Sources of Variability in Agronomic Weed Seed Predation: Time, Space, Habitat, and Hyperpredation

Weed seed predation is an ecosystem service that benefits farmers by decreasing seedbank inputs, thereby reducing weed pressure in subsequent growing seasons. Seed predation can be considerable, but is highly variable. Sources of variability may include time, space, habitat, and trophic interactions such as hyperpredation. Two experiments were conducted to measure the impacts of these sources of variability on weed seed predation rates in Maine mixed vegetable agroecosystems. Chapter One of this thesis describes a series of landscape-level field experiments conducted to quantify the effects of time, space, and habitat on seed predation rates. Seed assays, with and without vertebrate exclosures, were used to measure seed predation at spatially explicit sample sites across crop and non-crop habitats on a ‘typical’ Maine organic mixed vegetable farm. Total and invertebrate seed predation averaged 8% and 3% day-1, respectively. Motion-sensing wildlife cameras indicated that vertebrate seed predators included small mammals and birds. Pitfall trapping data indicated that one species of carabid, Harpalus rufipes, was highly dominant, comprising 66% of invertebrate seed predators captured within crop fields. Correlogram analysis showed that seed predation was randomly distributed in space. Based on linear mixed effects models, time and habitat were highly significant drivers of seed predation. Total seed predation varied between years of study, and both total and invertebrate seed predation decreased from August to October with winter’s approach. Total seed predation was greater in crop and riparian forest habitats than in mowed grass, meadow, or softwood forest. Generally, invertebrate seed predation was greatest at sites with moderate habitat complexity, while habitat type was the chief biotic determinant of vertebrate seed predation rates. In this study system, time and habitat were more important regulators of seed predation than was space. Chapter Two describes an experiment conducted to measure hyperpredation of H. rufipes and explore its effects on the weed seedbank. H. rufipes prefer sites with vegetative cover to fallow sites, preference speculated to be driven by predator avoidance behavior. To test this hypothesis, ‘hyperpredation assays’ were developed, in which live H. rufipes prey were presented to higher-order predators. Field trials were conducted to determine foremost if H. rufipes was subject to predation, and secondly, whether a) vegetative cover affords H. rufipes protection from hyperpredators, and b) high hyperpredation rates correspond with decreased invertebrate seed predation rates. Hyperpredation was 2.8% per day. Motion- sensing cameras indicated that H. rufipes’ predators included birds and small mammals. Neither a relationship between hyperpredation and vegetative treatment, nor an empirical relationship between hyperpredation and invertebrate seed predation were found. However, a simulation model predicted that hyperpredation at the rate observed could increase seedbank inputs by more than 17% annually. Additionally, complex habitats supported higher rates of hyperpredation than did simple habitats