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Hydrilla verticillata recently invaded a protected area in the Upper Paraná River floodplain. In a rare opportunity, when this non-indigenous species co-occurred with the native Egeria najas in a secondary channel of the river, we assessed its effects on assemblage of small-sized fish. Traps were used to catch fish inside macrophyte patches in periods of low and high water. We assessed fish abundance, biomass, diversity, richness and the community composition (using a DCA). The number of fish species did not differ between plants. Fish abundance and biomass were significantly higher during the low water period but plant species did not affect these attributes. Similarly, fish assemblage composition differed only between seasons. Our results indicate that the oscillations in the water level had a greater effect on the small-sized fish assemblage than the presence of the non-indigenous macrophyte. In addition, H. verticillata favors small fish assemblages such as the native does; however, we contend that our conclusions are valid only for habitats scarcely colonized by native plant species. The invasion of other habitats colonized by a high diversity of native species of macrophytes, which are key for small-sized fish, is a matter of concern.

Running title:Riparian plants and invasion resistance Summary 1. Riparian corridors are habitats that are prone to invasion by non-native plants, but the shade provided by trees that grow in riparian zones may limit the success of invasive species. Accepted ArticleThis article is protected by copyright. All rights reserved. 2. In this investigation, we conducted three complementary experiments to test the effects of shade levels, along with sediment type and biotic resistance (and their interactions), on the establishment of a highly invasive grass (Urochloa arrecta (Hack. ex T. Durand & Schinz) Morrone & Zuloaga).3. This invasive grass presented higher growth rates in sediment collected under riparian vegetation than in sediment collected at sites devoid of vegetation. Shade levels similar to those found underneath riparian vegetation suppressed the establishment of U. arrecta. In addition, when native macrophytes were planted a priori (i.e. before U. arrecta propagules arrived), the establishment of the invasive species was suppressed, even at lower levels of shade. Thus, biotic resistance was sufficient to prevent the establishment of the invasive grass at low shade levels. Therefore, our results indicated that although the sediment from riparian zones may favour the establishment of U. arrecta, shading by trees and the presence of native macrophytes suppress it. Synthesis and applications.Our results indicate that the shade provided by riparian trees and biotic resistance by macrophytes are likely the factors that negatively affect the establishment of the invasive macrophyte U. arrecta. A recommendation for management arising from our results is that restoring riparian corridors to prevent invasion is most likely more efficient if native macrophytes are planted along with the trees. This would be an efficient strategy to prevent invasion by U. arrecta and other invasive Poaceae during the early stages of riparian vegetation regeneration.

A good understanding of the mechanisms and magnitude of the impact of invasive alien species on ecosystem services and biodiversity is a prerequisite for the efficient prioritisation of actions to prevent new invasions or for developing mitigation measures. In this review, we identified alien marine species that have a high impact on ecosystem services and biodiversity in European seas, classified the mechanisms of impact, commented on the methods applied for assessing the impact and the related inferential strength, and reported on gaps in available information. Furthermore, we have proposed an updated inventory of 87 marine species in Europe, representing 13 phyla, which have a documented high impact on ecosystem services or biodiversity.Food provision was the ecosystem service that was impacted by the greatest number of alien species (in terms of both positive and negative impacts). Following food provision, the ecosystem services that were negatively affected by the highest number of alien species were ocean nourishment, recreation and tourism, and lifecycle maintenance, while the ecosystem services that were most often positively impacted were cognitive benefits, water purification, and climate regulation. In many cases, marine aliens were found to impact keystone/protected species and habitats. Thirty percent of the assessed species had an impact on entire ecosystem processes or wider ecosystem functioning, more often in a negative fashion. Forty-nine of the assessed species were reported as being ecosystem engineers, which fundamentally modify, create, or define habitats by altering their physical or chemical properties.The positive impacts of alien species are probably underestimated, as there is often a perception bias against alien species. Among the species herein assessed as high-impact species, 17 had only negative and 7 only positive impacts; for the majority (63 species), both negative and positive impacts were reported; the overall balance was often unknown. Although there is no doubt that invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, as it is based on expert judgement or dubious correlations, while only 13% of the reported impacts were inferred via manipulative or natural experiments. A need for stronger inference is evident, to improve our knowledge base of marine biological invasions and better inform environmental managers.

Hydrilla verticillata is an aquatic macrophyte recently found in the Upper Paraná River floodplain. This species has a high potential to reproduce and disperse to new sites, mainly through stem fragments. In this study, we evaluated the colonization (the ability of roots to develop), regeneration (the ability of lateral shoots to develop) and growth rates of this species using fragments of different lengths that originated from different plant parts. We selected parts with and without apical tips, each 5 and 10 cm long, and the following hypothesis was tested: the fragments with apical tips have greater colonization and regeneration abilities than fragments without apical tips, and these abilities depend on the fragment length. Our results showed that both fragments with and without tips presented the same number and dry weight of roots and shoots and thus did not differ in their colonization or regeneration abilities. The length was important for the colonization ability, as 10 cm fragments produced more roots than 5 cm fragments. The high colonization and regeneration abilities from different fragments coupled with the high growth rates exhibited by H. verticillata may account for its rapid spread in novel habitats and its competitive success over native species. Thus, we suggest that the inadequate management of this species, resulting in the formation of fragments, especially of large lengths, may have important implications with regard to its infestation of new sites. KEY WORDS: Fragmentation · Exotic species · Non-native species · Aquatic weedResale or republication not permitted without written consent of the publisher Aquat Biol 16: 197-202, 2012 1999, Coetzee et al. 2009), and, due to this feature, the mechanical removal or harvesting of this species is rarely successful. Accordingly, the management of this species has proven to be problematic (Hershner & Havens 2008). Stem fragments, which are mainly produced by the mechanical breakage of plants, float away from the original colonies and settle in the sediment (Madsen & Smith 1999). These fragments exhibit 2 survival strategies: (1) the development of roots and rapid establishment in the sediment (colonization) and (2) the development of new propagules that can be dispersed (regeneration) (Southwood 1988, Barrat-Segretain & Bornette 2000.Hydrilla verticillata recently invaded the Upper Paraná River Floodplain (UPRF) in Brazil (Sousa et al. 2009(Sousa et al. , 2010, and the potential for the spread of this exotic species is of particular concern because of the negative impacts associated with its occurrence in aquatic ecosystems (Posey et al. 1993, Hofstra et al. 1999, Wang et al. 2008. However, the main mechanism of dispersion/colonization of H. verticillata in the UPRF is most likely via small fragments (Sousa et al. 2009). Thus, to verify which type of fragments might increase the establishment success of this species, we evaluated the ability of H. verticillata to colonize, regenerate and grow from fragments that originated from differ...