Interspecific competition in germination of bird-dispersed seeds in a habitat with sparse tree vegetation in South Africa

Abstract

Background 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 and population recruitment. However, generally, there is limited knowledge on how deposited seeds interact/compete in the new microsite. We tested the hypothesis that the bird-dispersed seeds benefit from improved germination after their passage through the bird’s gut; and we investigated 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. Results Overall, the results partly supported the hypothesis. Germination trials with defecated seeds of five plant species compared with the manually depulped seeds showed that only Searsia lancea had significantly higher seed germination success and improved germination speed after passage through the bird gut while Ziziphus mucronata only benefited rapid germination. 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. Seed 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 Ehretia rigida did not germinate at all. Seed species diversity in the germination trays did not have a significant impact since the seeds of two former plant species consistently displayed significantly higher germination across the competition levels. Conclusions We conclude that different plant species respond differently to seed ingestion by birds, 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.

Determining the critical recruitment needs for the declining population of Olea europaea subsp. africana (Mill.) P.S. Green in Free State, South Africa

“…In the same manner, this study indicates unchanged O. europaea subsp. africana seed germination after passing through bird guts has been previously reported by Vukeya et al (2021). It is a likely result of possible frugivorous bird gut adaptations to the egestion of indigestible fruit material such as seeds (Jordano, 2000;Mokotjomela et al, 2021).…”

“…We found that the seeds of O. europaea subsp. africana germinated at a slower rate (up to 33 weeks), possibly because of the high resistance of their seed coat to scarification, within the gut of the vertebrates, to early release of seed dormancy (Mokotjomela, 2012;Vukeya et al, 2021Vukeya et al, , 2022. Slow germination might retard recruitment since Schupp (1993) argued that of utmost importance in the TA B L E 1 Potential seed dispersal effectiveness for the two groups of organisms that influenced recruitment processes in O. subsp.…”

“…Our results are not different since Bekele (2000) demonstrated that O. europaea subsp. africana seeds may persist for up to 35 months of seed dormancy, and this could be an adaptational mechanism to allow seed maturation and reduce seasonal competition for germination resources with other plant species (Orrock & Christopher, 2010;Vukeya et al, 2021).…”

The vegetation cover dynamics and potential drivers of habitat change over 30 years in the Free State National Botanical Garden, South Africa

“…Therefore, we argue that the observed changes in vegetation cover, which indicate an increase in woody species' cover between 1987 and 2017, can be partly attributed to the substantial seed rain associated with resident frugivorous bird species and colonial wintering birds (Vukeya et al 2020;Vukeya et al 2022). It has been shown that bird species' gut treatment of the seeds promotes the germination of the local woody plant species (Mokotjomela et al 2021;Vukeya et al 2021). Woody cover encroachment reportedly jeopardises grassland biodiversity and transforms grasslands and savannas into shrublands and woodlands (Archer et al 2001(Archer et al , 2017.…”

Seed dispersal phenology of encroaching woody species in the Free State National Botanical Garden, South Africa

“…Seed dispersal is important for reducing competition among siblings, species' colonisation of new sites, gene mixing and population persistence (Clark et al, 1999; Howe & Smallwood, 1982; Nathan et al, 2003). The phenology of seed dispersal is critical to the successful recruitment of species (Jordano, 2000; Stiles, 1980), since dispersed seeds require conducive conditions for germination (Footitt et al, 2018; Vukeya et al, 2021); and where dispersal vectors are migratory, fruit ripening and the arrival of the vectors should be synchronised (Johnson et al, 2019; Stiles, 1980). Phenology is a study of the timing of reproductive biological events (e.g.…”