Invasive species have become a hot topic lately, and for good reason. Many species of plant and animals have been introduced to new parts of the globe, and many are having devastating consequences, both for natural ecosystems, and for humans. Here I focus on plants, although invasive species are also a problem with animals, fungi, and microorganisms.
What is an invasive plant?
An invasive plant is a plant that has been introduced to an area outside its native range, and has naturalized, or established a self-sustaining wild population, and grows so aggressively that it causes problems for people and/or wild ecosystems.
So why do they become invasive?
Plants are uniquely adapted to grow in the areas in which they evolved. So how are invasive species able to thrive in new areas, with different climates?
In some cases, the species that become invasive even have some factors working against them. For example, Australian pine, growing in Florida, is poorly adapted to hurricanes, and English Ivy, an invasive in the northeastern U.S., is adapted to warmer winters and sometimes succumbs to the elements especially in prolonged cold spells with drying winds.
The main answer is that, in spite of these disadvantages, invasive plants have one huge advantage that allows them to out-compete other plants in their surroundings: almost nothing eats them. But why? To answer this question we need to look into insect-herbivore relationships, as the key lies largely in the interplay between the plants and insects.
Plant defenses against herbivores
Plants co-evolved with a landscape of herbivores, both insects, and larger herbivores like mammals. In order to survive without being completely eaten away by insects and larger animals, plants evolved defenses. These include chemical defenses, like bitter-tasting chemicals, poisons which deter or slow down or limit the ability of animals to eat them. They also include physical defenses, like thorns, which can keep large animals away, or fuzzy leaves, which can make leaves harder to eat.
Insect adaptation to plant defenses
Just as plants evolve to defend themselves from being eaten by insects and other herbivores, the insects evolve to eat the plants. Because insects are small, they do not have the luxury of a large liver (like cows, elephants, or even deer have) that would allow them to detoxify their bodies from a broad range of chemical compounds. Instead, insects are only able to have.
Insects thus end up specializing on groups or families of related plants. Most insects have a preferred host, and then a range of plants that they will be able to eat if their preferred host is not available. Some insects are more generalists, but most typically still have preferences for certain families or taxonomic groups of plants.
The plants and insects thus co-evolve. In natural ecosystems, each plant has a group of different insects that specialize in eating it, and the plant and its herbivores exist in a natural balance.
What happens when new plant species are introduced?
When plants are introduced to new regions, they typically are not brought over with the full range of insects that co-evolved to eat them. But the insects in the new location did not evolve with the new plants. Thus fewer insects eat the introduced plants.
With less pressure from insects, these plants gain a competitive advantage over the native plants, which still have to compete with the full range of insect herbivores they co-evolved with. These plants can then become invasive species.
The collapse of the food web
When invasive species become dominant, because there is a smaller array of herbivores eating them, there is less food for other animals and organisms higher up on the food web. For example, birds, spiders, and predatory insects, typically rely on herbivorous insects for food, and other animals in turn depend on these small predators.
