Increasing numbers of invasive alien plant (IAP) species are establishing around the globe and can have negative effects on resident animal species function and diversity. These impacts depend on a variety of factors, including the extent of invasion, the region and the taxonomic group affected. These context dependencies make extrapolations of IAP impacts on resident biota from region to region a substantial challenge.
Here, we synthesised data from studies that have examined the effects of IAPs on animal diversity in South Africa. Our focus is on ectothermic organisms (reptiles, amphibians and invertebrates).
We sourced relevant articles using keywords relating to (1) the effects of IAPs on species diversity (abundance, richness and composition), (2) the IAP and (3) the native ectotherm. We extracted the taxonomic and spatial coverage of IAPs and affected native species and assessed the extent of information given on potential mechanisms driving IAP impacts.
Across the 42 studies, IAPs had a decreasing or neutral effect on native animal abundance and richness and significantly changed species composition. This review highlighted the paucity of studies and the research deficits in taxonomic and geographic coverage and in the mechanisms underlying IAP impacts on ectotherms.
By assessing the status of knowledge regarding the impacts of IAPs on resident animal species in South Africa, this study identifies information gaps and research priorities at the country level with a view to informing monitoring and conservation efforts, such as alien plant removal and control programmes, and ensuring that endemic terrestrial animal diversity is maintained.
Invasive alien species are considered a major pressure on the current state of biodiversity globally (Butchart et al.
Species population and community metrics such as abundance, richness and composition can provide useful baseline data as indicators of animal diversity change between invaded and uninvaded areas. The direction and magnitude of effects of alien plant invasions on animal communities can, however, depend on a variety of factors, including the scale of the plant invasion (extent and density), the stage of invasion, and the region and taxonomic group affected (Kumschick et al.
Potential mechanisms through which invasive alien plants (IAPs) affect ectotherm diversity.
Whereas reports of negative impacts of invasive plant species are pervasive in the literature, positive effects have also been reported, for example, via increases in suitable habitat or net resources to recipient fauna (e.g. Schlaepfer, Sax & Olden
In this study, we aim to synthesise studies that have examined the effects of IAPs on animal diversity in South Africa. We concentrate this review on ectotherms (reptiles, amphibians and invertebrates) for several reasons. Firstly, their energy budgets are more directly influenced by the environment compared with endotherms (Gates
We searched the ISI Web of Science for relevant studies comparing abundance, richness or composition of terrestrial ectotherms between invaded and uninvaded sites in South Africa. Our search combined terms for (1) invasive plants; (2) native reptiles, amphibians and terrestrial invertebrates; and (3) effects on species abundance, richness or composition (shown in detail in
We gathered information from the studies on the location of the field sites and respective biomes (Mucina et al.
To assess the impacts of alien plants on native animal diversity, we focused on comparisons of species abundance or richness between sites with native and alien vegetation and classified the effects as positive (i.e. an increase in abundance or richness in sites with alien vegetation), negative (decrease) or neutral effect. Native vegetation sites included sites cleared of alien plants in two studies where authors indicated that sufficient time had elapsed for recovery of native vegetation. We incorporated comparisons based on original data, accumulation and rarefaction curves, and richness estimators (e.g. Chao1 and Chao2) but did not include diversity indices (e.g. Shannon’s or Simpson’s index) as their use was very variable across studies. For composition, comparisons were classified as alien vegetation resulting in a change or not in species composition. These comparisons were typically the result of ordination techniques based on similarity measures or cluster analyses. When studies presented the effects of alien plants on fauna for several functional or taxonomic groups (e.g. herbivores vs. predators and beetles vs. spiders) or for different habitat types (e.g. invaded by
Our first search for studies addressing the effects of invasive plants on ectotherm diversity in South Africa yielded 358 studies. Of these, only 42 were relevant for this review (
The 42 papers reviewed were published between 1985 and 2016, with a slight increase in the annual number of publications since 2000 (
Distribution of studies (a) per year of publication, (b) per taxonomic group (phylum) of native ectothermic organisms, and (c) per biome and taxonomic group of native ectothermic organisms. The map in (c) shows the distribution of study sites (black dots) in South Africa across seven biomes, with the shades of grey indicating the density of invasive plants.
Most studies compared sites along the coastal belt and adjacent interior in the Western Cape and KwaZulu-Natal provinces (
Distribution of studies per growth form of the invasive plant species studied and their classification according to (a) the NEM:BA regulations for control and management of invasive species and (b) their estimated impacts on biodiversity according to a published classification.
Alien plants had a larger decreasing effect on native species abundance compared to species richness (
Percentage of comparisons performed in the 42 studies reviewed that found invaded sites to have (a) positive (increased diversity), neutral or negative (decreased diversity) effects on native ectotherm species richness (
Impacts of specific invasive alien plants on abundance, richness and composition of several taxonomic groups of ectotherms.
Invasive alien plant species | Impacts on native species | ||
---|---|---|---|
Abundance | Richness | Composition | |
Negative: Insecta (1) | Negative: Insecta (1) | Different: Insecta (1) | |
Negative: Araneae, Lepidoptera, Formicidae (2) |
Negative: Odonata (3), Araneae and Formicidae (2), Araneae*, Coleoptera•, Diptera*, Hemiptera•, Hymenoptera• and Formicidae* (4) |
Different: Odonata (3, 5), Orthoptera*, Formicidae•, Hymenoptera• and Hemiptera• (4) |
|
Negative: Formicidae (6) | Neutral: Formicidae (6) | Different: Formicidae (6) | |
- | Neutral: Insecta and Arachnida (7) | - | |
- | Neutral: Araneae (8) | - | |
Negative: Araneae, Coleoptera, Hymenoptera, Lepidoptera, Formicidae (9) |
Negative: Araneae, Orthoptera, Hymenoptera, Lepidoptera, Formicidae (9) |
Different: Araneae, Orthoptera, Hymenoptera, Lepidoptera, Formicidae, Coleoptera, Diptera, Hemiptera (9) | |
- | - | Different: Formicidae (10) |
|
- | Negative: Hymenoptera, Opiliones and Amphipoda (11) | Different: Hymenoptera, Opiliones and Amphipoda (11) | |
Positive: Coleoptera (12) |
Neutral: Coleoptera (12), Araneae (12) | Different: Coleoptera (12) |
|
Neutral: Coleoptera (13) | Neutral: Coleoptera (13) | ||
- | - | Different: Formicidae and Araneae (10) |
|
Negative: Squamata (14) |
Negative: Squamata (14) |
Different: Collembola (15), Squamata (14) | |
- | Negative: Scarabaeidae (16) | Different: Scarabaeidae (16) | |
- | Positive: Anisoptera (17) |
Not different: Odonata (17) | |
- | Neutral: Insecta (18) | Different: Insecta (18) |
(1) Coetzee, van Rensburg & Robertson
For invasive alien plants listed in South Africa’s
Seventy percent of the studies investigating differences in species diversity between invaded and uninvaded habitats referred to potential mechanisms underlying the patterns found. These mechanisms included changes in habitat structure (
IAPs have been recognised as a threat to South Africa’s native biodiversity for more than two decades, with efforts to manage invasions underway through the Working for Water Programme since 1995 (Van Wilgen et al.
The studies reviewed in this synthesis covered some of the areas of South Africa most heavily invaded by alien plants such as
Our synthesis highlights a serious deficit in the knowledge of impacts of IAPs on terrestrial ectothermic groups other than arthropods, which mostly comprised insects and spiders. South Africa is known for its exceptional reptile and amphibian diversity and its high level of endemism; between 40% and 67% of its indigenous species of amphibians, chelonians, lizards and snakes are unique to the country (Bates et al.
It is well recognised that the impacts of alien plants on fauna are context-dependent and are shaped by a variety of factors, including the abundance and distribution of the IAP, the time since its introduction and the invasion history (e.g. rate of spread and lag times), the spatial extent of the study area, the degree of contrast of the alien plant form and function to the native vegetation, the ecosystem type and climatic conditions (e.g. seasonality) and the habitat preference of the animal species assessed (Kumschick et al.
Our synthesis also showed that opposite effects of alien plants on the same taxonomic group are found and generally depend on the animal group investigated, the type of IAP and the occurrence of other environmental stresses (cattle and habitat alteration). For example, alien plants can have decreasing (Samways & Grant
This review further revealed that most studies examining the impacts of alien plants on ectothermic animals employed the same methodological approach, comparing invaded and non-invaded sites. A very small proportion of studies included a gradient of alien plant abundance (e.g. Schreuder & Clusella-Trullas
Species abundance, richness and composition results provided useful data to compare key animal community changes between invaded and uninvaded habitats (albeit with the known problems associated with, e.g., comparison of species richness; Gotelli & Colwell
Overall, these metrics alone give little insight into the mechanisms underpinning community changes. Less than 13% of the studies reviewed here incorporated an experimental approach for testing for mechanisms. These are essential for describing processes underpinning patterns, distinguishing direct and indirect effects of alien plants on ecosystems (Hulme et al.
The current state of knowledge of the impacts of IAPs on resident ectothermic animal species in South Africa relies heavily on a few key studies, with distinct biases in geographic locations and taxonomic groups. In cases where detailed information is available, it is nevertheless clear that there are pronounced negative impacts of IAPs on terrestrial animal (ectotherm) species diversity. The mechanisms underlying these impacts are unclear, but here we highlight a few key abiotic and biotic processes that could be examined in future, especially if microenvironments determine key behaviours and life-cycle timing that lead to changes in population abundance. Such an integrated approach to the question of IAPs and their impact on native animal species diversity would be of direct value to monitoring and conservation efforts, such as alien plant removal and control programmes. At present, it is wholly unclear whether the removal of IAPs will be sufficient to allow recovery of native ectotherm biodiversity.
S.C.-T. is supported by the Centre for Invasion Biology, Stellenbosch University and the Incentive Funding for Rated Researchers from the South African National Research Foundation. R.A.G. is supported by a post-doctoral fellowship from the Centre for Invasion Biology, Stellenbosch University.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
Both S.C.-T. and R.A.G. contributed equally to the article.
Search terms used in the literature review on the ISI Web of Science.
Issue | Search terms |
---|---|
Location | “South Africa*” |
Invasive plants | ((invas* OR alien* OR non$nativ* OR exotic* OR introduced OR non$indigenous OR naturali?ed OR plantation*) AND (plant* OR vegetat* OR tree* OR shrub* OR grass* OR forest* OR forb* OR herb* OR vine* OR *weed*)) OR (invaded AND (habitat* OR site* OR plot*)) |
Reptiles | reptil* OR squamata OR snake* OR python* OR boa* OR cobra* OR mamba* OR viper* OR adder* OR colubrid* OR elapid* OR lizard* OR gecko* OR skink* OR chameleon * OR agama* OR monitor* OR lacertid* OR amphisbaenid* OR cordylid* OR testudine* OR chenolian* OR turtle* OR tortoise* OR terrapin* OR crocodylia OR crocodil* |
Amphibians | amphibian* OR frog* OR anura* OR tadpole* |
Terrestrial invertebrates | invertebrate* OR platyhelminthe* OR *worm* OR nematod* OR nematomorph* OR nemertea* OR acanthocephalan* OR annelid* OR oligochaet* OR leech* OR mollus* OR gastropod* OR snail* OR slug* OR tardigrad* OR onychophora* OR arthropod* OR crustacea* OR *lice OR “terrestrial crab*” OR amphipod* OR isopod* OR myriapod* OR centipede* OR millipede* OR chilopod* OR diplopod* OR chelicerat* OR Araneae OR arachnid* OR spider* OR Acari OR acarin* OR mite* OR tick* OR opiliones OR harvestm?n OR scorpion* OR hexapod* OR insect* OR apterygot* OR odonat* OR dragonfl* OR damselfl* OR orthoptera* OR grasshopper* OR cricket* OR isoptera* OR termite* OR mantodea* OR mantis* OR mantid* OR blattodea* OR cockroach* OR embioptera* OR webspinner* OR phasmid* OR phasmatodea* OR hemiptera* OR *bug* OR cicada* OR aphid* OR *hopper* OR thysanoptera* OR thrip* OR psocoptera* OR coleoptera* OR beetle* OR lepidoptera* OR butterfl* OR moth* OR diptera* OR *flies OR *fly OR mosquito* OR flea* OR hymenoptera* OR wasp* OR ant OR ants OR bee OR bees OR neuroptera* OR lacewing* OR antilon* OR pollinat* |
Effect on diversity | ((population* OR communit* OR assemblage* OR species) AND (abundan* OR richness OR ((population* OR communit* OR assemblage* OR species) AND (abundan* OR richness OR diversity OR composition OR evenness OR dominance OR equitability OR structure OR poor* OR impoverish*)) OR ((functional OR *genetic) AND diversity) |
Effect mechanism | (habitat* NEAR/3 (quality OR structure OR heterogeneity)) OR shad* OR thermal* OR hydrolog* OR micro$site* OR micro$climate* OsR micro$habitat* OR refuge* OR prey* OR activity OR thermo$regulat* OR behavio$r* OR bask* OR predat* OR competit* OR herbivo$r* OR resource* OR nutrient* OR fire* OR soil* OR sediment* OR locomoti* OR host* OR reproducti* OR toxic* OR poison* OR hybrid* OR disease* OR parasit* |
A first search combined terms for location, invasive plants, reptiles, amphibians and terrestrial invertebrates, and effects on diversity, whereas a second search replaced the terms for effects with terms for mechanisms.
TOPIC = “South Africa*” AND (((invas* OR alien* OR non$nativ* OR exotic* OR introduced OR non$indigenous OR naturali?ed OR plantation*) AND (plant* OR vegetat* OR tree* OR shrub* OR grass* OR forest* OR forb* OR herb* OR vine* OR *weed*)) OR (invaded AND (habitat* OR site* OR plot*))) AND ((reptil* OR squamata OR snake* OR python* OR boa* OR cobra* OR mamba* OR viper* OR adder* OR colubrid* OR elapid* OR lizard* OR gecko* OR skink* OR chameleon * OR agama* OR monitor* OR lacertid* OR amphisbaenid* OR cordylid* OR testudine* OR chenolian* OR turtle* OR tortoise* OR terrapin* OR crocodylia OR crocodil*) OR (amphibian* OR frog* OR anura* OR tadpole*) OR (invertebrate* OR platyhelminthe* OR *worm* OR nematod* OR nematomorph* OR nemertea* OR acanthocephalan* OR annelid* OR oligochaet* OR leech* OR mollus* OR gastropod* OR snail* OR slug* OR tardigrad* OR onychophora* OR arthropod* OR crustacea* OR *lice OR “terrestrial crab*” OR amphipod* OR isopod* OR myriapod* OR centipede* OR millipede* OR chilopod* OR diplopod* OR chelicerat* OR Araneae OR arachnid* OR spider* OR Acari OR acarin* OR mite* OR tick* OR opiliones OR harvestm?n OR scorpion* OR hexapod* OR insect* OR apterygot* OR odonat* OR dragonfl* OR damselfl* OR orthoptera* OR grasshopper* OR cricket* OR isoptera* OR termite* OR mantodea* OR mantis* OR mantid* OR blattodea* OR cockroach* OR embioptera* OR webspinner* OR phasmid* OR phasmatodea* OR hemiptera* OR *bug* OR cicada* OR aphid* OR *hopper* OR thysanoptera* OR thrip* OR psocoptera* OR coleoptera* OR beetle* OR lepidoptera* OR butterfl* OR moth* OR diptera* OR *flies OR *fly OR mosquito* OR flea* OR hymenoptera* OR wasp* OR ant OR ants OR bee OR bees OR neuroptera* OR lacewing* OR antilon* OR pollinat*)) AND (((population* OR communit* OR assemblage* OR species) AND (abundan* OR richness OR diversity OR composition OR evenness OR dominance OR equitability OR structure OR poor* OR impoverish*)) OR ((functional OR *genetic) AND diversity))
TOPIC = “South Africa*” AND (((invas* OR alien* OR non$nativ* OR exotic* OR introduced OR non$indigenous OR naturali?ed OR plantation*) AND (plant* OR vegetat* OR tree* OR shrub* OR grass* OR forest* OR forb* OR herb* OR vine* OR *weed*)) OR (invaded AND (habitat* OR site* OR plot*))) AND ((reptil* OR squamata OR snake* OR python* OR boa* OR cobra* OR mamba* OR viper* OR adder* OR colubrid* OR elapid* OR lizard* OR gecko* OR skink* OR chameleon * OR agama* OR monitor* OR lacertid* OR amphisbaenid* OR cordylid* OR testudine* OR chenolian* OR turtle* OR tortoise* OR terrapin* OR crocodylia OR crocodil*) OR (amphibian* OR frog* OR anura* OR tadpole*) OR (invertebrate* OR platyhelminthe* OR *worm* OR nematod* OR nematomorph* OR nemertea* OR acanthocephalan* OR annelid* OR oligochaet* OR leech* OR mollus* OR gastropod* OR snail* OR slug* OR tardigrad* OR onychophora* OR arthropod* OR crustacea* OR *lice OR “terrestrial crab*” OR amphipod* OR isopod* OR myriapod* OR centipede* OR millipede* OR chilopod* OR diplopod* OR chelicerat* OR Araneae OR arachnid* OR spider* OR Acari OR acarin* OR mite* OR tick* OR opiliones OR harvestm?n OR scorpion* OR hexapod* OR insect* OR apterygot* OR odonat* OR dragonfl* OR damselfl* OR orthoptera* OR grasshopper* OR cricket* OR isoptera* OR termite* OR mantodea* OR mantis* OR mantid* OR blattodea* OR cockroach* OR embioptera* OR webspinner* OR phasmid* OR phasmatodea* OR hemiptera* OR *bug* OR cicada* OR aphid* OR *hopper* OR thysanoptera* OR thrip* OR psocoptera* OR coleoptera* OR beetle* OR lepidoptera* OR butterfl* OR moth* OR diptera* OR *flies OR *fly OR mosquito* OR flea* OR hymenoptera* OR wasp* OR ant OR ants OR bee OR bees OR neuroptera* OR lacewing* OR antilon* OR pollinat*)) AND ((habitat* NEAR/3 (quality OR structure OR heterogeneity)) OR shad* OR thermal* OR hydrolog* OR micro$site* OR micro$climate* OsR micro$habitat* OR refuge* OR prey* OR activity OR thermo$regulat* OR behavio$r* OR bask* OR predat* OR competit* OR herbivo$r* OR resource* OR nutrient* OR fire* OR soil* OR sediment* OR locomoti* OR host* OR reproducti* OR toxic* OR poison* OR hybrid* OR disease* OR parasit*)
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