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Impact of Seriphium plumosum densification on Mesic Highveld Grassland biodiversity in South Africa

Citation

Graham, Susannah; Brown, Leslie R; Barrett, Alan S (2020), Impact of Seriphium plumosum densification on Mesic Highveld Grassland biodiversity in South Africa, Dryad, Dataset, https://doi.org/10.5061/dryad.2547d7wmt

Abstract

Mesic Highveld Grassland is important for biodiversity conservation, but is threatened by bush densification from Seriphium plumosum. This indigenous encroacher spreads rapidly and outcompetes other herbaceous species, changing the species composition and structure of grasslands. This study looks at three different densities of S. plumosum and how these affect grassland biodiversity within Telperion, Mpumalanga, South Africa.

An intermediate density of S. plumosum resulted in the highest species diversity (H = 2,26), a low density was moderately diverse (H = 1,96), and a high density was least diverse (H = 1,78). There were differences between the three densities in terms of plant species diversity, with the intermediate density being significantly more diverse (P < 0.01) than both the low and high densities. Findings indicate that there was a significant difference between the sites in terms of ecological successional status (P < 0,01). 

The presence of S. plumosum at low densities can be considered an integral part of the environment. It is important that in areas where S. plumosum occurs, it should be monitored. If this species is not in balance within its environment and it starts becoming dense, it will negatively affect the biodiversity, species composition and structure of the habitat.

Methods

1. Sampling design

Initial vegetation surveys were conducted in all three sites during February 2017 to calculate the S. plumosum frequency and establish the gradient of density between the three study sites. Grass and S. plumosum height was measured using a measuring stick at one meter intervals along a 100 m line transect, the transect was placed diagonally across the study site. At each of the three study sites all S. plumosum plants were counted in two 100 m2 (10 m x 10 m) sample plots. To determine species composition, species richness and species diversity, all plant species within twenty 1 m x 1 m quadrants were identified and counted within each of the study sites. The biomass of grass vegetation was determined using a Disc Pasture Meter (DPM) that was previously calibrated for the reserve (n = 100 readings were collected per sample site). Since we were interested in the grass component and its grazing value to grassland animal species, only open grass areas in the three sample sites were sampled with the DPM. The effect of S. plumosum on herbaceous biomass was not taken into account when calculating the total biomass for the three study sites. All plant species were classified into five successional classes (20), i.e. class 1 - pioneer annual; class 2 - pioneer perennial; class 3 - secondary succession; class 4 - secondary succession with anthropogenic disturbance; class 5 – climax.

 

2. Data analysis

Density – Using densities of S. plumosum plants in each of the two sample plots at each study site, means were calculated for each study site. Density was reported as the number of plants per hectare.

Alpha diversity - The Shannon–Wiener diversity index (21) was used to calculate alpha diversity. Index values obtained from diversity index calculations were insufficient for further statistical analysis and were converted into effective numbers (22). An ANOVA was used to determine whether there were significant differences between the three sites in terms of effective numbers of species (i.e. diversity). A post hoc Tukey test was run to determine where the significant difference/s were between the three sites.

Species evennessThe evenness of species was calculated using the Pielou’s Evenness Index (24).

Beta diversityBeta diversity (similarity) for the three sites was interrogated using the Extended Sørenson’s similarity index (33). The larger the index value, the greater the similarity. To understand the similarity between the individual sites, the Sorenson’s similarity index was calculated for each pair of sites.

Vegetation ecological successional classes A two-way classification chi-square test was used to determine if there was a significant difference between the 3 sites based on the observed frequency distribution of the 5 ecological successional classes.

Effect of key ecological variables on vegetation diversityA constrained correspondence analysis (CCA) ordination of the three sites (Grassland, Intermediate and Dense) was undertaken using the R software package (25), to determine which variables (effective number, biomass, S. plumosum height, grass height and ecological successional status/class) had the greatest effect on the vegetation diversity of the sites.

All statistical analyses were done using the IBM SPSS Statistics vs. 25 software package (23). The chosen critical significance level α wa s 0.05.

 

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4.            Pielou EC. The measurement of diversity in different types of biological collections. J Theor Biol [Internet]. 1966 Dec;13:131–44. Available from: http://linkinghub.elsevier.com/retrieve/pii/0022519366900130

5.            R Core Team. R: A language and environment for statistical computing [Internet]. Vienna, Austria: R Foundation for Statistical Computing; 2013. Available from: http://www.r-project.org/

6.            IBM Corp. IBM SPSS Statistics for Windows. Armonk, New York: IBM Corp.; 2016.

Usage Notes

Data cleaning

SPSS and R software package

Knowledge of plant ecology, plant succession, bush densifiation/encroachment, veld condition and palatability 

General knowledge of the area 

Funding

E Oppenheimer & Son