Competition among individuals and across species has long been recognized as an important driver of plant form and function. Few concepts in ecology are as classic and appealing as the concept of “a ghost of competition past” (Connell 1980). This idea has served us well in explaining and understanding how competition causes diversification of strategies by rewarding differing species with freedom from the pressure of competition. Diversification by niche differentiation is not always possible, however. Competition for essential resources whose availability are structured in a way that leaves little room for partitioning may not lead to diversification and relaxation of competition, but rather intensification of the competition through tragedies of the commons.
Questions remain about how these incentives drive plant strategies across environmental gradients. Are incentives stronger for individuals to invest in competition when resources are more abundant and individual density is higher? Here, focusing on belowground competition, I tease apart these questions with a simple model.
Specifically, I ask: How does the number of competitors an individual faces influence incentives for competition? How many competitors should an individual choose to compete with? And overall, how does individual-level competition change with resource availability?
Results/Conclusions
With a simple model of competition belowground, I find several surprising incentives on individuals to engage in competition. By manipulating only the number of competitors but keeping the rate of resource supply per individual constant, I find that plants benefit most from high investments in competition when confronted with only one competitor. Increasing the number of individuals competing decreases incentives for competition by decreasing the direct payoff to an individual in relative fitness.
Intuitively, increasing the cost of competition – a tax on sending roots toward a neighbors’ home space - decreases investment in competition. At zero cost plants act as we assume in mean fields models. But, adding a cost of competition has the surprising effect of improving the productivity elsewhere (allocations to structural biomass or reproduction). I find that plants that concentrate their roots to the fewest number of competitors possible (without being alone) will dominate in long term competition against those that spread their roots farther. Incentives for competitive overinvestments remain high across resource gradients.
Together these results lead to a more nuanced understanding of belowground competition whereby incentives for competition are more concentrated by individual than before thought.