Variation in demographic responses to abiotic and biotic factors within species
For one species (P. airoides ), we found clear evidence of demographic compensation along PC1 arising from a significant convex emergence relationship coupled with a significant negative seed production relationship. For a further five species we found similar compensatory trends along PC1 between emergence and seed production, but relationships were only significant for one of the vital rates (mainly emergence). This result highlights the risk of using single vital rates as proxies for fitness without considering how different vital rates may trade-off in their effects on population growth (Laughlin et al.2020; Klimeš et al. 2022). Since emergence trends were frequently different to population growth rate trends along PC1 in our study, we would caution against using emergence rate as a proxy for fitness in studies involving light availability gradients.
In contrast to the variation observed within species’ demographic responses to abiotic conditions, survival and seed production responses to neighbour abundance were almost always consistent with population growth responses to neighbour removal. Surprisingly, only two species experienced main effects of competition on population growth, A. calendula and P. debilis . Neither species had strong survival or seed production responses to neighbour abundance, illustrating the potential for weak vital rate relationships with the environment to result in strong relationships for population growth rate (Dahlgren & Ehrlen 2009). For these species, variation in survival and seed production respectively contributed most to the difference in population growth rates between the presence and absence of neighbours. Unlike Lyu and Alexander (2023) who revealed nearly ubiquitous demographic compensation in response to competition, we did not observe demographic compensation among vital rates in response to competition. Although main effects of competition were not widespread, for four species, competitive effects on population growth were only observed in low nutrient, high light conditions. This result reinforces that responses to competition may be stronger in high light, low nutrient environments in this system, and highlights the importance of interactions between abiotic and biotic factors for fitness.