Strange black border of image from theme_base

ggplot(data = mntd,aes(x = div, y = NTI,)) +
geom_point() +
theme_base() +
xlab("Plant species richness")NTI-plant_richness

as you can see, I use theme_base() to plot, then I export it as an image. However, this ".png" file has three black borders (top, bottom and left), maybe you need to insert it into powerpoint for better observation.
I wonder how to remove these black borders.

See the FAQ: How to do a minimal reproducible example reprex for beginners for how to illustrate a problem in a way that is reproducible. You just need representative data, as shown below with a built-in dataset.

ggplot(data = mtcars,aes(x = cyl, y = mpg)) + # trailing comma is unnecessary
  geom_point() +
  xlab("Plant species richness") + # generally better to put theme last
  # theme_base() # there is no theme_base in the ggplot2 package
  theme_classic() # this is the theme that reproduces the problem

Try theme_minimal, which eliminates the border. Come back with a separate question if you also want "How to eliminate grid lines from ggplot?"