Effects of small‐scale grassland fragmentation and frequent mowing on population density and species diversity of orthopterans: a long‐term study

Abstract 1. Habitat fragmentation is considered one of the major threats to invertebrate diversity in semi‐natural grassland. However, the effects of habitat fragmentation through mowing on the rich insect fauna of these grasslands have not been sufficiently investigated and experiments are especially rare. 2. We studied the impact of small‐scale grassland fragmentation on orthopterans over 7 years in an experiment which allowed us to additionally investigate the effect of frequent mowing on Orthoptera communities. 3. Overall, Orthoptera density and species richness increased over time. This was likely a result of increased small‐scale habitat heterogeneity and the provision of a short‐turf habitat suitable for xerophilous species. The fragmentation affected orthopteran density and species composition but not species richness whose response lagged behind the changes in abundance. 4. Responses differed between suborders. Ensifera density was higher in fragment than in control plots. Caelifera density did not differ between fragment and control plots. The mown matrix was an unsuitable habitat for most of the species, particularly within the Ensifera. 5. Our experiment shows that even small‐scale fragmentation can affect Orthopteran communities and that the effects became more pronounced over time. As the mown matrix was unsuitable for many Ensifera species, they may go locally extinct when large areas are mown simultaneously.     

High land‐use intensity exacerbates shifts in grassland vegetation composition after severe experimental drought

Climate change projections anticipate increased frequency and intensity of drought stress, but grassland responses to severe droughts and their potential to recover are poorly understood. In many grasslands, high land‐use intensity has enhanced productivity and promoted resource‐acquisitive species at the expense of resource‐conservative ones. Such changes in plant functional composition could affect the resistance to drought and the recovery after drought of grassland ecosystems with consequences for feed productivity resilience and environmental stewardship. In a 12‐site precipitation exclusion experiment in upland grassland ecosystems across Switzerland, we imposed severe edaphic drought in plots under rainout shelters and compared them with plots under ambient conditions. We used soil water potentials to scale drought stress across sites. Impacts of precipitation exclusion and drought legacy effects were examined along a gradient of land‐use intensity to determine how grasslands resisted to, and recovered after drought. In the year of precipitation exclusion, aboveground net primary productivity (ANPP) in plots under rainout shelters was ?15% to ?56% lower than in control plots. Drought effects on ANPP increased with drought severity, specified as duration of topsoil water potential ψ < ?100 kPa, irrespective of land‐use intensity. In the year after drought, ANPP had completely recovered, but total species diversity had declined by ?10%. Perennial species showed elevated mortality, but species richness of annuals showed a small increase due to enhanced recruitment. In general, the more resource‐acquisitive grasses increased at the expense of the deeper‐rooted forbs after drought, suggesting that community reorganization was driven by competition rather than plant mortality. The negative effects of precipitation exclusion on forbs increased with land‐use intensity. Our study suggests a synergistic impact of land‐use intensification and climate change on grassland vegetation composition, and implies that biomass recovery after drought may occur at the expense of biodiversity maintenance.     

Cumulative effects of land use,altered fire regime and climate change on persistence of Ceanothus verrucosus,a rare,fire‐dependent plant species

Mediterranean ecosystems are among the highest in species richness and endemism globally and are also among the most sensitive to climate and land‐use change. Fire is an important driver of ecosystem processes in these systems; however, fire regimes have been substantially changed by human activities. Climate change is predicted to further alter fire regimes and species distributions, leading to habitat loss and threatening biodiversity. It is currently unknown what the population‐level effects of these landscape‐level changes will be. We linked a spatially explicit stochastic population model to dynamic bioclimate envelopes to investigate the effects of climate change, habitat loss and fragm entation and altered fire regime on population abundances of a long‐lived obligate seeding shrub, Ceanothus verrucosus, a rare endemic species of southern California. We tested a range of fire return intervals under the present and two future climate scenarios. We also assessed the impact of potential anthropogenic land‐use change by excluding land identified as developable by local governments. We found that the 35–50 year fire return interval resulted in the highest population abundances. Expected minimum population abundance (EMA) declined gradually as fire return interval increased, but declined dramatically for shorter fire intervals. Simulated future development resulted in a 33% decline in EMA, but relatively stable population trajectories over the time frame modeled. Relative changes in EMA for alternative fire intervals were similar for all climate and habitat loss scenarios, except under the more severe climate scenario which resulted in a change in the relative ranking of the fire scenarios. Our results show climate change to be the most serious threat facing obligate seeding shrubs embedded in urban landscapes, resulting in population decline and increased local extirpation, and that likely interactions with other threats increase risks to these species. Taking account of parameter uncertainty did not alter our conclusions.     

Impacts of a mineral lick‐centred land use system on woody vegetation cover in an East African Savannah

Evaluation of woody vegetation changes with distance from a salt crater was conducted in the semi‐arid rangelands of southern Ethiopia. Data on live woody plants were collected over three seasons at 0, 1, 4, 6, 9 and 12 km from the salt crater. The density and diversity of woody plants differed significantly (p < .01) along the distance gradient. Six woody plant families were identified of which Fabaceae and Burseraceae were the dominant families. Acacia drepanolobium, Acacia nilotica, Commiphora africana and Acacia mellifera were among the severely encroaching woody species. There were high proportions of seedlings and saplings recorded closer to the salt crater showing a vigorous recruitment by woody plants. Woody plant encroachment along the 12‐km transect ranged from a low to severe encroachment, which could be translated into poor rangeland condition. Changes in soil characteristics increased grazing pressure and sedentary settlement around the salt crater, and the breakdown of traditional institutions seems to be major contributing factors to these vegetation changes. We suggest that severely encroached areas could be improved through a combination of methods such as bush clearing, prescribed fire, browsing animals and proper grazing management.     

Little shrimp left on the shelf: the roles that sea‐level change,ocean currents and continental shelf width play in the genetic connectivity of a seagrass‐associated species

Aim Caridean shrimp are diverse and abundant inhabitants of seagrass beds. Anthropogenic disturbances have already reduced and fragmented seagrass habitat, and the rate of change is likely to increase in the future. It is therefore becoming increasingly important to build a basis of understanding of connectivity among populations of seagrass‐associated fauna. Phycomenes zostericola is closely associated with seagrass and makes an ideal study species with which to explore patterns of connectivity and the influence of biogeographic boundaries and historical sea‐level changes on seagrass‐associated species. We hypothesized that strong currents and the high potential of P. zostericola for dispersal and adult movement would result, for the most part, in panmixia. We also hypothesized that if structure was evident, it would occur close to known biogeographic boundaries in Queensland. Location Phycomenes zostericola is an abundant shrimp species distributed throughout Queensland’s seagrass habitats. Nineteen seagrass sites from the Torres Strait Islands and Queensland coastlines were sampled. Methods Molecular sequence data for a 590 base pair fragment of the mitochondrial gene cytochrome c oxidase subunit I (COI) was analysed for 279 specimens of P. zostericola. Phylogeographic patterns were analysed using nested clade phylogeographic analysis (NCPA); an isolation‐by‐distance effect was tested using a Mantel test; the effect of biogeographic boundaries was tested using an analysis of molecular variance (AMOVA), and also a spatial analysis of molecular variance (SAMOVA); demographic expansions were tested for using Tajima’s D, Fu’s F_S and timing estimated using mismatch analysis; the timing of vicariant events was estimated using coalescent analysis (im program). Results Contrary to our original hypothesis, the strong marine currents are not a connective influence among populations of P. zostericola. Regional genetic structure and an isolation‐by‐distance effect are enhanced by existing coastal biogeographic boundaries. Population genetic structure and demographic history are intricately linked to the effects of a tumultuous Pleistocene sea‐level history on the Queensland continental shelf. Main conclusions Connectivity diminishes among populations of P. zostericola over scales larger than a few hundred kilometres. As seagrass habitats world‐wide become increasingly fragmented, low levels of connectivity will result in an isolated future for P. zostericola and other species reliant on seagrass as habitat.     

Impact of land‐use change to Jatropha bioenergy plantations on biomass and soil carbon stocks: a field study in Mali

Small‐scale Jatropha cultivation and biodiesel production have the potential of contributing to local development, energy security, and greenhouse gas (GHG) mitigation. In recent years however, the GHG mitigation potential of biofuel crops is heavily disputed due to the occurrence of a carbon debt, caused by CO₂ emissions from biomass and soil after land‐use change (LUC). Most published carbon footprint studies of Jatropha report modeled results based on a very limited database. In particular, little empirical data exist on the effects of Jatropha on biomass and soil C stocks. In this study, we used field data to quantify these C pools in three land uses in Mali, that is, Jatropha plantations, annual cropland, and fallow land, to estimate both the Jatropha C debt and its C sequestration potential. Four‐year‐old Jatropha plantations hold on average 2.3 Mg C ha^?1 in their above‐ and belowground woody biomass, which is considerably lower compared to results from other regions. This can be explained by the adverse growing conditions and poor local management. No significant soil organic carbon (SOC) sequestration could be demonstrated after 4 years of cultivation. While the conversion of cropland to Jatropha does not entail significant C losses, the replacement of fallow land results in an average C debt of 34.7 Mg C ha^?1, mainly caused by biomass removal (73%). Retaining native savannah woodland trees on the field during LUC and improved crop management focusing on SOC conservation can play an important role in reducing Jatropha's C debt. Although planting Jatropha on degraded, carbon‐poor cropland results in a limited C debt, the low biomass production, and seed yield attained on these lands reduce Jatropha's potential to sequester C and replace fossil fuels. Therefore, future research should mainly focus on increasing Jatropha's crop productivity in these degraded lands.     

Unusually high‐quality soil seed banks in a Midwestern U.S. oak savanna region: variation with land use history,habitat restoration,and soil properties

An overarching conclusion in the literature is that soil seed banks rarely contain many restoration‐target species and are often liabilities rather than assets to restoration. Our objective was to evaluate composition and spatial variation of seed banks and their potential contributions to restoration, including restoration‐target species such as rare species and those characterizing historical habitats. On 64 sites in a Midwestern U.S. oak savanna landscape, we sampled soil seed banks in seven habitat types (restored oak savannas, oak woodlands, and mesic prairies; unmanaged upland oak and mesic forests; and unmanaged and managed pine plantations). The germinable seed bank was exceptionally rich in restoration‐target species. In total for the 64 sites, seedlings of 127 species emerged from seed bank samples. Of the 101 native species, 56 were restoration‐target species, an unusually high number among seed bank studies. Restoration‐target species in seed banks included 13 threatened or endangered species, in addition to 43 other specialist species associated with high‐quality native habitats or on a floral list thought to characterize historical ecosystems. When analyzed across the 64‐site gradient, seed banks differed among the seven habitat types and varied with historical (1939) land use, recent management activities that restored open‐structured habitats, and biophysical gradients of tree density, soil drainage, and soil texture. While not all restoration‐target species were detected in the seed bank, the unusually high‐quality seed bank is a potential asset to restoration and was partly structured along environmental gradients across the landscape.