ECR Feature: Kevin Ma on the abundance and distribution of invasive mussels

Kevin Ma is a postdoc at Rhodes University. He is a biogeographer with an interest in marine organisms and their spatial structure. Kevin shares his recent work on the distribution of the invasive mussel, Mytilus galloprovincialis, in South Africa.

Kevin, sampling mussels during low tide.

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Institute. Rhodes University, South Africa

Academic life stage. Postdoc

Research themes. Spatial scales; Range shifts; Larval Ecology; Biological invasions; Early detection; Monitoring

Current study system. Native to the Mediterranean Sea, the mussel Mytilus galloprovincialis has been introduced to every continent except Antarctica. This highly invasive species has massive ecological consequences in southern Africa that range from outcompeting other species for space to driving increased abundances of endangered African oystercatchers. Its invasion of southern Africa continues to be one of our focal model systems to understanding marine biogeography, species interactions, and invasions. Moreover, as a relatively well-studied species, it is an ideal model species to look at long-term changes to better understand patterns of biological invasions, especially as it spreads from one biogeographic region to another.

Recent paper in JBI. Ma KCK, Gusha MNC, Zardi GI, Nicastro KR, Monsinjon JR, McQuaid CD. 2021. Biogeographic drivers of distribution and abundance in an alien ecosystem engineer: Transboundary range expansion, barriers to spread, and spatial structure. Journal of Biogeography.​

Motivation behind this paper. We were initially motivated to identify the contemporary warm range-edge of Mytilus galloprovincialis in southern Africa that might have shifted since their original invasion. Unexpectedly, our surveys revealed that its distributional limits had not changed substantially for the past decade or so. In fact, the warm range-edge has remained relatively stable at the transitional zone between two bioregions. We then wanted to know how this invader had interacted with biogeographic boundaries over the course of its invasion history in southern Africa. Consequently, the geographic scope of the study expanded by 1000s of km to encompass much of the southern African coastline that have been invaded by this mussel. Opportunistically, we were on the lookout for any new alien species (for early detection purposes) and any significant distributional changes of other established invasive species. In particular, we noted that another invasive mussel, Semimytilus algosus, an invader from Chile, has been spreading in the region, so this was reported in another study (Ma et al. 2020, in African Journal of Marine Science).

A patch of mussels, which consisted of both the invasive and native species

Methodologies. In addition to extracting occurrence records from the literature, we sampled an invasive mussel across 1000s of kilometres of the South African coastline to understand how its range had shifted over decadal timescales and assess spatial structure in abundance. Determining whether there are patterns in abundance across space can be challenging, because it depends on the scale of observation and the ecological processes operating at those scales. For example, the importance of wave action may be obvious at small scales, but hidden by biogeographic effects at larger scales. This can lead to contrasting patterns of deterministic versus stochastic abundance depending on the spatial scale sampled. We borrowed a technique that is commonly used on temporal datasets to examine heterogeneity in mussel abundance across space: wavelet analysis. By decomposing abundance values into different scales, wavelet analysis helped us identify the scales that exhibited significant structure in abundance, allowing us to speculate on the ecological processes most likely to explain these scale-dependent patterns.

Unexpected outcomes. Although the dominant mussels on rocky intertidal shores of southern Africa are the invasive Mytilus galloprovincialis and the native Perna perna, we also encountered another invasive mussel, Semimytilus algosus. Right away, we realised that S. algosus has extended its range since its distribution South Africa was last surveyed in 2010. In light of this unexpected information, we compiled all available historic and contemporary records of S. algosus to reconstruct its invasion history, first, for its invasion of South Africa (Ma et al. 2020 in African Journal of Marine Science) and, later, of Namibia and Angola (Ma et al. 2020 in PLoS ONE).

Exposed rocky shore at low tide.

Major results. Our recent paper clearly demonstrates that marine invasions along the South African coastline do not spread constantly over time and that biogeographic boundaries influence the rate of colonisation over decadal timescales. We hope that this contribution will help us better understand how marine biogeography drives (and hinders) the spread of biological invasions, especially the saltatory spread of alien ecosystem engineers. In addition, structure in the abundance of an invasive mussel was detected at multiple ranges of spatial scales, namely, scales of 120­–160 kilometres and of 400–990 kilometres. By identifying these dominant spatial scales, we hope that this will bring us one step closer to understanding large-scale ecological processes that together determine mussel abundance. Although the processes themselves are not exactly known to us, detecting structure at these two ranges of scales indicate that such processes are operating on mussel abundance at both intra-bioregional (scales of 120–160 km) and inter-biogeographical scales (scales of 400–900 km).

Next steps. At present, we would like to take a closer look at the range-edge dynamics of mussels by examining long-term changes in abundance and variation in endolithic infestation between range edge to centre populations.

If you could study any organism on Earth, what would it be? In addition to studying intertidal species, I also have a life-long fascination with ascidians (Tunicata: Ascidiacea). And, if I could, I would certainly like to study deep sea ascidians, especially macrophagous/carnivorous species. I imagine that there is more to know about their ecology, biogeography, taxonomy, and physiology. With the advent of ROVs, new records of deep-sea ascidians are being collected and documented, probably because specimens were damaged in the past when they were collected via dredging. Excitingly, new species are still being described in infrequently sampled regions of the world.

Anything else to add? It would be remiss not to mention the wealth of ecological data (in this case records of Mytilus galloprovincialis) that can be found in student theses. These records accounted for a substantial proportion of the historical dataset we used to reconstruct the geographic spread of this invader. Often student theses contain valuable details (e.g., dates, localities) that were not necessarily mentioned in the corresponding journal articles.

Morning view of rocky shores after a very early rise

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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