The effectiveness of using targeted grazing for vegetation management: a meta‐analysis

Targeted grazing reduces a widespread wetland plant invader with minimal nutrient impacts, yet native community recovery is limited

Targeted grazing to control undesirable plant species is increasingly of interest across a diversity of ecosystems, particularly as an alternative or complement to widely used herbicides. However, there are limited comprehensive evaluations of targeted grazing that evaluate both invasive species management effectiveness and potential negative effects on the ecosystem. Phragmites australis, a tall-statured, dense perennial invasive grass from Eurasia, is a pervasive problem in wetlands across the North American continent. As with many invasive species where management has historically relied on herbicides and resistance is a growing concern, land managers seek viable alternatives that have minimal negative ecosystem impacts. Grazing has been used for millennia to manage native Phragmites in Europe. Similarly, in its invasive range within North America, small-scale studies suggest Phragmites may be suppressed by grazers. Yet, the effectiveness of grazing at large scales and its effects on broader ecosystem properties remain largely unknown. We evaluated the influence of targeted grazing on vegetation, soil nutrients, and water nutrients over two years in large plots (∼300x the size of previous studies). We also tested the effects of mowing, a treatment that can be used to facilitate grazer access to large, dense Phragmites stands. In line with our predictions, we found that cattle grazing effectively suppressed invasive Phragmites over two years. Mowing reduced litter, and moderately reduced standing dead Phragmites, both of which suppress native plant germination in this system. However, these reductions in Phragmites were not accompanied by indications of native plant community recovery, as we had optimistically predicted. Despite the potential for grazing to reduce nutrient sequestration by plants and fertilize soils, we were surprised to find no clear negative effects of grazing on nutrient mobilization to groundwater or floodwater. Taken together, our findings indicate that targeted grazing, when implemented at broad scales over short time frames, is effective at achieving invasive plant management goals without sizable nutrient impacts. However, additional steps will be needed to achieve the restoration of diverse, robust native plant communities.

Systematic reduction of natural enemies and competition across variable precipitation approximates buffelgrass invasiveness (Cenchrus ciliaris) in its native range

Invasive grasses cause devastating losses to biodiversity and ecosystem function directly and indirectly by altering ecosystem processes. Escape from natural enemies, plant-plant competition, and variable resource availability provide frameworks for understanding invasion. However, we lack a clear understanding of how natural stressors interact in their native range to regulate invasiveness. In this study, we reduced diverse guilds of natural enemies and plant competitors of the highly invasive buffelgrass across a precipitation gradient throughout major climatic shifts in Laikipia, Kenya. To do this, we used a long-term ungulate exclosure experiment design across a precipitation gradient with nested treatments that (1) reduced plant competition through clipping, (2) reduced insects through systemic insecticide, and (3) reduced fungal associates through fungicide application. Additionally, we measured the interaction of ungulates on two stem-boring insect species feeding on buffelgrass. Finally, we measured a multiyear smut fungus outbreak. Our findings suggest that buffelgrass exhibits invasive qualities when released from a diverse group of natural stressors in its native range. We show natural enemies interact with precipitation to alter buffelgrass productivity patterns. In addition, interspecific plant competition decreased the basal area of buffelgrass, suggesting that biotic resistance mediates buffelgrass dominance in the home range. Surprisingly, systemic insecticides and fungicides did not impact buffelgrass production or reproduction, perhaps because other guilds filled the niche space in these highly diverse systems. For example, in the absence of ungulates, we showed an increase in host-specific stem-galling insects, where these insects compensated for reduced ungulate use. Finally, we documented a smut outbreak in 2020 and 2021, corresponding to highly variable precipitation patterns caused by a shifting Indian Ocean Dipole. In conclusion, we observed how reducing natural enemies and competitors and certain interactions increased properties related to buffelgrass invasiveness.

Displacement Effects of Conservation Grazing on Red Deer (Cervus elaphus) Spatial Behaviour

Conservation grazing uses semi-feral or domesticated herbivores to limit encroachment in open areas and to promote biodiversity. However, we are still unaware of its effects on wild herbivores. This study investigates the influence of herded sheep and goats on red deer (Cervus elaphus) spatial behavior by testing three a-priori hypotheses: (i) red deer are expected to avoid areas used by livestock, as well as adjacent areas, when livestock are present, albeit (ii) red deer increase the use of these areas when sheep and goats are temporarily absent and (iii) there is a time-lagged disruption in red deer spatial behavior when conservation grazing practice ends. Using GPS-telemetry data on red deer from a German heathland area, we modelled their use of areas grazed by sheep and goats, using mixed-effect logistic regression. Additionally, we developed seasonal resource selection functions (use-availability design) to depict habitat selection by red deer before, during, and after conservation grazing. Red deer used areas less during conservation grazing throughout all times of the day and there was no compensatory use during nighttime. This effect mostly persisted within 21 days after conservation grazing. Effects on habitat selection of red deer were detectable up to 3000 meters away from the conservation grazing sites, with no signs of either habituation or adaption. For the first time, we demonstrate that conservation grazing can affect the spatio-temporal behavior of wild herbivores. Our findings are relevant for optimizing landscape and wildlife management when conservation grazing is used in areas where wild herbivores are present.

Can Co-Grazing Waterfowl Reduce Brainworm Risk for Goats Browsing in Natural Areas?

Goats browsing in woodlands, whether for livestock production goals or vegetation management (e.g., targeted grazing to control invasive plants), are at risk of meningeal worm (Parelaphostrongylus tenuis) infection. Indeed, up to 25% incidence has been observed in goats employed in vegetation management. Infection, which occurs via the consumption of an infected gastropod intermediate host, is potentially deadly in goats. We experimentally tested whether co-grazing with waterfowl could reduce goats' exposure via waterfowl consumption of gastropods. Gastropods were sampled in a deciduous woodland before and after the addition of goats alone, goats and waterfowl, or a control with no animal addition. We found that goats browsing on their own increased the abundance of P. tenuis intermediate hosts; however, when goats co-grazed with waterfowl, these increases were not observed. Importantly, waterfowl did not significantly affect overall gastropod abundance, richness, or diversity. Thus, waterfowl co-grazing may effectively reduce goat contact with infectious gastropods without detrimentally affecting the gastropod community. While co-grazing goats with waterfowl may decrease their P. tenuis exposure risk, additional research is needed to confirm whether waterfowl can actually lower P. tenuis incidence.

Nature, nurture, and vegetation management: Studies with sheep and goats

Diet selection and preference by grazing animals are determined by genetic and environmental factors (i.e., nature and nurture) that interact and affect their efficacy for managing vegetation as targeted grazers. The effect of rearing environment on the consumption of leafy spurge by sheep and goats was investigated. We hypothesized that although rearing environment will affect the preference for chemically defended plants ultimately, the inherent ability to detoxify or eliminate phytotoxins will limit an animal's preference for them. The objective of this study was to determine if sheep would consume more of the invasive weed leafy spurge (Euphorbia esula) if they were raised by goat compared to sheep raised by sheep and goat raised by goat. Sheep were raised on leafy spurge-infested pastures by either their ewe (S) or a goat doe (FS) on which they were fostered within 24 hours of birth and parturition of lamb and doe, respectively. Does that fostered lambs also raised their own goat offspring (G) such that the same doe raised the FS and G animals. The rearing environment's effect on leafy spurge consumption was tested the following growing season by simultaneously grazing all animals on the same leafy spurge-infested rangeland and estimating percentage leafy spurge in their diet with either fecal near-infrared spectroscopy (f.NIR) or bite count. Goats consumed more leafy spurge as determined by either f.NIR (62.8%, P < 0.06) or bite count (71.9%, P < 0.01) than FS (35.2 % f.NIR, 39.3% bite count) or S (10.1 % f.NIR, 18.2% bite count). The FS consumed over twice as much leafy spurge as S and were numerically intermediate to G and S for leafy spurge consumption but not significantly different from the S sheep, most likely because one FS sheep did not eat leafy spurge during the evaluation period. Because leafy spurge is aversive to sheep but not goats, higher leafy spurge consumption by FS sheep is hypothesized to result from inoculation of their rumen microbes with microbes from the does capable of denaturing aversive phytotoxins in leafy spurge. The higher consumption of leafy spurge by G compared to FS shows that genetically determined physiological differences influence an animal's ability to ameliorate phytotoxins and determine the upper limit of an animal's preference for a chemically defended plant. It also indicated that in addition to the animal's genome, the genome of an animal's microbiome, which the mother may influence, can play an important role in diet selection.