Constructing a database of alien plants in the Himalayas to test patterns structuring diversity

Differences in the number of alien plant species in different locations
may reflect climatic and other controls that similarly affect native
species and/or propagule pressure accompanied by delayed spread from the
point of introduction. We set out to examine these alternatives for
Himalayan plants, in a phylogenetic framework. We build a database of
alien plant distributions for the Himalayas. Focusing on the
well-documented regions of Jammu & Kashmir (west) and Bhutan
(east) we compare alien and native species for (1) richness patterns, (2)
the degree of phylogenetic clustering, (3) the extent to which
species-poor regions are subsets of species-rich regions and (4)
continental and climatic affinities/source. We document 1470 alien species
(at least 600 naturalised), which comprise ~14% of the vascular plants
known from the Himalayas. Alien plant species with tropical affinities
decline in richness with elevation and species at high elevations form a
subset of those at lower elevations, supporting location of introduction
as an important driver of alien plant richness patterns. Separately,
elevations that are especially rich in native plant species are also rich
in alien plant species, suggesting an important role for climate (high
productivity) in determining both native and alien richness. We find no
support for the proposition that variance in human disturbance or numbers
of native species correlate with alien distributions. Results imply an
ongoing expansion of alien species from low-elevation sources, some of
which are highly invasive.