Sillett, T.S., Holmes, R.T. and Sherry, T.W. 2000. Impacts of a global climate cycle on population dynamics of a migratory songbird. Science 288, 2040-2042.
What was done
The authors studied the effects of the El Niņo Southern Oscillation (ENSO) on the population dynamics of black-throated blue warblers (Dendroica caerulescens) in their tropical winter quarters in Jamaica and their north temperate breeding grounds in New Hampshire, USA.
What was learned
According to the authors, the annual survival of black-throated blue warblers in Jamaica "was low in El Niņo years and high in La Niņa years." This result, they say, "is best explained by the impact of ENSO on local climate and a concomitant change in food availability for overwintering birds." Specifically, they note that reduced rainfall during El Niņo years in Jamaica "leads to a decreased amount of food available for warblers in the winter dry season and, hence, to lower survival."
Back in New Hampshire, it was found that black-throated blue warbler fecundity is limited by food availability in much the same way, via its effects on fledgling weight. Specifically, when adults are feeding nestlings and dependent juveniles in the summer (when food is most limited), lepidopteran larval biomass (the warblers' primary prey in summer) is positively correlated with the Southern Oscillation Index; and with less larval biomass available in El Niņo years than in La Niņa years, "fledglings weighed less in El Niņo years relative to La Niņa years," and their survival was likewise less.
What it means
Food supply means everything to all living creatures. When more is available, more individuals of each species survive, grow to maturity and successfully reproduce. As evidence of this fact, the authors cite research reports that reveal relationships of the same type they found for the black-throated blue warbler in seabirds, raptors, primates, rodents, arthropods and Pacific island passerines. Hence, as the air's CO2 content continues to rise, stimulating the productivity of essentially all plants on earth (see the various topics listed under the heading Growth Response to CO2 in our Subject Index), we can be assured that with a greater biospheric food supply, essentially all of earth's animal species will be benefited in ways that will tend to increase their populations.
Reviewed 9 August 2000