Islands of fertility: the invasive alien cactus Opuntia stricta creates and benefits from fertile islands
The sour prickly pear Opuntia stricta changes soil characteristics, such as nutrients and the level of enzymes, which are involved in nutrient cycling processes, and the abundance and diversity of soil bacterial communities, suggesting that extensive invasions could alter belowground ecosystem function.
Opuntia stricta, or sour prickly pear, has spread extensively in the Kruger National Park after its introduction in the 1950s. The seeds are widely dispersed by baboons, birds, and elephants eating fruit, and dislodged cladodes (i.e. leaves) grow readily into new plants. By the 1990s sour prickly pear covered large areas around Skukuza. Following extensive chemical control campaigns, and the introduction of the biological control agents, Cactoblastis cactorum (cactoblastis moth) and Dactylopius opuntiae (cochineal), further spread has been curbed to a large extent and former extensive, dense patches have collapsed into scattered individuals.
O. stricta will never be fully eradicated in the park and the nature of biological control means that populations will fluctuate through time. Previous work has shown that prickly pear affects native biodiversity, for example, O. stricta has an adverse impact on beetle assemblages in Kruger. However, no previous work has considered below-ground biodiversity and processes. To examine potential below-ground effects of prickly pear, we measured soil characteristics (pH, humidity and nutrients), enzymatic activities (phosphatase, β-glucosidase and urease), and the diversity and composition of bacterial communities, measured as operational taxonomic units (OTUs).
The invasive Opuntia stricta flourishing under the canopy of a native tree in the Kruger National Park
Woody plants in savannas often accumulate moisture and soil nutrients, and harbour unique bacterial communities under their canopies, and in this way create patches of local high fertility referred to as “islands of fertility”. To test whether prickly pear clumps create fertile “islands”, we compared soils under O. stricta, soils under the canopy of two indigenous trees, Vachellia nilotica (black thorn) and Spirostachys africana (tamboti) and soil in open patches with no prickly pear or tree cover. In line with the “fertility island” hypothesis, we found that the presence of both O. stricta and native trees increased soil water content and nutrients, the level of enzymes and the abundance and diversity of bacterial communities. Bacterial composition was also altered, while a total of 12704 OTUs were shared between invaded and uninvaded soils, 3815 OTUs were unique to invaded soils and 2198 OTUs to uninvaded soils.
In addition to soil analyses, we also undertook a germination experiment where we added prickly pear and tree seed to the various types of soils. Even though germination rates of O. stricta were higher in soils under trees compared to soils collected from open patches, O. stricta had lower and slower germination in previously invaded soils. This may suggest that O. stricta potentially releases phytochemicals to curb intraspecific (i.e. from the same species) competition. Tamboti and black thorn trees, on the other hand, germinated equally well in invaded and in uninvaded soils. These results suggest that the presence of a tree or shrub, whether an alien such as O. stricta, or a native plant, can create favourable conditions for the establishment and growth of other species.
What we have learnt from this work is that it is important to prevent the regeneration of widespread thickets because extensive invasion may have adverse effects on biodiversity and ecosystem functioning. At low levels of invasion however, those effects are likely to have a minimal and very localised negative effect, and perhaps even create fertile islands for recruitment of native species.
This article was originally published in the 2021/2022 Research Report.
The article is based on a publication in the journal Scientific Reports.
Novoa, A., L.C. Foxcroft, J-H. Keet, P. Pyšek and J.J. Le Roux. 2021. The invasive cactus Opuntia stricta creates fertility islands and benefits from those created by native trees in the Kruger National Park, South Africa. Scientific Reports 11:20748.
