CASCADING EFFECTS OF WATERBIRDS AND FISH ON BIODIVERSITY, ECOSYSTEM SERVICES, CLIMATE MITIGATION, AND CROP YIELDS IN CALIFORNIA RICE FIELDS

A critical challenge to maintaining viable and sustainable agroecosystems is conserving biodiversity and safeguarding ecological functions in working landscapes. Rice farms in California represent one of the few cases in which large-scale, intensive farms are explicitly managed to promote on-farm biodiversity. Winter flooding practices (when rice is not produced) allows growers to create critical habitat for migratory waterfowl and other wetland wildlife in an otherwise human-dominated landscape. Yet climate change threatens this success story. First, as severe droughts increase water prices, growers are beginning to embrace alternatives to winter flooding for rice straw decomposition. Second, like natural wetlands, flooded fields emit greenhouse gases (e.g., methane), raising climate change concerns. As a result, the acreage of flooded rice fields in California is currently in precipitous decline. Recently, a solution to these issues has emerged: fish introductions. Rice fields can provide substantial food resources for fish, with growth rates eclipsing those in wild populations. Rearing fish could thus incentivize flooding by providing growers with a valuable 'second crop,' while also mitigating methane emissions. Ultimately, however, our knowledge of how fish interact with waterbirds to structure on-farm biodiversity, ecosystem services, and rice production is still in its infancy.Our goal, in broadest terms, is to leverage a large-scale, manipulative experiment on a California rice farm to quantify cascading effects of winter flooding for waterbirds and fish introductions on outcomes of direct relevance to rice growers and conservationists. More specifically, we will experimentally exclude waterbirds in flooded fields, with and without introduced fish, to achieve three goals:1. Quantify the biological interactions between waterbirds and introduced fish.2. Trace cascading ecosystem service and agronomic effects of waterbirds and fish in flooded fields, alone and in combination.3. Measure the degree to which fish and/or waterbirds reduce greenhouse gas emissions.We have designed a series of concrete objectives to achieve these goals (listed below)Objective 1a: Compare fish growth rates and survival in the presence versus the absence of birds to assess trophic relationships between birds and fish.Because birds are known to depredate and compete with fish, we hypothesize the relationship between birds and fish will be largely antagonistic.Objective 2a: Quantify effects of birds and fish on straw decomposition rates.We hypothesize that by foraging in rice fields and masticating straw, waterfowl increase rice straw decomposition rates; however, mechanisms for how fish could increase straw decomposition are less clear.Objective 2b: Quantify effects of birds and fish on soil fertility.We hypothesize waterfowl increase nitrogen in the soil by feeding throughout a broad landscape and then congregating/defecating on rice fields. However on the vast majority of rice fields, fish are not likely importing nutrients in appreciable concentrations to rice fields (unlike birds); thus, we expect their effect on soil fertility to be minimal.Objective 2c: Quantify effects of birds and fish on weeds.We hypothesize that both birds and fish feed on weed seeds in the soil, thus reducing weed biomass at harvest.Objective 2d: Quantify effects of birds and fish on crop yields and nitrogen in rice plants.Given our hypotheses about weed suppression and soil fertility, we expect both birds and fish enhance rice yields and contribute to increased nitrogen in rice plants.Objective 3a: Quantify effects of birds and fish on greenhouse gas emissions.Fish may indirectly bolster methanotrophic bacteria, decreasing methane effluxes. Birds and fish may also increase dissolved oxygen, which could decrease methane emissions by increasing methanotroph activity and thus methane oxidation rates. Thus, we predict that both fish and birds decrease methane fluxes from rice fields.