DOI: 10.6084/m9.figshare.5755401.v1
title: Growth rates of native and invasive Azolla spp. fern species in mutual invasibility experiments

description: The dataset includes the final growth rates of rare species in mutual invasibility experiments between the native water fern <i>Azolla rubra </i>and the invasive <i>A. pinnata </i>in New Zealand. Growth rates are provided for each of the treatments outlined below.<br>The dataset consists of a single <b>.csv </b>text file, openly accessible through text editor or spreadsheet software. For each experiment the treatment, replicate, initial weight, final weight and population growth rate are provided.<br><br>The replicate column indicates the experiment as follows: - P=<i> A. pinnata</i> growing in monoculture; - R= <i>A. rubra</i> growing in monoculture; - PXR= <i>A. rubra </i>growing when rare with <i>A. pinnata</i> as resident;- RXP= <i>A. pinnata</i> growing when rare with <i>A. rubra</i> as resident<br><br>Specimens of Azolla pinnata were collected from Huntly, Waikato, central North Island. Specimens of A. rubra were collected in October 2015 at the Travis Wetland Nature Heritage Park, Christchurch.<br>The experiments were run for eight weeks between November 30th, 2015 and January 25th, 2016 using 4L buckets (256 cm2 surface area) as mesocosms. Each species was grown in two separate treatments: alone and in the presence of a resident population of its competitor. Each treatment had a total of ten replicates and 1L of filtered water was used in each replicate,<br>For a more detailed description of methodology and population growth calculations please see the related publication.<br><b>Background to related study</b>Despite considerable evidence of alien plants impacting the fecundity, productivity and abundance of native plant species support for alien plant species causing the widespread decline of native species is rare. Coexistence theory proposes that the outcome of competition can be predicted through the invasion criterion, measured as a positive population-level growth rate of each species when that species is rare. Here we make use of coexistence theory to examine the likelihood of persistence of a native water fern (Azolla rubra) following invasion by an alien congener (Azolla pinnata) which has progressively been displaced the native through much of New Zealand. We evaluate the ease of coexistence between the two water fern species, using experimental measurements of population-level growth rates. We show that the alien A. pinnata has a higher fitness than A. rubra, which hinders coexistence between the two species. These experimental results match the rapid expansion of A. pinnata and the apparent decline of A. rubra observed in nature. Our study serves as a prediction of the capability of A. pinnata to replace its native congener, highlights the importance of fitness differences on invasion success, and the value of analyses of species coexistence to predict invasion dynamics and impacts. Using experiments to test coexistence mechanisms between aliens and natives is a valuable approach to predict invasion outcomes that would be more consistent with current attempts to classify alien species in relation to their longer term impacts on native species populations and their extinction risk.<br>