- 2018年02月07日15:45 来源：小站整理
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The sea otter is a small mammal that lives in waters along the western coast of North America from California to Alaska. When some sea otter populations off the Alaskan coast started rapidly declining a few years ago, it caused much concern because sea otters play an important ecological role in the coastal ecosystem. Experts started investigating the cause of the decline and quickly realized that there were two possible explanations: environmental pollution or attacks by predators. Initially, the pollution hypothesis seemed the more likely of the two.
The first reason why pollution seemed the more likely cause was that there were known sources of it along the Alaskan coast, such as oil rigs and other sources of industrial chemical pollution. Water samples from the area revealed increased levels of chemicals that could decrease the otters' resistance to life-threatening infections and thus could indirectly cause their deaths.
Second, other sea mammals such as seals and sea lions along the Alaskan coast were also declining, indicating that whatever had endangered the otters was affecting other sea mammals as well. This fact again pointed to environmental pollution, since it usually affects the entire ecosystem rather than a single species. Only widely occurring predators, such as the orca (a large predatory whale), could have the same effect, but orcas prefer to hunt much larger prey, such as other whales.
Third, scientists believed that the pollution hypothesis could also explain the uneven pattern of otter decline: at some Alaskan locations the otter populations declined greatly, while at others they remained stable. Some experts explained these observations by suggesting that ocean currents or other environmental factors may have created uneven concentrations of pollutants along the coast.
Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all. Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all.
First, the pollution theory is weakened by the fact that no one can really find any dead sea others washing off on Alaskan beaches. That's not what you would expect if infections caused by pollution started killing a lot of otters. On the other hand, the fact that it's so hard to find dead otters is consistent with the predator hypothesis. If an otter is killed by a predator, it's eaten immediately so it can't wash up on shore.
Second, although orcas may prefer to hunt whales, whales have essentially disappeared from the area because of human hunters. That means that orcas have had to change their diet to survive and since only smaller sea mammals are now available, orcas have probably started hunting those. So it probably is the orcas that are causing the decline of all the smaller sea mammals mentioned in the passage - the seals, the sea lions and the sea otters.
And third, the uneven pattern of otter decline is better explained by the orca predation theory than by the pollution theory. What happens to otters seems to depend on whether the location where they live is accessible to orcas or not. In those locations that orcas can access easily, the number of sea otters has declined greatly. However, because orcas are so large, they can't access shallow or rocky locations. And shallow and rocky locations are precisely the types of locations where sea otter populations have not declined.
The lecturer and the reading passage suggest two competing theories, the predation theory vs. the pollution theory, to explain why the sea otter population is in rapid decline.
The professor reasons that the absence of dead sea otters washed up the coast suggests that their decline is not caused by sea pollution but rather by sea predators who consume their bodies after killing them. In contrast, the reading passage attributes the death of sea otters to pollution, citing evidence of increased sources of ocean contaminants which lead to greater vulnerability to infections.
Furthermore, the lecturer argues that orcas are likely factors in the disappearance of sea otters, because the scarcity of whales, their usual prey, has left them with no other choice but to start hunting smaller mammals like the seals for food. The reading passage, on the other hand, rules out this theory based on the orca’s praying habit, and instead approves of the pollution theory as the only explanation for the decline of both large and small sea mammals across the entire ecosystem.
Finally, according to the lecturer, the uneven pattern of sea otter decline corresponds to the distribution of the orcas; she argues that the fact that their population has declined most rapidly where orcas are most prevalent further validates the predation theory. However, the reading passage argues that changeable environmental factors, which lead to different concentrations of pollutants, better explains the varying pattern of sea otter decline.