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.

We investigated seed dispersal phenology of bird-dispersed and encroaching woody species in the Free State National Botanical Garden (FSNBG), South Africa, over a 12month period. Results from defecated seeds' collection in 17 roosting sites show that a total of 22,161 seeds belonging to 14 woody species under 11 families were eaten and dispersed by foraging birds with seeds of six species being frequently dispersed.The native species had a larger number of dispersed bird-ingested seeds than the alien and invasive species. Seeds' collection suggested that out of five fruiting species, birds mostly preferred the red (Ziziphus mucronata, Ehretia rigida and Viscum rotundifolium) and black fruits (Olea europaea subsp. africana and Diospyros lyciodes). Ziziphus mucronata and O. subsp. africana seeds were dispersed in significantly higher numbers between March and July (mean ± SE: 47.1 ± 11.6; 51.4 ± 9.0), while Searsia lancea and E. rigida were mostly dispersed between August and December (221.6 ± 74.1; 373.6 ± 91.5)-a dichotomy that could minimise competition for dispersal agents while sustaining the resident bird species. Over 480 h of camera trapping, 22 bird species were captured, of which seven displayed high foraging frequency, and were resident in behaviour, while six species were migratory and colonial avian frugivores, including two alien bird species. We conclude that the reported bush encroachment is a likely result of high propagule pressure by resident frugivorous birds and colonial wintering birds in FSNBG, together with low fruit diversity in the grassland habitat.

As a conservation strategy, the South African National Biodiversity Institute (SANBI) establishes biodiversity gardens in areas with unique vegetation types that are vulnerable to extinction. The study aimed to (1) determine the vegetation cover dynamics of the Free State National Botanical Garden (FSNBG) over a 30-year period (1987–2017), focusing on different vegetation classes; (2) evaluate the ecological integrity of the Critical Biodiversity Area 1 (CBA1) vegetation using species abundance and vegetation cover; and (3) quantify potential conservation threats that may be drivers of vegetation cover changes. The “moderate vegetation cover” and “dense vegetation cover” had increased by 25.1 ha and 8.6 ha respectively in the FSNBG. Woody vegetation cover expanded significantly over the past 30-year period, suggesting “bush” encroachment. Shannon–Wiener diversity indices showed high overall plant species diversity of CBA1 vegetation type (H = 3.5), with the vegetation cover remaining high (79.6 ± 15.9%), 50 plant species no longer existing, suggesting reduced taxonomic richness. Major conservation threats included the presence of 27 alien and invasive plant species interspersed within different vegetation patches and anthropogenic habitat fragmentation in the past 19 years (i.e. covering ~ 18% of the buffer zone). We conclude that increased vegetation cover is associated with bush encroachment and we recommend interventions to reduce the population density of woody plants and establish permanent vegetation monitoring plots.