Spatial Pattern Formation in an Insect Host-Parasitoid System

Maron and Harrison set out to empirically test the result of many spatially explicit predator-prey models that patchy distributions in prey population density can arise on a homogeneous landscape. These predictions come from models which assume that predators control prey and that predators are the better dispersers. Their system of western tussock moth outbreaks in central coastal California seems to resemble this model system. Outbreaks of these moths tend to remain spatially restricted and patchily distributed despite widespread availability of host plants. The larvae and flightless female moths have very low dispersal while their flighted parasitoids are far better dispersers. Prior work had shown that parasitism was higher just beyond the edge of prior outbreaks than it was within them (a local positive effect). The authors set out to test whether there is a parasitoid mediated negative effect of proximity to prior moth outbreaks on the growth and size of new outbreaks (an intermediate distance negative effect). They began tussock moth outbreaks on two paired bushes at 50 m intervals from the site of a prior outbreak and monitored survival and fecundity on these bushes. One bush in each pair had ground predators (such as spiders and rodents) excluded and the other did not. Generally they found a positive effect of ground predator exclusion on caterpillar abundance. They also found that the number of successfully pupating individuals per bush increased substantially with distance from the outbreak. This effect did not vary by ground predator exclusion treatment suggesting that the distance based survival was driven by the mobile parasitoids. This paper dissects a nice clean example of a real-world predator prey system which displays pattern formation and demonstrates that one of the requisite theoretical mechanisms is present. Experiments such as this were and are necessary to validate theoretical models which make predictions about the processes driving spatial pattern formation in nature.

Maron, John L., and Susan Harrison. “Spatial pattern formation in an insect host-parasitoid system.” Science 278.5343 (1997): 1619-1621.