Professor University of California - Los Angeles Los Angeles, California, United States
Abstract: With environmental change driving shifts in species’ abundances and distributions from local to global scales, understanding the processes that shape species’ distributions is an increasingly urgent goal of ecological research. Of particular interest are processes that can create mismatches between species’ intrinsic performances and realized distributions. For example, competitive interactions can exclude species from otherwise favorable environments, potentially hindering species’ responses to environmental change. Yet, despite their purported importance, competitive interactions remain a large source of uncertainty in the predictability of species’ distributions for several reasons. For one, the strength of competition can change depending on the environmental context. However, the detailed demographic data needed to quantify such context-dependent interactions (e.g. from field experiments or community time series) are challenging to collect at the scales of species’ geographic distributions. This is further complicated when multiple environmental factors simultaneously vary across geographic gradients. In light of these challenges, carefully designed mesocosm experiments that characterize species’ demographic responses to important environmental gradients can facilitate the scaling of insights from local competition to geographic distributions.
We conducted a greenhouse experiment to quantify how pairwise competition between plant species changes along a water supply gradient. We grew two California annual plant species—a grass (Festuca microstachys) and a forb (Plantago erecta)—among different densities of either intra- or interspecific competitors, under six different watering treatments designed to emulate the range of rainfall these species experience across their geographic ranges, totaling 480 pots. We measured height, biomass, and lifetime fecundity for focal individuals, and estimated per capita competitive effects. Using height as a preliminary fitness proxy, height in the absence of competitors did not exhibit clear patterns with respect to watering treatment, although F. microstachys tended to be shortest in the driest treatment. In addition, we found evidence for competition across most treatments, with competitive effects trending stronger in drier treatments. Across all treatments, intraspecific competitive effects for F. microstachys were stronger than for P. erecta, but no clear pattern was identified for interspecific competitive effects. These preliminary results illustrate the complex direct and indirect pathways through which water supply can influence competitive dynamics. Additional analyses will focus on insights gleaned from parameterizing a demographic model of annual plant competition. Results of these analyses will be discussed in the context of geographic gradients in rainfall.
