cannot add ggproto objects together

ggplot2 is the name of the package but, the function you need to use from that package is ggplot().


ggplot(data_sum, aes(x = habitat, y = biomass, fill = habitat)) +
    geom_bar(position = position_dodge(.9), stat = "identity") +
    geom_errorbar(aes(ymin = biomass - se, ymax = biomass + se), width = .1) +
    xlab("Habitat") +
    ylab("Predator biomass (g.m-2)") 

If this doesn't solve your problem, please provide a proper REPRoducible EXample (reprex) illustrating your issue.