Chapter 20

The red-backed salamander. The introduction of earthworms from Europe and Asia to northern forests that historically lacked earthworms has caused a decline in leaf litter and soil insects. This has led to a sharp decline in the abundance of red-backed salamanders.

Photo by George Grall/National Geographic Stock.

Earthworms play an important role in terrestrial ecosystems. As they burrow in the ground to consume detritus, they aerate compact soils, which allows water to better percolate into the ground. Earthworms seem to be everywhere in North America; you may have seen them on sidewalks and roads during rainy days or in the soil if you have ever worked in a garden. Therefore it is surprising that many common species of earthworms are not native to North America but were introduced from Europe and Asia in the eighteenth century. The northern temperate and boreal forests of North America did not previously have any earthworms, but the introduced worms are now having a profound effect on the movement of energy in these northern ecosystems.

Scientists hypothesize that northern forests completely lack native species of earthworms because the glaciers that advanced over the region during past ice ages eliminated all life. Since the ice receded 10,000 years ago, many other animal species have since returned to the area. However, the native earthworms that survived in the southern United States have a very slow rate of dispersal and have not yet arrived in many habitats in the northern United States. When there are no worms present, the leaf litter in these forests is primarily decomposed by soil fungi and microbes. However, this changed when European colonists inadvertently introduced European and Asian worms to the northern regions of North America. These worms have a high rate of dispersal and they tolerate a wide range of ecological conditions.

The introduced earthworms are still in the process of spreading into northern forests, assisted by the construction of logging roads and anglers who use the worms as bait. As consumers of dead leaves, the invasive worms can consume much of the energy available in the detritus and move many of the nutrients deeper into the soil where young plants cannot access them. The worms also leave much less energy for other organisms, such as the soil fungi that specialize on decomposing dead leaves. This is a major shift in the movement of energy. In addition to moving nutrients away from the forest floor, earthworm activity leaves a thinner layer of leaves and other organic matter which leads to drier soil conditions. These changes in abiotic conditions dramatically alter the soil food web. For example, in 2009 researchers reported that the recent arrival of introduced worms into the forests of New York and Pennsylvania caused a nearly 50 percent reduction in leaf litter. This, in turn, resulted in a substantial decline in the abundance of soil insects such as springtails, ants, and beetles. The decline in the insects also meant that there was less energy available for predators of the insects, such as the red-backed salamander (Plethodon cinereus). In areas with high densities of introduced earthworms, the decline in leaf litter and soil insects resulted in an 80 percent reduction in the number of salamanders. While adult salamanders can consume earthworms, young salamanders consume the much smaller soil insects.

Introduced earthworms. Northern temperate and boreal forests have lacked native earthworms since glaciers eliminated them during past ice ages. Today these forests are being invaded by species of earthworms that have been inadvertently introduced from Europe and Asia.

Photo by Oxford Scientific/Getty Images.

Salamanders aren’t the only consumer affected by the earthworm invasion. In 2012 researchers reported that a forest-dwelling ovenbird (Seiurus aurocapilla) was declining in areas of Wisconsin and Minnesota where the introduced earthworms had invaded. While the exact mechanisms were not clear in this case, the researchers suspected that the decline in the ovenbirds was in part because earthworms reduced the availability of insects on the forest floor, something similar to what caused the decline of the red-backed salamander.

The story of the invasive earthworms demonstrates that species depend on energy flowing between producers, detritivores, and consumers. Changes in energy flow between trophic groups can have major impacts on the species that inhabit an ecosystem. In this chapter, we will explore the flow of energy through food webs and the dynamics of energy movement through the ecosystem.

In Chapter 1 we noted that the ecosystem approach to ecology focuses on the transfer of energy and matter among living and nonliving components within and between ecosystems. The amount of energy that fuels ecosystems and the efficiency with which it is transferred through trophic levels determine the number of trophic levels in communities and ecosystems. The amount of energy available and the efficiency of its transfer determine the biomass of organisms that exist at each trophic level and the amount of energy that is left behind for scavengers, detritivores, and decomposers. In this chapter, we will discuss the movement of energy in ecosystems including the importance of primary producers and the flow of energy throughout the food web. In the next chapter, we will concentrate on how matter, in the form of key chemical elements, cycles around ecosystems.