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Marc Rius - Research

I am broadly interested in the ecological and evolutionary mechanisms that determine species ranges and maintain populations within these ranges. My main research interests include community ecology, population genetics, biogeography, and conservation biology, with a special focus on marine ecosystems. I am acutely interested in biological invasions as they not only represent tremendous conservation challenges, but also serve as a rich source of knowledge. They epitomise large scale biogeographic ‘experiments’ that can provide otherwise unattainable insights into fundamental ecological and evolutionary processes.

 Global distribution of the Pyura stolonifera species complex

My research predominantly focuses on marine species that are ecosystem engineers. Many marine invertebrates from lower trophic levels fall into this category and are among the most abundant and diversified groups across the world’s oceans. These animals are responsible for the majority of biological invasions that occur along coastal areas and are easily manageable for use in experiments. Taken together, they are an ideal group to address fundamental questions in ecology, evolution and biogeography.

Ecosystem engineer species as habitat modifiers

My research can be divided in six sections:

1. Reconstructing routes of invasion in the sea

The ever-increasing environmental impacts derived from human activities have extensively modified evolutionary trajectories of populations across species ranges. Biological invasions are both direct and indirect consequences of such large-scale habitat alteration, and constitute a crucial factor shaping biodiversity and biogeographic patterns worldwide. There is growing recognition that marine biological invasions are often extremely complex due to the high prevalence of multiple sources and non-independent introductions. However, the relative importance of the trade-off between losses of genetic diversity due to bottleneck processes during colonisation, and high genetic diversity as a result of multiple origins or introduction events, remains to be fully understood. In order to track routes of invasion of a widely distributed species, I have developed new molecular tools and implemented innovative analytical techniques that allow contrasting a wide range of complex scenarios. 
General questions include:
- Can the native range be reliably identified? If so, are admixture processes responsible for the genetic composition of the introduced range?
- Are colonisations generally non-independent?
- In what sequence have introduced populations been colonized?
- Are genetically-diverse populations a result of a single introduction of a large number of genetically heterogeneous individuals, or of recurrent introductions?

Relevant Publications:
Rius et al. 2012.PLoS ONE pdf
Rius & Shenkar 2012.Marine Pollution Bulletin pdf
Rius et al. 2008.Molecular Ecology Resources pdf
                                            Rius et al. 2008.Diversity and Distributions pdf

2. Understanding the naturalization of non-indigenous species

The mechanisms that determine the introduction, establishment and spread of non-indigenous species can only be correctly interpreted by first obtaining rigorous historical data. In addition, acquiring ecological data on life-history traits within its introduced range is vital to understand relevant key characteristics of the invasion process such as generation time and adaptive phenotypic plasticity. I have been involved in several collaborative projects, which have included researchers from Japan, South Africa, Spain and the United States of America, and assessed the magnitude and characteristics of marine and estuarine introductions along coastlines using both a regional and global approach.

Substratum dominated by the introduced ascidian
Microcosmus squamiger in the Mediterranean Sea  

Relevant Publications
Mead et al. 2011.Journal of Natural History pdf
Rius & Griffiths 2011.Book chapter pdf
Mead et al. 2011.Biological Invasions pdf
Rius et al. 2009.Biological Invasions pdf
Turon et al. 2007.Journal of Experimental Marine Biology and Ecology pdf
Robinson et al. 2005.African Journal of Marine Science pdf

3. Cryptic diversity and species delimitation

Many of the world’s coastal regions contain a large proportion of marine species that cannot be clearly identified due to a lack of systematic, biogeographical and historical evidence. The increasing availability of DNA sequence data has improved this situation, resulting in an increase in the identification of cryptic biodiversity. In an innovative study, we combined a morphological analysis with genetic data to unravel cryptic speciation within a widespread species complex that is highly abundant along temperate coastal regions of the southern hemisphere, namely the Pyura stolonifera species complex. This species complex is popularly known as cunjevoi in Australia and as redbait in South Africa.

Our research has revealed that in Australasia this complex comprises three species (two new to science) that are both morphologically and genetically distinct, and not one species (Pyura praeputialis) as previously thought. This work has been highlighted in both scientific and popular articles, including a piece entitled ‘Molecular view reassesses species’ published in Frontiers in Ecology and Environment, where the controversial 
topic of species delimitation is discussed and framed around the assessment and conservation of global biodiversity.

Relevant Publications:
Rius & Teske 2011.Zootaxa pdf
Teske, Rius et al. 2011.BMC Evolutionary Biology pdf

                                               Extensive aggregates of Pyura stolonifera

Dorsal tubercle of Pyura dalbyi

4. Maternal effects and the importance of early life-history stages

Most organisms have complex life-cycles whereby individuals undergo dramatic developmental transitions before reaching adulthood. 

Larva of Pyura herdmani obtained by artificial 
fertilization in the laboratory

While studies have investigated key community traits such as biotic resistance during adult stages, there is little understanding of the relative importance of processes affecting early life-history stages and their carry-over effects. I found that lethal and non-lethal effects on early life-history stages can determine the viability of adults of native sessile species. In addition, I studied selection pressures in a species with complex life-cycles, in which mothers must balance the fecundity costs of increasing propagule size with the fitness benefits of increased propagule performance.
Questions addressed include:
- Are settlement preferences a good predictor of adult field distribution?
- Is biotic resistance occurring evenly across multiple life-history stages?
- Are offspring size effects consistent across the entire life cycle?

Relevant Publications
Pineda et al. 2012.PLoS ONE pdf
Rius et al. 2010.Functional Ecology pdf
Rius et al. 2010.Marine Ecology Progress Series pdf
Rius et al. 2009.Oecologia pdf

5. Species interactions and mechanisms of coexistence

Studies on species interactions have traditionally focused on negative interactions such as competition and predation, but there is increasing recognition that positive interactions are more important than previously thought. I have conducted experiments assessing the relative role of competition and facilitation in determining the distribution of marine foundation species. In addition, and considering that abiotic stresses can directly influence the outcome of species interactions, I analysed species interactions along gradients of conditions.
The results untangled the influence of abiotic factors in mediating biotic interactions and coexistence between newcomers and well-established species. Through this work I addressed questions such as:
- Are positive and negative interactions temporally and spatially partitioned?
- How are species interactions maintained over long periods?

Relevant Publications
Rius et al. 2011.Marine Pollution Bulletin pdf
Rius & McQuaid 2009.Basic and Applied Ecology pdf
Zardi et al. 2006.Marine Biology pdf 
Rius & McQuaid 2006.Marine Biology pdf

Wave action is one of the primary causes of disturbance
on rocky shores, often preventing dominant competitors 
from monopolizing primary space

6. Marine Protected Areas

Marine Protected Areas (MPAs) are crucial for conservation biology and management. Most studies of MPAs focus on the recovery of top predators with fewer studies focusing on lower trophic level organisms. In my early work, I assessed the impact of recreational exploitation along protected and non-protected coastlines in southern Europe, as well as how inaccessible or formally protected areas are beneficial to the recovery of exploited populations. In South Africa, I studied how ancestral subsistence exploitation is reconciled with modern day conservation enforcement.

Human harvesting is an ancestral activity

Relevant Publications:
Rius & Zabala 2008.Aquatic Conservation: Marine and Freshwater Ecosystems pdf
Rius 2007.Scientia Marina pdf
Rius et al. 2006.South African Journal of Science pdf
Rius & Cabral 2004.Scientia Marina pdf

Aerial surveys along the South African coast to assess patterns of exploitation pressure