The answer is an increase in the other grazing animals.
Since impala and other grazing animals use the same food resources of the African plains, this is an example of competition. Competition is an interaction between members of the same species or between different species. If resources, such as food, water, sunlight, territory, are limited and several species depend on these resources, then, they must compete with other species to gain the resources. The more successful species in this competition will have more food resources for themselves, leaving less successful species (in this case impala) with less food. Without enough food, the population of the impala will decrease.