Predation causes a lot of suffering. Still, many people, especially ecologists, defend predation by arguing that it benefits ecosystems. However, the idea that predation enhances something like ecosystem health or biodiversity is very disputable. What does it even mean to benefit an ecosystem? What is the health of an ecosystem? What is biodiversity?
In this article I argue that it’s complicated. Yes, sometimes some predators are beneficial for some measures of ecosystem health for some ecosystems, but many times predators are also harmful. We cannot generally conclude that predation benefits ecosystems. The benefits of predation are inconsistent. Therefore the ecosystem health argument is not a valid defense of predation. The harms of predation to animal welfare are more important than the inconsistent benefits to ecosystems. Animal welfare is more important than ecosystem health, because animals themselves care about their own welfare, whereas ecosystems do not care about their health. Ecosystems don’t care about anything.
Biodiversity is a mess
How can we measure the health of an ecosystem? One approach is the notion of biodiversity: a more biodiverse ecosystem is assumed to be a healthier ecosystem. But what is biodiversity? There are many measures of biodiversity, and they all face some weird properties.
Let’s start with the simplest, perhaps most common measure of biodiversity: the number of species. This measure is arbitrary: why counting the number of species and not for example the number of genera, or the number of families, or the number of orders, or the number of classes, or the number of any other biological taxon? Perhaps you insist that there is something special about the taxon of species. Very well then, let’s use the number of species.
Consider a world with two ecosystems, north and south of a river. The northern ecosystem contains two species: a rabbit species and a fox species. So the biodiversity equals 2. The southern ecosystem also contains two species: a hare species and an elephant species. Now consider a second world where the northern ecosystem has three species, rabbits, hares and foxes, and the southern ecosystem has those same three species. In this example, each ecosystem in the second world has a higher biodiversity than each ecosystem in the first world. So it seems obvious that the second world has more biodiversity, right? But if we consider the whole world as one ecosystem, the first world has biodiversity 4 (rabbits, hares, foxes and elephants), which is higher than the biodiversity of the second world (rabbits, hares and foxes). It is a paradox: A is bigger than B, C is bigger than D, but A plus C is smaller than B plus D. Conclusion: this measure of biodiversity, which is very simple, does not allow for simple aggregation.
Let’s make it more complicated. The above measure of biodiversity does not take into account the species abundances, the number of individuals of each species. Take the northern ecosystem in the second world, with rabbits, hares and foxes. Suppose rabbits are very abundant and hares are very rare. Still, the biodiversity equals 2, the same as an ecosystem with a more equal population distribution of rabbits and hares. We can redefine biodiversity to take into account the species abundances. In fact, there are many ways to do so. So there are an infinite amount of possible definitions of biodiversity. According to all those definitions, an ecosystem with 50 rabbits and 50 hares is more biodiverse than an ecosystem with 90 rabbits and 10 hares.
And even taking into account abundances might not be enough. We can expect that the number of large animals, like elephants, is much lower than the number of small animals like hares. We can also expect that the number of predators like foxes is lower than the number of prey like rabbits. Does a definition of biodiversity have to take into consideration those expected abundances? Is an ecosystem with 50 hares and 50 elephants more biodiverse than a more realistic ecosystem with 90 hares and 10 elephants?
It is not over yet. Hares and rabbits are more similar than hares and elephants. The Belgian hare, for example, is actually a rabbit (a breed of the European rabbit species), but looks more like a hare. It could have been the case that rabbits and hares can procreate and have fertile offspring, in which case rabbits and hares are different subspecies of the same species (according to one of the many definitions of species). Instead of three species, the second world only contains two species. With a small genetic mutation, such that rabbits and hares can no longer interbreed with each other, biodiversity suddenly increases with one species.
The same sudden increase in biodiversity can even happen when whole animal populations are killed. There is this notion of a ring species, such as the Ensatinasalamander in California. There are several Ensatina populations, forming a horseshoe shape around the mountains. The south-western population can interbreed with the north-western, so they form the same species. The north-western can interbreed with the northern population, so they also form the same species. Similarly, the northern population can interbreed with the north-eastern, which can interbreed with the south-eastern population. All these populations form the same salamander species, right? But the south-western and the south-eastern cannot interbreed. So suppose we kill the north-western, northern and north-eastern populations. All of them! Then the two remaining populations, who cannot interbreed, become two separate species. If biodiversity is measured in terms of the number of species, and a species is defined as the group of individuals who can interbreed, then killing populations can increase biodiversity.
This all shows that defining biodiversity is a real mess. Defining and measuring ecosystem health more generally, is also a real mess.
Native predators are a mess
The reintroduction of wolves in Yellowstone National Park is the most cited example of predators benefitting biodiversity. Through a trophic cascade, wolves change the abundances (population seizes) of many other species in Yellowstone. However, using this example to demonstrate the benefits of predation, is cherry picking, because it neglects the many counter-examples where the introduction of predators negatively affect biodiversity. Think of rats on New Zealand islands, brown tree snakes on the island of Guan and small Indian mongooses in South America. Those mongooses were brought to Central and South America and some Asian islands, to solve the problems caused by other predators: they control rat and snake populations. So predators like rats and snakes cause a problem and other predators were introduced to solve that problem but in fact those introduced predators caused similar problems themselves: the mongooses decrease populations of many birds, mammals and reptiles such as hawksbill turtles.
The arrival of predators on islands shows an inconsistent attitude towards predation. Many people believe that ecosystems with low levels of predation contain a lot of animal suffering, because the animals in those areas overpopulate and then die from starvation or infectious diseases. This would be especially the case on islands, because on islands the overpopulated animals cannot easily migrate to other places. Yet, especially on islands, the introduction of predators is widely considered, also by ecologists, to be negative. The predators hunt too much.
You can argue that these counter-examples are invalid because these predators are exotic species that never lived on those islands or continents, whereas wolves lived in Yellowstone some centuries ago. Wolves are native predators in Yellowstone. Perhaps only native predators are beneficial for ecosystems.
But this notion of a native species is debatable. Around 10.000 years ago, the last saber-toothed tiger lived in North America. Suppose we could resurrect this extinct species of large cats. Should we do that and then reintroduce those tigers in Yellowstone? Are they native to Yellowstone as they once lived in Yellowstone? Does that mean that the reintroduction of those tigers will benefit Yellowstone’s biodiversity? Or did the extinction occur too long ago, such that those species should now be considered as exotic to Yellowstone?
In any case, the saber-toothed tiger belongs to the family of Felidae or cats. There are large cats in Yellowstone, such as bobcats, lynxes and cougars. So the family of cats is native to Yellowstone. Of course saber-toothed tigers, and especially the Smilodon populator, are bigger than those other cats, so they may have a very different effect on the ecosystem of Yellowstone than bobcats, lynxes and cougars. If we can’t resurrect and reintroduce the saber-toothed tiger, what about introducing another large cat that still exists today, belongs to the same family of Felidae that is native to Yellowstone and could probably survive well in Yellowstone: the Siberian tiger? Yes, Siberia is far away from Yellowstone, so Siberian tigers do not live in Yellowstone. As a subspecies of the species of tigers (Panthera tigris), the Siberian tiger is exotic to Yellowstone. There are no animals belonging to the species of tigers roaming around in Yellowstone. But as a subspecies of the family of Felidae, those Siberian tigers are native to Yellowstone: there are Felidaeroaming around in Yellowstone. Felidae are native to Yellowstone, Siberian tigers are Felidae, so that means… Well, is the Siberian tiger native or not? Furthermore it is possible that there is a small acre in Yellowstone where no wolf ever stepped. Does that mean that wolves are exotic to that acre?
The more you think about it, the more messy this notion of ‘native predator’ becomes. Let’s try it one last time, by looking at the definition of an exotic or non-native species: “a species living outside its native distributional range, but which has arrived there by human activity, directly or indirectly, and either deliberately or accidentally.” This definition is highly arbitrary. First, “a species”: why use this biological taxon and not another taxon, such as “a genus” or “a class”? Second, “distributional range”: how large is a distributional range and what shape does it have? Third, “by human activity”: why by human activity and not by mammal activity? (Humans are mammals after all.) Think about whale barnacles: crustacea who attach themselves to baleen whales and migrate with them, eventually accidentally arriving at new places in the ocean. Or why not arriving at an island on driftwood or mats of vegetation? Lizards hop from island to island in the Bahama’s on natural rafts, lemurs arrived on Madagascar and monkeys crossed the ocean from Africa to South-America around 30 million years ago.
The claim that native predators are beneficial and exotic predators are harmful to ecosystems is unlikely to be true, due to the arbitrariness in the definition of an exotic species.
Inconsistent examples of predation benefitting biodiversity
Even if we take one of the many measures of biodiversity that considers species abundances, and even if we define nativeness of a species and consider only native predators, it is not obvious that predation by native predators benefits biodiversity, that ecosystems with more predation have more biodiversity. It is striking that so many articles that defend predation contain so many obvious inconsistencies.
Take the article “The ecological importance of predators.“ In only two pages, this article contains many contradictions. There are even contradictions in the same sentence. One sentence claims that wolves are good: “The reintroduction of wolves, however, restores these habitats…”, but then states that wolves are also bad in the sense that they reduce coyote populations: “… and additionally reduces coyote populations…” But wait, coyotes are bad, because reducing their numbers increases antelope populations: “… thereby boosting pronghorn antelope and other small mammal populations.” But no, reading further, coyotes are good: “Coyotes, therefore, play an important role in maintaining the balance of species diversity within their ecosystems.” Wait, coyotes are bad again, because they reduce predator populations such as badgers: “In cases where coyote numbers have been successfully reduced, other mesopredators such as foxes, badgers and raccoons, which coyotes often compete with and sometimes prey on, have increased significantly, thereby altering the surrounding ecosystem.” But wait, predators such as foxes and badgers are bad: “‘Mesopredator release,’ as this phenomenon is called, has been shown to decrease overall species diversity and density of smaller prey such as bird and rodent populations.” Oh wait, some of those predators, in particular badgers, turn out to be good after all: “When foraging, badgers use their strong sense of smell to locate prey and then penetrate the soil in targeted areas. This digging provides aeration and nutrient mixing, and helps maintain moisture to the soil, all of which aid in recruiting native plant species.” Can you still follow?
Here is another example: “Top predators in crisis”. It repeats the claim that wolves are bad, because they cause a decline of the coyote population: “Today, the number of coyotes in the national park has declined, and in the core wolf areas, there is, on average, one-third of the density of coyotes compared to other areas of Yellowstone. The wolf considers its ‘smaller cousin’ a competitor that must be killed — and coyotes have moved their dens to wolf-free areas of the park.” Coyotes are good, because they reduce the population of other predators such as cats, who are bad because they kill birds: “Coyotes help control numbers of both domestic and feral cats, which kill native birds.” Large sharks are bad, because they negatively affect populations of small sharks: “Between 90 and 99 per cent of 11 large shark species have disappeared in the Atlantic off the coast of the United States. All of these species hunt small sharks and rays, and biologists have recorded an increase in the populations of 12 out of 14 of the small shark and ray species.” But small sharks are bad: “These, in turn, have decimated local stocks of commercially important mollusks.” Finally, top predators such as killer whales are bad, because they endanger threatened species such as otters: “But sea otters face another danger: lower fish stocks in the region have cut off the main food source for local seals and sea lions, which in turn were hunted by killer whales. Now killer whales (Orcinus orca) have begun to hunt sea otters, and the seaweed forests are once again at risk.”
We are not done yet. As it happens, wolves and coyotes can interbreed. The offspring are called coywolves and these hybrids of wolves and coyotes happen to be fertile. According to one of the many definitions of species that refer to the possibility of having fertile offspring, wolves and coyotes belong to the same species. So we cannot say that the species of wolves was reintroduced in Yellowstone, because that species has always been there, namely as coyotes. Yes, the above mentioned articles say that the species of wolves is good because wolves decrease the population of the species of coyotes who were bad, but if wolves and coyotes belong to the same species, is that species good or bad? Perhaps you insist that wolves and coyotes belong to different species, but then at least you have to accept the common classification in biology, that wolves and coyotes belong to the same genus of Canis. So let’s repeat the question: is the genus Canis good or bad? And did the abundance of Canis animals increase or decrease after the reintroduction of wolves?
All these contradictions, what a mess!
Ecologists actually don’t truly believe that predation benefits ecosystems. Otherwise they would propose to increase the predation level in ecosystems. After all, if decreasing the predation level has negative effects on ecosystems, then increasing the predation level should in general have positive effects, unless you believe that the current predation level is optimal. But that would be a sheer coincidence, like blindly picking a random point on the globe, and by pure luck you are pointing at the highest mountain, Mount Everest. The rule is simple: if one direction goes down, the other goes up, unless you happen to be on the top. There is no good reason why the current predation level is the top. So, ecologists would be in favor of increasing the predation level, for example by introducing more predators and creating new predator species. They would be in favor – if it were possible – of genetic modification to breed new predator species or to carnivorise herbivores. All predator species are carnivorized herbivores by evolution, because herbivory evolved first, before carnivory. Or why not propose to resurrect dinosaurs like the tyrannosaurus?
Who controls the predators?
People argue that predators are beneficial because they control prey populations. Without predators, prey populations will reproduce too much, beyond the carrying capacity of the ecosystem, thereby decreasing populations of other species. And concerning animal welfare: without predators, prey will overpopulate the ecosystem and eventually will starve to death due to lack of food.
But these arguments apply also to predator populations. If there is no predator species controlling a predator population, the uncontrolled predator population will also grow beyond the carrying capacity. There are not enough prey, so many predators will starve to death. We need a predator species to control a predator population. But than that higher level predator also has to be controlled by another predator species, and so on to infinity. We need a food chain of infinite length: predators who eat predators who eat predators… Every level has to be controlled by a higher level. If you believe that a finite food chain is optimal for ecosystem health, that there is a level in the food chain that does not have to be controlled by predation, there is no reason why the length of the food chain should be say four instead of two trophic levels. An ecosystem with two trophic levels consists of plants and plant-eaters (herbivore animals), without predators.
Predators play God
If predation is bad, and if we have safe and effective interventions in nature that decrease predation and improve animal welfare and biodiversity (for example herbivorize predators), then doing those interventions is good. However, many people are against massive interventions in nature that decrease predation, believing that such interventions are something like ‘playing God’.
This objection against intervening in nature doesn’t make sense, because by all possible interpretations, predators themselves are playing God. Here are three interpretations of ‘playing God’. Predators are playing God according to all three interpretations.
1. Determining the fate of others. By killing their victims, predators determine the fate of prey animals. Predators determine who to hunt and kill. They intentionally do so.
2. Influencing the identities of others. Predation influences the evolution of prey animals. Through natural selection, the genes, bodies and minds of prey animals change, because prey animals evolve towards having stronger limbs to run away from predators, having camouflage colors to hide from predators, having better senses to spot predators, being more anxious to look out for predators,… Prey animals would not look the way they do and feel the way they do if not for the predators.
3. Influencing the functioning of ecosystems on large scales. The theory of trophic cascades and keystone predators demonstrates that predators can have a large influence on ecosystems. Removing or reintroducing predators can have large knock-on effects. Top predators control populations lower in the food chain. The reintroduction of wolves in Yellowstone is the most known example of this. Those wolves caused massive changes in the ecosystem. Even the flow of rivers changed (due to more beavers because of more trees because of fewer tree-eating elks). For ecologists who studied Yellowstone, and definitely for the wolves themselves, the predatory behavior of the wolves had unforeseen consequences. The wolves never assessed the safety of their massive interventions. They did not do an environmental impact study of their predation. They intentionally preyed on animals, intervened at large scale, caused large changes, all without thinking about the consequences. They were playing God, but completely blindfolded.
Conclusion
The claim that predation is overall good, is not convincing. The harms of predation are very obvious: all the prey animals are harmed. But the benefits are very dubious. According to some weird, arbitrary definitions of biodiversity or ecosystem health, sometimes predators are beneficial, but many times predation is bad. Articles that point at the benefits of predation to ecosystems contain many inconsistencies or are cherry picking only the positive examples and neglecting the many counter-examples. We don’t see ecologists proposing the idea to introduce more predators in nature, create new predator species, recreate the T-rex, reintroduce large cats in Yellowstone (like the Siberian tiger as a replacement of the extinct saber-tooth tiger), introduce exotic predators on islands to control populations of native herbivores,… And why would that chosen, arbitrary notion of ecosystem biodiversity be more important than animal welfare? At least the animals themselves care about their welfare, whereas ecosystems don’t care about any of the notions of biodiversity or ecosystem health.
