植食性哺乳动物觅食功能反应模型机制的检验

植食性哺乳动物在食物密集斑块的觅食为 型功能反应。在新鲜苜蓿叶片构成的食物密集斑块上 ,以高原鼠兔作为实验动物 ,检验植食性哺乳动物的觅食功能反应及其模型机制。食物大小可调节高原鼠兔的口量 ,尽而控制其瞬时摄入率 ;高原鼠兔觅食叶片的口量 S与瞬时摄入率 I存在渐近的函数关系 ,为 型功能反应 ;高原鼠兔的食物收获率 B随口量 S的增加呈非线性递减 ;最大处理速率 Rmax的测定值与模型的预测值极为近似 ;瞬时摄入率 I的测定值与模型的预测值线性回归显著 ( P<0 .0 1 )。研究结果充分验证了提出的假设 :植食性哺乳动物 型功能反应模型能有效地预测其摄入率的动态 ;植食性哺乳动物收获与咀嚼间的竞争能调其收获率和摄入率     

Prey or host searching behavior that leads to a sigmoid functional response in invertebrate predators and parasitoids

Invertebrate predators and parasitoids have long been characterized as having a hyperbolic (Type 2) functional response. Modifications were made to Holling's sand paper disc experiment which consisted of limiting the initial period of search during which a host must be contacted. Failure to contact a host during this initial period causes the predator to emigrate from the search area. The modification generated a sigmoid (Type 3) functional response. This response resulted from the low probability of encountering a host during the initial period of search at low host densities in the time allotted. A limited period of search has been found in several insect parasitoids. Such a strategy would minimize the time (energy) spent per offspring produced by minimizing the time invested in searching microhabitats in which hosts are scarce or absent.     

Increased O-GlcNAc levels during reperfusion lead to improved functional recovery and reduced calpain proteolysis

We have previously shown that preischemic treatment with glucosamine improved cardiac functional recovery following ischemia-reperfusion, and this was mediated, at least in part, via enhanced flux through the hexosamine biosynthesis pathway and subsequently elevated O-linked N-acetylglucosamine (O-GlcNAc) protein levels. However, preischemic treatment is typically impractical in a clinical setting; therefore, the goal of this study was to investigate whether increasing protein O-GlcNAc levels only during reperfusion also improved recovery. Isolated perfused rat hearts were subjected to 20 min of global, no-flow ischemia followed by 60 min of reperfusion. Administration of glucosamine (10 mM) or an inhibitor of O-GlcNAcase, O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc; 200 microM), during the first 20 min of reperfusion significantly improved cardiac functional recovery and reduced troponin release during reperfusion compared with untreated control. Both interventions also significantly increased the levels of protein O-GlcNAc and ATP levels. We also found that both glucosamine and PUGNAc attenuated calpain-mediated proteolysis of alpha-fodrin as well as Ca(2+)/calmodulin-dependent protein kinase II during reperfusion. Thus two independent strategies for increasing protein O-GlcNAc levels in the heart during reperfusion significantly improved recovery, and this was correlated with attenuation of calcium-mediated proteolysis. These data provide further support for the concept that increasing cardiac O-GlcNAc levels may be a clinically relevant cardioprotective strategy and suggest that this protection could be due, at least in part, to inhibition of calcium-mediated stress responses.     

Linking grazing response of species abundance to functional traits in the Tibetan alpine meadow

Whether grazing response of species abundance can be predicted by plant functional trait remains a challenge untested in a specific ecosystem. We measured species abundance and relevant functional traits for 30 common component species in a moderately grazed and a control community over 3 years (2005, 2006 and 2007) in a Kobresia capillifolia dominated alpine meadow on the Tibetan Plateau. Our objective was to examine species response to grazing and to test the relationship between the response of species abundance and functional traits in grazed and control habitats. Our results showed: i) in terms of species relative abundance and saturated height (the maximum height of an adult individual), most component species significantly decrease in response to moderate grazing and the effect differed among species and between functional groups. ii) The response of species abundance was significantly negatively correlated with saturated height, but not correlated with specific leaf area or seed size. We concluded that the response direction of species abundance to grazing can be predicted by functional traits, whereas it is a weak predictor of the extent of grazing response in species abundance.     

A functional assessment of the response of grassland vegetation to reduced grazing and abandonment

Question: Predicting the impact of land‐use change on vegetation is vital to understanding how biodiversity and ecosystem function may respond. Is it correct to assume that abandonment is an extreme form of grazing reduction? Location: Borders and central Scotland. Methods: The analysis used data sets from two identical experiments where the impacts of two unfertilized, extensively grazed treatments and one unfertilized abandoned treatment were compared against the species dynamics of a pasture subject to normal, productive grazing management over a 16‐year period. Initial multivariate analysis using Principal Response Curves was used to assess if particular traits were associated with either extensive or abandoned treatments, and was checked using univariate tests of individual traits. RLQ analysis followed by clustering into response groups was used to assess if species behaved in a similar manner between sites. Results: For many traits/attributes the shift in value or proportion was approximately linear across the extensification treatments as grazing was removed. However, certain traits showed step changes and quadratic responses. Leaf dry matter content, an important effect trait, was in the latter group. Most traits/attributes and species behaved similarly at the two sites. However, traits such as regenerative strategy, seed length, longevity and mass and seed bank type behaved differently, indicating that they are not predictable response traits. Conclusion: The results indicate that responses to grazing removal during extensification are largely straightforward and largely independent of species pool. However, there are discrepancies that suggest that simple analyses of the impacts of land‐use changes such as grazing reduction may hide more complex responses.     

The functional grazing response of a phytoplanktivorous fish Oreochromis mloticus to mixtures of toxic and non-toxicstrains of the cyanobacterium Microcystis aeruginosa

Small (10 g) tilapia ( Oreochromis niloticus ) were exposed to pure and mixed populations of toxic and non-toxic strains of the cyanobacterium Microcystis aeruginosa (100% toxic, 50% toxic, 25% toxic, 0% toxic) at two particle concentrations (1 × 10⁶ and 5 × 10^sparticles ml⁻¹). At both concentrations there was a progressive decrease in grazing rate as the percentage of toxic cells increased. Differences in opercular beat rates, and hence the volumes of water passed over the gills, were also recorded among treatments, opercular beat rates decreasing as the percentage of toxic cells increased. Although in all treatment groups with toxic cells present, the medium had detectable levels (>250 ng I⁻¹) of extracellular microcystin-LR toxin present, grazing was correlated with particle-bound rather than extracellular levels.     

Interactive response of herbivores,soils and vegetation to annual burning in a South African savanna

Abstract Despite decades of research, the primary factors determining savanna structure remain elusive – a conundrum termed ‘the savanna problem’. After 47 years of annual burning in Terminalia woodland and Acacia/Combretum savanna on sandy, granite‐derived soils in the southern Kruger National Park, South Africa, a dense cover of trees and shrubs persists on some burnt plots and is largely absent from others. We postulated that intense browsing pressure by antelope and other herbivores prevents recruitment of trees in burnt plots and that herbivores concentrate on plots that are richest in nutrients. Herbivore abundance did not show a relationship with soil macronutrients and we consequently investigated micronutrient status. The reduction in tree cover as a result of annual burning was positively correlated with mass of herbivores (15–1500 kg) (r ² = 0.61, n = 8). This index of herbivore abundance was in turn positively correlated with total Zn (r ² = 0.64, n = 8). Other indices of herbivore abundance showed significant relationships with total clay content and total Mn. We suggest that herbivores concentrate on sites with greater clay content (possibly due to a greater availability of micronutrients), and that tree cover can remain relatively dense under a regime of annual burning if browsing pressure is not intense. The long‐term burn experiments in the Kruger National Park savanna provide a platform for unravelling the savanna problem. Determining possible interactions between soil properties, herbivory and fire is a step in this direction.     

Stream periphyton response to grazing and changes in phosphorus concentration

Grazing by the large caddisfly larva, Dicosmoecus gilvipes (Trichoptera; Limnephilidae), drastically reduced periphyton biomass in laboratory channels at a current velocity of 20 cm s^–1. Reduction in biomass as chl a and AFDW ranged from 88 to 93% and 82 to 85%, respectively. On average, grazing rate increased with in-channel SRP (soluble reactive phosphorus) content from 6 to 10 µg 1^–1. Grazing rates averaged 25.9–29.3 µg chl a m^–2 d^–1 and 10.8–12.2 µg chl a mg^–1 d^–1 based on area and grazer biomass, respectively, with most variability among treatments being due to the grazing effect. Grazing tended to shift the algal community increasingly to filamentous blue-green algae regardless of enrichment. After three weeks, Phormidium comprised over 61% of the community in grazed treatments but only 35% in ungrazed treatments. The stalked diatom Gomphonema comprised only 4% of the grazed community, but 11% in the three ungrazed channels with similar values for Scenedesmus. A model that includes grazing was calibrated to the data and produced a reasonable expectation of periphyton biomass over a range in SRP concentrations. While the model with constant grazer abundance predicts a gradually increasing grazed biomass as SRP increases, grazer production in natural streams may actually increase to accommodate the increased food production.     

Selection of food patches by sympatric herbivores in response to concealment and distance from a refuge

Small herbivores face risks of predation while foraging and are often forced to trade off food quality for safety. Life history, behaviour, and habitat of predator and prey can influence these trade‐offs. We compared how two sympatric rabbits (pygmy rabbit, Brachylagus idahoensis; mountain cottontail, Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush‐steppe of western North America respond to amount and orientation of concealment cover and proximity to burrow refuges when selecting food patches. We predicted that both rabbit species would prefer food patches that offered greater concealment and food patches that were closer to burrow refuges. However, because pygmy rabbits are small, obligate burrowers that are restricted to sagebrush habitats, we predicted that they would show stronger preferences for greater cover, orientation of concealment, and patches closer to burrow refuges. We offered two food patches to individuals of each species during three experiments that either varied in the amount of concealment cover, orientation of concealment cover, or distance from a burrow refuge. Both species preferred food patches that offered greater concealment, but pygmy rabbits generally preferred terrestrial and mountain cottontails preferred aerial concealment. Only pygmy rabbits preferred food patches closer to their burrow refuge. Different responses to concealment and proximity to burrow refuges by the two species likely reflect differences in perceived predation risks. Because terrestrial predators are able to dig for prey in burrows, animals like pygmy rabbits that rely on burrow refuges might select food patches based more on terrestrial concealment. In contrast, larger habitat generalists that do not rely on burrow refuges, like mountain cottontails, might trade off terrestrial concealment for visibility to detect approaching terrestrial predators. This study suggests that body size and evolutionary adaptations for using habitat, even in closely related species, might influence anti‐predator behaviors in prey species.     

Increased competition does not lead to increased phylogenetic overdispersion in a native grassland

That competition is stronger among closely related species and leads to phylogenetic overdispersion is a common assumption in community ecology. However, tests of this assumption are rare and field‐based experiments lacking. We tested the relationship between competition, the degree of relatedness, and overdispersion among plants experimentally and using a field survey in a native grassland. Relatedness did not affect competition, nor was competition associated with phylogenetic overdispersion. Further, there was only weak evidence for increased overdispersion at spatial scales where plants are likely to compete. These results challenge traditional theory, but are consistent with recent theories regarding the mechanisms of plant competition and its potential effect on phylogenetic structure. We suggest that specific conditions related to the form of competition and trait conservatism must be met for competition to cause phylogenetic overdispersion. Consequently, overdispersion as a result of competition is likely to be rare in natural communities.     

Morphological and compositional changes in a planktonic bacterial community in response to enhanced protozoan grazing

We analyzed changes in bacterioplankton morphology and composition during enhanced protozoan grazing by image analysis and fluorescent in situ hybridization with group-specific rRNA-targeted oligonucleotide probes. Enclosure experiments were conducted in a small, fishless freshwater pond which was dominated by the cladoceran Daphnia magna. The removal of metazooplankton enhanced protozoan grazing pressure and triggered a microbial succession from fast-growing small bacteria to larger grazing-resistant morphotypes. These were mainly different types of filamentous bacteria which correlated in biomass with the population development of heterotrophic nanoflagellates (HNF). Small bacterial rods and cocci, which showed increased proportion after removal of Daphnia and doubling times of 6 to 11 h, belonged nearly exclusively to the beta subdivision of the class Proteobacteria and the Cytophaga-Flavobacterium cluster. The majority of this newly produced bacterial biomass was rapidly consumed by HNF. In contrast, the proportion of bacteria belonging to the gamma and alpha subdivisions of the Proteobacteria increased throughout the experiment. The alpha subdivision consisted mainly of rods that were 3 to 6 microm in length, which probably exceeded the size range of bacteria edible by protozoa. Initially, these organisms accounted for less than 1% of total bacteria, but after 72 h they became the predominant group of the bacterial assemblage. Other types of grazing-resistant, filamentous bacteria were also found within the beta subdivision of Proteobacteria and the Cytophaga-Flavobacterium cluster. We conclude that the predation regimen is a major structuring force for the bacterial community composition in this system. Protozoan grazing resulted in shifts of the morphological as well as the taxonomic composition of the bacterial assemblage. Grazing-resistant filamentous bacteria can develop within different phylogenetic groups of bacteria, and formerly underepresented taxa might become a dominant group when protozoan predation is the major selective pressure.     

Patterns in grass silicification: response to grazing history and defoliation

Summary Morphologically distinct populations of a North American perennial grass, Agropyron smithii, collected from a heavily grazed prairie dog (Cynomys ludovicianus) colony (PDC) and a grazing exclosure (EX), were grown in an environmental chamber to determine whether: (1) leaf silicon (Si) concentrations are greater in plant populations which differentiated under heavy grazing pressure, and (2) leaf silicification is inducible by defoliation. Mean shoot Si concentration of nondefoliated plants was greater in the PDC population (2.2%) than the EX population (1.9%) over the 18 wk experiment, largely as a result of differences in Si concentrations in leaf blades. However, leaf Si concentration was lower in defoliated plants of each population than in nondefoliated plants, indicating that leaf silicification was not an inducible herbivore defense mechanism in A. smithii. The higher leaf Si concentrations from the heavily grazed population may be associated with grazingrelated environmental stresses such as a warmer, drier microclimate or with morphological characteristics related to grazing tolerance or avoidance.     

Phytoplankton responses to experimental enhancement of grazing pressure and nutrient recycling in a small Andean lake

1. Three series of field experiments with different zooplankton species composition and biomass were performed in a small lake in the south Andes. We attempted to measure the responses of phytoplankton species resulting from grazing mortality and stimulation of growth by nutrient recycling.
2. Nanoflagellates contributed substantially to total phytoplankton cell abundance. Chrysochromulina parva represented 93.4%, 92.2% and 95.9% of total phytoplankton density in December, January and February, respectively. This fraction was reduced in all treatments with increasing zooplankton biomass.
3. A negative relationship was obtained between C. parva cell numbers and increase in dissolved P. On the other hand, a significant positive relationship between the abundance of the diatom Aulacoseira granulata and P concentration was observed. These results indicate that the ungrazed diatom was able to capitalise on the increase in nutrient availability.
4. As a net result of the increase or decrease of algal species we observed a change in the nano:net phytoplankton relationship. The outcome of three‐day incubations with increased zooplankton biomass was an increasing importance of net phytoplankton.
5. The results indicate the importance of the indirect effects of zooplankton (through nutrient recycling) in the increase in diatoms, and the role of grazing as a growth‐limiting factor for the flagellate C. parva .