> Haiti [...] is steadily losing arable land. But it still has more arable land than the entire Dominican Republic, despite being only about half as large in terms of territory! So the D.R. actually had — and has — less opportunity to farm.
Seems almost the opposite of your argument, if we substitute "plains that are not salt marshes" for "arable land". I don't know which is true or whether you're actually measuring something substantially different.
While on Haiti, it says that of the total arable land of 550,000 hectares, 125,000 hectares are suited for irrigation, and of those only 75,000 hectares actually have been improved with irrigation.[1]
So while I'm sure Haiti could do better than what they are doing now, it seems curious to say that it is more suited for farming than the Dominican Republic. I think the author might have misread numbers.
[0]https://en.wikipedia.org/wiki/Economy_of_the_Dominican_Repub...
[1]https://en.wikipedia.org/wiki/Economy_of_Haiti#Agriculture,_...
They don't share the same geography or natural resources at all, most of the settled, economically important places both in Haiti and DR are in the plains but DR has more plains than the whole area of Haiti, while Haiti's few plains are mostly salt marshes with cities in the more hospitable areas.