Cage Matching: Head to Head Competition Experiments of an Invasive Plant Species from Different Regions as a Means to Test for Differentiation

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

Intersexual Differences in the Strength of Selection on Condition in Drosophila melanogaster

Variation in condition can result from environmental heterogeneity or genetic variation affecting resource acquisition and processing ability. Although condition should be positively correlated with the fitness of both males and females, the strength of selection on condition may differ between the sexes due to differences in reproductive variance. Sexual selection on male condition has been proposed to reduce mutation load, but only if selection is greater on males than females. To investigate the strength of selection on condition, the quality of the larval environment was used to manipulate the condition of Drosophila melanogaster. Additionally, selection was measured when the availability of key resources for females (live yeast) and males (access to females) were altered. Overall, selection was found to be stronger on males than females. However, selection on males weakened under a female-biased sex ratio, whereas selection on females was not significantly affected by the abundance of live yeast.MAS

Body mass data

Body mass dat

Dev Timing-ALL(w-M&F)

Average development time per via

Adult fitnes (over days 1-3)

Adult fitnes (over days 1-3

Data Coding Info

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Tests of the relative interaction index (Rii) contrasting performance in mixtures to that in monocultures.

<p>Mixtures refers to the performance of individuals of <i>C. solstitialis</i> in competition with seeds sown from two regions, Argentina (AR) and California (CA), at 2, 10, and 20 seeds per pot or in monocultures, i.e. seeds from only one invaded region. Two-tailed t-tests were used to determine whether the mean Rii values were significantly different from 0 (at p<0.01).</p

A summary of the generalised linear models used to test the importance of density, region, and density by region on the four responses measured in this greenhouse experiment of <i>C. solstitialis</i>.

<p>Seeds collected from two invaded regions were tested (California and Argentina), densities included 1, 2, 5, 10, and 20 seeds per pot, and details for the responses are reported in the text. Proportionate germination and survival were tested with logistic models and relative growth rates (rgr) with linear models. Bold denotes significant effects.</p