Invasion of alien macroalgae in different Mediterranean habitats

The study evaluated different macroalgal invasions in the main Mediterranean coastal habitats on hard bottom. Biodiversity, species composition and structure of macroalgal assemblages were compared among non-invaded areas and areas invaded by the Chlorophyta Caulerpa racemosa var. cylindracea and by the turf-forming Rhodophyta Womersleyella setacea in three different habitats: shallow rocky bottom, deep rocky bottom and dead matte of the seagrass Posidonia oceanica. Results showed that alien macroalgae constituted a relevant component of benthic assemblages in invaded areas of the Mediterranean Sea. Assemblages invaded by Womersleyella setacea and Caulerpa racemosa showed lower values of diversity and large differences in the structure and species composition related to non-nvaded assemblages. The species that mostly suffered from invasion were erect species reproducing sexually; moreover, the dominance of W. setacea led to low abundance of native filamentous algae, while C. racemosa colonization seemed particularly threatening for encrusting algae. All the studied habitats appeared highly invasible by alien macroalgae, even if W. setacea appeared more invasive in deeper habitats, while colonization of C. racemosa seemed more serious in shallower habitats; the dead matte of P. oceanica represented a suitable substrate for the spread of both species. Differences among assemblages in different habitats were reduced in invaded areas.     

<Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> as a potential source of organic matter for benthic consumers: evidences from a stable isotope analysis

Caulerpa racemosa var. cylindracea has been invading various types of substrates in wide areas throughout the Mediterranean Sea. However, the effects of the distribution of this alga on zoobenthos are scanty. The aim of this study was to evaluate the effects of C. racemosa invasion on the feeding habits of some zoobenthic taxa inhabiting the upper infralittoral rocky shores. This was done by identifying the isotopic N and C ratios of several potential food sources and testing differences in isotope composition among the taxa collected from areas invaded and not-invaded areas by C. racemosa. Results suggest that C. racemosa detritus was a significant food source for the polychaete Syllis prolifera, the gammarid Corophium sextonae and the gastropods Cerithium rupestre and Pisinna glabrata. They would also suggest that stable carbon and nitrogen isotope ratios deserve further attention as a possible valuable approach to understand the overall effect of C. racemosa spread on the trophic interactions among the species.     

Loss of consumers alters the effects of resident assemblages on the local spread of an introduced macroalga

Despite the great interest for the role played by resident assemblages in regulating biological invasions, few studies have assessed how these can influence the spread of exotic species that have successfully established or have included more than one trophic level. On shallow rocky reefs, we assessed how the effects of different benthic assemblages on the spread of an invasive alga, Caulerpa racemosa, are influenced by alterations in the density and species composition of the resident sea urchin assemblage. In order to simulate herbivore species loss scenarios, assemblages dominated by different morphological groups of algae (i.e. turfs or encrusting corallines) or experimentally cleared plots (i.e. bare rock) were exposed to grazing by different combinations of species (Arbacia lixula and Paracentrotus lividus) and densities (natural, ?50% and ?100% of natural densities) of urchins. Algal turfs and encrusting corallines generally facilitated C. racemosa. Manipulating urchins assemblages did not affect the cover and density of fronds of C. racemosa. In contrast, halving the density of P. lividus favoured the penetration of stolons of C. racemosa, consistently among algal assemblages. Other effects of urchins varied among algal assemblages, indicating interactions between trophic levels. In algal turfs, the total removal of urchins caused a decrease in the penetration of stolons, while it enhanced the length of fronds, indicating a shift in the growth form of this clonal plant and, ultimately, a depression of its spreading ability. In bare or encrusting corallines dominated surfaces, the removal of urchins had positive effects on the penetration of stolons and on the length of fronds of C. racemosa, irrespective of one or two species being manipulated and for the intensity of their removal. Our results show that tradeoffs between negative and positive effects of herbivores, varying according to relative densities of species and to the direction and strength of the effects of resident plant assemblages, can influence local rates of spread of C. racemosa. Thus, not only facilitation of exotics by natives can be key in enhancing the spread of exotic species, but it can occur between organisms at different trophic levels. Finally, our findings have important implications for the management of C. racemosa, in view of the progressive domination of shallow rocky reefs by algal turfs.     

Dinucleotide microsatellite markers in the genus <Emphasis Type="Italic">Caulerpa</Emphasis>

Caulerpa spp. are clonal green marine algae which often act as invasive species when growing outside their native biogeographical borders. Over the two past decades, Caulerpa taxifolia has spread along the Mediterranean coast, presently occurring at 70 sites and covering nearly 3,000 ha of subtidal area. New genetic markers (microsatellites) have been developed to assess clonal structure and genetic diversity of recently established populations of the invasive species C. taxifolia and Caulerpa racemosa in comparison with populations of the native Caulerpa prolifera in the Mediterranean. Our results show that nine polymorphic markers have been developed for C. prolifera, seven for C. taxifolia, and three for C. racemosa. Genetic diversity in Caulerpa was assessed in two geographical scales: one at a population scale where 40 thalli units were collected from C. prolifera in Cala d’Or, Mallorca, Spain, and another at a species scale, where 30 sample units were analyzed for C. prolifera, 24 for C. taxifolia, and 24 for C. racemosa from different sites in the Mediterranean, Atlantic, and Pacific Ocean. Number of alleles, expected heterozygosity, and marker amplification success are provided in each case.     

Peracarid assemblages of <Emphasis Type="Italic">Zostera</Emphasis> meadows in an estuarine ecosystem (O Grove inlet,NW Iberian Peninsula): spatial distribution and seasonal variation

The Galician rias are singular and complex estuarine systems of great economic importance. Seagrasses are key elements of the ecosystem and favor the maintenance of high species diversity in benthic communities. Nevertheless, the ecological role of seagrass meadows in the Galician rias has been only partially assessed. Peracarid crustaceans are an important component of soft-bottom faunas and have great importance for the structure of benthic assemblages. In this work, species diversity, patterns of distribution and seasonal fluctuations of peracarids (Crustacea, Peracarida) are studied in estuarine sediments colonized by two species of Zostera (Z. marina and Z. noltii) at the O Grove inlet (Ría de Arousa, Galicia, NW Iberian Peninsula). The spatial distribution of peracarid assemblages was characterized by high numerical dominances due to a few species, particularly tanaidaceans. The temporal study at a Z. marina meadow showed a strongly seasonal pattern defined by great fluctuations of the amphipod population, the latter being the dominant group in abundance and number of species. The highest numbers of species and individuals were observed in September, with minimum values in March. Analyses pointed out a high correlation among the granulometric features of the studied bottoms and the faunistic attributes. Nevertheless, the presence of the seagrasses should influence in a major way the hydrodynamic and sedimentary features of the habitat and utterly the spatial and temporal patterns observed in the peracarid assemblage in the O Grove inlet.     

The indirect role of nutrients in enhancing the invasion of Caulerpa racemosa var cylindracea

The effects of nutrients dynamics on biological invasions in marine habitats have not been fully investigated. The present study aimed to evaluate the role of nutrient pollution in determining the successful invasion of the introduced seaweed Caulerpa racemosa var. cylindracea in a Mediterranean subtidal rocky habitat. To this end, a manipulative field experiment was carried out by supplying the water column with nutrients for a one-year period. Afterwards C. racemosa was transplanted in both nutrient-enriched and control macroalgal assemblages. Results showed that the penetration and biomass of C. racemosa in the assemblages previously treated with nutrients were higher than in control ones, indicating that communities stressed by nutrient pollution are more vulnerable to invasion. The nutrient enrichment changed the structure of natural macroalgal assemblages, mainly by encouraging the growth of turf species. No significant effects were detected for the erect and prostrate layers and for the number of species, suggesting that the susceptibility of the community to invasion may depend more on the morpho-functional identity of macroalgae rather than on the diversity of assemblages. Changes in the availability of seawater nutrients may foster the spread of C. racemosa, both by enhancing its growth and eroding the natural resistance of macroalgal assemblages. Thus, the maintenance of good water quality may play an important role in containing both C. racemosa and other introduced seaweeds.     

Do native herbivores provide resistance to Mediterranean marine bioinvasions? A seaweed example

Generalist herbivores in marine ecosystems are poorly examined for their potential to serve as a source of biotic resistance against algal invasion. We assessed how one of the main generalist herbivores in Mediterranean rocky reefs (the sea urchin Paracentrotus lividus) affects Lophocladia lallemandii and Caulerpa racemosa, two algal invaders with strong detrimental effects on native benthic communities. In a comparison of sea urchin gut contents to algal community composition, strong preferences were exhibited, leading to no relationship between consumption and availability. Both C. racemosa and L. lallemandi were abundant in algal assemblages (>60% occurrence), but C. racemosa (20% of diet) was consumed more than L. lallemandi (3.5%). Experimental enclosures of sea urchins (12 sea urchins * m⁻²) were carried out in locations where L. lallemandii was already established and C. racemosa was rare (new invasion) or abundant (established invasion). C. racemosa was negatively affected by sea urchins only when it was rare, and no effect was detected when the alga was already abundant. Results for L. lallemandi were exactly opposite: urchins limited seasonal increases in L. lallemandi in highly-invaded areas. Because of the small amount of direct consumption of L. lallemandi, its decrease in abundance may be related to the grazing of native algae where L. lallemandii is attached. Overall, our results show that high densities of native herbivores may reduce invasive algae at low densities, due to a combination of direct and indirect effects, but it has no significant effect in highly-invaded areas.     

Invasive macrophytes in a marine reserve (Columbretes Islands,NW Mediterranean): spread dynamics and interactions with the endemic scleractinian coral Cladocora caespitosa

The invasive algae Lophocladia lallemandii and Caulerpa racemosa are becoming an important threat to benthic assemblages in the Mediterranean Sea. Both species were first detected in Illa Grossa Bay (Columbretes Islands Marine Reserve, NW Mediterranean) in 2006, and their invasion was monitored until 2012. L. lallemandii showed a rapid outburst, spreading around the entire bay in just 2 years and showing the highest abundances between 5 and 10 m of depth (82.07 ± 3.53 % (±SE) in 2011). Caulerpa racemosa showed a slower but steady spread and remained in deeper areas during the first years; however, drastic changes in the depth distribution, with algae invading toward shallower areas, were noted beginning in 2010 and reached abundances of 57.76 ± 1.07 % (±SE) between 10 and 20 m of depth in 2011. Illa Grossa Bay hosts one of the most important populations of the endemic coral Cladocora caespitosa. This study is the first to quantitatively assess interactions between the coral and invasive algae. Although both invasive species L. lallemandii and C. racemosa had overlapping distributions with C. caespitosa, we did not find any lethal or sublethal effects of either invasive algal species. On the other hand, C. caespitosa exhibited toxic activity, which could explain the low overgrowth of living colony parts by C. racemosa.     

Effects of mean intensity and temporal variability of disturbance on the invasion of <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> (Caulerpales) in rock pools

Disturbance is a key factor influencing the invasibility of habitats and assemblages. This relationship was extensively studied in terrestrial systems, but it was scarcely tested in the marine environment. We investigated experimentally the interactive effects of changes in the intensity and temporal variability of mechanical disturbance by boulders on invasion dynamics of the green alga Caulerpa racemosa var. cylindracea in littoral rock pools. We tested the hypothesis that the success of invasion of C. racemosa would be (1) greater under large than under low intensity of disturbance, (2) greater under large than under low temporal variability of disturbance and that (3) interactive effects could also occur, with variability of disturbance magnifying the effects of intensity. C. racemosa was virtually absent in pools maintained under high intensity of disturbance, independently of temporal variability. High intensity of disturbance was also associated with lower density and length of fronds and thinner diameter of the stolons of the alga. The total number of native taxa and the abundance of encrusting coralline algae increased under high intensity of disturbance. Differently, turf-forming algae were positively affected by temporal variability of disturbance, while canopy-forming algae did not respond to experimental treatments. Our results suggest a direct negative effect of the most severe experimental conditions on the spread of C. racemosa in rock pools. This likely overwhelmed likely concomitant positive and negative effects mediated by resident organisms. The results of this study help anticipating invasion dynamics of C. racemosa in rock pools under climate change scenarios, in which both intensity and temporal variability of extreme meteorological events are predicted to increase.     

Caprellid assemblages (Crustacea: Amphipoda) in shallow waters invaded by <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> from southeastern Spain

Growth of the invasive algae Caulerpa racemosa var. cylindracea in shallow habitats may influence the faunal assemblage composition. We studied its effects on caprellid assemblages associated with shallow-water habitats of hard and soft bottoms from the SE Iberian Peninsula (native rocky-bottom algae, C. racemosa from hard and soft bottoms, and Caulerpa prolifera, Cymodocea nodosa and Posidonia oceanica from soft bottoms). Samples were taken in two different sampling periods (September 2004 and March 2005). A total of seven caprellid species were identified, with important differences in their distribution in different habitats. Total abundance of caprellids was very high in March on native algae on hard bottoms, and on C. racemosa on both soft and hard bottoms. On both hard and soft bottoms, abundances of Caprella hirsuta recorded from C. racemosa were low. On the other hand, a higher abundance of other species, namely C. acanthifera, C. santosrosai, Phtisica marina and Pseudoprotella phasma, was recorded from C. racemosa. The results indicate that C. racemosa may have a positive influence on some caprellid species, while seasonal changes are also evident. It is concluded that introduced C. racemosa may serve as a new habitat, promoting and maintaining caprellid populations in shallow Mediterranean habitats.     

Effects of <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> on prey availability: an experimental approach to predation of amphipods by <Emphasis Type="Italic">Thalassoma pavo</Emphasis> (Labridae)

Alien plant species, such as Caulerpa racemosa var. cylindracea, that invade Mediterranean marine vegetated habitats can affect habitat structure. In turn, changes in habitat structure may affect the associated invertebrate assemblages, either through changes in habitat selection or as a result of altered predation efficiency. In order to test for effects of changes in habitat structure resulting from colonization by C. racemosa on prey availability for predators, the importance of amphipods as a trophic resource in natural vegetated habitat was first assessed, and later experiments were undertaken to assess the effects of the alien alga on predation by Thalassoma pavo of two dominant amphipods: Elasmopus brasiliensis (Gammaridea) and Caprella dilatata (Caprellidea). Laboratory experiments were conducted in separate aquaria with five vegetation habitat types: Halopteris scoparia, Jania rubens, C. racemosa without detritus, C. racemosa with detritus, Cymodocea nodosa, together with controls. The vegetation was first defaunated, and then 30 amphipods were introduced to each aquarium and exposed to a single Thalassoma pavo individual for 1 h, after which the fish’s gut contents were examined. Consumption (per fish per hour) of caprellids (11.7 ± 1.4) was higher overall than that of gammarids (8.7 ± 1.5) and likely reflects different microhabitat use by amphipods, which affects susceptibility to predators. Consumption of amphipods also varied by habitat type. The highest predation rate was found in the C. nodosa habitat (12.7 ± 2.19) and the lowest in the C. racemosa habitats with detritus (4.1 ± 1.78) and without detritus (5.2 ± 0.55), which did not differ. The pattern of predation across habitats, however, was similar for both caprellid and gammarid amphipods, indicating a more general effect of habitat on amphipod predation. Our findings showed that invasive species such as C. racemosa can decrease feeding by predators such as T. pavo. Changes in predator–prey interactions could have consequences for food web support in the Mediterranean.     

Comparative study of the growth of the two co-occurring introduced green algae Caulerpa taxifolia and Caulerpa racemosa along the Tuscan coast (Italy,western Mediterranean)

Caulerpa taxifolia and C. racemosa are tropical green algae now found in the Mediterranean, where the former was accidentally introduced and the latter presumably also introduced; both species have spread rapidly and invaded a variety of habitats. Since 1996, the two algae have co-inhabited a region along the Tuscan coast near Livorno. The present study has been carried out to evaluate the invasive capacity of the two species. After one year, the increase in surface area was 67.5 % for C. taxifolia and 284.8 % for C. racemosa, while the increase in patch number was 11.4 % and 121.4 % respectively. The stolon growth of the two algae followed a similar trend, as is showing the same temporal variations with the maximum growth rate between August and October. At the end of the study period, the increase in stolon length and number was significantly higher in C. racemosa than in C. taxifolia. The results of the present study show a higher invasive potential in C. racemosa than in C. taxifolia, although further studies are necessary to evaluate the competition between the two algae and the effect of their simultaneous presence on the benthic community.     

Insect diversity and trophic structure differ on native and non‐indigenous congeneric rushes in coastal salt marshes

Displacement of native plant species by non‐indigenous congeners may affect associated faunal assemblages. In endangered salt marshes of south‐east Australia, the non‐indigenous rush Juncus acutus is currently displacing the native rush Juncus kraussii, which is a dominant habitat‐forming species along the upper border of coastal salt marshes. We sampled insect assemblages on multiple plants of these congeneric rushes in coastal salt marshes in Sydney, New South Wales, Australia, and compared the abundance, richness, diversity, composition and trophic structure between: (i) J. acutus and J. kraussii at invaded locations; and (ii) J. kraussii at locations either invaded or not invaded by J. acutus. Although J. acutus supported a diverse suite of insects, species richness and diversity were significantly greater on the native J. kraussii. Moreover, insect assemblages associated with J. kraussii at sites invaded by J. acutus were significantly different from, and more variable than, those on J. kraussii at non‐invaded sites. The trophic structure of the insect assemblages was also different, including the abundance and richness of predators and herbivores, suggesting that J. acutus may be altering consumer interactions, and may be spreading in part because of a reduction in herbivory. This strongly suggests that J. acutus is not playing a functionally similar role to J. kraussii with respect to the plant‐associated insect species assemblages. Consequently, at sites where this non‐indigenous species successfully displaces the native congener, this may have important ecological consequences for community composition and functioning of these endangered coastal salt marshes.     

Response of macroalgal assemblages from rockpools to climate change: effects of persistent increase in temperature and CO2

Anthropogenically induced global climate change has important implications for marine ecosystems with unprecedented ecological and economic consequences. Climate change will include the simultaneous increase of temperature and CO₂ concentration in oceans. However, experimental manipulations of these factors at the community scale are rare. In this study, we used an experimental approach in mesocosms to analyse the combined effects of elevated CO₂ and temperature on macroalgal assemblages from intertidal rock pools. Our model systems were synthetic assemblages of varying diversity and understory component and canopy species identity. We used assemblages invaded by the non‐indigenous canopy forming alga Sargassum muticum and assemblages with the native canopy species Cystoseira tamariscifolia. We examined the effects of both climate change factors on several ecosystem functioning variables (i.e. photosynthetic efficiency, productivity, respiration and biomass) and how these effects could be shaped by the diversity and species identity of assemblages. CO₂ alone or in combination with temperature affected the performance of macroalgae at both individual and assemblage level. In particular, high CO₂ and high temperature (20°C) drastically reduced the biomass of macroalgal assemblages and affected their productivity and respiration rates. The identity of canopy species also played an important role in shaping assemblage responses, whereas species richness did not seem to affect such responses. Species belonging to the same functional effect group responded differently to the same environmental conditions. Data suggested that assemblages invaded with S. muticum might be more resistant in a future scenario of climate change. Thus, in a future scenario of increasing temperature and CO₂ concentration, macroalgal assemblages invaded with canopy‐forming species sharing response traits similar to those of S. muticum could be favoured.     

Illegal trawling and induced invasive algal spread as collaborative factors in a Posidonia oceanica meadow degradation

Posidonia oceanica, a key seagrass species of the Mediterranean Sea, shows clear signs of regression throughout the Mediterranean and illegal trawling is recognized as one of the main causes. We examined the condition of a P. oceanica meadow in Alykes Bay (Zakynthos Island, western Greece), a typical Mediterranean littoral area where illegal trawling is common practice, in respect to the total area affected, and in terms of possible ecological substitution. A side scan sonar (SSS) survey of the seafloor provided an image of the condition of the meadow and biological sampling evaluated the ecological status in affected meadow areas. SSS images revealed that trawling has a serious effect on the meadow, with 11% of the vegetated area being abraded, and the affected areas were also found to be fully colonized by the invasive alga Caulerpa racemosa. Moreover, unusually high densities of the polychaete Sabella pavonina were detected in the affected areas among C. racemosa fronds. Recolonization by P. oceanica of the affected meadow areas that have been colonized by C. racemosa seems improbable considering the allelopathic interactions between the species, with the alga displaying phytotoxic properties through caulerpenyne production and deterioration of the sediment quality.     

Biogeographical patterns of algal communities in the Mediterranean Sea: Cystoseira crinita‐dominated assemblages as a case study

Aim The aim of this study was to describe the composition, community structure and biogeographical variation of subtidal algal assemblages dominated by the brown alga Cystoseira crinita across the Mediterranean Sea. Location The Mediterranean coast, from Spain (1°25′ E) to Turkey (30°26′ E). Methods Data on the species composition and structure of assemblages dominated by the species C. crinita were collected from 101 sites in nine regions across the Mediterranean Sea. Multivariate and univariate statistical tools were used to investigate patterns of variation in the composition of the assemblages among sites and regions, and to compare these with previously defined biogeographical regions. Linear regressions of species richness versus longitude and versus latitude were also carried out to test previously formulated hypotheses of biodiversity gradients in the Mediterranean Sea. Results The main features characterizing C. crinita‐dominated assemblages across the Mediterranean included a similar total cover of species, a similar cover of C. crinita, and consistency in the presence of the epiphyte Haliptilon virgatum. Biogeographical variation was detected as shifts in relative abundances of species among regions, partly coinciding with previously described biogeographical sectors. A significant positive correlation was found between species richness and latitude, while no significant correlation was detected between species richness and longitude. Main conclusions The patterns of variation in community structure detected among the studied regions reflected their geographical positions quite well. However, latitude seemed to contribute more to the explanation of biological patterns of diversity than did geographical distances or boundaries, which classically have been used to delimit biogeographical sectors. Moreover, the positive correlation between species richness and latitude reinforced the idea that latitude, and possibly temperature as a related environmental factor, plays a primary role in structuring biogeographical patterns in the Mediterranean Sea. The lack of correlation between species richness and longitude contradicts the notion that there is a decrease in species richness from west to east in the Mediterranean, following the direction of species colonization from the Atlantic.     

Benthic assemblages of wetlands invaded by Tarebia granifera (Lamarck, 1822) (Caenogastropoda: Thiaridae) in the iSimangaliso Wetland Park,South Africa

Tarebia granifera is a freshwater/estuarine gastropod invading many tropical and sub-tropical areas around the world. This snail is native to southeast Asia and was accidentally introduced into South Africa during the last decade. The current study investigated shallow-water benthic assemblages of different invaded and uninvaded localities across locations spanning a large range of environmental conditions in the iSimangaliso Wetland Park. Using a correlation-based approach, we found that native benthic assemblages were more closely associated with environmental conditions than with densities of T. granifera. However, there were significant negative correlations between T. granifera abundance and Shannon Diversity at two of the invaded locations. This alien species has successfully invaded, and become dominant in, different types of water bodies with different assemblage compositions and physico-chemical characteristics, ranging from freshwater ponds to saline estuaries and lakes. The current data set is presented as an essential baseline for future studies. It is recommended that future work focuses on specific localities, in order to determine if changes in diversity are driven by non-native species or by other disturbances (e.g., climate change).     

Habitat heterogeneity promotes the coexistence of exotic seaweeds

Despite the progressive accumulation of exotic species in natural communities, little effort has been devoted to elucidating the mechanisms underpinning the coexistence of invaders in environmentally and biologically heterogeneous systems. The exotic seaweeds, Asparagopsis taxiformis and Caulerpa racemosa, exhibit a segregated distribution on Mediterranean rocky reefs. A. taxiformis dominates assemblages in topographically complex habitats, but is virtually absent on homogenous platforms. In contrast, C. racemosa achieves extensive cover in both types of habitat. We assessed whether differences in their distribution were generated by biotic interactions (between invaders and/or between invaders and natives) or by environmental constraints. Three models were proposed to explain seaweed distribution patterns: (1) invaders inhibit one another; (2) native assemblages, differing between complex and simple habitats, prevent the establishment/spread of one invader, but not that of the other; and (3) environmental conditions regulate the establishment/persistence of the seaweeds in different habitats. We removed the dominant invader and resident assemblages in each type of habitat. Moreover, A. taxiformis thalli were transplanted into the habitat dominated by C. racemosa to establish whether its failure to colonize the simple habitat was due to the lack of propagules or post-recruitment mortality. C. racemosa spread in the complex habitat was not influenced by the removal of resident assemblages, but it was slightly enhanced by A. taxiformis removal. Neither C. racemosa removal nor that of resident assemblages promoted A. taxiformis colonization and survival in simple habitats. Our results suggest that heterogeneity in environmental conditions can promote invader coexistence by mitigating the effects of negative biotic interactions. Therefore, the accumulation of introduced species in native communities does not necessarily imply established invaders fostering further invasion.     

Tracking the origin of the invading Caulerpa (Caulerpales, Chlorophyta) with Geographic Profiling, a criminological technique for a killer alga

The application of the Geographic Profiling technique (with the “Rossmo formula”) proved to be effective in assessing the spreading origin of invading species of Caulerpa in the Mediterranean. Geoprofiling is a technique more frequently used in criminology. We applied this method to an algal invasion for the first time. The method was calibrated with the distribution data of Caulerpa taxifolia, whose spreading in the Mediterranean Sea started from the aquarium of Monaco. This is the first time that Geographic Profiling is calibrated on a data set of sites of presence of a biological invader, of which the spreading origin is known. The application on Caulerpa racemosa var. cylindracea showed that the probable spreading center of the alga should be located in Western Sicily for the Mediterranean and in the southern part of Tenerife for the Canary Islands (Las Galletas, a fishing village). The spreading centers correspond to areas with ports, indicating that the spreading of this alga should be related to (fishing) boats. This result confirmed the opinion of other authors. The parameters used in the Rossmo Formula, obtained through calibration with the known origin of C. taxifolia invasion for identifying the origin of the spread of C. racemosa var. cylindracea may be extended to other algae with similar propagation mode and similar habitat requirements.