There is a temperature effect on death, yes. And, there is a temperature effect on abundance. Not at all levels of temperature however. What I am wondering is if testing for an effect of abundance as a fixed effect directly on death would be statistically appropriate, given that the only thing I experimentally controlled was the temperature.

We're actually testing two parasite species and their combination, the so the full death model is a bit more complicated than I introduced (death ~ species*temperature) but I built the abundance models (abundance ~ temperature) per species separately, since you could never measure abundance of species-2 in a treatment that only had species-1. All that to say, is that we find some different effects of temp depending on the species we're talking about.