P. alpina , this means adopting a hermaphroditic rather than a male phase in a given season) places a heavier burden on a plant’s resources than flowering just through its male function (Schlessman 1991; Zhang et al 2014; Bialic-Murphy et al 2020): smaller plants were more likely to produce a single male flower, whereas larger plants produced more flowers and flowers with both male and female functions. Indeed, we found that larger individuals of P. alpina made a greater absolute and relative allocation to their female function than smaller plants, with a significantly steeper positive slope of absolute allocation to the female than male function.…”

“…alpina, this means adopting a hermaphroditic rather than a male phase in a given season) places a heavier burden on a plant's resources than flowering just through its male function (Schlessman 1991;Zhang et al 2014;Bialic-Murphy et al 2020): smaller plants were more likely to produce a single male flower, whereas larger plants produced more flowers and flowers with both male and female functions. Indeed, we found that larger individuals of P.…”

“…Empirical assessment of the marginal costs of male versus female function in terms of growth and survivorship indicates that plants often invest more heavily in their female function than their male function (Obeso 2002). This is particularly evident in dioecious species in which adult sex ratios of reproducing individuals are male-biased as a result of the greater mortality or less frequent flowering of females that bear a higher cost of reproduction (Sinclair et al 2012; Barrett and Hough 2013; Field et al 2013), but it is also evident in hermaphroditic species in which plants that have invested heavily in their female function are less likely to flower the following year than those that have reproduced as males (Schlessman 1991; Zhang et al 2014; Blake-Mahmud and Struwe 2019; Bialic-Murphy et al 2020).…”

Sex ratios, damage and distribution ofMyrianthus holstiiEngl.: a dioecious afromontane forest tree

“…We suspect that while some variation will always arise, much larger differences have likely arisen in recent centuries as densities of larger fauna, such as elephants, have fluctuated far outside prehistoric norms as populations have been eliminated from some forests and restricted to others (Sheil 2020). The consequences of variable conditions on dioecious populations remain an important subject of research (Bialic-Murphy et al 2020, Tonnabel et al 2017.…”

Invasion‐induced root–fungal disruptions alter plant water and nitrogen economies

“…To capture the effect of the Alliaria treatment on plant growth, we randomly selected N = 501 individuals across the three species and measured aboveground size in June-July in 2018 and 2019. Plant biomass was estimated using speciesspecific allometric equations previously developed (Brouwer et al 2015;Bialic-Murphy et al 2019;Bialic-Murphy et al 2020).…”