Shift In Dominant Species Research Articles

Summer heat and drought extremes trigger unexpected changes in productivity of a temperate annual/biannual plant community

One consequence of climate warming is an increase in frequency and intensity of heat and drought extremes. The need for realistic experiments examining the effects of extreme events, including their possible interaction, is therefore high. Here, we imposed heat and drought events in a full factorial design, on experimental plant communities consisting of three temperate, annual or biannual herbaceous species. The extremes were simulated in either early or late summer (July or August), as summer is the season where they are expected to have the greatest impact. The 10-day heat waves were simulated with infrared lamps and drought (either 20 or 17 days) was created by withholding water input and removing the water table.During exposure, effects of drought extremes on leaf physiology were predominantly negative in July, and absent in August. Effects of heat extremes were positive, negative or neutral depending on species and period. Heat and drought were additive in July, but interacted in August. The aforementioned negative effects were not reflected in total (aboveground plus belowground) end-of-season community biomass, which, surprisingly, was stimulated in response to drought extremes in both periods. Effects of heat extremes varied but never reduced biomass, likewise to combined heat and drought. This increase in total community biomass originated exclusively from stimulated root growth. The exact mechanism for this unexpected result could not be ascertained, but greater whole-plant nitrogen stocks clearly indicated enhanced nutrient availability. The latter may have arisen from increased net mineralization or from greater root exploration under the influence of “mid-season drought”. Shifts in species dominance apparently also played a role, as species with pronounced allocation to roots were released from competition.This study demonstrates that community responses to climate extremes are not necessarily negative. The observed responses depended on the soil moisture conditions they induced, which in our experiments varied with the timing of the imposed extremes, even within the same season.

Needle accumulation rate model-based reconstruction of palaeo-tree biomass in the western subalpine Alps

An appropriate bioproxy is required to decipher Holocene tree biomass dynamics from the stand scale in relation to local processes such as disturbance or global climate change. Here we used plant litter collected in traps placed in subalpine forests, and data on the surrounding stands, to develop calibration equations for converting the observed macroremain accumulation rates to tree biomass (basal area) values. The needle accumulation rate (NAR) was modeled for Larix decidua and Pinus cembra. We then used the calibration equation developed from the trapped macroremains to reconstruct past tree biomass for sedimentary Holocene series from two subalpine lakes in the Alps. Our data show that NAR is significantly correlated with basal area. We found a clear overrepresentation of L. decidua NAR compared with its real basal area. This distortion potentially masks the occurrence of P. cembra, another important functional species of subalpine ecosystems, when macroremains are not calibrated. Without calibration, the use of NAR to describe past plant biomass always leads to an overestimation of L. decidua biomass and an underestimation of P. cembra biomass. Several shifts between the dominance of the two species, which were masked when using unadjusted NAR, were apparent and occurred at both sites. By comparing the reconstructed basal areas with fire frequencies, we found that P. cembra biomass accumulation preceded the increase of fire frequency and that fire frequencies superior to 0.0085 fire/yr could induce a long-term loss of resilience of cembra pine forest to the benefit of larch. This results to a slight dominance of Larix biomass from 2500 to 2000 cal. BP until the present day at the two sites. Our results provide increased understanding of tree biomass dynamics associated with specific vegetation phases, and shifts in dominant species, and highlight the needs to understand the causes of these shifts and identify how such processes are related to local environmental conditions.

Efectos de la extracción no controlada de madera sobre la comunidad y estructura de tamaños de los manglares de Alvarado, Veracruz, México

En este trabajo se analizó el efecto de la extracción no controlada de madera sobre la comunidad y estructura de tamaños de los manglares de Alvarado. Para realizar ésto se muestrearon 15 sitios de los cuales se obtuvo la composición florística, altura y área basal. Los efectos sobre la estructura de tamaños se evaluaron con un análisis de la distribución de individuos por categoría de tallas. Se documentó la presencia de Avicennia germinans, Laguncularia racemosa y Rhizophora mangle . Los parámetros estructurales fueron similares a los registrados en otros manglares de la zona tropical de México; sin embargo, los sitios con mayor influencia humana presentan cambios en la dominancia de especies. El análisis por categoría de tamaños indica que L. racemosa tiene problemas de regeneración debido principalmente a la extracción de individuos < 10 cm de diámetro de tallo. Los datos sugieren que de continuar con la extracción actual de tallos de L. racemosa , la situación local de la especie puede estar en riesgo.

Competition between the cyanobacterium Microcystis aeruginosa and the diatom Cyclotella sp. under nitrogen-limited condition caused by dilution in eutrophic lake

The improvement of water quality in Lake Tega, Japan, has been carried out by dilution, causing the shift of dominant species from Microcystis aeruginosa to Cyclotella sp. in summer. The disappearance of Microcystis blooms would be related to dilution, but the detail effect has not been understood yet. In this study, the effect of nitrate concentration on the competition between M. aeruginosa and Cyclotella sp. was investigated through the single-species and the competitive culture experiments. The single-species culture experiment indicated that the half saturation constants for M. aeruginosa and Cyclotella sp. were 0.016 and 0.234 mg N L−1, representing that M. aeruginosa would possess a higher affinity to nitrate. On the other hand, the maximum growth rate for Cyclotella sp. was obtained as 0.418 day−1, which did not represent a significant difference with 0.366 day−1 obtained for M. aeruginosa. The competitive culture experiment revealed that Cyclotella sp. completely dominated over M. aeruginosa at the nitrate concentrations of 0.5 and 2.5 mg N L−1. The dominance of Cyclotella sp. could be attributed to the difference in the abilities of nitrate storage as well as nitrate uptake. One of the possibilities for the disappearance of Microcystis blooms caused by dilution as observed in Lake Tega could be due to the decrease in nitrate concentration, and the lower N:P ratio seemed not to relate to Microcystis blooms.

Managing the forest for more than the trees: effects of experimental timber harvest on forest Lepidoptera

Studies of the effects of timber harvest on forest insect communities have rarely considered how disturbance from a range of harvest levels interacts with temporal variation in species diversity to affect community resistance to change. Here I report the results of a landscape-scale, before-and-after, treatment-control experiment designed to test how communities of forest Lepidoptera experience (1) changes in species richness and composition and (2) shifts in species dominance one year after logging. I sampled Lepidoptera from 20 forest stands allocated to three harvest treatments (control, even-aged shelterwood or clearcuts, and uneven-aged group selection cuts) within three watersheds at Morgan-Monroe State Forest, Indiana, USA. Moths were sampled from all forest stands one year prior to harvest in 2007 and immediately post-harvest in 2009. Species composition was most significantly affected by temporal variation between years, although uneven-aged management also caused significant changes in lepidopteran community structure. Furthermore, species richness of Lepidoptera was higher in 2007 compared to 2009 across all watersheds and forest stands. The decrease in species richness between years, however, was much larger in even-aged and uneven-aged management units compared to the control. Furthermore, matrix stands within the even-aged management unit demonstrated the highest resistance to species loss within any management unit. Species dominance was highly resistant to effects of timber harvest, with pre- and post-harvest values for Simpson diversity nearly invariant. Counter to prediction, however, the suite of dominant taxa differed dramatically among the three management units post-harvest. My results suggest that temporal variation may have strong interactions with timber harvest, precipitating loss of nearly 50% species richness from managed stands regardless of harvest level. Even-aged management, however, appeared to leave the smallest "footprint" on moth communities. Timber harvest also created greater heterogeneity in species dominance among management units, suggesting that forest watersheds may diverge in post-harvest recovery transients relative to unlogged watersheds in Morgan-Monroe State Forest. Because moth communities appear to be highly sensitive to group-selection cuts, shelterwood cuts, and clearcuts, it will be important to retain unlogged concessions within management units to resist landscape-level loss of Lepidoptera.

Characterization and quantification of the nickel resistant microbial community in activated sludge using 16S rDNA and nickel resistance genes

The effect of nickel on the microbial community in the activated sludge of a sequencing batch reactor (SBR) reactor was investigated by continuously dosing nickel from 60 to 240 mg Ni(II) L−1. The diversity of the microbial community was investigated by polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis of the variable V3 region of the bacterial 16S rDNA. The experimental results showed that the community structure changed significantly after dosing with nickel with a shift in dominant species, the disappearance of some original species and the emergence of some new species. The existence of a nickel resistant gene was also investigated using PCR. The obtained nickel resistance gene had a maximum homology with the plasmid pMOL30 of Ralstonia metallidurans CH34. The quantitative real‐time PCR results indicated that the quantity of the nickel resistance gene was related to the nickel concentration loaded to the reactor.

Zooplankton species composition is linked to ocean transport in the Northern California Current

In the Northern California Current (NCC), zooplankton communities show interannual and multiyear shifts in species dominance that are tracked by survival of salmon populations. These zooplankton community changes correlate with the Pacific Decadal Oscillation (PDO) index: a 'warm-water' copepod species group is more abundant during warm (positive) phases of the PDO and less abundant during cold (negative) phases; the reverse occurs for a 'cold-water' species group. The observed relationship led to the hypothesis that the relative dominance of warm/cold-water copepods in the NCC is driven by changes in the horizontal advection of surface water over different phases of the PDO. To test this hypothesis, variation in surface water advection to coastal regions of the NCC over the period of 1950-2008 was investigated using a Regional Ocean Modeling System (ROMS) and passive tracer experiments, then was compared with zooplankton collected off Oregon since 1996. Results showed that surface water advection varied with the phase of the PDO; the low-frequency component of advection anomalies strongly correlated with copepod species composition (R > 0.9). During positive phases of the PDO, current anomalies were northward and onshore, resulting in transport of warmer waters and the associated copepods into the region. During negatives phases, increased equatorward current anomalies led to a copepod community that was dominated by cold-water taxa. Our results support the hypothesis that climate-driven changes in basin-scale circulation controls copepod community composition in the NCC, and demonstrate that large-scale climate forcings downscale to influence local and regional ecosystem structure.

Sitzung 6, Vortrag 2: Primäre Pflege – Kontinuität und Sicherheit für den Patienten im Schmerzmanagement

In recent years, Pinus sylvestris die-off and mortality events have occurred across all its range of distribution, usually associated with recurrent droughts induced by climate change. A shift in canopy dominance towards other better adapted co-existing species can be expected, especially in populations located close to their climatic tolerance limits. Herein, we tested, along a local elevational gradient, whether canopy opening resulting from die-off and mortality favours the growth of a non-dominant co-existing tree species (Quercus pubescens) established in the sub-canopy, in comparison to P. sylvestris sub-canopy trees. We also tested whether the growth of both species is associated with local climatic suitability for these species (extracted from SDMs) or, alternatively, with direct measures of micro-climatic variables. Finally, the effect on tree growth of other micro-local factors such as competition, canopy closure and micro-topography was also tested. Sub-canopy tree growth was enhanced overall by canopy opening resulting from P. sylvestris canopy die-off, but this response was stronger in P. sylvestris trees, reinforcing the self-replacement of this species after die-off. This higher growth rate is related to modifications in the micro-local climate (higher temperatures in the wettest quarter). Conversely, Q. pubescens is less sensitive to micro-local climate conditions but it can grow faster than P. sylvestris on stands with no canopy die-off or mortality. In contrast, climatic suitability extracted from SDMs was negatively related to sub-canopy P. sylvestris growth and had no effect on Q. pubescens. These contrasting results support observations at plot scale that P. sylvestris self-replacement is better explained by local environmental conditions than by values of climatic suitability obtained from regional-scale data-sets. Nevertheless, these climatic suitability measures remain consistent with the overall pattern of low seedling recruitment observed in previous works at the rear edge of species' distribution. This study reveals that short-term shifts in species dominance at a local scale will not necessarily occur in the studied P. sylvestris forests following die-off. This finding endorses the notion that micro-local environment and species traits (i.e., light and temperature tolerance, life-history strategies) modulate the capacity for resilience in rear-edge populations that would probably be prone to collapse otherwise.

Sitzung 2, Vortrag 2: Leitliniengerechte medikamentöse Behandlung des Rückenschmerzes

In recent years, Pinus sylvestris die-off and mortality events have occurred across all its range of distribution, usually associated with recurrent droughts induced by climate change. A shift in canopy dominance towards other better adapted co-existing species can be expected, especially in populations located close to their climatic tolerance limits. Herein, we tested, along a local elevational gradient, whether canopy opening resulting from die-off and mortality favours the growth of a non-dominant co-existing tree species (Quercus pubescens) established in the sub-canopy, in comparison to P. sylvestris sub-canopy trees. We also tested whether the growth of both species is associated with local climatic suitability for these species (extracted from SDMs) or, alternatively, with direct measures of micro-climatic variables. Finally, the effect on tree growth of other micro-local factors such as competition, canopy closure and micro-topography was also tested. Sub-canopy tree growth was enhanced overall by canopy opening resulting from P. sylvestris canopy die-off, but this response was stronger in P. sylvestris trees, reinforcing the self-replacement of this species after die-off. This higher growth rate is related to modifications in the micro-local climate (higher temperatures in the wettest quarter). Conversely, Q. pubescens is less sensitive to micro-local climate conditions but it can grow faster than P. sylvestris on stands with no canopy die-off or mortality. In contrast, climatic suitability extracted from SDMs was negatively related to sub-canopy P. sylvestris growth and had no effect on Q. pubescens. These contrasting results support observations at plot scale that P. sylvestris self-replacement is better explained by local environmental conditions than by values of climatic suitability obtained from regional-scale data-sets. Nevertheless, these climatic suitability measures remain consistent with the overall pattern of low seedling recruitment observed in previous works at the rear edge of species' distribution. This study reveals that short-term shifts in species dominance at a local scale will not necessarily occur in the studied P. sylvestris forests following die-off. This finding endorses the notion that micro-local environment and species traits (i.e., light and temperature tolerance, life-history strategies) modulate the capacity for resilience in rear-edge populations that would probably be prone to collapse otherwise.

Sitzung 12, Vortrag 2: Perioperative Thromboseprophylaxe: Neue und etablierte Antikoagulanzien

In recent years, Pinus sylvestris die-off and mortality events have occurred across all its range of distribution, usually associated with recurrent droughts induced by climate change. A shift in canopy dominance towards other better adapted co-existing species can be expected, especially in populations located close to their climatic tolerance limits. Herein, we tested, along a local elevational gradient, whether canopy opening resulting from die-off and mortality favours the growth of a non-dominant co-existing tree species (Quercus pubescens) established in the sub-canopy, in comparison to P. sylvestris sub-canopy trees. We also tested whether the growth of both species is associated with local climatic suitability for these species (extracted from SDMs) or, alternatively, with direct measures of micro-climatic variables. Finally, the effect on tree growth of other micro-local factors such as competition, canopy closure and micro-topography was also tested. Sub-canopy tree growth was enhanced overall by canopy opening resulting from P. sylvestris canopy die-off, but this response was stronger in P. sylvestris trees, reinforcing the self-replacement of this species after die-off. This higher growth rate is related to modifications in the micro-local climate (higher temperatures in the wettest quarter). Conversely, Q. pubescens is less sensitive to micro-local climate conditions but it can grow faster than P. sylvestris on stands with no canopy die-off or mortality. In contrast, climatic suitability extracted from SDMs was negatively related to sub-canopy P. sylvestris growth and had no effect on Q. pubescens. These contrasting results support observations at plot scale that P. sylvestris self-replacement is better explained by local environmental conditions than by values of climatic suitability obtained from regional-scale data-sets. Nevertheless, these climatic suitability measures remain consistent with the overall pattern of low seedling recruitment observed in previous works at the rear edge of species' distribution. This study reveals that short-term shifts in species dominance at a local scale will not necessarily occur in the studied P. sylvestris forests following die-off. This finding endorses the notion that micro-local environment and species traits (i.e., light and temperature tolerance, life-history strategies) modulate the capacity for resilience in rear-edge populations that would probably be prone to collapse otherwise.

Ecology, changes in fisheries, and energy estimates in the middle stretch of the River Ganges

The River Ganges is the most important of all Indian rivers. It is being harnessed for several onsite and offsite benefits for various sections of society. Through varied man-induced interventions, over the years, the river system has recorded changes in its ecological functions, especially at some stations of the middle stretch viz., Kanpur, Varanasi and Allahabad. An attempt has been made in the present paper to understand changes with regard to key parameters of water quality, fish species composition and catches in this stretch, covering the highly populated major states of Uttar Pradesh and Bihar. For this purpose, data available over the years and some new data were utilized. These data reveal that siltation over the years has lowered water volumes and depths at various sections in the middle stretch with over 90% of the sediment comprising sand. The water quality in terms of pH remained higher (7.0 to 8.8) at some stations while dissolved oxygen remained lower (4.5 to 5.8 ppm) in comparison to upper and lower river stretches, suggesting progressive eutrophication. Other parameters, especially nutrients, also showed variations, coupled with variability in plankton density and primary productivity in this stretch reflecting changes in ecological functions. These changes coupled with various engineering interventions has resulted in drastic changes in water flows in the main river channel, the cumulative impact is evident in the drop in fish catches from 932 kg km−1 in the 1960s to 382 kg km−1 in the recent past, and the shift in species dominance from Indian major carps to less economic varieties. This has affected the livelihoods of fishers. The general lack of concern and awareness among the river authorities on requirements of river fishery management has been observed to be main reasons for these changes. Possible policy and governance interventions required to restore the river for sustainable fishery have also been suggested in this paper.

Multiple levels of nitrogen applied to an oligohaline marsh identify a plant community response sequence to eutrophication

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 417:73-82 (2010) - DOI: https://doi.org/10.3354/meps08808 Multiple levels of nitrogen applied to an oligohaline marsh identify a plant community response sequence to eutrophication Sean A. Graham*, Irving A. Mendelssohn Department of Oceanography and Coastal Sciences, School of the Coast and Environment, Louisiana State University, Energy, Coast & Environment Building, Baton Rouge, Louisiana 70803, USA *Email: sgraham@lsu.edu ABSTRACT: We enriched experimental plots in a Sagittaria lancifolia L. dominated oligohaline marsh for 4 yr with one of 4 levels of nitrogen (N) (0, 50, 200, or 1200 kg N ha–1 yr–1) in combination with one of 2 levels of phosphorus (P) (0 or 131 kg P ha–1 yr–1) to investigate nutrient limitation of primary production and plant community- and species-level responses to nutrient enrichment. Overall, significant changes in ecosystem structure and function occurred with N enrichment only; P enrichment had no significant effect alone or in interaction with N. Both 200 and 1200 kg N ha–1 yr–1 stimulated above-ground plant production. Enrichment with 1200 kg N ha–1 yr–1 also increased S. lancifolia tissue N:P ratio, reduced S. lancifolia N and P resorption during senescence, and altered the relative dominance of the 3 dominant species, but had no effect on species richness. We conclude that (1) N limits above-ground primary production in this oligohaline marsh; (2) the plant response to N enrichment is sequential: moderate N loading stimulated plant production only, while high N loading maintained the elevated production and also altered plant tissue nutrients and species dominance, but not species richness; (3) N enrichment beyond the assimilation capacity of the vegetation drives changes in ecosystem structure caused by altered plant nutrient cycling; and (4) linear changes in species dominance with increasing N enrichment suggest that further nutrient enrichment in time or quantity may result in a shift in species dominance and reduced species richness. KEY WORDS: Marsh · Nutrient enrichment · Nitrogen limitation · Primary production · Relative dominance · Nutrient resorption · Louisiana Full text in pdf format PreviousNextCite this article as: Graham SA, Mendelssohn IA (2010) Multiple levels of nitrogen applied to an oligohaline marsh identify a plant community response sequence to eutrophication. Mar Ecol Prog Ser 417:73-82. https://doi.org/10.3354/meps08808 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 417. Online publication date: November 04, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Dynamics of Typha domingensis spread in Eleocharis dominated oligotrophic tropical wetlands following nutrient enrichment

Recent agricultural intensification in tropical countries has led to increased nutrient input and eutrophication of wetland ecosystems. Higher nutrient levels often lead to changes of vegetation structure and, eventually, shift in species dominance and loss of ecosystem services. We studied the dynamics of species shift in a manipulative nutrient enrichment experiment (+N, +P, +N&P) in oligotrophic wetlands of northern Belize distributed along a salinity gradient. We monitored spread and biomass accumulation of an introduced single individual of Typha domingensis within a 4 years period. The focus was on speed of the spreading and the relative importance of neighbouring ramets in this process. Large differences were found between control and N addition plots versus P and N&P addition plots. The ramets planted in control and N plots died or barely survived, while ramets in P and N&P plots grew vigorously and almost completely outcompeted original vegetation represented by Eleocharis spp. Final numbers of ramets were 2 and 576 per 100 m2 for control and N versus P and N&P plots. The filling dynamics of P-enriched plots of differing salinity changed in time. The spreading was delayed in low salinity plots compared to high and medium salinity plots, although it finally reached comparable rates and values. We attribute this delay to originally denser vegetation and less suitable soil conditions in low salinity plots than to a direct salinity effect. Eventually, the number of ramets stabilized and often even decreased, probably due to self-thinning. Spatiotemporal model extrapolating observed vegetative spread suggested that in P-enriched conditions, a clone originating from a single individual is able to cover 1 ha plot completely within 9 years. We conclude that P-enrichment strongly increases the possibility of fast takeover of Belizean wetlands by Typha domingensis. Eventually, such species change can highly increase potential larval habitat for malaria transmitting mosquitoes.

Subsidy or stress? Tree structure and growth in wetland forests along a hydrological gradient in Southern Europe

In forested wetlands, hydrology exerts complex and sometimes compensatory influences on tree growth. This is particularly true in semi-arid ecosystems, where water can be both a limiting resource and a stressor. To better understand these relationships, we studied hydrologic and edaphic controls on the density, growth, tree architecture and overall productivity of forested wetlands dominated by the tree species Alnus glutinosa and Salix atrocinerea in Southern Europe. We sampled 49 plots set within 21 stands in the Atlantic coastal zone of the Iberian Peninsula, and quantified woody composition, size structure (diameter and height), and radial growth using dendrochronology. Plots were grouped into three saturation classes to compare tree growth characteristics (tree density, degree of sprouting, live basal area and productivity) across levels of saturation. We used Principal Component Analysis (PCA) to create integrated explanatory factors of hydrology, soil nutrient status and soil texture for use in linear mixed models to predict stand characteristics. Increased site saturation favoured a shift in species dominance from Alnus to Salix and resulted in a higher degree of multi-stemmed tree architecture (‘shrubbiness’), particularly for Alnus. Radial growth was negatively correlated with long-term soil saturation; however, annual productivity on a per-tree basis varied by species. Alnus growth and tree density were negatively correlated with waterlogging and fine-textured soils, possibly due to anaerobiosis in the rooting zone. In contrast, Salix growth was more influenced by nutrient limitation. Overall site productivity as measured by annual basal area increment decreased with prolonged saturation. In summary, soil saturation appears to act as a chronic stressor for tree species in this ecosystem. However, these species persist and maintain a dominant canopy position in the most permanently flooded patches through increased sprouting, albeit at a reduced rate of overall biomass accumulation relative to well-drained sites. The diversity in functional responses among wetland forest species has important implications for the conservation and management of these ecosystems. The sustainable management of these ecosystems is directly tied to their vulnerability to changing hydrological conditions. Non-equilibrium modifications to the hydrologic regime from land use and climate change, which are particularly severe in semi-arid regions, may further decrease productivity, integrity and resilience in these stress-adapted communities.

Species shift drives decomposition rates following invasion by hemlock woolly adelgid

Insect and disease outbreak is an important cause of selective species removal and accompanying functional change in North American forests. Outbreak of hemlock woolly adelgid, Adelgies tsugae– HWA, is causing selective removal of eastern hemlock Tsuga canadensis at a regional scale. Impacts of outbreak-caused canopy mortality and shifts in dominant species on litter decay were compared across sites that range in HWA-caused canopy damage and subsequent canopy dominance by black birch Betula lenta. Senescent litter from eastern hemlock, black birch, and equal litter mixes were decomposed in the field for 36 months within nine sites in Connecticut and Massachusetts USA. Mass loss and % N accumulation of black birch was 65% and 52% greater compared to eastern hemlock. In contrast, outbreak related canopy damage increased litter mass loss by 11.5% in high mortality stands relative to uninfested stands but canopy damage had no impact on % N dynamics. Non-additive effects of litter mixing influenced chemical dynamics of decaying litter; black birch accumulated less N and eastern hemlock accumulated more N compared to each species decaying alone. However, these changes offset and mixed litter bags overall showed no differences in N dynamics compared to values from each species decaying alone. In eastern hemlock stands invaded by hemlock woolly adelgid, canopy damage influences the rates and dynamics of decay but species differences between hemlock and black birch leaf litter are the dominant mechanisms of decomposition changes and a long-lasting driver of increased N cycling rates. Species shifts may be the dominant driver of altered ecosystem processes for other insect outbreaks, particularly when replacement species have very different characteristics regulating decomposition and N cycling.

Intercanopy community structure across a heterogeneous landscape in a western juniper‐encroached ecosystem

AbstractQuestions: How does landscape position influence biotic and abiotic attributes of western juniper (Juniperus occidentalis) encroached ecosystems? How does intercanopy plant community structure respond to changes in soil moisture and temperature based on juniper cover and topographic position?Location: Steens Mountain, southeast Oregon, USA.Objectives: Competition with western juniper modifies plant community composition, alters soil hydrology, and reduces plant productivity. Research is needed to understand these influences across heterogeneous landscapes. This study characterizes the relationship between juniper encroachment and soil water, soil temperature, topographic position, and intercanopy plant community structure.Methods: Using a completely randomized block design, plant density and cover, percent bare ground, percent soil moisture, soil temperature, heat accumulation, and elevation were sampled in 10 m2 plots representing low (<1%), moderate (∼14%), and high (∼27%) juniper cover at four aspects. The relationship and difference between vegetation patterns and environmental variables were analyzed using AOV, NMS, and MRPP (α=0.1). Indicator species analysis tested for shifts in dominant species along ecological gradients.Results: Soil moisture remained higher in low juniper cover sites than moderate and high juniper cover sites. North‐facing sites had highest soil moisture at 5 cm depth with low and moderate juniper cover levels. With increasing soil temperature from May to June, soil moisture declined by 19.7% at 5 cm depth. Achnatherum lemmonii and Pseudoreogneria spicata occurred in closed juniper stands while Achnatherum occidentale and Leymus cinereus were common when encroachment was limited.Application: This approach can be used to predict ecosystem response to western juniper encroachment across heterogeneous landscapes.

Maintenance of diversity altered by a shift in dominant species: implications for species coexistence

Diversity can be maintained by the biological characteristics of species within commu- nities, particularly dominant species. Invasions often result in shifts of dominant species. Despite the number of studies on invasion ecology, few have determined their long-term effect on diversity. In the present study, we coupled short- and long-term studies to examine the relationship between invasive species (colonial ascidians or sea squirts) and diversity and to discuss their implications for species coexistence. Diversity patterns between 2 panel studies (1979 to 1982; 2003 to 2006), one conducted before the establishment of invasive colonial ascidians, revealed an increase in species diversity. Short-term recruitment studies designed to elucidate mechanisms behind the patterns observed in our long-term studies suggest contrasting methods of maintaining diversity. Between 1979 and 1982, Mytilus edulis was a consistent spatial occupant whose hard shell provided secondary space for col- onization by other species. In contrast, the 2003 to 2006 community was dominated by the seasonally abundant invasive colonial ascidians Botrylloides violaceus and to a lesser extent Didemnum vexil- lum, which do not provide secondary substrate. These species either senesce after reproduction or undergo seasonal regression and free space for colonization by other species. These studies suggest that the coexistence of species has shifted from a community in which diversity was maintained by secondary substrates to a community in which diversity is maintained by primary substrates. Addi- tionally, they suggest that the community is undersaturated and could support more species, particu- larly those that seasonally utilize a limited resource (free space).

A chronosequence analysis of forest recovery on abandoned agricultural fields in Nicaragua

Species composition, diversity and population structure of woody species recovered on three abandoned agricultural fields after 4, 9 and 14 years were characterized. Seedlings, saplings/poles and mature trees were identified and counted in 70 plots of 100 m2 in each abandoned site. A total of 13, 29 and 22 families represented by 17, 48 and 44 species were registered in 4, 9 and 14 year-old stands, respectively. There was a shift in dominant species across successional stages. Lonchocarpus acuminatus had the highest importance value in the 4-year old stand, whereas, Myrospermun frutescens, Guazuma ulmifolia and Cordia alliodora had the highest importance value in the 9-year-old-stand and Caesaeria corymbosa, Muntingia calabura, Gliricidia sepium and Tabebuia rosea in the 14-year old stand. The total stem density increased from 5011 to 9631 individuals per hectare as the age of abandonment increased from 4 to 14 years. The total basal area of individuals ≥ 1cm d.b.h. also increased with the age of abandonment. Overall, small individuals (< 10 cm dbh) contributed to more than half of the total basal area. Species diversity was the highest in the 9-year old stand followed by 14- and 4-year old stands. We concluded that floristic composition of secondary forests recovers rapidly to the mature forest level compared to structural attributes, which is consistent with the general successional trajectories of tropical dry forest.

Pros and cons of using seabirds as ecological indicators

Climate change and overfishing are increasingly causing unanticipated changes in marine ecosystems (e.g. shifts in species dominance). In order to understand and anticipate these changes, there is a crucial need for indicators that summarise large quantities of information into a few relevant and accessible signals. Seabirds have been suggested as good candidates for ecological indicators of the marine environment; however, few studies have critically evaluated their value as such. We review the role of seabirds as ecological indicators, and discuss their limitations and drawbacks, as compared to other types of indicators. In addition, we highlight the statistical consequences of inverse inference when using seabird data as indicators. We discuss the use of integrated indices and the use of seabirds as autonomous samplers of the marine environment. Finally, we highlight the necessary steps preceding the use of seabirds as indicators. We conclude that, in order to use seabird time series properly, the use of recent advances both in statistics and in remote sensing is a way to move forward. This, along with the assessment of their usefulness, should enable us to use seabird indicators appropriately for managing urgent conservation problems.

Post-eruption succession of macrofaunal communities at diffuse flow hydrothermal vents on Axial Volcano, Juan de Fuca Ridge, Northeast Pacific

Abstract Hydrothermal vents harbor dense aggregations of invertebrate fauna supported by chemosynthesis. Severe tectonic events and volcanic eruptions frequently destroy vent communities and initiate primary succession at new vents on ridge-crest submarine lava flows. An eruption on Axial Volcano (∼1500 m depth), a seamount on the Juan de Fuca Ridge (JdFR) in the northeast Pacific Ocean, occurred in January 1998, which created new substratum and vents. This study examines the development of the macrofaunal vent assemblages associated with tubeworms (Ridgeia piscesae) at eight diffuse flow vents over the following 3 years. Biological collections by suction of lava surfaces also characterized “pre-tubeworm assemblages”. Coupled fluid sampling showed an overall decrease in temperature, sulphide, and sulphide-to-heat ratios over 3 years as well as large spatial variability across the new vents. We examined collections of pre-eruption diffuse flow vent assemblages at Axial Volcano to compare the stages of new community development to “mature” vents. Mature vent assemblages are characterized by two major community types dominated by limpets (Lepetodrilus fucensis) and alvinellid polychaetes (Paralvinella pandorae and/or P. palmiformis). The following post-eruption succession patterns emerged. First, R. piscesae tubeworms took up to 3 years to establish aggregations at the new vents, and the majority of pre-tubeworm assemblages were dominated by grazing polychaetes. Second, species colonized quickly and by 30 months after habitat creation >60% of Axial's species pool had arrived at the new vents; abundance at mature vents predicted colonization success with some notable exceptions. Third, shifts in species dominance occurred rapidly and by Year 3 new vent assemblages resembled mature, pre-eruption communities. In general, tubeworm assemblages were dominated by alvinellid polychaetes (P. pandorae and P. palmiformis) in the first 2 years post-eruption, with limpets (L. fucensis) becoming more numerous in Year 3. Fourth, successional trajectories corresponded to four habitat variables: the timing of R. piscease recruitment, vent age, maximum vent fluid sulphide-to-heat ratios, and maximum vent fluid temperatures. The two mature community types (dominance by limpets or alvinellid polychaetes) seem to relate in part to vent differences in fluid properties, with limpets more prevalent than alvinellids at diffuse vents with lower maximum sulphide-to-heat ratios and temperatures. We present a general model of post-eruption succession for Juan de Fuca Ridge diffuse flow vents. The model incorporates potential abiotic and biotic drivers of community development and identifies six successional stages: a pre-tubeworm assemblage, four tubeworm assemblage states determined by the relative dominance of alvinellid polychaetes and the limpet, and a final senescent stage when venting wanes. Future eruptions will allow for this model to be tested through direct observation and experimentation.