In Central California, southern sea otter populations have rebounded to close to 3,000 individuals (from fewer than 100 a century ago). In the case of the sea otter and other recent examples, reestablishment of the population and previous food web dynamics have been more complex. In the case of the Yellowstone wolves, reintroduction has been largely successful and has reversed some of the ecological imbalance, but the balance within the food web may never return to its original stable state. This phenomenon, when the path to recovery differs from the path of disruption, is called hysteresis. However, the path back to a stable food web is not always the reverse of the environmental pathway that led to destabilization. Instead, predation heavily influences mortality throughout the year. Now, young elk no longer die in bulk from starvation during winter. It also affected mortality trends in the elk population. Repercussions included a decrease in the once overabundant elk population, a downstream increase in vegetation growth, and a rebound in the populations of smaller animals. National Park Service reintroduced wolves into Yellowstone National Park beginning in the mid-1990s, dramatically stabilizing some aspects of the food web and ecosystem. As scientists have learned more about trophic cascades, they have begun to discuss and implement efforts to reintroduce some top predators back into their ecosystems. Without enough plants to hold the soil together or shade the ground, the overgrazing of the area also led to river bank erosion and changes in water temperature due to lack of shading. This loss of vegetation negatively impacted beaver and bird populations. The loss of gray wolves led to an explosion of the elk ( Cervus canadensis) population and subsequent overgrazing of plants, including immature trees. In the 1900s, settlers competed with wolves for large prey, such as bison and deer, and hungry wolves inevitably began instead to prey on livestock and were thus hunted themselves by ranchers protecting their flocks. A decline in the gray wolf ( Canis lupus) population had a similar effect in the U.S. This type of relationship within a food web, in which the feeding habits of a predator trickle down and influence lower levels of the food web beyond the predator’s immediate prey, is called a trophic cascade. The absence of sea otters from the food web resulted in decreased ecosystem diversity. The shrinking forests of algal seaweed forced fish, which used to find shelter within them, into alternate habitats. This bloated population of invertebrates ate away whole forests of kelp to support themselves. Populations of sea urchins and other marine invertebrates, natural prey to sea otters, skyrocketed. When the otter population dropped during the fur-trade era, it initiated a cascade of events that altered the region’s food web. However, the damage to the ecosystem has taken its toll. society has evolved to prioritize species conservation efforts and regulate activities, such as the harvesting of furs, people have gained interest in the preservation and reintroduction of once-hunted apex predators. North American participation in the global fur trade heavily influenced this near-extinction. California coast dropped from more than 10,000 in the 1700s to fewer than 100 by the early 1900s. The southern sea otter population off the U.S. The sea otter is an apex predator, a carnivore at the top of its local food chain. In several ecosystems, over-hunting has left once-top predators unable to regain a foothold in their previous ecosystems. This is just one example of how a human-made, near- extinction event has altered complex multiorganism dynamics connected with a top predator. Due to these lingering effects on the food web, sea otter populations have since had a hard time rebounding. In the case of sea otters, overhunting decimated the sea otter population, and the removal of this top predator from its environment had a negative effect that cascaded down throughout the ecosystem. However, it also took a toll on wildlife not only did it affect the animal populations coveted for their soft furs (for example, sea otters ), but it also affected the broader food web. The booming fur trade of the 19th century catalyzed westward expansion and helped establish the United States as a rising force in the global economy.
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