The Revitalization of Monterey Bay

sea otter

Monterey Bay is well-known for its sea otter population. (Photo credit: worldswildlifewonders/Shutterstock)

It wasn’t all that long ago that the Monterey Bay was a gloopy mess and one of the most polluted places on the Pacific Coast. Today it is considered to be one of the most productive marine ecosystems on the planet. What changed? How did a once-neglected and degraded region become one of the most revered and studied ecosystems on Earth? [Read more…]

Termites Lay Foundation for the Savanna Ecosystem

a cheetah sits atop a termite mound

Termites may hold the key to savanna ecosystem health. (Photo credit: Graeme Purdy/iStock/Getty Images)

The abundant grasses, iconic grazers, and legendary predators of the African savanna ecosystem all depend on one thing: fertile soil. Although it has long been known that termites play an important role in recycling nutrients in soil, researchers are now discovering that termites may do more than just contribute to the savanna ecosystem, they may make the whole thing possible. [Read more…]

Top Predators and Ecosystem Health

The presence of predators is a key indicator of ecosystem health. (Photo credit: Doug Smith/NPS)

Ecologists have long known that the health of an ecosystem depends on productivity at its base. Known as “bottom-up” control, nutrients available at the bottom of a food web dictate the abundance of organisms at each trophic level above it. But now mounting evidence is turning the bottom-focused theory on its head, pointing to the key role played at the very top of the food web.

Although they make up little of an ecosystem’s total biomass, top predators help regulate the entire web. For example, sea otters are a top predator in the kelp forests of the Pacific Ocean. When the sea otter population dropped, sea urchins went unchecked and decimated the kelp forests. As a result, many other species that depended on the kelp forests also declined, including raptors, shorebirds, fish, and invertebrates. Now that sea otters are protected and their numbers are back up, the kelp forests and the species  that depend on them are recovering as well.

If such chain reactions, or trophic cascades, are common to other ecosystems, there could be major implications for conservation. Top predators of terrestrial ecosystems such as wolves, bears, jaguars, and lions have declined throughout the world due to human activities. In North America and Eurasia, wolves in particular have been eradicated from many areas where they were once widespread. If the wolf populations were required to keep their prey populations in check, we would expect to see much higher numbers of deer, caribou, elk, and moose in those areas. We would also see changes in the plants the herbivores consume, as well as impacts to the biodiversity those plant communities harbor.

A recent study at Oregon State University set out to look at that very data. Researchers analyzed published data on populations of wolves, bears, and cervids (members of the deer family) in boreal and temperate forests of North America and Eurasia from 42 separate studies. They used remote sensing imagery to identify productive forests and historical range maps to determine areas where wolves and bears have gone locally extinct.

The investigators used statistical analyses to compare cervid density in forests where top predators remain to areas where they have been removed. What they found was strong evidence for “top-down” control of herbivores by predation. The average cervid density was almost six times greater in areas with wolves than areas without them.

Many authors of the reviewed studies also noted impacts of unrestricted cervid populations on the rest of the ecosystem. They observed declines in specific tree and shrub species, as well as changes to invertebrate communities. Other effects included increased stream bank erosion, which has led to changes in stream shape and fish habitat.

The researchers’ analysis also revealed that, although humans hunt deer, humans do not adequately stand in to fill the role of the top predator. The reason may have to do with how humans hunt compared with how large mammalian carnivores hunt. Apparently, the presence of predators does more than just limit a population’s numbers; it also affects how they behave.

When wolves were reintroduced to Yellowstone National Park in the mid-1990s after almost a 100-year absence, elk populations not only shrunk but also changed their behavior. They moved to safer areas, avoiding the riparian ecosystems which have since made a striking comeback. Vegetation came back to the stream banks along with beavers, beaver dams, and the fish and bird habitats they create.

As a strategy for habitat restoration, then, reintroduction programs or protection programs for top predators may yield far-reaching results. But they must be implemented with careful planning. Wolves have not historically been ranchers’ best friends, and their reintroduction in and around Yellowstone has been rife with controversy. One organization, Defenders of Wildlife, has come up with a creative solution to overcome ranchers’ losses due to wolves by paying them for killed livestock.

However, one thing is becoming clear from the mounting data on the complexities of ecosystem regulation: no one can do it quite like the top predators. It’s a job best left to the wolves.

More to Explore

The Reintroduction of Wolves to Yellowstone 
Top-Down Regulation
Defenders of Wildlife

The Native Species That Could

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When an “exotic” species is brought to a new environment and makes its home there, everything changes. Many native species die out as their biotic and abiotic resources are taken over or are fundamentally altered. Enormous damage is done as the invasive species spreads faster than people can eradicate it. Examples of destructive invasions can be found everywhere in the United States, from zebra mussels in the Great Lakes to kudzu in the South. However, new evidence is providing the hopeful message that not all native species necessarily succumb to invasions. As a striking example of evolution in action, scientists have found that some native species are adapting to the changes brought on by invaders, and can do so rapidly.

For example, toxic cane toads were brought to Australia in 1935. Today, less than 30 generations later, some species of snakes have evolved as a response. Australian black snakes from toad-invaded areas have acquired an increased resistance to the toad’s toxin. In addition, the black snake and other species, including the death adder, have evolved smaller head sizes in areas where the cane toad is present. The snakes with smaller heads cannot eat toads that are large enough to kill them, so natural selection has favored those with smaller heads.

Likewise, some plants in invaded areas have shown signs of adaptation. Cheatgrass is an invasive grass that has spread across the deserts of the Intermountain West. It sprouts earlier in the spring than the native grasses and saps nutrients from the soil. Cheatgrass is extremely flammable and has led to an increase in wildfires in the region. Once a fire has wiped out the existing vegetation, the invasive grass spreads farther across the open land.

Scientists compared several hundred native perennial grasses from communities with and without cheatgrass. Each plant was divided in two and brought to a greenhouse. One plant of each pair was grown with cheatgrass to measure its ability to survive and compete. They found that plants from the invaded areas had adapted by sprouting earlier, and two species began flowering earlier in order to better compete with the cheatgrass.

In another study, scientists found that a native grass called big squirreltail has adapted to cheatgrass invasion by developing smaller shoots and larger roots. The altered squirreltail roots take back the nutrients that cheatgrass has been stealing from the native plants and suppress the cheatgrass’ growth.

Other scientists have even found evidence of coevolution, in which both the native species and the invasive species have evolved in response to each other. Garlic mustard is a stubborn invasive herb that releases a compound called sinigrin. Sinigrin kills fungi that native plants need to absorb nutrients from the soil. However, a native species known as clearweed has shown increased resistance to garlic mustard. Garlic mustard has, in turn, adapted by releasing more sinigrin in areas where more native species are present. What may at first appear to just be a clump of boring-looking weeds, may actually be an example of a dramatic evolutionary arms race in progress.

Questions:
1. Just because one population of a native species has evolved a response to an invader doesn’t mean another will, given the chance. How might strategic transplantation be used in the management of invasive species? Give a specific example.

2. Consider another invasive species that you already know about. Design an investigation to find out if any native species are adapting to the presence of the invasive species.

3. How could scientists determine whether genetic changes have occurred in adapted native species? What might be done with this information?

4. Speculate on the long-term consequences of native and invasive species evolution.

More to Explore
 Native Plants Evolve to Fight Off Invading Species
Coevolution and Evolutionary Arms Races
Native Species Fight Back: First Evidence of Coevolution Between Invasive, Native Species
Native Perennial Grasses Show Evolutionary Response to Bromus tectorum (Cheatgrass) Invasion

Society and the Environment: Conserving Top Predators

wolf

Successful reintroduction of wild wolves in the American West has led to significant changes in the ecosystem.

Return of Wolves
By the early 1900s wolves had been virtually eliminated from most of their native range in the United States. They were hunted vigorously because they killed livestock. In Yellowstone National Park, wolves were hunted to extinction.

[Read more…]

Making a Difference: A Little Piece of Cajun Prairie

collecting seed

Charles Allen is shown here collecting seeds from a compass plant at a Cajun prairie remnant. The leaves of the compass plant face east to catch the sun.

Cajun prairie is a distinct grassland, named for the settlers who lived there. It once covered more than 2.5 million acres of southwest Louisiana. Today, only about 100 acres of Cajun prairie remain. If the work of two biologists and many volunteers pays off, however, a little piece of Cajun prairie will always exist in Louisiana.

[Read more…]