The Invasibility of Marine Algal Assemblages: Role of Functional Diversity and Identity

Abstract

The emergence of the biodiversity-ecosystem functioning debate in the last decade has renewed interest in understanding why some communities are more easily invaded than others and how the impact of invasion on recipient communities and ecosystems varies. To date most of the research on invasibility has focused on taxonomic diversity, i.e., species richness. However, functional diversity of the communities should be more relevant for the resistance of the community to invasions, as the extent of functional differences among the species in an assemblage is a major determinant of ecosystem processes. Although coastal marine habitats are among the most heavily invaded ecosystems, studies on community invasibility and vulnerability in these habitats are scarce. We carried out a manipulative field experiment in tide pools of the rocky intertidal to test the hypothesis that increasing functional richness reduces the susceptibility of macroalgal communities to invasion. We selected a priori four functional groups on the basis of previous knowledge of local species characteristics: encrusting, turf, subcanopy, and canopy species. Synthetic assemblages containing one, two, three, or four different functional groups of seaweeds were created, and invasion by native species was monitored over an eight-month period. Cover and resource availability in the assemblages with only one functional group showed different patterns in the use of space and light, confirming true functional differences among our groups. Experimental results showed that the identity of functional groups was more important than functional richness in determining the ability of macroalgal communities to resist invasion and that resistance to invasion was resource-mediated.

Complementarity in marine biodiversity manipulations: Reconciling divergent evidence from field and mesocosm experiments

“…This second difference is also illustrated by the comparison of 2 experiments examining the effect of functional group diversity on invasion success. When Arenas et al (31) manipulated functional group richness by assembling small monocultures of each functional group growing on natural rock pieces into various combinations, the species largely remained as separate monocultures, interacting weakly and failing to exhibit niche differentiation via canopy layering (31). No effect of richness on invasion by other seaweeds was observed in this experiment.…”

Historical invasions of the intertidal zone of Atlantic North America associated with distinctive patterns of trade and emigration

“…One possibility is that disturbance regimes may have played a role in these invasions, as they do in many modern ones (40). For example, an experimental study in SW England (41) showed that F. serratus had poor invasibility into functional groups (e.g., canopy and turf) that are more common on Atlantic shores than on Gulf shores because of the friable rock (e.g., sandstone, slate) and strong ice scour of Gulf shores (Pictou shipping begins in late April and ends in December because of blockage of the Gulf by sea ice, Table S3). Additionally, the significant amounts of rock ballast dumped underwater created hard substratum below the winter ice pack; this may have facilitated successful colonization by F. serratus on newly dumped ballast.…”

Facilitation of the introduced green alga Caulerpa racemosa by resident algal turfs: experimental evaluation of underlying mechanisms

“…C. racemosa reproduces vegetatively from fragments and its ability to overgrow other sessile organisms, both invertebrates and algae, is not limited by the availability of free space. In contrast, the creation of patches of bare space, maximising the availability of resources such as space and light, was effective at enhancing the biomass of marine invaders characterised by reproductive strategies different from those of C. racemosa (sea squirts in Stachowicz et al 2002; ephemerals and the canopy-forming alga Fucus serratus in Arenas et al 2006; Codium fragile ssp. tomentosoides in Bulleri & Airoldi 2005).…”

“…Other studies have attempted to shed some light on the mechanisms regulating invasion success (Mack et al 2000, Pimentel et al 2000. There is, in fact, convincing evidence that resistance to invasion varies greatly among recipient assemblages (Levine & D'Antonio 1999, Lonsdale 1999, Kennedy et al 2002, Arenas et al 2006.…”