Research Article |
Corresponding author: Petr Pyšek ( pysek@ibot.cas.cz ) Academic editor: Sven Jelaska
© 2020 Petr Pyšek, Martin Hejda, Jan Čuda, Guin Zambatis, Klára Pyšková, Sandra MacFadyen, David Storch, Robert Tropek, Llewellyn C. Foxcroft.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Pyšek P, Hejda M, Čuda J, Zambatis G, Pyšková K, MacFadyen S, Storch D, Tropek R, Foxcroft LC (2020) Into the great wide open: do alien plants spread from rivers to dry savanna in the Kruger National Park? NeoBiota 60: 61-77. https://doi.org/10.3897/neobiota.60.54608
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Protected areas play an important role as refuges from invasive species impacts on biodiversity. Within the MOSAIK (Monitoring Savanna Biodiversity in the Kruger National Park) project, plant species were recorded in a representative set of 60 plots, 50 × 50 m in size, across the entire KNP, distributed so as to cover a range of savanna habitats, i.e. perennial rivers, seasonal rivers and dry crests, and two main bedrock types (granite and basalt). The data were used to assess the role of rivers in the dispersal of alien plants and study whether the alien plant species spread from rivers to open dry savanna. The resulting dataset provided the first thorough information on the spatial distribution of naturalised alien plants in KNP. In total, we recorded 20 plant species that are alien to the park, four of them considered invasive: Parthenium hysterophorus, Opuntia stricta, Xanthium strumarium and Zinnia peruviana. The most widespread species in KNP was Tridax procumbens, recorded in 11 plots (i.e. 18% of all sampled), four other species were found in > 10% of the plots. One species, Bidens bipinnata, was not previously reported from the park and represents a new record. The majority of aliens were concentrated along perennial rivers (60% of all occurrences), but some were repeatedly recorded at seasonal rivers as well and two of the most invasive species in KNP, Opuntia stricta and Parthenium hysterophorus, occurred also on dry crests away from water. The average number of alien species per plot was low (1.6), as was their mean percentage contribution to all species in a plot (2.2%), but some plots harboured as many as seven species and contributed up to 11.9%. Moreover, only 21 plots (35%) were alien-species free. In terms of the total species number per habitat, perennial rivers had significantly more aliens than crests and were marginally significantly richer than seasonal rivers. By recording all naturalised alien species occurring in the plots – many of them are not invasive but may become so in the future – and by using the GloNAF database of global distribution of naturalised species, we assessed the invasion potential of the recorded species.
alien species richness, crest, habitat, perennial river, plant invasion, protected area, savanna, seasonal river
The majority of protected areas worldwide are vulnerable to invasions, with very few completely free of alien species (
One of the iconic protected areas included into the SCOPE programme is the Kruger National Park (KNP) in South Africa. Established in 1898, it is the largest game reserve in South Africa and one of the oldest national parks in the world (
The Kruger National Park situated between latitudes 22°19'40"S to 25°31'44"S and longitudes 30°53'18"E to 32°01'59"E, with location of the 60 sampled sites, separated according to habitat and distributed across the four land systems. The size of the symbols indicates the number of alien plant species recorded in the plot.
There are about 360 alien plant species currently recorded in KNP (
In response to the escalating importance of plant invasions, KNP has initiated a number of programmes aimed at preventing and mitigating incursions of alien species (
To contribute to closing this gap, we use our data collected by the MOSAIK (Monitoring Savanna Biodiversity in the Kruger National Park) project aimed at studying biodiversity across the entire KNP, within four distinct land systems with variable supply of water and contrasting geologies. Here we aim to (i) describe the distribution of alien plant species, (ii) assess to what extent alien plants are confined to rivers as the main introduction pathways and dispersal vectors, versus how commonly they occur in drier habitats away from rivers and (iii) identify potentially invasive species of the future.
The data analysed in this paper were collected within the MOSAIK project between 2018 and 2020. MOSAIK’s primary objective is to sample plant and animal (mammals, birds, bats and moths) biodiversity in habitats across different land systems in KNP (as defined by
Images of habitats that were considered in the Kruger National Park study: A perennial river, B seasonal river and C dry crest. The plots were located in the vicinity of the rivers, near the river beds and within the crest.
Plants were sampled during two rainy seasons, 16 January to 4 February 2019 and 17 January to 3 February 2020. All vascular plant species were recorded in each 2500 m2 plot and their abundance estimated visually using the Braun-Blanquet cover-abundance seven-grade scale (
Species that are alien to South Africa were selected for analyses in this paper. To assign species an alien status, we followed geographical criteria broadly accepted in the invasion literature, referring to species introduced by humans to regions outside their native range (see
To assess the invasion potential of the alien species recorded in KNP, we extracted information on their global naturalisation success from the GloNAF (Global Naturalized Alien Flora) database (
Differences amongst habitats and bedrock in the mean numbers of alien species in plots were tested by using a Linear Mixed-Effects Model (LMM) (
A log-linear model (
In total, we recorded 20 plant species that are classified as naturalised aliens to KNP (Table
Overview of alien plant species recorded in savanna habitats in the Kruger National Park. Total number of records, separately for basalt and granite bedrock, frequency of occurrence in plots (n = 60) and the range of covers are given (one cover value indicates that the species occurred in plots with the same cover). Species that are currently considered as invasive in KNP are marked with * (based on
Species | Family | Life history | Origin | Occurrences | Frequency (%) | Basalt | Granite | Cover (%) | Naturalised (globally/in Africa) |
---|---|---|---|---|---|---|---|---|---|
Tridax procumbens | Asteraceae | a | central America | 11 | 18.3 | 3 | 8 | 0.1 | 146/55 |
Bidens biternata | Asteraceae | a | East Asia (Himalayas) | 10 | 16.7 | 4 | 6 | 0.1–15.0 | 31/29 |
Malvastrum coromandelianum | Malvaceae | a, p, ss | North America | 10 | 16.7 | 5 | 5 | 0.1 | 161/29 |
Parthenium hysterophorus* | Asteraceae | p | North America | 9 | 15.0 | 4 | 5 | 0.1 | 119/13 |
Alternanthera pungens | Amaranthaceae | p | tropical America | 8 | 13.3 | 5 | 3 | 0.1–2.5 | 124/35 |
Bidens bipinnata | Asteraceae | a | Asia, North America | 6 | 10.0 | 2 | 4 | 0.1–15 | 88/26 |
Gomphrena celosioides | Amaranthaceae | a, p | tropical South America | 6 | 10.0 | 2 | 4 | 0.1 | 94/43 |
Acanthospermum hispidum | Asteraceae | a | tropical America | 5 | 8.3 | 3 | 2 | 0.1–2.5 | 128/49 |
Portulaca oleracea | Portulacaceae | a | Eurasia | 4 | 6.7 | 3 | 1 | 0.1 | 311/56 |
Melanthera scandens | Asteraceae | p | tropical to subtropical Africa | 4 | 6.7 | 3 | 1 | 0.1 | 12/12 |
Litogyne gariepina | Asteraceae | p | obscure | 3 | 1.7 | 2 | 1 | 0.1 | – |
Xanthium strumarium* | Asteraceae | a | America1 | 3 | 5.0 | 2 | 1 | 0.1 | 147/18 |
Opuntia stricta* | Cactaceae | p | North America | 3 | 5.0 | 0 | 3 | 0.1 | 84/10 |
Achyranthes aspera | Amaranthaceae | a, p | Mediterranean | 2 | 3.3 | 2 | 0 | 0.1 | 160/52 |
Zinnia peruviana* | Asteraceae | a | North to South America | 2 | 3.3 | 1 | 1 | 0.1 | 45/9 |
Opuntia ficus-indica | Cactaceae | p | North America | 1 | 1.7 | 0 | 1 | 0.1 | 139/40 |
Argemone ochroleuca | Papaveraceae | a | North America | 1 | 1.7 | 1 | 0 | 0.1 | 96/15 |
Chenopodium album agg. | Amaranthaceae | a | Eurasia | 1 | 1.7 | 1 | 0 | 0.1 | 298/28 |
Datura inoxia | Solanaceae | p, ss | North America | 1 | 1.7 | 1 | 0 | 0.1 | 126/29 |
Verbesina encelioides | Asteraceae | a | South America | 1 | 1.7 | 0 | 1 | 0.1 | 88/12 |
The most widespread species in KNP was Tridax procumbens, recorded in 11 plots (i.e. 18%), other species recorded in more than 10% of plots being Bidens biternata, Malvastrum coromandelianum, Parthenium hysterophorus and Alternanthera pungens (Table
Distribution of alien species in the Kruger National Park according to the savanna habitats delimited within the MOSAIK project (perennial rivers, seasonal rivers, dry crest). Numbers of occurrences (n = 20 per habitat) are shown. Species with * are considered invasive in KNP.
In terms of distribution of the recorded species by habitats, the majority were concentrated at perennial rivers. Some species, for example, Alternanthera pungens, Gomphrena celosioides and Acanthospermum hispidum, occurred almost exclusively in this habitat, whilst others, for example, Bidens biternata, Malvastrum coromandelianum and Parthenium hysterophorus, were repeatedly recorded also at seasonal rivers and Opuntia stricta, Parthenium hysterophorus and Tridax procumbens on the crests, too (Fig.
The majority of species did not prefer any particular bedrock, with the exception of four species occurring more frequently on granites: Tridax procumbens (eight records on granites vs. three on basalts), Bidens bipinnata, Gomphrena celosioides (four vs. two) and Opuntia stricta (three records exclusively on granite). The species occurring more often on basalt bedrock were Alternanthera pungens (three vs. five) and Melanthera scandens (three vs. one) (Table
The average number of alien species per plot was relatively low, 1.6 ± 1.7 (mean ± S.D.), but only 21 plots out of 60 were alien free, meaning that 65% of plots harboured some alien species. The maximum number of alien species per plot was seven. On average, the alien species made up 2.2% (range 0–11.9%) of all species in a plot. The numbers of alien and native species in plots were not correlated (r = 0.067, DFresid = 58, p = 0.609).
Testing the average number of aliens per plot (Fig.
Level of invasion by bedrock and habitat. A Mean numbers ± S. D. of species per plot (n = 20 per habitat) B total species numbers and C percentage of alien species amongst all species in a plot are shown for particular factors. The habitats bearing the same letter were not significantly different in the respective characteristics; the effect of bedrock was not significant.
In total, there were 17, 11 and 8 species recorded at perennial rivers, seasonal rivers and on the crest, respectively, and the total numbers of alien species in a habitat (Fig.
Venn diagram showing the sharing of alien species by habitats in the Kruger National Park. Tridax procumbens, Malvastrum coromandelianum and Parthenium hysterophorus were the species recorded at all three habitats.
The percentage of alien species per plot (Fig.
Of the 20 alien species recorded in total, 16 were found on granites and 17 on basalts, with corresponding averages per plot 1.6 ± 1.9 and 1.5 ± 1.5, respectively. Neither the main effect of bedrock, nor the bedrock × habitat interaction had significant effects on the mean number of aliens per plot (LMM: deletion test, DFmodel = 6 vs. 7, L-ratio = 0.895, p = 0.344, and DFmodel = 7 vs. 9, L-ratio = 0.294, p = 0.634, respectively; Fig.
It has been suggested that the negative impacts of plant invasions in protected areas in African savannas are less dramatic than in the savanna regions and ecosystems in the Neotropics and Australia.
Due to research conducted mostly in the temperate areas, rivers have long been recognised as major pathways of alien plant introduction to new regions; they are highly prone to invasion by alien plants, largely because of their dynamic hydrology that makes them conduits for efficient dispersal of propagules (
However, we found that the threat of invasion beyond the main perennial rivers and adjacent floodplain areas, where the major invaders are concentrated (
It needs to be pointed out, however, that alien species recorded in our plots mostly occur in low abundance. Bidens bipinnata occasionally reached up to 15% of cover and Alternanthera pungens and Acanthospermum hispidum ~5%. Aliens also account for a rather small proportion of the total plant richness; on average, there were less than two alien species per plot, with maximum of seven and contribute less than 3% to the total plot richness. However, in two plots at perennial rivers, alien species contributed 11.9% and 9.2% and additional seven plots harboured more than 5% of aliens. This, together with the fact that almost three quarters of all sampled plots had at least one alien species, indicates that KNP needs to monitor the occurrence of these species, ideally on a regular and systematic basis. Our detailed survey covered, in cumulative terms, 15 hectares and, extrapolating the figures to the total park area, implies that alien plant species are already a fairly common phenomenon throughout the whole park.
The study was supported by grant no. 18-18495S (Czech Science Foundation), EXPRO grant no. 19-28807X (Czech Science Foundation), long-term research development project RVO 67985939 (Czech Academy of Sciences) and projects UNCE204069 and PRIMUS/17/SCI/8 (Charles University). The project was registered as PYSK1432 with SANParks. LCF thanks SANParks and acknowledges support from the DSI-NRF Centre for Invasion Biology, Stellenbosch University. Thanks are to our guards Obert Mathebula, Thomas Rikombe, Desmond Mabaso, Herman Ntimane, Annoit Mashele, Isaac Sedibe, Priska Rikombe and Velly Ndlovu for keeping us safe in the field. We thank Elizabete Marchante, Nina Šajna and Sven Jelaska for helpful comments on the manuscript.