The Roe deer will adapt. Births will go down for a time, but over time the survivors will be the offspring of those who adapted best and as time goes on and such survivors dominate the Roe deer population the Roe deer birth rates will come back up - barring any other adverse conditions, such as shrinkage of habitat in response to growing human habitat - the MOST LIKELY CURRENT CAUSE of more Roe deer offspring surviving.
If you read the companion letter the data was derived from an “enclosed forest” - the only way they can get accurate numbers. It’s an established fact that when deer populations reach a certain level of overcrowding they suffer precipitous declines. Presumably as they approach that maximum the rate of growth declines and then goes negative. How do they know that the deer population is not reaching its maximum level?