In wintertime, it’s often getting dark in Princeton by the time I head home from the office to scrounge up some dinner. Along the half-mile path, I regularly walk or bike within few meters of the local herd of white-tailed deer. There are at least five or six animals that circulate among the tiny patches of trees and streams at the south end of campus. The university deer are just a fraction of the estimated 450-500 that roam the 16 km2 town of Princeton. That’s almost 40 deer per km2, well above the state of New Jersey’s recommended 20-25 per km2. Indeed, much of the northeast U.S. is forced to deal with dense, growing deer populations thanks to the removal of wolves, forest recovery over the last century following the westward shift of American agriculture, and a suburbanization-associated decline in hunting.
All these extra ungulates come with costs. For one, Princeton has paid hundreds-of-thousands of dollars between 2001 and 2015 to professional sharpshooters who controversially culled over 2700 deer from the population. However, hunting costs pale in comparison to those of the 300-plus deer-vehicle collisions that occurred each year in Princeton before the hunts were organized!
There are unequivocal ecological costs to deer overabundance, too. Over-browsing decreases the diversity of understory vegetation, even years after deer browsing is reduced, and can facilitate the spread of invasive species by reducing competition with native plants.
Direct effects on vegetation can also bounce back up the food chain to affect other consumers. In our new paper “Does primary productivity modulate the indirect effects of large herbivores? A global meta-analysis” published in the Journal of Animal Ecology, Rob Pringle and I analyzed data from 67 studies of large mammalian herbivore (various deer species, but also, antelope, cape buffalo, elephant, wildebeest, and zebra) impacts on the species richness and abundance of other animals. We were able to include studies of a wide range of responding taxa, from frogs and polychaetes to rodents and beetles.
Overall, herbivores negatively affected both the abundance and richness of other animals, probably by consuming vegetation that other species would use as food or habitat. This suggests that the deer irruptions in the northeast U.S., and similar deer population booms in Europe and Japan, are reducing the richness and abundance of other animals. As just one example, Common Nightingales in eastern England preferred habitat within areas fenced off from roe deer, fallow deer, and Reeve’s muntjac.
The negative effects of herbivores were also stronger in less productive ecosystems (e.g., deserts and drier savannas compared to forests). This pattern is probably due to faster regrowth, and therefore renewed availability to other consumers, of plants damaged by herbivores in more productive habitats. There is also a greater absolute amount of plant material available to begin with in these environments, perhaps buffering the effects of large herbivores.
It is concerning that the impacts of changing large mammal populations are greater in less productive ecosystems. Large herbivore populations have been declining in African savannas for several decades, and ongoing climate change is drying out some of the same habitats. Drought-driven decreases in primary productivity coupled with declining large herbivore populations could mean strong changes to savanna communities in the near future.
Hippopoatamus (top) and sable antelope (bottom) were just two of the many large herbivores whose populations were decimated in Gorongosa National Park, Mozambique during the country’s 1977-1992 civil war. Today, Pringle Lab and other scientists are working to understand the impacts of those decline on other savanna species, and to assist the ongoing restoration of the mammal fauna.
I work in one savanna that experienced catastrophic mammal declines, Gorongosa National Park, Mozambique. During the country’s 1977-1992 civil war, soldiers took up arms in the park and hunted their way through the large mammal herds. Some species, once thousands of animals strong, were left with only single digit populations, and over 95% of all large herbivores were eradicated in Gorongosa. We’re just beginning to scratch the surface of how these declines affected Gorongosa’s animal community, but anecdotally grasshoppers populations may have grown following release from competition with the once-abundant large mammals. [in-line grasshopper photo & caption] In other savannas, experimental removals of large mammals have led to an increase in rodents, and both groups of small herbivores can have big impacts on savanna vegetation. Our findings bear particular consideration in light of the fact that southern Africa is experiencing its worst drought and hottest temperatures in 50 years.
Slideshow of large mammals which declined heavily in Gorongosa National Park. Photos by Josh Daskin.