Scale_x_discrete breaks are not appearing

I'm creating a rank abundaunce curve for some species data. The y axis is the abundaunce (numeric) and the x axis is the factor "rank" of each species (one with the most abundance gets rank #1, and so forth)

This data is between rank 1-400, and so I want to set my own x axis limits ( i dont want to see every single label).

I confirmed that "ab" (which is abundaunce) is numeric and rank.freq (which is the rank) is a factor.

But, the x-axis scale just never shows up. What am I doing wrong?

rank.abun<- ggplot(HEMI ,aes(x =reorder(rank.freq, -ab), y = ab))+
 geom_bar(stat = "identity")+
  scale_x_discrete(breaks = c("700", "1000", "2000", "3000", "4000"), labels = c("700", "1000", "2000", "3000", "4000"))+ ## not sure why this isn't working
  labs(x = "Abundance Rank", y = "Abundance"); rank.abun

the data

     X Species freq     ab rank.freq
330 330    COUM  197 178.39       330
472 472    PEBR  139 149.50       472
548 548    KRGR  119 207.04       548
555 555    PECE  117  59.52       555
657 657  CAMI12   98  97.53       657
702 702   PEGR2   92 139.20       702

and the resulting plot rank abundance

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