Abstract
Many hypotheses are prevalent in the literature predicting why some plant species can become invasive. However, in some respects, we lack a standard approach to compare the breadth of various studies and differentiate between alternative explanations. Furthermore, most of these hypotheses rely on ‘changes in density’ of an introduced species to infer invasiveness. Here, we propose a simple method to screen invasive plant species for potential differences in density effects between novel regions. Studies of plant competition using density series are a fundamental tool applied to virtually every aspect of plant population ecology to better understand evolution. Hence, we use a simple density series with substitution contrasting the performance of Centaurea solstitialis in monoculture (from one region) to mixtures (seeds from two regions). All else being equal, if there is no difference between the introduced species in the two novel regions compared, Argentina and California, then there should be no competitive differences between intra and inter-regional competition series. Using a replicated regression design, seeds of each species were sown in the greenhouse at 5 densities in monoculture and mixed and grown till onset of flowering. Centaurea seeds from California had higher germination while seedlings had significantly greater survival than Argentina. There was no evidence for density dependence in any measure for the California region but negative density dependence was detected in the germination of seeds from Argentina. The relative differences in competition also differed between regions with no evidence of differential competitive effects of seeds from Argentina in mixture versus monoculture while seeds from California expressed a relative cost in germination and relative growth rate in mixtures with Argentina. In the former instance, lack of difference does not mean ‘no ecological differences’ but does suggest that local adaptation in competitive abilities has not occurred. Importantly, this method successfully detected differences in the response of an invasive species to changes in density between novel regions which suggests that it is a useful preliminary means to explore invasiveness.
Highlights
IntroductionInvasion is primarily a biogeographical issue as it involves the movement (either intentionally or accidentally) of a species from one region to another [3,4]
Understanding the success of invasive plants is not necessarily simple [1,2]
Negative density dependence was detected in the germination of seeds from Argentina (Table 1, significant density6region effect, Fig. 1, best fit curvilinear r2 = 0.97 on summarized data) while survival of plants responded positively at first to increasing seed densities but began to decrease – for the California populations (Fig. 1, best fit curvilinear r2 = 0.77 on summarized data)
Summary
Invasion is primarily a biogeographical issue as it involves the movement (either intentionally or accidentally) of a species from one region to another [3,4] The application of this filter as a means to infer differences is powerful, and there are a variety of broad applications such as (i) population-level experiments, i.e. comparison of success in home versus away regions [5,6], assessment of variation in dominance in novel ranges [7], or gradient studies [8,9] and (ii) individual-based tests such as evidence for differences in plasticity [10], genetics [11,12], fitness [13], or ecotypic differentiation in morphology such as size [14,15]. We propose a simple experimental method using the biogeographical filter as a first approximation to test whether there is evidence for differences in an invasive species in any response characters in competition when introduced to more than one novel region
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