El Niño effects on the dynamics of terrestrial ecosystems

New studies are showing that the El Ni?o Southern Oscillation (ENSO) has major implications for the functioning of different ecosystems, ranging from deserts to tropical rain forests. ENSO-induced pulses of enhanced plant productivity can cascade upward through the food web invoking unforeseen feedbacks, and can cause open dryland ecosystems to shift to permanent woodlands. These insights suggest that the predicted change in extreme climatic events resulting from global warming could profoundly alter biodiversity and ecosystem functioning in many regions of the world. Our increasing ability to predict El Ni?o effects can be used to enhance management strategies for the restoration of degraded ecosystems.     

El Niño effects on soil seed bank dynamics in north-central Chile

The soil seed bank was monitored in four 75×75 m plots over 6 years (1990–1995) in an arid thorn scrub community in north-central Chile. Sixty-six species were identified. Total seed densities ranged from 2,000 to 42,000/m². Average mass of shrub seeds was significantly greater than that of other growth-forms. Between 70 and 90% of the seeds were less than 1 mg, with those in the 0.51–1.00 mg size class being most numerous. Seed densities were highly variable between years as well as within years, but were also closely associated with plant cover patterns and rainfall regime. Higher seed densities were found in wet years, and in samples taken in early summer and early autumn (i.e., after seed set); the lowest seed densities were in late winter (i.e., after annual plant germination). The annual plant species with the highest cover were also the most abundant in the soil seed bank and exhibited the largest seed density fluctuations. In general, seed densities were 5- to 10-fold higher during the 1991–1992 El Niño/southern oscillation (ENSO) years than non-ENSO years, showing the importance of this phenomenon for seed bank replenishment in the arid region of Chile.     

The impact of El Niño events on populations of mesopelagic hydromedusae

For over 10 years, the midwater ecology group at MBARI has compiled video and accompanying physical data with the ROV Ventana operating in mesopelagic depths of Monterey Bay, CA in order to elucidate patterns in midwater ecology. Two El Niño events have occurred during this time period, in 1991–92 and in 1997–98. The oceanographic metric of spiciness combines temperature and salinity data into one sensitive measurement. Although temperature and salinity measurements alone revealed no clear patterns, clear signals of spiciness were observed that corresponded to water mass intrusions into the deep waters of the bay during the two El Niño events. During these events, some seldom-seen species were observed in high numbers in the midwater, while historically common species became rare. During non-El Niño years, the leptomedusa Mitrocoma cellularia(A. Agassiz, 1865) was common in the surface waters (0–50 m) of Monterey Bay, but it was not abundant at depth, while the trachymedusa Colobonema sericeum Vanhöffen, 1902 was found in relatively high numbers at mesopelagic depths. During the last two El Niño events, M. cellulariawas observed in higher numbers at mesopelagic depths, whereas C. sericeum was scarce. M. cellularia was found in a wider range of temperatures, salinities, and dissolved oxygen values than was C. sericeum. Transport and tolerance hypotheses are proposed to explain differences in the presence and numerical density of the medusae.     

Positive effects of “El Niño” on macrozoobenthos inhabiting hypoxic areas of the Peruvian upwelling system

Summary Soft-bottom macrozoobenthos was sampled at monthly intervals between September 1981 and September 1984 at a normally hypoxic site (34 m depth) in Ancón Bay (Peru). Temperature, salinity, and dissolved oxygen were measured and related to changes in community structure. Large increases in the number of species, density, biomass, and diversity of macrozoobenthos were observed during the 1982–1983 El Niño (EN) thermal anomaly and for 1 year afterwards. These favourable changes were mainly associated with increased oxygen concentrations found in water masses near the bottom. Results are consistent with the hypothesis that the processes of post-catastrophic recovery of macrozoobenthos on marine soft bottoms are largely predictable, although certain particular mechanisms seem to exist for post-hypoxic recovery during El Niño in areas of the Peruvian coastal upwelling.Contribution No. 55 of the Peruvian-German Cooperative Fisheries Project (PROCOPA-Callao) and No. 63 of the Alfred-Wegener-Institut für Polar- und Meeresforschung (AWI-Bremerhaven)     

Nonstationary influence of El Niño on the synchronous dengue epidemics in Thailand

BackgroundSeveral factors, including environmental and climatic factors, influence the transmission of vector-borne diseases. Nevertheless, the identification and relative importance of climatic factors for vector-borne diseases remain controversial. Dengue is the world''s most important viral vector-borne disease, and the controversy about climatic effects also applies in this case. Here we address the role of climate variability in shaping the interannual pattern of dengue epidemics.ConclusionThe underlying mechanism for the synchronisation of dengue epidemics may resemble that of a pacemaker, in which intrinsic disease dynamics interact with climate variations driven by El Niño to propagate travelling waves of infection. When association with El Niño is strong in the 2–3-y periodic mode, one observes high synchrony of dengue epidemics over Thailand. When this association is absent, the seasonal dynamics become dominant and the synchrony initiated in Bangkok collapses.     

El Niño droughts and their effects on tree species composition and diversity in tropical rain forests

In this study I investigated the effects of the extreme, 1997/98 El Niño related drought on tree mortality and understorey light conditions of logged and unlogged tropical rain forest in the Indonesian province of East Kalimantan (Borneo). My objectives were to test (1) whether drought had a significant effect on tree mortality and understorey light conditions, (2) whether this effect was greater in logged than in undisturbed forest, (3) if the expected change in tree mortality and light conditions had an effect on Macaranga pioneer seedling and sapling densities, and (4) which (a)biotic factors influenced tree mortality during the drought. The 1997/1998 drought led to an additional tree mortality of 11.2, 18.1, and 22.7% in undisturbed, old logged and recently logged forest, respectively. Mortality was highest in logged forests, due to extremely high mortality of pioneer Macaranga trees (65.4%). Canopy openness was significantly higher during the drought than during the non-drought year (6.0, 8.6 and 10.4 vs 3.7, 3.8 and 3.7 in undisturbed, old logged and recently logged forest, respectively) and was positively correlated with the number of dead standing trees. The increase in light in the understorey was accompanied by a 30 to 300-fold increase in pioneer Macaranga seedling densities. Factors affecting tree mortality during drought were (1) tree species successional status, (2) tree size, and (3) tree location with respect to soil moisture. Tree density and basal area per surface unit had no influence on tree mortality during drought. The results of this study show that extreme droughts, such as those associated with El Niño events, can affect the tree species composition and diversity of tropical forests in two ways: (1) by disproportionate mortality of certain tree species groups and tree size classes, and (2) by changing the light environment in the forest understorey, thereby affecting the recruitment and growth conditions of small and immature trees.     

El Niño,volcanism, and global climate

The June 1991 eruption of Mt. Pinatubo in the Philippines produced one of the greatest volcanic aerosols in the last hundred years. The estimated net decrease of radiation may have peaked at 10% in the tropics. What was the impact of the Pinatubo aerosol on regional and global climate? Besides the expected net cooling of the average global surface temperature, correlation studies indicate that other types of climate anomalies may also be expected. These include the appearance of an El Niño event, decreased Indian monsoon rainfall, fewer tropical storms in the north Atlantic Ocean in 1991–1993, and normal to above normal winter rainfall in California in 1991/92, all of which were observed. A proposed physical mechanism for the almost-simultaneous occurrence of this constellation of climate anomalies is presented. The results of correlation studies between low-latitude volcanic aerosols and the El Niño/Southern Oscillation are presented in some detail as one example. The correlation between Indian monsoon rainfall and tropical storms in the north Atlantic Ocean is also shown and is updated for the most recent 5 years.     

The El Niño / Southern Oscillation and Australian vegetation

The El Niño-Southern Oscillation (ENSO) phenomenon has a marked effect on Australia's rainfall. The tendency for major Australian droughts to coincide with ENSO “events” (i.e. anomalously warm sea surface temperatures in the east equatorial Pacific), and for extensive wet periods to accompany “anti-ENSO” events, is well documented. Also well-known is the partial predictability of Australian rainfall anomalies provided by ENSO. Some other ENSO-related characteristics of interannual fluctuations of Australian rainfall are lesswidely recognised, viz:

- rainfall variability is very large

- droughts and wet periods have time scales of about one year

- they exhibit very large (continental) spatial scales

- they tend to be phase-locked with the annual cycle

- they are often followed/preceded by the opposite rainfall anomaly.

The character of Australian rainfall fluctuations is thus very different from that of areas where the influence of ENSO is weak, Europe for instance. Rainfall in some other areas, notably southern Africa and India and parts of the Americas, is also strongly affected by ENSO and shares some of the above characteristics. The relevance of these ENSO-related characteristics of Australian rainfall to its vegetation will be discussed. Australian native vegetation is adapted to these characteristics, especially in the semi-arid inland where ENSO's influence is strong. Most introduced plants are not adapted to ENSO and this has sometimes complicated their use here. The combination of ENSO-related rainfall fluctuations and European land-use strategies has resulted in some very rapid, unpredicted and undesirable changes in vegetation in the past two centuries. It has also increased the risk of soil erosion. Recognition of the real character of Australian rainfall fluctuations may help avoid further degradation of soil and vegetation.     

Effects of the 1991-92 El Niño on scleractinian corals of the Costa Rican central Pacific coast

Coral communities on the central Pacific coast of Costa Rica were affected during the 1991-92 El Ni?o warming event. More than 57% of all observed colonies at three localities (Parque Nacional Manuel Antonio, Punta Cambutal, and Parque Marino Ballena) were bleached. Mortality during this El Ni?o was much lower (approximately 9%) than in previous events. Psammocora spp. accounted for approximately 66% of dead corals, while massive (Porites lobota, Pavona spp.) and branching (Pocillopora spp.) for approximately 34%. Our results suggest that the observed bleaching in P. lobata was related to zooxanthellar densities and not to changes in pigment concentrations: only chlorophyll a varied between normally pigmented and bleached colonies at one locality (Ballena). Site differences in zooxanthellar densities or their pigment concentrations, may not be the result of the bleaching event itself, because a percentage of dead corals and zooxanthellar densities of bleached colonies seems to follow a trend with the exposure to tidal regimes and currents at each site. Local oceanographic conditions can be influencing the zooxanthellar densities and their response to the warming, together with intrinsic differences between colonies as well. The impact of this event can be considered serious given the short period of time that elapsed between El Ni?o related mortalities and the slow reefs recovery, the mode of reproduction of reef building species, and the anthropogenic-originated disturbances which affect the coral communities and reefs of the Costa Rican central Pacific coast.     

A new generation of real-time environmental monitoring systems to study the impact of El Niño on disease dynamics

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El Niño as a Window of Opportunity for the Restoration of Degraded Arid Ecosystems

Most arid ecosystems have suffered from severe overexploitation by excessive wood harvesting, overgrazing, and agriculture, resulting in depletion of vegetation biomass and soil erosion. These changes are often difficult to reverse due to positive feedbacks that tend to stabilize the new situation. In this paper, we briefly review evidence for the idea that different states in these ecosystems might represent alternative equilibria and present a graphic model that summarizes the implications for their response to changing environmental conditions. We show how, in the light of this theoretical framework, climatic oscillations such as El Niño Southern Oscillation (ENSO) could be used in combination with grazer control to restore degraded arid ecosystems. We also present evidence that, depending on grazing pressure, ENSO episodes can trigger structural and long-lasting changes in these ecosystems.     

Impacts of El Niño-Southern Oscillation Events on the Distribution of Wintering Raptors

Abstract We report the effects of El Niño-Southern Oscillation (ENSO) events on the distribution and abundance of 3 raptor species at continental, regional, and landscape scales. We correlated values from the southern oscillation index (SOI), an index of ENSO phase and strength, with Christmas Bird Count data over a 30-year period. We investigated the relationship between the SOI and winter raptor distributions at 3 spatial scales: continental (central United States), regional (TX, USA), and landscape (3 roadside transects within TX). At the continental scale, ENSO events resulted in regional shifts for American kestrel (Falco sparverius), northern harrier (Circus cyaneus), and red-tailed hawk (Buteo jamaicensis) winter abundances. As expected, these shifts were northward during El Niño (warm) winters, and southward for red-tailed hawks and northern harriers during La Niña (cold) winters. Within Texas, northern harrier distributions shifted towards arid west Texas during wet El Niño winters but were restricted to mesic coastal Texas during dry La Niña winters. Red-tailed hawk abundance increased in eastern Texas during La Niña winters responding to cooler than normal temperatures throughout the northern Midwest. Data from local roadside transects over a 3-year period encompassing 2 El Niño winters and one La Niña winter supported the abundance patterns revealed by continental and regional data, and added evidence that fluctuations in winter abundances result from demographic pulses as well as spatial shifts for wintering populations. This study underscores the need for long-term monitoring at both local and regional spatial scales in order to detect changes in continental populations. Short-term or local studies would have erroneously assumed local population declines or increases associated with ENSO events, rather than facultative movements or demographic pulses supported by this study.     

Effects of El Niño drought on seedling dynamics in a seasonally dry tropical forest in Northern Thailand

As El Niño is predicted to become stronger and more frequent in the future, it is crucial to understand how El Niño-induced droughts will affect tropical forests. Although many studies have focused on tropical rainforests, there is a paucity of studies on seasonally dry tropical forests (SDTFs), particularly in Asia, and few studies have focused on seedling dynamics, which are expected to be strongly affected by drought. Seedlings in SDTFs are generally more drought-tolerant than those in the rainforests, and the effects of El Niño-induced droughts may differ between SDTF and tropical rainforests. In this study, we explored the impact of El Niño-induced drought at an SDTF in northern Thailand by monitoring the seedling dynamics at monthly intervals for 7 years, including a period of strong El Niño. The effects were compared between two forest types in an SDTF: a deciduous dipterocarp forest (DDF), dominated by deciduous species, and an adjacent lower montane forest (LMF) with more evergreen species. El Niño-induced drought increased seedling mortality in both the forest types. The effect of drought was stronger in evergreen than in the deciduous species, resulting in higher mortality in the LMF during El Niño. However, El Niño increased seedling recruitment only in the DDF, mainly because of the massive recruitment of the deciduous oak, Quercus brandisiana (Fagaceae), which compensated for the mortality of seedlings in the DDF. As a result, El Niño increased seedling density in the DDF and decreased it in the LMF. This is the first long-term study to identify the differences in the impacts of El Niño on seedlings between the two forest types, and two leaf habits, evergreen and deciduous, in Southeast Asia. Our findings suggest that future climate change may alter the species composition and spatial distribution of seedlings in Asian SDTFs.     

Drought and the consequences of El Niño in Borneo: a case study of figs

Borneo has a perhumid climate but occasional severe droughts have an important impact. Droughts may affect the composition and size structure of plant communities through differential mortality or, via their impact on the availability of plant resources, affect plant–animal interactions. From January to April 1998, northern Borneo suffered a very severe drought linked to the El Niño Southern Oscillation event of 1997–1998. In this article, the impacts of this drought on the rain forest at Lambir Hills National Park, Sarawak, are considered with special reference to a keystone plant group, the figs. Small fires entered the edge of the forest from the roadside, killing saplings, climbers, and understory trees. Community-wide mortality for adult trees was 0–7 times higher than in nondrought years, with larger trees showing a greater proportional increase. In figs, mortality was significantly higher in pioneers, but hemiepiphytes and roadside species were unaffected. Phenology was substantially affected. Leaf and flower/fruit production decreased or ceased during the drought and increased suddenly following renewed rain. Pollinators of dioecious figs became locally extinct during the drought, and other plant–animal interactions may also have been disrupted. The frequency and severity of droughts has increased substantially in the past three decades, and climate models suggest this may be the result of global warming. The impacts of the 1998 drought at Lambir Hills National Park suggest that, should this trend continue, a substantial alteration of habitats and overall loss of biodiversity can be expected in Borneo.     

Repercussions of El Niño: drought causes extinction and the breakdown of mutualism in Borneo

Figs (Ficus spp.) and their species-specific pollinators, the fig wasps (Agaonidae), have coevolved one of the most intricate interactions found in nature, in which the fig wasps, in return for pollination services, raise their offspring in the fig inflorescence. Fig wasps, however, have very short adult lives and hence are dependent on the near-continuous production of inflorescences to maintain their populations. From January to March 1998 northern Borneo suffered a very severe drought linked to the El Niño-Southern Oscillation event of 1997-1998. This caused a substantial break in the production of inflorescences on dioecious figs and led to the local extinction of their pollinators at Lambir Hills National Park, Sarawak, Malaysia. Most pollinators had not recolonized six months after the drought and, given the high level of endemism and wide extent of the drought, some species may be totally extinct. Cascading effects on vertebrate seed dispersers, for which figs are often regarded as keystone resources, and the tree species dependent on their services are also likely. This has considerable implications for the maintenance of biodiversity under a scenario of climate change and greater climatic extremes.     

“El Niño” events recorded in dry-forest species of the lowlands of northwest Peru

The northwest coast of Peru (5°S, 80°W) is very sensitive to and impacted by the climate phenomenon El Niño-Southern Oscillation (ENSO). Though mainly desert, this warm, dry region contains an equatorial dry forest. We report the first dendrochronological studies from this region and identify several species that have dendrochronological potential. Short ring-width chronologies of Palo Santo (Bursera graveolens) show a well-developed response to the ENSO signal over the last 50 years and good inter-site correlations. Preliminary isotopic studies in Algarrobo (Prosopis sp.) also show evidence of the 1997–98 El Niño event. ENSO events have a strong effect on the variability in the growth of several species and thereby on the economy of rural communities where the wood is used for housing, cooking, furniture, tools, fodder and medicinal uses. The extensive use of wood in archeological sites also offers the possibility of ultimately developing longer records for some of these species.     

Genetic impact of a severe El Niño event on Galápagos marine iguanas (Amblyrhynchus cristatus)

The El Niño-Southern Oscillation (ENSO) is a major source of climatic disturbance, impacting the dynamics of ecosystems worldwide. Recent models predict that human-generated rises in green-house gas levels will cause an increase in the strength and frequency of El Niño warming events in the next several decades, highlighting the need to understand the potential biological consequences of increased ENSO activity. Studies have focused on the ecological and demographic implications of El Niño in a range of organisms, but there have been few systematic attempts to measure the impact of these processes on genetic diversity in populations. Here, we evaluate whether the 1997–1998 El Niño altered the genetic composition of Galápagos marine iguana populations from eleven islands, some of which experienced mortality rates of up to 90% as a result of El Niño warming. Specifically, we measured the temporal variation in microsatellite allele frequencies and mitochondrial DNA diversity (mtDNA) in samples collected before (1991/1993) and after (2004) the El Niño event. Based on microsatellite data, only one island (Marchena) showed signatures of a genetic bottleneck, where the harmonic mean of the effective population size (N_e) was estimated to be less than 50 individuals during the period between samplings. Substantial decreases in mtDNA variation between time points were observed in populations from just two islands (Marchena and Genovesa). Our results suggests that, for the majority of islands, a single, intense El Niño event did not reduce marine iguana populations to the point where substantial neutral genetic diversity was lost. In the case of Marchena, simultaneous changes to both nuclear and mitochondrial DNA variation may also be the result of a volcanic eruption on the island in 1991. Therefore, studies that seek to evaluate the genetic impact of El Niño must also consider the confounding or potentially synergistic effect of other environmental and biological forces shaping populations.     

Mass Mortality of Coral Reef Ascidians Following the 1997/1998 El Niño Event

In April/May each year from 1995 to 2000, ascidians were sampled randomly with 35 1m² quadrats from three different reef habitats (intertidal reef tops, coastal reef walls and shallow-bank reefs) at four replicate localities (Praia do Forte, Itacimirim, Guarajuba and Abai) in northern Bahia (Brazil). As the sampling period included the 1997/1998 El Niño event, the most severe on record, for the first time these results allow a quantitative assessment of the impact of this major environmental stressor on the biodiversity of associated coral reef ascidians. Across all reef habitats, 22 ascidian species were recorded from three different orders (Aplousobranchia, Phlebobranchia and Stolidobranchia). After El Niño, all species showed significantly altered densities (ANOVA, F=602.90, p<0.0001); many species were absent from the reefs within 2 years of the El Niño period, but densities of Lissoclinum perforatum (all reefs) and Echinoclinum verrilli (subtidal reefs) increased significantly from 1998 onwards. Univariate and multivariate analyses confirmed that significant changes in assemblage composition had occurred. BIOENV analysis identified turbidity, mean temperature and cloud cover as the main factors best explaining these assemblage changes. Our results suggest that although the 1997/1998 El Niño had a differential effect on the species contributing to the ascidian assemblage of Brazilian coral reefs, most species disappeared and those remaining are likely to enhance reef degradation through their bioeroding activities.