Sunday 12 February 2023

Benefits of a Deadly Predator

A basic concept in ecology is that of the trophic level, a concept that lines up life forms by where they sit in the food chain. Plants are at the bottom level, taking energy directly from the sun, then the next level up are the herbivores, which eat the plants. Above that we have primary predators eating the herbivores, and then secondary predators eating the primary ones. Because the transfer of energy is never perfect, each level will be smaller than the one below it, with fewer individual life forms in it, thus forming a pyramidal pattern.

The reality is more complex than this. Secondary predators also eat herbivores, omnivores are common, really large herbivores aren't likely to be eaten by anything, there may be more than two steps in the chain of carnivores, and we can't forget the detritivores and fungi. And so on. So what we actually have is a "trophic web", a complicated set of interactions where some animals don't fit neatly into a single level on the pyramid. Nonetheless, that doesn't make the basic idea useless and one of the concepts it leads to is the apex predator - the large carnivores that have no predators of their own (at least as adults).

Because each trophic level has to be smaller than the one below it, apex predators are often rare. Because they are also often a threat either to humans directly or to their livestock, they often end up persecuted and become endangered species. (Whether we ourselves are apex predators is a matter of definition, given the amount of plant matter we normally eat). Where they do survive, however, they can be crucially important to their ecosystems, having all manner of effects on the organisms "below" them in the chain.

Perhaps the most widespread land-based mammalian apex predator is the puma/cougar/mountain lion (Puma concolor). Before the coming of the white man, they were found through essentially the entirety of the continental Americas south of the tundra. That's no longer true today, since, aside from the tiny exclave of the Florida panther, they are now only found in the west of the US and Canada. They're also largely absent from Uruguay and uncommon in Peru and parts of Bolivia. But even so, that still leaves them living from at least central British Columbia, through the western US and most of Latin America, right the way down to the Straits of Magellan on the edge of the Antarctic Ocean.

That makes it more widespread than literally any other native mammal in the Americas and covers almost every climate type you could care to mention aside from lowland tundra and ice cap. Deserts, pine forests, jungles, grassland, the works. In 1996, their population in the US and Canada was estimated at around 15,000 and there are presumably many more than that in Latin America, especially in the depths of the Amazon where they would be difficult to count. So, unlike say, tigers, they are not an endangered species or even especially close to that status. 

But their worldwide population is thought to be declining, partly due to general human population growth and its attendant urban areas, roads, and so forth, but also due to unregulated hunting and deliberate killing of what can certainly be a dangerous predator. A review of the last 70 years of English and Spanish language research papers about pumas took a look at how the presence of this particular apex predator benefits the wider environment.

(I'll note in passing that the choice of these two languages, while certainly helpful if we don't want to ignore much of the research in Latin America, might still pass over some of the work conducted in Brazil. Although, given that English is, like it or not, the lingua franca of science, that may be less than you'd think.)

The most obvious way that apex predators such as pumas affect their ecology is basically by eating other animals, thus potentially reducing their population. This could be a good thing if the numbers of prey animals would otherwise be too high thus, for example, eating too many of the plants and damaging the ecosystem in that way. However, this review found little evidence that this is true of pumas (possibly there's little opportunity for it) even though they do undoubtedly affect the population of animals such as mule deer and elk and, more worryingly, on potential prey that are already rare, such as bighorn sheep in Arizona.

When it comes to the more common animals, however, the overall effect is less than one might expect. This is probably because the primary threats to animals such as deer are reduction in their own food supply (for example, due to prolonged drought or other bad weather) and hunting by humans, both of which typically outweigh the danger of being eaten by pumas. For example, a review published in 2020 found that reducing the number of pumas in an area does increase the survival of very young elk but does nothing for the overall population numbers - the adults are just more likely to die early for other reasons.

But predators don't have to kill herbivores to have an effect on them. The sheer risk that they might be eaten could lead prey animals to avoid certain areas, essentially allowing the predator to control the ecosystem through the power of fear. Unsurprisingly, elk don't tend to visit areas where pumas (or wolves) are known to hunt, and vicuña avoid such places at night when the pumas are more likely to be on the prowl. In at least some places, this has been shown to be a positive benefit, leading to greater diversity and lushness of plant growth in places where predators scare away the herbivores - an effect known as a "trophic cascade". 

A related concept is the "intraguild cascade" where the presence of apex predators affects the abundance of other, smaller carnivores that are not quite at the apex which, in turn, affects the animals that those carnivores feed on. For instance, pumas are known to kill coyotes and they reduce the numbers of foxes and ocelots, if only by driving them away from areas where they are common. On the other hand, carnivores that are too small for the pumas to bother with, such as skunks, may benefit from the presence of the apex predators because, for them, a smaller number of coyotes and bobcats is a good thing.

In fact, the skunks gain a double benefit because they are also scavengers and can eat the carcass remains that pumas leave behind. The more pumas, the more carcasses. Indeed, those coyotes that get killed by the pumas are largely doing so while trying to feed on the pumas' own leftovers. More specialist scavengers, such as condors, also benefit from puma-created carrion and one estimate is that pumas provide a staggering 1,500 tons of meat per day across the whole of their range. It's apparently good for beetles too, and the nutrients soaking into the soil from all those corpses may create nitrogen hotspots to benefit the local plantlife.

One additional benefit that the presence of pumas provides comes from a study in South Dakota which showed that the return of pumas to the state saved its inhabitants $1.1 million per year... due to the reduced chance of their car hitting a deer. That's in addition to the more direct benefits of tourists paying to watch pumas in Patagonia.

All in all, pumas interact with their ecosystem and even the human environment in multiple ways. It's true that pumas, like other large apex predators, can be dangerous and they do occasionally kill humans (mainly children) but it's a very rare event - Washington state has apparently only recorded two such deaths in the last century. Even then, it is possible to mitigate against that, through education and physical modifications to inhabited areas. 

Like any other animal, apex predators are a key part of the trophic web and their loss can affect the local ecology in ways that could well be detrimental.

[Photo by "NaturesFan1226", from Wikimedia Commons.]

1 comment:

  1. Puma, cougar, mountain lion, leon, suçuarana, onça-parda, onça-baia, onça-vermelha