Which of the following best describes an environmental problem associated with a sudden decrease in the population of a keystone species in an ecosystem?
(a) A sudden decrease in population of a keystone species is always caused by human activities.
(b) The loss of a keystone species will cause a sharp increase in the primary production in an ecosystem.
(c) A sudden decrease in the population of a keystone species will result in increased gene flow in a population.
(d) The loss of a keystone species can have a cascading effect on the populations of other species in its food web.