Lower rates seldom yield adequate forage production to justify the expense.Traditionally, nitrogen applications have been made in the fall near the time of the first rains. In regions of high rainfall and where heavy winter grazing has occurred, the forage may become extremely nitrogen deficient in the spring, even though nitrogen was applied the previous fall. Under these circumstances, spring applications of nitrogen fertilizer may be beneficial, but this practice has not been adequately evaluated on annual rangelands. Where rainfall is not great enough to leach all of the fertilizer nitrogen out of the soil, and plant nitrogen uptake is insufficient to use all of the fertilizer nitrogen, there may be a carryover response to nitrogen fertilization during the next growing season. In the 1950s, many grazing trials were conducted to demonstrate the response of range livestock gains to rangenitrogen fertilization. Carryover effects were assessed in 13 of the tests. In all but one test there was an appreciable carryover effect from fertilization, the additional gain being equivalent to about 50 percent of the first-year effects on the average. Part of the gains in these studies should be credited to the phosphorus and sulfur also applied, but the amount of credit to be given cannot be determined with the available data. Without applied nitrogen or a good stand of legumes, there is usually no response to phosphorus or sulfur on annual rangelands of California.Fertilizer can be applied from the ground or by aircraft. Large, inaccessible, rough, cannabis racks and rocky ranges are usually fertilized by aircraft. Fertilizer application equipment and tractors are usually restricted to use on rangeland where slopes are less than 20 to 30 percent and the surface is relatively free of rocks or other obstructions to the equipment.
The analysis of range sites on a given ranch during a range management planning process will help to identify those areas that can be treated from the ground and those that must be treated from the air.Fall nitrogen fertilization generally increases the protein content in annual grasses and broad-leaved forbs early in the growing season. However, an increase in protein in winter is not beneficial, since there is typically adequate protein for animal needs in unfertilized pasture at that time of year. The primary benefit from nitrogen in the early part of the season is an increase in dry matter production. As the season advances, the protein levels may decrease more rapidly in plants fertilized at moderate nitrogen levels than in those not fertilized. As a result, at the end of the growing season fertilized plants are often lower in protein than are unfertilized plants. Exceptions may occur in very dry spring seasons when moisture becomes limiting and plants are unable to grow to their full potential, thus drying up before growth dilutes the nitrogen to a low level. Yearly application of nitrogen generally increases the percentage of grasses and forbs. The particular grasses or forbs that increase will depend upon the grazing or clipping management of the pasture in question. For example, slender wild oats or ripgut brome often become dominant where nitrogen fertilizer is applied to ungrazed plots. In similarly treated plots that are heavily grazed, soft chess may become dominant. This is due to the greater tillering ability of soft chess when grazed as compared to wild oats or ripgut, which tiller poorly. Moderate to heavy grazing pressure tends to reduce the impact of fertilizer on botanical composition.Invasive species have a multitude of ecological and socio-economic effects, and can play a strong role in ecosystem functioning .
However, eradication of invasive species is expensive and difficult: globally, only ~50% of eradication attempts are successful, and success rates are substantially lower in semi-natural habitats relative to man-made environments such as greenhouses . Additionally, invasive species can become community mainstays and assume novel ecological roles in the environments they invade . In some cases, reducing invasive species can have strong habitat implications for native species of conservation concern. Therefore, if the invading species is eradicated without other restoration activities, native organisms may experience negative consequences . Scenarios like these are increasingly common around the world and present an urgent dilemma: how can natural resource managers minimize negative effects of invasive species without depleting native taxa that have come to rely on them? We address this question through our case study in the Sacramento–San Joaquin River Delta.In this case study, we examined whether water hyacinth management activities influence invertebrate communities that may support fishes of the Sacramento–San Joaquin River Delta . The Delta is part of the largest estuary on the Pacific Coast of the Americas, and serves as a critical link between California’s water supply, aquatic species, and human populations. The Delta faces many challengesthat have been described extensively by natural resources agencies and researchers . Aquatic invasive species are of great concern in the Delta because they can affect ecological communities, water distribution, commerce, recreation, and other human industries . For these and other reasons, the Delta is an ecosystem with novel features—“abiotic, biotic, and social components that, by virtue of human influence, differs from those that prevailed historically, having a tendency to self-organize and manifest novel qualities without intensive human management” .
Water hyacinth is one of the most visible invaders in the Delta, because it is a floating aquatic weed and is found nearly Delta-wide. Historically, much of the Delta, including our study location, was characterized by freshwater emergent wetlands . Vegetated littoral zones like these are important for producing invertebrate biomass as food for threatened and endangered juvenile salmonids and other fishes that forage on insects and zooplankton . However, only a small proportion of historic Delta freshwater wetlands remain today . In the absence of the once-abundant Delta littoral native plant communities, invertebrates must use available habitats. Today, the littoral habitat includes water hyacinth throughout the Delta.Water hyacinth is a floating aquatic macrophyte native to the Amazon basin. It has invaded aquatic ecosystems around the world, affecting human endeavors as well as abiotic and biotic ecosystem elements. For example, Water hyacinth can block sunlight and alter turbidity levels; decrease phytoplankton production, dissolved oxygen , and nutrient levels; and influence heavy metal concentrations . Water hyacinth can also clog navigable water ways, displace native vegetation, alter nutrient cycling, and change sediment dynamics . There is limited research that describes the role of water hyacinth in structuring and sustaining invertebrate communities. However, Villamagna et al. determined that, in general, water hyacinth increases habitat complexity but decreases food availability for invertebrates. Toft et al. demonstrated that—compared to a native macrophyte —water hyacinth had lower macroinvertebrate densities, and that the invertebrates found on water hyacinth were less prevalent in fish diets. Even so, water hyacinth has been a feature of the Delta for ~70 years, is widely dispersed, extremely abundant, and provides complex habitat for invertebrates. Given that water hyacinth is a major physical and biological feature of the Delta and serves as habitat for invertebrates that are common in the diets of some Delta fishes , it is incumbent upon Delta managers to consider the implications of water hyacinth management on the species that use it as habitat.In accordance with the Harbors and Navigation Code , the California Parks Division of Boating and Waterways is the lead agency responsible for cooperating with state, local, and federal agencies in “identifying, detecting, controlling, and administering programs to manage invasive aquatic plants in the Sacramento–San Joaquin Delta, its tributaries, and the Suisun Marsh.” In cooperation with the California Department of Fish and Wildlife, CDBW is tasked with evaluating the threat of aquatic invasive species to the environment, economy, and human health. Consequently, CDBW has undertaken programs to control water hyacinth, Brazilian waterweed , indoor grow rack and South American sponge plant in the Delta. The CDBW uses several techniques in its water hyacinth control program, including the use of herbicides . Though it is imperative that water hyacinth in the Delta be managed—given its wide distribution, ability to block navigation, rapid growth rate, and sheer abundance—weed management activities in other ecosystems demonstrate that such actions may also alter habitat, hydrology, water quality, and food resources for aquatic invertebrates that are associated with invasive macrophytes . The CDBW prepares biological assessments of their management activities—including justification of the amount of herbicide applied—for regulatory review. In an effort to reduce the environmental effects of their operations over time, they also provide logistical support for on-going research as part of their adaptive management process. Widespread herbicide application in the Delta creates a mosaic of living and decaying water hyacinth that can persist for at least 4 weeks before the decaying material dissipates.
Little is known about the macroinvertebrate communities within these decaying water hyacinth mats. It is difficult to predict how management will affect macroinvertebrates because decaying water hyacinth releases nutrients and organic particles that support the food web, but also feed bacterial communities that may drive DO down and subsequently negatively affect macroinvertabrates and zooplankton . We hypothesized that herbicide applications under current management protocols would reduce the abundance and diversity of aquatic invertebrates because they would alter structural and biological habitat.This study provides valuable information for the “evaluate and respond” component of the Delta Water Hyacinth Control Program , which employs adaptive management: a systematic approach for improving resource management by learning from management outcomes . This study also serves as a case study example for generating experimentally derived evidence to support adaptive management programs in other systems where water hyacinth is present . To ensure best practices in their management and policy operations, the CDBW has employed this work in their Section7 Biological Assessment with the United States Fish and Wildlife Service. This case study is also pertinent to management of novel ecosystems, where even non-native species can have important ecological roles . Since water hyacinth is intensively managed with herbicides on a global scale, this work has broad applicability and provides an example for future hypothesis-driven adaptive management efforts involving invasive plants’ roles in ecosystem functioning.Experimental sites were in the central Delta, California , in water hyacinth mats that surround Bacon Island . We chose to focus our study on sites that surrounded Bacon Island because they had predictable herbicide treatment dates. Other site-selection criteria included: mats of floating water hyacinth that were likely to remain in place for the study’s duration; mats at control sites that would remain untreated with herbicide; habitat characteristics similar enough to be comparable between and among treatment and control sites; and no other management activities by the CDBW.Using a Before, After, Control, Intervention experimental design, we established five sampling sites to receive herbicide treatment with glyphosate, each paired with a control site that would not receive any herbicide treatment. CDBW applied glyphosate treatments along with AgriDex at treatment locations. Treatment dates varied among sites during spring and summer 2015 . During herbicide application, the CDBW left untreated buffer strips to comply with the agency’s fish passage protocols that protect migrating and resident fish . However, for treated sites, we assumed that treatment effects would be detected throughout the site, across treated and untreated strips. To ensure this assumption matched reality, we randomly sampled across the entire spatial extent of mats at both treatment and control sites.At each site, we sampled approximately 1 month before the herbicide treatment to ensure that we had pre-treatment data. We sampled each location again approximately 4 weeks post-treatment. We used this post-treatment lag-time to ensure that herbicide effects were as uniform as possible at each of the treatment sites. Since the control locations did not receive any herbicide treatment, we describe them in terms of before and after the treatment date to indicate the point in time when herbicides were applied at comparable treatment locations. CDBW staff collected water-quality measures throughout the treatment season in accordance with National Pollution Discharge Elimination System permitting requirements using a Hach HQ30 meter and Luminescent Dissolved Oxygen probe for DO measures on the periphery of water hyacinth mats. For each sampling event, we used a numbered grid overlaid on a graphical representation of each weed mat, and a random number generator to select four portions of the mat to sample.