The prickly pear cactus (Opuntia stricta) is a species native to the Americas. It was introduced to Australia in the 19th century, and its population quickly exploded, becoming unusually abundant in the country. This phenomenon can be explained by the fact that the prickly pear cactus had no coevolved herbivores in Australia.

a) No Coevolved Herbivores:
Coevolution is a process in which two or more species reciprocally influence each other's evolution. In the case of plants and herbivores, coevolution often involves the development of defense mechanisms by plants to deter herbivory, and counter-adaptations by herbivores to overcome these defenses. Over time, as plants and herbivores interact and adapt to each other, a balance is usually reached, and herbivores keep plant populations in check.

However, when a plant species is introduced to a new environment where it has no coevolved herbivores, it may lack effective defenses against herbivory. This can result in unrestricted population growth, as the plant is not being consumed by local herbivores. This is known as the enemy release hypothesis.

In the case of the prickly pear cactus in Australia, it had no coevolved herbivores to feed on it and keep its population in check. As a result, it was able to spread rapidly and establish itself in various habitats, including grasslands, woodlands, and coastal areas.

The absence of coevolved herbivores allowed the prickly pear cactus to outcompete native vegetation and dominate the landscape. It also had a negative impact on biodiversity, as it reduced the availability of resources for other plant species and disrupted native ecosystems.

Overall, the lack of coevolved herbivores in Australia was a key factor in the unusually abundant growth of the prickly pear cactus after its introduction. This highlights the importance of coevolutionary relationships in maintaining ecological balance and preventing the spread of invasive species.