Consistent demographic patterns among species with similar life histories in similar habitats have led to suggestions that species with known vital rates can serve as proxies, or “approximate models”, for predicting vital rates and ecological properties of species that have not been studied, or for rare species for which data may be difficult to obtain. An implicit assumption of these comparisons is that similar vital rates in different species are caused by similar responses to the same ecological mechanisms, but this ignores the possibility that, even for similar species in the same habitat, convergent vital rates may reflect very different aspects of the environment. Further, most demographic studies are descriptive and estimate only realized vital rates, rather than determining the driving mechanisms. I first showed that two, federally-listed, sympatric annual plants in coastal California, Gilia tenuiflora arenaria and Chorizanthe pungens pungens have very similar survival and seed set in the same years, and that these rates correlate strongly with rainfall. I then identified the mechanisms determining these vital rates in a field experiment, showing that, despite strong correlations with rainfall, the survival and reproduction of each species were driven by different stochastic factors and processes: Gilia by herbivory, and Chorizanthe by rainfall. Knowing which causal agents drive demographic patterns, and how they affect individual populations, is critical both for ecological theory, for successful management and conservation and for predicting population viability with shifts in climatic regimes.