A comparison of bird foraging preferences for fruits of indigenous and alien shrubs and seed dispersal potentials in the Cape Floristic Region

[摘要] ENGLISH ABSTRACT: This study tested the proposal that fleshy-fruited alien shrubs competed more effectively for aviandispersal services than indigenous shrubs. Several different quantitative measures of increasingcomplexity were applied to test bird foraging preferences for fruits of two established alien shrubs(Lantana camara, Solanum mauritianum), two emergent alien shrubs (Myoporum tenuifolium,Pittisporum undulatum) and two indigenous shrubs (Chrysanthemoides monilifera, Olea europaeasubsp. africana). Overall the simplest quantitative measure, namely that of bird visitation frequencyidentified foraging preferences of individual bird species for fruits of alien and indigenous shrubs.Moreover, even the more complex measures of numbers of foraging birds, foraging times andproducts of these in the computed consumption intensity and seed dispersal efficiency werepositively correlated to visitation frequency. There was a distinct preference of several, especiallyheavily fugivorous, bird species for fruits of established alien than indigenous shrubs and severalmoderately fugivorous bird species preferred fruits of emergent than established alien shrubs.Intricate photographic and experimental approaches were applied to obtain precisenumerical data on seed removal rates by birds from the alien and indigenous shrubs. Deficienciesassociated with the experimental approach included its inability to discriminate betweenasynchronous fruit production and fruit ripening, individual foraging bird species, and fruitconsumption by other fruit foraging vertebrates. The advantages of the photographic approachwere in its provision of a detailed permanent record of individual foraging bird species, their totalnumbers and precise foraging times and the proportions of whole fruits consumed, as well as otherbehavioural foraging traits. Tested also was the proposal that frugivorous birds remove largeramounts of fruits from plants with high fruit production and nutritional contents and that thegermination of the bird-ingested seeds is enhanced. In compliance with this proposal, canopy fruitmass and monosaccharide content were all positively correlated with the total numbers of seedsremoved by birds and viable seeds excreted by birds. Only defecated seeds of the indigenous C.monilifera and alien L. camara displayed enhanced germination following their ingestion by birdsSeed dispersal distances of alien and indigenous shrubs by birds were estimated frompublished records of bird body masses, gut retention times of consumed seed and flight speeds.Estimated seed dispersal distances ranged between 0.41 and 0.81 km for the subset of heavily andmoderately frugivorous bird species examined. However, their foraging distances derived from birdringrecapture records were much greater, these peaking between 2.5 and 50 km in some heavilyfrugivorous bird species.It is concluded 1. that bird visitation frequency be applied as the most efficient and practicalquantitative measure for gauging bird foraging preferences in future surveys which should involvelarge complements of indigenous, emergent and established alien species, 2. that flight speeds and gut passage times of ingested alien and indigenous seeds need to be measured in local birdspecies so long distance seed dispersal potentials can be more precisely determined and 3. thatcontrol measures should be focused on eradicating especially emergent alien species with fleshyfruits from urban environments to prevent their transport by birds into adjacent natural areas.

[发布机构] Stellenbosch University