Does pollination limit tolerance to browsing in Ipomopsis aggregata ?

Abstract

Ungulate browsing of flowering stalks of the semelparous herb Ipomopsis aggregata leads to regrowth of lateral inflorescences, a response that has been reported to yield overcompensation in some cases (browsed plants with higher reproductive success than unbrowsed), but undercompensation in others. Little is known about the mechanisms that cause such variable tolerance to herbivory. We explored one possible mechanism--variation in effects of browsing on pollination--by clipping I. aggregata inflorescences to mimic browsing, observing subsequent visits by pollinators and nectar-robbers, and adding pollen by hand to flowers of some clipped and unclipped plants. Clipping reduced floral display size and increased inflorescence branching, but neither hummingbirds, the primary pollinators, nor nectar-robbing bumblebees showed any preference for unclipped versus clipped plants. Clipping delayed flowering; this shift in phenology caused clipped plants to miss the peak of hummingbird activity and to have lower per-flower visitation rates than unclipped controls in one year, but to have greater overlap with birds and higher visitation rates in the subsequent year. In three sites and 2 years, clipped plants exposed to natural pollination suffered extreme undercompensation, producing on average only 16% as many seeds as unclipped controls. This was not directly attributable to clipping effects on pollination, however, because clipped plants were unable to increase fecundity when provided with supplemental pollen by hand. Taken altogether, our results suggest that compensation was constrained less by indirect effects of browsing on pollination than by its direct impacts on resource availability and hence on the ability of plants to regrow lost inflorescence tissue and to fill seeds. Exploring the physiological and developmental processes involved in regrowth of inflorescences and provisioning of seeds is a promising future direction for research designed to understand variation in browsing tolerance.

The effects of nutrient addition on floral characters and pollination in two subalpine plants, Ipomopsis aggregata and Linum lewisii

“…Mule deer, Odocoileus hemionus, is the primary herbivore of I. aggregata, consuming portions of individuals during the reproductive phase of these plants (Paige and Whitham 1987;Sharaf and Price 2004). Herbivory was assessed in I. aggregata by whether or not each plant was browsed.…”

Interactions between nectar robbers and seed predators mediated by a shared host plant, Ipomopsis aggregata

“…Early season herbivory delays Xowering and lowers the per Xower risk to Hylemya (Juenger and Bergelson 1998;Juenger et al 2005). However, herbivory exacerbates the eVects of nectar robbing because plants that bloom late in the season incur higher rates of robbing (Sharaf and Price 2004;Irwin 2006). In some years, but not all, nectar robbing increases as the season progresses (Irwin and Maloof 2002;Irwin 2006), and oviposition by Hylemya usually declines over the season (A. K. Brody, unpublished data).…”

Bottom‐up multitrophic effects in resprouting plants

“…Several studies have highlighted the influence of biotic and abiotic factors on flowering phenology (Brody 1997, Sola andEhrle´n 2007). Floral phenological shifts may promote negative effects on plants, for instance if pollinator peaks are missed; but the flowering delay may compensate the loss of pollination through temporal escape from pre-dispersal seed predators (Sharaf and Price 2004). However, the delay in flowering onset of resprouted plants increased susceptibility of C. pungens to seed predation.…”

“…Rapid plant regrowth following mammalian browsing and fire often increases tissue nutritional content, decreases chemical or physical defenses, alters morphology, phenology, and reproduction of plants (Maschinski and Whitham 1989, Strauss and Agrawal 1999, Spiller and Agrawal 2003, Sharaf and Price 2004, Nakamura et al 2006. Resprouts tend to have higher nutritional quality and often are less defended, which makes them more susceptible to chewing herbivores (Danell and Huss-Danell 1985, Spiller and Agrawal 2003, Nakamura et al 2006, Utsumi et al 2009).…”