The most significant gaps are in understanding year-to-year variability in the abundance of different species: how many there are this year, and how many there are the following year, and how that relates to changing weather or changing climate. What is the normal range of change we can expect that we wouldn't necessarily have to attribute to human effects, but to something in the longer term, like climate change?
On October 24th, 2017. See this statement in context.