I have a species phylogeny and am interested in studying diversification dynamics, I am using the (R package DDD][1] to assess whether density dependent vs time dependent diversification has occurred. In particular, I am using the function bd_ML() and I am trying to understand the description for the time dependent model tdmodel 2, which is described as "a stepwise decline in speciation rate as in diversity dependence without extinction".

I am a little unsure what "as in diversity dependence without extinction" means, as it is very clear in the description of the function that these are testing for diversity independent models. Does it mean to say that there is no way to determine whether the tree follows a stepwise time-dependent model vs a diversity dependent model without extinction?

Any insight appreciated.

Thanks!