Livestock grazing affects vernal pool specialists more than habitat generalists in montane vernal pools

Questions

Do livestock grazing and seasonal precipitation structure species composition in montane vernal pools? Which grazing and precipitation variables best predict cover of vernal pool specialists and species with broader habitat requirements? Is vernal pool species diversity correlated with livestock exclosure, and at what spatial scales?

Location

Montane vernal pools, northeast California, USA.

Methods

Vegetation was sampled in 20 vernal pools, including pools where livestock had been excluded for up to 20 years We compared plant species composition, functional group composition and species diversity among sites that varied in grazing history and seasonal precipitation using CCA and LMM.

Results

Although vernal pool specialists were dominant in montane vernal pools, over a third of plant cover was comprised of species that occur over a broad range of wetland or upland environments. The species composition of vernal pool plant communities was influenced by both livestock grazing and precipitation patterns, however the relative effects of these environmental variables differed by functional group. Livestock exclosures favoured perennial vernal pool specialists over annual vernal pool specialists. In contrast, the cover of habitat generalists was more strongly influenced by seasonal precipitation than livestock grazing. At small spatial scales, species richness and diversity decreased as the number of years a pool had been fenced increased, but this relationship was not significant at a larger spatial scale.

Conclusions

Both livestock grazing and seasonal precipitation structure the montane vernal pool plant community. We found that livestock grazing promotes the cover of annual vernal pool specialists, but at the expense of perennial vernal pool specialists. Wetter vernal pools, however, support higher cover of wetland generalist species regardless of whether pools are grazed.     

Loss of biodiversity and hydrologic function in seasonal wetlands persists over 10 years of livestock grazing removal

Ecological restoration provides a means to increase biodiversity in ecosystems degraded by natural and human‐induced changes. In some systems, disturbances such as grazing can be key factors in the successful restoration of biodiversity and ecological function, but few studies have addressed this experimentally, especially over long time periods and at landscape scales. In this study, we excluded livestock grazing from plots within a grassland landscape containing vernal pools in the Central Valley of California for 10 years and compared vernal pool hydrology and plant community composition with areas grazed under an historic regime. In all 10 years, the relative cover of native plant species remained between 5 and 20% higher in the grazed versus ungrazed plots. This effect was particularly prominent on the pool edges, though evidence of invasion into the pool basins was evident later in the study. Native species richness was lower in the ungrazed plots with 10–20% fewer native species found in ungrazed versus grazed plots in all years except the first year of treatment. Ungrazed pools held water for a shorter period of time than pools grazed under an historic regime. By the ninth year of the study, ungrazed pools took up to 2 weeks longer to fill and dried down 1–2 weeks sooner at the end of the rainy season compared to grazed pools. The results of this study confirm that livestock grazing plays a key role in maintaining biodiversity and ecosystem function in vernal pools.     

Grazing Lawns in Terai Grasslands,Royal Bardia National Park,Nepal1

We compared the community structure, nutritive quality, and aboveground biomass of grazing lawns (patches of shortgrass communities) to neighboring grasslands in the Terai of western Nepal. Grazing lawns differed from the adjacent grasslands in species composition and community structure. Species diversity and species richness were higher on grazing lawns (H = 1.60, S = 20.93) than the grasslands (H = 0.97, S = 8.97). Fencing that excluded grazers for 150 days made areas of grazing lawns indistinguishable from neighboring grasslands in terms of plant height and biomass. Growing shoots of forage from grazing lawns had higher digestibility, crude protein, and sodium than forage from the grasslands. Grazing lawns appear to be maintained by continuous grazing and are enriched by deposition of urine, dung, and by certain plant species not found in the the adjacent grasslands.     

A Long‐Term Comparison of Hydrology and Plant Community Composition in Constructed Versus Naturally Occurring Vernal Pools

Successful restoration of ephemeral wetlands worldwide is particularly challenging, given the often‐precise relationship between hydrological features and plant community dynamics. Using a long‐term experiment in vernal pool restoration, we compare hydrological and vegetative characteristics of constructed pools with those of adjacent, naturally occurring reference pools. Although constructed and reference pools were similar in maximum water depth and duration of inundation at the beginning of our experiment in 2000, constructed pools were shallower and inundated for shorter periods by 2009. Native vernal pool species were able to establish populations in many constructed pools, and seeding sped their establishment. Comparing seeded plots in constructed pools with unseeded plots in reference pools, we found no significant difference in the cover of seeded species, native species, or exotic species in most years. In recent years, however, native species have declined in both constructed and reference pools. Finally, the cover of native vernal pool species was positively and non‐linearly associated with both water depth and seeding treatment. We conclude that the establishment of appropriate hydrological conditions was necessary, but not sufficient to promote successful performance of vernal pool species in constructed pools. Constructed pools with hydrologic conditions similar to those of reference pools were more likely to support populations of native vernal pool plant species, but only seeded pools were similar to reference pools in abundance of native cover. Most importantly, hydrological conditions in experimental pools have worsened since their construction, which may hamper persistence of native species in this restoration effort.     

Effects of different vegetative substrates on algal composition in vernal mesocosms

We investigated the response of an algal assemblage to different vegetative substrates in controlled vernal mesocosms. Litter was collected from four vascular plant communities (Deciduous Forest, Macrophyte, Old Field and Pinus stands) and the litter was used to line the benthos of the vernal mesocosms. The development and response of the algal assemblage in treatment and control mesocosms was tracked biweekly for a period of 56 days. A repeated-measure MANOVA and Bonferroni (Dunn) post-hoc test indicated that the Pinus treatment produced a significantly greater biomass than all other treatments. The Pinus treatment mesocosms had acidic (4.5–4.7) water when sampled on day 42, which continued until the completion of the study. The greatest levels of species richness and diversity were recorded from the Deciduous Forest and Old Field treatments. Algal assemblage analyses indicated that there was much overlap in community structure between various litter treatments and algae alone (no litter). Most of the vernal mesocosm treatments were dominated by the filamentous chlorophytes Mougeotia, Oedogonium and Ulothrix. These results suggest that, in this study, the vegetative litter of vernal mesocosms (with the exception of Pinus) exhibits limited independent influence on the developmental trajectories of algal communities.     

Impact of different‐sized herbivores on recruitment opportunities for subordinate herbs in grasslands

Abstract. Potential effects of herbivores on plant species diversity depend on herbivore size, species and density. In this study we examine the effect of different‐sized herbivores (cattle and rabbits) on recruitment of subordinate herbs in grasslands. We show that in a grazed floodplain, grassland plant species richness is mainly determined by the presence of many species of subordinate herbs. These herbs experience high colonization and extinction rates. We conclude that the creation of colonization opportunities for subordinate herbs plays a crucial role in maintaining plant species richness in productive grasslands. We found that cattle disperse large amounts of seeds via their dung, over ten times more than rabbits. Rabbits create more and on average larger bare soil patches than cattle. In a field experiment artificial disturbances improved germination success tremendously for four tested herb species. We found that bare soil is the best regeneration site, while cattle dung gave a too strong nutrient stimulus, resulting in tall vegetation and therefore light limitation. These results can be confirmed with results from field monitoring plots where plant species richness was positively related to the occurrence of bare soil patches. Therefore both large and small herbivores have a major impact on dispersal and colonization, but for different reasons. Cattle are identified as most important for seed dispersal whereas rabbits have a main effect as creators of disturbances. These results emphasize the importance of distinguishing between herbivore species in assessing their (potential) effects.     

Conservation tool or threatening process? management implications of interactions of cattle with vegetation and land at the Vale of Belvoir reserve

A recently purchased private reserve of high conservation value has been grazed in summer by cattle since the early nineteenth century. We ask whether the cattle are causing continuing damage and whether they are necessary to maintain any conservation values. We used five‐year‐old exclosures to determine the effects of cattle on wetland vegetation, mapped damage to soils and landforms and mapped cattle dung deposition in relation to the distributions of rare and threatened plant species. Cattle impacts on wetland plants were minor. Cattle damage to landforms by pugging and bank collapse was frequent near unfenced water bodies. Rare and threatened species were largely on well‐drained ground, while cattle dung was concentrated in wetlands and near water bodies. We conclude that cattle grazing is impacting some conservation values and is not necessary for conservation purposes. Destocking should be accompanied by careful monitoring of threatened plant populations. If necessary, alternative planned disturbances can provide regeneration niches for rare and threatened plants.     

Grazer exclusion alters plant spatial organization at multiple scales,increasing diversity

Grazing is one of the most important factors influencing community structure and productivity in natural grasslands. Understanding why and how grazing pressure changes species diversity is essential for the preservation and restoration of biodiversity in grasslands. We use heavily grazed subalpine meadows in the Qinghai‐Tibetan Plateau to test the hypothesis that grazer exclusion alters plant diversity by changing inter‐ and intraspecific species distributions. Using recently developed spatial analyses combined with detailed ramet mapping of entire plant communities (91 species), we show striking differences between grazed and fenced areas that emerged at scales of just one meter. Species richness was similar at very small scales (0.0625 m²), but at larger scales diversity in grazed areas fell below 75% of corresponding fenced areas. These differences were explained by differences in spatial distributions; intra‐ and interspecific associations changed from aggregated at small scales to overdispersed in the fenced plots, but were consistently aggregated in the grazed ones. We conclude that grazing enhanced inter‐ and intraspecific aggregations and maintained high diversity at small scales, but caused decreased turnover in species at larger scales, resulting in lower species richness. Our study provides strong support to the theoretical prediction that inter‐ and intraspecific aggregation produces local spatial patterns that scale‐up to affect species diversity in a community. It also demonstrates that the impacts of grazing can manifest through this mechanism, lowering diversity by reducing spatial turnover in species. Finally, it highlights the ecological and physiological plant processes that are likely responding to grazing and thereby altering aggregation patterns, providing new insights for monitoring, and mediating the impacts of grazing.     

Ecosystem-phase interactions: aquatic eutrophication decreases terrestrial plant diversity in California vernal pools

Eutrophication has long been known to negatively affect aquatic and terrestrial ecosystems worldwide. In freshwater ecosystems, excessive nutrient input results in a shift from vascular plant dominance to algal dominance, while the nutrient-species richness relationship is found to be unimodal. Eutrophication studies are usually conducted in continuously aquatic or terrestrial habitats, but it is unclear how these patterns may be altered by temporal heterogeneity driven by precipitation and temperature variation. The California vernal pool (CVP) ecosystem consists of three distinct phases (aquatic, terrestrial, and dry) caused by variation in climatic conditions. The purpose of this study was to test the hypothesis that resource addition during the aquatic phase results in increased algal abundance, which reduces vascular plant cover and richness of the terrestrial phase upon desiccation. We used mesocosms layered with CVP soil, in which treatments consisted of five levels of nitrogen and phosphorous added every 2 weeks. Resource addition increased available phosphorus levels and algae cover during the aquatic phase. Increased algal crusts resulted in decreased vascular plant percent cover and species richness. Few significant patterns were observed with individual plant species and total biomass. The phosphorus-plant richness relationship was not significant, but species composition was significantly different among the low and high treatment comparisons. These results highlight a neglected effect of eutrophication in seasonal habitats. Interactions among ecosystem phases clearly require more attention empirically and theoretically. Management and restoration of temporally heterogeneous habitat, such as the endemic-rich CVP, need to consider the extensive effects of increased nutrient input.     

Dung beetle diversity and functions suggest no major impacts of cattle grazing in the Brazilian Pantanal wetlands

1. Dung beetles perform relevant ecological functions in pastures, such as dung removal and parasite control. Livestock farming is the main economic activity in the Brazilian Pantanal. However, the impact of cattle grazing on the Pantanal's native dung beetle community, and functions performed by them, is still unknown. 2. This study evaluated the effects of cattle activity on dung beetle community attributes (richness, abundance, biomass, composition, and functional group) as well as their ecological functions (dung removal and soil bioturbation) in the Pantanal. In January/February 2016, dung beetles were sampled and their ecological functions measured in 16 sites of native grasslands in Aquidauana, Mato Grosso do Sul, Brazil, 10 areas regularly grazed by cattle and six control ungrazed areas (> 20 years of abandonment). 3. In all, 1169 individuals from 30 species of dung beetles were collected. Although abundance, species richness, and biomass did not differ between grasslands with and without cattle activity, species composition and functional groups differed among systems. Large roller beetles were absent from non‐cattle grasslands, and the abundance, richness, and biomass of medium roller beetles was higher in those systems. 4. Despite causing changes in species/functional group composition, the results of this study show that a density compensation of functional groups in cattle‐grazed natural grasslands seems to have conserved the ecological functions (dung removal and soil bioturbation), with no significant differences between systems. 5. Therefore, these results provide evidence that cattle breeding in natural grasslands of the Brazilian Pantanal can integrate livestock production with the conservation of the dung beetle community and its ecological functions.     

Ecosystem-level effects of bioturbation by the tadpole shrimp <Emphasis Type="Italic">Lepidurus packardi</Emphasis> in temporary pond mesocosms

An example of ecosystem engineering gaining attention in aquatic systems is bioturbation, the disruption of sediment at the water–sediment interface due to burrowing and foraging. One consequence of bioturbation can be increased turbidity from suspended sediment, which generally inhibits macrophyte growth and reduces ecosystem functioning. Conversely, bioturbation may promote invertebrate species richness by unearthing dormant cysts. Temporary-pond crustaceans are not widely regarded as agents of bioturbation, but on the basis of aquaria observations we hypothesized that certain taxa can disturb the sediment and create highly turbid water. We tested this hypothesis by removing crustaceans from mesocosms lined with vernal pool soil. Compared to this treatment group, mesocosms containing crustaceans had extremely high turbidity from suspended sediment, as well as reduced total macrophyte cover. We also found clear compositional differences in macrophyte communities between treatments, driven largely by differences in water physicochemistry, including turbidity. Regression analysis linked most of the bioturbation to the endangered notostracan Lepidurus packardi Simon 1886, which was a strong predictor of turbidity in our mesocosms. We also found a trend toward increased crustacean species richness in our mesocosms in the presence of this taxon. An analysis of published data from King et al. (1996) suggests that this trend may extend to natural vernal pools. Overall, our results suggest that L. packardi may have large effects on vernal pool communities, likely mediated in part through its disturbing of the sediment.     

Herbivory and drought interact to enhance spatial patterning and diversity in a savanna understory

The combination of abiotic stress and consumer stress can have complex impacts on plant community structure. Effective conservation and management of semi-arid ecosystems requires an understanding of how different stresses interact to structure plant communities. We explored the separate and combined impacts of episodic drought, livestock grazing, and wild ungulate herbivory on species co-occurrence and diversity patterns in a relatively productive, semi-arid Acacia savanna. Specifically, we analyzed 9 years of biannual plant community data from the Kenya long-term exclosure experiment, a broad-scale manipulative experiment that has excluded different combinations of large mammalian herbivores from 18 4-ha plots since 1995. During droughts, we observed low species diversity and random species co-occurrence patterns. However, when rain followed a major drought, areas exposed to moderate cattle grazing displayed high species diversity and evidence of significant species aggregation. These patterns were not apparent in the absence of cattle, even if other large herbivores were present. To explore possible mechanisms, we examined patterns separately for common and rare species. We found that aggregation patterns were likely driven by rare species responding similarly to the availability of open micro-sites. Our results indicate that in a productive, fire-suppressed savanna, the combination of periodic drought and moderate cattle grazing can enhance plant biodiversity and fine-scale spatial heterogeneity by opening up space for species that are otherwise rare or cryptic. Our findings also emphasize that domestic herbivores can have significantly stronger impacts on plant community dynamics than wild herbivores, even in an ecosystem with a long history of grazing.     

Identification of co-occurring Branchinecta fairy shrimp species from encysted embryos using multiplex polymerase chain reaction

Morphological identification of many fairy shrimp species is difficult because distinguishing characters are restricted to adults. We developed two multiplex polymerase chain reaction assays that differentiate among three Branchinecta fairy shrimp with distributional overlap in southern California vernal pools. Two of the species are federally listed as threatened. Molecular identification of Branchinecta from cysts allows for species surveys to be conducted during the dry season, expanding the timeframe for population assessment and providing a less intrusive method of sampling sensitive vernal pool habitats.     

Plant Colonizers Shape Early N-dynamics in Gopher-mounds

Disturbances by fossorial mammals are extremely common in many ecosystems, including the California annual grassland. We compared the impact of juveniles of four common plant colonizers (Aegilops triuncialis, Cerastium glomeratum, Aphanes occidentalis and Lupinus bicolor) on the pools and fluxes of N in mounds created by pocket gophers (Thomomys bottae Mewa). The mechanisms and magnitude of biotic N retention differed among plant species. In mounds colonized by Cerastium, Aphanes and Lupinus, the microbial N pool was significantly larger than the plant N pool, as is typical in California grasslands in the early spring, whereas in mounds colonized by Aegilops, there was a more equal distribution of biotic N between plant and microbial pools. A 1-day ¹⁵N pulse field experiment demonstrated that plant species significantly differed in their effects on the distribution of isotopic N, with the N-fixing Lupinus leaving most (82%) ¹⁵N as inorganic N in soil, whereas more ¹⁵N was immobilized in plants or otherwise removed from the available soil pool in mounds colonized by other species. The impacts of early colonizers on N dynamics suggest that the identity of plant species that initially colonize gopher mounds may have important consequences on the dynamics of the overall grassland community.     

Traditional Cattle Grazing in a Mosaic Alkali Landscape: Effects on Grassland Biodiversity along a Moisture Gradient

Extensively managed pastures are of crucial importance in sustaining biodiversity both in local- and landscape-level. Thus, re-introduction of traditional grazing management is a crucial issue in grassland conservation actions worldwide. Traditional grazing with robust cattle breeds in low stocking rates is considered to be especially useful to mimic natural grazing regimes, but well documented case-studies are surprisingly rare on this topic. Our goal was to evaluate the effectiveness of traditional Hungarian Grey cattle grazing as a conservation action in a mosaic alkali landscape. We asked the following questions: (i) How does cattle grazing affect species composition and diversity of the grasslands? (ii) What are the effects of grazing on short-lived and perennial noxious species? (iii) Are there distinct effects of grazing in dry-, mesophilous- and wet grassland types? Vegetation of fenced and grazed plots in a 200-ha sized habitat complex (secondary dry grasslands and pristine mesophilous- and wet alkali grasslands) was sampled from 2006–2009 in East-Hungary. We found higher diversity scores in grazed plots compared to fenced ones in mesophilous- and wet grasslands. Higher cover of noxious species was typical in fenced plots compared to their grazed counterparts in the last year in every studied grassland type. We found an increasing effect of grazing from the dry- towards the wet grassland types. The year-to-year differences also followed similar pattern: the site-dependent effects were the lowest in the dry grassland and an increasing effect was detected along the moisture gradient. We found that extensive Hungarian Grey cattle grazing is an effective tool to suppress noxious species and to create a mosaic vegetation structure, which enables to maintain high species richness in the landscape. Hungarian Grey cattle can feed in open habitats along long moisture gradient, thus in highly mosaic landscapes this breed can be the most suitable livestock type.     

The population biology of mitigation: impacts of habitat creation on an endangered plant species

Conservation practitioners widely agree that optimal conservation strategies will maximize the amount of genetic variation preserved in target taxa, but there is ongoing debate about how that variation should be distributed through restoration and mitigation activities. Here, we evaluate the impacts of ~10 years of mitigation on the population genetic structure of Limnanthes vinculans, a state- and federally-listed endangered plant species restricted to ephemeral vernal pool wetlands in the Santa Rosa Plain of California. Using microsatellite loci to estimate patterns of neutral molecular variation, we found that created pools support similar levels of variation in L. vinculans as natural pools. Habitat creation and seed translocation have not disrupted the largest-scale patterns of population structure across the species range, but a concentration of mitigation activity towards the range center has reduced the extent of isolation-by-distance operating in this region and shifted the location of at least one genetic boundary. Patterns of genetic variation among populations in remnant vernal pools reveal that gene flow has historically occurred beyond the scale of individual pools at the center of the species range, while small genetic populations have differentiated around the range margins. On average, L. vinculans in created pools exhibit less cover and more restricted local distributions than those in remnant pools, but these patterns were driven by two particularly productive natural sites rather than consistent differences between natural and created sites. We conclude that mitigation activities have changed the historical patterns of gene flow within the species range to a moderate degree, that these changes will likely impact remnant pools through gene flow, and that current created sites provide less heterogeneous habitat for L. vinculans than do natural pools. Studies that track individual plants will be needed to determine if the changes in gene flow due to mitigation will have positive or negative impacts on the demographic and microevolutionary trajectories of L. vinculans. More generally, this study provides a retrospective analysis of the outcome of managing an endangered plant species through intensive mitigation, and yields several insights to inform future conservation strategies.     

Restoration genetics of the vernal pool endemic Lasthenia conjugens (Asteraceae)

Restoration of habitat for endangered species often involves translocation of seeds or individuals from source populations to an area targeted for revegetation. Long-term persistence of a species is dependent on the maintenance of sufficient genetic variation within and among populations. Thus, knowledge and maintenance of genetic variability within rare or endangered species is essential for developing effective conservation and restoration strategies. Genetic monitoring of both natural and restored populations can provide an assessment of restoration protocol success in establishing populations that maintain levels of genetic diversity similar to those in natural populations. California’s vernal pools are home to many endangered plants, thus conservation and restoration are large components of their management. Lasthenia conjugens (Asteraceae) is a federally endangered self-incompatible vernal pool annual with gravity- dispersed seeds. Using the molecular technique of intersimple sequence repeats (ISSRs), this study assessed levels and patterns of genetic variability present within natural and restored populations of L. conjugens. At Travis Air Force Base near Fairfield, California, a vernal pool restoration project is underway. Genetic success of the ecologically based seeding protocol was examined through genetic monitoring of natural and restored populations over a three-year period. Genetic diversity remained constant across the three sampled generations. Diversity was also widely distributed across all populations. We conclude that the protocol used to establish restored populations was successful in capturing similar levels and patterns of genetic diversity to those seen within natural pools. This study also demonstrates how genetic markers can be used to inform conservation and restoration decisions.     

Interactive effects of fire and grazing on structure and diversity of Mediterranean grasslands

Abstract. The separate and combined effects of fire and cattle grazing on structure and diversity of productive Mediterranean grasslands in northern Israel were examined within a set of climatically and edaphically similar sites. Cover and height of green and dry plants in winter, and species richness and diversity in spring, were measured in paired transects on both sides of cattle fences, and on both sides of boundaries of both incidental and experimentally lit fires. Early in the first growing season after a fire, plant cover as well as height of green plants were reduced, compared to unburnt grassland. These structural effects of fire were similar to the effects of grazing, but they were greater in ungrazed than in grazed grasslands, indicating a fire-grazing interaction. The effects of fire were considerably attenuated in the second growing season after the fire. Species richness and diversity tended to be higher in grazed than in adjacent ungrazed grasslands. Richness consistently increased after a fire only in grazed grasslands with a strong perennial component. In ungrazed grasslands, and in predominantly annual grasslands, fires reduced species richness and diversity at least as often as they increased it. Fire and grazing should be regarded as two agents with distinct and interactive effects on the community, rather than as two alternative mechanisms of a general disturbance factor.