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M. Scherer-Lorenzen, H. Olde Venterink, and H. Buschmann
Fig. 10.1 Spatial pattern of total inorganic nitrogen deposition in the early 1990s
(mgNm –2 year –1 ; reprinted from Galloway et al. 2004, with permission from the author)
At a continental or regional scale, however, we found some evidence for a
positive correlation between N deposition and invasive species abundance.
We combined data on inorganic nitrogen wet deposition from nitrate and
ammonium, extracted from the US National Atmospheric Deposition Program/National
Trends Network (NADP 2006), with estimates of nonnative
plant species abundance in North American ecoregions, provided by the
PAGE project of the World Resources Institute (WRI 2000, based on data of
the US World Wildlife Fund WWF). This shows that the proportion of exotic
species in forest ecoregions of the USA clearly increases at higher levels of wet
N deposition (Fig. 10.2). By contrast, no such pattern emerges for corresponding
data from grassland ecoregions (data not shown). Of course, the relationship
depicted in Fig. 10.2 is no absolute proof of a causal link between N
enrichment and invasibility, because other factors determining invader success
do also change in the vicinity of centers of N emission, such as disturbance,
traffic, or seed input. Nevertheless, it convincingly demonstrates the
potentially accelerating effects on invasibility resulting from complex interactions
between various drivers of environmental change.