2019

De Bona, Sebastiano

Invasive populations are a major driver of biodiversity loss. Most invasions are discovered after the population is established, spreading, and often adapting to the new ecological conditions, hampering effective eradication. Understanding the spatial dynamics of introduced populations, and how these change throughout the invasion, is crucial to predicting their spread and restricting their harm. In this thesis, I studied density-dependent dispersal, habitat use, and changes in population spread after the introduction of guppies (Poecilia reticulata) in montane streams. This was done by combining the analyses of long-term individual-based data with short-term manipulations in the field, and with the mathematical modelling of spread. Introduced populations were shown to grow rapidly after translocation, and attain densities beyond those of natural populations if the habitat is disturbed. The study highlighted that the effect of density on habitat use is scale-dependent: at the landscape scale, guppies occupy habitat patches according to the ideal free distribution; at the local scale, size-dependent responses to density suggest that large individuals displace smaller ones from good quality microhabitats, implying an ideal despotic distribution. Moreover, density at both the local (within habitat patch) and landscape (whole stream) scales was shown to affect dispersal. Finally, guppies were found to be successful invaders upon introduction, but to rapidly lose their invasive potential as they adapt to limiting resources in the environment. In the later phases of the invasion, individual dispersal distance negatively affects the speed of spread of the population. This counterintuitive result can be explained when considering how population growth and individual dispersal interact in determining population spread. I conclude that 1) disturbance can favour invasions; 2) studies of spatial dynamics should be explicit about the scale examined and; 3) integrative approaches are crucial to understanding the spread of introduced populations.