Reduced competition enhances community temporal stability under conditions of increasing environmental stress

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Jan Douda, Jana Doudová and Alena Havrdová, Ecology, Czech University of Life Sciences Prague, Prague, Czech Republic

Background/Question/Methods
The stress-gradient hypothesis predicts a switch from competition to facilitation, under
increasing environmental stress. However, it is unclear how important is the change in competition–facilitation
balance (i.e., the net outcome of plant–plant interactions) along the stress gradient in the
regulation of community temporal stability (i.e., the inverse of temporal variability in total biomass).
Increasing environmental stress may enhance community temporal stability by reduced competition
or eventually by leading to facilitative interactions between the dominant and subordinate species.
Here, we present the results of a 5-yr mesocosm experiment that demonstrates the effects of interspecific
interactions on the temporal stability of a riparian community across different drought-stress scenarios.
We constructed artificial communities of dominant species (Carex elata) and three subordinate
species to simulate the independent effects of environmental stress and interspecific interactions.
Results/Conclusions
Using removal of the dominant species, we evaluated the interplay of various mechanisms regulating the temporal
stability of the subordinate species (competition–facilitation balance, species asynchrony, and
dominant species stability). By simultaneous testing of these stabilizing mechanisms, we show their
importance differs depending on environmental variability and harshness. The predominant role is
taken by species asynchrony in a seasonally dry environment, whereas in a permanently dry environment,
the importance of reduced competition increases. Reduced competition was stabilizing, in particular
through increased total community biomass, whereas species asynchrony increased total
community biomass and decreased biomass variation. These results suggest experiments and simulations
that exclude interspecific interactions may not offer realistic predictions of the effects of changing
hydrological regimes on ecosystem functioning.