Week 5 Science Determine how an invasive species—the zebra and quagga mussel—aff

Question

Week 5 Science

Determine how an invasive species—the zebra and quagga mussel—affects other species in the freshwater lake. Use the animation to help you come up with an answer to the following:

Why do you see increases and decreases in the invasive species population?

What are the implications associated with these alterations to the ecosystem as a whole?

The Effects of Zebra and Quagga Mussels Introduced into a Freshwater Lake

As you have learned, population dynamics are caused by the biotic potential of the population and the effects of environmental resistance. When there is minimal environmental resistance impacting a population, it will exhibit a population explosion. One reason for minimal resistance could be factors that no longer regulate a population (e.g., predator decline or resource increases). Another reason for a population explosion is the introduction of an invasive species. Invasive species are species foreign to an ecosystem and are not immediately regulated by the environmental restraints of the particular ecosystem that they invade. This in turn allows their populations to grow seemingly uncontrolled and to displace other indigenous populations. Examples of such an invasive species into North America are dreissenid mussels, commonly known as zebra and quagga mussels. Their introduction into the Great Lakes has caused economic hardship and a reorganization of the ecosystem. This has led, in part, to pollution-causing effects that can be linked to an alga known as Cladophora.

Ecosystems are webs of intricately balanced interactions, what happens when a new species is introduced that uses a disproportionate share of the ecosystem’s resources?

Explanation / Answer

Zebra mussels and quagga mussels are virtually identical both physically and behaviorally. Originally from Eastern Europe, these tiny trespassers were picked up in the ballast water of ocean-going ships and brought to the Great Lakes in the 1980s. By 1990 zebra mussels and quagga mussels had infested all of the Great Lakes. In her five-year lifetime, a single quagga or zebra mussel will produce about five million eggs, 100,000 of which reach adulthood.

The offspring of a single mussel will in turn produce a total of half a billion adult offspring. Zebra and quagga mussels feed on small organisms called plankton that drift in the water. The 10 trillion quagga and zebra mussels blanketing the bottom of the Great Lakes filter water as they eat plankton and have succeeded in doubling water clarity during the past decade. Clear water may look nice to us, but the lack of plankton floating in the water means less food for native fish.

Where quagga and zebra mussels co-exist, quagga mussels appear to outcompete zebra mussels, and quagga mussels can colonize to depths greater than those achieved by zebra mussels and are more tolerant of colder water temperatures. Zebra mussels were found at densities of around 899 per square meter, but quagga mussels now dominate at 7,790 mussels per square meter. The shells of both mussel species are sharp and can cut people, which forces the wearing of shoes when walking along infested beaches or over rocks. Mussels adhering to boat hulls can increase drag, affect boat steering, and clog engines, all of which can lead to overheating and engine malfunctions. Ecological problems also result from mussel invasions. Zebra and quagga mussels can kill native freshwater mussels in two ways:

(1) attachment to the shells of native species can kill them, and

(2) these invasive species can outcompete native mussels and other filter feeding invertebrates for food.

This problem has been particularly acute in some areas of the USA that have a very rich diversity of native freshwater mussel species. The encrusting of lake and river bottoms can displace native aquatic arthropods that need soft sediments for burrowing. In the Great Lakes this had led to the collapse of amphipod populations that fish rely on for food and the health of fish populations has been severely affected.

Zebra and quagga mussel invasions create an immense financial burden because of the need to continuously and actively manage these pests. Critically, niche-filling by introduced species could facilitate their evolution into invasive genotypes. If introductions often succeed where competition for resources is low, then establishing populations may have novel opportunities to exploit available resources to increase their fitness and spread. In plants, invading individuals are often larger than their native conspecifics, consistent with an increase in the resources available for growth.

An alternative explanation for the life history evolution that we observe here could be an adaptive acceleration of growth and reproduction to avoid summer drought. Though possible, we found that biomass both accumulated faster and remained higher in the invaders throughout life indicating that either increased resource exploitation (as we argue here) or re-allocation of resources must underlie gains in biomass. The loss of natural enemies during long-distance dispersal has often been hypothesized to facilitate invasiveness by allowing the re-allocation of resources away from defense functions. In the face of declines in native biodiversity, the widespread opportunity for non-native species to fill vacated niches has led some biologists to suggest that introductions might be beneficial for recipient communities, as lost functional roles of natives species are replaced by newcomers.

We see for the first time that a plant has evolved to increase its resource use where competition in its niche is low, and by doing so has achieved higher fitness and the ability to spread more aggressively. Our results caution that native biodiversity loss may make ecosystems more vulnerable to the evolution of new invaders with adverse ecosystem and economic impacts, as introduced species evolve in response to altered communities.

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