A forest set to a biological clock
In the dense understory of Chile's Valdivian temperate rainforest, a species of native bamboo, Chusquea quila, follows a remarkable life cycle. This is not an annual event. Instead, vast populations of this bamboo flower in unison, an event known as gregarious or mast flowering, only once every 15 to 70 years. After this single, massive reproductive effort, the parent plants die. This synchronous bloom and subsequent death is an evolutionary strategy which floods the ecosystem with an enormous quantity of seeds—up to 50 million per hectare—overwhelming seed predators and ensuring some will survive to germinate.
The trigger for this synchronous event is not fully understood, but it appears to be controlled by a kind of internal genetic clock. Regardless of age or location, all plants derived from the same ancestral stock will flower at the same time. This creates a dramatic and widespread ecological pulse. Following the mass seeding, the dense bamboo thickets, known as quilantales, die off, creating large gaps in the forest understory and drastically altering light conditions on the forest floor for years. The next generation of Chusquea quila takes about five years to grow over two meters high, slowly re-establishing the dense undergrowth.
The "Ratada" and its consequences
The sudden superabundance of bamboo seeds provides a massive food source for seed-eating animals, particularly rodents. This leads to a dramatic population explosion of the long-tailed pygmy rice rat, Oligoryzomys longicaudatus. This phenomenon is known locally as a "ratada," or rat plague. The rodent populations, normally sparse, can multiply to extreme densities, creating a ripple effect through the food web.
This boom in the rodent population in turn supports a rapid increase in their predators. Birds of prey like the Magellanic horned owl (Bubo magellanicus) and predators such as the culpeo fox (Lycalopex culpaeus) experience a population surge fueled by the plentiful rats. The "ratada" has a significant public health dimension. Oligoryzomys longicaudatus is the primary reservoir for the Andes virus (ANDV), a hantavirus that causes a severe and often fatal respiratory illness in humans called Hantavirus Cardiopulmonary Syndrome (HCPS). The increased number of rodents and their potential incursion into human-populated areas following the depletion of the seed stock elevates the risk of human transmission. The case fatality rate for HCPS can be as high as 40%.