Japan's reproductive rate and population density have decreased significantly over the past 20 years, and suicide only accounts for a small percentage of that decline.
Most of the decline is accounted for by more Japanese never reproducing and the ones who do reproduce having only one child.
Illnesses among the Japanese have decreased dramatically, making them the world's longest-lived nation. As far as "erratic behavior" is concerned, the Japanese have lower rates of institutionalization and incarceration than many countries. These rates declined and longevity increased all the while Japan's population was still becoming denser (1945-1985).
And, of course, the Japanese are not a species.
“And, of course, the Japanese are not a species.”
Valid corrections, except that last one. :P