Interspecific competition in germination of bird-dispersed seeds in a habitat with sparse tree vegetation in South Africa
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Avian frugivores provide directed seed dispersal services that contribute in shaping the native plant communities and facilitate population persistence. By transporting and scarifying the seeds during ingestion, avian frugivores reduce the competition with siblings, and may improve the germination which is critical for dispersal effectiveness. However, generally, there is limited knowledge on how deposited seeds interact/compete in the new microsite. Therefore, we investigated 1) whether bird-dispersed seeds benefit from improved germination after their passage through the bird’s gut; and 2) the potential impact of seed density on competition at the microsites by determining whether seed density and species diversity influence germination in the Free State Province, South Africa. Overall, the results partly supported the hypothesis. Germination trials with defecated seeds of five plant species compared with the manually depulped seeds showed that despite prolific seed germination observed in Ziziphus mucronata, only Searsia lancea seeds had significantly high seed germination after passage through the bird gut. Overall, there was a significant correlation between seed size and the germination of bird-ingested seeds except in Olea subsp. africana possibly due to possession of extremely hard protective seed cover. Seeds competition experiments pointed to Z. mucronata and O. subsp. africana having significant germination performance that was positively correlated to seed density and seed size while E. rigida did not germinate at all. Seed species diversity in the germination trays did not have significant impact since only two former plant species consistently displayed significantly higher germination across the competition levels. We conclude that not all bird-defecated seeds have their germination improved, and that further long-term tests for germination physiological responses of the seeds’ samples used in this study are required since poor germination observed in other tree/shrub species cannot be attributed to competition solely.