Seed dispersers as disease vectors: bird transmission of mistletoes seeds to plant hosts (original) (raw)
Ecology
The relationship between mistletoes and birds has been studied from the perspectives of mutualism and seed dispersal. Here, we emphasize the role that avian dispersers play as agents of mistletoe seed transmission to plant hosts. We describe the patterns of transmission of the seeds of Tristerix aphyllus, an endophytic Chilean mistletoe, on two of its columnar cacti hosts (Eulychnia acida and Echinopsis skottsbergii) by the Chilean Mockingbird Mimus thenca. In north-central Chile, these cacti grow in relatively discrete subpopulations on north-facing slopes. We measured variation in seed transmission within 10 subpopulations varying in species composition, host density, parasite density, parasite prevalence (defined as the percentage of hosts infested in a given population), and disperser abundance. Seed transmission was independent of species, but was strongly de- pendent on prior parasitism. Parasitized individuals received seeds much more frequently than expected from their relative abundance. We found no correlation between the density of hosts and seed transmission. We found strong positive correlations, however, between parasite prevalence and seed transmission to both parasitized and nonparasitized hosts. Seed transmission of T. aphyllus seeds by M. thenca appeared to be frequency- rather than density- dependent. Seed transmission was also tightly and positively correlated with the abundance of seed-dispersing birds at each site. Because bird abundance and parasite prevalence were correlated, we conducted path analysis to disentangle their relative effect on seed trans- mission. A model including only the direct effect of bird abundance and the indirect effect of parasite prevalence through bird abundance explained roughly the same variance as a full model including both the direct and indirect effects of bird abundance and prevalence on seed transmission. Apparently, variation in bird abundance was the main determinant of variation in transmission. We suggest that mistletoes, host plants, and the birds that disperse mistletoe seeds are systems well suited for studies of the ecological and evolu- tionary dynamics of disease transmission.
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This paper discusses the interaction between birds, mistletoes and host plants, focusing on 2 desert mistletoes, i.e. Tristerix aphyllus and Phoradendron californicum. The seeds of the species are primarily dispersed by the Chilean mockingbird (Mimus thenca) and Phoradendron californicum, respectively. In the first section, patterns of variation in mistletoe infection are outlined. This section identifies the contrasting scales at which these patterns are found and the mechanisms that shape them. In the second section, a unified framework is proposed for the study of mistletoe populations. This framework emphasizes the role of mistletoes as plant parasites, recognizes that they have a patchy metapopulation-like structure and accentuates the fact that their seeds are dispersed by mutualistic birds.
The mistletoe Tristerix verticillatus (Loranthaceae) parasitizes within a small area of the Yerba Loca Nature Sanctuary near Santiago, Chile, three co-occurring hosts: Schinus montanus (Anacardiaceae), Fabiana imbricata (Solanaceae) and Berberis montana (Berberidaceae). Previous studies suggest that T. verticillatus may be favoured when parasitizing S. montanus relative to the other two host species. We hypothesize that infection of S. montanus is not proportional to its local abundance or appearance, that S. montanus is more intensively parasitized than other available hosts, and that host provenance is a determinant of the fate of the infecting seed. We compare the incidence of infection of T. verticillatus in relation to local availability and appearance variables, and the intensity of infection of T. verticillatus, on the three co-occurring host species.We then test the effects of host provenance on mistletoe seed establishment success with a seed cross inoculation experiment varying the donor and receptor hosts. Finally, we test whether there are differences in establishment success between manually processed seeds and seeds defecated by the avian disperser Mimus thenca (Passeriformes: Mimidae). Our results show that the three hosts have an aggregated spatial distribution. Schinus montanus was parasitized at a higher rate than expected by its local availability and appearance, and inoculated seeds showed differential development depending on the origin of the seeds: seeds from T. verticillatus parasitizing S. montanus inoculated to S. montanus twigs showed higher germination and lower mortality than seeds from T. verticillatus parasitizing F. imbricata inoculated to S. montanus twigs. Furthermore, seeds defecated by the avian disperser, M. thenca, had higher adherence and reduced mortality when compared to manually processed seeds. The disproportional host infection found is discussed in terms of the differential establishment of mistletoe seeds, morphological characteristics of hosts and the behaviour of dispersing birds.
Adamawa State University Journal of Scientific Research (ADSUJSR
Montane habitats are generally rare and disproportionately distributed around the world; and are rich in biodiversity with a high concentration of endemic species of conservative interest. These rare forests are highly threatened and their remnants are often surrounded by deforested landscapes typically dominated by high human population. These high concentrations of humans within and around these unique landscapes often exert undue pressures on the fragile ecosystem leading to habitat degradation and species loss. Restoration through dispersal actions by some vertebrates is therefore crucial to the survival of these threatened landscapes. Individuals each of Agelanthus brunneus and Globimetula braunii were monitored on different host tree species at both forest edge and fragments to record various avian dispersers. A total of 13 bird species, from 11 genera and 10 families were recorded, with only four species (African thrush, African green pigeon, Yellow rumped tinker bird, and Western green tinker bird) identified as mistletoe dispersers. Yellow rumped tinker bird was the most dominant disperser and moved the highest number (70) of seeds. Avian species abundance in concert with host-mistletoe pair combinations partially predicted species' relative contributions to fruit dispersal of the two mistletoe species.
Implications of movement patterns of a dietary generalist for mistletoe seed dispersal
Austral Ecology, 2011
Mistletoes are dispersed primarily by frugivorous birds and have highly aggregated distributions at multiple scales. Mistletoe specialist frugivores have been found to intensify infections within infected hosts and stands, and this is considered the most likely mechanism underlying clumped mistletoe distributions at these scales. How these patchy infections first develop and whether seed dispersers also contribute to aggregated mistletoe distributions at landscape and regional scales have not been evaluated. Here we predict the mistletoe seed shadow of a dietary generalist (spiny-cheeked honeyeater Acanthagenys rufogularis Aves: Meliphagidae), by combining our observations of movements via radio telemetry with previous data on gut passage times to estimate seed dispersal curves for individual birds. There was considerable variation in movements and inferred seed dispersal between individuals, with non-breeding birds predicted to regularly transport Amyema quandang (Santalales: Loranthaceae) seeds up to 700 m; well beyond the boundaries of an existing mistletoe infection. As the first work to consider explicitly the distance component of mistletoe seed dispersal by dietary generalists, this study poses further questions about the relative seed dispersal roles of dietary generalists and mistletoe specialists. Moreover, our findings highlight considerable intraspecific variation in movement and foraging behaviour, suggesting gender and reproductive status of birds should be considered explicitly when quantifying seed dispersal services.
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