Seed predators provide a valuable ecosystem service to farmers by reducing densities of weed seeds, and,
in turn, densities of weed seedlings they must manage. The predominant invertebrate weed seed predator
in Maine, USA, agroecosystems is the carabid beetle Harpalus rufipes DeGeer. Pitfall trapping has
shown that H. rufipes prefers sites with vegetative cover to fallow sites, preference speculated to be driven
by predator avoidance behavior. To test this hypothesis, ‘second-order predation assays’ were developed,
in which live H. rufipes prey were presented to second-order predators. Field experiments were
conducted to determine foremost if H. rufipes was subject to second-order predation, and secondly,
whether (a) vegetative cover affords H. rufipes protection from second-order predators, and (b) high rates
of second-order predation correspond with decreased invertebrate seed predation rates. Two 72-h experiments
were conducted (mid August and September 2012) at crop and non-crop sites across a 28 ha
diversified farm in Stillwater, ME, USA.
Second-order predation was 2.8% per day. Based on images from motion-sensing cameras, H. rufipes’
predators included birds and small mammals. Neither a relationship between second-order predation
and vegetative treatment, nor an empirical relationship between second-order predation and invertebrate
seed predation were detected. However, a simulation model predicted that 2.8% per day secondorder
predation could increase the number of seeds entering the seedbank by more than 17% annually.
Additionally, complex habitats supported higher rates of second-order predation than did simple
habitats.