Fellow gardeners in Arlington have recently noticed the exoskeletons of what one person described as an “insect monster,” a large dead cicada. It has sparked a discussion about why we are seeing so many cicadas this year.
Our normal crop of annual cicadas have been joined by another less common group of periodic cicadas. These periodic cicadas have long incubation periods in unusual yearly increments. The 13 and 17 year cicadas emerge in different parts of the country on those unusual yearly increments. That breeding cycle is a marvel of evolutionary biology. Both numbers are prime numbers, not divisible by any other number. Most other creatures, plant and animal, operate on more regular cycles, annually or biennially. The cicadas long incubation period based on the prime numbers means that no predator can hang around waiting for an annual feast as predators need regular food sources. The unusual breeding cycle means there is no other creature that is dependent on them as a food source. Birds and mammals will eat them when they arise from the ground but the great numbers of cicadas arriving en masse means that many will survive and breed again despite predation.
The advantage of reproducing in great bursts is also seen in the plant world with mast years among trees. In a mast year, more than enough fruits and nuts are produced to feed the local wildlife yet allow enough other seeds to germinate and take root. The year after a mast year, reproduction rates among consumers (like squirrels and raccoons) booms, as they have full bellies and are healthy. When food production falls in the years after masting, the now large animal population faces a food crisis and they die in large numbers. A few years of environmental balance follow. When a mast year returns, the consuming population has again fallen to a level that allows some seeds to germinate.
Since trees in an area are often genetically related and climate plays a big role in when mast years happen, tress sharing a mast cycle can cover large stretches of land. If you think back to the years when there have been a lot of road-kill squirrels, it was probably after a mast year, as the expanded squirrel population ventures further and to more dangerous places in search of food.
Mast years do not alternate annually with low production years, usually taking a few years between peak production years instead. That gives them an advantage against an animal population that might otherwise also have evolved to a biennial reproductive cycle. So it seems that prime numbers and long reproductive cycles are keys to the reproductive success of some plants and critters like cicadas.