If female butterflies can produce 100 to 1000 eggs, why aren't there more butterflies in the world? Many factors play a part in controlling the size of butterfly populations, as they do for most living organisms. These factors fall into three main categories:
- environmental conditions, including weather, space constraints, and food limitations;
- accidents, including being crushed by cars, falling branches, or larger animals' feet; and
Although all three of these play important roles in reducing natural populations, this section focuses on predation.
Predation can be defined as an event in which one organism kills and consumes another organism or uses its nutrients without killing it. People often think of showy vertebrate predators first, such as lions chasing down a gazelle or birds feeding on butterflies they catch in mid-flight. But smaller invertebrate predators such as insects, spiders, bacteria, and viruses kill many more butterflies in most natural populations than do frogs, lizards, birds, toads, mice, and other vertebrates. Invertebrate predators also include parasites, organisms that take nutrients and resources from other organisms without killing them directly (although parasitism can result in death), and the specialized form of parasites called parasitoids, insects that consume other insects from the inside out.
All butterflies have mechanisms that help protect them from predation. Some species are camouflaged: they either look like something else, such as a leaf or stick, or blend in with their backgrounds. Others have patterns that make them appear to be a bigger animal, such as eyespots on wings. Still other butterflies protect themselves through behavior. Adults can fly off, while larvae can live together in groups or drop to the ground suddenly if a predator approaches. Some butterflies also have mechanical or chemical defenses. Mechanical defenses are things like hairs, spines, or bristles that make it harder for the predator to get a good grasp on the larva, while chemical defenses make the butterfly less appetizing (maybe a noxious smell or a bad taste).
Monarchs have an effective chemical defense. When they eat milkweed, they sequester the poisonous cardenolides (also called cardiac glycosides) in the milkweed. Cardenolides are poisonous to vertebrates (although maybe not to invertebrates, bacteria, and viruses), and most Monarchs face little predation from frogs, lizards, mice, birds, and other species with backbones.
But being poisonous doesn't help the Monarchs after a predator has already killed and tried to eat them. Monarchs need some way to warn off predators before they become lunch. Monarchs do this through their warning coloration, or bright colors (yellow, orange, black, and white). This coloration warns potential predators that the animal contains poisonous chemicals. Warning coloration may work particularly well in adult butterflies because the hard body and wings allows a predator to bite the adult, taste the poison, and release the butterfly without killing it. It is common to see butterflies with beak-sized sections gone from their wings. Large wings help the butterfly escape relatively unharmed. Since Monarch wings contain distasteful cardenolides, one bite may discourage further attack without killing the adult.
Early studies of Monarchs showed that when a blue jay ate a Monarch butterfly, it vomited shortly afterwards due to the cardenolides. That bird also learned to never eat another Monarch butterfly. Like this early study, most research about Monarch predation has focused on bird predation, especially on adults, and researchers have found a few species that eat Monarchs despite the cardenolides. However, invertebrates that eat the larvae are probably more important Monarch predators and thus play a more significant role in controlling Monarch population size. Despite their importance, scientists have done much less research on invertebrate predation of Monarchs. This is an important hole, since the cardenolides Monarchs sequester may not affect invertebrates in the same way. As you read about the studies of vertebrate predation on Monarchs, and about the little information we have on invertebrate predation, think about ways you might investigate the impact of invertebrate predators on Monarch butterfly populations.
Introduction : At Overwintering Sites : By Invertebrates, Parasitoids & Disease