The Effect of El Niño-Southern Oscillation on the Birds of the Galápagos Islands

Melody Shieh
GEOL388
July 12, 2002

El Niño, translated as "The Child" and technically known as El Niño-Southern Oscillation (ENSO), occurs every two to eleven years. This phenomenon is predominantly characterized by an anomalous rise in sea surface temperature (SST) and rainfall. During non-ENSO periods, the cool, weak, eastward winds and warm, strong westward winds maintain a balance in the west Pacific region. Storm clouds accumulate and condensate above the ocean's warm surface spots. This region is normally far west from South America. Strong winds pushing the warm water layer westward enable the cooler, nutrient-rich layer of the ocean to upwell near the coast of South America. This layer is called the thermocline, and is normally deeper on the west side and shallower on the east side (Suplee 81).

Every two to eleven years, the westward winds weaken and eastward winds strengthen, pushing those ocean hotspots further east, closer to South America. This switch in the two wind speeds causes changes in pressure and, coupled with water expansion due to warmer temperature, results in a sea level decrease in the western Pacific and a sea level increase in the eastern Pacific. The storm clouds thus travel with the areas of increased SST and condensate in the central Pacific region, near the GalápagosIslands. Due to this shift in wind direction, warmer western Pacific waters flow over the cool waters of the east Pacific. This causes equalization in temperature of the top layer of the ocean and pushes the nutrient-rich waters to lower depths, preventing them from upwelling. This can last for about ten months, spanning the breeding cycles and gestation periods of many species of birds and plants. Once the main phase of ENSO passes, the decay process begins: the eastward and westward Pacific winds exchange magnitudes. The ocean hot spot begins to travel back west, thereby allowing the thermocline to resume its upwelling and reestablish its shallowness in the east. Approximately a year and a half after ENSO, Pacific weather conditions return to normal.

The diversity and abundance of marine life-and consequently marine bird life-in the GalápagosIslands is largely attributed to the upwelling of the nutrient-rich thermocline. When El Niño deprives this continuous wash of sustenance from the various species of underwater life in the GalápagosIslands, they must either migrate or, in the case of those less mobile, die. The same conditions apply to the survival of sea birds. Those species that prefer specific locales or distances from the island coasts are forced to fly farther and longer to find migrated fish to feed the younger generation. Some areas are rendered uninhabitable to sea birds by increased rainfall and sea level. The sea birds affected by ENSO are the Galápagospenguin (Spheniscus mendiculus), flightless cormorant (Nannopterum harrisi), waved albatross (Diomedea irrorata), Audubon's shearwater (Puffinus lherminieri), wedged-rumped storm petrel (Oceanodroma thethys), brown pelican (Pelecanus occidentalis), blue-footed booby (Sula nebouxii), masked booby (Sula dactylatra), red-footed booby (Sula sula), swallowtailed gull (Creagrus furcatus), dark-rumped petrel (P. phaeopygia), red-billed tropicbird (Phaethon aethereus), great frigatebird (F. minor), magnificent frigatebird (Fregata magnificens), and lava gull (Larus fuliginosus). Land birds with exceptional responses to ENSO are Darwin's finches (Geospiza) and the Galápagosmockingbird (Nesomimus parvulus) (Coulter, et. al. 14,438).

The GalápagosIslands, described by Charles Darwin as a "living laboratory of evolution," (Grove 3), have been repeatedly subjected to conditions caused by ENSO that often devastate many species of life inhabiting the islands or the surrounding areas. During the 1982-83 and 1986-87 ENSO, the increase in SST temperature around and rainfall upon the islands both directly and indirectly affected the various species of land and sea birds. Consequently, the land birds, namely Darwin's finches and the Galápagosmockingbirds, experienced a thriving successful El Niño season while most species of sea birds suffered malnutrition and reproductive failure.

The 1982-83 ENSO is the most intensely destructive meteorological disturbance in recorded history. Rainfall levels in 1983 in the central Pacific exceeded fourteen times the levels of the preceding five years (Grant & Grant 57). The SST surrounding the Galápagosarchipelago reached up to four degrees Centigrade above normal, a record high. For sea bird reproduction, this was a disaster. The Galápagospenguin, Audubon's shearwater, wedge-rumped storm petrel, brown pelican, swallow-tail gull, blue-footed, red-footed, and masked booby all interrupted their breeding patterns. No attempts at reproduction were even made, and existing nests were abandoned. Since penguins are non-migratory, the food supply shortage devastated their numbers from 12,000 to 2,000, and those who did not or could not swim to farther water bodies to find food starved (The Antarctic Connection). The booby nestling population was also greatly depleted because their usual arid nesting sites were now abound with vegetation (Gibbs, et. al. 441). The blue-footed booby in particular suffered massive failures in reproduction caused by diseases spread by the explosion of mosquito colonies emerging from the heightened temperature and moist atmosphere (Anderson 211). Birds nesting closer to the coast in higher wind speeds were not so affected by mosquitoes. The red-footed boobies, the only species of booby that feed farther from the islands, were thus able to sustain their existing population by eating flying fish, halfbeaks, and needlefish (Grove 7). Other sea birds were not so lucky and were forced to emigrate beyond the boundaries of their usual territory. Among those sea birds that attempted to reproduce yet failed miserably with high infant mortality rates are the flightless cormorant, waved albatross, great frigatebird, and magnificent frigatebird. Constant rain drowned newborns soon after hatching; eggs often rested in puddles and would perish in the low rainwater temperature.

Conversely, several species of sea birds actually fared quite well during the 1982-83 ENSO. The dark-rumped petrel, red-billed tropicbird, and lava gulls were able to successfully continue their breeding cycle. For example, petrel fledglings born during this ENSO period obtained greater mass later in life than those born in previous years (Coulter, et. al. 14,441). However, the reproductive and fledgling growth rate decreased as a result of the food shortage. Plankton-feeding squid are the dark-rumped petrel's primary prey; increased SST caused a decrease in plankton levels, threatening the squid. During ENSO, the petrel nestlings stayed in their nest longer than those of previous years. Perhaps the growth delay is due to non-ideal climate conditions that compel the petrel to actually slow down its growth process to prevent high-energy intake needs, and as a result prevent starvation when adult petrels are unable to find food (Cruz 163).

While some sea birds had the excess energy to attempt reproduction during 1983, others were in desperate simply to survive. Usually shy creatures, the Galápagospenguins oftentimes swam as far as Academy Bay near Puerto Ayora and Santa Cruz to find food, no longer concerned with human intruders. The swallow-tail gull, a species that breeds only in the Galapagos, left the islands completely and were recorded found along the coasts of Ecuador and Peru (Grove 7). Scavengers such as the lava gull found limitless food in the archipelago among the corpses of sea lions, marine iguanas, and blue-footed boobies. Even though the great frigatebird failed in its reproductive attempts, it was able to steal enough food from the red-footed boobies, tropicbirds, and lava gulls to survive well (Grove 8).

During the 1986-87 ENSO, which caused much less severe conditions as the previous ENSO, only the wedge-rumped storm petrel, swallow-tail gull, and all three types of booby were significantly affected. The petrel and swallow-tail gull both abandoned their nests; the petrels left their territory altogether. The blue-footed booby, however, suffered mortality and malnutrition on similar levels as in the 1982-83 ENSO. This is perhaps due to the fact that while the two other types of booby find their food supply in multiple types of fish, the blue-footed booby feeds almost exclusively on the sardine (Sardinops sagax) (Anderson 213). The red-footed and masked boobies experienced little to no reproductive failure during 1986-87. "These healthy [red-footed booby] nestlings contrasted markedly with the starving chicks and corpses in the adjacent blue-footed colony." (Anderson 211). Records show that blue-footed boobies were the most sensitive to climate changes and often suffer extreme losses during typical ENSO years because of their limited palate and insistence on breeding inland while their red-footed and masked cousins were not affected. Not only did ENSO cause direct physical destruction to the sea bird fledglings and eggs from storms and high wind speeds, but it also indirectly caused failure after failure of natural processes essential to their lives.

While the Galápagossea birds were suffering from malnutrition, starvation, relocation, reproductive failure, and death, life inland during the 1982-83 ENSO was flourishing. Increased rainfall and temperature caused a land vegetation explosion. Areas usually arid and semi-arid were now blossoming with lush, green flora. Seed biomass increased 80 percent due to rapid growth of preexisting plants; as the seeds gestated and began to mature, the density of the vegetation grew exponentially (Grant & Grant 58). Certain plant species such as the Croton scouleri gestated and flowered within an amazingly short amount of time. Others such as species of Abutilon and Sida reached record heights, sometimes four times as tall (Grant & Grant 58). One species, however, did not fare as well: Opuntia helleri, the tree-like cactuses. The seeds of the Opuntia are important to the granivorous Darwin's finches (Gibbs & Grant 1737). Unfortunately, the cacti were often covered by such a dense overgrowth of vines that they failed to photosynthesize. Increased groundwater intake caused the pads to swell and grow heavy, falling off or pulling the entire plant to the ground (Grant & Grant 59). The ideal nesting grounds for male finches usually contain the Opuntia cactus and almost all nests were built on Opuntia bushes (Grant & Grant 59). Due to the failure of Opuntia to benefit from the ENSO rains, Darwin's finches began nesting in areas with little or none of these cacti while only about half remained in the bushes.

The two most common species of finch on the island Daphne Major (chosen by researchers Gibbs and Grant because of its small size and conduciveness for study of almost every finch on the island) are the medium ground finch (Geospiza fortis) and the cactus finch (Geospiza scandens). As granivours, these finches gained an abundance of food from the eruption in plant life caused by rainfall as displayed in Fig. 1. (Gibbs & Grant 1737)

Caterpillars are essential for feeding the young for at least a short time while they are developing (9). Fortunately, they appeared in mass quantities and undoubtedly were the strongest positive affect on the finch nestling survival rate. Mosquitoes and fleas that were deathly to the blue-footed boobies became another supply of food (Grove 7). The breeding season more than doubled in duration, enabling the finches to breed continuously, yielding as many as six to eight times as many clutches and four times more young (Gibbs & Grant 1739). Meanwhile, the infant mortality rate was not blessed with the same fortune. During the 1982-83 ENSO, there was a higher infant mortality rate in finches than any other breeding season. Deaths were due to not only fierce rainstorms and winds, which often caused the females to abandon their eggs, but predators were also prevalent. The Galápagosmockingbird (Nesomimus parvulus) and the short-eared owl (Asio flammeus) were able to conceal themselves more effectively with the newly thickened vegetation and terrorized finch eggs and nestlings. Also, those finches that nested in Opuntia bushes suffered losses from collapsing stalks and falling pads that crushed their nests. However, since the reproduction yield was still high, overall finch population nearly quadrupled in 1984.

The other species of land bird most noted for its excellent survival during the 1982-83 ENSO is the Galápagosmockingbird. Aforementioned, their food supply consisted in part of the many abandoned and unsupervised finch nestlings, of which there were many. Furthermore, the mockingbirds obtained extra sustenance from the bodies of thousands of marine iguanas starving or dead from lack of sea lettuce and inability or unwillingness to eat the red algae that sprung up in its place due to increased SST (Grove 5). However, later research revealed that the effects of the 1982-83 and 1986-87 ENSO events played an even deeper role in changing the patterns of the mockingbirds.

In 1989, Robert Curry and Peter Grant studied the social organization of this territorial and cooperatively breeding bird. An aspect of mockingbird society not present in the finches is "...some individuals, termed helpers, provide care for young that are not their own offspring." (Curry & Grant 442). Also, their food supply, consisting of arthropods and a variety of fruits, is constant year-round, which allows the mockingbird groups to remain in one area all their lives and establish a more complex social system. Since mockingbirds are so territorial, increased density of population caused by decreased juvenile deaths became an obstacle to breeding. This is not to say that mockingbirds did not suffer high mortality as the finches did; most problems arose in the adult age group. According to Curry and Grant, an epizootic occurred during the 1982-83 ENSO, "we saw eighty mockingbirds with conspicuous disease symptoms." (Curry & Grant 448). Viruses were more likely transferred between males from territorial fights and overall aggressive behavior. Higher adult male mortality rate caused an imbalance in the sex ratio within groups. Despite these setbacks, mockingbird females yielded clutch sizes of up to six eggs and many more nestlings than non-ENSO years (Curry & Grant 451). The most nestlings were born during the 1986-87 ENSO season because the rains and high winds associated with the 1982-83 ENSO were not present to threaten the nests.

In both cases of the finch and mockingbird, clutch size and nestling density had sharply increased during the 1982-83 and 1986-87 ENSO. It is important to examine the age structure of these two societies caused by this baby boom. In the year after ENSO, there was an extreme imbalance towards the 0.5 - 1.5 year-old age group versus the older groups in both bird species. This population level among Darwin's finches apparently persisted through the following years, establishing itself as a semi-permanent evolutionary development with very recent and significant scientific implications (Gibbs & Grant 1743). Among the mockingbird society, this bottom-heavy age structure affected more than simply their numbers. Because of the increased group proportion in yearlings, the high bird density forced the normally extremely territorial mockingbird to live in close quarters. It is undetermined if this is due to the low dominance status in a fraction of the new yearlings (Curry & Grant 456). Plural groups, breeding by young birds that do not have their own territory, sprung forth in larger numbers because of lack of motivation. However, not all mockingbirds shed this territorial instinct and dispersed to establish their own natal territories. Overrun by nestlings, the surviving yearling mockingbirds left their natal territory to take the place of those groups or adult birds that had died from the storms in 1983. Typically, mockingbirds are unable to breed during their first year of life and so become helpers until they disperse or begin a plural group. As a result, many reproductive failures were due to attempts to breed by young, inexperienced birds. Clearly, the 1982-83 ENSO was intense enough to infiltrate even the very social core of these birds. This event has been marked as an evolutionary landmark, where changes in several of these species, bird and plant, have actually persisted and perhaps will continue to persist for an indefinite amount of time.

The magnitude of the 1982-83 ENSO stretched far beyond the GalápagosIslands. In fact, several individuals from certain species of bird that had never been seen before on the islands, were sighted after periods of heavy rainstorms and high wind speeds had passed. Among them were the black tern (Chlidonias niger), commonly found on the Pacific coasts of Columbia and Peru, the rose-breasted grosbeaks (Pheucticus ludovicianus), usually residing western Ecuador, the eared dove (Zenaida auriculata), belonging to a western Columbian subspecies, and an eastern kingbird (Tyrannus tyrannus) (Curry & Stoleson 505-506). Cattle egrets had never bred in the islands until 1982, when they became regular visitors or maybe even residents. During the mixing wind currents of ENSO, Galapagan resident birds in mid-flight were blown off course and often found themselves on islands to where they would never normally travel. Particulary the dark-billed cuckoo (Coccyzus melacoryphus), cattle egrets (Bubulcus ibis), smooth-billed anis (Crotophaga ani), and the medium ground finch were all reported spotted on the island Genovesa; none of these species had ever been sighted there before. The dark-billed cuckoo was actually able to nest and bear one fledgling, "the first breeding record of this species on Genovesa." (Curry & Stoleson 506). Other species of plants or animals might have been tossed to foreign islands by the strong crosswinds during the 1982-83 ENSO that may be unknown. In an archipelago, such as the GalápagosIslands, where each island's endemic and introduced species is so closely monitored and controlled; where tourists must wash their shoes after stepping off each island; where certain introduced animals are actually being hunted and eliminated this could be a serious threat to rare or endangered species of plant or animal. Most resident species of birds are very territorial and have already established their nesting grounds for each season; new species of birds with new breeding patterns and differing levels of aggressiveness may become a huge problem in preserving the islands' pristine environment.

Since the 1982-83 ENSO, no other ENSO event has ever been as destructive. The Galápagossea birds suffered massive decreases in population about a year after the event. The flightless cormorant experienced a 50 percent reduction in population, and the Galápagospenguin population decreased from 12,000 to 2,000 birds. There are only about 1,000 flightless cormorants living in the GalápagosIslands, and they have been deemed 'rare' by several endangered species establishments (WWF Global). The cormorant's population returned a few years after ENSO, but the penguin's population did not. There are still only about 2,000 Galápagospenguins residing in the islands. The other species of sea birds were able to resume breeding immediately or shortly after ENSO and replenished their numbers within a short time. This was mainly due to an increase in the sea bird to fish food ratio after high mortality rates killed off a large fraction of the preexisting birds, which enabled the females of each species to breed several times a season. Unfortunately, the Galápagospenguin is one of the few endemic species of bird to the archipelago; their inability to recover from their population decline and the rapidity with which their numbers dropped is concerning for possible serious future ENSO occurrences. Overall, the 1982-83 ENSO affected life on the GalápagosIslands on many levels. The first and most obvious was the physical destruction, causing high mortality rates in sea and land birds alike.

The second was the increased SST causing marine life to die or migrate, and thus starving the sea birds. The third is the increased air temperature and rainfall causing land vegetation to bloom like never before and land birds to thrive. The fourth was increased land bird density-due to higher adult and young survival from an abundant food supply-, which caused changes in the social structure and age distribution of Darwin's finches and the Galápagosmockingbird. The fifth and last effect of the 1982-83 ENSO is the persisting increased level of population of Darwin's finches and the irreparable loss of life of the Galápagospenguin. The 1986-87 ENSO was much more representative of a typical ENSO season, and future advances in weather predicting will be able to map out successive ENSO occurrences and maybe discover how such a powerful and unpredictably destructive cycle came to exist in nature.

Works Cited

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Coulter, Malcolm C. & Valle, Carlos A. "Present Status of the Flightless Cormorant, GalápagosPenguin and Greater Flamingo Populations in the GalápagosIslands, Ecuador, After the 1982-83 El Niño." The Condor, Vol. 89 (1987): pp. 276-281.

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Gibbs, Lisle H. et. al. "Effects of the 1982-83 El Niño Event on Blue-Footed and Masked Booby Populations on Isla Daphne Major, Galapagos." The Condor, Vol. 89 (1987): pp. 440-442.

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Grove, Jack S. "At the Heart of El Niño: Too Warm Waters Surround the GalápagosIslands." Oceans, Vol. 17, Issue 3 (1984): pp. 3-8.

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