Nanosilver inhibits nitrification and reduces ammonia‐oxidising bacterial but not archaeal amoA gene abundance in estuarine sediments

Silver nanoparticles (AgNPs) enter estuaries via wastewater treatment effluents, where they can inhibit microorganisms, because of their antimicrobial properties. Ammonia‐oxidising bacteria (AOB) and archaea (AOA) are involved in the first step of nitrification and are important to ecosystem function, especially where effluent discharge results in high nitrogen inputs. Here, we investigated the effect of a pulse addition of AgNPs on AOB and AOA ammonia monooxygenase (amoA) gene abundances and benthic nitrification potential rates (NPR) in low‐salinity and mesohaline estuarine sediments. Whilst exposure to 0.5 mg L^?1 AgNPs had no significant effect on amoA gene abundances or NPR, 50 mg L^?1 AgNPs significantly decreased AOB amoA gene abundance (up to 76% over 14 days), and significantly decreased NPR by 20‐fold in low‐salinity sediments and by twofold in mesohaline sediments, after one day. AgNP behaviour differed between sites, whereby greater aggregation occurred in mesohaline waters (possibly due to higher salinity), which may have reduced toxicity. In conclusion, AgNPs have the potential to reduce ammonia oxidation in estuarine sediments, particularly where AgNPs accumulate over time and reach high concentrations. This could lead to long‐term risks to nitrification, especially in polyhaline estuaries where ammonia‐oxidation is largely driven by AOB.     

Estuarine fish communities respond to climate variability over both river and ocean basins

Estuaries are dynamic environments at the land–sea interface that are strongly affected by interannual climate variability. Ocean–atmosphere processes propagate into estuaries from the sea, and atmospheric processes over land propagate into estuaries from watersheds. We examined the effects of these two separate climate‐driven processes on pelagic and demersal fish community structure along the salinity gradient in the San Francisco Estuary, California, USA. A 33‐year data set (1980–2012) on pelagic and demersal fishes spanning the freshwater to marine regions of the estuary suggested the existence of five estuarine salinity fish guilds: limnetic (salinity = 0–1), oligohaline (salinity = 1–12), mesohaline (salinity = 6–19), polyhaline (salinity = 19–28), and euhaline (salinity = 29–32). Climatic effects propagating from the adjacent Pacific Ocean, indexed by the North Pacific Gyre Oscillation (NPGO), affected demersal and pelagic fish community structure in the euhaline and polyhaline guilds. Climatic effects propagating over land, indexed as freshwater outflow from the watershed (OUT), affected demersal and pelagic fish community structure in the oligohaline, mesohaline, polyhaline, and euhaline guilds. The effects of OUT propagated further down the estuary salinity gradient than the effects of NPGO that propagated up the estuary salinity gradient, exemplifying the role of variable freshwater outflow as an important driver of biotic communities in river‐dominated estuaries. These results illustrate how unique sources of climate variability interact to drive biotic communities and, therefore, that climate change is likely to be an important driver in shaping the future trajectory of biotic communities in estuaries and other transitional habitats.     

The temporal variation of intertidal nematodes in the Westerschelde I. The importance of an estuarine gradient

Biannual meiobenthic sampling (Spring and Autumn) was carried out in 1983–1989 at two fine sandy intertidal stations in the Westerschelde estuary. Both stations are exposed daily for more than one hour and are situated in the polyhaline and the mesohaline zone of the estuary. Average density data of non-selective deposit-feeders > predators > epigrowth-feeders > selective deposit-feeders for both stations are presented in spite of different nematode species composition. No difference between Spring and Autumn nor trend over 7 years could be detected. Higher total nematode densities are found at the polyhaline station (average 3200 ind. 10 cm^?2) in comparison with the mesohaline station (average 2300 ind. 10 cm^?2), a difference mainly due to higher non-selective deposit-feeders and predators densities in the polyhaline station. Each year, heterogeneous variance is found for all feeding types at the mesohaline station, but only for epigrowth-feeders and predators at the polyhaline station. The higher nematode density at the polyhaline station is probably caused by the more stable nematode structure. An unstable nematode temporal pattern at the mesohaline station is suggested to be combined with the detritus food chain system in the mesohaline zone. The unstable estuarine habitats are mainly caused by their upstream effects: the River Schelde, which clearly influences the stability of the nematode communities.     

Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.     

Environmental Variables and Planktonic Communities in Two Ponds of El Hondo Wetland (SE Spain)

The annual cycle of physical and chemical variables and plankton dynamics was studied in two shallow ponds (East and West Ponds) of the El Hondo wetland, an ecosystem of international importance. Water conductivity increased up to 31–49 mS cm^–1 as water level decreased due to high evaporation and minimal water inputs. Initially considered mesohaline, the waters were reclassified as polyhaline during the hot season. EP was subject to successive desiccation‐flooding cycles, and flooding of the dried sediment caused the release of high concentrations of nitrogen and phosphorus compounds, which quickly depleted. The algal species composition was typical of eutrophic ecosystems, and the chlorophyll content indicated that EP was eutrophic and WP mesotrophic. Phytoplanktonic species richness and diversity were low in both ponds. Algal assemblages, in terms of biovolume, were mainly dominated by Dinophyceae in EP and by Cryptophyta in WP. The zooplankton community was dominated by Rotifers (Brachionus and Hexarthra), although Copepods and Ciliates were also important. Different water inputs to the ponds, partial drying in EP during the warm season with the subsequent higher increment of salinity, and the presence of dense populations of submerged macrophytes in WP, explain the differences in plankton communities found between the two ponds.     

The sensitivity of annual grassland carbon cycling to the quantity and timing of rainfall

Global climate models predict significant changes to the rainfall regimes of the grassland biome, where C cycling is particularly sensitive to the amount and timing of precipitation. We explored the effects of both natural interannual rainfall variability and experimental rainfall additions on net C storage and loss in annual grasslands. Soil respiration and net primary productivity (NPP) were measured in treatment and control plots over four growing seasons (water years, or WYs) that varied in wet‐season length and the quantity of rainfall. In treatment plots, we increased total rainfall by 50% above ambient levels and simulated one early‐ and one late‐season storm. The early‐ and late‐season rain events significantly increased soil respiration for 2–4 weeks after wetting, while augmentation of wet‐season rainfall had no significant effect. Interannual variability in precipitation had large and significant effects on C cycling. We observed a significant positive relationship between annual rainfall and aboveground NPP across the study (P=0.01, r²=0.69). Changes in the seasonal timing of rainfall significantly affected soil respiration. Abundant rainfall late in the wet season in WY 2004, a year with average total rainfall, led to greater net ecosystem C losses due to a ～50% increase in soil respiration relative to other years. Our results suggest that C cycling in annual grasslands will be less sensitive to changes in rainfall quantity and more affected by altered seasonal timing of rainfall, with a longer or later wet season resulting in significant C losses from annual grasslands.     

Defining winter trophic habitat of juvenile Gulf Sturgeon in the Suwannee and Apalachicola rivermouth estuaries, acoustic telemetry investigations

Three automated listening post‐telemetry studies were undertaken in the Suwannee and Apalachicola estuaries to gain knowledge of habitats use by juvenile Gulf Sturgeons (Acipenser oxyrinchus desotoi) on winter feeding grounds. A simple and reliable method for external attachment of small acoustic tags to the dorsal fin base was developed using shrink‐tubing. Suspending receivers on masts below anchored buoys improved reception and facilitated downloading; a detection range of 500–2500 m was realized. In the Apalachicola estuary, juvenile GS stayed in shallow water (< 2 m) within the estuarine transition zone all winter in the vicinity of the Apalachicola River mouth. Juvenile GS high‐use areas did not coincide with high density benthic macrofauna areas from the most recent (1999) benthos survey. In the Suwannee estuary, juveniles ranged widely and individually throughout oligohaline to mesohaline subareas of the estuary, preferentially using mesohaline subareas seaward of Suwannee Reef (52% of acoustic detections). The river mouth subarea was important only in early and late winter, during the times of adult Gulf Sturgeon migrations (41% of detections). Preferred winter feeding subareas coincided spatially with known areas of dense macrofaunal benthos concentrations. Following a dramatic drop in air and water temperatures, juvenile GS left the river mouth and estuary, subsequently being detected 8 km offshore in polyhaline open Gulf of Mexico waters, before returning to the estuary. Cold‐event offshore excursions demonstrate that they can tolerate full‐salinity polyhaline waters in the open Gulf of Mexico, for at least several days at a time. For juvenile sturgeons, the stress and metabolic cost of enduring high salinity ( Jarvis et al., 2001 ; McKenzie et al., 2001 ; Singer and Ballantyne, 2002 ) for short periods in deep offshore waters seems adaptively advantageous relative to the risk of cold‐event mortality in shallow inshore waters of lower salinity. Thus, while juveniles can tolerate high salinities for days to weeks to escape cold events, they appear to make only infrequent use of open polyhaline waters. Throughout the winter foraging period, juvenile GS stayed primarily within the core area of Suwannee River mouth influence, extending about 12 km north and south of the river mouth, and somewhat seaward of Suwannee Reef (< 5 km offshore). None were detected departing the core area past either of the northern or southern acoustic gates, located 66 and 52 km distant from the river mouth, respectively.     

Assessing estuarine benthic quality conditions in Chesapeake Bay: A comparison of three indices

Legislation in US and Europe has been adopted to determine the ecological integrity of estuarine and coastal waters, including, as one of the most relevant elements, the benthic macroinvertebrate communities. It has been recommended that greater emphasis should be placed on evaluating the suitability of existing indices prior to developing new ones. This study compares two widely used measures of ecological integrity, the Benthic Index of Biotic Integrity (B-IBI) developed in USA and the European AZTI's Marine Biotic Index (AMBI) and its multivariate extension, the M-AMBI. Specific objectives were to identify the frequency, magnitude, and nature of differences in assessment of Chesapeake Bay sites as ‘degraded’ or ‘undegraded’ by the indices. A dataset of 275 subtidal samples taken in 2003 from Chesapeake Bay were used in this comparison. Linear regression of B-IBI and AMBI, accounted for 24% of the variability; however, when evaluated by salinity regimes, the explained variability increased in polyhaline (38%), high mesohaline (38%), and low mesohaline (35%) habitats, remained similar in the tidal freshwater (25%), and decreased in oligohaline areas (17%). Using the M-AMBI, the explained variability increased to 43% for linear regression, and 54% for logarithmic regression. By salinity regime, the highest explained variability was found in high mesohaline and low polyhaline areas (53–63%), while the lowest explained variability was in the oligohaline and tidal freshwater areas (6–17%). The total disagreement between methods, in terms of degraded-undegraded classifications, was 28%, with high spatial levels of agreement. Our study suggests that different methodologies in assessing benthic quality can provide similar results even though these methods have been developed within different geographical areas.     

Stability of Bacterial Composition and Activity in Different Salinity Waters in the Dynamic Patos Lagoon Estuary: Evidence from a Lagrangian-Like Approach

We employed a Lagrangian-like sampling design to evaluate bacterial community composition (BCC—using temporal temperature gel gradient electrophoresis), community-level physiological profiles (CLPP—using the EcoPlate? assay), and influencing factors in different salinity waters in the highly dynamic Patos Lagoon estuary (southern Brazil) and adjacent coastal zone. Samples were collected monthly by following limnetic–oligohaline (0–1), mesohaline (14–16), and polyhaline (28–31) waters for 1 year. The BCC was specific for each salinity range, whereas the CLPPs were similar for mesohaline and polyhaline waters, and both were different from the limnetic–oligohaline samples. The limnetic–oligohaline waters displayed an oxidation capacity for almost all organic substrates tested, whereas the mesohaline and polyhaline waters presented lower numbers of oxidized substrates, suggesting that potential activities of bacteria increased from the polyhaline to oligohaline waters. However, the polyhaline samples showed a higher utilization of some simple carbohydrates, amino acids, and polymers, indicating a shortage of inorganic nutrients (especially nitrogen) and organic substrates in coastal saltwater. The hypothesis of bacterial nitrogen limitation was corroborated by the higher Nuse index (an EcoPlate?-based nitrogen limitation indicator) in the polyhaline waters and the importance of NO₂ ^?, NO₃ ^?, low-molecular-weight substances, and the low-molecular-weight:high-molecular-weight substances ratio, indicated by the canonical correspondence analyses (CCAs). Our results demonstrate the important stability of microbial community composition and potential metabolic activity in the different water salinity ranges, which are independent of the region and time of the year of sample collection in the estuary. This is a quite unexpected result for a dynamic environment such as the Patos Lagoon estuary.     

Scales of spatiotemporal variability in macroinvertebrate abundance and diversity in monsoonal streams: detecting environmental change

1. We quantified spatial and temporal variability in benthic macroinvertebrate species richness, diversity and abundance in six unpolluted streams in monsoonal Hong Kong at different scales using a nested sampling design. The spatial scales were regions, stream sites and stream sections within sites; temporal scales were years (1997–99), seasons (dry versus wet seasons) and days within seasons. 2. Spatiotemporal variability in total abundance and species richness was greater during the wet season, especially at small scales, and tended to obscure site‐ and region‐scale differences, which were more conspicuous during the dry season. Total abundance and richness were greater in the dry season, reflecting the effects of spate‐induced disturbance during the wet season. Species diversity showed little variation at the seasonal scale, but variability at the site scale was apparent during both seasons. 3. Despite marked variations in monsoonal rainfall, inter‐year differences in macroinvertebrate richness and abundance at the site scale during the wet season were minor. Inter‐year differences were only evident during the dry season when streams were at base flow and biotic interactions may structure assemblages. 4. Small‐scale patchiness within riffles was the dominant spatial scale of variation in macroinvertebrate richness, total abundance and densities of common species, although site or region was important for some species. The proportion of total variance contributed by small‐scale spatial variability increased during the dry season, whereas temporal variability associated with days was greater during the wet season. 5. The observed patterns of spatiotemporal variation have implications for detection of environmental change or biomonitoring using macroinvertebrate indicators in streams in monsoonal regions. Sampling should be confined to the dry season or, in cases where more resources are available, make use of data from both dry and wet seasons. Sampling in more than one dry season is required to avoid the potentially confounding effects of inter‐year variation, although variability at that scale was relatively small.     

Spatiotemporal Variation in Shallow-Water Freshwater Fish Distribution and Abundance in a Large Subtropical Coastal Lagoon

Patos Lagoon is located off the southern Brazilian coast and represents one of the largest coastal lagoons in the world. We estimated hydrological and physicochemical conditions associated with spatial variation in the abundance and diversity of freshwater fishes along the lagoon, and inter-annual variability in abundances of freshwater fishes occurring in its estuarine zone. During our study, the region experienced two periods of average rainfall and two periods with above-average rainfall. The characids Astyanax eigenmaniorum and Oligosarcus jenynsii and the siluriform Parapimelodus nigribarbis were the most abundant freshwater fishes in the estuary during wet periods when water levels were higher and salinity was lower. Increases in abundance of these species in the estuarine area, all of which members of primary-division freshwater families, apparently were associated with pulses of reproduction and passive transport from freshwater habitats located near middle and upper lagoon reaches. Abundance of species from secondary freshwater families, such as poeciliids and cichlids, were less correlated with hydrological conditions, and their patterns of occurrence in the estuary suggest active migration from nearby freshwater habitats draining into this area. Findings indicate that freshwater discharge in the basin and expansion/retraction of freshwaters in the middle-upper lagoon determined patterns of freshwater fish abundance and species richness in the estuarine zone.     

Multi‐century tree‐ring precipitation record reveals increasing frequency of extreme dry events in the upper Blue Nile River catchment

Climate‐related environmental and humanitarian crisis are important challenges in the Great Horn of Africa (GHA). In the absence of long‐term past climate records in the region, tree‐rings are valuable climate proxies, reflecting past climate variations and complementing climate records prior to the instrumental era. We established annually resolved multi‐century tree‐ring chronology from Juniperus procera trees in northern Ethiopia, the longest series yet for the GHA. The chronology correlates significantly with wet‐season (r = .64, p < .01) and annual (r = .68, p < .01) regional rainfall. Reconstructed rainfall since A.D. 1811 revealed significant interannual variations between 2.2 and 3.8 year periodicity, with significant decadal and multidecadal variations during 1855–1900 and 1960–1990. The duration of negative and positive rainfall anomalies varied between 1–7 years and 1–8 years. Approximately 78.4% (95%) of reconstructed dry (extreme dry) and 85.4% (95%) of wet (extreme wet) events lasted for 1 year only and corresponded to historical records of famine and flooding, suggesting that future climate change studies should be both trend and extreme event focused. The average return periods for dry (extreme dry) and wet (extreme wet) events were 4.1 (8.8) years and 4.1 (9.5) years. Extreme‐dry conditions during the 19th century were concurrent with drought episodes in equatorial eastern Africa that occurred at the end of the Little Ice Age. El Niño and La Niña events matched with 38.5% and 50% of extreme‐dry and extreme‐wet events. Equivalent matches for positive and negative Indian Ocean Dipole events were weaker, reaching 23.1 and 25%, respectively. Spatial correlations revealed that reconstructed rainfall represents wet‐season rainfall variations over northern Ethiopia and large parts of the Sahel belt. The data presented are useful for backcasting climate and hydrological models and for developing regional strategic plans to manage scarce and contested water resources. Historical perspectives on long‐term regional rainfall variability improve the interpretation of recent climate trends.     

Seasonal abundance of anopheline mosquitoes and their association with rainfall and malaria along the Matapí River,Amapí, Brazil

Three communities separated by 1.5–7.0 km, along the Matapí River, Amapá State, Brazil, were sampled monthly from April 2003 to November 2005 to determine relationships between seasonal abundance of host‐seeking anophelines, rainfall and malaria cases. Out of the 759 821 adult female anophelines collected, Anopheles darlingi Root (Diptera: Culicidae) was the most abundant (56.2%) followed by An. marajoara Galvão & Damasceno (24.6%), An. nuneztovari Gabaldón (12.4%), An. intermedius (Chagas) (4.4%) and An. triannulatus (Neiva and Pinto) (2.3%). Vector abundance, as measured by human landing catches, fluctuated during the course of the study and varied in species‐specific ways with seasonal patterns of rainfall. Anopheles darlingi and An. triannulatus were more abundant during the wet‐dry transition period in June to August, whereas An. marajoara began to increase in abundance in February in two villages, and during the wet‐dry transition in the other village. Anopheles nuneztovari and An. intermedius increased in abundance shortly after the rains began in January to February. A generalized linear mixed model (GLMM) analysis of 32 consecutive months of collections showed significant differences in abundance for each species by village and date (P < 0.0001). Correlations between lagged rainfall and abundances also differed among species. A strong positive correlation of An. darlingi abundance with rainfall lagged by 4 and 5 months (Pearson's r = 0.472–0.676) was consistent among villages and suggests that rainfall may predict vector abundance. Significant correlations were detected between numbers of malaria cases and abundances of suspected vector species. The present study shows how long‐term field research may connect entomological and climatological correlates with malaria incidence.     

Relationships among fire frequency, rainfall and vegetation patterns in the wet–dry tropics of northern Australia: an analysis based on NOAA-AVHRR data

Aim To quantify the regional‐scale spatio‐temporal relationships among rainfall, vegetation and fire frequency in the Australian wet–dry tropics (AWDT). Location Northern Australia: Cape York Peninsula, central Arnhem, central Kimberly, Einasleigh Uplands, Gulf Fall Uplands and northern Kimberley. Methods Monthly ‘fraction of photosynthetic active radiation absorbed by green vegetation’ (fAPAR) was decomposed into monthly evergreen (EG) and monthly raingreen (RG) components using time‐series techniques applied to monthly normalized difference vegetation index (NDVI) data from Advanced Very High Resolution Radiometer (AVHRR) imagery. Fire affected areas were independently mapped at the same spatio‐temporal resolution from AVHRR imagery. Weather station records were spatially interpolated to create monthly rainfall surfaces. Vegetation structural classes were derived from a digitized map of northern Australian vegetation communities (1 : 1,000,000). Generalized linear models were used to quantify relationships among the fAPAR, EG and RG signals, vegetation structure, rainfall and fire frequency, for the period November 1996–December 2001. Results The fAPAR and EG signals are positively correlated with annual rainfall and canopy cover, notably: EG_{closed forest} > EG_{open heathland} > EG_{open forest} > EG_woodland > EG_{open woodland} > EG_{low woodland} > EG_{low open woodland} > EG_{open grassland}. Vegetation height and fAPAR are positively correlated, excluding the special case of open heathland. The RG signal is highest where intermediate annual rainfall and strong seasonality in rainfall coincide, and is associated with vegetation structure as follows: RG_{open grassland} > RG_woodland > RG_{open forest} > RG_{open heathland} > RG_{low woodland} > RG_{open woodland} > RG_{low open woodland} > RG_{closed forest}. Monthly RG tracks monthly rainfall. Annual proportion of area burnt (PB) is maximal where high RG coincides with low EG (open grassland, several woodland communities). PB is minimal in vegetation where both RG and EG are low (low open woodland); and in vegetation where EG is high (closed forest, open heathland). Conclusions The RG–EG scheme successfully reflects digitally mapped tree and grass covers in relation to rainfall. RG–EG patterns are strongly associated with fire frequency patterns. PB is maximal in areas of high RG, where high biomass production during the wet season supports abundant fine fuel during the dry season. PB is minimal in areas with high EG, where relatively moist fuel limits fire ignition; and in areas with low EG and RG, where a relative short supply of fuel limits fire spread.     

Short and long-term movement and site fidelity of juvenile Haemulidae in back-reef habitats of a Caribbean embayment

In the context of a main project that aims to recover modern data on diatom distribution applicable to paleosalinity reconstructions in coastal areas of Southern South America, the composition and distribution of dead diatom assemblages in the littoral zone of the Quequén Salado estuary (Argentina) were studied. Diatom zones were defined along the estuarine gradient by cluster analysis and related to the salinity range and sediment composition by Canonical Correspondence Analysis. Four diatom zones were identified. A mixture of marine, brackish and freshwater diatoms, probably allochthonous, characterized the inlet (zone I). Marine/brackish taxa, represented mainly by Paralia sulcata dominated zone II, characterized by polyhaline conditions and sandy sediments. Zone III was characterized by mesohaline conditions, muddy sediments and the dominance of the estuarine diatom Amphora helenensis. Brackish/freshwater and freshwater diatoms dominated the headwaters (zone IV), where salinity was always below 5‰. The comparison of Quequén Salado diatom assemblages with previous results from the Quequén Grande estuary showed a similar taxonomic composition between both estuaries. However, differences in the salinity ranges of the estuaries (related to differences in the degree of human impact and tidal range) lead to a displacement in their spatial distribution along the longitudinal estuarine axis. This paper contributes to the knowledge of the ecological requirements of South American estuarine diatoms and provides useful data for paleosalinity reconstructions in the region. Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users Handling editor: J. Padisak     

The role of El Niño–Southern Oscillation in the dynamics of a savanna large herbivore population

Our understanding of large‐scale climatic phenomena and dynamics of large herbivore populations comes principally from research in northern regions with temperate, seasonal climate and animal communities with relatively low species diversity. To assess the generality of that perspective, we investigated effects of El Niño–Southern Oscillation (ENSO) on population dynamics of African buffalo Syncerus caffer inhabiting a semi‐arid savanna with variable rainfall. We used linear and nonlinear‐threshold models to investigate relationships between population parameters and explanatory variables affecting forage conditions (seasonal rainfall, Southern Oscillation Index [SOI]). El Niño‐related droughts in 1982–1983 and 1991–1992 were associated with strongly negative population change, a pattern expected to coincide with a decrease in normally high and constant adult survival. Consistent with that nonlinear pattern, we detected threshold relationships between wet‐season rainfall and population change. Juvenile recruitment was described best by linear relationships with dry‐season. Because ENSO operates primarily through wet‐season rainfall, whereas population dynamics were also related to dry‐season rainfall, SOI did not have the predictive ability of individual weather components.     

The mid-domain effect applied to elevational gradients: species richness of small mammals in Costa Rica

Aim The objective of this study was to comprehensively document and examine the alpha and gamma patterns of species richness in non-volant, small mammals (rodents, shrews and mouse opossums) along a tropical elevational gradient. These data were used to determine the support for existing hypotheses of species richness encompassing mid-domain null models, as well as climatic, and community overlap hypotheses. Location Field studies were conducted along a Caribbean slope of the Río Peñas Blancas watershed in the north-eastern region of Costa Rica between 750 and 1850 m at 10 sampling sites. Methods Species richness and abundances of small mammals were surveyed for four seasons including three temporal replicates at each of five elevational sites: late wet season (2000), early wet season (2001), and dry season (2002), and one spatial replicate at five different sites within the same elevations during the late wet season (2001). Species richness at elevations below 700 m was compiled from specimen records from 23 US national and international collections. Predictions of a null model based solely on geometric constraints were examined using a Monte Carlo simulation program, Mid-Domain Null. Results In 16,900 trap nights, 1561 individuals from 16 species were captured. Both alpha and gamma species richness peaked at mid-elevation between 1000 and 1300 m, with richness declining both at higher and lower elevations. Most of the empirical curves of species richness occur within 95% prediction curves of the mid-domain model, although deviations from the null model exist. Regression of the empirical richness on the null model predictions explained nearly half of the variation observed (r² = 0.45, P = 0.002). Main conclusions The geometric constraints of montane topography appear to influence the diversity pattern of small mammals, although climatic conditions including an intermediate rainfall and temperature regime, and distance from the persistent cloud cap also are correlated with the pattern of species richness. The predictions of productivity, and community overlap hypotheses are not supported with the empirical data.     

Long‐term changes in semi‐arid vegetation: Invasion of an exotic perennial grass has larger effects than rainfall variability

Abstract. Questions: This paper examines the long‐term change in the herbaceous layer of semi‐arid vegetation since grazing ceased. We asked whether (1) there were differences in the temporal trends of abundance among growth forms of plants; (2) season of rainfall affected the growth form response; (3) the presence of an invasive species influenced the abundance and species richness of native plants relative to non‐invaded plots, and (4) abundance of native plants and/or species richness was related to the time it took for an invasive species to invade a plot. Location: Alice Springs, Central Australia. Methods: Long‐term changes in the semi‐arid vegetation of Central Australia were measured over 28 years (1976–2004) to partition the effects of rainfall and an invasive perennial grass. The relative abundance (biomass) of all species was assessed 25 times in each of 24 plots (8 m × 1 m) across two sites that traversed floodplains and adjacent foot slopes. Photo‐points, starting in 1972, were also used to provide a broader overview of a landscape that had been intensively grazed by cattle and rabbits prior to the 1970s. Species’abundance data were amalgamated into growth forms to examine their relationship with environmental variation in space and time. Environmental variables included season and amount of rainfall, fire history, soil variability and the colonization of the plots by the exotic perennial grass Cenchrus ciliaris (Buffel grass). Results: Constrained ordination showed that season of rainfall and landscape variables relating to soil depth strongly influenced vegetation composition when Cenchrus was used as a covariate. When Cenchrus was included in constrained ordination, it was strongly related to the decline of all native growth forms over time. Univariate comparisons of non‐invaded vs impacted plots over time revealed unequivocal evidence that Cenchrus had caused the decline of all native growth form groups and species richness. They also revealed a contrasting response of native plants to season of rainfall, with a strong response of native grasses to summer rainfall and forbs to winter rainfall. In the presence of Cenchrus these responses were strongly attenuated. Discussion: Pronounced changes in the composition of vegetation were interpreted as a response to removal of grazing pressure, fluctuations in rainfall and, most importantly, invasion of an exotic grass. Declines in herbaceous species abundance and richness in the presence of Cenchrus appear to be directly related to competition for resources. Indirect effects may also be causing the declines of some woody species from changed fire regimes as a result of increased fuel loads. We predict that Cenchrus will begin to alter landscape level processes as a result of the direct and indirect effects of Cenchrus on the demography of native plants when there is a switch from resource limited (rainfall) establishment of native plants to seed limited recruitment.     

Vegetative Phenology and Growth of a Facultatively Deciduous Bamboo in a Monsoonal Climate1

Canopy closure, leaf flush, and ramet recruitment in Bambusa arnhemica, a semelparous, clumping bamboo from the Australian monsoonal tropics, were monitored monthly for 2.5 years at three sites along a flood gradient. Bambusa arnhemica was facultatively deciduous, remaining evergreen at a downslope riparian site but suffering total loss of canopy on a hillside for up to 4 mo during the dry season. Leaf flush was flexible, occurring after as little as 25 mm of rain at the onset of wet season, in response to unusual dry season storms, and apparently also in response to fire independent of rainfall. New culms emerged soon after leaf flush early in the wet season. Culm growth took place during the middle and late wet season, with peak elongation rates of 15–30 cm/day. Some growth continued into the dry season, mostly on branches and leaves of new culms at riparian sites. Not all culms completed elongation before the onset of the dry season, and those that did not were permanently stunted. The demands of culm elongation may limit the occurrence of bamboo in wet‐dry climates to areas with predictable and sustained wet season rainfall, but the flexibility of branching and leaf processes facilitates coping with, and permits exploitation of less predictable pre‐ and postmonsoonal rains. The bamboo growth form and phenological patterns differ markedly from those of dicotyledonous trees and shrubs.