Demographic Feedback Between Clonal Growth and Fragmentation in an Invasive Seaweed

Abstract

Many abundant plants, invertebrates, and seaweed are clonal, and this allows the formation of high-density aggregations, foraging, and the placement of modules into new space, and rapid rates of expansion. For these species, population density and rates of expansion are functions of recruitment of asexual modules and post-recruitment vegetative growth and survivorship. In this study, we provide the first experimental test of the relative importance of these two processes in determining the abundance of a clonal seaweed using Caulerpa taxifolia, an invasive green alga that spreads rapidly and reaches very high abundance. We asked two main questions: What is the relative importance to abundance (biomass) of vegetative stolon growth and fragment recruitment during expansion of established patches? Does greater fragment recruitment result in greater abundance in established patches? Vegetative growth of stolons underpinned patch expansion. Plots with stolons growing into them always had a greater abundance than plots where stolons were removed, even when fragment recruitment was increased. Greater recruitment only resulted in greater abundance when stolons were absent, a situation analogous to the establishment of new populations. Although post-recruitment processes were more important in determining abundance during patch expansion, there was greater ambient fragment recruitment when stolons were present compared to when they were absent, and as the abundance of C. taxifolia increased, demonstrating an important feedback between stolon growth, abundance, and fragment recruitment. In established patches, greater fragment recruitment over six months (six levels ranging from 0 to 480 recruits x m(-2) x mo(-1)) had no effect on biomass. Our experiments demonstrate that the rapid expansion and high abundance of invasive C. taxifolia are underpinned by post-recruitment vegetative growth and, during expansion, by a feedback between vegetative growth and asexual fragmentation.

Ecophysiological studies of the non-indigenous speciesGracilaria vermiculophylla(Rhodophyta) and its abundance patterns in Ria de Aveiro lagoon, Portugal

“…The capacity for vegetative propagation is fundamental to the success of some NIS (Nyberg & Wallentinus, 2005;Wright & Davis, 2006), leading to higher rates of successful colonization (Norton & Mathieson, 1983). Gracilaria vermiculophylla was always abundant and reproductive even though mature male gametophytes were never observed during the sampling period.…”

Human-mediated transport determines the non-native distribution of the anemone Nematostella vectensis, a dispersal-limited estuarine invertebrate

“…The capacity to reproduce asexually has been often cited as a characteristic conducive to invasion success (Sakai et al 2001), though the correlation has been reported most thoroughly in plant systems (Ruesink & Collado-Vides 2006, Wright & Davis 2006, Burns 2008, Milbau & Stout 2008. Only a few studies have explored the role of asexual reproduction in the successful invasions of invertebrate taxa (Ting & Geller 2000, Facon et al 2003, Stadler et al 2005, and none have directly tested the hypothesis that asexuality contributes to invasiveness.…”

Spatial refuges and associational defenses promote harmful blooms of the alga Caulerpa sertularioides onto coral reefs

“…Glynn, personal communication). Blooms were concentrated on the fore-reef slope adjacent to established oV-reef patches of Caulerpa, suggesting that the alga moved from the oV-reef reservoirs by vegetative growth of established thalli and fragmentation, both of which are known to be eVective mechanisms of population spread within the genus Caulerpa (Smith and Walters 1999;Wright and Davis 2006). Blooming Caulerpa overgrew coral and caused nearly 100% mortality of prostrate, nodular coral species (Psammocora spp.…”