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Fly by night – seed dispersal of invasive alien plants by fruit bats

Wahlberg's Epauletted Fruit Bat

Epomophorus wahlbergi feeding on Melia azedarach. Photo: S.D. Johnson.

Say the word ‘bat’ and we think Dracula, vampires and blood. Rarely do we think of fruit bats chewing on juicy fruits and of the vital ecological services they provide while we sleep soundly at night. However, these services which many plants rely on, now threaten the ecosystems within which they occur — as these bats also disperse invasive alien plants. Some of the worst global invasive alien plant species are fleshy-fruited. An important factor contributing to their success is the rapid, loose mutualisms that they form with local frugivores. Furthermore, fruits of these plants often offer greater nutritional rewards than indigenous fruits and are therefore preferred over indigenous fruits. Often it is birds that are recognised as key dispersers of invasive alien plants; fruit bats are rarely considered as key dispersers. This is surprising, as on a global scale bats are known for their importance as dispersers, especially with regards to maintaining forest ecosystems. While avian frugivores are limited by their gape size, fruit bats can consume larger fruits. These are usually carried away to feeding sites where fruit pulp and seeds are spat out following the extraction of the fruit juice. Smaller seeds, such as those of figs, may be ingested. Fruit bats often cover large distances between feeding and roosting sites, resting in non-fruiting trees and crossing open expanses, which avian frugivores will not do. This facilitates the establishment of new satellite plant populations.

A recent article published in the journal Biological Invasions by PhD student Lorinda Jordaan and colleagues investigated whether Wahlberg’s epauletted fruit bats, Epomophorus wahlbergi, would influence the germination of seeds of invasive alien plants. This was done by feeding fruits of four invasive alien plant species — Psidium guajava, Melia azedarach, Eriobotrya japonica, and Morus alba to the bats. Seeds were extracted from individual bat cages and planted in separate soil trays in a greenhouse where daily seedling counts were conducted. Manually de-pulped seeds and whole fruits were planted a controls.

Fruit bats consumed up to twice as much invasive alien fruit (per gram body mass) than co-occurring native avian frugivores have been shown to do, and are therefore capable of dispersing proportionately more seeds per individual. Germination was enhanced by the removal of fruit pulp, as seeds contained within whole fruits germinated less than bat-processed and de-pulped seeds. In general these invasive plant species germinated rapidly and profusely, which suggests that they may rely on continuous dispersal events and not on seed banks.

Palette ridges

Palette ridges of E. wahlbergi. Photo credit: Prof. C.T. Downs.

To achieve effective management, the multifaceted nature of plant invasions must be considered. One facet that has been given insufficient attention in this regard in South Africa is the role of frugivores as dispersers of invasive alien plants. Results from this study provide insights into the germination patterns of these invasive alien plants and highlight the need for the role of various dispersers to be recognised as key components in the invasion process. Such information will improve screening protocols and management strategies of current and potential invasive alien plants.

Jordaan L. A., Johnson S. D., Downs C. T. (2011) Wahlberg's Epauletted Fruit Bat (Epomophorus wahlbergi) as a potential dispersal agent for fleshy-fruited invasive alien plants: effects of handling behaviour on seed germination Biological Invasions DOI: 10.1007/s10530-011-0131-7.

To read the paper, click here.