Invasion biology of Trichocorixa verticalis in Doñana, SW Spain

Abstract

Biological invasions are one of the top threats to biodiversity and ecosystem functioning worldwide, and fresh waters are among the most invaded ecosystems in the world. To be successful, an invader must possess qualities that allow invasion in the new habitat, but besides these qualities its success also depends on the interactions between its traits, the traits of the invaded community, and many other contingent factors. A recent addition to the list of alien invertebrate species in European fresh waters is the North American Trichocorixa verticalis (Hemiptera: Corixidae). To date, T. verticalis is the only established alien waterbug in these ecosystems. In the 18 years since its first detection in the Iberian Peninsula, T. verticalis has increased its area of distribution in and around Doñana, in the Guadalquivir delta, and also in other areas of conservation interest including Ramsar wetlands and Nature Reserves in Andalucia. In general, it is highly dominant and abundant in permanent saline waters, where native Corixidae are rare, but it is rare in fresh waters, where native Corixidae dominate. This thesis focuses on four main determinants of successful invasions (plasticity, resource competition, parasites and facilitative interactions among invaders) to investigate why T. verticalis dominates in saline waters while it is rare in fresh waters. In addition, we explored the success of a restoration project for macroinvertebrates in new ponds where T. verticalis is known to be an abundant breeder. In addressing these topics, we apply an invasive-native comparative approach to both experimental and field data collected in Doñana. In Chapter 1 we show experimentally that T. verticalis possesses broader physiological plasticity than native corixids when exposed to different conditions of temperature and salinity, and its physiological tolerance to both heat and freezing increases following exposure to high conductivities. In Chapter 2 we investigate the niche partitioning between native and invasive corixids from different ecosystems by means of carbon (C) and nitrogen (N) stable isotopes. We reveal strong resource partitioning between species in permanent ponds, but also some degree of niche overlap in unstable temporary sites. In Chapter 3 we describe the role of parasite infections during the invasion. T. verticalis show higher total parasite (water mite) prevalence, mean total abundance infection and mean infection than native corixids in low salinity waters, whereas mites are not present in saline waters. In Chapter 4 we examine experimentally the role of facilitative interactions among invaders. T. verticalis invasion does not seem to be promoted by a lower predation rate by alien predators compared to native corixids. In contrast, owing its smaller size it suffers higher predation rates by Odonata larvae. Finally, in Chapter 5 we investigate the value of the Caracoles restoration project in supporting aquatic macroinvertebrates. T. verticalis is known to be an abundant breeder in these ponds. We find that, although new ponds differ from reference sites in abiotic conditions, they become representative and even surpass the levels of local invertebrate richness, diversity and abundance 6-7 years after restoration. However, differences in the abundance and distribution of invasive species between waterbody types and inundation periods may have strong effects on the patterns of species composition, especially for the Hemiptera. The integration of these results sheds light on the role of salinity for the invasion success of T. verticalis, and helps to elucidate why it is still rare in fresh waters. In addition, we also provide important insights on the potential impacts that T. verticalis may have on native Corixids in the future.