Avian frugivory of Cinnamomum camphora in an urban environment: visiting bird species and the implications for seed dispersal and seedling emergence

Abstract

Alien invasive plant species have become a major threat to biodiversity worldwide. Through mutualistic relationships with local frugivorous bird species, many fleshy-fruited invasive plant species have established populations that have become difficult to monitor and control. Cinnamomum camphora (L.) Presl. (Laureceae) is an alien invasive plant species found across many countries including Australia and South Africa. Cinnamomum camphora has been classified as different invasive categories in only five of the nine provinces in South Africa. Cinnamomum camphora has been classified as Category 1b in four provinces (Eastern Cape, Limpopo, KwaZulu-Natal) and Category 3 in the Western Cape (with the exception of ‘Champion trees’. However, C. camphora has not been categorised in the remaining four provinces (Gauteng, North West, Northern Cape and Mpumalanga), leading to the possibility of the species becoming highly invasive without correct monitoring and control of already established populations. Cinnamomum camphora is adapted to growth in disturbed habitats, meaning that dispersal is possible within urban environments. This study aimed to investigate how bird species responded to the production and ripeness of C. camphora fruit, as well as the implications for seed dispersal and seedling emergence. Bird observations and fruit production and ripeness sampling showed that there was an increase in the number of visiting birds as fruit production increased during the fruiting season (February to June) and as a higher proportion of fruit became ripe. The primary bird visitor species were a combination of both seed predators and legitimate seed dispersers with 35.3% and 52.9% of bird observations respectively. The most common bird was the Rock Dove (Columba livia), a known seed predator, with 26.4%, while the most common seed iii

disperser was the Karoo Thrush (Turdus smithi) with 19.1 % of total bird observations. Primary activities consisted of birds consuming C. camphora fruit and birds found resting in C. camphora trees. Seedling emergence testing in a greenhouse trial showed that C. camphora seeds that remained within intact fruit were still able to successfully emerge (and hence had germinated in the soil) even when previous studies suggested otherwise. Percentage seedling emergence over the 12 month greenhouse trial period for the four seed/fruit stages were 22.6%, 38.3%, 40.8% and 38.2% for the unripened fruit, ripened fruit, shed fruit and consumed fruit stages respectively. Seedlings from seeds that had been ingested and voided by birds took significantly longer to emerge compared to seeds that had been within intact fruit stages (ripened, shed and unripened). Unripened fruit took a significantly shorter time for seedlings to emerge than the other three stages. Furthermore, consumed seeds planted in both April and May of 2016 took a significantly longer time to emerge than the other three fruit stages. Seed viability using tetrazolium on the remaining seeds in the soil showed that only 15.5% of all C. camphora seeds remained viable after 12 months of the seedling emergence trial. The majority of seeds were either unviable, 29%, or had decomposed due to factors such as fungal infections or predation by insect larvae. However, while the seed viability was not significantly different between the four fruit/seed stages, bird ingestion resulted in a slower rate of emergence, indicating that birds reduced the overall seed vigour of C. camphora. Overall it was found that birds served mainly as C. camphora dispersal agents and not as both dispersers and germination mediators, except that birds lowering the rate of seedling emergence, not percentage emergence. This makes C. camphora populations potentially difficult to control and monitor, especially within urban environments, and suggests that the populations are likely to spread further via bird dispersal.