Jumping worms, scientifically known as Amynthas species, have a detrimental effect on tree populations in North America primarily because they decrease the population by eating tree seedlings. These invasive worms consume organic matter, including leaf litter, which is essential for soil health and tree seedling establishment. As they alter the soil ecosystem, they can inhibit the growth of young trees, leading to a decrease in tree populations.
Therefore, the correct response is:
They decrease the population by eating tree seedlings.