Effects of the invasive macroalgae Gracilaria vermiculophylla on two co-occurring foundation species and associated invertebrates

Mads Thomsen,David Schiel,Peter Stæhr,Lars Nejrup

doi:10.3391/ai.2013.8.2.02

Abstract

Non-native foundation species occasionally invade habitats occupied by native foundation species. Little is known, however, about reciprocal effects of native and non-native foundation species and cascading effects on organisms that depend on foundation species. In a 3factorial field experiment, we tested for: (1) reciprocal effects between the invasive red alga Gracilaria vermiculophylla and two native cooccurring foundation species, the mussel Mytilus edulis and the seagrass Zostera marina; and (2) effects on mobile macro-invertebrates associated with the three foundation species. We found a negative effect of G. vermiculophylla on the above-ground biomass of Z. marina and a positive effect of M. edulis on Z. marina below-ground biomass, but no other significant effects between the three foundation species. Both M. edulis and G. vermiculophylla had positive effects on invertebrate richness and diversity, but Gracilaria also had positive effects on densities of most invertebrate taxa. Additional correlation analyses showed that the abundance of invertebrates increased with the biomass of G. vermiculophylla, indicating density-mediated habitat cascades in invaded seagrass beds. The strong facilitation by G. vermiculophylla could be related to its complex morphology that creates a 3D mosaic of structures and interstitial spaces for different sizes of invertebrates to occupy. Although these results are constrained by the experimental design (one month duration, small plot size, embedded in seagrass meadow, relatively low invader density, invader physically attached with pegs) we suggest that our findings are typical when an invader is structurally complex and mainly occurs in localized patches.

Highlights

Invasive non-native species typically have a negative impact on the abundance and diversity of native species and community structure (Vilà et al 2011)
We found nearsignificant effects (p < 0.1) of Mytilus on Zostera above-ground biomass and on the Z×G interaction
‘Lost’ mussels from the M+ plots had probably re-positioned themselves outside of the plot center

Summary

Introduction

Invasive non-native species typically have a negative impact on the abundance and diversity of native species and community structure (Vilà et al 2011). Non-native species can be foundation species (Dayton 1972) that increase biodiversity by creating and modifying habitats for other organisms (Rodriguez 2006; Wallentinus and Nyberg 2007). Non-native foundation species that invade habitats that lack functionally similar native foundation species are likely to cause positive effects on associated invertebrates, because these organisms may use the non-native species as food, habitat, for stress amelioration, and protection from grazers and predators (Rodriguez 2006; Wallentinus and Nyberg 2007; Thomsen et al 2010). Predicting invasion impacts on native foundation species and associated communities will be even more complicated in habitats where multiple native foundation species co-exist. Few field experiments have tested how marine invaders affect multiple foundation species and their associated communities, making it difficult to understand and predict invasion impacts in many natural systems

Methods

Results

Conclusion