Wolves grow forests; elephants engineer savannas

The Scottish island of Rùm, from which wolves have been absent for 250 to 500 years, provides a view of the likely final outcome of predator loss and elevated herbivory in many temperate forests. Rùm has transitioned over this same period from a forested environment to a treeless island. [from Trophic Downgrading of Planet Earth (Science, 15 July 2011)]

Many people have probably heard of how the returning of wolves to Yellowstone National Park affected the entire landscape: trees grew tall again along streams, which brought beavers back into the area, and increased songbird populations. (Here is one article on the subject.)

It’s not a unique phenomenon; as the paper in Science points out, this kind of effect – a trophic cascade brought about by the presence of an “apex consumer” – has been documented in every kind of ecosystem there is, whether on land or in the water. Some of the other examples I’ve seen press about have also focused on large carnivores – but in other ecosystems, it is a large herbivore who has been documented as holding the apex position, and in others, the “top” is held by relatively small creatures, like starfish.

A trophic cascade doesn’t always have to involve a top predator. Back in the late 1960s, zoologist Robert Paine removed the starfish known as “ochre stars” from sample areas on the coast of Washington state. Active, aggressive hunters, ochre stars gorge on mussels. Lose that single predator, and thick beds of rapidly proliferating mussels take over, squeezing out barnacles, algae, snails, sponges, tube worms, sea squirts, and the rest of the remarkable mix of marine life usually at home in the intertidal zone. With ochre stars, Paine introduced the idea of a “keystone species,” defining it as one that exerts an outsized effect within an ecosystem, not necessarily through sheer size, number or biomass, but because of the pivotal role it plays.

. . .

In another type of trophic cascade involving a keystone species, the lead role is played by a vegan: the African elephant. Consuming upward of 400 pounds of plants per day in the case of large males, these titans rip down tree limbs for meals, girdle tree trunks by stripping off nutritious bark, and simply push smaller trees over to get at the branches. As for shrubs, elephants not only eat the stems but tear whole plants to pieces, uproot others and trample still more underfoot while foraging.

When fully grown, elephants are virtually immune to predators, and herds tend to increase until checked by drought, food shortages or disease. At high population levels, their quest for food can transform wooded habitats into open plains. In cooperation with wildfire, elephants also maintain existing savannas by removing the woody plants that inevitably invade grasslands. Elephants shape the woodland-savanna balance in an ecosystem by determining the proportions of grazers such as gazelles, zebras and wildebeest to woody plant browsers such as giraffes and kudus, and of lions prowling the plains to leopards waiting to pounce from an overarching tree limb. That is, elephants shaped the communities in which our primate ancestors developed, stood upright and started walking toward the future.

Ecologists sometimes refer to elephants as “ecosystem architects” or “ecosystem engineers.” Here in North America, grizzlies serve the same function. In addition to their effects as predators and scavengers of hoofed animals, the big bears fertilize streamside habitats with their waste and the remains of the salmon they eat. (Both are rich in nitrogen and other essential nutrients.) Grizzlies also distribute thousands of seeds from shrubs after eating the berries, and they rank as the chief animal earth-movers of the upper elevations in portions of the Rockies. Excavating acre upon acre to get at hibernating rodents and the bulbs and starchy roots of various herbs, they bring up nitrogen from deeper soil levels just as a farmer does when tilling fields. The seeds that fall into such freshly turned soil yield a more robust crop of new alpine and subalpine plants than seeds in undisturbed patches. . . [Source]

Some of the other benefits to an ecosystem having its creatures returned are that they can decrease the presence of invasive species which are posing a threat to other native members of that ecosystem. In Ireland and Scotland, the non-native grey squirrels, which have had detrimental impacts on the landscape (including pushing the smaller native red squirrel out of place) are vanishing in areas where pine martens are left alone to do their pine marten thing, or even return, rather than being hunted out of existence by humans.

It now seems that many exotic species, like grey squirrels, that appear to present intractable problems do so only because they are moving into depleted ecosystems. They become invasive and destructive because there is nothing left to restrain them. American mink, for example, are a major problem in Europe where there are no otters, proliferating rapidly and wiping out water voles, birds and other species. But when otters, which are highly territorial, move in, they drive the mink out. White-tailed eagles, which have recently been reintroduced to the Hebrides, but once lived throughout Britain, prey heavily on mink and, according to a study in Finland, keep them out of areas they would otherwise occupy.

There might be no grey squirrel problem – in fact there might be no grey squirrels here at all – had pine martens not been eliminated across most of their range, primarily by gamekeepers.

. . .

Meanwhile, the Game and Wildlife Conservation Trust, which I see as a greenwashing agency for the shooting industry (how many conservation groups do you know that teach children to use shot guns and run courses on snaring, lamping and trapping?), is campaigning to reduce pine marten populations in Scotland. Yes, reduce. It claims that it wants to do so to protect capercaillies: the giant grouse that also once lived across much of Britain but are now confined to a few glens in Scotland. But there is no evidence that pine martens are implicated in the capercaillie’s decline: in fact the capercaillie is doing best where pine martens are also thriving, and doing worst where the predator continues, illegally, to be persecuted.

Another predator being persecuted by humans is the Australian dingo – despite there being evidence that when left alone, it too keeps other (non-native) species in check, which allows the return of native plants and animals.

. . . dingoes are very effective at limiting the populations of smaller predators and wild herbivores. By doing so small prey species are able to recover and vegetation condition improves. Persecution of dingoes across the continent explains patterns of high-density fox populations on the one hand and extinctions of marsupial and native rodent species on the other. “Humans with poisons, traps and guns have not been able to benefit biodiversity in this way” Wallach said. [Source]

We tested the idea that state shifts to invasive dominance are symptomatic of losses in ecosystem resilience, due to the suppression of apex predators. This concept was investigated in Australia where the high rate of mammalian extinctions is largely attributed to the destructive influence of invasive species. Intensive pest control is widely applied across the continent, simultaneously eliminating Australia’s apex predator, the dingo (Canis lupus dingo). We show that predator management accounts for shifts between two main ecosystem states. Lethal control fractures dingo social structure and leads to bottom-up driven increases in invasive mesopredators and herbivores. Where control is relaxed, dingoes re-establish top–downregulation of ecosystems, allowing for the recovery of biodiversity and productivity. [Source; a paper by Wallach and others]

The Science article also talks about how other factors, not just the populations of different organisms, are affected by the presence of certain keystone species:

Recent research suggests that the disappearance of these animals reverberates further than previously anticipated, with far-reaching effects on processes as diverse as the dynamics of disease; fire; carbon sequestration; invasive species; and biogeochemical exchanges among Earth’s soil, water, and atmosphere.

Interactions between predators and herbivores affects plant populations, which then interacts with climate to impact wildfire, as well as changing soil health. And if things change in aquatic systems, that can affect how gases like CO2 move between water and atmosphere.

Summary: stop eradicating species, let them come back, and the overall ecosystem can see multiple benefits – invasive species vanish, other native species come back in abundance, the entire system starts repairing itself, much better than our attempts to “control” the non-natives (or to persecute natives who are now developing large populations – look up any articles on the planned and actual killings of cormorants and sea lions in Oregon, who are preying too much on salmon, which reduces the populations of already-endangered fishes . . . of course we “can’t” reduce the biggest threats to salmon populations: dams and other human-made impediments to river health).

Meanwhile, wolves may lose their state legal protection in eastern Oregon, because more than 4 breeding pairs live there (a whopping great 7 pairs, with something like 34 total wolves), and in other states “hunting derbies” – sporting competitions, basically – to kill wolves and coyotes continue.

Under the state’s wolf plan, the Oregon Fish and Wildlife Commission can consider removing the eastern packs from the state’s endangered species list once that population bar is met.

Before the new population threshold was met, ranchers could only take wolves caught in the act of injuring or killing livestock. Now they can take wolves caught chasing livestock under some circumstances. Ranchers on private land also no longer need a permit to use beanbags, rubber bullets or other “non-lethal injurious harassment,” on wolves. [Source]

I realize that bureaucracies have to develop rules that they then must follow, but given that there is much more at stake than whether or not an entirely tiny number of wolves (how could four breeding pairs be considered enough genetic variation for a healthy long-term population???) may live without legally being harassed and killed by humans, the old standards – and the status of whether a species is “endangered” or not – seem very out of date and totally inadequate; it appears that laws and regulations haven’t kept up with what the science says, nor do the people who live in areas where big predators are returning seem aware of the full impact these animals have on their ecosystems – the focus is always about the terrible impact the wolves have on ranchers’ livestock. Give them another several decades without the wolves, and they might see some pretty terrible long-term impacts on the surrounding forests and other lands. (I wonder, again, what the long-term impact on forests is from the depletion of salmon, lamprey, and other fish populations, and I worry that we won’t see the forests dying, or other wide-scale ecosystem impacts, until it is too late to let the fishes live normally again.) But that’s the problem – it takes a long time to see just how bad it really is to eradicate one predator to improve one particular aspect of human life (here’s Science again:

The omnipresence of top-down control in ecosystems is not widely appreciated because several of its key components are difficult to observe. The main reason for this is that species interactions, which are invisible under static or equilibrial conditions, must be perturbed if one is to witness and describe them. Even with such perturbations, responses to the loss or addition of a species may require years or decades to be come evident because of the long generation times of some species. Adding to these difficulties is the fact that populations of large apex consumers have long been reduced or extirpated from much of the world. The irony of this latter situation is that we often cannot unequivocally see the effects of large apex consumers until after they have been lost from an ecosystem, at which point the capacity to restore top-down control has also been lost. Another difficulty is that many of the processes associated with trophic downgrading occur on scales of tens to thousands of square kilometers, whereas most empirical studies of species interactions have been done on small or weakly motile species with short generation times that could be manipulated at small spatial scales.


About Fjothr Lokakvan

More or less Northern Tradition polytheist.
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