Temporal Changes In Species Composition Research Articles

Effect of Variable Rates of Daily Sampling of Fly Larvae on Decomposition and Carrion Insect Community Assembly: Implications for Forensic Entomology Field Study Protocols

Experimental protocols in forensic entomology successional field studies generally involve daily sampling of insects to document temporal changes in species composition on animal carcasses. One challenge with that method has been to adjust the sampling intensity to obtain the best representation of the community present without affecting the said community. To this date, little is known about how such investigator perturbations affect decomposition-related processes. Here, we investigated how different levels of daily sampling of fly eggs and fly larvae affected, over time, carcass decomposition rate and the carrion insect community. Results indicated that a daily sampling of <5% of the egg and larvae volumes present on a carcass, a sampling intensity believed to be consistent with current accepted practices in successional field studies, had little effect overall. Higher sampling intensities, however, slowed down carcass decomposition, affected the abundance of certain carrion insects, and caused an increase in the volume of eggs laid by dipterans. This study suggests that the carrion insect community not only has a limited resilience to recurrent perturbations but that a daily sampling intensity equal to or <5% of the egg and larvae volumes appears adequate to ensure that the system is representative of unsampled conditions. Hence we propose that this threshold be accepted as best practice in future forensic entomology successional field studies.

Temporal Changes and Prescribed-Fire Effects on Vegetation and Small-Mammal Communities in Central Appalachian Forest, Creek, and Field Habitats

We quantified changes in vegetation and small-mammal communities over a 3-year period in paired creek, forest, and field sites in the central Appalachian Mountains. Prescribed burns were applied to field sites in 2008. Data were collected at all sites during summers of 2008 (pre-burn for fields), 2009 (ca. 2–4 months post-burn for fields), and 2010 (ca. 14–16 months post-burn for fields). In 19,640 trap-nights across 3 years, we captured 605 individuals of 14 small-mammal species. Sørenson index showed substantial differences in mammal communities between 2008 pre-burn and 2009/2010 post-burn fields (<10% similarity for all pre- to post-burn comparisons). Creek and forest habitats showed markedly greater year-to-year similarities (46–82%). Unlike mammals, vegetation and habitat structure showed little change over time. Minimal changes in preand post-burn fields suggest that field vegetation at these sites recovered rapidly after the low-intensity surface fires. In contrast, fire appears to have had a profound effect on small-mammal communities in fields, as documented by dramatic temporal changes in species composition and abundance and little evidence of recovery after 16 months postburn. As many managed fields in this region are burned on 3-year rotations, this potential impact of prescribed fire on small-mammal communities is important. Additional studies are needed to determine whether small-mammal populations are strongly affected by conditions during prescribed burns (i.e., direct effects on species mortality and emigration), or if the changes we observed reflect natural cyclical patterns (annual or multi-annual periodicities) in these populations.

Nekton response to freshwater inputs in a temperate European Estuary with regulated riverine inflow

The aim of this 12-year study was to assess the nekton (fish, decapod crustaceans) response to freshwater inputs (rainfall, dam discharges) in a temperate estuary with regulated riverine inflow. Although interannual variability in river discharges to the Guadalquivir estuary has been extremely high since the construction of a dam in 1930, a significant decreasing trend in the dam's discharges has been observed in the last 80years. During this study, an alternation of wet, standard and dry years occurred in the estuarine area but no significant long-term trend was observed. River discharge, in turn, showed a considerable interannual variability and a significantly decreasing long-term trend. Freshwater inputs had an immediate effect on estuarine salinity and turbidity, and consequently on prey availability (mysids). Although 124 nektonic species were collected, only 47 of them (adding up to 99.7% of total abundance) were regularly present in the estuary: 32 marine migrants, 13 estuarine species and 2 diadromous species. Well-defined temporal changes in species composition and abundance yielded clear seasonal patterns in the estuarine nektonic community. Considerable intermonth and interannual changes were occasionally observed relating to freshwater inputs, mainly in winter/autumn of wet years. Thus, within each two-month period, some significant interannual differences in the nektonic community were also observed, with marine migrants tending to be more abundant in dry years. However, changes in the studied nektonic community did not show long-term trends. In conclusion, natural and human-controlled freshwater inputs currently play a significant role in determining the physicochemical conditions and the biota of the Guadalquivir estuary. However, although freshwater input seemed to transitorily affect the estuarine nekton, either directly (flushing out) or indirectly (through changes in salinity, turbidity and prey availability), a quick reestablishment of the estuarine nekton (strong resilience) was observed following freshwater inputs together with the recovery of environmental conditions within the estuary.

Species composition, reproduction, and body size of mud crabs, Scylla spp., caught in Urado Bay, Japan

Species composition, reproduction and body size of mud crabs, genus Scylla de Haan, 1833, were investigated using gill nets from October 2008 to October 2009 in Urado Bay, Japan. Three mud crab species were identified in the area, with S. paramamosain Estampador, 1949 being the dominant species (74% of the catch), followed by S. serrata (Forskal, 1775) (23%) and S. olivacea (Herbst, 1796) (3%). We found temporal changes in species composition with abundances of S. serrata increasing towards the summer season. Ovigerous females of S. paramamosain and S. olivacea occur between January and October, peaking in the warm (May-July) and rainy (June-July) seasons, but we found no berried females of S. serrata. The female-biased sex ratios of S. paramamosain and S. serrata, and the larger body size of females during the period from autumn (November) to early spring (April), suggest that large gravid females may have migrated offshore before the spawning season. The mean body size of females and males of S. paramamosain and S. serrata tended to increase and the mating activity was high between May and October, showing the moulting and growth season.

Variations, trends and patterns of fish landings in large tropical reservoirs

AbstractTemporal variations of fish yields in four major reservoirs in Thailand (Ubolratana; Sirindhorn; Srinakarin; Vajiralongkorn) were investigated with the use of long‐term fish landing data (≥20 years). The long‐term variations in fish yield, measured as the coefficient of variation of yearly yield, ranged mostly between 50% and 100%. For short‐term variations, the means of the relative variation (85%) were larger than the absolute variation (63%). This finding indicates that short‐term variations were inversely related to fish yield and that a higher uncertainty occurs when fish catches are low. The stocked exotic species exhibited higher variations than the indigenous species. The trend analyses indicated some species had sharply declined fish landings, while some species were quite stable (i.e. reservoir‐adapted species). Stocked species tended to increase in relatively shallow reservoirs, compared to the deep reservoir. Fish landing data for each reservoir were patternized, using the self‐organizing map, indicating temporal trends of chronological order. The differences among clusters in each reservoir were with respect to the weight of each species in the fish landings in each year, and temporal changes in species composition in the reservoirs, which would primarily be attributed to the environmental changes followed by anthropogenic pressures. The mean trophic level (τ) fluctuated, resulting from changes in species composition and weight of fish landing, as well as fish stocking programmes.

Understorey Vegetation Stability and Dynamics in Unmanaged Boreal Forests Along a Deposition Gradient in Sweden

The aims of this study were to investigate spatial patterns and temporal changes in understorey vegetation at four forest catchments forming a depositional gradient. Inventories of the bottom and field layers were carried out in the 1990s and repeated after 5-14 years, depending on catchment. It was hypothesized that changes and patterns in ground vegetation would be related to changes and patterns in N and S deposition. The data were analyzed using Ellenberg indices and multivariate methods. All catchments showed temporal changes in species composition. Analyses of the bottom layer were confounded by a change of field staff, but after accounting for this observer effect, differences in species composition between the catchments remained. Within catchments, the changes in species composition were unrelated to N or S deposition. Relationships between environmental factors, expressed as Ellenberg indices, and compositional patterns differed between catchments although Ellenberg indices showed small temporal changes.

A comparative study of ancient sedimentary DNA, pollen and macrofossils from permafrost sediments of northern Siberia reveals long‐term vegetational stability

Although ancient DNA from sediments (sedaDNA) has been used to investigate past ecosystems, the approach has never been directly compared with the traditional methods of pollen and macrofossil analysis. We conducted a comparative survey of 18 ancient permafrost samples spanning the Late Pleistocene (46-12.5 thousand years ago), from the Taymyr Peninsula in northern Siberia. The results show that pollen, macrofossils and sedaDNA are complementary rather than overlapping and, in combination, reveal more detailed information on plant palaeocommunities than can be achieved by each individual approach. SedaDNA and macrofossils share greater overlap in plant identifications than with pollen, suggesting that sedaDNA is local in origin. These two proxies also permit identification to lower taxonomic levels than pollen, enabling investigation into temporal changes in species composition and the determination of indicator species to describe environmental changes. Combining data from all three proxies reveals an area continually dominated by a mosaic vegetation of tundra-steppe, pioneer and wet-indicator plants. Such vegetational stability is unexpected, given the severe climate changes taking place in the Northern Hemisphere during this time, with changes in average annual temperatures of >22 °C. This may explain the abundance of ice-age mammals such as horse and bison in Taymyr Peninsula during the Pleistocene and why it acted as a refugium for the last mainland woolly mammoth. Our finding reveals the benefits of combining sedaDNA, pollen and macrofossil for palaeovegetational reconstruction and adds to the increasing evidence suggesting large areas of the Northern Hemisphere remained ecologically stable during the Late Pleistocene.

CHARACTERIZING SPECIES DISTRIBUTIONS BY PRODUCTIVITY AND MORTALITY RATES IN MULTISPECIES MODELS

Abstract The temporal change in species composition is one dimension that may be used to characterize ecosystems. Multispecies interactions and species abundance (or biomass) distributions can be analyzed by an N‐species Lotka–Volterra (LV) system of equations. We derive a statistical approximation to the stationary probability distribution of relative species biomass for a system of interacting species. The distribution is parameterized by the mean and variance of the LV species interaction coefficients and can exhibit a variety of shapes. Given this distribution, one can describe the state of the system, and subsequent perturbations within the system, in terms of shifts in the resulting stable distribution. Theoretical distributions with known properties were compared to distributions estimated from simulations and good agreement was found. Our analytical result could be used to develop “ecosystem indicators” for describing changes in the state of species within an ecosystem.

Vegetation change in Southeast Greenland? Tasiilaq revisited after 40 years

Questions: Are there changes in species composition of the oceanic, Low-Arctic tundra vegetation after 40 years? Can possible changes be attributed to climate change? Location: Ammassalik Island near Tasiilaq, Southeast Greenland. Methods: Species composition and cover of 11 key vegetation types were recorded in 110 vegetation survey plots in 1968–1969 and in 11 permanent plots in 1981. Recording was repeated in 2007. Temporal changes in species composition and cover between the surveys were tested using permutation tests linked with constrained ordinations for vegetation types, and Mann–Whitney tests for individual species. Changes in vegetation were related to climate change. Results: Although climate became warmer over the studied period, most of the vegetation types showed minor changes. The changes were most conspicuous in mire and snowbed vegetation, such as the Carex rariflora mire and Hylocomium splendens snowbed. In the C. rariflora mire, species number and cover of vascular plants and cover of bryophytes increased, whereas in the H. splendens snowbed species numbers of vascular plants, bryophytes, and also lichens increased. Lichen richness increased in the Carex bigelowii snowbed and cover of bryophytes in the Salix herbacea snowbed. No such changes occurred in the Alchemilla glomerulans meadow, Alchemilla alpina snowbed and Phyllodoce coerulea heath. There was no change of species composition within the Salix glauca scrub, A. alpina snowbed, lichen grassland and the Empetrum nigrum and Phyllodoce coerulea heaths. Most changes resulted from increasing frequency or cover of some species; there were very few decreasing species. Most of the increasing species indicate drier substrate conditions. Conclusions: Only minor changes in species composition and cover were detected in the vegetation types studied. These changes were probably caused by milder winters and warmer summers during the years before the 2007 sampling. Climate warming may have reduced the duration of snow cover and soil moisture, particularly in snowbed and mire habitats, where species composition change was most pronounced. However, its magnitude was insufficient to cause a major change in species composition. Thus, on the level of plant community types, tundra vegetation near Tasiilaq was rather stable over the last 40 years.

Contrasting seasonal niche separation between rare and abundant taxa conceals the extent of protist diversity

With the advent of molecular methods, it became clear that microbial biodiversity had been vastly underestimated. Since then, species abundance patterns were determined for several environments, but temporal changes in species composition were not studied to the same level of resolution. Using massively parallel sequencing on the 454 GS FLX platform we identified a highly dynamic turnover of the seasonal abundance of protists in the Austrian lake Fuschlsee. We show that seasonal abundance patterns of protists closely match their biogeographic distribution. The stable predominance of few highly abundant taxa, which previously led to the suggestion of a low global protist species richness, is contrasted by a highly dynamic turnover of rare species. We suggest that differential seasonality of rare and abundant protist taxa explains the—so far—conflicting evidence in the ‘everything is everywhere’ dispute. Consequently temporal sampling is basic for adequate diversity and species richness estimates.

Species composition and richness of avifauna in an urban area of southern Brazil

The urban environment has become an object of study by ornithologists, who seek to understand the relationships between this new habitat and the birds that survive within it. Aiming to investigate the composition and richness of the avifauna present at the State University of Maringá and identify temporal changes in species composition, the obtained results were compared to the study accomplished by Krügel (1998) in this same location. Thus, we performed field samplings from August 2007 to July 2008, using transects, with visual and auditive identification. We recorded 74 species belonging to 27 families from 12 orders, during 72 hours of observation. Most species were constant in the study area. The predominant trophic categories were insectivores (46%) and omnivores (20%). Comparing the analyzed periods, we verified a reduction in the number of species, besides the record of new species for the studied location.

Temporal changes in species composition of mixedwood stands in northwest New Brunswick: 1946–2008

Patterns of softwood (SW) – hardwood (HW) change from 1946 to 2006 in 32 unharvested mixedwood (MW) stands in northern New Brunswick were analyzed using aerial photographs (1946, 1966, 1982, and 2006), sampled, and related to disturbance and stand conditions. Five stand development patterns were identified based on 1946 SW content (70%–80%, termed SW versus 30%–60%, termed MW) and change in SW content from 1946 to 2006 (SW-stable, SW-declining, MW-fluctuating, MW-stable, or MW-declining). Species composition was surprisingly changeable over this 60-year period, with change in SW content varying from +18% to –62%. High canopy cover reduction from 1946 to 1966 resulted from balsam fir ( Abies balsamea (L.) Mill.) mortality due to old age and a 1950s spruce budworm ( Choristoneura fumiferana Clem.) outbreak plus birch ( Betula sp.) dieback. SW-stable stands that maintained SW composition from 1946 to 2006 (+7%) had more red spruce ( Picea rubens Sarg.) than all other classes in which SW declined by 15%–47%. SW-declining stands were located on southerly aspects (189°) and had higher mean elevations (423 m) than other classes. Results suggest that balsam fir – tolerant HW MW stands may be naturally transitional due to disturbance, species, and stand conditions, which has significant implications for forest management designed to maintain static proportions of MW and SW stands.

Temporal changes in species composition in Fescue Prairie: relationships with burning history, time of burning, and environmental conditions

Historically, fires occurred throughout the year in the Fescue Prairie of Canada, but little is known about plant community responses to burning at different times of the year. Composition of plant communities was determined annually for 6 years after burning one or three times in a remnant Fescue Prairie in central Saskatchewan. A multiple-response permutation procedure indicated that plant community composition was different in the two burning histories (P < 0.001) and among times of burning (P < 0.001). Variables related to plant community composition after burning were evaluated using Non-metric Multidimensional Scaling. Three gradients explained 86% of the variation in composition of plant communities. Environmental conditions leading up to burning and at the time of burning correlated poorly with species composition. Differences in composition of plant communities were attributed primarily to burning history, cumulative precipitation in the 12 months before sampling, cumulative cold-stress days in the 12 months before sampling, cold-stress days in March and April, and months since burning. Plant communities burned once responded negatively to increasing cold-stress days while those burned three times responded positively to cold-stress days. Cover of Festuca hallii and Symphoricarpos occidentalis was 88 and 350% greater after one burn as compared to three burns, whereas cover of Carex obtusata, Carex pensylvanica, and Elymus lanceolatus was 126, 53, and 220% greater after three burns than after one burn. Festuca hallii, Galium boreale, Pulsatilla patens ssp. multifida, Symphoricarpos occidentalis, and Symphyotrichum ericoides had the highest indicator value of a single burn; Carex obtusata, Elymus lanceolatus, and Koeleria macrantha had the greatest indicator values for sites burned three times. Longer-term effects of burning history exert a strong influence on plant community composition while short-term conditions after burning, namely, precipitation and cold-stress days, appear important in controlling species responses and composition of plant communities in Fescue Prairie.

Temporal changes in species composition in permanent plots across the Shawnee National Forest, Illinois, USA1

Abstract Species composition in three strata of 63 permanent plots across two physiographic divisions (Ozark Hills and Shawnee Hills) of the Shawnee National Forest (SNF) in southern Illinois was resampled 3–6 years after initial establishment with the objective to assess short-term changes in species composition. Environmental data collected at each plot included overhead canopy cover, elevation, solar radiation, slope, and a suite of nine soil properties. Basal area of the trees in the Ozark Hills Division did not change, but total basal area and basal area of Carya ovata, Fraxinus americana, Quercus rubra, and Ulmus alata increased in the Shawnee Hills Division. In the woody understory, there was an increase in the density of Acer saccharum, C. florida, F. americana, and U. alata in the Shawnee Hills Division, and a decrease in density of C. florida and Fagus grandifolia in the Ozark Hills Division. The decrease in C. florida in the Ozark Hills Division is likely due to an infestation of Discula destru...

Diversity-Stability Relationships of an Alpine Plant Community under Simulated Environmental Change

We examined the relationship between plant community diversity (species richness, evenness) and stability in an alpine Dryas octopetala heath exposed to four years of warming (open top chambers) and nutrient addition at Finse, southern Norway. Furthermore, we examined if different functional types responded differently to the interaction between environmental change and initial community diversity. We used the temporal change in species composition, calculated as the sum of change in sample scores in detrended correspondence analyses, as a measure of variability (opposite of stability). Under ambient conditions, a high initial species richness was associated with less stability of the total species composition. Under experimental warming, initial high species richness and evenness were associated with a more stable subsequent vascular species composition. Vascular stability decreased, however, with higher species richness under nutrient addition. When warming and nutrient addition were combined, high initial evenness was associated with more stable bryophyte composition, whereas high species richness was associated with reduced lichen stability. Thus, the degree and direction of the diversity-stability relationship depended on the type of environmental perturbation, the responding functional type, and on the diversity parameter used. The large variation in diversity-stability relationships is likely an outcome of complex species interactions and environmental factors influencing community diversity.

Long‐term impacts of nitrogen and sulphur deposition on forest floor vegetation in the Northern limestone Alps, Austria

Abstract.Question: Are there effects of long‐term deposition of airborne nitrogen and sulphur on the forest floor vegetation from permanent plots collected in 1993 compared to 2005.Location: Northern limestone Alps in Austria.Methods: Single species responses were analysed by correlating trends in cover‐abundance values, as derived from marginal models, with Ellenberg indicator values. Changes in the species composition of plots were analysed by correlating changes in mean Ellenberg indicator values with the displacement of plots within a multidimensional scaling ordination.Results: Trends in single species abundance were positively correlated with indicator values of soil pH but were independent of nutrient availability. A general trend towards the homogenisation of vegetation, due to convergent time vectors of the relevés, became obvious. Oligotrophic sites previously situated at the distal ends of ordination axes shifted towards the centre since they were enriched by species preferring mesotrophic conditions. The bulk of plots with intermediate site conditions hardly showed any trends. A concomitant analysis demonstrated that temporal changes in species composition exceed the variation in cover abundance estimates among different field botanists.Conclusions: N deposition can lead to a homogenisation of forest floor vegetation. Larger limestone areas with diverse soil conditions, such as the Northern limestone Alps in Austria, as a whole are thus negatively affected by airborne N deposition. Nevertheless, the vegetation was at least as strongly affected by an increase of basiphilous species as a result of decreasing S deposition.

The diet of European Shag Phalacrocorax aristotelis, Black-legged Kittiwake Rissa tridactyla and Common Guillemot Uria aalge on Canna during the chick-rearing period 1981–2007

Chick diet of European Shags Phalacrocorax aristotelis, Black-legged Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge at Canna was investigated over a 27- year period. The diet was mainly composed of Sprats Sprattus sprattus, Lesser Sandeels Ammodytes marinus and members of the Gadidae (a variety of species but mainly Trisopterus spp. and Whiting Merlangius merlangus). Other groups (ten families of fish, crustaceans, cephalopod molluscs and polychaete worms) were of minimal importance. Lesser Sandeels dominated the diet of young Black-legged Kittiwakes and European Shags, Sprats the diet of young Common Guillemots, whereas gadid otoliths were by far the commonest items found in pellets regurgitated by older European Shags. There were few significant temporal changes in species composition or the size of prey taken over the 27 years and the results confirm earlier findings that gadids are a normal and important part of the diet of seabirds at this colony.

Spatial changes in the distributions of deep-sea “Cerviniidae” (Harpacticoida, Copepoda) and their associations with environmental factors in the bathyal zone around Sagami Bay, Japan

To estimate species turnover rates on scales of several tens of km in deep-sea benthic animals, we analyzed spatial and inter-annual changes in species diversity and composition of cerviniids, a typical group of deep-sea harpacticoids, at stations in and around Sagami Bay, central Japan. Associations with environmental factors were also investigated. Generally, bathymetrical patterns in diversity of benthos are unimodal and peak at depths of 2,000–3,000 m. In Sagami Bay, cerviniid diversity did not follow this trend; both species richness and evenness were negatively correlated with water depth. Multivariate analyses [detrended correspondence analysis (DCA) and non-metric multi-dimensional scaling] suggested that temporal changes in species composition of cerviniids are smaller than spatial changes that occur on horizontal scales of several tens of km. Community structure does not change completely on these scales in the bathyal zone around Sagami Bay. DCA also showed that bathymetrical changes in species composition can be regulated by certain factors associated with water depth.

HOLOCENE DIATOMS (BACILLARIOPHYCEAE) FROM FAEROE ISLANDS FJORDS, NORTHERN ATLANTIC OCEAN. II. DISTRIBUTION AND TAXONOMY OF MARINE TAXA WITH SPECIAL REFERENCE TO BENTHIC FORMS

Diatoms were studied in sediment cores SKPC-10, SKPC-08, SKPC-01 and LINK-1, retrieved from the Faeroe Islands fjords and the adjacent waters of the Northern Atlantic Ocean. The diatom record encompasses the Holocene sediments beginning from the middle Holocene. The record shows distinct temporal changes in species composition and is dominated by marine forms. Earlier studies showed that inundation of the fjords by Northern Atlantic waters took place about 7700–6400 years BP. Saline waters inflow forced drastic changes in the diatom species composition in the study area. Unlike other areas of the Northern Atlantic, the Faeroe Islands fjords sediments are characterized by well preserved diatom floras. The diatoms are predominantly of planktonie origin (Paralia sulcata, Thalassiosira nordenskioeldii, Thalassionema nitzschioides, Thalassiosira angulata and Rhizosolenia hebetatd); however, the relatively shallow coring sites result in fairly abundant and diverse benthic flora (Odontella aurita, Grammatophora marina, Grammatophora angulosa var. islandica, Cocconeis costata, Rhabdonema minutum, and Rhabdonema arcuatum). A special focus of this paper is the distribution and taxonomy of the benthic marine taxa, either sediment dwelling or epiphytic.

Seasonal and annual variability in an epifaunal community in the German Bight

While the benthic infauna of the North Sea has been studied intensively over the past decades, few studies have focused on the larger mobile epifauna. Studies carried out to date have described the distribution of epifaunal communities over the whole of the North Sea, but variability within the identified communities, which occurs on a much smaller scale, has so far remained unstudied. This is the first study to describe seasonal and annual variability of an epifaunal assemblage in the German Bight area, where environmental conditions are highly variable. The benthic community was sampled with a 2 m beam trawl from 1998 to 2001. The echinoderms Ophiura albida and Asterias rubens and the crustacean Pagurus bernhardus were the dominant species caught throughout the study period. Overall the species composition of the catches was relatively consistent, while abundances of dominant species fluctuated considerably between sampling periods. Differences between sampling periods were not only influenced by the abundances of dominant species, but also by less dominant species such as Ophiura ophiura, Astropecten irregularis, Corystes cassivelaunus, Crangon crangon and Aphorrahis pespelicani. The abundances of these species varied annually and seasonally in the assemblage. Clear differences between summer and winter in the species composition, abundance and biomass were identified. Annual and seasonal changes were most likely linked to migratory movements of epifauna into and out of the area under investigation. Temporal changes in species composition and abundance correlated best with water temperatures, while the spatial distribution of the total biomass over the whole sampling period was correlated with sediment characteristics. Anthropogenic influences such as fishing activity and chronic large-scale eutrophication are thought to have influenced the community on a long-term basis, but have been considered unlikely causes for the short-term variability described by this study.