Effects of grazing intensity on plant richness and diversity: a meta‐analysis

Most of our knowledge of the effect of grazing on grassland structure is based on grazed–ungrazed contrasts. The effects of grazing in the most common scenario, where grazing intensity varies from low to high grazing intensity, are less known. The objectives of this paper were to 1) quantify the effect of stocking rates on species richness and diversity of grasslands world‐wide, and 2) evaluate the response under different environmental and experimental conditions. We conducted a meta‐analysis of experiments with at least two levels of controlled stocking rates and evaluated their effect on species richness and diversity. The results showed that the response of richness and diversity to either reducing or increasing stocking rate from a moderate level mostly fell within the range  25% or  5 species. Mean response of species richness and diversity to increasing stocking rate from moderate to high levels was negative. Mean response to lowering stocking rate from moderate levels was not different from zero. However, overall, species richness significantly decreased as stocking rate increased. The response of richness and diversity to stocking rate was not related to mean precipitation, productivity or aridity. However, the most negative responses of richness to stocking rate were larger in arid, low productivity systems than in subhumid and humid systems. The effects of grazing on richness and diversity found in this review were smaller than the effects on species composition shown by the literature. Thus, grazing drastically changes species composition, but the net change of species and diversity is much smaller.     

Frequency-dependent seed size selection on Cryptocarya alba (Mol.) Looser (Lauraceae): testing the effect of background

Seed size affects the probability of seed predation. Large seeds should be preferred over small seeds but the selective responses of seed predators may also be frequency-dependent. Seed predators may prefer the most common seed sizes (apostatic selection), the rarest ones (antiapostatic selection) or even be unresponsive to the size of seeds. Moreover, seed density may further modify the selective responses of seed predators. We expect that at a low seed density seed consumption should be concentrated on common seed sizes, and at high seed density it should be concentrated on rare seed sizes, as common ones act as a background that makes rare phenotypes more conspicuous (the effect of background). We tested this prediction in a field experiment with seeds of Cryptocarya alba (Lauraceae) at La Campana National Park, central Chile. We presented large and small seeds in two contrasting seed densities (ten and 100 seeds per m²) and at five frequencies of large seeds: 0.1, 0.25, 0.5, 0.75 and 0.9. After 28 days we measured the proportions of the two size classes in the remaining seeds. Large seeds were always preferred to small seeds, even when they were at low frequency. Contrary to the predictions, at low seed density consumption was antiapostatic whereas at high seed density selection was independent of frequency. We discuss the causes and consequences of such selective responses expressed by seed predators.  © 2005 The Linnean Society of London , Biological Journal of the Linnean Society , 2005, 84 , 137–142.     

Long-Term Effects of Biosolids on Revegetation of Disturbed Sagebrush Steppe in Northwestern Colorado

A study was conducted to evaluate the long‐term effects of biosolids amendment on restoration of disturbed sagebrush steppe habitat in northwestern Colorado. Twenty‐four years after biosolids amendment, soil fertility and plant community development were studied in replicated plots receiving various biosolids amendments on two different substrates. The two substrates used were a subsoil, determined to have low initial fertility, and a topsoil over retorted shale substrate, determined to have relatively high initial fertility. Results suggest that biosolids amendments have long‐lasting effects on soil fertility and plant community composition, but these effects vary between the two substrates that were utilized. Within the plots established on subsoil, the long‐term effect of biosolids was a reduction in plant species diversity and dominance by perennial grasses. On the topsoil substrate, there was a decrease in perennial grasses and an increase in shrub dominance with increasing biosolids. Results demonstrate the importance of considering initial soil conditions, seed mixture, and biosolids application rate when using biosolids for restoration of disturbed sagebrush steppe habitat. The long‐term effects of the biosolids treatments at this site demonstrate the need to consider restoration treatment effects over longer and more ecologically meaningful time frames.     

Do Birds Alter Levels of Parental Investment Based on Nest‐Site Habitat Features?

Many habitats have undergone rapid changes over the past century as a result of anthropogenic activities. Organisms can respond to changes in their environment in many ways, including how much they invest in a given reproductive bout. Optimality theory and life history theory together have provided a framework for understanding how individuals make decisions about the risks and rewards of investing high levels of resources into their offspring within the context of current vs. future reproduction. Shrubland birds in the United States often breed in habitat that has been impacted by humans to varying degrees and provide an ideal system for examining whether organisms alter their levels of parental investment based on the habitat around the nest site. In this study, we assessed levels of nest‐defense behavior during a simulated nest predation event in four shrubland birds: American robins (Turdus migratorius), gray catbirds (Dumetella carolinensis), brown thrashers (Toxostoma rufum), and northern cardinals (Cardinalis cardinalis). We found that thrashers and catbirds exhibited significantly higher levels of nest defense compared to robins and cardinals and that birds with larger broods acted more aggressively in defending them. We also found that thrashers acted less aggressively as the proportion of shrubland around the nest increased and as the proportion of developed land around the nest decreased and that cardinals acted less aggressively as the proportion of developed land around the nest increased. These patterns suggest that nest‐defense behavior can covary with features of the landscape surrounding the nest, but that the factors shaping that behavior can differ by species.     

Distribution and species diversity patterns of Vitex negundo var. heterophylla shrublands in North China

生境差异是影响植物群落及其物种多样性分布格局的重要因素。该研究基于370个荆条(Vitex negundo var. heterophylla)灌丛的野外调查样方, 结合多元回归树(MRT)、显著性分析、Pearson相关分析等技术手段, 分析了华北地区荆条灌丛及其物种多样性的空间分布格局。结果表明: 荆条灌丛的经纬度均跨越10°以上, 可分为高、中、低纬度分布区; 分布海拔则从117 m至1 248 m跨越了1 000 m以上, 也可分为高、中、低海拔分布区。所有分布区中荆条灌丛草本层的Shannon-Wiener指数、Simpson指数和均匀度指数均高于灌木层。灌木层物种多样性随着纬度的降低而降低, 随着海拔的升高而降低; 草本层相反, 物种多样性随着纬度降低而升高, 随着海拔升高而升高。灌木层和草本层的物种丰富度(Shannon-Wiener指数)与水、热、地形等环境因子显著相关, 其中灌木层物种多样性随着气温、坡位升高而显著减少, 随着降水的增加而显著增加;草本层则相反。     

Fire and resprouting in Mediterranean ecosystems: insights from an external biogeographical region, the mexical shrubland

We investigated modes of regeneration of dominant species of the mexical vegetation after fire. The mexical shrubland shows a remarkable structural, morphological, and floristic similarity to Mediterranean-type vegetation and is considered a relict of the Madro-Tertiary Geoflora under a non-Mediterranean climate. This vegetation provides an ideal scenario to test the role of fire in Mediterranean ecosystems because historical fire occurrence is absent and the species assembly is constituted mostly by Madro-Tertiary elements and Neotropical species (some of them, endemic species from Mexico). The existence of congeneric species of the California chaparral allows us to determine the regeneration ability of these communities after fire in relation to resprouting and seeding strategies, which are widespread modes reported in the Mediterranean-type vegetation. By the experimental application of fire in the two biogeographical groups of species, we tested the hypothesis that low resprouting ability of California congeneric species (Madro-Tertiary species) after fire would indicate that fire has played an important selective force in the resprouting habit. A low resprouting ability in the Neotropical group of species would suggest that fire has molded the set of species dominating fire-prone environments.Our results indicated that resprouting is a widespread trait in the mexical species characterized by the presence of lignotubers and burls. Resprouting can be considered an ancient trait, probably linked to losses of aboveground biomass, that became a pre-adaptation in Mediterranean fire-prone communities. The Neotropical group of species showed less ability to regenerate after fire, and small plants were more likely to die after disturbance in this group than in the Madro-Tertiary group. The resprouting feature and the seeder strategy of other species after a fire in the mexical shrubland are similar to Mediterranean-type ecosystems, emphasizing their common origin and the relevance of phylogenetic and biogeographical studies to explain current patterns of vegetation.     

Female biased sex ratios in Pistacia lentiscus L. (Anacardiaceae)

Sex ratios of populations of the dioecious shrub Pistacia lentiscus L. (Anacardiaceae) were studied. Several hypotheses concerning biased sex ratios were tested. The expected pattern of male preponderance in stressful habitats was not found. The populations located in a microclimatic gradient, such as a slope, did not display a male-biased sex ratio on the stressful middle slope. The populations located in a climatic gradient did not display a male-biased sex ratio in the more xeric habitats. Testing the hypothesis of female preponderance when pollen grain competition exists, we found a significant correlation in the direction opposite to that predicted by this hypothesis. Low density of individuals (an estimate of pollen density) correlates with a high preponderance of females but the sex ratio approaches 1:1 when density increases. This correlation should have an upper threshold in 1:1 because male-biased sex ratios have never been found.     

增温对窄叶鲜卑花高寒灌丛生物量碳和氮分配的影响

文章以青藏高原东部窄叶鲜卑花灌丛为研究对象,采用开顶式生长室(OTCs)模拟增温实验(+1.2℃),分析增温对灌木层和草本层各器官碳、氮分配的影响及其影响因素,以揭示青藏高原东部高寒灌丛灌木层和草本层碳、氮分配对增温的响应策略。结果显示:(1)增温使窄叶鲜卑花灌木层叶、粗根、细根碳库显著增加18.8%、7.7%和139.4%,使灌木层细根氮库显著增加153.9%;增温使草本层地上和地下部分碳库显著增加60.4%和130.5%,使草本层地上和地下部分氮库显著增加46.1%和124.0%。(2)增温使灌木层茎和粗根碳分配比例显著降低18.9%和16.2%,使灌木层叶、茎和粗根氮分配比例显著降低25.2%、23.3%和14.4%,使灌木层细根碳、氮分配比例显著增加86.5%和96.2%;增温使草本层地上部分碳、氮分配比例显著降低19.5%和18.9%,使草本层地下部分碳、氮分配比例显著增加15.6%和24.8%。(3)Pearson相关分析和多元线性回归分析结果表明,空气温度和土壤微生物生物量是影响灌木层地上碳、氮分配的主要因子,能解释其变异的72.0%以上;土壤温度、土壤有机碳含量和土壤脲酶活性是影响灌木层地下碳、氮分配的主要因子,能解释其变异的92.0%以上;土壤有机碳含量、土壤转化酶和脲酶活性是影响草本层地上和地下碳、氮分配的主要因子,能解释其变异的92.8%以上。(4)土壤氮素有效性对灌木层和草本层生物量碳、氮分配的影响不显著。研究表明,气候变暖情景下,青藏高原东部窄叶鲜卑花高寒灌丛的灌木层和草本层植物通过提高地下部分碳、氮分配,进而更好地适应外界环境温度的提高。