Cryptic seedling herbivory by nocturnal introduced generalists impacts survival, performance of native and exotic plants

Abstract

Although much of the theory on the success of invasive species has been geared at escape from specialist enemies, the impact of introduced generalist invertebrate herbivores on both native and introduced plant species has been underappreciated. The role of nocturnal invertebrate herbivores in structuring plant communities has been examined extensively in Europe, but less so in North America. Many nocturnal generalists (slugs, snails, and earwigs) have been introduced to North America, and 96% of herbivores found during a night census at our California Central Valley site were introduced generalists. We explored the role of these herbivores in the distribution, survivorship, and growth of 12 native and introduced plant species from six families. We predicted that introduced species sharing an evolutionary history with these generalists might be less vulnerable than native plant species. We quantified plant and herbivore abundances within our heterogeneous site and also established herbivore removal experiments in 160 plots spanning the gamut of microhabitats. As 18 collaborators, we checked 2000 seedling sites every day for three weeks to assess nocturnal seedling predation. Laboratory feeding trials allowed us to quantify the palatability of plant species to the two dominant nocturnal herbivores at the site (slugs and earwigs) and allowed us to account for herbivore microhabitat preferences when analyzing attack rates on seedlings. The relationship between local slug abundance and percent cover of five common plant taxa at the field site was significantly negatively associated with the mean palatability of these taxa to slugs in laboratory trials. Moreover, seedling mortality of 12 species in open-field plots was positively correlated with mean palatability of these taxa to both slugs and earwigs in laboratory trials. Counter to expectations, seedlings of native species were neither more vulnerable nor more palatable to nocturnal generalists than those of introduced species. Growth comparison of plants within and outside herbivore exclosures also revealed no differences between native and introduced plant species, despite large impacts of herbivores on growth. Cryptic nocturnal predation on seedlings was common and had large effects on plant establishment at our site. Without intensive monitoring, such predation could easily be misconstrued as poor seedling emergence.

Temporal variation in extrafloral nectar secretion in different ontogenic stages of the fruits ofAlibertia verrucosaS. Moore (Rubiaceae) in a Neotropical savanna

“…Optimal defense theory can also help to explain this pattern, one most herbivores in regions with high temperature (e.g. Neotropical savanna) tend to be strictly nocturnal (Fagundes et al 1996;Strauss et al 2009;Byk & Del-Claro 2010). Moreover, due to the plants reduce greatly the nectar secretion when this resource is removed in low frequency (Koptur 1983;Pyke 1991;Heil et al 2000), the ants' visitation on EFN of A. verrucosa may also have night foraging in arid and semi-arid regions (Briese & Macauley 1980;Oliveira et al 1995).…”

“…In regions with high temperature and low humidity during the day (e.g. Neotropical savanna), most herbivores display strong nocturnal activity due to their ecophysiological limitations (Fagundes et al 1996;Strauss et al 2009;Byk & Del-Claro 2010). Therefore, a change in the nectar secretion during periods with high herbivory and/or high levels of water stress can reduce energy loss in 'superfluous' EFN secretion, and reduce the risk of EFN fungal infections (Heil et al 2000;Holland et al 2010;Yamawo et al 2012a).…”

Conifer growth and reproduction in urban forest fragments: Predictors of future responses to global change?

“…We suspect that factors common to disturbed, but not necessarily urban, forests reduce seedling recruitment. For example, introduced species, which we observed at some urban and rural sites, are known to negatively impact native species through competition (Mack et al 2000) or herbivory (Nystrand and Granström 2000;Pigot and Leather 2008;Strauss et al 2009). Regardless of cause, the lack of regeneration is consistent with other observations of early life-history stage trees in urban areas (Broshot 2007;Lehvävirta and Rita 2002;Zipperer 2002).…”

Plant invasions, generalist herbivores, and novel defense weapons

“…Because generalist herbivores are widespread and present in both the native and nonnative ranges of invaders and apparently cannot be escaped, ecologists have focused almost exclusively on the release of invaders from specialists and the potential of reintroducing specialists to control invaders (Mu¨ller-Scha¨rer and Schaffner 2008). However, there are few if any generalist consumers of plants that do not have preferences, and the preferences of generalists can completely transform the composition and diversity of plant communities (Huntly 1990, Strauss et al 2009). Furthermore, recent studies have suggested that broad or single-family native generalists avoid some invasive plant species , Wikstro¨m et al 2006, Parker and Gilbert 2007, Han et al 2008, Jogesh et al 2008, Tallamy et al 2010; but see Lind and Parker 2010).…”