Evaluating the Interacting Influences of Pollination, Seed Predation, Invasive Species and Isolation on Reproductive Success in a Threatened Alpine Plant

Paul D Krushelnycky,Anna Traveset

doi:10.1371/journal.pone.0088948

Abstract

Reproduction in rare plants may be influenced and limited by a complex combination of factors. External threats such as invasive species and landscape characteristics such as isolation may impinge on both pollination and seed predation dynamics, which in turn can strongly affect reproduction. I assessed how patterns in floral visitation, seed predation, invasive ant presence, and plant isolation influenced one another and ultimately affected viable seed production in Haleakalā silverswords (Argyroxiphium sandwicense subsp. macrocephalum) of Hawai’i. Floral visitation was dominated by endemic Hylaeus bees, and patterns of visitation were influenced by floral display size and number of plants clustered together, but not by floral herbivory or nearest flowering neighbor distance. There was also some indication that Argentine ant presence impacted floral visitation, but contradictory evidence and limitations of the study design make this result uncertain. Degree of seed predation was associated only with plant isolation, with the two main herbivores partitioning resources such that one preferentially attacked isolated plants while the other attacked clumped plants; total seed predation was greater in more isolated plants. Net viable seed production was highly variable among individuals (0–55% seed set), and was affected mainly by nearest neighbor distance, apparently owing to low cross-pollination among plants separated by even short distances (>10–20 m). This isolation effect dominated net seed set, with no apparent influence from floral visitation rates, percent seed predation, or invasive ant presence. The measured steep decline in seed set with isolation distance may not be typical of the entire silversword range, and may indicate that pollinators in addition to Hylaeus bees could be important for greater gene flow. Management aimed at maintaining or maximizing silversword reproduction should focus on the spatial context of field populations and outplanting efforts, as well as on conserving the widest possible range of pollinator taxa.

Highlights

Threatened and endangered plant species often suffer from a complex host of threats that may impinge on various stages of their life histories [1,2]
Understanding the nature of each of these dynamics in isolation may often be of intrinsic interest, it is evident that taking each into account becomes necessary when evaluating the importance of any single factor, and when assessing overall patterns in net seed production
Floral Visitation At least 19 insect taxa were seen in a total of 1102 floral visits

Summary

Introduction

Threatened and endangered plant species often suffer from a complex host of threats that may impinge on various stages of their life histories [1,2] This includes factors that limit fecundity [3], understanding the reproductive ecology of threatened or rare plants can be crucial for planning mitigation and recovery strategies. Reproductive success in plants is influenced by their intrinsic characteristics, the animals that interact with them, and the spatial context of the community in which they are embedded [5,6,7,8] This applies both to pollination and seed predation dynamics, two key determinants of viable seed output. Understanding the nature of each of these dynamics in isolation may often be of intrinsic interest, it is evident that taking each into account becomes necessary when evaluating the importance of any single factor, and when assessing overall patterns in net seed production

Methods

Results

Discussion

Conclusion