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.