Macrofaunal responses to pit-mound patch dynamics in an intertidal mudflat: local versus patch-type effects

Biogenic structures created via feeding activities have varying effects on soft sediment communities, altering population dynamics, creating a temporal mosaic of successional patches, and ultimately increasing variation at different spatial scales. This study focused on assessing population responses to pits (P) and mounds (M) in an intertidal mudflat (River Plym, England) in order to determine (a) how these structures (max size 1400 cm²), and their formation, affected infauna and (b) to what extent local variations in ambient communities may have affected infaunal responses. Densities in 0- and 1-week-old pits were significantly lower than in ambient and mound areas for most of the dominant infauna over the study period. Abundances were higher in mounds for some species and responses to both structures differed among juveniles and adults. Recovery to ambient levels in P-M systems took about 3 weeks, although their physical characteristics only lasted about 2 weeks. Correlations among ambient densities and adjacent P-M systems were mixed, but suggested decoupling between local ambient and P-M dynamics for some taxa either in relation to age of the patch and/or size of the individuals. At the community level, local differences appeared to be a significant source of variation in new pits, but differences were swamped out when recruitment was high. Differences were more evident in older P-M systems. These results show that sources of local variation in infaunal dynamics can be attributed to a combination of small-scale disturbance/recovery processes and spatially and temporally changing ambient conditions. Identifying extant spatial and temporal small-scale variation can be a critical component of understanding and interpreting larger-scale dynamics of soft-sediment systems.     

DISTURBANCE EFFECTS OF DEPOSIT FEEDING ON MICROALGAL COMMUNITY STRUCTURE AND MECHANISMS OF RECOLONIZATION1

Benthic microalgae (BMA) are important primary producers in intertidal and shallow subtidal sediments, serving as a vital food resource for heterotrophs. BMA also release extracellular polymeric secretions that inhibit resuspension of sediments. Key ecological parameters such as abundance, productivity, and species composition of BMA each contribute to the character of these roles. Our primary objectives were to (i) assess the importance of biotic disturbance to the structure of sedimentary microalgal communities and (ii) identify principal modes of recolonization. We employed field comparative studies to test whether deposit feeding by two invertebrates (Leptosynapta tenuis and Balanoglossus aurantiacus) caused removal of BMA, and manipulative experiments to assess rates and mechanisms of recolonization. Both deposit feeders were determined to significantly reduce BMA biomass via ingestion; however, little change in community composition was observed. Recovery of these disturbed patches was followed over the period of intertidal exposure. We distinguished between potential recolonization methods of migration and regrowth by monitoring fecal coils incubated naturally on underlying sediments (regrowth + migration treatment), hydrogen‐peroxide‐treated coils incubated on ambient sediment (migration only), and coils that were incubated on 0.2 μm filters and thereby isolated from underlying sediment (regrowth only). BMA biomass recovery was significant in <3 h, with migration from underlying sediments the dominant means of recolonization. Surprisingly, recovery appeared somewhat slower when natural egesta were exposed to underlying sediments (migration + regrowth treatments) as compared to migration into peroxide‐treated coils (migration only). This counterintuitive result was due to the dynamic, bidirectional vertical migrations of diatoms in surficial sediments.     

Macrofaunal recolonization on intertidal mudflats: effect of sediment organic and sand content

Following concerns over how differences in dredged material properties affects recovery following dredged material deposition on mudflats, a field manipulation experiment was conducted to investigate the effects of increased sediment organic content (from 0.9% to 2.8%) and sand content (from 12.0% to 47.0%) on macrofaunal recolonization and sediment properties on an intertidal mudflat, Crouch Estuary, Essex, UK. The manipulated sediments were placed in 1-m² plots, 10 cm deep, in a randomised block design and macrofauna and sediments sampled after 1 week, 1, 3, 6, and 12 months. The results suggested that while increased sand content had no discernible effect on sediment property changes, increased organic content resulted in marked reductions in redox potential at 1-, 2- and 4-cm sediment depths and increased shear strengths. These sediments exhibited a distinct ‘cracking’ appearance on their surfaces. Similarly, while macrofaunal recovery was only affected by increased sand content in the short-term (1 month), total numbers of individuals, number of species and diversity took longer to recover in sediments with increased organic contents. Recolonization proceeded via a gradual increase in the abundance of those taxa present in ambient sediments rather than via a distinct successional sequence. Recovery was species-specific with two species (Hediste diversicolor (Müller) and Hydrobia ulvae (Pennant)) recovering to ambient levels within the first week of the experiment. Multivariate analyses indicated that the community assemblages of the low-organic content treatments had recovered after 12 months while those of the high-organic content treatments were still significantly different to those of the controls at the end of the experiment. We propose that the relatively rapid recovery and lack of successional sequence generally observed in all sediment types of this study can be explained by the opportunistic nature of the ambient assemblage and by the dispersive nature of the dominant species. Reduced sediments and/or increased shear strengths led to a slow recovery in high-organic content treatments. These results indicate that a careful consideration of the organic content of fine-grained dredged material must be made during the licensing process of beneficial use schemes.     

Biotic Disturbance,Recolonization, and Early Succession of Bacterial Assemblages in Intertidal Sediments

The role of disturbance in structuring natural microbial communities has been largely unexplored. Disturbance associated with invertebrate ingestion can reduce bacterial biomass and alter metabolic activities and compositions of bacterial assemblages in marine sediments. The primary objectives of the research presented here were to test whether ingestion by a taxonomically diverse group of deposit feeders constituted a disturbance, and to determine the mechanisms by which bacterial assemblages recover following deposit-feeder ingestion. To test the question of disturbance, we compared fresh egesta vs surficial sediments with respect to bacterial assemblage structure. In emersed intertidal sediments, microbial recovery could be due to regrowth of bacterial populations surviving gut passage or to immigration from adjacent sediments. To differentiate between these modes of recolonization we used field manipulative experiments to exclude migration by isolating freshly extruded fecal coils of three deposit-feeding species from surrounding sediments. We then followed the quantitative and qualitative recovery in egesta and sediments through time using epifluorescence microscopy and PCR-DGGE analysis of 16S rDNA. Our findings indicate that (1) the degree and nature of the disturbance to bacterial assemblages from deposit feeding varies among invertebrate taxa, (2) recovery was significant but incomplete over 3 h, and (3) recolonization of biotically disturbed sediments is dominated by immigration.This revised version was published online in November 2004 with corrections to Volume 48.     

Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities

Intense disturbance may locally destroy patches of habitat and shape the landscape into a mosaic of reassembling communities. The development of ecosystem properties during such community reassembly is poorly understood. In intertidal bare sediments, trophic relations between microphytobenthos or heterotrophic bacteria and macrofauna invertebrates may guarantee fundamental ecosystem properties such as carbon flow through the food web. We studied the dynamic relation between reassembling macrofauna communities and such microbial carbon flow during recovery after severe disturbance. We deliberately induced prolonged hypoxia in winter and early summer and allowed recolonisation for periods of two and five months. Carbon flow was quantified from basal resources (microphytobenthos and bacteria) to intermediate consumers using ¹³C as a tracer. Within the period of study (5 months), microbial carbon flow fully recovered, although macrofauna diversity was still very low compared to the natural communities (ranging from 6 to 17 species). More than 90% of microbial carbon flow to macrofauna was due to the consumers that recolonised within two months. Two of these species were dominant contributors to microphytobenthos carbon transfer to fauna. Furthermore, at an early stage of reassembly, this ecosystem property was remarkably similar when disturbance took place at different times of the year (winter or early summer), although there were differences in assemblage composition and functional diversity. We conclude that species assemblages and ecosystem function developed relatively independently in this benthic system. We discuss which ecological factors may have caused such non-parallel development of macrofaunal communities and carbon flow.     

Recovery of a northern New England salt marsh plant community from winter icing

High latitude salt marsh plant communities are frequently exposed to conspicuous winter ice disturbances, which trigger secondary succession. In this paper, we document the recovery of a northern New England salt marsh from a severe winter icing event in 1998. Ice disturbances that killed plants but that left the underlying peat intact recovered rapidly. However, ice damage that killed plants and removed the underlying peat, led to areas of physiologically harsh edaphic conditions, specifically waterlogged and anoxic soils that limited plant recolonization. A transplant experiment revealed that only the most stress-tolerant plants were capable of invading the most stressful portions of ice disturbances. A second experiment that artificially dried disturbance patches accelerated patch recovery. These data suggest that recovery from intense ice disturbance is dependent on stress-tolerant plants invading edaphically harsh disturbances, eventually facilitating the recolonization of the community. This process likely takes longer than a decade for full recovery to occur in the areas where both plants and the peat base are removed.     

Dense aggregations of tube-building polychaetes: response to small-scale disturbances

Many species of tube-building polychaetes form dense aggregations or ‘patches’ within marine soft-bottom habitats. Although the ecological importance of such patches has been well studied, the effect of small-scale disturbances within patches has not been investigated. On the intertidal sandflat Drum Sands, Firth of Forth, Scotland, the tube-building spionid Pygospio elegans Claparède forms patches approximately 1-1.5 m², wherein its density is significantly greater than in surrounding sediments. This study explicitly investigates whether the initial communities colonising small-scale disturbances within P. elegans patches differ in terms of community structure and colonisation mode from those of surrounding sediments.Defaunated sediments (1/125 m²) were implanted into the sediments within and outside patches during April, August and December 1997, and sampled after 3 weeks. The results indicated that some species colonised patches and non-patches in significantly different numbers, at least at certain times of the year. Following these controlled disturbances, P. elegans larvae colonised patches in significantly higher numbers compared to non-patch areas during April 1997, but showed the opposite trend in August 1997 when larval recruitment was relatively low. Adult P. elegans colonised patch sediments in higher numbers than non-patch ones during August and December. Capitella capitata (Fabricius) consistently colonised all defaunated areas in relatively high numbers while Cerastoderma edule (L.) and Macoma balthica (L.) colonised non-patch azoic sediments in significantly higher numbers than patch azoics during April 1997. Multivariate analyses, together with nonparametric analysis of similarities (ANOSIM) tests, revealed that the community composition of the disturbed sediments was significantly different between patches and non-patches during April 1997 only. These analyses also indicated that there was a strong temporal effect on community composition since the April community was markedly different from those of August and December 1997. Sediment measurements taken during December suggested that the observed faunal differences were unlikely to have been a response to differences in bulk sediment characteristics or redox potential.It is proposed that the initial recolonisation differences were likely to have been due to the near-bed hydrodynamic effects of P. elegans tubes creating local sediment stabilisation. The ecological implications of differential recruitment following small-scale disturbances within patches compared to outside patches are discussed. The ability of P. elegans patches to enhance successful conspecific colonisation, as demonstrated here, may explain the longevity of such patches. Their persistence promotes alpha diversity and small-scale patchiness, together with providing a local source of colonists for the initial recovery of large-scale disturbances.     

Responses of Benthic Infauna to Large-Scale Sediment Disturbance in Corpus Christi Bay, Texas

Responses of benthic infauna to large scale disturbance by dredged material placement were studied at eight paired (placement and reference) areas in Corpus Christi Bay, Texas in 1995 and 1996. Total infaunal abundance, taxa richness, and the biomass of annelids and molluscs in placement areas no longer differed from that of reference areas approximately one year after placement. Annual secondary production for annelids and molluscs did not differ between placement and reference areas, however mean (± standard error) total annual secondary production at reference areas (53.6 ± 5.8 g AFDW/m²/yr) was greater than that of placement areas (26.2 ± 2.5 g AFDW/m²/yr) due to higher brittle star densities in reference areas. Differences in community structure between placement and reference areas returned to pre-placement levels one year after disturbance. The magnitude of changes in sediment grain size related to disturbance was positively correlated with the degree of convergence of placement area communities with pre-placement and reference communities. Sediment compaction was consistently associated with infaunal taxonomic composition (BIOENV) and increased linearly with time after disturbance. The use of the term “recovery” with reference to recolonization of dredging-related disturbance implies a return to pre-placement ecological conditions that are frequently neither a reality nor a practical expectation for areas that are subject to repeated disturbances. Our characterization of biological responses to dredged material disturbance targeted benchmarks that were linked to both pre-disturbance conditions and differences between disturbed and neighboring undisturbed areas and indicate that impacts persisted less than one year.     

Response of benthic invertebrates to natural versus experimental disturbance in a Swiss prealpine river

1. The crucial point of disturbance experiments in streams is the extent to which they can simulate a natural spate. Ideally, disturbance experiments should proceed side by side with a phenomenological study to allow a direct comparison. In the present study conducted in a prealpine Swiss river, the River Necker, fortuitous events made such a comparison possible. 2. In summer 1994, we took Surber samples one day before and on several sampling dates after a major flood (recurrence interval ≈ 5 years), which was followed by a long period of uniform discharge in a river characterized by frequent spates. Beginning 19 days after this flood, patches of the stream bed (≈ 9 m²) were physically disturbed by kicking and raking. 3. The degrees of reduction in the total number of individuals and the dominant taxa were similar after both types of disturbance, as were the recolonization patterns of Rhithrogena spp., Leuctra spp. and Hydracarina. Chironomidae, Baetis spp., Simuliidae, Pentaneurini and Corynoneura/Thienemanniella spp. showed a distinct lag phase after the flood before recolonization began, whereas there was no such lag phase after the experiment. Therefore, the time needed to recover to pre-flood densities was longer for these taxa. Nevertheless, recolonization rates and patterns after the lag phase were similar to those after the experimental disturbance. 4. Size-class measurements indicated that recruitment from egg hatching may have been more important after the flood than after the experimental disturbance for Rhithrogena spp., but not for Chironomidae, Baetis spp., Simuliidae, Pentaneurini and Leuctra spp. Invertebrate drift was probably the most important pathway of recolonization after both types of disturbance. 5. Our experiment allowed a realistic simulation of several important effects of the large flood on the invertebrate community. Smaller spates that induce substratum movement at a spatial scale similar to our experimental plots are much more common than large floods in the River Necker. For these spates, our experiment should provide an even more realistic simulation of natural disturbance.     

Influence of flood timing on the recovery of macrophytes in a former river channel

Over two vegetation cycles we compared the recovery of macrophytes from flood disturbances that occured at different seasons (July vs December) on patches of a former channel of the Rhône River, France. Some patches were disturbed twice; others were disturbed either in summer or in winter; others were never disturbed and were used as controls.The recovery rate of the vegetation was estimated from the duration of recolonization of the disturbed areas and of growth of the recolonizing species. The influence of the summer disturbance appeared to be strong because the disturbance occurred when the development of the vegetation was maximum. The influence of the winter disturbance was apparently much lower since most species had already declined at this time because of their phenology. The repetition of the two disturbances on the same patch had little influence on the vegetation community.In all cases, the recovery of the vegetation occurred rapidly, both for total vegetation cover and species richness. By the following spring, no significant differences appeared between disturbed and reference patches. The effect of the disturbances varied according to the phenology of the plants, and the macrophyte community studied was more sensitive in summer than in winter.     

Recovery of riverine vegetation after experimental disturbance: a field test of the patch dynamics concept

The Patch Dynamics Concept predicts different recovery patterns of communities after disturbance according to the spatial and temporal heterogeneity of the habitat. The aim of this study was to test the predictions arising from the Patch Dynamics Concept on the recovery of macrophyte communities after an experimental disturbance. The test was based on the comparison of the vegetation recovery in three stations located on former channels of the Rhône River, France, differing by their temporal heterogeneity, which was estimated by the frequency of flood scouring. In each station the experimental disturbance was set up by uprooting the aquatic plants in three experimental sets comprising four 1 m²-plots. The aquatic plants were surveyed in these sets as well as in their reference sets from July to November 1991.As predicted, the most frequently disturbed station recovered its species richness (19 species) and its vegetation cover in less than two months. The biological traits of the species occurring in this station are considered as r-strategies. The species colonizing the experimental sets were not necessarily present on the sets before the disturbance, but occurred regularly on other sites station. The rapidity of the community recovery demonstrates the high resilience of this ecosystem. According to the unpredictable character of its recolonization, the macrophyte community of this station could be said to be founder controlled with competitive lottery for establishment.The station with intermediate temporal heterogeneity was overgrown by some r-selected species but the community recolonization was predictable and this station had intermediate resilience. Its species richness was low (6 species).The less frequently disturbed station presented low resilience with a slow recovery of its community (more than 5 months); the species richness (4 species) and the traits of some species were related to K-strategies while others were related to r-strategies. The macrophyte community of this station could be said to be dominance controlled.Different patterns of recovery of the macrophyte communities of former channels of the Rhône River could be depicted according to their temporal heterogeneity; these patterns were consistent with the hypotheses arising from the Patch Dynamics Concept. However, the competitive lottery appeared to be limited at the scale of our experiment.     

洋山港潮间带大型底栖动物群落结构及多样性

2009-2010年在洋山港海域大洋山岛和圣姑礁进行四个季度潮间带大型底栖动物生态学研究。共采集到大型底栖动物61种,以广布种为主,部分为河口低盐种,其中软体动物22种,环节动物16种,节肢动物12种,苔藓动物5种,腔肠动物4种,星虫动物和棘皮动物各1种。优势种为短滨螺(Littorina brevicula)、多齿围沙蚕(Perinereis nuntia)、日本笠藤壶(Tetraclita japonica)、齿纹蜒螺(Nerita yoldii)和特异大权蟹(Macromedaeus distinguendus)。丰度和生物量在不同季节明显不同(P<0.05):丰度的最高值出现在春季,为(3204.9±837.84)个/m²,最低值出现在秋季,仅为(2213.2±731.27)个/m²;生物量的最高值则出现在夏季(2233.2±1493.42)g/m²,冬季最少,仅为(819.95±484.80) g/m²。大洋山岛和圣姑礁的丰度和生物量具有较大差异(P<0.05):年均丰度以大洋山断面较高,为(3090±742.74)个/m²,圣姑礁断面较低,为(2133±372.51)个/m²;而年均生物量则以圣姑礁断面较高,为(1711.1±1180.76) g/m²,大洋山断面较低,仅为(1028.5±627.61) g/m²。运用ABC曲线、等级聚类和MDS对大型底栖动物群落结构分析发现:大洋山潮带间大型底栖动物群落尚未受到干扰或干扰较轻,群落结构相对稳定;而圣姑礁断面的潮间带大型底栖动物群落受到了中度的干扰,群落结构稳定性下降。与15年前的研究相比,圣姑礁大型底栖动物的密度和生物量均有大幅度的降低,这与洋山港海域盐度的升高、生态环境变化和人为干扰强度增加等有关。     

Recolonization pattern of the polychaete Lanice conchilega on an intertidal sand flat following the severe winter of 1995/96

The recolonization process of Lanice conchilega was studied on an intertidal sand flat in the northern Wadden Sea after an intertidal population had been wiped out during the severe winter of 1995/96. After very low recruitment in the first two years, the population fully recovered in the third year after its destruction. By then, the distribution pattern closely resembled the pattern before the severe winter with >95% of all Lanice occurring below –0.5 m mean tidal level and dominance of the density categories 6–50 individuals (ind) m^–2 and 51–200 ind m^–2. Lanice larvae have been observed to attach preferentially on the tubes of adults. However, the high recruitment in the third year was also seen in areas devoid of adult Lanice, indicating that settlement facilitation by adults was of little importance for the recolonization process. Instead, we suggest that the observed recruitment pattern was a consequence of a large-scale decline in the metapopulation in the tidal basin and the onshore coastal subtidal which resulted in insufficient larval supply onto the tidal flats and low recruitment in the first two years following the severe winter. Received in revised form: 7 May 2001 Electronic Publication     

长山列岛南部三岛岩相潮间带群落多样性格局

对长山列岛南部南长山、北长山和大黑山岛屿岩相潮间带群落种类组成的调查表明,3岛屿岩礁潮间带的群落结构差异不大,表现在决定群落结构和外貌的前5位优势种群及其优势度序列基本相同;而砾石潮间带的群落结构存在一定差异,表现在北长山群落中优势种群及其优势度序列显著地区别于南长山和大黑山,但后二者趋同-3岛屿岩礁潮间带的景观和群落外貌与结构由东方小藤壶(Chthamalus chal-lengeri)、牡蛎（Ostrea spp.)、大型藻类和短滨螺(Littorina breuicula)控制,砾石潮间带则由东方小藤壶、大型藻类和平背蜞（Gaetice depressus)决定．群落多样性由物种丰富度(R)、均匀度(J)和多样性(HB、Hlv、DR、DIv)衡量,3岛屿岩礁潮间带群落的各种多样性指数呈现一致性,群落多样性以北长山最高,大黑山最低;而砾石潮间带群落的各种多样性指数存在一定差异,南长山高于北长山和大黑山,北长山潮间带群落中物种分布的均匀度最高,南长山和大黑山趋同．比较3岛屿两种基质潮间带的群落多样性发现,砾石潮间带高于岩礁潮间带。     

Impacts of boat-generated waves on macroinfauna: Towards a mechanistic understanding

Increasing evidence suggests that the waves produced by even low-wash vessels can have a sizeable impact on infaunal assemblages in otherwise sheltered estuaries. Although this impact is widely regarded to be a consequence of wash coarsening sediment grain-size, it may be due to a number of alternative mechanisms which include enhanced turbidity, decreased larval supply, changed resource availability and/or erosion of animals from the sediment. To increase understanding of impacts resulting from boat-wash, this study tested alternate mechanisms by which intertidal infauna along the Parramatta River, Sydney, Australia are impacted by wash from local ferries. Specifically, to discriminate between impacts of wash resulting from changes to sediment characteristics and impacts directly resulting from the altered hydrodynamic regime, cores of intertidal sediment were transplanted between a no-wash and a wash zone and within each of these zones. It was predicted that if wash indirectly determines the structure of assemblages by altering characteristics of the sediment: (1) sediment granulometry and/or mineralogy would differ between cores sourced from the wash zone and cores sourced from the no-wash zone; and (2) over a 6-week period during which sediment characteristics of transplanted cores do not appreciably change, assemblages transplanted across zones would remain more similar to assemblages at their origin than their destination. To the contrary, sediment grain-size did not differ among sites according to their location in wash or no-wash zones and the destination of sediment cores was more important than their origin in determining the structure of macroinvertebrate assemblages. Following transplant into the no-wash zone, the abundances of capitellids and nereids in cores of sediment from the wash zone rapidly increased in abundance. In cores of sediment transplanted from the no-wash zone to the wash zone, the abundances of these taxa decreased. The results of this study therefore indicate that impacts of boat-wash to infauna are not always a consequence of changes to sediment grain-size distribution. Thus, in order to successfully manage ecological impacts of boat-wash, other potential mechanisms such as erosion of sediments and changing resource availability also need to be considered.     

Recolonization by benthic invertebrates after experimental disturbance in a Swiss prealpine river

1. Although the crucial point of disturbance experiments in streams is the extent to which they can simulate an actual spate, this aspect has been widely neglected in the design of such studies. Similarly, the influence of the specific hydrological disturbance regime of a stream on its benthic community has received much theoretical attention in recent years, but hypotheses have rarely been tested in the field. 2. Our field experiment compared the structure of the benthic invertebrate community in the prealpine River Necker in north-eastern Switzerland with predictions of the patch dynamics concept about the faunal composition of frequently disturbed streams. We also compared the resistance and resilience of the invertebrates between two sites in the River Necker. A similar substratum composition at both sites, but higher shear stress values both at baseflow and bankfull discharge at site 2, implied a higher disturbance frequency at the latter site. Five patches of stream bed of ≈ 9 m² were disturbed by kicking and raking at each site, while five similar areas served as controls. From each plot, six Surber samples were taken: the first immediately after the disturbance, and the following five 1, 3, 6, 10 and 30 days later. 3. Resilience of the total benthic invertebrate fauna was high. The total number of individuals recovered to undisturbed densities within 30 days at site 1 and 6 days at site 2. Taxon richness recovered within 3 days. In accordance with theory, taxa with high recolonization rates made up a major percentage of the total number of individuals, especially in disturbed plots. However, this percentage was lower at site 2 in spite of the higher disturbance frequency at this site. Rhithrogena spp., Leuctra spp. and the Simuliidae recovered faster to undisturbed densities at site 2. In contrast, absolute recolonization rates of these taxa were higher at site 1, where total invertebrate densities were more than twice as high as at site 2. 4. Our results suggest that the time since the last disturbance should be considered as an important factor in studies of benthic invertebrate communities in prealpine rivers, because disturbances can alter the community structure. In frequently disturbed streams, very short sampling intervals may be needed to detect differences in taxon-specific colonization rates. The specific hydrological disturbance regime of such streams is also important, because even within-stream differences in the resilience of the benthic invertebrate community are possible.     

The Interactive Effects of Pulsed Grazing Disturbance and Patch Size Vary among Wetland Arthropod Guilds

Pulse disturbances and habitat patch size can determine community composition independently or in concert, and may be particularly influential on small spatial scales for organisms with low mobility. In a field experiment, we investigated whether the effects of a pulsed disturbance that simulated a grazing event varied with habitat patch size. We focused on the short-term responses of multiple co-occurring emergent salt marsh arthropods with differing levels of mobility and dispersal potential. As part of a marsh restoration project, two types of emergent marsh structures were created: small circular mounds (0.5 m diameter) separated by several meters of aquatic habitat, and larger, elongated terraces (>50 m long). Study plots (0.25 m²) were established on both structures; in a subset of plots, we simulated a pulsed grazing disturbance event by clipping the aboveground tissue of emergent plants, primarily Spartina alterniflora. At the end of the two-month recovery period, Ischnodemus (Hemiptera: Blissidae) density was over 50% lower in disturbed treatments within both large (terrace) and small (mound) patches. Predatory spider treatment responses were similar to Ischnodemus responses, suggesting a trophic relationship between those two arthropod groups. Alternatively, spiders may have been directly affected by the loss of shelter in the disturbed plots. Prokelisia (Homoptera: Delphacidae), which are generally more mobile than Ischnodemus, were not affected by disturbance treatment or by patch size, suggesting the potential for rapid recolonization following disturbance. Larval stem borers decreased by an order of magnitude in disturbed plots, but only in the large patches. In general, the disturbance effects of vegetation removal on arthropod density and community composition were stronger than patch size effects, and there were few interactions between pulsed disturbance and patch size. Rather, emergent marsh arthropod responses to disturbance and habitat area treatments were linked to the dispersal potential and mobility of each individual taxon.     

灌丛化对黄土高原草地植物群落结构和地上生物量的影响

中国北方草地普遍出现灌丛化现象,灌丛化改变植物群落结构、植物多样性和生产力,直接影响着草地生态保护与可持续利用.该研究以黄土高原灌丛化草地为研究对象,通过植被调查,分析比较不同坡向的灌丛斑块与禾草斑块植物群落结构(物种组成、优势种及物种多样性)和地上生物量的差异.结果发现:(1)灌丛化草地不同坡向对物种多样性及地上生物...     

Vole disturbances and plant diversity in a grassland metacommunity

We studied the disturbance associated with prairie vole burrows and its effects on grassland plant diversity at the patch (1 m²) and metacommunity (>5 ha) scales. We expected vole burrows to increase patch-scale plant species diversity by locally reducing competition for resources or creating niche opportunities that increase the presence of fugitive species. At the metacommunity scale, we expected burrows to increase resource heterogeneity and have a community composition distinct from the matrix. We measured resource variables and plant community composition in 30 paired plots representing disturbed burrows and undisturbed matrix patches in a cool-season grassland. Vole disturbance affected the mean values of nine resource variables measured and contributed more to resource heterogeneity in the metacommunity than matrix plots. Disturbance increased local plant species richness, metacommunity evenness, and the presence and abundance of fugitive species. To learn more about the contribution of burrow and matrix habitats to metacommunity diversity, we compared community similarity among burrow and matrix plots. Using Sorenson’s similarity index, which considers only presence–absence data, we found no difference in community similarity among burrows and matrix plots. Using a proportional similarity index, which considers both presence–absence and relative abundance data, we found low community similarity among burrows. Burrows appeared to shift the identity of dominant species away from the species dominant in the matrix. They also allowed subordinate species to persist in higher abundances. The patterns we observed are consistent with several diversity-maintaining mechanisms, including a successional mosaic and alternative successional trajectories. We also found evidence that prairie voles may be ecosystem engineers.