DOI: 10.5061/dryad.3bk3j9kpx
title: Alien plant species are precursors for invasion: a case study of Alternanthera brasiliana (L.) Kuntze in Ile-Ife (Nigeria)

description: The impact of Alternanthera brasiliana on vegetation and soil seed bank
 was assessed in Ile-Ife, Nigeria. Ten sample plots, 10 m x 10 m each, were
 established in invaded plant communities with high density of
 Alternanthera brasiliana and adjacent uninvaded plant communities where
 the weed species has low density. In each sample plot, twenty 1 m x 1 m
 quadrats were randomly laid and all rooted plant species were identified
 and counted. Post-dispersal soil seed bank was collected by randomly
 taking five core samples of top soil per sample plot to estimate the soil
 seed bank density and floristics of the sites. The species composition of
 soil seed bank was compared with that of the above-ground vegetation so as
 to assess the invader’s impact on the vegetation using Sorensen’s index of
 similarity. The results showed that Alternanthera brasiliana invasion
 significantly impacted on the species diversity (t = 5.27; df = 18; p =
 0.0003) and evenness of species distribution (t = 4.50; df = 18; p =
 0.00005) in the aboveground vegetation, and the species diversity (t =
 5.37; df = 18; p = 0.00004) and evenness of species distribution (t =
 6.19; df = 18; p &lt; 0.0001) in the soil seed bank. This study
 concluded that Alternanthera brasiliana has significantly caused
 alterations in key parameters of the aboveground vegetation and those of
 the soil seed bank. It is likely that with increasing resident time, these
 alterations might increase more significantly to enhance the spread of
 Alternanthera brasiliana.
description: <strong>Study area</strong> The
 study was carried out in some abandoned sites interspersed within the
 Obafemi Awolowo University estate in the City of Ile-Ife, in the
 southwestern Nigeria (Fig. 1). The university estate covers approximately
 56.09 square kilometers (Makinde <em>et al</em>. 2021).
 Ile-Ife is found in the lowland rainforest type (Keay 1959) (latitudes 7°
 30′ to 7° 35′ N and longitude 4° 30′ to 4° 35′ E, 213–457 msl) (Hall
 1969). Ile-Ife has a short dry season that persists for approximately four
 months (November to March) with the rainy season occupying the remaining
 months with an average annual temperature of 25.5 °C, average annual
 rainfall of 1302 mm, average relative humidity of 82%, annual solar
 radiation of 164.30 Wm<sup>-2</sup>, and average annual wind
 speed of 2.06 kmhr<sup>-1</sup> (Atmospheric Physics Research
 Group 2013). The land use system of the university estate is apportioned
 as follow: 18% of the area constitutes built infrastructure, 22% serves as
 agricultural and research farmland, with 60% remaining undeveloped
 (Makinde <em>et al.</em> 2021). <em>Alternanthera.
 brasiliana </em>is widely distributed through the university estate.
 For this study, invaded sites with close proximity to uninvaded sites
 (<em>A. brasiliana</em> absent or at very low density) were
 selected for sampling. The invaded site were located between latitudes
 7°31.391′ to 7°31.531′ N and longitudes 4°31.676′ to 4°31.872′ E while the
 uninvaded sites ranged between latitudes 7°31.198′ to 7°31.207′ N and
 longitudes 4°31.737′ to 4°31.751′ E. The study area is underlain by rocks
 of the Basement Complex of Precambrian age (De Swardt 1953), with soils
 belonging to the Ultisols class (USDA 1975, Uwalaka and Muoghalu
 2021). <strong>Impact on aboveground vegetation
 parameters</strong> To determine ecological
 impact of the plant invader on the invaded plant communities, ten 10 m ×
 10 m sample plots (termed large plots) were established in invaded plant
 communities with high cover or density of <em>A.
 brasiliana</em> and adjacent uninvaded plant communities where
 <em>A. brasiliana </em>is not present or had very low cover or
 density in the Obafemi Awolowo University estate. Within the ten large
 plots, twenty 1 m × 1 m quadrats were randomly placed and sampled to
 estimate the density and frequency of species in each large plot. In each
 quadrat, all rooted plant species were identified and counted. To
 determine the percentage cover of species, five 10 m line transects were
 randomly laid using a measuring tape in each large plot. A single cover
 pin was then perpendicularly dropped at every metre point along each
 transect according to the method of Evans and Love (1957), modified by
 Uwalaka and Muoghalu (2021). The “Lucky draw randomisation method” was
 used by generating random numbers to produce points where quadrats were
 laid, and transects were sampled. All plant species parts that contact the
 pin was recorded to estimate the cover of the species in each large plot
 (Uwalaka and Muoghalu 2021). The percent cover was then calculated as the
 number of ‘hits’ per species divided by the total number of pins dropped
 multiplied by 100 (Uwalaka and Muoghalu 2021). Voucher specimens of the
 plant species which could not be identified in the field were collected
 and identified in the IFE Herbarium of the Department of Botany, Obafemi
 Awolowo University, Ile-Ife, Nigeria. Species nomenclature follows that of
 POWO (http://www.plantsoftheworldonline.org/).
 <strong>Impact on soil seed bank
 parameters</strong> From the same plots where
 aboveground vegetation parameters were measured, soil seed bank samples
 were collected after seed rain by randomly taking five core samples of the
 top soil (0–20 cm depth) per large plot between December 2017 and February
 2018 after the resident plant species (invader and other plant species)
 had completed their growth cycles and their seeds had been dispersed. The
 soil samples were air-dried, and the debris, stone and gravel were removed
 by passing the soil samples through a 4 mm mesh sieve. The soil was thinly
 spread in perforated 45 cm-diameter plastic trays and placed in a
 screenhouse at the Department of Botany, Obafemi Awolowo University,
 Ile-Ife, at an average temperature of 39 °C, light intensity of 3517 lux
 and humidity level of 54% (Uwalaka and Muoghalu 2021). To ensure that
 seeds were not accidentally introduced into the trays, three trays
 containing post-dispersed soil seed bank samples that had been sterilised
 in an oven at 105 °C for 48 hours to ensure deactivation of the seeds
 present in the soil, were used as control (Uwalaka and Muoghalu 2021).
 Germinated seedlings were counted, identified and removed every week and
 the soil stirred to bring seeds buried in the soil to the surface.
 Unidentified germinated seedlings were transplanted into planting bags,
 watered daily until they flowered and were then identified. The seed bank
 study lasted 12 months. <strong>Data and
 statistical analysis</strong> Species density in
 the aboveground vegetation was estimated as mean density ± 95% confidence
 interval per hectare for each species. Species richness was estimated as
 the number of species in each plot per site. The sum of relative density,
 relative frequency and relative cover was used to estimate the species
 importance value of each species in the invaded and uninvaded communities
 (Cottam and Curtis 1956) which was in turn used to assess the dominance of
 each species in the communities. To determine the relationship between the
 aboveground vegetation and soil seed bank, the species composition of the
 site types was compared using Sorensen's index of similarity with the
 formula: ISS =  x 100. Where; C is the number of species common
 to both seed bank and standing vegetation, A is the number of species in
 seed bank, B is the number of species in standing vegetation. Indicator
 Species Analysis (ISA) was performed on the dataset using the
 ‘Indicspecies’ package in R, after grouping the plots into two,
 <em>viz</em>. invaded and uninvaded. The plots served as
 indicators (row) while the species were the vector (column). The
 importance value index of the species, which was the measure of abundance,
 was used as factor in the dataset. Principal Component Analysis (PCA) was
 further used to identify the impact of <em>A. brasiliana
 </em>on the invaded site using the importance value index of each
 species as standard of abundance. The Chi-square test of independence was
 carried out to test for the association between plant species forms in
 aboveground vegetation and their corresponding soil seed banks in both
 invaded and uninvaded sites. An independent samples
 <em>t</em>-test was used to compare the means of the total
 species density, diversity and evenness between the invaded and uninvaded
 plots of the aboveground vegetation as well as those of the invaded and
 uninvaded soil seed banks. Plants were divided into guilds (native,
 invader of interest and other alien species), and the contributions of
 species to the aboveground vegetation and soil seed bank were expressed as
 a percentage. The percentage contributions were then used to test for the
 influence of the presence of other alien species in the aboveground
 vegetation and seed bank of both invaded and uninvaded sites using a
 contingency test. The analysis was done using the PAleontological
 STatistics (PAST) version 3.17 software (Hammer <em>et
 al</em>. 2001).
description: The files are in text and excel formats.
description: # Alien plant species are precursors for invasion: a case study of
 Alternanthera brasiliana (L.) Kuntze in Ile-Ife (Nigeria)
 [https://doi.org/10.5061/dryad.3bk3j9kpx](https://doi.org/10.5061/dryad.3bk3j9kpx) The excel file contains two different two site types (aboveground vegetation and soil seed bank) where *Alternanthera brasiliana* is present in high abundance (invaded) and present in low abundance or absent (uninvaded). The data contains the densities across all ten plots of the invaded and uninvaded sites, as well as the percentage contribution of each species to the total site density. Ten plots were examined for each of the sites and presented as mean and 95% CI. All rows in columns A and B are string variables comprising the taxa (species names and their families), with the last row of columns A and B merged to signify the total (sum) of all the corresponding columns from C to P. The R code file contains the code used in the R software to perform the indicator species analysis of the aboveground vegetation. The R code was broken down to show the package needed for the analysis as well as the functions needed to perform the analysis.