Grazing History Research Articles

Recovery of plant species composition and ecosystem function after cessation of grazing in a Mediterranean grassland

Short- and long-term changes in species composition, plant biomass production, and litter decomposition after cessation of grazing were examined in a Mediterranean grassland with high dominance of annual species and strong seasonality in biomass production. Short-term changes were assessed during three consecutive years in plots previously exposed to different grazing pressures and compared to plots in long-term (30–40 years) exclosures. Short-term cessation of grazing led in the short-term to an increase in relative biomass of annual crucifers and tall annual and perennial grasses, while biomass of annual legumes, annual thistles and short annual grasses decreased. Consequently, similarity increased between vegetation recently excluded from grazing and vegetation in long-term protected plots. Our research showed that in systems with high dominance of grasses and annual species, the rapid changes in plant species composition that occur after grazing cessation were associated with a fast recovery of the potential for biomass production to levels found in long-term protected plots, while litter decomposition rate did not change even after long-term cessation of grazing. Moreover, previous history of grazing did not affect plant litter decomposition, despite higher litter quality in grazed treatments. This study provides new insights about the processes involved in the diverse responses of ecosystem functions resulting from shifts in species composition associated with grazing cessation and land use change in Mediterranean grasslands.

Fire and grazing impacts on silica production and storage in grass dominated ecosystems

Grassland ecosystems are an important terrestrial component of the global biogeochemical silicon cycle. Although the structure and ecological functioning of grasslands are strongly influenced by fire and grazing, the role of these key ecological drivers in the production and storage of silicon represents a significant knowledge gap, particularly since they are being altered worldwide by human activities. We evaluated the effects of fire and grazing on the range and variability of plant derived biogenic silica stored in plant biomass and soils by sampling plants and soils from long-term experimental plots with known fire and grazing histories. Overall, plants and soils from grazed sites in the South African ecosystems had up to 76 and 54% greater biogenic silica totals (kg ha−1), respectively, than grazed North American sites. In North American soils, the combination of grazing and annual fire resulted in the greatest abundance of biogenic silica, whereas South African soils had the highest biogenic silica content where grazed regardless of burn frequency. These results as well as those that show greater Si concentrations in grazed South African plants indicate that South African plants and soils responded somewhat differently to fire and grazing with respect to silicon cycling, which may be linked to differences in the evolutionary history and in the grazer diversity and grazing intensity of these ecosystems. We conclude that although fire and grazing (as interactive and/or independent factors) do not affect the concentration of Si taken up by plants, they do promote increased silicon storage in aboveground biomass and soil as a result of directly affecting other site factors such as aboveground net primary productivity. Therefore, as management practices, fire and grazing have important implications for assessing global change impacts on the terrestrial biogeochemical cycling of silicon.

Controls of Aboveground Net Primary Production in Mesic Savanna Grasslands: An Inter-Hemispheric Comparison

Patterns and controls of annual aboveground net primary productivity (ANPP) are fundamental metrics of ecosystem functioning. It is generally assumed, but rarely tested, that determinants of ANPP in one region within a biome will operate similarly throughout that biome, as long as physiognomy and climate are broadly consistent. We tested this assumption by quantifying ANPP responses to fire, grazing history, and nitrogen (N) addition in North American (NA) and South African (SA) savanna grasslands. We found that total ANPP responded in generally consistent ways to fire, grazing history, and N addition on both continents. Annual fire in both NA and SA consistently stimulated total ANPP (28–100%) relative to unburned treatments at sites with deep soils, and had no effect on ANPP in sites with shallow soils. Fire did not affect total ANPP in sites with a recent history of grazing, regardless of whether a single or a diverse suite of large herbivores was present. N addition interacted strongly and consistently with fire regime in both NA and SA. In annually burned sites that were not grazed, total ANPP was stimulated by N addition (29–39%), but there was no effect of N fertilization in the absence of fire. In contrast, responses in forb ANPP to fire and grazing were somewhat divergent across this biome. Annual fire in NA reduced forb ANPP, whereas grazing increased forb ANPP, but neither response was evident in SA. Thus, despite a consistent response in total ANPP, divergent responses in forb ANPP suggest that other aspects of community structure and ecosystem functioning differ in important ways between these mesic savanna grasslands.

Fine-Scale Spatial Genetic Structure in Perennial Grasses in Three Environments

Past research has shown that changes in grazing-resistance traits may be associated with genetic changes in plant populations. Little is known about spatial genetic relationships within plant populations (spatial genetic structure) and any grazing effects on these relationships. Here we present observations of the fine-scale spatial genetic structure in three grass species in semiarid environments (Arizona, Mexico, and Argentina). In each environment, populations of a dominant grass species were sampled from two sites with contrasting livestock grazing histories. Plant genotypes were described with the use of amplified fragment length polymorphism markers. In Arizona, populations of sideoats grama (Bouteloua curtipendula var. caespitosa Gould and Kapadia) differed in that one has never experienced livestock grazing, whereas cattle have grazed the other. In the other two environments, populations exposed to long-term heavy grazing were examined, along with those that experienced either moderate grazing (Mexico, blue grama [Bouteloua gracilis {Willd. ex Kunt} Lag. ex Griffiths]) or extended exclusion of livestock (Argentina [Poa ligularis Nees ex Steud.]). Based on independent analysis of each population, we observed no differences in average gene diversity between populations of each species. With the use of analysis of molecular variance we found slight but significant genetic differentiation between populations with different grazing histories in Arizona and Argentina. Significant genetic structure was present in all populations and indicated an inverse relationship between spatial and genetic distance. Interestingly, this relationship was most pronounced in the cattle-free sideoats grama population, suggesting larger genetic neighborhood areas in the absence of livestock. Less distinct differences in spatial genetic structure associated with grazing history were evident in the other two species. We hypothesize that livestock grazing may lead to increased homogeneity in genetic structure at the landscape scale. Effectively examining this hypothesis presents many experimental challenges. Investigaciones conducidas anteriormente ha mostrado que los cambios en las características a la resistencia al pastoreo quizá están asociados con los cambios genéticos en la población de las plantas. Muy poco se conoce acerca de la relación espacial genética entre la población de las plantas (la estructura genética espacial) y cualquier efecto del pastoreo en esta relación. Aquí presentamos observaciones a una escala fina de la estructura espacial genética en tres especies de gramíneas en un medio ambiente semiárido (Arizona, México, y Argentina). En cada medio ambiente, las poblaciones de las especies de gramíneas dominantes fueron muestreadas en dos sitios con historia de pastoreo contrastantes. Los genotipos de las plantas fueron descritos utilizando marcadores amplificados de fragmentos de polyformismo. En Arizona, las poblaciones de navajita (Bouteloua curtipendula var. caespitosa Gould and Kapadia) difieren en que nunca ha estado expuestas al pastoreo del ganado mientras que en las otras poblaciones han estado expuestas al pastoreo. En las otras dos zonas las poblaciones expuestas al pastoreo pesado por largo tiempo fueron examinadas junto con aquellas poblaciones que experimentaron pastoreo moderado (México, blue grama [Bouteloua gracilis {Willd. ex Kunt} Lag. ex Griffiths]) o una prolongada exclusión del pastoreo (Argentina [Poa ligularis Nees ex Steud.]). Basándose en un análisis independiente de cada población, observamos que no había diferencia en el promedio de la diversidad de genes entre la población de cada especie. Utilizando análisis de la varianza molecular se encontró una diferencia ligera pero una significante diferenciación genética entre las poblaciones con diferente historial de pastoreo en Arizona y Argentina. Significante estructura genética estuvo presente en todas las poblaciones y se encontró una relación inversa entre la distancia genética y la espacial. Interesantemente, esta relación estuvo más marcada en la población de navajita (sideoats grama) donde no existió el pastoreo, sugiriendo una mayor acercamiento genético en las aéreas donde el ganado estuvo ausente. Menor evidente fueron las diferencias en la estructura espacial genética relacionada con la historia del pastoreo en las otras dos especies. Tenemos las hipótesis que el pastoreo por el ganado puede incrementar la homogeneidad en la estructura genética a niveles de paisaje. El examen efectivo de esta hipótesis presenta varios retos experimentales.

Restoration of Degraded Grazing Lands through Grazing Management: Can It Work?

Abstract Grazing lands are the most degraded land use type in the world, particularly in arid and semiarid areas, as a result of improper human activities such as overgrazing coupled with drought. For restoration of degraded grazing lands, large‐scale projects are implemented, which include extensive vegetation improvements (e.g., reseeding, weed control, shrub plantations, reforestations, etc.). Such interventions, apart from being very expensive, often create environmental problems. In situations where the abiotic function of the degraded grazing land has not been irreversibly damaged, application of appropriate grazing management is an ecologically viable solution to their restoration. This is especially necessary for grazed lands with a long history of grazing by large herbivores, including livestock. Excluding domestic animals from such ecosystems may lead to several ecological problems such as loss of biodiversity and devastating wildfires. Appropriate grazing management should include an adjusted stocking rate to the grazing capacity of the restored land, the right kind of animal species, and an appropriate grazing system. In addition, grazing management should be implemented on the basis of a plan that is part of the restoration project. It is concluded that grazing management should become a priority option in restoration of the biotic function of degraded grazing lands, especially in those that have had a long history with the presence of domestic animals.

Functional response in habitat selection and the tradeoffs between foraging niche components in a large herbivore

How herbivore behaviour is influenced by changes in resource levels is central for understanding trophic interactions. We examined whether foraging tradeoffs change with food levels by comparing habitat selection and space use within and between two neighbouring, predator‐free Svalbard reindeer populations. The populations faced different food levels due to contrasting grazing history. Summer resource selection in radiocollared females was assessed by a multi‐dimensional niche approach based on habitat variables obtained from a satellite image (e.g. the normalised difference vegetation index, NDVI) and a digital terrain model. The population at the overgrazed Brøggerhalvøya faced overall lower plant cover, biomass and primary productivity (i.e. lower NDVI) than the population at Sarsøyra. At Brøggerhalvøya, most reindeer selected for productive habitat when choosing home range and patches within the home range. In contrast, habitat selection at Sarsøyra was more affected by abiotic conditions such as moisture, which may influence plant quality. Here, reindeer used patches with even less biomass than the average reindeer at the poorer Brøggerhalvøya. Such a difference in habitat preference with different habitat availability (a functional response in habitat selection) probably reflected increased selection for high‐quality forage at the expense of high forage quantity at Sarsøyra. Accordingly, a negative relationship between habitat productivity and home range size was only present across individuals within Brøggerhalvøya, where forage quantity was the important foraging niche component. Individuals having poor (and large) home ranges apparently could not compensate for this by higher patch selectivity compared to individuals with richer home ranges. The results indicate changes in foraging tradeoffs at contrasting resource levels and that strong interactions occur between habitat selection, space use and the foraging niche structure in the absence of predation.

Regional fire history based on charcoal analysis of sediments from nine lakes in western Mongolia

Fires are common in grassland regions of the world, and the frequency and severity of fire is linked to climate-driven changes in fuel loads. Western Mongolia is dominated by grasslands but the fire history of this region is largely unknown. We reconstructed modern fire (48 lakes) and historical fires (9 lakes) using sediment charcoal. Modern fuel loads were estimated using a combination of clipped plots, satellite-based estimates of annual aboveground net primary productivity (NPP) and NPP modeled from annual temperature and precipitation. Loss-on-ignition and environmental magnetics of lake sediments were analyzed as proxies for climate. We found little evidence for modern or historical fire in the landscape, as charcoal was absent from the surface sediments of 34 of 48 lakes. Charcoal influxes were uniformly low, averaging from 0.002 to 0.028 mm2/cm2 per yr, over the past 1200 years at nine lakes, and the past 6000—5000 years at two of the lakes with longer sediment records. In the modern landscape, livestock grazing has eliminated most of the fuels necessary to carry a fire, as measured fuel loads (27.3±4.9 g/m2) were only ~20% of aboveground annual NPP estimated using MODIS Imagery or modeled from climate data. The historical absence of fire may indicate a longer history of intensive grazing than sometimes assumed, and cultural prohibitions against burning may also play a role. Regional summary indicated a >50% decrease in charcoal influxes since AD 1600 at most sites which may be related to lower temperatures or greater aridity during the `Little Ice Age'. Alternatively this decrease in charcoal influxes may reflect increases in livestock numbers or increased local concentrations because of restrictions on the movement of animals coincident with the establishment of Manchu rule in the late seventeenth century.

Pyric Herbivory: Rewilding Landscapes through the Recoupling of Fire and Grazing

Our understanding of fire and grazing is largely based on small-scale experimental studies in which treatments are uniformly applied to experimental units that are considered homogenous. Any discussion of an interaction between fire and grazing is usually based on a statistical approach that ignores the spatial and temporal interactions on complex landscapes. We propose a new focus on the ecological interaction of fire and grazing in which each disturbance is spatially and temporally dependent on the other and results in a landscape where disturbance is best described as a shifting mosaic (a landscape with patches that vary with time since disturbance) that is critical to ecological structure and function of many ecosystems. We call this spatiotemporal interaction pyric herbivory (literal interpretation means grazing driven by fire). Pyric herbivory is the spatial and temporal interaction of fire and grazing, where positive and negative feedbacks promote a shifting pattern of disturbance across the landscape. We present data we collected from the Tallgrass Prairie Preserve in the southern Great Plains of North America that demonstrates that the interaction between free-roaming bison (Bison bison) and random fires promotes heterogeneity and provides the foundation for biological diversity and ecosystem function of North American and African grasslands. This study is different from other studies of fire and grazing because the fires we examined were random and grazing animals were free to roam and select from burned and unburned patches. For ecosystems across the globe with a long history of fire and grazing, pyric herbivory with any grazing herbivore is likely more effective at restoring evolutionary disturbance patterns than a focus on restoring any large vertebrate while ignoring the interaction with fire and other disturbances.

Grazing responses in herbs in relation to herbivore selectivity and plant traits in an alpine ecosystem

Herbivores shape plant communities through selective foraging. However, both herbivore selectivity and the plant's ability to tolerate or resist herbivory may depend on the density of herbivores. In an alpine ecosystem with a long history of grazing, plants are expected to respond to both enhanced and reduced grazing pressures, and the interaction between plant traits and changes in species abundance are expected to differ between the two types of alteration of grazing regime. To understand the mechanisms behind species response, we investigated the relationship between sheep selectivity (measured in situ), plant traits and experimentally derived measures of change in species abundance as a response to the enhancement (from low to high density) or cessation (from low to zero density) of sheep grazing pressure over a six-year time period for 22 abundant herb species in an alpine habitat in south Norway. Sheep selected large, late-flowering herbs with a low leaf C/N ratio. Species that increased in abundance in response to enhanced grazing pressure were generally small and had high root/shoot ratios, thus exhibiting traits that reflect both resistance (through avoidance) and tolerance (through regrowth capacity) strategies. The abundance of selected species remained stable during the study period, and also under the enhanced grazing pressure treatment. There was, however, a tendency for selected species to respond positively to cessation of grazing, although overall responses to cessation of grazing were much less pronounced than responses to enhanced grazing. Avoidance through short stature (probably associated with increased light availability through the removal of tall competitors) as well as a certain amount of regrowth capacity appear to be the main mechanisms behind a positive response to enhanced grazing pressure in this study. The plant trait perspective clearly improves our insight into the mechanisms behind observed changes in species abundance when the disturbance regime is altered.

Breeding habitat use and the future management of the critically endangered Southern Corroboree Frog

Summary The Southern Corroboree Frog (Pseudophryne corroboree) is one of Australia's most critically endangered frog species. The species occurs entirely within Kosciuszko National Park, which has a history of cattle grazing (up to the 1970s). A consequence of cattle grazing has been a significant reduction in the extent of montane and sub‐alpine peat‐bog systems that the species uses as breeding habitat. Furthermore, climate change and associated increased wildfire frequency is expected to further reduce the extent and quality of peat bogs throughout the Australian Alps. In this study, we investigated habitat selection for breeding pools and nest sites within peat‐bog systems in order to inform the conservation management of the species and guide other management practices being undertaken in peat bogs where this species occurs. Occupancy of breeding males at bog pools was found to be positively associated with increasing pool area, water depth and mid‐day temperature, and negatively associated with extent of bare substrate. The majority of breeding pools identified were ephemeral. Nest sites within vegetation where males call and where females deposit their eggs were located at mid‐elevations in a range of vegetation types, with the majority of nests being within moss and sedge dominated by Sphagnum cristatum and Empodisma minor. We also found that male nest sites were not randomly distributed within the edges of pools, but were more often located in areas of loose vegetation. These results highlight the potential sensitivity of the Southern Corroboree Frog to predicted changes in peat‐bog systems resulting from climate change such as earlier drying and a possible reduction in the size of bog pools. A monitoring programme focused on key features of the breeding habitat should be undertaken to provide a basis for developing and assessing management actions implemented in peat bogs occupied by this species.

Evolution of maternal effects: past and present.

It has been said that many original ideas reflect a convergence of related thought that coalesces into a unified representation of what many have been thinking. This is certainly true for the subfield of maternal effects evolution. The study of maternal effects has a long history. The first two

Holocene climatic change and the nomadic Anthropocene in Eastern Tibet: palynological and geomorphological results from the Nianbaoyeze Mountains

Our study provides detailed information on the Lateglacial landscape and vegetation development of Tibet. Based on a suite of geomorphological and palynological proxy data from the Nianbaoyeze Shan on the eastern margin of the Tibetan Plateau (33°N/101°E, 3300–4500 m asl.), we reconstruct the current state as a function of climate history and the longevity of human influence. Study results constrain several major phases of aeolian sedimentation between 50–15 ka and various glacier advances during the Late Pleistocene, the Holocene and the Little Ice Age. Increased aeolian deposition was primarily associated with periods of more extensive glacial ice extent. Fluvial and alluvial sediment pulses document an increase of erosion starting at 3926 ± 79 cal yr B.P., coinciding with cooling (Neoglacial) and a growing anthropo-zoogenic influence. Evidence for periglacial mass movements indicate that the late Holocene cooling started at around 2000 cal yr B.P., demonstrating increased surface activity under the combined effects of human influence and climate deterioration. The onset of peat growth generally depended on local conditions that include relief, meso-climate and in more recent times also on soil compaction due to animal trampling. We distinguish three initiation periods of peat growth: 12,700–10,400 cal yr B.P. for flat basins inside last glacial terminal moraines; 7000–5000 cal yr B.P. for the main valley floors; and 3000–1000 cal yr B.P. for the higher terrace surfaces. The Holocene vegetation history started with an open landscape dominated by pioneer shrubs along braided rivers (<10,600–9800 cal yr B.P.), followed by the spreading of conifers ( Picea, Juniperus, Abies) and Betula-trees accompanied by a successive closing of the vegetation cover by Poaceae, Cyperaceae and herbs (9800–8300 cal yr B.P.). First signs of nomadic presence appear as early as 7200 cal yr B.P., when temperatures were up to 2 °C warmer than today. Forest remained very patchy with strong local contrasts. During the following cooling phase (5900–2750 cal yr B.P.) the natural vegetation was transformed by nomadic grazing to Bistorta-rich Kobresia pygmaea-pastures. Modern nomadic migration routes were established at least 2200 years ago. Overgrazing and trampling led to the shrinking of Bistorta and the spreading of annual weeds. Short-lived cold events (8000, 6200, 3500 cal yr B.P.) impacted on the vegetation only temporarily. As the transformation of the natural Poaceae-rich vegetation into Kobresia-pastures modified the influence of the Tibetan Plateau (“hot plate”) on the monsoon system, our data even point to an early start of a nomadic (!) Anthropocene nearly 6000 years ago. Against the background of a very long grazing history, modern Tibet must be seen as a cultural landscape.

Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali)

The patterns of the changes in woody plant population densities, size and species composition is documented and discussed for 24 rangeland sites monitored from 1984 to 2006 in Gourma (Mali). The sites are sampled along the North–South bioclimatic gradient on each of the main soils and levels of grazing intensity. Site woody plant populations range from extremely sparse on shallow soils, to scattered on sandy soils, to open forest in temporarily flooded clayed soils, and to narrow thickets on hard pans. Three different methods contributed to assess and monitor woody plant density and canopy cover. In the short term woody populations were struck by the 1983–1984 droughts irrespective of their edaphic situation and location along the bioclimatic gradient. Drought induced mortality was not more severe under drier climate within the Sahel gradient but occurred sooner after drought in shallow soils, and with a lag of a year or two on flooded clay soils. No evidences were found of higher mortality rates in stands with history of intense grazing. Although rainfall remained below average for a decade after the drought, active recruitment of woody plants occurred in all sites starting as soon as 1985. Recruitment proceeded by successive cohorts, often with short-living perennial undershrubs and pioneer shrubs settling first. Acacia species were among the first to settle or re-establish, especially on the sites most intensively grazed. The release of competition due to drought induced mortality and to the reduction of herbaceous cover contributed to the success of the recruitment. The species composition change that resulted could first be interpreted as a shift toward a more arid tolerant flora, then some diversification occurred since the mid 1990s that could indicate a possible return to previous composition in the long term, confirming the resilience Sahel vegetation.

Multispecies Allometric Models Predict Grass Biomass in Semidesert Rangeland

Multispecies allometric models to predict grass biomass may increase field study efficiency by eliminating the need for speciesspecific data. We used field measurements during two growing seasons to develop single-species and multispecies regression models predicting the current year’s aboveground biomass for eight common cespitose grass species. Simple and stepwise regression analyses were based on natural log expressions of biomass, basal diameter, and height, and a dummy variable expression of grazing history. Basal diameter had the strongest relationship with biomass among single-species (adjusted R 2 5 0.80 to 0.91) and multispecies (adjusted R 2 5 0.85) models. Regression slopes (b) for diameter among single-species (b 5 1.01 to 1.49) and the multispecies (b 5 1.25) models suggests that biomass will double when diameter increases ,75%. Height and grazing history added little predictive value when diameter was already in the model. When applied to actual populations, biomass estimates from multispecies models were within 3–29% of estimates from the single-species models. Although the multispecies biomass–size relationship was robust across the cespitose life-form, users should be cautious about applying our equations to different locations, plant sizes, and population size-structures.

Multi-decadal increases in shrub abundance in non-riverine red gum (Eucalyptus camaldulensis) woodlands occur during a period of complex land-use history

The history of land-use was examined in Eucalyptus camaldulensis Dehnh. woodland in the Victoria Valley of the Grampians National Park, south-eastern Australia, to help interpret changes in vegetation there during the last 50 years. We used aerial photography and dendrochronological data to quantify the amount of, and the rate of change in, the woody-vegetation cover between 1948 and 1997, and historical data to document land-use changes during this time. Aerial photographs indicated that in 1948, 56% of study area had &lt;50% cover of woody plants. By 1997, 90% of the study area had &gt;50% woody-plant cover. The native shrub Leptospermum scoparium J.R. Forst &amp; G. Forst (Myrtaceae) was predominantly responsible for the increases in cover. Demographic analyses indicated that recruitment has been ongoing rather than episodic; large numbers of shrubs, however, have recruited since 1994. We hypothesise that the vegetation changes observed are likely a response to changes in land-use that have occurred since European occupation. Increased woody-plant cover followed the removal of sheep grazing in the long-term absence of fire. It is very likely that the long history of stock grazing, coupled with selective logging and associated soil disturbance, initiated a change in understorey vegetation by reducing competitive native tussock grasses and fuel loads to carry fires and this reduction was initially responsible for the encroachment of shrubs into the woodland. Recruitment has been ongoing in the absence of any recent land-use changes (most utilisation ceasing after the declaration of National Park status in 1984) and hence, this transformation from species-rich herbaceous woodland to shrubby woodland is expected to continue in the future.

Ground-dwelling beetle community in beech forests of Sado Island, Japan: comparison with the mainland

Island ecosystems differ from mainland ecosystems in their long-term isolation. On Sado Island, Japan, cattle grazing in the past gradually changed forests into grasslands, thereby altering the diversity of indigenous insects. We compared ground-dwelling beetle diversity on Sado Island to that on the nearby mainland. Contrary to previous reports, the island isolation effect resulted in lower γ and β diversity than on the mainland, because of the low species richness of beetles with low dispersal abilities. Few flightless beetles could invade Sado Island, even when the beech forests on the island were relatively adjacent to those on the mainland. Such a history of ecosystem formation could be the general pattern for continental-island mountain forests that were never connected to those of the mainland. We did not observe low α diversity of ground-dwelling beetles, suggesting that the cattle grazing history of the island has not changed environmental conditions for ground-dwelling beetles enough to reduce their diversity. In addition, cattle grazing increases the abundance of merdivorous insects by increasing the abundance of their food resource.

HERBIVORY MEDIATES GRASS–ENDOPHYTE RELATIONSHIPS: COMMENT

In the recent paper by Koh and Hik (2007), the pattern of Neotyphodium spp. endophyte infection frequency is shown to be related with the gradient of grazing pressure by two native herbivores, pikas (Ochotona collaris) and hoary marmots (Marmota caligata) in Yukon Territory (Canada). The grazing gradient is determined by the behavior of herbivores, being high ‘‘on’’ boulder fields and low ‘‘far’’ from there. The mechanism invoked to explain the pattern of infection frequency is the relative fitness of endophyteinfected and uninfected plants along this gradient. According to the defensive mutualism and optimal defense theories (Clay 1993, Zangerl and Rutledge 1996), infected hosts with alkaloid-based deterrents would be favored where grazing was consistently high, while uninfected plants or infected plants without deterrents would be favored where herbivory was low or absent. To reinforce the hypothesis, a trial was carried out to evaluate pikas selectivity for endophyteinfected and uninfected plants sampled from ‘‘on þ near’’ and ‘‘far’’ from boulder fields. Herbivores were able to discriminate endophyte-infected and uninfected Festuca altaica plants from boulder fields, but they did not discriminate when plants came from meadows. In summary, the authors conclude that grazing pressure and herbivory selectivity is the major factor generating the pattern in endophyte infection frequency. Hence, endophyte infection is viewed as an adaptive trait in the environment with high grazing pressure. In our opinion, Koh and Hik (2007) present little information supporting that differences in endophyte infection frequency and herbivore avoidance of infected grasses were mainly a consequence of the distinct longer term grazing history of the grasses mediated by the deterrence fungus effect. We propose that the effect of herbivores on infection frequency might as well be mediated through selection of phenotypic traits not related to antiherbivory (e.g., compensatory growth ability [Karban and Baldwin 1997, Sullivan et al. 2007]), or through an interaction with other environmental factors that can affect both partners to a different extent (Hill et al. 2005, Rasmussen et al. 2007). A usual observational approach to study broad ecological patterns is to find out simple relationships between the distribution and abundance of organisms and any environmental factor. However, this procedure may result in oversimplifications leading to wrong conclusions (see the case of shared enemies discussed by Holt and Lawton 1994). For the specific case of Neotyphodium endophytes, ecological mechanisms underlying patterns of endophyte infection frequency are quite complex (Faeth 2002). In fact, Koh and Hik (2007) acknowledge that ‘‘variations at the population level in the deterrence ability of infected tillers, frequency of infection, and hyphal densities are thought to be produced by multiple factors (genetic and environmental)’’ among which they cite ‘‘differences in grazing pressure.’’ Different theoretical and modeling approaches have indicated that the endophyte infection frequency in local populations may result from some of the following mechanisms: (1) relative fitness of endophyte-infected and uninfected plants, (2) vertical transmission efficiency of endophyte in infected plants, and (3) migration of infected and uninfected individuals among connected local populations (Clay 1993, Ravel et al. 1997, Saikkonen et al. 2002, Gundel et al. 2008). Here, we discuss each of those mechanisms and confront them with the arguments presented by Koh and Hik (2007). Our aim is to show that relative fitness may or may not be related to herbivory deterrence and that the other two mechanisms have been almost completely neglected. 1. The relative fitness of endophyte-infected and uninfected related plants has been always used to explain natural variations in endophyte infection frequency in grass populations (Clay 1993, Clay and Schardl 2002).— By this mechanism, it is predicted that the endophyte infection frequency tends to be high if endophyte enhances the host plant fitness or, alternatively, the endophyte goes to extinction if it depresses the fitness of the plants (Clay 1993). An ecological trait can be considered adaptive if it has a direct impact on fitness in natural environments. In the case of the endophyte infection, to consider it as adaptive based solely on the observed correlation between infection frequency and grazing level may be problematic. Even when the correlation is observed, it is difficult to determine whether the ecological trait (endophyte infection) per Manuscript received 14 December 2007 ; revised 11 March 2008; accepted 18 April 2008. Corresponding Editor: J. N. Klironomos. 1 IFEVA-Facultad de Agronomia UBA/CONICET, Catedra de Ecologia, Av. San Martin 4453 (C1417DSE) Buenos Aires, Argentina. 2 E-mail: gundel@agro.uba.ar

Vegetation and seedbank response to Eragrostis lehmanniana removal in semi‐desert communities

Summary Eragrostis lehmanniana (Lehmann lovegrass) is an invasive perennial bunchgrass in semi‐desert grasslands in the USA. We removed E. lehmanniana using glyphosate at three locations in south‐eastern Arizona in 2003 and 2004 to evaluate the community response. We also sampled the seedbank five times during the course of the experiment to characterise viable seeds at the sites. Two of the sites with histories of livestock grazing responded similarly to the continued removal of E. lehmanniana, with significant increases in native cover and species richness, mainly attributable to herbaceous annual species. Over the course of the study, progressively fewer E. lehmanniana seeds and higher numbers of native annual herb seeds were germinated from seedbank samples of treated plots. At the third site, characterised by a history of intense agricultural use and drier conditions than the other sites, there was a large decrease in E. lehmanniana cover in all plots, followed by an increase in aggressive non‐native species as well as native grasses. The seedbank at this site suggested potential for restoration, with native perennial grass species present in increasing quantities. The results of this study have implications for site restoration, should a means of permanently removing E. lehmanniana be found.

Sahelian rangeland response to changes in rainfall over two decades in the Gourma region, Mali

Summary Twenty-five rangeland sites were monitored over two decades (1984–2006) first to assess the impact of the 1983–1984 droughts on fodder resources, then to better understand ecosystem functioning and dynamics. Sites are sampled along the south–north bioclimatic gradient in Gourma (Mali), within three main edaphic situations: sandy, loamy-clay and shallow soils. In addition, three levels of grazing pressure where systematically sampled within sandy soils. Located at the northern edge of the area reached by the West African monsoon, the Gourma gradient has recorded extremes in inter-annual variations of rainfall and resulting variations in vegetation growth. Following rainfall variability, inter-annual variability of herbaceous yield increases as climate gets dryer with latitudes at least on the sandy soils sites. Local redistribution of rainfall explains the high patchiness of herbaceous vegetation, especially on shallow soils. Yet spatial heterogeneity of the vegetation does not buffer between year yield variability that increases with spatial heterogeneity. At short term, livestock grazing during the wet season affects plant growth and thus yield in direction and proportions that vary with the timing and intensity of grazing. In the longer term, grazing also impinges upon species composition in many ways. Hence, long histories of heavy grazing promote either long cycle annuals refused by livestock or else short cycle good quality feed species. Primary production is maintained or even increased in the case of refusal such as Sida cordifolia , and is lessened in the case of short cycle species such as Zornia glochidiata . These behaviours explain that the yield anomalies calculated for the rangelands on sandy soils relative to the yield of site less grazed under similar climate tend to be negative in northern Sahel where the scenario of short cycle species dominates, while yield anomalies are close to nil in centre Sahel and slightly positive in South Sahel where the refusal scenario is more frequent. Because grazing promotes short cycle species, grazed rangelands respond faster to droughts. Year to year changes in species composition are abrupt as expected from the transient soil seed stock. However, some decadal trends in species composition are identified, with a wave of pioneer species following the 1983–1984 droughts, and a more progressive diversification and return to typical Sahel flora from 1992 onwards.

Dwarf morphology of the annual plant Persicaria longiseta as a local adaptation to a grazed habitat, Nara Park, Japan

AbstractI conducted a common garden experiment using three populations of an annual species, Persicaria longiseta, with different grazing histories to compare plant morphology and its plasticity in response to grazing among populations. Seeds collected from the populations were assigned to clipping and nutrient treatments. Regardless of the treatment conditions, after 5 months individuals of the Nara Park population (long history of deer grazing) exhibited shorter shoots, smaller leaves and lower plant mass and developed a larger number of shoots and inflorescences compared with the Miwa and Heijo populations (no grazing history). Across nutrient conditions, the three populations exhibited few differences in plasticity in response to clipping. I also conducted a field experiment in which cultivated individuals from the three populations were transplanted to the Nara Park site. Results of the transplant experiment revealed that the dwarf morphology reduced grazing damage in such a way that small‐formed individuals of the Nara population exhibited significantly higher survival and a lower grazing rate than large‐formed individuals from the Miwa and Heijo populations. In conclusion, the dwarf morphology of P. longiseta in the Nara population is likely to have evolved as a genetically stable form as a result of selection pressure from deer grazing. Although phenotypic plasticity contributed in part to the development of the dwarf forms, I observed no evidence of evolution of plasticity specific to Nara Park.