LONG-TERM DYNAMICS OF MEDITERRANEAN FRUGIVOROUS BIRDS AND FLESHY FRUITS - A 12-YEAR STUDY

The relationship between fleshy-fruited plants and their vertebrate se ed dispersal agents often has been depicted as subject to important in terannual variation, but no study has thus far documented such variati on on a long-term basis. This paper presents the results of a 12-yr in vestigation on fleshy-fruited plants and avian frugivores in a Mediter ranean montane locality of southeastern Spain. The main objective was to document patterns and correlates of long-term variation in the comp osition and abundance of fruits and birds, with particular reference t o seed dispersers. During October-December (''autumn'' period) 1978-19 90, abundance of ripe fruits and birds was assessed in a 4-ha plot in dense, well-preserved sclerophyllous scrub, by means of counts in perm anent plots and mist-netting, respectively. Diet composition and fruit preference patterns of Erithacus rubecula and Sylvia atricapilla, the two most abundant seed dispersers, were also investigated over the sa me period, using fecal sample analyses. Possible consequences to the b irds of annual variation in fruit supply and diet composition were inv estigated using data on fat deposition levels and recapture rates of m ist-netted individuals. Total fruit abundance (i.e., mean ripe fruit d ensity of all species combined) fluctuated among years between 5.4 +/- 11.1 fruits/m(2) (mean +/- 1 SD; 1986) and 77.1 +/- 78.0 fruits/m(2) (1989) and was positively related to the amount of rainfall in the pre ceding spring. Not all fruiting species bore ripe fruits every year, a nd among those species that did, fruit density fluctuated asynchronous ly and to variable degrees. Seven out of 13 species exhibited signific ant supra-annual periodicity in fruit abundance, with fluctuation peri ods ranging from 2 to 6 yr. Variation in the abundance of each of the six most abundant fruit species was unrelated to annual variation in r ainfall. The autumn bird assemblage at the study site was made up of y ear-round resident species (54.1% of captures, all years combined) tha t were largely fruit predators (feeding on pulp or seeds without perfo rming dispersal) and of autumn-winter resident species (45.5% of captu res) that were largely seed dispersers. Bird abundance, all species co mbined, ranged between 27.1 and 61.5 captures/100 net-hours for 1987 a nd 1986, respectively. Depending on year, seed dispersers made up 25.6 -75.4% of the total captures, and fruit predators made up 20.9-69.7%. The relative importance of nonfrugivores was always negligible (1.1-9. 9%). No correlation existed across years between total fruit abundance and the capture rates of all bird species combined, seed dispersers, or fruit predators. Annual variation in the abundance of seed disperse rs was positively related to November mean maximum temperature. At the individual species level, S. atricapilla capture rates were correlate d with the abundance of the fruits of Phillyrea latifolia, a species e xhibiting extreme annual fluctuations. Annual variation in the importa nce of fruits in the diet of S. atricapilla and E. rubecula was not si gnificantly related to changes in fruit abundance. Composition of the fruit diet of these species fluctuated markedly among years, and there was little agreement between composition of the diet and of the fruit supply. Certain fruit species were significantly preferred, and other s avoided, by both S. atricapilla and E. rubecula. Ranking of interspe cific fruit preferences remained consistent among years and was relate d to differences in carbohydrate and lipid content of fruit pulp. Fat deposition levels of S. atricapilla and E. rubecula did not vary among years and were not significantly related to fruit abundance, percenta ge of fruit volume in the diet, or contribution of lipid-rich fruits t o the diet. Return rates of individuals of these species to the study locality in successive wintering periods were very low, and not signif icantly related to diet composition or fruit abundance. Prevalence of abiotic over biotic determinants of annual variations, extensive decou pling of the long-term temporal dynamics of fruits and dispersers, and the remarkable ''indifference'' of frugivores to variations in the fr uit supply all point to the non-equilibrial nature of this assemblage of fleshy-fruited plants and their avian dispersers.