Trout killing elk

One of the main intellectual draws of ecology is its deceptive complexity. Whilst topics can appear at first glance quite simple, a little study usually drags up a handful of interactions right away; illuminating the countless intricacies that make up the world we live in. It’s for this reason that I’d like to talk to you about how a paper published this month shows how trout in Yellowstone are killing elk.

Yep, honest.

At 136 square miles Yellowstone Lake is the largest body of fresh liquid water over 7,000 feet in North America. Historically it has had a very abundant population of cutthroat trout, which have been a food source for 42 species of mammals and birds, including the endangered bald eagle, white pelican, otter, black bear, mink, osprey, and the grizzly bear.

Grizzly bears awake after their long hibernations hungry, very hungry. After such a long sleep, having burnt off their fat deposits it is crucial that the animals get some energy-rich food, to replenish what they have lost. As cutthroat trout spawn at around this time in Yellowstone Lakes shallower tributaries they were ideal for this, easily providing high levels of fat and protein per fish caught.

However the spectre of invasive species has reared its ugly head and at some point around 1985 the species ‘lake trout’ was illegally introduced to Yellowstone Lake. Lake trout feed on cutthroat trout, and are a poor food-substitute as rather than spawning in shallow streams they instead do it on the lake bottom, making them inaccessible to the majority of terrestrial predators. Through predation by lake trout, it has been estimated that since 1990 numbers of cutthroat trout in some of the lake’s tributaries have been reduced by up to 90%.

Compared to other grizzly bear habitats in North America Yellowstone has less abundant nutritious plant matter, with low amounts of berries being produced, and so the area’s grizzlies are mostly carnivorous. In the past one of their main sources of nutrition was the abundant spawning trout, but with these much reduced they have had few opportunities but to begin switching to feeding on the newborns of animals calving within Yellowstone.

It is thought that in 1997 when cutthroat trout were still relatively abundant the park’s bears would have consumed 5656kg of trout per year. For this food source to be replaced by elk it has been estimated that between 297 and 476 elk calves are being consumed annually. With this increase in predation on calves there is of course a reduction in the number of elk reaching adulthood, causing meaningful changes in this species’ population dynamics. Some simulations have shown that this has been enough to reduce calf survival by up to 16%.

Changes in the number of hooved animals in Yellowstone have until recently been viewed as a simple consequence of the introduction and recovery of the ecosystem’s large carnivores, but this shows that, as always ‘it’s more complicated than that’. Both wolves and grizzly bears are increasing in numbers, but it is only grizzlies that feed so heavily on elk calves, with the current situation an unseen consequence of a seemingly unrelated introduction.

The introduction of a single invasive trout species to the lake has thus managed to affect a species of large herbivores that spends a significant portion of their lives up to 140km away.  28 species of animal and bird in Yellowstone were thought to depend on the spawning cutthroat trout and so the wider effects of their decline are likely to be very large, though the implications of this remain to be seen. Invasive species are often benign but can sometimes have large and widespread impacts that are difficult to foresee.

It is worth remembering in closing that the park’s management is working to reduce the number of lake trout in an attempt to help reduce their greater influence on the Yellowstone ecosystem. Such work is unglamorous and underappreciated, and often first in line around the world when budget cuts come around, despite it having potentially dramatic implications for the surrounding areas. Similarly monitoring programmes worldwide struggle for funding against ‘big’ research, though they are vital to uncover phenomena as shown here, which is of course needed for remediation to ever be possible. Money and human effort can often be vital to keeping such wondrous areas healthy.

Dave Bennett/@goldenmole

Elk image by Julie Falk, Trout image by NOAA Great Lakes Environmental Research Laboratory