What Are Trophic Cascades? This Will Blow Your Mind

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This short video is undoubtedly one of the most heart warming things I have watched in a long time. It renewed my faith in the power of our planet to work in harmony when left to it’s own devices. The intricate balance of nature is shown in all it’s glory. Whatever your faith, this will reaffirm it for you.

The title…How Wolves Change Rivers is a little obscure, but you will see that it is actually accurate. So how can wolves change rivers? The phenomena is known as a Trophic Cascade, and is the reason why we absolutely need ‘Apex Predators‘, such as sharks, to remain at the head of the various eco-systems on the planet.


But before I get ahead of myself and start talking about the science, let me break down the wonderful facts covered in the video.

So How Do Wolves Change Rivers?

The video focuses on the changes documented in the Yellowstone National Park in the United States, since they reintroduced wolves in 1995. The wolves were originally eliminated in the 1920s, due to a government policy which allowed them to be exterminated. As a result, the eco-system collapsed.

A relatively small number of grey wolves were released into the park in 1995, thanks to the Endangered Species Act, and the results have been astonishing. Of course, as expected, they killed a number of deer, which was what had been hoped. Over the years humans had tried to control the growing deer population, which had devastated the vegetation there.

So of course the wolves did kill some deer, but more than that, they drastically changed the behaviour of the deer, forcing them to live in smaller pockets of the forest and thereby allowing the vegetation to replenish. Some of the trees increased in height by 5 times in the space of six years.

The increase in trees and plants had the effect of encouraging various birds back to the park, and along with them insects. Beavers also returned, happy to have wood to gnaw on. The beavers built dams, and this in turn created habitats for even more creatures. Otters, fish and reptile numbers rose.

Beaver
Beaver

The wolves also killed a number of coyotes, which meant that the numbers of mice and smaller rodents began to rise. This increase in prey encouraged other predators such as hawks, weasels and badgers.

Going back to the wolves for a moment, the carrion left over by their hunting activities led eagles and bears to forage for leftovers. Bears also enjoyed the berries that were in abundance growing in the forest since the vegetation recovered.

The growing number of bears also reinforced the impact of the wolves in hunting the deer in the park.

So all of this is impressive, but how does it impact the rivers, beyond the beaver dams? This is mainly the result of the increased depth of the forest. There was less erosion on the banks of the river, pools began to form and the river meandered less. And so, the physical geography of the park also changed! All from reintroducing a few wolves and allowing nature to take it’s course….allowing a Trophic Cascade.

The following quote from Aldo Leopold takes a step back in time to 1949, to when the wolves were first eliminated. And here we see the opposite effect.

‘Since then I have lived to see state after state extirpate its wolves. I have watched the face of many a new wolfless moun- tain, and seen the south-facing slopes wrinkle with a maze of new deer trails. I have seen every edible bush and seedling browsed, first to ane- mic desuetude, and then to death. I have seen every edible tree defoli- ated to the height of a saddle horn.’

What Exactly is A Trophic Cascade?

A trophic cascade is a series of events which occur as a result of the natural behaviour of predators at the top of an ecosystem. The definition below is from nature.com

Trophic cascades are powerful indirect interactions that can control entire ecosystems. Trophic cascades occur when predators limit the density and/or behavior of their prey and thereby enhance survival of the next lower trophic level.

They occur across a diverse range of ecosystems, on land, in rivers and seas. The fact that the control comes from the predator down has surprised scientists who previously believed that such drastic changes in ecosystem could only be controlled from the bottom up. The following is from a review of studies into trophic cascades, which shows the extent to which we still need to understand this ecological phenomena.trophic-cascade-wolves

New studies are documenting trophic cascades in theoretically unlikely systems such as tropical forests and the open ocean. Together with increasing evidence of cascades, there is a deepening understanding of the conditions that promote and inhibit the transmission of predatory effects. These conditions include the relative productivity of ecosystems, presence of refuges and the potential for compensation. However, trophic cascades are also altered by humans. Analyses of the extirpation of large animals reveal loss of cascades, and the potential of conservation to restore not only predator populations but also the ecosystem-level effects that ramify from their presence.

The effect has been seen in many systems aside from mountain forests, and we will explore a few here.

Kelp Beds

In South East Alaska a group of islands has given us another fantastic example of a tropic cascade in action. Sea otters, sea urchins and macro algae (kelp) levels are the three trophic levels at play here. Otter levels approached levels near extinction in some areas due to hunting by humans for their skins. In these areas the population of grazing sea urchins boomed, resulting in a greatly diminished amount of kelp.

In contrast, in those environments where the otters were present, the sea urchins were controlled and the kelp beds were strong and healthy – allowing other organisms to thrive there. This suggests that the predator allows for the whole-ecosystem to remain healthily in balance. (Estes & Duggins 1995).

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Tropical Rain Forests

In the heart of the Venezuelan rain forest, a series of islands was unnaturally created through man’s actions. The balance of predators across the individual islands varies greatly, and this was taken as an opportunity by Terborgh et al. to investigate the trophic cascades upon each after a period of seven years in 2001.

The levels of predators of invertebrates (including birds, lizards and spiders), seed predators (rodents) and herbivores (howler monkeys, leaf cutter ants) were assessed when compared with the presence of vertebrate predators on each.

Those islands without apex predators were found to have a huge abundance of seed predators and herbivores – which resulted in fewer seedlings and saplings of canopy forming trees. It had originally been thought that trophic cascades were of little or no consequence in an eco-system as complex as that found in the rain forest – but this showed the truth. The top-down control had a dramatic impact on the health of the entire system, and removal of predators could actually destabilise the forest itself.

Salt Marshes

Salt marshes have been considered a perfect illustration of an eco-system which is regulated from the bottom-up. The majority of plants are largely unpalatable, and are controlled by nutrients and physical environment. It was believed that the nutrients and conditions dictated the plants which grew there, which in turn affected the larger life forms which lived there. However, it has recently been revealed that the control could actually come from the top down.

Various studies have reported that predators may actually play an enormous part in these flourishing eco-systems, meaning that changes occur due to both top-down and bottom-up influences. Silliman & Bertness reported in 2002 that a common snail controlled the growth of marsh plants by farming fungus on live plant tissue. If the number of snails grows too large, they can kill the grasses in the marsh, converting the marshes to open mudflats. However, when species of blue crab were present, the snails were controlled and the decimation of the salt marshes was prevented.

In Conclusion

The lesson that should be taken from these examples is that we are only now beginning to uncover the complex, indirect relationships between different levels within each ecosystem. For so long we have viewed predators as those which take life, without understanding the extent to which they give life, to those further down the trophic levels and to the planet itself.

It seemed illogical that carnivores could have a positive impact on plant life and the physical shape of the planet – but that seems to be just what is happening. Historical human intervention has in many cases done little more than devastate an entire eco-system….the exact opposite of what we were trying to do.

We may indeed have had good intentions, but we were not able to see the bigger picture. That is a lesson we are still learning in so many ways on the planet still. Only when we replace the missing pieces of the puzzle do we see the full implications of removing them in the first place. Nature can balance itself beautifully without our intervention, and it is awe inspiring to watch.

What of you think? Is this something you have seen before? We would love to hear your thoughts.

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