Guest Author: William Ishmael
Herbivores, Carnivores, Omnivores
If you were paying attention in biology class, you might remember a lesson about food pyramids, food chains and food webs in natural ecosystems. Basically, a food pyramid is a depiction of how the sun’s energy flows through an ecosystem, sustaining life for all the plants and animals. Soils and vegetation form the wide bottom layer of the pyramid. Herbivores (prey species) who feed on the plants are in the narrower middle of the pyramid and predators who feed on the herbivores form the still-narrower tip of the pyramid. Plants are dependent on sunlight, photosynthesis and soil nutrients…..herbivores are dependent on plants for food and cover….predators are dependent on herbivores. Quite simple.
As omnivores who eat a combination of meat and vegetables, us humans share the intermediate levels of the pyramid with other omnivores such as bears and raccoons. True carnivores, such a wolves, mountain lions and bobcats are at the top of the pyramid. However, since the domestication of animals such as livestock, poultry and even fish, we are no longer dependent on wild prey species even though we put millions of pounds of wild game and fish meat in the freezers every year. When times are tough we can go to the grocery store for a steak or pound of bacon or a pen-reared salmon. Wild predators don’t have that option.
Still, humans continue to “compete” with predators and many people continue to believe that fewer predators will result in more wild game meat for us. Is predator control a sound wildlife management practice? Not likely. For instance, coyotes are known to be one of the most persecuted wild animals in North America. Every tool in the box has been invented and used to eliminate coyotes from the landscape yet they continue to thrive and are now in every habitat imaginable, including downtown Chicago and our suburban backyards. These so-called “varmints” are incredibly mobile and adaptable and take advantage of any food source available. Shooting or trapping coyotes with the aim of increasing deer or other prey species results in fewer coyotes in the short term but opens up new territories for dispersing young coyotes to move in and fill the territorial gaps. More importantly, fewer coyotes means more food available for the remaining coyotes which results in larger litter sizes and higher pup survival. So in return, soon after the shooting ends, there’ll likely be the same number or more coyotes.
Will these sorts of hunting efforts make a difference in local prey species abundance? Not likely in the long run.
Removing Predator Populations = Negative Chain Reaction
Recent research shows that dealing with the tip of the food pyramid just screws things up for everyone and will more than likely have long-lasting negative effects on the species we are trying to protect.
Removal of top-level predators allows populations of herbivores to rapidly increase in the short-term, causing overconsumption of the native plants and trees that provide food and cover for them, as well as for other wildlife species. Studies of widely different species such as predatory starfish, large-mouthed bass, sea otters and African wild dogs show that removal of these “keystone” species resulted in long-term damage to vegetation and eventual massive declines in prey abundance as the prey species destroyed their own food sources. Predators (including us) kill and eat prey species but they also instill what is now being called a “landscape of fear” in their prey populations. The fear of getting preyed upon keeps the prey species moving or hidden in cover, reducing the amount of time they have to consume and use their habitats.
Understanding the backfire of predator control is nothing new. Famed ecologist and Professor of Wildlife Management at the University of Wisconsin came to this conclusion more than 100 years ago after he and his colleagues in the U.S. Forest Service shot up a pack of wolves in the mountains of southern New Mexico. After watching the wolves die or limp off into the brush he realized that messing with the tip of the pyramid is a bad idea.
“I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer…for while a buck pulled down by wolves can be replaced in two or three years, a range pulled down by too many deer may fail of replacement in as many decades.”
– Aldo Leopold, A Sand County Almanac 1949
Short-term gains result in long-term losses. Again, quite simple.
The average whitetail in the upper Midwest eats about 6 pounds of forage per day, or about a ton of food per year. One might assume there is more than plenty of food out there for every deer to eat and live a healthy well-fed life. But is there really? Over-consumed deer feeding areas soon become dominated by plants and trees that deer (and other wild herbivores) won’t eat. Honeysuckle and buckthorn provide some cover but very little food value. Sure, an agricultural-dominated landscape provides lots of food for deer in summer and fall but deer basically eat a diet of woody plants for about 5 months of the year. Oaks provide cover and an incredible amount of nutritious food (ie. acorns) for deer and many other species of wildlife, including many game species. During winter, deer browse heavily on young oaks and, over time, can cause a shift in the type of trees that are allowed to grow, changing a woodlot from high quality food and cover into a low quality, over-used biological desert. How many baby oaks do you have in your woodlot? What will replace the mature oak trees when they are dead and gone?
Life With and Without Predators
Deer and other prey species have evolved incredibly high reproductive rates to compensate for predation (including hunting harvests). In the upper Midwest it’s not unusual for a population of deer to increase by a factor of 1.5 every year if they are well-fed and not affected by diseases such as Chronic Wasting Disease and pneumonia. A herd of 1,000 deer in March will become a herd of 1,500 by September even in the presence of predation. Yes, predators such as wolves, bears, coyotes, bobcats and even foxes will kill deer fawns and even yearling or adult deer in poor body condition as a result of poor quality habitat or disease. But mother nature has two strings to her bow. The breeding season for deer (the “rut”) takes place in a fairly narrow window of time each fall when the majority of adult does are bred. The timing of the fall rut is aimed at ensuring that fawns will be born 200 days later when there is ample cover for them as well as food for their nursing mothers. Whitetails in the upper Midwest literally “flood the market” with newborn fawns over a two-week time span in late May and early June. There’s also lots of other young prey species out there at the same time so prey abundance is at its peak and, although predators kill fawns, many will survive to add to the growing population.
So, knocking off the tip of the pyramid may seem like a good strategy for those who want to increase prey abundance but, in the long run, all those efforts will just result in a short-term overabundance of prey that causes long-term damage to it’s food sources and habitat and the strategy ultimately backfires.