### Instruction:
The reduction in wild grain populations lead to what type of crop being cultivated?

### Input:
Around 4000 BCE, the Saharan climate started to become drier at an exceedingly fast pace. This climate change caused lakes and rivers to shrink significantly and caused increasing desertification. This, in turn, decreased the amount of land conducive to settlements and encouraged migrations of farming communities to the more tropical climate of West Africa. During the first millennium BCE, a reduction in wild grain populations related to changing climate conditions facilitated the expansion of farming communities and the rapid adoption of rice cultivation around the Niger River.

### Response:
The reduction in wild grain populations lead to the rapid adoption of rice cultivation around the Niger River.