The relative effects of predation and competition on ecological communities has long been debated. This debate is complicated when species compete for resources under certain conditions and have a predator-prey relationship under others. Here, we experimentally explore the mechanisms underlying this transition between competition and predation using a three species in a guild of grassland wolf spiders. Wolf spiders are an excellent group to study these processes because they directly compete for food and interactions are strongly size dependent (i.e., large spiders eat small spiders). We predicted that as the difference between two spiders in relative body size increased, so would the probability of predation and thus the behavioral response of spiders to the perceived threat. We first developed a bio-mechanical model of spider movement to predict when size dependent differences between two spiders in power and acceleration would be sufficiently large to allow predation. We then tested these model predictions using behavioral assays of paired individuals in which size differences were manipulated, both within and between species. In addition to identifying predation events, we used automated tracking to quantify effects of size differences on spider movement and behavior.
Results/Conclusions
We tested 81 unique pairs of wolf spiders, and observed predation in 28% of the trials. Interactions transitioned from competition to predation when the smaller individual was approximately 30% the size (wet mass) of the larger individual. This inflection point differed between con- and heterospecific pairings, with more gradual transitions observed in conspecific assays. We conclude that wolf spiders have a behavioral response when encountering guild members that is highly dependent on the relative size differences between the two individuals. The extent of size differences contributes to whether individuals are competitors or predator-prey, and these processes likely have important population- and community-level consequences.