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Animal Migrations to the Americas Before the Pleistocene, North and South America contained their own distinctive sets of animals. Marsupials abounded in South America and the horse flourished in North America. With the emergence of the Panamanian land bridge between the two Americas, a great migration, or swapping of animal species, began. Marsupials (mammals with no placenta but with a pouch in which their young develop), sloths, and other animals such as glyptodonts, which looked like an armadillo, headed north.

This is also known as exponential growth. Population Dynamics What happens to population size over time under exponential growth? If r is negative, the population declines quickly to extinction. However, if r is positive, the population increases in size, slowly at first and then ever more quickly. ” Exponential growth is often seen in small populations, which are likely to experience abundant resources. J-shaped growth is not sustainable however, and a population crash is ultimately inevitable.

This adds to the cost of water treatment. biomagnification increasing levels of toxic chemicals through each trophic level of a food chain Mercury pollution has been recognized as a serious, chronic, and widespread danger in many waterways. Even very small amounts of mercury can cause serious physiological effects or even death. Because mercury is not normally found in food or water, no organisms have developed the ability to process and excrete mercury. So it collects in tissues until toxic levels are reached.