Competition, both within and between species, is a critical driver of ecological structure and function due to its role in regulating resource acquisition. However, key questions remain unanswered, such as what are the behavioral drivers of competition for individual organisms and how do these drivers scale up to affect the structure and diversity of communities. For example, one generally overlooked mechanism of species coexistence is interference competition, which arises out of behavioral interactions among competing predators, resulting in a reduction in per capita foraging rates. We explore these ideas using functional response experiments of wolf spiders feeding on wingless fruit flies, where we not only vary prey density but also predator density. Our use of automated tracking of these feeding trials enables us to identify every spider-spider interaction, and to directly quantify the effect of each interaction on subsequent spider movement, foraging decisions and ultimately per-capita consumption rate.
Results/Conclusions
As expected, the average per capita consumption rate of flies by spiders decreased as spider density increased, showing that interference competition exists in this system. Our use of automated image-based tracking enabled us to explore the behavioral underpinnings of this interference competition. Broadly, the effect of any specific spider-spider interaction on their behavior and consumption rate was strongly size dependent. When spiders of a similar size encountered each other, both spiders tended to move immediately away from the other spider and to ignore prey for some duration (even early on in the experiments when spiders were hungry). When spiders of different sizes encountered each other, the interference on the smaller spider was much higher than for the larger spider, presumably because size-based cannibalism is common in wolf spiders. We are using results of these experiments to develop a generalized theory to make quantitative predictions of the effect of interference competition on the relative abundance and size structure of spider communities in nature, which should yield novel insights into the maintenance of biodiversity.