Evaluating relative species competition strength is a central question in community ecology, with strong implications for invasion ecology. Models assessing invader's competition consider three components: distribution, abundance and per-capita impact. However, relative strength and interactions among these factors remain unclear when applied to specific invasion scenarios. Here, we focused on the interaction between abundance and per-capita impact. We hypothesized the plant performance will be impacted and vary by the indirect and direct effects that neighboring species triggers at different abundances and scales. We conducted a one-year replacement experiment between two dominant grasses in subtropical grasslands (the native Axonopus fissifolius and the non-native Paspalum notatum) at Buck Island Ranch in central Florida, USA. Thirty fenced plots (1 m x 3 m each) representing a gradient (15 levels) of increasing non-native groundcover and decreasing native groundcover were set up in November 2017. We transplanted individuals of these two species in subplots (12 subplots per plot, 2-4 transplants per subplot, 36 transplants per plot; 1080 plants in total) within the 30 fenced plots in a 2*2 factorial design (native/non-native groundcover * 2/4 focal plant individuals). Leaf length, number and plant biomass were evaluated per individual at the beginning and end of the experiment along with plot species composition and soil nutrients.
Results/Conclusions
Over 90% of transplants of each grass species survived until harvest (>92 %; 11 months after transplant). Our results indicated significant differences in leaf length/number and plant biomass between subplots (native/non-native groundcover) treatment. Both P. notatum and A. fissifolius performed better when they were transplanted in non-native P. notatum groundcover subplots. They also indicated evidence of interactions between subplot (native/non-native groundcover) treatments (direct effects) and the gradient of increasing P. notatum /decreasing A. fissifolius groundcover (indirect effects) treatments. It was clear that as the increasing of P. notatum groundcover in the whole plot, the competition environment became more favorable for both focal species. Although the magnitude of the responses to direct and indirect competitive effects was relatively weak, the leaf length, number and biomass of the two focal species show consistent and similar patterns regarding the direction of plant performance. The study allowed us to evaluate the indirect and direct effects from different scales of abundance. We found evidence of these effects caused by environmental variation introduced from changing abundance scales and levels.