Spatial and temporal patterns of native and invasive ascidian assemblages in a Southern California embayment

Abstract

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.

Seasonal heterogeneity provides a niche opportunity for ascidian invasion in subtropical marine communities

“…Based on the seawater parameters obtained from our study sites, it can be generalized that temperature, salinity, chlorophyll-a and volatile suspended solids are most variable between seasons. Temperature and salinity are important environmental stressors affecting the performance, distribution and abundance of native and non-native species (Canning-Clode et al, 2011;Coma et al, 2000;Tracy and Reyns, 2014). Hong Kong is characterized by a summer with high temperature and a decrease of salinity due to heavy rainfall, and a winter with low temperature and high salinity (Morton and Morton, 1983).…”

“…The recruitment and abundance of marine invertebrates (including non-native species) through seasons is a result of species-specific interactions with the environment (Tracy and Reyns, 2014). Therefore, in temperate regions, where seasonal changes are stronger with a larger range of temperature fluctuations, species recruitment and abundance patterns are apparently marked (Stachowicz and Byrnes, 2006;Freestone et al, 2009).…”

“…In subtropical regions, where environmental seasonality varies between temperate and tropical conditions, the recruitment and abundance of marine species clearly respond to seasonal changes (Morton and Morton, 1983;Lam, 2001;Huang, 2003). Studies have reported that some non-native species recruit under seasonal and local spatial conditions in which native species recruitment is lower (Osman et al, 2010;Tracy and Reyns, 2014), suggesting that non-native species may use available temporal niches according to their abilities to respond to environmental changes.…”

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“…Thiyagarajan and Qian (2003); Tracy and Reyns (2014) mentioned that temperature is an important factor which can affect the community composition, distribution and diversity of ascidians.…”

“…Furthermore, invasive ascidians have ability to a) change community structure (Tracy & Reyns, 2014), b) they can lead to the loss of native genotypes (Katsanevakis et al, 2014), c) they can modify habitats, d) they can affect ecosystem processes and trophic relationships in the food chain, e) they can economically affect fishing and aquaculture activities and also f) they can introduce new diseases and pathogens or toxic substances (Marques et al, 2013).…”