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Summary1. Geographic information systems (GIS) have recently proved useful for estimating the environmental niche of species across broad geographic regions. However, the application of these nichebased GIS techniques has yet to be extensively applied to local systems. The assumptions of the methods are transferable across scales: species exist across a range of habitats, and these habitats represent a gradient of suitability that can be characterized using multivariate environmental data in association with known species occurrences. 2. Habitat availability and species' niche characteristics are often considered to be important predictors of population density. However, seasonal habitat variability and stochastic events have been hypothesized to limit interactions among species and confound the relationship between available habitat, species niche characteristics and community structure. This research examines the relationship between environmental niche breadth and niche position and population density among species of stream fishes in a seasonally variable environment using a novel application of fine-scale GIS data. 3. Niche breadth and niche position were estimated for 11 species in a local assemblage using five fine-scale (0AE5 m) GIS-based environmental data sets collected during four different times of the year (July, October, January, April). We compared niche measures to variation in population density among species to determine whether environmental niche characteristics, in the context of available habitat, explain variation in numbers of individuals among species. 4. Variation in population density among species in the October sample was predicted by niche breadth measures (R 2 = 0AE752), while variation in population density among species in the January sample was predicted by a model incorporating measures of both niche breadth and niche position (R 2 = 0AE953). Measures of niche breadth and niche position were not correlated with variation in population density in the July and April samples. 5. Species presence and local abundance are often assumed to be predictable based on the availability of suitable habitat. However, little effort has been directed at understanding the influence of both niche breadth and niche position on local abundance. Our results suggest that the amount and distribution of available habitat can be a strong predictor of interspecific variation in population density, even in seasonally variable environments.

This is the first attempt to outline the occurrence of selected invasive alien vertebrates in Greece, since up to now, there are no "official" or scientific reports except from sporadic sightings and anecdotal stories. Records on the occurrence of: Lithobates catesbeianus, Trachemys scripta (T. s. elegans and T. s. scripta), Neovison vison, Myocastor coypus, Nyctereutes procyonoides and Ondatra zibethicus were requested through a pan-Hellenic survey. According to the results, the coypu (Myocastor coypus) appears to be the most widely distributed of all species, having conquered practically all wetlands of Western and Central Greece with populations exceeding, in most cases, 20 individuals each. On the contrary, there is merely one unconfirmed record of the raccoon dog (Nyctereutes procyonoides) while there were no sightings at all for the muskrat (Ondatra zibethicus). The American mink (Neovison vison) was recorded in the northwestern part of Greece. Regarding the alien herpetofauna, the bullfrog (Lithobates catesbeianus) seems to be still confined in Crete where it was originally introduced, while the red-eared slider (Trachemys scripta) appears mostly in Crete, Attiki prefecture, and a few more places, showing most probably an underestimated distribution.

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