Figure 1. Opuntia ficus-indica (L.) Mill in Limpopo Province, South Africa, Photos taken by K.E. Ravhuhali and O. Hawu.

Table 1. Scientific, common name, origin and usefulness of prickly pear species.

Taxon	Origin	Use	References
Opuntia leucotricha and O. rastrera	Mexico	Health benefits, domestic animal feedstuff, water supply, ornamental purposes	[21][22][23][24][25][26]
Opuntia humifusa	Canada	Human consumption, fodder crop, prevent soil erosion, pharmaceutical (emergency hydration)	[27][28][29]
Opuntia monacantha	Spain, South America	Human consumption and health and pharmaceutical benefits, chemical industrial uses	[30][31][32][33]
Opuntia engelmannii	south-central, south-western United States and northern Mexico	Cultivated as an ornamental or as live hedge and the fruit is edible, health benefits	[25][34][35]
Opuntia microdasys	central and northern Mexico	Animal feedstuff, pharmaceutical and health benefits, human consumption	[36][37]
Opuntia megacantha	North America and Mexico	Health benefits and animal feedstuff, human consumption	[38][39][40]
Opuntia stricta	Australia, central America	Health benefits and animal feedstuff, human consumption, living fences	[18][33][41][42]
Opuntia cochenillifera	South Africa	Both traditional medicinal and edible purposes	[43][44]
Opuntia ficus-indica	Ethiopia	Health benefits, animal feedstuff and fruits for humans	[45]
Opuntia ficus-indica	Ethiopia	Health benefits, animal feedstuff and fruits for humans	[45]
Opuntia ficus-indica	Argentina and Mediterranean region	Health benefits, animal feedstuff and fruits for humans	[46][47]
Opuntia dillenii	Italy	Health benefits, animal feedstuff and fruits for humans	[33]
Opuntia ficus-indica	Chile	Health benefits, animal feedstuff and fruits for humans	[33]

2. Growing Conditions and Varieties of Prickly Pear

Climatic fluctuations have long been suggested as a key driving power of the diversification of the cacti family and their distribution ^[48]. The species of Opuntia is highly adaptive to any climatic conditions and soil and it is also resistant to drought ^[49]. It was reported that there is little information regarding the response of the invasive Opuntia species on climatic changes ^[50][51] and its effect on biodiversity. Since Opuntia spp. grow in arid and semi-arid environmental regions, a high proportion of the species is threatened by extinction as a result of overexploitation and land use change ^[52][53]. Conversely, under global warming, most environments are becoming more arid ^[53] such that Opuntia will inhabit areas that are not suitable, with climatic changes resulting in an expansion of the geographical collection of the species in the near future ^[53], alongside a reduction in rainfall and drop in carbon dioxide ^[54]. The diversification mechanism of Opuntia could be compounded with the availability of arid areas, and climatic changes are suspected of playing a significant role ^[55][56]. Prickly pear is a drought-resistant and sustainable feed source for livestock ^[53]. The variation in climatic conditions, water shortages and an increase in the human population require plants—which are of significant value to livestock and humans—to adapt to these factors for sustainable production. In semi-arid environments, plants for livestock would be from those species that can adapt and produce in severe hot and dry regions ^[9]. Genus’ such as Opuntia normally thrive in conditions of high temperatures, elevated levels of carbon dioxide, and low rainfall, and are characterised by increased cladodes productivity and extensive growth of their root systems ^[57]. On uncultivated lands, the dry matter (DM) yield from Opuntia can surpass 2000 kg ha^{−1 [58]}. The DM content of different Opuntia spp. Is varied between 61 and 105 g kg⁻¹ DM ^[12].

2.1. Temperature

Temperature is one of the determinants of natural plant distribution ^[59]. Different plants respond to temperature sensitivity differently with leaves being known as more sensitive than stems ^[60]; however, Nobel and Bobich ^[61] reported that the roots are more sensitive than stems. In South Africa, the Opuntia species are normally found in arid and semi-arid regions with high temperatures ^[9]. Drennan and Nobel ^[62] have also stated that the normal temperature for Opuntia species root growth is between 27–30 °C and some branches can be ruined by ambient temperature (−16 °C) depending on the type of species ^[63]. Opuntia plants can also survive in temperatures above 65 °C ^[64]. Nobel and De le Barrera ^[65] observed the tolerance of Opuntia at the age of 10 years and above to a low temperature of 2–6 °C, which normally prevents the plant from dying during freezing times. In a study conducted by Snyman et al. ^[66] in South Africa, it was highlighted that freezing temperatures during spring time did not result in the death of the Opuntia plant but rather had a negative impact on the plant moisture stress, and phonological stage. Water stress reduces irradiance on the physiological responses in basal mature cladodes ^[67]. Nobel ^[68] and Valdez-Cepeda et al. ^[63] found difficulties when assessing the strength of severe cold and frost on Opuntia in semi-arid areas, as there was little occurrence of cold and frost in those regions, whereas in other regions, low temperatures during winter periods become the major limiting factors in the cultivation of the species. High temperatures (above 30 °C) can decrease the photosynthetic process by up to 70% and low temperatures (lower than 0 °C) create irreversible damage to the cladode tissue. High temperatures (greater than 30 °C) during the day and greater than 20 °C during the night produce a higher number of new cladodes than fruits, which is ideal for fodder production ^[69].

2.2. Soil and Water

In order to have sustainable agriculture production, plant species which are salt tolerant and adapt well during drought are ideal for most arid and semi-arid regions ^[70]. Opuntia spp. adapt well in poor soils and some varieties adapt and survive in severely degraded soils with a limited or no nutrient supply ^[71]. Gajender et al. ^[70] highlighted that some Opuntia spp. are moderately tolerant to salt while being sensitive to alkaline conditions and that they can fail to adapt when the pH is more than 9. Apart from other nutrients, water is another limiting environmental parameter of plant growth ^[72] and several studies have reported the adaptability of Opuntia to water scarcity ^[73][74]. Opuntia is a drought tolerant species, which normally survives under moisture stress ^[75] and performs well in an area with limited water supply and under a rainfall range of 100–300 mm ^[76]. It uses a shallow and horizontally spread root system to access what little moisture is available ^[75]. The root adaptation of this species adds to the classical physiological and structural modification of CAM plants to tolerate severe drought periods ^[70][75][77]. Nobel ^[78] and Edvan et al. ^[79] indicated that this species’ adaptability to drought is also due to its ability to store water in its shoots and to fix CO2 during the night leading to reduced transpiration due to low night temperatures compared to the daytime. Edvan et al. ^[79] further stated that the species’ water use efficiency is six times greater than legume plants, and three times greater than herbaceous plants.

3. The Effect of the Invasive Opuntia Species on Biodiversity and Ecosystems

The researcheuthors correctly linked the effects of Opuntia on biodiversity, vegetation and an ecosystem ^[80][81] and there is information in the literature regarding the invasiveness of alien species and their impact on an environment ^[80][81], such as the utilisation of 7% of South African water resources to the detriment of native plants. Most of the Opuntia spp. in South Africa were sourced from the USA around 1914 ^[82]. This species now covers most areas around the world including Ethiopia and the Mediterranean Basin, France, Egypt, Greece, Italy, Libya, Turkey and South Africa ^[83][84], and it is regarded invasive by several countries such as Australia, Ethiopia, Mauritius, Yemen, the United states and Madagascar ^[85]. In line with the latest list of invasive plant species by the Conservation of Agricultural Resources Act No 43 of 1983 in South Africa, this species was categorised in the category number 1b and is regarded as a weed, meaning it is not supposed to be planted ^[81][86]. The negative impact of this species is associated with a disturbance of natural vegetation through a reduction in grazing capacity and causing injuries to people and livestock ^[87]. Mokotjomela et al. ^[88] indicated that birds are some of the animals that spread the cacti seeds in the arid areas of South Africa. Jones et al. ^[89] and Shackleton et al. ^[41] highlighted that the Opuntia species are regarded as an engineer of ecosystems due to its ability to modify the indigenous plant species habitats and also because it prevents livestock movement as it forms a thicket. Shackleton et al. ^[41], Pyšek et al. ^[90] and Seebens et al. ^[91] stressed that the introduction of many Opuntia around the world resulted in them becoming major invaders which have social and ecological costs. This is in agreement with Githae ^[92] and Tesfay and Kreyling ^[93], who found a significant homogenisation of the plant species composition and richness, and a poor rangeland condition leading to land degradation as influenced by the presence of Opuntia ficus-indica, and this leads to some plant species suffering because of its presence.