Trade-offs between seed size and biotic interactions contribute to coexistence of co-occurring species that vary in fecundity

Despite theoretical advances, the ecological factors and functional traits
that enable species varying in seed size and fecundity to coexist remain
unclear. Given inherent fecundity advantages, why don’t small-seeded
species dominate communities? In perennial grasslands, we evaluated
whether small-seeded species are less tolerant of competition from the
community dominant bunchgrass than large-seeded species but also less
vulnerable to seed predation by mice. We also explored whether trade-offs
involving competitive tolerance include two other functional traits,
height and leaf mass per area (LMA). We added seeds of 17 forb species to
plots where bunchgrass competition and rodent seed predation were
manipulated across sites varying in bunchgrass productivity and thus
competitive intensity. Seeds were added at densities mimicking
interspecific variation in fecundity among target species. Standardizing
for differences in fecundity (i.e. seed input; which enabled us to
evaluate inherent interspecific differences in susceptibility to biotic
interactions), bunchgrass competition more greatly reduced recruitment and
establishment of small vs. large-seeded species, whereas rodent seed
predation more greatly reduced the recruitment of large- versus
small-seeded species. Plant height and LMA were unrelated to the
competition effect size. Small-seeded species abundance decreased across
sites increasing in bunchgrass productivity, whereas this was not the case
for large-seeded species. For adult plants but not seedlings, community
weighted functional trait means (CWM) for seed size, height, and LMA
increased in plots with versus without bunchgrass competition and the CWM
for seed size and height also increased at sites with greater bunchgrass
productivity (for adults only). In contrast, rodent seed predation had no
significant effects on CWM seed size. At the end of the experiment, adult
abundance positively correlated with plant fecundity in plots lacking
bunchgrass, indicating the inherent advantages accrued to high fecundity
small-seeded species. However, with bunchgrass competition, abundances
were equalized across species due to reduced competitive tolerance of high
fecundity small-seeded species. Synthesis: Our results suggest that
coexistence among subordinate forb species varying in seed size and
fecundity is in-part due to a trade-off involving competitive tolerance
and fecundity, mediated by seed size and associated functional traits.