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If any r get too small, the probability of an interaction with a distant prey can become zero numerically, resulting in a log-likelihood of -Inf. Moreover, any predators in the data that have only 1 prey will likely drive r to be arbitrarily small, countered only by the prior.

In Williams et al. 2010, they fix r and c to small values for primary producers. These parameters aren't really meaningful for primary producers and there's nothing to inform c, as long as r gets small enough that the range doesn't contain anything.

In that paper, they also fix r to a small value for specialists with only one prey. Not fixing it could cause numerical problems with r getting shrunk arbitrarily small, but the prior might be able to prevent that.

From the log-likelihood being something other than a number or -Inf. Not sure how this is happening (maybe a 0/0 in the phi caclulation?), but need to at least try to catch it and return -Inf instead.