The carrying capacity of a population refers to the maximum number of individuals that an environment can sustain over time, given the resources available. To determine which of the options could cause the manatee population to drop below its carrying capacity, let's analyze each choice:
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A phytoplankton bloom that blocks sunlight and decreases seagrass growth: This could lead to a reduction in the availability of food for manatees, which primarily eat seagrasses. A decrease in food availability could indeed cause the population to drop below its carrying capacity.
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A change in freshwater flow from a river that decreases salinity slightly and causes a higher density of the seagrasses: If this change leads to an increase in seagrass availability, it could support a larger manatee population rather than causing a decline. Therefore, this option is unlikely to cause a drop in the population.
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A disease that kills off fish: This would likely impact the fish population but may not directly affect manatees, which primarily graze on seagrass. However, if the disease disrupts the ecosystem significantly (e.g., affecting the food web), it could indirectly affect manatee populations, though it is not a direct cause.
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Hunting by humans: This would directly reduce the manatee population through mortality, leading to a decrease below the carrying capacity.
From this analysis, options 1 and 4 are the most likely to cause the manatee population to drop below its carrying capacity.