Fitness and niche differences are both important in explaining responses of plant diversity to nutrient addition

Plant species loss due to eutrophication is a common phenomenon in
temperate perennial grasslands. It occurs in a non-random fashion and is
usually explained by increased competitive size asymmetry between
co-occurring winner (tall species with optima in productive habitats) and
loser species (small-statured plants typical for unproductive habitats).
It remains unclear why nutrient addition decreases diversity in
communities consisting of losers only, whereas it has little effect on
winner-only communities. Here, I used the framework of modern coexistence
theory to explore fertilization-driven changes in fitness and niche
differences between different combinations of field-identified winner (W)
and loser (L) species. I experimentally estimated competition parameters
for plant species pairs constructed from a pool of eight species,
including pairs of species from the same (WW, LL) and different species
categories (LW) grown for approximately two years in control and
fertilized conditions. Concurrently, I also followed plant species
diversity in mesocosm communities constructed from the same species pool
(four-species communities including winners, losers, or both) exposed to
control and nutrient addition. I found that nutrient addition can reduce
but, unexpectedly, also promote species coexistence depending on the type
of species pairs. Whereas nutrient addition eroded coexistence of losers
with winners, but also with other losers, treatment had the opposite
effect on the persistence of winner species. Fertilization induced large
fitness differences between species in loser-winner and loser-loser
combinations, but had little effect on the fitness differences of species
within the winner-winner combination. In addition, the persistence of
winner pairs was promoted by larger niche differences compared to loser
species, irrespective of soil nutrients. The differences in how nutrient
addition modified coexistence at the pairwise level were reflected by
differences in evenness of multispecies communities assembled from the
corresponding species categories. These results suggest that the effect of
eutrophication on plant species richness cannot simply be explained by an
increased competitive asymmetry. To fully understand the effect of
fertilization on the diversity of temperate grasslands, interspecific and
intraspecific interactions should be explored while considering
differences in species’ ecological optima.