Tools for Critical Thinking in Biology is a book of stories about doing science. Most of these stories are incomplete because science is a work in progress, therefore one of my great pleasures is learning about new research that extends the stories in the book. Two new books tell exciting stories that complement stories in my book, and I encourage you to read these new books to learn more about how science works. The new books are Heart of a Lion: A Lone Cat’s Walk across America, by William Stolzenburg, and Serendipity: An Ecologist’s Quest to Understand Nature, by James A. Estes. Heart of a Lion describes a remarkable journey by a young male mountain lion from South Dakota to Connecticut and illustrates what we can learn from purely observational evidence, and Serendipity is Jim Estes memoir of his lifetime studying sea otters and killer whales in the Aleutian Islands.
Scientists use many different tools to learn how the world works. These tools include ways of studying nature such as comparative studies and experiments. Some studies use fancy equipment such as DNA sequencers; others use complex statistical methods. Regardless of the approach used and the technology employed, all science depends on reliable observations as the basic source of information. I illustrate this idea in my book with two brief stories about the evaluation of observational evidence in studies of ivory-billed woodpeckers and wolverines. In 2004, a birder in Arkansas reported the first sighting of an ivory-billed woodpecker since 1944, but extensive follow-up work didn’t confirm this observation, so ivory-billed woodpeckers are still considered extinct. In 2008, a graduate student set up remotely activated cameras to study pine martens in northern California. One of her cameras recorded a wolverine, the first definitive evidence of this rare and elusive carnivore in California since 1922. The graduate student and her team collected hair and fecal samples which yielded DNA confirming that the subject of their photo was a wolverine, probably a migrant from 650 kilometers away in Idaho.
Mountain lions once occupied most of the western hemisphere, but were extirpated east of the Rocky Mountains following European settlement. Except for a small, isolated population in Florida, the easternmost population in North America is now found in the Black Hills of South Dakota. Mountain lions are solitary and territorial, and young disperse from their natal areas at 12 to 18 months of age. By dispersing, young males avoid being killed by adult males that defend territories encompassing the ranges of several females.
Biologists documented dispersal of mountain lions from the Black Hills in the 1990s by attaching radio transmitters to young animals. Some dispersers went west, looking for openings in the matrix of mountain lion territories in the Rockies where they could establish territories of their own. Other dispersers went east, where opportunities to establish territories were virtually limitless but opportunities to mate were scarce or nonexistent, though the dispersers didn’t know this. Will Stolzenburg tells the story of one such disperser in Heart of a Lion. The hero of this story is a juvenile male that left its birthplace in the Black Hills in summer 2009. After a journey of more than 2000 miles through Minnesota, Wisconsin, Michigan, probably Canada, and New York, the lion was killed by a car in southwestern Connecticut in June 2011. Unlike other dispersers that traveled shorter distances, this lion did not carry a radio transmitter that enabled researchers to follow him day-by-day. His story as told by Stolzenburg is about the use of bits and pieces of observational evidence to reconstruct his route. This evidence includes sightings of tracks, photographs from remotely activated cameras, video recordings, hair samples, and scat. Researchers extracted DNA from the hair and scat that the lion left along his route, confirming that a single male had left these samples in Minnesota, Wisconsin, and New York and that this same male was the one killed by a car in Connecticut. This is similar to the wolverine story I told in my book, but much more detailed since the mountain lion’s journey was traced from beginning to end.
The scientific consensus is that mountain lions no longer live in eastern North America, but many locals believe that mountain lions were never completely extirpated from this region. As explained by Stolzenburg, this belief is based on numerous reported sightings over many years in many locations. Some of these observations were supported by photographs that turned out to be domestic cats, golden retrievers, and various other animals. In other cases, people really did see mountain lions, but ones that had escaped from captivity. In 2011, the US Fish and Wildlife Service reviewed the evidence for persistence of mountain lions in eastern North America and concluded that none of it was credible. Besides giving us an important history of our evolving relationship with wildlife, Stolzenburg shows how observational evidence can be evaluated for reliability and can contribute to our understanding of nature.
Serendipity: An Ecologist’s Quest to Understand Nature by Jim Estes is the second book I encourage you to read. I discuss the complexity of causation in Chapter 8 of my book using the ecology of sea otters in the Aleutian Islands as a case study. Jim Estes started research on sea otters in in the Aleutians in 1970 when he was a graduate student and has studied them ever since. Estes discovered that sea otters are a keystone species that has dramatic impacts on nearshore environments in the Pacific Ocean. Sea otters eat sea urchins, which graze on large marine algae called kelp. Where sea otters are absent, the ocean floor near shore is carpeted by sea urchins; where sea otters are present, sea urchin populations are much smaller and forests of kelp develop. These kelp forests provide habitat for fish, which provide food for eagles and other predators.
After working in the Aleutians for 20 years, Estes and his coworkers observed a sharp drop in the population of sea otters during the 1990s, which they eventually attributed to predation on otters by killer whales. With a declining sea otter population, sea urchins began to increase and kelp was reduced, changing many nearshore environments from kelp forests to sea urchin barrens. This is an example of a trophic cascade, in which a top predator influences the abundance of species below it in the food chain, including the vegetation at the base of the food chain.
Serendipity: An Ecologist’s Quest to Understand Nature is a wonderful memoir of Estes’ long career in science, including not only the details of what he did and learned but also an explanation of the thinking that led him from one project to another during his career and discussion of the broader implications of his work. Many of us can identify chance events that changed the direction of our lives; Estes entitled his book Serendipity because it shows how he was able to capitalize on these events in his own life to make fundamental contributions to our understanding of ecology. Read his book if you want to learn more about sea otters and killer whales and, more generally, about how science works.
Ecologists have identified several examples of trophic cascades driven by top predators in recent years. I discuss wolves in Yellowstone National Park as well as killer whales in the Aleutians in Chapter 8 of my book, while Will Stolzenburg devoted an entire book called Where the Wild Things Were: Life, Death, and Ecological Wreckage in a Land of Vanishing Predators to this topic. Stolzenburg emphasizes implications for conservation of new knowledge about trophic cascades.
While writing this posting, I learned about a new study that nicely illustrates an implication of a trophic cascade that may be surprising to you. Several ecologists used a mathematical model to predict the effects of recolonization of eastern North America by mountain lions. According to the model, control of eastern deer populations by mountain lions would reduce collisions of vehicles with deer, resulting in 155 fewer human deaths, 21,400 fewer human injuries, and savings of $2.1 billion over a period of 30 years. Between 1890 and 2008, mountain lions attacked humans 153 times and killed humans 21 times in the entire US and Canada, so occasional attacks and deaths would occur in the east if mountain lions were reintroduced, but these would almost certainly be far less than the number of injuries and deaths avoided due to the reduction in deer-vehicle collisions.