1: Ask students: Why do aspen trees need wolves?

It turns out that aspens need wolves to help keep populations of ungulates like elk in check – an elk’s favourite food is the young shoots that aspen send up as they try to reproduce. Some areas around Banff townsite, for example, are so overrun by elk (because wolves rarely come close to the townsite) that there are no young aspen trees, and the ecosystem is changing as a result.

2: Tell students that this is an example of an ecosystem enigma. An enigma is a strange or odd observation, one that seems different from expected. Challenge students to “put on their ecologist hats” as they try to answer the following ecosystem enigmas…

Why do beavers need to be in the area in order for Yellowthroat warblers to flourish?

Beavers create swamps and ponds surrounded by low-lying shrubs – the favourite habitat of the Yellowthroat warbler.

Why are pollutants found in high alpine lakes that have never even been visited by humans?

The answer has to do with the “leapfrog effect.” Scientists are finding that chemicals (such as toxaphene, spread to control cotton pests in the U.S.) used in lower latitudes move northwards in air currents and only come to earth again where they are trapped by precipitation, such as the copious snow that falls in the Canadian Rockies.

Why do fences built to keep elk off the Trans-Canada Highway inside Banff National Park attract coyotes onto the road, where they are sometimes killed?

The fences work. Elk stay out of the highway area, allowing the grass inside the fence and beside the highway to grow tall. These grasses provide excellent cover for mice – and mice attract coyotes, who can squeeze through holes in the fence to get to the food. Unfortunately, they may stray too close to the highway and are killed by vehicles.

Why do these same fences sometimes mean death for Bighorn Sheep?

A coyote herd in the area of Lake Minnewanka learned how to use the fence to trap Bighorn Sheep against it, eventually causing the extinction of a whole herd of sheep.

Why does the presence of numerous elk in the Bow Valley threaten the very existence of the moose population – even though there’s enough food for both species?

For decades, elk have been carriers of a nasty parasite called a liver fluke, a large slug-like parasite that tunnels into the liver of ungulates. Elk have grown used to this parasite, and it doesn’t seem to affect them. But if a moose is infested with more than a couple of liver flukes, it may die. Increasing numbers of elk in the Bow Valley increases the chance that moose populations are infested with liver flukes.

Why does a beautiful ornamental plant, purple loosestrife, decrease duck populations in an area?

Purple loosestrife is a popular plant used in gardening. Unfortunately, this non-native/introduced species is very productive: each plant can disperse 2 million seeds per year. Once these seeds enter a waterway, the plant continues to grow and overtake wetlands, choking out native vegetation. Native herbivores eat around the plants, essentially eating themselves out of food. When purple loosestrife, a plant introduced to North America in the late 1800s, overtakes an area, wildlife populations, including ducks, suffer.

3: Ask the students: Why do the numbers of songbirds decrease when an ecosystem loses its large, tertiary consumers/predators?

These images can be used to answer this interesting question. The lesson for each image is described below:

Image #1
Start by asking students: If the population of wolves goes down in an ecosystem, how would it affect the population of song sparrows?

Accept all answers – but tell students that in this activity the habitat itself is assumed not to change, and that there is no direct predator/prey relationship – wolves don’t eat sparrows.

Image #2
It turns out that when wolf numbers decrease, so do sparrow populations.

Can you think of some kind of interaction or interrelationship that can be used to explain this?

Accept some guesses and theories from students, asking them to back up their answers and suggest how they would prove or disprove their theories if they were biologists.

Images #3 and #4
Tell students that animals such as raccoons are called mesopredators – they do not sit at the top of the food chain, but they definitely prey on the eggs and young found in any sparrow nest they might find on the ground (i.e. they are secondary consumers).

Image #5
This image shows the whole picture.