Increasing uncontrolled poaching acts as some kind of <em>"selective pressure"</em>.
Animals that carry the genes to grow tusks are removed from the population, and most of the surviving animals are those that do not have tusk and that carry the gene for tusklessness.
Tuskless animals get to reproduce more frequently, transmitting the tusklessness gene to their progeny, and hence, increasing its frequency in the population.
Explanation:
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The secondary consumers will be able to survive, as well as decomposers, as they kill the primary consumers, which depends on the plants, and teaches them to their offspring