Counteradaptation and Integration into the Biotic Community - Alien Species and Evolution - page 252

rodinogaster ) of Tasmania combines the feeding behaviors of two robin
species inVictoria, foraging on the ground, in tree canopies, and by hawk-
ing insects in the air. Only two species of canopy-feeding, insectivorous
thornbills ( Acanthiza spp.) are present in Tasmania, compared to four
species in southern Victoria. Two of the Victoria species are specialist
canopy feeders. In Tasmania, the two species of thornbills that are present
have expanded their feeding activity into the canopy zone.
Changes in tarsus length and in bill shape and length are shown by
many of the passerine birds in Tasmania that have shifted their feeding
behavior relative to equivalent species in Victoria (Keast 1970). True
trunk-foraging species of sittellas ( Neositta spp.) and tree-creepers ( Climac-
teris spp.), widespread in mainland Australia, are absent from Tasmania.
Several birds from groups that are not trunk-foragers on the mainland
have moved into this niche in Tasmania.These species tend to have longer
bills, which are more effective for probing crevices, than their mainland
relatives. Other species, such as the pink robin, that have expanded ground
foraging activity show a lengthened tarsus.
In addition to these adaptive shifts in habitat use, foraging behavior,
and morphology by many species, double invasions of Tasmania from
mainland Australia have occurred in two bird genera.These invasions have
given rise to sibling species of foliage-gleaning insectivores, which are
among the groups most poorly represented in Tasmanian forests (Keast
1970). Thus, by a combination of speciation, habitat and foraging shifts,
and evolutionary adjustments, the bird fauna of Tasmania has achieved a
high degree of integration over the past few thousand years.
Long-term Evolutionary Integration
One of the most interesting examples of a geologically recent plant
invader of North America is the creosote bush ( Larrea tridentata ), which is
the dominant woody shrub of the Sonoran, Mojave, and Chihuahuan
deserts. The genus Larrea is of South American origin, and four species
of this genus now occur in Argentina. These species include L. divaricata ,
a sister species of L. tridentata (Hunter et al. 2001). Several other genera
closely related to Larrea also occur in South America. It is generally agreed
that L. tridentata reached North America by long-distance dispersal, prob-
ably as seeds carried by migratory birds (Wells and Hunziker 1976)
during a period of climatic aridity corresponding to glacial periods in
the northern hemisphere. These periods of aridity may have allowed the
South American L. divaricata to spread farther north than its current