Prickly pears displace native plants
Plants classified as being in the genus Opuntia are members of the cactus family. They are commonly known as prickly pears in English speaking countries.
Prickly pear cacti have been spread around the world deliberately to serve as 1) animal fodder 2) used as ornamentals and hedging plants 3) support the cochineal industry. That was the plan. The reality played out quite differently.
The misplaced
Species of the genus Opuntia are notoriously difficult to to classify as they hybridise readily. Two of the best known species in terms of invasiveness are O. stricta (erect prickly pear) and O. monocantha (common prickly pear). This case study focuses largely on these two species, out of the approx. 200 species in the genus.
Structure and Ecology
Plants consist of fleshy ovoid green segments called pads or cladodes. These branch out repeated from the stem (which may be very short). Short irritant bristles and long pointed spines erupt from little bumps on the cladode surface. Flowers develop on the periphery of the cladode and after pollination, primarily by bees, turn into fruits. Cladodes (or pads as they are sometimes called) are formed by vegetative budding.
O. monacantha is native to a region with a principally tropical savannah climate with dry summers and wet winters. In its native range it is notable as a species that prefers coastal sites, sandy soils and dunes, being rarer inland. However, following introduction it has proved itself well adapted to range of other climates, especially more arid regions, warm temperate regions and those with wet summers and dry winters. O. monacantha can tolerate high temperatures and long dry seasons, and is also able to tolerate a certain degree of frost. It can grow at altitudes up to 2000 m.
O. monacantha prefers free draining sandy or loamy soils and is intolerant to waterlogging. Being a coastal species it is also tolerant of saline soils and partially tolerant to salt spray.1
O. stricta is tolerant of a range of environmental conditions although it does require a free draining soil.
Other adaptations to drought include a protective epidermis covered by a thick waxy waterproof cuticle and a shallow root system with surface 'rain roots' enabling the plant to exploit light rains.2
Reproduction
Opuntia plants can multiply vegetatively by segments that root where ever they contact the ground. This characteristic of the cacti initially baffled and frustrated efforts to eradicate the weed because peoples' first instinct was to hack at the plants which only caused further spreading.
Opuntia plants can reproduce sexually by seed found in the fruits. The seeds are tough and are often distributed by birds who have consumed the tasty fruit.
The displaced
In regions lacking appropriate natural predators, prickly pear will run rampant and blanket areas displacing the vast majority of plants growing in the locale.
This has the knock on effect of disrupting the natural vegetation which is central to ecosystem. There is insufficient food for local consumers to eat because they are not adapted to eat the cactus vegetation. The disruption ripples through and hence destabilises the entire food web
The consequences
Where is the invasion?
Case Study - Australia
Prickly pear has been introduced into multiple locations around the world. One of the best documented tales of how this has played out is the invasion of Australia in the the 19th and early 20th century. It is thought that prickly pear was first brought to Australia at the end of the 18th century by the British First fleet. They were trying to keep the red dye used for their uniforms available, so the they brought with them from Europe Opuntia plants to keep the cochineal bugs alive as this bug was the source of their pigment.
In Australia, throughout the 19th and early 20th century period, prickly pair spread like wildfire in Australia. According to The prickly pear story,
Prickly pear infestations covered 10 million acres (4 million ha) of land by 1900 and 58 million acres (24 million ha) by 1920. Despite every control effort, it was estimated that the plant’s rate of advance was 2.4 million acres (1 million ha) per year.
After manual removal, people tried poisoning the plants with an arsenic salt, and while effective, this was expensive, time consuming and very dangerous to people applying the salt and then, it left toxic residue in the soil.
The solution to the problem was finally found in the 1920s. In its native environment, predators kept the cacti under control. One of these predators is the larva of a South American moth with a resplendent name, Cactoblastis cactorum.
Following mass rearing of cactoblastis, 10 million eggs were distributed in 61 localities throughout affected areas during 1926 and 1927 and a further 2.2 billion eggs were released between 1927 and 1931.
It worked, and because the larvae only ate the flesh of the Opuntia spp. cacti, it did not cause more problems than it solved as in the cane toad case.
Some species of Dactylopius ( the cochineal bugs) were also successfully used for biocontrol.
See it for yourself
Drag the green line from side to side to view how the photograph changes from during infestation to after biocontrol.
Video examining the almost disaster of the prickly pear invasion of Australia
Prickly pear is promoted as a desirable garden plant in areas where it is not native. This becomes problematic as average temperatures rise. Due to climate change , temperature ranges in the European Alps are changing and there are now more icefree days. Prickly pear is becoming a pest in the mountains, replacing edelweiss in areas of Switzerland.
The skin of prickly pear plants is sometimes utilised as "vegan leather".