How El Nino Events Affect Terrestrial Ecosystems Worldwide

El Niño is a recognizable, but often misunderstood phrase. The effects of this phenomenon are what are most commonly referred to as El Niño in general conversation. However scientists prefer to distinguish between the cause and its effects by using the term El Niño events to refer to the climatic consequences. Recognizing these causes and effects is important so that scientists may predict future events and mitigate their outcomes. El Niño events are significant in that they involve the oceans, the landmasses, and the atmosphere worldwide. Its effects endure for many years (Holmgren et al. 2001). I will describe the climactic interactions that lead up to these events and how these events ultimately have devastating repercussions for terrestrial ecosystems.

A series of climactic interactions initiate all El Niño events. The term El Niño is used to refer to a warm, equatorial ocean current flowing towards South America from the west. When the Peru current flowing north from the Antarctic abruptly veers west near Ecuador, it collides with the El Niño current. During El Niño events, the normal pressures over Tahiti and Darwin, Australia switch, and associated trade-winds reverse. This is termed the southern oscillation (Ricklefs 2001). The Peru Current is weakened as a result. Warm water from the west, driven by the El Niño current, builds-up along the coast of South America, disrupting upwelling and marine ecosystems (Ricklefs 2001). The combined irregularities in pressure and sea-surface temperature are used to calculate the ENSO (El Niño Southern Oscillation) index. This index is useful in predicting El Niño events. The frequency of this phenomenon ranges from every one to 11.5 years, and its magnitude is virtually unpredictable (Jaksic 2001). The ensuing effects of these events reverberate worldwide. The warmer ocean water in the equatorial Pacific evaporates more easily (Jaksic 2001), thus the formation of clouds intensifies. The subtropical jet stream carries this moisture east (Ricklefs 2001). Excessive precipitation is observed in equatorial Africa, the contiguous United States east of the Rocky Mountains, and western equatorial and southeastern South America (Ricklefs 2001). Excessive dryness is observed in southern Africa, Madagascar, northern Australia, Indonesia, and eastern equatorial South America (Ricklefs 2001).

Although precipitation is often a limiting factor for autochthonous producers, excessive moisture is not always beneficial at the ecosystem level. A cascade of explosive growth overburdens the land. Deserts see dramatic increase in greening and flowering. Massive germination of annuals, marked increase in growth, flowering and fruit production, and a sharp increase in the seed bank in the soil here are common (Holmgren et al. 2001). Increased precipitation, which can reach up to four to ten times greater than average, leads to increased primary production. This leads to increased populations in herbivores, and subsequently carnivores (Holmgren et al. 2001). Small herbivorous and granivorous rodent populations can reach up to twenty times their normal levels (Holmgren et al.2001)! These larger populations of herbivores can overgraze the land. Because these populations cannot be sustained, they decline in the drier years following (Holmgren et al. 2001).

The opposite extreme does not tend to be any more beneficial for terrestrial ecosystems. Those areas that are unseasonably dry following ENSO events also resonate with devastation. In agricultural areas crops fail and resources become scarcer. Consequently, livestock have difficulties finding adequate grazing areas. El Niño induced droughts can incite devastating wildfires. Explosive growth from previous years leads to excessive fuel once dry (Letnic et al. 2005). These wildfires ravage the dry lands, wiping out forests and prairies and the populations that reside there. Fluctuations toward either temperature extreme generally despoil terrestrial ecosystems.

The scope of ENSO events' effects on terrestrial ecosystems is quite broad, affecting terrestrial food chains from the bottom-up and the top-down. The same event may temporarily allow some species to thrive and others to decline. When considering the periodic nature of ENSO events, it follows that regular population fluctuations are often a direct result of these events. Small mammal populations in the arctic often fluctuate every three to five years (Samelius et al. 2007). The dramatically higher than normal level of food is termed a resource pulse, and commonly impacts the trophic ecology in many systems worldwide (Samelius et al. 2007). ENSO events cause resource pulses. The bottom-up effects of these pulses on populations are further complicated by the top-down effects of predators. Population increases in one species of prey may cause the top predator to focus on this species, and other prey species are then able to flourish (Ostfeld and Keesing 2000). Thus a case can be made for ENSO events complicating terrestrial ecosystems around the world, including the individual populations within them.

It is apparent that El Niño events are detrimental beyond the areas of their origin, Tahiti and Australia. Here I have given support that these events are devastating to terrestrial ecosystems. But the aftermath of the southern oscillation is notable globally. Even a tiny insect on the opposite side of the world can feel the effects. Ecologists should be aware that this and other climactic events can disturb ecosystems in ways we might not first suspect, and we should aim to research and understand these events quickly and thoroughly so that their worst effects can be forecast and mitigated.

Literature Cited

Holmgren, M., M. Scheffer, E. Ezcurra, J. R. Gutierrez, and G. M. J. Mohren. 2001. El Niño effects on the dynamics of terrestrial ecosystems. Trends in Ecology and Evolution 16(2):89-94.

Jaksic, F. M. 2001. Ecological effects of El Niño in terrestrial ecosystems of western South America. Ecography 24:241-250.

Letnic, M., B. Tamayo, and C. Dickman. 2005. The responses of mammals to La Niña (El Niño Southern Oscillation)-associated rainfall, predation, and wildfire in central Australia. Journal of Mammalogy 86(4):689-703.

Ostfeld, R. S., and F. Keesing. 2000. Pulsed resources and community dynamics of consumers in terrestrial ecosystems. Trends in Ecology and Evolution 15:232-237.

Ricklefs, R.E. 2001. The Economy of Nature. 5th ed. W. H. Freeman and Company, New York, 550pp.

Samelius, G., R. T. Alisauskas, K. A. Hobson, and S. Lariviere. 2007. Prolonging the arctic pulse: long-term exploitation of cached eggs by arctic foxes when lemmings are scarce. Journal of Animal Ecology 76:873-880.