Seed dispersal effectiveness: A comparison of four bird species feeding on seeds of invasive Acacia Cyclops in South Africa

Seed dispersal effectiveness (SDE) for vertebrates is determined as a product of quantity (numbers of seeds dispersed) and quality (probability that a dispersed seed becomes a new adult plant) components. There is a limited understanding of seed dispersal effectiveness for different avian vectors in South Africa, yet birds are important for long-distance dispersal for native species, and are implicated in the dispersal of many invasive plant species such as Acacia cyclops. Consequently, SDE was investigated in four bird species that are common seed dispersers in South Africa, of which two are frugivorous species (the Knysna turaco Tauraco corythaix and the red-winged starling Onychognathus morio), and two are granivorous species (the red-eyed dove Streptopelia semitorquata and the laughing dove Streptopelia senegalensis). Individuals of these species were caged and fed mature seeds of A. cyclops to determine quality of seed treatment in the gut. SDE was computed as a product of germination rates of gut-passed seeds of A. cyclops (i.e. quality) and the average bird body mass (i.e. proxy for seed load as quantity component) for the four bird species. Results show that frugivorous birds had significantly greater SDE than the granivorous bird species. SDEs for respective bird species also showed notable differences: the Knysna turaco had highest SDE followed by the non-significantly different red-eyed dove and red-winged starling, while the laughing dove had the lowest SDE of all bird species. However, it is likely that the two ubiquitous dove species, and both the colonial and nomadic red-winged starling, might have higher SDEs associated with large home ranges than the Knysna turaco with its more restricted geographic range. The results highlight the previously overlooked importance of doves and the other birds inmediating the invasion of A. cyclops, and the dispersal processes for the seeds of other plants included in their diet

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

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

Comparing germination metrics of Opuntia ficus-indica and O. robusta between two sets of bird species (Pied Crows and two smaller species)

The fleshy fruits of exotic Opuntia ficus-indica (L.) Mill and Opuntia robusta (Haw.) Haw (Cactaceae family) are consumed and dispersed by many vertebrates, which likely influences their invasion success. We tested whether seed ingestion by Pied Crows (Corvus albus) and other smaller birds (African Pied Starling Lamprotornis bicolor and the Red-eyed Bulbul Pycnonotus nigricans) improve the germination and speed in O. ficus-indica and O. robusta. Controlled germination trials for two Opuntia species were set up using the seeds extracted from faecal material, depulped seeds, and intact fruits. Overall, results show that seed germination for O. robusta was significantly higher than for O. ficus-indica. There were significant differences in seed germination between treatments of two Opuntia species, with the total mean germination of seeds defecated by the Pied Crows being equivalent to that of seeds defecated by the other smaller birds but significantly greater than the other treatments (i.e., depulped seeds and intact fruit). We noted that removal of the fruit pulp from seeds significantly improved germination in both Opuntia species compared to intact fruits. The O. robusta seeds defecated by the smaller birds had significantly greatest and most accelerated germination of all other treatments followed by the seeds defecated by the Pied Crows. Seeds of O. ficus-indica defecated by the Pied Crows had significantly greater germination than the seeds ingested by smaller birds. We conclude that seed germination success in fleshy fruits including those of alien species, varies with seed treatment by different vertebrate dispersal agents, and their net effect on seeds, may have consequences on species invasion success

Potential seed dispersal distances of native and non-native fleshy fruiting shrubs in the South African Mediterranean Climate Region

Bird flight distances for the small Zosterops capensis, the medium-size Pycnonotus capensis and the large Colius striatus were extracted from these birds’ initial ring and subsequent recapture locations and expressed on equivalent per km bases. The products of the bird-ring recapture records in nine different flight distance categories and daily consumption rates by these birds of seeds of two native (Chrysanthemoides monilifera and Olea europaea spp. africana) and two alien (Lantana camara and Solanum mauritianum) shrubs were used to construct seed dispersal curves. The dispersal distances to which ingested seeds were theoretically restricted were computed from the product of the retention time of seed in the birds’ guts and their flight speeds using published functions. All three bird species displayed thin long-tailed seed dispersal curves characterized by peaks at distances below 1 km which declined progressively with increasing distances, the tails extending to distances of up to 400 km. Flight distances corresponding with predicted seed gut retention times were 9.4 km in the small Z. capensis, 17.8 km in the medium size P. capensis and 21.2 km in the large C. striatus. These potential seed dispersal distances were much greater that the frequently reported long distance seed dispersal threshold of 1 km by frugivorous birds in fragmented landscapes

An appraisal of seed enumeration and videographic techniques for determining seed removal rates by birds

We examined the efficacy of seed enumeration and videographic techniques for determining seed removal by birds from indigenous (Chrysanthemoides monilifera and Olea europaea subsp africana) and alien (Lantana camara and Solanum mauritianum) shrubs at different study sites in the Cape Floristic Region. The seed enumeration technique involved counting the numbers of fruits and associated seeds removed monthly by birds, excluding those naturally abscised, from the shrub canopy. The videographic technique involved visual counts from images of the numbers of fruits and associated seeds consumed by birds over specific time intervals captured by a digital camcorder. Daily seed removal rates by all birds, irrespective of species, measured by both techniques were similar with no significant interactions evident between measuring techniques, site and shrub species. Both techniques displayed higher seed removal from tiny-seeded S. mauritianum than other shrub species; this was also evident among individual bird species. However, the seed enumeration technique was unable to discriminate between foraging organisms, contamination of traps by wind-blown fruits abscised from neighbouring branches and fruit theft from the canopy and the traps. In contrast, the videographic technique provided permanent visual and time-lapse records for individual foraging bird species allowing greater measurement precision and interpretation of fruit removal behaviour by birds. We recommend use of the videographic technique over the seed enumeration technique for studying vertebrates’ seed removal in a detailed manner

Frugivorous birds visit fruits of emerging alien shrub species more frequently than those of established alien and native shrub species in the South African Mediterranean climate region

We compared daily visitation frequency indices by 4 large (>150 g), 7 medium-size (50–150 g), 5 small (30–50 g) and 8 and tiny (b30 g) frugivorous bird species on fleshy fruits of two native shrubs (Olea europaea subsp. africana and Chrysanthemoides monilifera subsp. monilifera), two established alien shrubs (Solanum mauritianum and Lantana camara) and two emerging alien shrubs (Myoporum tenuifolium and Pittosporum undulatum) at nine different sites in the Cape Floristic Region. Large, medium-size and tiny birds as groups displayed significantly higher visitation frequency indices on fruits of both emerging alien shrub species than the other shrub species. Small birds as a group displayed insignificantly different visitation frequency indices on fruits of both emerging and established alien shrub species but significantly higher visitation frequency indices on fruits of both emerging and established alien shrub species than on fruits of the native shrub species. However, there were significant differences in foraging frequency indices of the bird species included within each of these body size groups on fruits of the different shrub species. Among the large birds, Columba guinea and among the medium size birds Sturnus vulgaris, Streptopelia senegalensis, Turdus olivaceus and Onychognathus morio all exhibited significantly higher visitation frequency indices on fruits of both emerging alien shrub species than on fruits of the other shrub species. These findings indicate that alien plant control measures should be focused on eradicating localised populations of emerging aliens to limit preferential consumption of their fruits by birds and consequent dispersal of their seeds that germinate readily into natural areas.Andrew Mellon Foundation; Oppenheimer foundatio

Biotic Interactions as Mediators of Biological Invasions: Insights from South Africa

This chapter provides an overview of the researchers and research initiatives relevant to invasion science in South Africa over the past 130 years, profiling some of the more recent personalities, particularly those who are today regarded as international leaders in the field. A number of key points arise from this review. Since 1913, South Africa has been one of a few countries that have investigated and implemented alien plant biological control on a large scale, and is regarded as a leader in this field. South Africa was also prominent in the conceptualisation and execution of the international SCOPE project on the ecology of biological invasions in the 1980s, during which South African scientists established themselves as valuable contributors to the field. The development of invasion science benefitted from a deliberate strategy to promote multi-organisational, interdisciplinary research in the 1980s. Since 1995, the Working for Water programme has provided funding for research and a host of practical questions that required research solutions. Finally, the establishment of a national centre of excellence with a focus on biological invasions has made a considerable contribution to building human capacity in the field, resulting in advances in all aspects of invasion science—primarily in terms of biology and ecology, but also in history, sociology, economics and management. South Africa has punched well above its weight in developing the field of invasion science, possibly because of the remarkable biodiversity that provided a rich template on which to carry out research, and a small, well-connected research community that was encouraged to operate in a collaborative manner