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We studied population dynamics of red squirrels in a group of small forest fragments, that cover only 6.5% of the total study area (4664 ha) and where distances to the nearest source population were up to 2.2 km. We tested effects of patch size, quality and isolation and supplementary feeding on patch occupation during 1995-99. Larger patches and patches with supplementary feeding had a higher probability of being occupied. No patch < 3.5 ha was ever occupied. No effects of isolation were found, suggesting that the forest habitat in the study area is not sufficiently fragmented to influence red squirrel distribution across patches. For medium sized patches (3.7-21 ha), that were occupied some years, there was an increase in patch occupation over the years, even though overall population size tended to decrease. These patches had a high turnover, especially of males. Patches in which the squirrel population went extinct were recolonized within a year. For patches that were at least some years occupied. squirrel density depended on patch quality only. No effects of patch size isolation and winter temperature on population density were found. These data suggest that in our study area habitat fragmentation has no effect on local squirrel density and that the random sample hypothesis explains the distribution pattern across patches.

Niche conservatism, the hypothesis that niches remain constant through time and space, is crucial for the study of biological invasions as it underlies native-range based predictions of invasion risk. Niche changes between native and non-native populations are increasingly reported. However, it has been argued that these changes arise mainly because in their novel range, species occupy only a subset of the environments they inhabit in their native range, and not because they expand into environments entirely novel to them. Here, using occurrences of 29 vertebrate species native to either Europe or North America and introduced into the other continent, we assess the prevalence of niche changes between native and non-native populations and assess whether the changes detected are caused primarily by native niche unfilling in the non-native range rather than by expansion into novel environments. We show that niche overlap between native and non-native populations is generally low because of a large degree of niche unfilling in the non-native range. This most probably reflects an ongoing colonization of the novel range, as niche changes were smaller for species that were introduced longer ago and into a larger number of locations. Niche expansion was rare, and for the few species exhibiting larger amounts of niche overlap, an unfilling of the niche in the native range (e.g. through competition or dispersal limitations) is the most probable explanation. The fact that for most species, the realized non-native niche is a subset of the realized native niche allows native-range based niche models to generate accurate predictions of invasion risk. These results suggest that niche changes arising during biological invasions are strongly influenced by propagule pressure and colonization processes, and we argue that introduction history should be taken into account when evaluating niche conservatism in the context of biological invasions.

Despite its relevance for the persistence of populations, the ecological mechanisms underlying habitat use decisions of juvenile birds are poorly understood. We examined postfledging habitat selection of radio-tracked juvenile middle spotted woodpeckers Dendrocopos medius at multiple hierarchically-nested spatial scales in NW Spain. At the landscape and home range scales, old oak forest was the most used and selected habitat, young oak forests and pine plantations were avoided, and riverside forests were used as available. At a lower scale, birds selected larger diameter trees for foraging. Home ranges had higher densities of large deciduous trees (mainly oaks Quercus spp., but also poplars Populus spp. and willows Salix spp. >22 cm and >33 cm DBH) selected for foraging by juveniles than non-used areas. These results suggest that foraging conditions may drive, at least partly, habitat use decisions by juvenile birds. We also discuss the potential influence of intraspecific competition, the search for a future breeding territory in the early postfledging period and predation avoidance on habitat use decisions by juvenile birds. Contrary to previous studies on migrant forest birds, postfledging juvenile woodpeckers selected the same habitat as for the breeding adults (i.e. old oak forest), indicating that migrant and resident specialist avian species may require different conservation actions. Conservation strategies of woodpecker populations should consider the protection of old oak forests with high densities of large trees to provide suitable habitat to breeding adults and postfledging juveniles. The habitat improvement for this indicator and umbrella species would also favour other organisms that depend on characteristics of old-growth oak forests.

There is a large and growing number of alien species in ecosystems all over the world. In view of this, management efforts should concentrate on invasives that have detrimental effects on native biota. However, action against invasives is often hindered by a lack of relevant ecological information such as the expected distribution and impact of the invader. Our aims were to identify the habitat characteristics that influence ring-necked parakeet abundance in Belgium and to assess the effects of competition for nesting cavities with native hole-nesters. We determined the abundance of parakeets and native hole-nesters in 44 study sites using point counts. We examined the relationship between parakeet numbers and a set of habitat and landscape variables and to assess the effect of competition, we studied the relationships between the number of parakeets and the number of native hole-nesters. We found that parakeet abundance differs between vegetation types based on dominant tree species, and that abundance is higher in forests or parks surrounded by built-up areas. Parakeet numbers were also strongly associated with cavity density, suggesting that this may be a limiting factor. Of the two native hole-nesters considered to be the most vulnerable to competition (nuthatch and starling), we found a negative association with parakeet numbers for the nuthatch only. No relations were found with starlings or any of the other hole-nesting species examined. These findings should be considered in the debate concerning the possible need of action against this species.

Density-dependent dispersal can be caused by various mechanisms, from competition inducing individuals to emigrate (positive density-dependence) to social crowding effects impeding free movement (negative density-dependence). Various spatial population models have incorporated positively density-dependent dispersal algorithms, and recent theoretical models have explored the conditions for density-dependent dispersal (DD) to evolve. However, while the existence of DD is well documented in some taxa such as insects, there is no clear picture on its generality in vertebrates. Here I review the available empirical data on DD in birds and mammals, focusing mainly on variation in dispersal between years and on experimental density manipulations. Surprisingly few studies have explicitly focused on DD, and interpretation of the available data is often hampered by differences in approach, small sample sizes and/or statistical shortcomings. Positive DD was reported in 50 and 33% of the selected mammal and bird studies, respectively, while two studies on mammals (out of eight) reported negative DD. Among bird studies, DD was more often reported for emigration rates or long-distance recoveries than for average distances within finite study areas. Experimental studies manipulating densities (mainly on mammals) have consistently generated positive DD, typically showing reduced emigration in response to partial population removal. Studies that examined dispersal in relation to seasonal changes in density (small mammals only) have more often reported negative DD. Studies that compared dispersal between sites differing in density, also show a mixture of positive and negative DD. This suggests that dispersal changes in a more complex way with seasonal and spatial density variation than with annual densities, and/or that these results are confounded by other factors differing between seasons and sites, such as habitat quality. I conclude that both correlational and experimental studies support the existence of positive, rather than negative, density-dependent dispersal in birds and mammals.

Few studies are available that analyse variation in dispersal rates between populations. Here we present data on the degree of local recruitment (LR) of great and blue tits (Parus major, P. caeruleus) in a large number of nest-box plots in northern Belgium that vary in size, degree of isolation and population density, These plots have been studied for varying lengths of time over the past 40 yr. As expected, I.R was higher among male than female birds, and this difference was most pronounced in blue tits. Regardless of species and sex, more local recruits were found in larger plots but also in plots with a higher population density, Thus, LR increased with population size (number of pairs) but levelled off in the largest populations at ca 50% for male birds, LR was higher in forest fragments compared to plots inside continuous forest suggesting that fragmentation reduces exchange among local populations. However. LR was not related to the degree of isolation of individual fragments. We also found a weak but significant increase in LR with productivity (number of nestlings) of plots, but no relationship with other demographic variables.

We studied the winter foraging niches of tits and related species in deciduous forest fragments varying in size between 1 and 30 ha (plus one forest of 200 ha) in order to investigate the influence of forest fragmentation on foraging niches. Very few correlations between niche structure (foraging niche, width and overlap) and forest size or isolation turned out to be significant. This implies that either the species that disappear in small fragments are those that suffer most from competition (making the effect unmeasurable), or that competition is relatively unimportant for niche structure. In any case we find no evidence that foraging niches are strongly affected by the changes (in habitat and/or community structure) associated with fragmentation.

We studied establishment in summer territories by first-year nuthatches Sitta europaea both in a large forest and in a set of small and isolated forest fragments (2 yr in each area). In the large forest, vacant territories were rapidly taken up by newly formed pairs of juveniles. In the fragments, settlers arrived at a slower rate, more of them remained unpaired, and more territories remained vacant at the end of the dispersal period. Furthermore, territories were taken up in a highly predictable order related to territory quality in the large forest, but not so in the fragments. We suggest that dispersal is more costly in a highly fragmented habitat, reducing the number of settlers at the population level, and reducing opportunities for pair formation and habitat selection at the individual level.

Predation on artificial nests was studied in Belgian deciduous forest fragments between 1 and 200 ha. Predation rates were compared to fragment size, distance from the forest edge, time period (three replicates), and nest type (ground and tree). Logistic regression analysis showed that overall nest predation did not vary with distance from the edge, forest size, and time period. Birds represented over 70% of all predator attacks but their importance decreased in larger areas and away from the forest edge where mammals were responsible for much of the nest predation. It is concluded that the effect of habitat fragmentation depends on the composition of the local predator community.