Which parts of speech must agree in standard written english?

Nathan is an ecologist. he’s studying the effect of secondary consumers on primary consumers. his current research focuses on a population of blue jays and crows. they eat caterpillars and mice. if the number of blue jays increases, what are the likely effects on other populations? 1the number of crows will increase. 2the number of mice will decrease. 3the number of mice will remain the same. 4the number of caterpillars will decrease.