Actually they make up for decreased fertility. The stats are very clear planet wide; as women become more educated and or economically independent, fertility drops. So as one child families by choice--not coercion--cut the next generation, the non-conceived are replaced by immigrants. Secondly, as a recent media story noted, the farmers in our state KNOW they need the underpaid workers but they support noisy anti-immigrant pols as a way of intimidating the workers. Demand honest wages,I call ICE/CBP.
59 years ago a CBS Reports episode detailed the awful conditions of farmworkers in the US--Harvest of Shame. What has changed is the proportion of immigrants as poor whites and blacks born here moved on to slightly better jobs.
The figures for 2017 show a slight decrease in CA residents--the well educated moving in slightly outnumbered by those leaving for less diversity.