4. Discussion
Our study showed how non-invaded bird communities retained higher
taxonomical and functional diversity (TD and FD respectively) thaninvaded communities in a highly human-modified landscape,
supporting the idea that higher species diversity shapes community
resistance to invasions (Gerhold et al., 2011; Lososová et al., 2015).
Moreover, we found that it is more likely that alien birds occupy novel
parts of the functional space in bird communities characterised by low
TD and FD (invaded vs invaded no alien ), but that they do
not fully compensate for the taxonomic and functional biodiversity loss
caused by the absence of native species in invaded sites. Our findings
also highlighted how these trends were stronger in less human-modified
and more heterogeneous areas, whereas the diversity metrics were
negatively affected in more human-modified areas. Finally, these results
were similar (but stronger) when we considered those communities where
the Feral Pigeon was present as invaded (i.e. it was treated as
an alien species), suggesting that this ubiquitous bird has more
characteristics of an alien than native species.
4.1. Diversity metrics and community types
In our study, the higher TD and FD found in non-invaded thaninvaded bird communities (Test 1) across land-uses likely
suggests a general negative impact of alien species on native bird
communities inhabiting the area. TD and FD were also higher innon-invaded than invaded no alien communities (i.e.,invaded communities considering only native species; Test 2).
This impact could reflect in a loss of ecosystem functioning in the
invaded bird communities (e.g. through loss of functional groups; Flynn
et al., 2009; Cadotte et al. 2011). For example, high functional
divergence (FDiv) infers a high niche differentiation in the community
(Cosset & Edwards, 2017), suggesting that the non-invadedcommunities provide a more diverse pool of functional processes thaninvaded communities (Edwards et al., 2013). Similar reasoning
could be applied when accounting for functional evenness (FEve), since
high values of FEve infer efficient use of resources by species in the
community (Lee & Martin, 2017). Therefore, non-invaded showed
higher resilience (i.e. FEve) than invaded communities (Test 1),
but lower susceptibility to disturbance than invaded no aliencommunities (Test 2). This is consistent with the biotic resistance
hypothesis stating that higher diversity should promote resistance to
invasion (Ordonez, 2010).
Following Test 3 (invaded vs invaded no alien ), our
results showed how alien species were functionally different from native
species of the resident invaded community assemblages. In fact, FDiv was
higher in invaded than invaded no alien communities. This
suggests that alien species provide a set of functional traits that are
different from those of the native species. For example, in Portugal,
the common waxbill (Estrilda astrid ) has successfully established
in unsaturated communities, occupying a marginal niche (Batalha et al.,
2013). Therefore, most alien species may show different adaptations to
native species, leading to higher niche differentiation of theinvaded communities, suggesting that alien species usually occupy
a different ecological niche in those communities (Hejda & de Bello
2013). Nonetheless, the FEve showed no significant difference betweeninvaded vs invaded no alien communities. Thus, despite
that alien species have a higher niche differentiation in invadedcommunities, they do not provide a more efficient use of resources than
the native species in the community.
From a PD point of view, we found that invaded communities had
similar Mean Pairwise Distance (MPD) to both non-invaded andinvaded no alien communities (Test 1 and 3), while species ininvaded no alien communities were more clustered (lower MPD) than
species in non-invaded communities (Test 2). The latter, along
with the TD and FD results, supports the resistance hypothesis. In fact,
higher PD could infer a community characterized by several adaptations,
leading to increased competition and hampering alien species spread
(Ketola et al., 2017). Finally, since the addition of alien species
closes this phylogenetic gap with non-invaded communities (Test
1), it is more likely that alien species are favoured in more clustered
communities (Lososová et al., 2015), occupying novel phylogenetic space
in the invaded community (e.g. Psittacula krameri ).
These patterns were also partially true when we compared the diversity
metrics within land-uses to exclude the effect of environmental
filtering on bird community assemblage (Loiola et al., 2018).
Nonetheless, MPD was higher for invaded than non-invadedbird communities of urban areas, suggesting a higher vulnerability to
alien species invasion (i.e. a phylogenetic gap). Moreover, no
differences were found between community types within forest, either due
to the low proportion of invaded communities, or because alien
bird species seem to be weakly influenced by environmental filtering
compared to its effect on the structure of native bird communities
(Lazarina et al., 2022).