Citizen science shows how predation risk effects can reduce lifetime fitness

Submitted by editor on 26 March 2019.Get the paper!

Simple observations can sometimes lead to unexpected scientific discoveries. In 1996, wildlife biologist and educator Dick Thiel was musing about the abundance of North American porcupines (Erethizon dorsatum) near his home in central Wisconsin. That winter he started a mark-recapture study in nearby Sandhill Wildlife Area with help from secondary school students. In 2001, one of those students, Matt Schuler observed an unusual animal track while surveying part of the wildlife area. Matt and Dick confirmed that the track belonged to a fisher (Pekania pennanti), a predator that was extirpated from the region nearly 100 years ago, and began tracking both porcupines and fishers. Twenty years of data collected by hundreds of students revealed more than we expected about porcupine behaviour, nutrition, predation risk effects, demography, and evolutionary responses.

Porcupines are found across much of North America; from as far north as Alaska and as far south as Mexico. They are generally solitary animals that forage in tree canopies, and maintain dens in hollow trees or rock crevasses. Porcupines use quills to defend themselves from predators, but that does not mean they are immune to predation risk effects. The presence of fishers alters individuals’ foraging behaviours and can heighten physiological stress. Using data from Sandhill Wildlife Area, we learned that the combination of food, climatic conditions, non-consumptive effects of predation, and lethal predation all influence the nutritional state and recruitment of porcupines. Following these discoveries, we wondered: could these effects have lifetime fitness consequences?

We measured female growth and reproductive success before and after fishers recolonized the region, facilitating analyses of natural variation arising from both predation risk and environmental conditions. Our results indicate that individual females experiencing predation risk from fishers grew slower and gave birth to fewer offspring. Additionally, simulations show that predation risk alone can lead to population declines, and that populations can only grow when females invest more energy into reproduction or adult survival. If females only invest energy in juvenile survival, the population will continue to decline. Overall, we found that the accumulation of predation risk can reduce lifetime reproductive success in wild mammals. These results have wide reaching implications for the management and conservation of wild mammals, especially naïve populations experiencing increased predation risk from colonizing or invasive predators.

Dick answered his first question over a decade ago. However, more questions arose as students marked and recaptured porcupines each winter. Having seen how risk effects can affect porcupines, we wonder: how do individual risk effects contribute to the evolution and structure of other wild predator-prey systems? 

The authors through Philip DeWitt

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