V2关于主旨题那个,选项貌似没有反对pottery是用来storage food那么简单,选项都很长,大概每个都是三行,我选了一个是gaps in the earlier explanation warrant an alternative explanation of social competition。。。还有一个选项跟这个很像,我纠结了一下,估计选错了= =
Biomagnification means that the level of toxin in animals' tissues rises as one moves up the food chain. For instance, as larvae eat algae, fish eat the larvae, and bigger fish eat smaller fish, the toxin present in the algae becomes increasingly concentrated; top predators like swordfish and polar bears end up with the highest doses in their tissues. This can happen with stable, fat-soluble chemicals that aren't easily excreted in urine or feces. Biomagnification was first studied in the late 1960s in aquatic food webs, explains Frank Gobas, professor at Simon Fraser University and leader of the study. To screen chemicals, scientists began using a property known as Kow, which indicates how readily a chemical dissolves in water compared with fat and thus predicts how easily it will move from a fish's blood lipids into water through its gills. Low-Kow, or more water soluble, chemicals don't build up in the fish food chain and were assumed to be safe.
Environmental chemists realized, however, that this assumption might not hold in food chains involving mammals and birds because their lungs are in contact with air, not water. This means that many chemicals that are relatively soluble in water and therefore don't accumulate in fish might remain in the tissues of land animals if they aren't volatile enough to easily move from the lungs into the air (predicted by a property called Koa). Supporting this idea, some organic chemicals that don't biomagnify in fish appeared to be doing so in other wildlife and humans.
To explore this hypothesis, Gobas and graduate student Barry Kelly and colleagues collected plant and animal tissue samples— from lichens to beluga whales killed in Inuit hunts—in the Arctic, where, because of weather patterns and cold temperatures, organic pollutant leve ls are high. They tested the samples not only for known POPs but also for several chemicals with a low Kow but high Koa, which suggested they might biomagnify in air-breathing animals. The measured levels of contaminants for various animals in aquatic and land food webs were similar to those predicted from a bioaccumulation model incorporating Koa and Kow, suggesting the model was correct. Chemicals with low Kow and high Koa stood out as potentially risky.
V2关于主旨题那个,选项貌似没有反对pottery是用来storage food那么简单,选项都很长,大概每个都是三行,我选了一个是gaps in the earlier explanation warrant an alternative explanation of social competition。。。还有一个选项跟这个很像,我纠结了一下,估计选错了= =