IMPROVING THE ASSESSMENT OF SPREAD AND IMPACTS OF FRESHWATER INVASIVE SPECIES
Freshwater habitats occupy a small fraction of Earth’s surface, but they host 6-10% of global biodiversity and are considered endemism hotspots. Moreover, they provide ecological services for more than 6 billion people worldwide, thus their conservation is particularly relevant to preserve both biodiversity and human well-being. Despite their importance, though, freshwater environments are generally less studied than terrestrial habitats. Among the factors threatening global freshwater biodiversity, Freshwater Invasive Species (FIS) play a key role. FIS can impact ecosystems through multiple pathways and in some areas they represent a major portion of freshwater biodiversity, both in terms of number of species and biomass. Moreover, their introduction rate has not yet start decreasing. For these reasons, FIS are considered to be among the main drivers of freshwater biodiversity loss. Understanding the drivers of their distribution and the consequences of their presence on local communities is an essential prerequisite to enable efficient management measures. Despite numerous tools potentially appliable to FIS studies that have been developed in the last decades, their application to this field, as assessed in Chapter 1, appears limited. Moreover, despite the growing interests for this topic, certain relevant aspects of it remain underinvestigated. The aim of this thesis was to improve the assessment of spread and impacts of FIS using different approaches. In Chapter 2, I investigated the current distribution and spread of an invasive crayfish, Procambarus clarkii, in Italy using data coming from multiple data sources: published literature, grey literature, experts’ communications, and online forums. Eradication of FIS is often feasible only when the focal species has yet to establish a relevant population. As there could be a time lag between the first occurrence of a FIS in a certain area and its publication in peer-reviewed journals, using data coming from multiple data sources allows a much more rapid evaluation of the current situation, potentially enabling prompt management actions. Moreover, certain information remains confined within the grey literature and other unconventional sources. In the case of P. clarkii in Italy, 33% of the data used to create the distribution maps were not published in peer-reviewed journals. Despite the risk of biases inherent in these sources, they represent an important tool. Studies on FIS and management plans often require comprehensive information on their distribution, thus including these unconventional data sources may greatly improve conservation of freshwater environments. In Chapter 3, I assessed how different methodological choices affect measures of niche dynamics. Niche conservatism is a prerequisite for Species Distribution Models (SDMs) transferability across space and time. SDMs are widely used in biological invasion studies to infer the potential distribution of an invasive species in its exotic range or under future climatic conditions. Previous studies have reported low levels of niche conservatism in FIS, but whether these findings reflected actual biological mechanisms or were related to the selected modelling approaches was yet to be assessed. FIS distribution may be shaped by different limiting factors than terrestrial species, thus variable selection may be extremely important for model ability to correctly measure the species niche. Moreover, FIS are limited in their spread by the occurrence of freshwater environments, thus certain background selection criteria may fail to describe the actual portion of accessible environments. For these reasons, I tested the effects of 14 different combinations of ecological variables and background selection methods on nine different niche dynamic measures in 40 FIS. I found relevant effects of the different modelling approaches on niche dynamic measures, suggesting that the high rates of niche shift found in FIS may be at least partially explained by apparent shifts unrelated to biological mechanisms. These findings question the reliability of SDMs used to predict potential distribution of FIS and highlight the lack of more in-depth assessments on the optimal strategies to get realistic measure of niche shifts in aquatic species. In Chapter 4, I assessed the effects of multiple environmental characteristics, including the presence of introduced predators (fish and Procambarus clarkii) on adult occurrence and breeding probabilities of seven amphibian species in Northern Italy. Assessing the effects of environmental features and stressors on local aquatic biodiversity is not an easy task, and this is particularly true for species with complex life cycles, as their different life stages can be differently impacted by the same variable. Certain life stages, though, have low detection probabilities, requiring high sampling efforts, implying a trade-off between model reliability and resources needed. I used multistate occupancy models to evaluate the responses of adult occurrence and breeding probabilities to the same parameters and the detection probabilities of adults versus eggs and larvae, then I used the results to assess the optimal monitoring strategy (adults-only versus adults, eggs, and larvae) under different levels of resource availability. Assessing the effects of FIS on native biodiversity is essential to plan management actions, thus when dealing with organisms with complex life cycles, such as amphibians, monitoring programs should be focused on collect data on multiple life stages. In Chapter 5 I evaluated the effects of drought-related breeding pond scarcity and presence of invasive predators on the breeding strategies of two frog species in Northern Italy. Climate change is among the main threats to global biodiversity, and the combined effects of unavailability of freshwater habitats related to climate change and FIS may impact breeding success of amphibian species. Some anurans show high philopatric behavior, while others tend to select breeding sites based on ecological characteristics, such as the presence of predators. Breeding site scarcity may force anurans to breed in the few remaining waterbodies or, alternatively, to skip reproduction. The presence of predator FIS may complicate this choice. Using data from 27 clusters of breeding sites of Rana dalmatina and R. latastei monitored from 2017 to 2023, I found that the former shower higher levels of plasticity, being able to exploit the remaining waterbodies to breed in case of breeding site scarcity, while the latter responded to site scarcity only with a reduction in the number of breeding females. Moreover, both species showed no avoidance behavior toward the invasive Procambarus clarkii, despite its detrimental effects on amphibian larvae. These findings confirmed that plastic species may limit negative effects of climate change thanks to wide ecological tolerance, but endemic species are much more endangered by it. Moreover, they highlighted that predator avoidance may not occur in case of recently introduced invasive species. In this thesis I underlined the importance of assessing spread and impacts of FIS, particularly from a conservation perspective. I found that unconventional data sources may complement peer-reviewed data in order to improve management actions, and that there is an urgent need for a consensus on the best approach to assess FIS niche dynamics. Both occurrence data and niche assessments are needed to create maps of potential distribution, that usually represent the basis for management plans. Moreover, this thesis highlighted the differential effects of FIS on different life-stages of organisms with complex life cycles, such as amphibians, showing that collecting data on multiple life stages may result in better assessments of the status of local populations. Finally, I showed that endemic species with narrow ecological niches may be extremely endangered by the pressures exerted by climate change and FIS, thus requiring careful protection. The role of invasive species in shaping global biodiversity is expected to increase in the next future, thus the findings of this thesis may be particularly relevant for conservation of aquatic habitats.
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