Assembly of nonnative floras along elevational gradients explained by directional ecological filtering

Abstract

Nonnative species richness typically declines along environmental gradients such as elevation. It is usually assumed that this is because few invaders possess the necessary adaptations to succeed under extreme environmental conditions. Here, we show that nonnative plants reaching high elevations around the world are not highly specialized stress tolerators but species with broad climatic tolerances capable of growing across a wide elevational range. These results contrast with patterns for native species, and they can be explained by the unidirectional expansion of nonnative species from anthropogenic sources at low elevations and the progressive dropping out of species with narrow elevational amplitudes--a process that we call directional ecological filtering. Independent data confirm that climatic generalists have succeeded in colonizing the more extreme environments at higher elevations. These results suggest that invasion resistance is not conferred by extreme conditions at a particular site but determined by pathways of introduction of nonnative species. In the future, increased direct introduction of nonnative species with specialized ecophysiological adaptations to mountain environments could increase the risk of invasion. As well as providing a general explanation for gradients of nonnative species richness and the importance of traits such as phenotypic plasticity for many invasive species, the concept of directional ecological filtering is useful for understanding the initial assembly of some native floras at high elevations and latitudes.

Fig. 1.

Global decreases in nonnative species richness with elevation. The relationship between nonnative species richness in plots along roadsides and elevation in eight mountain regions (stars) around the world. For model parameters and statistics, see Table S1.

Fig. 2.

The elevational ranges (lines) and mean elevation of occurrence (points) of the nonnative and native (box) species recorded in each mountain region ordered by elevation of maximum occurrence. Also shown are the results of a nestedness analysis of species composition in relation to elevation in each region. The analysis tests the hypothesis that the compositions of species-poor, high-elevation sites are nested subsets of the composition of species-rich, low-elevation sites. Only results for the nestedness (nest) of sites and their Z scores are presented (values increase with increasing nestedness). Note that significantly negative Z scores indicate that species composition is less nested than would be expected by chance (20). Note also that the analysis was performed on the presence–absence matrices and not the species ranges shown in the figure.

Fig. 3.

The relationship between elevational range and the maximum elevation reached by nonnative species (circles and solid lines) in each region (B–I). Note that the circles are logically constrained to the gray-shaded one-half of the plots. Colored circles indicate species with ranges that fall exclusively within the lower (blue) or upper (red) halves of the elevational gradient in each region, whereas black circles indicate species whose ranges occupy both halves. The proportion of species in each category is given as colored numbers. A shows the expected relationship if species elevational ranges are distributed randomly along the elevational gradient, with approximately equal proportions of species ranges at low and high elevations; 95% confidence intervals for this expectation in each region are indicated by dashed lines and by black numbers for the proportion of species with ranges exclusively at high and low elevations. In all regions, note the lack of species with ranges falling exclusively at high elevation and the significantly steeper increase in range size with elevation than expected with random range placement. For N values, see Fig. 2.

Fig. 4.

The relationship between elevational range size and the maximum elevation reached by native species (circles and solid lines). Note the much higher proportion of species found exclusively in the upper one-half of the gradients compared with nonnative species (Fig. 3). For N values, see Fig. 2, and for an explanation of symbols, see Fig. 3.

Fig. 5.

The relationship in the six New World regions between the maximum elevation reached by European species and (A) the number of floristic zones that they occupy in Europe and (B) their Landolt temperature indicator values (1, arctic-alpine; 2, subalpine-boreal; 3, montane; 4, colline; 5, lowland/southern European; n = 276 and n = 270 for A and B, respectively). AU, Australia; CC, central Chile; CS, southern Chile; HI, Hawaii; MT, Montana; OR, Oregon.