Book Review: David Wilcove's No Way Home

Food is undoubtedly one of the main seasonally abundant resources that motivates species to migrate. Ultimately, when it comes to food, migrants are taking advantage of differences in climate and in many cases, as Wilcove explains, it depends on rainfall. Numerous European songbirds migrate south to Sahel in Africa during Europe's winter because this area resembles that of their European breeding grounds (Wilcove 2008, p. 28). While Europe is covered in snow, this region of Africa is rich in food resources, namely insects and seeds, which are tied to the previous winter's rainfall. Higher rainfalls make the wintering area much more abundant in critical food resources. Additionally, rainfall amounts are also positively correlated with the breeding populations of these migratory songbirds because more food in the wintering areas allows the birds to put on more fat reserves, thus increasing their chance of survival while migrating back to their European breeding grounds (Wilcove 2008, p. 29). Wildebeests also migrate based on seasonal rains for abundant and nutrient-rich food sources. During the dry season, herds leave the Serengeti because the lack of rainfall cannot sustain plant life or herds of animals that rely on vegetation for food (Wilcove 2008, p. 84).

As soon as rainfall begins vegetation sprouts into enough food for the traveling herds, but once the rain shifts out of an area the grass stops growing and the wildebeest must move to where the rain has shifted (Wilcove 2008, p. 85). Similarly, Wilcove states that "it appears as though Serengeti migration is driven by the need to acquire particular nutrients" (p.86). In No Way Home, Wilcove includes the fact that the wildebeest move around the grassland where the seasonal rains and mineral-packed soils create extremely nutrient-rich grass, which is needed for female wildebeest lactation and the growth of new calves (Wilcove 2008, p. 86). Another example of migrants using food as a seasonally fluctuating resource is bellbirds. Wilcove notes that these birds migrate between high and middle-elevation forests in order to take advantage of fruit that comes into season during different times of the year in forests at different latitudes and elevations. The changing abundances in fruit over the different regions and seasons have caused bellbirds to adopt migration (Wilcove 2008, p. 42).

Migratory monarch butterflies use seasonal fluctuations in both food (and thus prime egg-laying habitat) and temperatures to their advantage. In terms of food, monarch butterflies have timed their northern migration from Mexico to Canada to overlap with the spring and summer emergence of milkweed plants, the only type of food that newborn caterpillars will eat (Wilcove 2008, p. 63). However, monarch butterflies also use temperature fluctuations as another major resource. In fall when temperatures in the U.S. and Canada begin to get colder, the butterflies migrate south to the Michoac¡n mountains where temperatures are colder than nearby lower altitudes.

The butterflies cling to fir trees, barely moving and rely on cold, just-above-freezing temperatures to keep their metabolic rate low and fat reserves high. Too high of temperatures would deplete their fat reserves and decrease their survival rate during the migration back north, but too low of temperatures would freeze them to death (Wilcove 2008, p. 60).

Before being nearly wiped out by settlers, food and shelter were two important seasonally abundant resources for bison on the Great Plains. During the spring, summer, and fall, the herds of bison moved around the grassland, which is rich in vegetation during these seasons (Wilcove 2008, p. 108). The warm temperatures allowed them to forage in the grasslands but during cold winter months, the open grasslands of the Great Plains were harsh and offered little relief from the conditions for the bison, which instead moved to woodlands and used the available shelter and vegetation to their advantage (Wilcove 2008, p. 107). While food is indeed one of the main motivating factors for migration, resources like shelter and temperatures are also just as crucial and many migrating species rely on fluctuations in climate and rainfall to provide them with access to these resources year-round.

In No Way Home, Wilcove talks about many of the anthropogenic problems that affect migratory species and in many cases migratory behavior has caused further endangerment of certain species. One way that animals are endangered by their migratory behavior is that these populations often migrate to or from countries where they don't have a protected status from the government. As Wilcove explains, these developing countries are often battling with social problems and don't have the resources to put towards protecting migratory birds: "Chronically underfunded, facing immense and immediate challenges of poverty and political instability, these countries simply cannot afford to give birds the same attention they receive in northern nations" (Wilcove 2008, p. 49).

Numerous developing countries, including those in Asian, Latin America, and Africa, serve as stopover locations, but in countries where there is no protected status, all kinds of migratory birds can be hunted for recreation or sold as food (Wilcove 2008, p. 34). The dangers of migratory behavior also occur in bison that normally stay in Yellowstone National Park but will occasionally migrate during particularly harsh winters where there is more available food. But often the bison will migrate through privately owned land and less they can be returned to the park they are killed due to fear that they could spread disease to cattle grazing on the same land (Wilcove 2008, p. 118).

Wilcove also mentions that the migratory behavior of right whales also endangers them because they come directly in contact with fishing vessels and they "migrate along one of the busiest coastal transportation routes in the world -- Collision with ships and fatal entanglements in fishing gear have replaced harpoons as the major threat to its survival" (p. 137). Sea turtle reproduction is threatened by their migratory behavior since they must lay their eggs on beaches and make the trek from the sea to these sandy areas. Beaches are extremely risky places to lay eggs since everything from egg predators like raccoons to beach developments can severely decrease the survival of these turtles who are much safer in the water than on land (Wilcove 2008, p. 150). Of all the migratory species included in No Way Home, it seems as though salmon are threatened the most by their migratory behavior back through freshwater streams. During their migration back to their natal stream for reproduction, several obstacles threaten their survival. Salmon must cross dams that are built across rivers and streams and although dams have fish ladders, not all fish are able to cross over the dams. Similarly, dams can ruin essential salmon ecosystems by disrupting the flow pattern of the river and creating pools of slow-moving and warm water, both of which are unsuitable for salmon (Wilcove 2008, p. 173).

Agriculture near these fragile freshwater streams poses a huge threat as pesticide and fertilizer runoff enters the water and alters the chemistry beyond what the fish, especially the eggs and developing salmon, can tolerate (Wilcove 2008, p. 172). Despite all the examples of how migratory behavior threatens the survival of migrants, migratory behavior can also offer protection. A prime example is in the Serengeti where resident populations have declined much more drastically than migratory populations, which is most likely due to the boundaries of the reserve being changed over time. Additionally, habitat has had less of an impact because migrants often use different routes (Wilcove 2008, p. 93-94). In conclusion, due to the spread of human developments, migratory animals are often threatened by their migratory behavior but in some cases it can protect their survival.

Throughout No Way Home, Wilcove presents examples of migratory species and it is evident that there are commonalities and differences between them. One major commonality between migratory species is that they have significant impacts on the ecosystems they inhabit and often these populations will aid in an environmental cycle that promotes the health of that ecosystem and its other inhabitants, including migrating populations returning the next year. A prime example of this is the Serengeti's wildebeest and zebra populations, which induce grazing succession after completely grazing an area into a wasteland. By stripping the ground of drying and dead vegetation and depositing vast amounts of feces and urine, the ecosystem can renew itself by growing healthy and protein-rich vegetation using the nutrients from the waste products. This provides food for other species, like the Thomson's gazelles, and ultimately allows the ecosystem to support a greater number of grazing species (Wilcove 2008, p. 88). In Costa Rica, bellbirds help disperse fruit tree seeds by consuming the fruit and leaving the seeds behind in their feces as they perch in sunny areas, which is an ideal place for the fruit trees to grow. If bellbird populations begin to decline it could interfere with tree reproduction, which has the potential to reduce food sources for other animals that rely on the fruit (Wilcove, 2008, p. 43). Salmon play an important role in the ecology of streams since their feces, sperm, eggs, and decay carcasses help boost nitrogen and phosphorus concentrations, which then increase the production of phytoplankton and zooplankton, two major food sources for small fish and juvenile salmon in the streams (Wilcove 2008, p. 187).

Although there are certain similarities between migratory species, there are also important differences, including the ways they navigate. In many of Wilcove's example of migratory species, he talks about the general hypothesized mechanisms behind animal migration and it is clear that migratory species utilize different cues to guide them to their destination. For example, monarchs use a sun compass to regulate their circadian clock and scientists have demonstrated this by "jet-lagging" the butterflies, which responded to the position of the sun and flew in the wrong direction (Wilcove 2008, p. 64). Unlike monarch butterflies, some scientists have hypothesized that whales use topographical features, like familiar landmarks and water depth, to guide them. Whales are incredibly difficult to test in a lab setting, but it is possible that they use more than one type of navigational system (Wilcove 2008, p. 64). Salmon, on the other hand, rely on a scent that became imprinted on them one to three years ago to lead them back to their natal stream for reproduction. Researchers have tested this hypothesis by imprinting on them a different smell and by adding that smell to a completely different river, which the fish chose over their actual natal river (Wilcove 2008, p. 171).


References Cited

Wilcove, D. S. 2008. No Way Home. Island Press, Washington D.C.