Using the negative binomial regression count model with 95%
confidence intervals, the increase in the number of species offered
in the most recent time period 1990–2011 was significant compared to time periods 1900–1929, 1930–1959, and 1960–1989. The
95% confidence interval of time period 1990–2011 did not overlap
with the 95% confidence intervals of the other time periods (Fig. 2).
There was no significant difference in the number of species offered
between time periods 1900–1929, 1930–1959, and 1960–1989.
A polynomial contrast indicated a significant linear (p < 0.05) and
quadratic (p < 0.05) trend with time.