Microbiological study of the body wall lesions of the echinoid Tripneustes gratilla

The microbiota of the body wall lesions of the echinoid Tripneustes gratilla, initiated by the grazing action of the parasitic gastropod Vexilla vexillum, was investigated with a pluridisciplinary approach. Parasitised sea urchins showed body wall lesions strongly infected by bacteria that progressed through the test and reached the coelomic cavity after ca. 1 mo. We report here on the bacterial community observed in lesions of echinoids collected in situ and on the bacteria that successively appeared during laboratory experiments. Two Alphaproteobacteria, a CFB (Cytophaga-Flavobacterium-Bacteroides) bacterium, 3 Vibrio species and Exiguobacterium aestuarii were identified in the field-collected lesions by 16S rDNA sequencing. The last 4 bacteria were cultured and each induced the disease when inoculated on scalpel-made wounds, with 100% of the individuals infected within 2 d. Scalpel-induced scarifications tended to heal within 3 wk, while gastropod-induced lesions evolved into disease, suggesting a role of Vexilla vexillum in the development of the infection. Denaturing gradient gel electrophoresis (DGGE) and sequencing suggest that (1) bacteria associated with healthy integument were not present in the lesions and were thus not responsible for their infection, (2) Alphaproteobacteria with close phylogenetic affiliation with other bacteria involved in several diseases affecting marine invertebrates were present, and (3) these Alphaproteobacteria were present from the beginning of the infection and appeared earlier in the infection than other bacteria such as CFB bacteria.     

Echinoid grazing as a structuring force in coral communities: Whole reef manipulations

To examine the effects of echinoid grazing on shallow water coral community structure, the entire Diadema antillarum Philippi population (> 3000 individuals) was eliminated from a patch reef in Discovery Bay, Jamaica W.I. in January 1974. All remaining regular echinoids, notably including Echinometra viridis A. Agassiz (> 7000 individuals), were removed from one-half of the same reef 6 months later.Where Echinometra was present, its grazing was highly patchy, even at densities of up to 50/m². On the other hand, the effects of grazing by Diadema were more uniformly distributed at equivalent or lower densities.Percent-cover of adult corals was enhanced in the presence of Echinometra viridis, however, it was greatly reduced in the absence of all echinoid grazing as a result of massive overgrowth by algae. Success of coral recruitment (density of coral spat) increased in the absence of Diadema due to the alleviation of biological disturbance imposed by this urchin's feeding activities. The presence of Echinometra viridis, even at the high densities naturally present on the reef, did not deter coral settlement.Coral diversity was highest in the presence of all echinoids, intermediate in the presence of E. viridis (without Diadema), and lowest in the absence of all echinoids. This was due to the disproportionately successful recruitment and resultant high relative abundance of Agaricia at low echinoid densities. Favia, Millepora, Helioseris and Acropora all exhibited universally low recruitment levels. Predation by Diadema and Echinometra is responsible for damaging larger coral colonies, particularly Porites.As echinoid species composition and density was altered, the species composition of benthic algae and associated invertebrates shifted. In this way, the array of species competing for space with coral spat shifted as did their competitive relationships. Major epifaunal competitors included spirorbid polychaetes, various foraminifera (particularly Gypsina), and sponges. Algal competitors included filamentous chlorophytes, corallines (e.g. Jania), and numerous other red algae; the species composition of algal competitors shifted through time. Competitive success in corals varied as echinoid density changed and was species-specific with respect to coral genus. Interspecific competition between coral spat (i.e., interspecific aggression via extracoelenteric digestion) was negligible under all treatments.The presence of Echinometra viridis in high densities (in the absence of Diadema) created conditions where growth and fusion in Agaricia spat were optimized. Under these conditions, fusion allows a young coral colony to attain a large enough size to survive damage incurred from grazing or from competitive overgrowth, decreasing the probability of whole-colony mortality during the early stages of development.     

Local and regional scale recovery of Diadema promotes recruitment of scleractinian corals

The phase change from coral to macroalgal dominance on many Caribbean reefs was exacerbated by the mortality of the echinoid Diadema antillarum in 1983–1984, and until recently, this sea urchin has remained rare on reefs throughout the western Atlantic. By the late 1990s, Diadema started to reappear in large numbers on some Jamaican reefs, and by 2000, the high densities were correlated with significantly greater abundances of juvenile corals. Here, we show that dense populations of Diadema now occur over a multi-kilometre-wide scale at six locations scattered along a 4100 km arc across the entire Caribbean. In all cases, these dense populations are found in shallow water (< 6 m depth) on outer reef communities and are associated with reduced macroalgal cover and enhanced coral recruitment. We conclude that population recovery of Diadema is occurring at both local and regional scales, and that grazing by this echinoid is creating conditions favouring the recruitment of corals.     

Effects of grazing by Diadema antillarum Philippi (Echinodermata: Echinoidea) on algal diversity and community structure

In Discovery Bay, Jamaica W.I., densities of Diadema antillarum Philippi were experimentally manipulated over a range of 0–64 individuals · m^?2 with the aid of enclosures in shallow water. The effects of grazing by this echinoid on the algal community structure of a coral reef were compared under conditions of primary and post-primary succession.Algal biomass decreased as Diadema density was increased, particularly under primary succession conditions. A similar response was noted under post-primary conditions, but here biomass was also dependent on species composition and abundance of algae prior to experimental alterations of Diadema density. Peyssonnelia sp. was found to be particularly resistant to echinoid grazing, surviving even the most intense grazing. Within 11 months, algal diversity decreased significantly in an exponential fashion as Diadema density increased. This occurred under conditions of both primary and post-primary succession. Diversity, irrespective of index, was not maximized at intermediate grazing pressures. No single species of alga dominated the benthic community at even the lowest Diadema densities.Species composition of algae in the benthic community prior to changes in grazing pressure and lack of successful recruitment of a potential dominant were found to have a major influence on the response of the algal community to the treatments. Each of these factors can influence both the relationship between algal diversity and grazing pressure and the rate at which the community approaches an initial plateau of diversity in its earliest stages of succession.     

Spatial variation in the demography and population dynamics of a perennial shrub (Atriplex vesicaria) under sheep grazing in semi‐arid Australian rangelands

Atriplex vesicaria Heward ex Benth. (Chenopodiaceae) is a widespread perennial shrub in southern Australia's chenopod rangelands but is sensitive to grazing. A detailed investigation of the demography and population dynamics of A. vesicaria under sheep grazing was conducted over 6.5 years at a range of sites across a typical paddock to assess the long‐term effects of grazing on the species and elucidate the mechanisms of population change under grazing. The effects of rainfall on recruitment and mortality were also examined. Six‐monthly censuses of all A. vesicaria individuals were conducted in permanent grazed and ungrazed plots at sites located across an 1100‐ha paddock. Grazing increased adult shrub mortality close to water and reduced recruitment over a broader area of the paddock, but seedling survival did not appear to be affected by grazing. As a result of these changes, the population declined on grazed plots up to 2200 m from water during the study, but the decline was greatest closer to water. The population was most dynamic at the sites furthest from the water point where it was unaffected by grazing because of the greater recruitment and mortality of young plants, but because these processes balanced out over time, population density was effectively unchanged by the end of the study. Although statistical models indicated that six‐monthly rainfall did not explain temporal variation in recruitment or mortality, rainfall nevertheless has a central role in both processes. In particular, longer periods of favourable rainfall and drought appear to have an important influence on recruitment and mortality, respectively, with heavy grazing during a drought period increasing mortality. Occasional shortages of seed or rains occurring during the warmer months when seed germination is limited possibly explain poor recruitment at sites unaffected by grazing following good rainfall.     

Brood‐parasite‐induced female‐biased mortality affects songbird demography: negative implications for conservation

Parasites, of all sorts, can profoundly affect host population dynamics. Parasites commonly cause sex‐biased mortality and this can add to their impact. Female‐biased mortality in particular can destabilize dynamics and promote population collapse. We previously reported in a correlative study that brown‐headed cowbird Molothrus ater brood parasitism of song sparrows Melospiza melodia appears to cause female‐biased host nestling mortality. Here, we report results from ‘infestation’ and ‘de‐infestation’ experiments designed to test whether brood parasitism causes female‐biased mortality, and we document the resulting demographic impact using a simulation model. Experimental cowbird infestation of song sparrow nests halved the proportion of female host nestlings (0.31±0.07 vs 0.59±0.06; infested vs unparasitized nests at day 6) replicating the halving reported in naturally cowbird‐parasitized nests (0.28±0.01 vs 0.57±0.05; parasitized vs unparasitized). De‐infestation of naturally cowbird‐parasitized nests in turn wholly eliminated any effect on the proportion of female host nestlings (0.53±0.13 vs 0.54±0.06; de‐infested vs unparasitized) confirming that brood parasitism is the cause. This halving of the proportion of females fledging is likely to be as significant as nest predation in affecting population dynamics, based on the elasticities derived from our demographic model (–0.50 vs –0.59). Experimental infestation reduced the testosterone levels, begging behaviour, and body mass of six day old female host nestlings, whereas males were largely unaffected, suggesting that it is the exacerbation of intra‐brood competition that may be primarily responsible for the resulting female‐biased mortality. The brown‐headed cowbird is invasive in most of North America and has been implicated in regional population declines of many native species. We suggest that female‐biased host offspring mortality is likely to be commonplace among the 144 host species the cowbird successfully parasitizes, and we discuss the negative implications for songbird conservation, given the projected demographic impact.     

Ectoparasites and age-dependent survival in a desert rodent

Host age is one of the key factors in host–parasite relationships as it possibly affects infestation levels, parasite-induced mortality of a host, and parasite distribution among host individuals. We tested two alternative hypotheses about infestation pattern and survival under parasitism in relation to host age. The first hypothesis assumes that parasites are recruited faster than they die and, thus, suggests that adult hosts will show higher infestation levels than juveniles because the former have more time to accumulate parasites. The second hypothesis assumes that parasites die faster than they are recruited and, thus, suggests that adults will show lower infestation levels because of acquired immune response and/or the mortality of heavily infested juveniles and, thus, selection for less infested adults. As the negative effects of parasites on host are often intensity-dependent, we expected that the age-related differences in infestation may be translated to lower or higher survival under parasitism of adults, in the cases of the first and the second hypotheses, respectively. We manipulated ectoparasite numbers using insecticide and assessed the infestation pattern in adult and juvenile gerbils (Gerbillus andersoni) in the Negev Desert. We found only a partial support for age-dependent parasitism. No age-related differences in infestation and distribution among host individuals were found after adjusting the ectoparasite numbers to the host’s surface area. However, age-related differences in survival under parasitism were revealed. The survival probability of parasitized juveniles decreased in about 48% compared to unparasitized hosts while the survival probability of adults was not affected by ectoparasites. Our results suggest that the effect of host age on host–parasite dynamics may not explicitly be determined by age-dependent differences in ectoparasite recruitment or mortality processes but may also be affected by other host-related and parasite-related traits.     

内蒙古典型草原群落实生苗动态对刈割和放牧干扰的响应

该文通过刈割和放牧样地的实生苗动态观测,试图揭示刈割和放牧干扰对冰草(Agropyron cristatum)、克氏针茅(Stipa krylovii)和糙隐子 草(Cleistogenes squarrosa )等 3种禾草实生苗出生、存活和死亡季节动态的影响,以及这种效应是如何受气温和降水影响的。利用有重复测 量的方差分析,比较了不同刈割留茬高度(0、5和15 cm)与放牧处理之间在实生苗密度、死亡率和出现率的时间动态方面的差异。结果表明：3 种禾草的实生苗密度自5月下旬至8月中旬一直呈增加态势。刈割在一定程度上抑制了克氏针茅的实生苗出现率,原因可能是相对高大的克氏针 茅在前一年刈割时会被去掉相当多的生殖部分,其种子数量必然受到影响,导致实生苗出现率的降低。同时,刈割对相对低矮的糙隐子草实生 苗有正向效应 ,即促进其实生苗的出现,可能的原因是刈割使植被高度降低、植被稀疏,植被冠层以下至地面的光照较好、地温较高,同时位 于下层的糙隐子草受到的高大植株的竞争压力减小。生长季早期和晚期实生苗死亡率较高,这与这两个时期的干旱和低温有关,而自6月中旬至 7月初之间的死亡率小高峰也是先前的干旱少雨导致的。3种禾草实生苗的大规模出现几乎都在5～7月,这与该时期相对适宜的气温、地温和水 分条件有关。对实生苗更新相关过程 (如种子萌发、休眠、种子库存量等)进行更加深入细致的研究,将进一步揭示种子生产和有性繁殖在群落 过程中的作用。     

紫黑翼蚌钩介幼虫寄主鱼的人工筛选及寄生包囊形成观察

选取16种鱼类, 对紫黑翼蚌(Potamilus alatus)钩介幼虫寄主鱼进行人工筛选试验, 结果表明: 仅在淡水石首鱼(Aplodinotus grunniens)上获得变态发育的稚蚌, 寄生变态率为(49.6±9.4)%, 但是过量寄生将导致淡水石首鱼的死亡。除眼斑拟石首鱼(Soiaenops ocellatus)外, 寄生在其他14种淡水鱼类均不可能实现钩介幼虫的变态。进一步扫描电镜观察显示: 钩介幼虫可在与淡水石首鱼同科的眼斑拟石首鱼鳃丝寄生并形成完整的包囊, 且在大规格鱼体形成包囊的速度明显快于幼鱼。但在寄生和淡水低渗双重胁迫下, 眼斑拟石首鱼出现极高的死亡率, 提示应进一步开展淡化驯养以提高眼斑拟石首鱼寄生后的成活率。其中黄颡鱼(Pelteobagrus fulvidraco)、小口黑鲈(Micropterus dolomieu) 、蓝鲶(Ictalurus furcatus)寄生后1—2d内脱落的幼虫几乎全部死亡, 推测3种鱼体中可能存在紫黑翼蚌幼虫的致死因子。综合研究表明: 紫黑翼蚌是目前发现对寄主鱼选择性最为专一的蚌类, 而眼斑拟石首鱼具有作为替代寄主鱼的潜能。     

内蒙古典型草原群落实生苗动态对刈割和放牧干扰的响应

 该文通过刈割和放牧样地的实生苗动态观测,试图揭示刈割和放牧干扰对冰草(Agropyron cristatum)、克氏针茅(Stipa krylovii)和糙隐子 草(Cleistogenes squarrosa )等 3种禾草实生苗出生、存活和死亡季节动态的影响,以及这种效应是如何受气温和降水影响的。利用有重复测 量的方差分析,比较了不同刈割留茬高度(0、5和15 cm)与放牧处理之间在实生苗密度、死亡率和出现率的时间动态方面的差异。结果表明：3 种禾草的实生苗密度自5月下旬至8月中旬一直呈增加态势。刈割在一定程度上抑制了克氏针茅的实生苗出现率,原因可能是相对高大的克氏针 茅在前一年刈割时会被去掉相当多的生殖部分,其种子数量必然受到影响,导致实生苗出现率的降低。同时,刈割对相对低矮的糙隐子草实生 苗有正向效应 ,即促进其实生苗的出现,可能的原因是刈割使植被高度降低、植被稀疏,植被冠层以下至地面的光照较好、地温较高,同时位 于下层的糙隐子草受到的高大植株的竞争压力减小。生长季早期和晚期实生苗死亡率较高,这与这两个时期的干旱和低温有关,而自6月中旬至 7月初之间的死亡率小高峰也是先前的干旱少雨导致的。3种禾草实生苗的大规模出现几乎都在5～7月,这与该时期相对适宜的气温、地温和水 分条件有关。对实生苗更新相关过程 (如种子萌发、休眠、种子库存量等)进行更加深入细致的研究,将进一步揭示种子生产和有性繁殖在群落 过程中的作用。     

Burning and Grazing Management in a California Grassland: Growth, Mortality, and Recruitment of Nassella pulchra

Abstract Annual grasslands in California are often managed with seasonal grazing and prescribed burning on the assumption that such practices have long‐term benefits for native species. Mature native perennial bunchgrasses, particularly Nassella pulchra (purple needlegrass), are often the focal species, although very little is known about responses at different life history stages. Thus, important questions remain about long‐term population dynamics of both mature plants and seedling recruitment. In plots receiving repeated grazing and burning events over 7 years, mortality of mature plants was threefold higher on mounds than on intermounds and likely reflected increased competition intensity associated with increased resource availability in deeper soil. Burning and grazing treatments had strong positive effects on basal area of mature N. pulchra. However, plants in grazed plots that were not burned contained considerable standing dead biomass. Topographic location strongly influenced growth as intermound plants grew relatively more than mound plants, but the effects on growth of burning and grazing did not vary with topographic location. In mapped plots N. pulchra recruitment was very low, and overall density dropped an average of 31%. However, a significant time‐by‐burning effect indicated that survival was significantly higher in burned plots. After 7 years of repeated treatments, effects of burning and grazing management on mature N. pulchra were positive but not for all phenological stages. Understanding long‐term influence of management on bunchgrass populations may not be easy to determine because short‐term results may not reflect long‐term responses and some life cycle dynamics may be observed only over very long periods.     

Clonal regeneration of an arrow bamboo, <Emphasis Type="Italic">Fargesia qinlingensis</Emphasis>, following giant panda herbivory

Characteristics of giant panda herbivory sites and clonal regeneration of an arrow bamboo Fargesia qinlingensis following giant panda grazing were studied in the Qinling Mountains of China. Three types of plots were located in a pandas’ summer habitat in 2002: herbivory (naturally grazed by giant pandas), clipped (simulated panda herbivory), and control. Average area of herbivory sites was 2.92 m² and average distance from herbivory sites to the closest tree (dbh > 10 cm) was 1.0 m. Pandas avoided thin bamboo culms with basal diameters <5 mm. Average height of stumps of culms grazed by panda was 0.67 m and average density of grazed culms was 9.0 culms m⁻². Annual culm mortality rate was significantly greater in herbivory and clipped plots than in control plots while annual recruitment rate was not significantly different among the three plot types in 2003. Neither recruitment rate nor mortality rate were significantly different among the three plot types in 2004. Annual recruitment rate was significantly greater than annual mortality rate only in control plots in both 2003 and 2004, suggesting static ramet dynamics in disturbed plots (herbivory and clipped). Density of new shoots was not significantly different, but basal diameter of new shoots was significantly less in herbivory plots compared to control plots in 2002. Differences of annual mortality rate and growth of new shoots found between control plots and herbivory plots suggest that clonal regeneration of F. qinlingensis culms was negatively affected by giant panda grazing. Therefore, no evidence of a clonal integration compensatory response to panda herbivory was found in F. qinlingensis.     

Competition,facilitation or mediation via host? Patterns of infestation of small European mammals by two taxa of haematophagous arthropods

1. We studied the effect of flea infestation on the pattern of tick (Ixodes ricinus and Ixodes trianguliceps) infestation on small mammals. 2. We asked (1) whether the probability of an individual host being infested by ticks was affected by its infestation of fleas (number of individuals and species) and (2) whether the abundance and prevalence of ticks in a host population was affected by the abundance, prevalence, level of aggregation, and species richness of fleas. 3. The probability of a host individual being infested by ticks was affected negatively by flea infestation. At the level of host populations, flea abundance and prevalence had a predominantly positive effect on tick infestation, whereas flea species richness had a negative effect on tick infestation. 4. The effect of flea infestation on tick infestation was generally greater in I. ricinus than in I. trianguliceps, but varied among host species. 5. It can be concluded that the effect of fleas on tick infestation of small mammals may be either negative or positive depending on the level of consideration and parameters involved. The results did not provide support for direct interactions between the two ectoparasite taxa, but suggested population and community dynamics and the defence system of the hosts as possible factors.     

Direct and indirect effects of grazing constrain shrub encroachment in semi‐arid Patagonian steppes

Question: What are the long‐term effects of grazing exclusion on the population structure and dynamics of, and interactions among, three dominant shrub species? Location: Grass‐shrub Patagonian steppe, Chubut, Argentina. Methods: Permanent plots were established in grazed paddocks and paddocks excluded from grazing in representative Patagonian rangelands. Shrub abundance, population size‐structure, short‐term (two 3‐yr periods) and long‐term (matrix models) population dynamics, and neighborhood interactions of three native and codominant shrub species (Mulinum spinosum, Senecio filaginoides and Adesmia volckmanni) were measured and analysed using different statistical approaches. Results: The total density of shrubs was 74% higher in paddocks excluded from grazing, owing mainly to increases in Mulinum (80%) and Senecio (68%) species. However, differences in size structure between ungrazed and grazed paddocks were only detected in Mulinum. Demographic rates differed between shrub species, time‐periods and grazing conditions. In particular, recruitment in the short term (especially in wet years) and population growth rate in the long term (λ) were higher in paddocks excluded from grazing only in Mulinum populations. Senecio populations showed a marginal increase in recruitment and mortality independent of the grazing condition in the wet and dry period. Grazing exclusion modified the balance of neighborhood interactions among the three shrub species. In grazing‐exclusion paddocks, there was a balance between positive and negative interspecific interactions, while in grazed paddocks there were more negative intraspecific and interspecific interactions, resulting in a net negative balance of neighborhood interactions. Conclusions: Our understanding of woody encroachment in arid rangelands can be informed through evaluation of direct and indirect effects of grazing exclusion on the abundance and demography of dominant woody species. In Patagonian arid steppes, the occurrence of woody encroachment in rangelands excluded from grazing can be explained by altered responses in plant‐animal and plant‐plant interactions among shrub species.     

Patterns of shell penetration by Chorus giganteus juveniles (Gastropoda: Muricidae) on the mussel Semimytilus algosus

Patterns are described for shell penetration by the sublittoral muricid snail Chorus giganteus during predatory attacks on the mussel Semimytilus algosus. Location, form and size of shell penetrations were observed in relation to the size of the predator. The results suggested that positions of the perforations on the mussel shells were related to size of the attacking snail. Smaller snails perforated areas near the shell ligament and in the central zone of the shell, while larger snails more frequently attacked shell borders, principally on the ventral side. These observations may be related to: (a) changes in the process of manipulation of the prey during development of the foot and the shell tooth of the predator, (b) changes in internal structure of the snails related to the shell perforation mechanism, or (c) learned behavior acquired experientially by the snails during early growth. Although in other studies of muricid penetration patterns larger boreholes made in shells of the prey were positively correlated with increasing predator size, this relation did not appear to hold with C. giganteus, as larger specimens often made relatively small shell perforations. Areas of boreholes made in the mussel shells by this snail varied from 0.01 to 1.1 mm(2), and were unusually variable in size and shape, especially when compared with literature results on bores characteristic of other muricid species.     

吉林蛟河针阔混交林乔木幼苗组成与月际动态

以吉林省蛟河42 hm²针阔混交林动态监测样地为平台, 在样地内选取224个1 m × 1 m幼苗监测小样方, 基于2013年6至8月每月中旬的幼苗监测数据, 对乔木幼苗物种组成、数量特征、月际动态及幼苗与同种大树之间的关系进行分析。结果表明: (1)该群落共调查到乔木树种幼苗16种。物种组成在6月和7月间变化不大, 但与8月差异较大。物种组成在各样方间表现出极大的差异性, 不同种幼苗出现的样方数为1-159。(2) 6月至8月, 幼苗个体从1722株减少到1214株, 降幅达29.5%。不同种幼苗在个体数量上变异较大, 个体数超过100的幼苗有色木槭(Acer mono)、水曲柳(Fraxinus mandschurica)、紫椴(Tilia amurensis)和东北槭(Acer mandshuricum), 四者个体数之和占所有物种个体总数的84.26%; 不同种幼苗密度变异性较大, 只有色木槭、水曲柳、紫椴和东北槭密度大于1株·m^-2, 其他种幼苗密度均较低。(3)新生幼苗更新、死亡格局表现出明显的种间差异与月际间差异, 7月新生幼苗总体死亡率(48.9%)显著高于8月(28.3%); 整个调查季(6-8月), 新生幼苗总体死亡率(40.56%)明显高于多年生幼苗(7.34%)。就幼苗更新情况而言, 该研究群落一年中更新主要集中在6月。(4)色木槭、紫椴、杉松(Abies holophylla)幼苗的密度与同种成年个体的胸高断面积之和之间存在显著的正相关关系(p = 0.006、0.013、0.037), 表明三者幼苗密度随周围母树多度的增加而增加, 其他物种幼苗密度与同种个体数目或同种胸高断面积之和未表现出显著的相关关系。     

Increased inter-colony fusion rates are associated with reduced COI haplotype diversity in an invasive colonial ascidian Didemnum vexillum

Considerable progress in our understanding of the population genetic changes associated with biological invasions has been made over the past decade. Using selectively neutral loci, it has been established that reductions in genetic diversity, reflecting founder effects, have occurred during the establishment of some invasive populations. However, some colonial organisms may actually gain an ecological advantage from reduced genetic diversity because of the associated reduction in inter-colony conflict. Here we report population genetic analyses, along with colony fusion experiments, for a highly invasive colonial ascidian, Didemnum vexillum. Analyses based on mitochondrial cytochrome oxidase I (COI) partial coding sequences revealed two distinct D. vexillum clades. One COI clade appears to be restricted to the probable native region (i.e., north-west Pacific Ocean), while the other clade is present in widely dispersed temperate coastal waters around the world. This clade structure was supported by 18S ribosomal DNA (rDNA) sequence data, which revealed a one base-pair difference between the two clades. Recently established populations of D. vexillum in New Zealand displayed greatly reduced COI genetic diversity when compared with D. vexillum in Japan. In association with this reduction in genetic diversity was a significantly higher inter-colony fusion rate between randomly paired New Zealand D. vexillum colonies (80%, standard deviation ±18%) when compared with colonies found in Japan (27%, standard deviation ±15%). The results of this study add to growing evidence that for colonial organisms reductions in population level genetic diversity may alter colony interaction dynamics and enhance the invasive potential of newly colonizing species.     

Disentangling the effects of land use, shrub cover and climate on the invasion speed of native and introduced pines in grasslands

Aims To determine how changes in land use, climate and shrub cover affect the invasion dynamics of native (Pinus sylvestris L.) and introduced (Pinus nigra Arn. subsp. nigra) pines in grasslands. To analyse how these factors interact and affect seedling recruitment, a bottleneck in the lifecycle of many trees. Such information is required to manage the dynamics of these species. Location Grands Causses, calcareous plateaus (Southern France). Methods We used both published and unpublished demographic and dispersal data to assess population growth and invasion speed of invading pines. A demographic and spatially explicit model, which included density dependence and stochasticity in dispersal, demography and environment, was run for different scenarios of sheep grazing pressure (nil, extensive or intensive), shrub cover (0, 10 or 20%) and drought frequency (past‐to‐present or future). For each scenario, population growth rate, invasion speed and elasticity of invasion speed to each demographic and dispersal parameter were computed. Results Grazing was the main factor for limiting invasion speed. Shrub cover reduced tree spread under nil or extensive grazing pressure, but increased it under intensive grazing pressure. Although dry years led to nil seedling establishment rates, an increase in their frequency had surprisingly few effects on pine invasion speed. This last result remained unchanged when very dry years, inducing seedling, but also sapling mortality were introduced. In most environmental conditions, population growth rate and invasion speed were higher for the introduced than for the native pine. Elasticity analysis highlighted the importance of demographic parameters on invasion speed, notably adult and sapling survival. Main conclusion Tree invasion speed may rely at least as much on human activities, like sheep grazing, tree cutting and non‐native trees introduction, as on changes in climate factors. Therefore, human activities need to be explicitly taken into account in the prediction and management of tree dynamics.     

A fence-line contrast reveals effects of heavy grazing on plant diversity and community composition in Namaqualand,South Africa

Changes in plant species richness and community composition were investigated across a fence separating heavily grazed communal and lightly grazed commercial farming systems in Namaqualand, South Africa. No significant differences in plant species richness between communal and commercial farming systems were detected either locally within individual plots or overall across all plots. Within-plot, richness of species tolerant of grazing, such as annuals and geophytes, has increased, while the richness of large palatable shrub species has decreased on the communal rangeland. In terms of plant cover, species' responses to grazing were strongly associated with growth form. Annuals and geophytes formed the majority of grazing increasers, while large, presumably palatable, shrubs and leaf succulents were characteristic grazing decreasers. An investigation into population processes of five shrub species revealed that heavy grazing on the communal rangeland has resulted in: reduced size of palatable shrub species; reduced flower production and seedling recruitment of palatable species; increased density and recruitment of the unpalatable shrub, Galenia africana. Reductions in shrub volume, reproductive output and seedling recruitment were most marked in the palatable shrub Osteospermum sinuatum and were in the order of 90%. The results are further discussed in terms of their relevance to rangeland dynamics and the current land use practices of the region.     

Contrasting effects of grazing on the early stages of woody encroachment in a Neotropical savanna

Woody encroachment in savannas represents an ecological process of current global interest given its negative impact on ecosystem functioning, particularly on forage production. Traditional savanna models propose competition and niche differentiation as the main mechanisms allowing tree-grass coexistence. Demographic models, instead, propose abiotic and biotic factors as bottlenecks controlling vital rates and transitions from seeds to adult trees. The role played by domestic grazing on woody encroachment is yet controversial. Here, using a multistage tree life approach, we combine both models and evaluate the role of grazing and herbaceous vegetation on woody recruitment in a Neotropical savanna dominated by Vachellia caven, a successful and widely spread encroacher tree species. We performed three experiments to evaluate seed predation, seedling emergence and survival of V. caven by manipulating cattle grazing (grazed and ungrazed areas) and herbaceous vegetation presence (vegetated and unvegetated). Finally, we combined the results of the three experiments to estimate the probability of plant recruitment across these experimental factors. Grazing decreased seed predation by half, did not modify seedling emergence and decreased seedling survival. Herbaceous vegetation did not affect seed predation nor seedling emergence rate, but increased seedling survival. Overall, the net effect of grazing on V. caven recruitment was neutral since the increase in seed availability due to the reduction in seed predation rate was compensated by the negative effect of grazing on seedling survival. Our analysis revealed that cattle grazing and herbaceous vegetation had contrasting effects on the seed and seedling life stages. We propose that in order to restrain the early stages of encroachment, cattle grazing pressure could be managed following the seasonality of demographic tree transitions. Through rotational grazing amongst paddocks, stocking rates could be relaxed during the primary dispersal stage to maximize granivory, and then increased to enhance the chance of seedling consumption and trampling.