Summary
-
(1)
We report some effects of an annually variable and unpredictable rainfall upon Darwin's Finches on the Galápagos. Finch numbers, foraging and food supply were studied on I. Daphne Major in December of 1973 and 1977, and compared. 1973 was the second of two successive wet years and 1977 was a drought year.
-
(2)
Seed numbers and biomass were approximately one order of magnitude lower in the drought year than in the wet year. Small and soft seeds were absolutely and relatively rarer in the drought year than in the wet year.
-
(3)
Similarly finch numbers and biomass were approximately one order of magnitude lower in the drought year than in the wet year. Numbers of G. scandens declined less than did number of G. fortis. Both species exhibited unabalaced sex ratios, in favour of males, in the dry year in contrast to balanced sex ratios in the wet year. Male scandens were heavier on average in the wet year, but male fortis were heavier in the dry year.
-
(4)
The foraging of scandens, a cactus (Opuntia) specialist, was similar in the two years. The foraging of fortis in the dry year differed from foraging in the wet year in three important respects: fortis devoted a disproportionate amount of time to feeding on small seeds while tending to avoid seeds of Opuntia, they fed more on floral and extra-floral parts of Opuntia and they fed on Tribulus cistoides, a large and hard fruit which was absent from their diet in the wet year. As a consequence of feeding more on Opuntia, fortis foraging was more similar to scandens foraging in the dry year than in the wet year.
-
(5)
The results are discussed in relation to expectations from competition theory. The decline in numbers in relation to a decline in food supply was expected, but a convergence in diets was not. The convergence is attributed to the recent renewal of a single resource, Opuntia flowers, against a background of general resource scarcity. Diet overlap and limitation of numbers by food provide indirect evidence of interspecific competition; scandens, with an included niche, was competitively superior to fortis.
Similar content being viewed by others
References
Abbott I, Abbott LK, Grant PR (1977) Comparative ecology of Galápagos ground finches (Geospiza Gould): evaluation of the importance of floristic diversity and interspecific competition. Ecological Monographs 47:151–184
Cody ML (1974) Competition and the structure of bird communities. Univ. Press; Princeton
Colwell RK, Futuyma DJ (1971) On the measurement of niche breadth and overlap. Ecology 52:567–576
Curio E, Kramer P (1965) On plumage variation in male Darwin's Finches. Bird-Banding 36:27–44
Gifford EW (1919) Field notes on the land birds of the Galápagos Islands and of Cocos Island, Costa Rica. Proceedings of the California Academy of Sciences Ser 4, 2:189–258
Goodall DW (1973) Sample similarity and species correlation. In: Tuxen R (ed) Ordination and classification of communities Handbook of vegetation science Part V Junk The Hague p 106–156
Grant PR, Smith JNM, Grant BR, Abbott I, Abbott LK (1975) Finch numbers owl predation and plant dispersal on Isla Daphne Major, Galápagos. Oecologia (Berlin) 19:239–259
Houvenaghel GT (1974) Equatorial undercurrent and climate in the Galápagos Islands. Nature 250:565–566
Hurlbert SH (1978) The measurement of niche overlap and some relatives. Ecology 59:67–77
Krebs JR (1978) Optimal foraging decision rules for predators. In: Behavioural Ecology, Krebs JR, Davies NB, Sinauer Massachusetts p 23–63
Lack D (1947) Darwin's Finches. Univ Press Cambridge
MacArthur RH (1972) Geographical Ecology. Harper and Row New York
MacArthur RH, Pianka ER (1966) On optimal use of a patchy environment. American Naturalist 100:603–609
McNair J (1979) A generalized model of optimal diets. Theoretical Population Biology 15:159–170
Miller RS (1967) Patterns and process in competition. Advances in Ecological Research 3 Cragg JB, Academic, New York p 1–73
Morel GJ, Morel MY (1974) Recherches écologiques sur une savane Sahelienne du Ferlo septentrional Sénégal: Influence de la sécheresse de l'année 1972–1973 sur l'avifaune. La Terre et la Vie, 28:95–123
Pianka ER (1976) Competition and niche theory. Theoretical ecology principles and applications. May RM Blackwell Oxford
Pyke GH, Pulliam HR, Charnov EL (1977) Optimal foraging; a selective review of theory and tests. Quarterly Review of Biology 52:137–154
Schoener TW (1974a) Resource partitioning in ecological communities. Science 185:27–39
Schoener TW (1974b) The compression hypothesis and temporal resource partitioning. Proceedings of the National Academy of Science USA 71:4169–4172
Smith JNM, Grant PR, Grant BR, Abbott IJ, Abbott LK (1978) Seasonal variation in feeding habits of Darwin's ground finches. Ecology 59:1137–1150
Southwood TRE (1966) Ecological methods with particular reference to the study of insect populations. Methuen London
Westoby M (1978) What are the biological bases of varied diets? American Naturalist 112:627–631
Whittaker RH (1960) Vegetation of the Siskiyou Mountains, Oregon and California. Ecological Monographs 30:279–338
Wiggins IL, Porter DM (1971) Flora of the Galápagos Islands. Univ Press Stanford
Wyrtki K, Stroup W, Patzert W, Williams R, Quinn W (1976) Predicting and observing El Niño. Science 191:343–346
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Grant, P.R., Grant, B.R. Annual variation in Finch numbers, foraging and food supply on Isla Daphne Major, Galápagos. Oecologia 46, 55–62 (1980). https://doi.org/10.1007/BF00346966
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00346966