Trait-habitat associations explain novel bird assemblages mixing native and alien species across New-Zealand landscapes

Aim: Species introductions have reshaped island faunas for the last 200
years, often threatening native biodiversity. Approximately equal numbers
of native and alien species currently co-occur in the New Zealand
avifauna, but they show distinct habitat use. Antagonistic interactions,
habitat affinities and legacies of introduction history may concur to
explain their segregation along habitat gradients. To investigate these
processes, we explored how habitat, ecological traits and introduction
history relate with the current composition of bird assemblages. Location:
New Zealand Taxon: Birds Methods: We analysed 917 bird point counts spread
along habitat and elevation gradients in the Canterbury region, South
Island, and related 10 ecological traits to landscape composition using a
three-table ordination method known as ‘RLQ analysis’, accounting for
spatial autocorrelation and phylogeny. We tested whether alien species’
positions in the RLQ were related to proxies of introduction history.
Results: Eighteen endemic, 11 native and 19 alien species were distributed
along a gradient from forest to open-habitat assemblages, in relation to
foraging mode, nesting site and body size. A second gradient segregated
species between native and exotic forests according to territoriality,
sedentarity and diet. Traits accounted for the separation of native and
alien bird species in forests, but not in open habitats. Phylogenetic
signals emerged from the separation of native and alien species by forest
type, and spatial structures suggested a landscape-level, rather than
regional or local determinism. These correlations were independent of
introduction history, although open-habitat assemblages tended to host
alien species introduced later in time.  Main conclusions:
Habitat type and resource availability explain the spatial partitioning of
New Zealand bird assemblages between native and alien species more
consistently than competitive exclusion. We conclude that trait-mediated
ecological differences among species have likely played a predominant role
in species’ segregation among landscapes, while maintaining endemic bird
assemblages in native forests.