Mechanisms Influencing the Spread of a Native Marine Alga

Abstract

Like invasive macrophytes, some native macrophytes are spreading rapidly with consequences for community structure. There is evidence that the native alga Caulerpa filiformis is spreading along intertidal rocky shores in New South Wales, Australia, seemingly at the expense of native Sargassum spp. We experimentally investigated the role physical disturbance plays in the spread of C. filiformis and its possible consequences for Sargassum spp. Cleared patches within beds of C. filiformis (Caulerpa habitat) or Sargassum spp. (Sargassum habitat) at multiple sites showed that C. filiformis had significantly higher recruitment (via propagules) into its own habitat. The recruitment of Sargassum spp. to Caulerpa habitat was rare, possibly due in part to sediment accretion within Caulerpa habitat. Diversity of newly recruited epibiotic assemblages within Caulerpa habitat was significantly less than in Sargassum habitat. In addition, more C. filiformis than Sargassum spp. recruited to Sargassum habitat at some sites. On common boundaries between these two macroalgae, the vegetative growth of adjacent C. filiformis into cleared patches was significantly higher than for adjacent Sargassum spp. In both experiments, results were largely independent of the size of disturbance (clearing). Lastly, we used PAM fluorometry to show that the photosynthetic condition of Sargassum spp. fronds adjacent to C. filiformis was generally suppressed relative to those distant from C. filiformis. Thus, physical disturbance, combined with invasive traits (e.g. high levels of recruitment and vegetative growth) most likely facilitate the spread of C. filiformis, with the ramifications being lower epibiotic diversity and possibly reduced photosynthetic condition of co-occurring native macrophytes.

Beyond the border: Effects of an expanding algal habitat on the fauna of neighbouring habitats

“…With negative effects on the physiology of species of Sargassum, and rapid recolonisation of disturbed patches, this alga appears to be replacing other habitat-forming macroalgae on these reefs (Zhang et al, 2014). Patches colonised by C. filiformis support a low diversity assemblage (this study, Zhang et al, 2014), and our results indicate that this alga also affects the abundance and richness of fauna on neighbouring habitats in close proximity.…”

“…Dense patches of C. filiformis accumulate sediments more quickly than neighbouring patches of Sargassum spp. (Zhang et al, 2014) and tolerance to sedimentation has been inferred as a likely mechanism for the success of this species (Glasby et al, in press). Finally, increased abundances of predators associated with C. filiformis, or altered interaction strengths along habitat boundaries (e.g., Warry et al, 2009), could affect the abundance of epifauna on S. linearifolium when in close proximity to this alga.…”

“…Yet, tests of the effects on fauna of native range expanding marine macrophytes are rare (but see Arrontes, 2002). Caulerpa filiformis is a green alga native to south-east Australia that appears to have increased in abundance over the last century, with increased sedimentation (Glasby et al in press;Zhang et al, 2014) and loss of co-occurring brown algae due to pollution (Coleman et al, 2008) suggested as causal factors. It is now a dominant species on many rocky reefs in the Sydney region (Glasby et al, in press;Pillmann et al, 1997;Zhang et al, 2014) and can form mono-specific patches hundreds of meters in area.…”

“…Caulerpa filiformis is a green alga native to south-east Australia that appears to have increased in abundance over the last century, with increased sedimentation (Glasby et al in press;Zhang et al, 2014) and loss of co-occurring brown algae due to pollution (Coleman et al, 2008) suggested as causal factors. It is now a dominant species on many rocky reefs in the Sydney region (Glasby et al, in press;Pillmann et al, 1997;Zhang et al, 2014) and can form mono-specific patches hundreds of meters in area. It colonises new areas by the dispersal of fragments and the vegetative growth of stolons (Khou et al, 2007) and its establishment is facilitated by disturbance, colonising disturbed patches more quickly than co-occurring macroalgae (Zhang et al, 2014).…”

“…It is now a dominant species on many rocky reefs in the Sydney region (Glasby et al, in press;Pillmann et al, 1997;Zhang et al, 2014) and can form mono-specific patches hundreds of meters in area. It colonises new areas by the dispersal of fragments and the vegetative growth of stolons (Khou et al, 2007) and its establishment is facilitated by disturbance, colonising disturbed patches more quickly than co-occurring macroalgae (Zhang et al, 2014).…”