However this time of prosperity was not to last. Already, by the end of the fourteenth century New Zealand's environment was beginning to change. The climate became cooler, the podocarp forest retreated, and the moa population began to decline. The changing environment affected those who relied upon moas and seals for food and forced them to develop more effective techniques for catching birds and fish. The largest settlements of the early centuries lost their importance and declined. The population declined because of emigration north to regions where one could cultivate kumara (sweet potatoes).