Key Issues for Effective Seed Dispersal
Effective seed dispersal is critical for forest regeneration, and many animal species fulfil this important ecological role. Globally, rising anthropogenic pressures have led to habitat loss and forest fragmentation. Under such situations, the significance of various seed-dispersing agents increases manifolds in the context of forest regeneration and recruitment. This holds true especially for fruits with large seeds.
The importance of large bodied animals as seed dispersers is well-understood. For example, larger fish seem to be particularly important as seed dispersers; the probability of seed dispersal increases up to 28% with every centimetre increase in fish length. Similarly, elephants are considered ‘mega-gardeners’ of the forests. However, larger species are also the most vulnerable to habitat loss and poaching. And herein lies the importance of understanding the seed dispersal effectiveness of smaller, common species which are characterized by dietary flexibility and the ability to adapt to a vast range of habitats (ecologically-resilient species). For instance, in the forests of tropical and subtropical Asia where forests are degraded or native mammal fauna is overtly poached, the widespread and disturbance-tolerant macaques may be the only seed dispersers, especially for the large fruit/seed-bearing species. Indeed, at the Buxa Tiger Reserve, West Bengal, rhesus macaques dispersed, through spitting or defecation, 84% of the 49 plant species they fed on. Ninety-six percent of the handled seeds were found undamaged, and 61% of the species for which germination tests were performed had enhanced germination. Fifty percent of monitored seeds germinated in situ and 22% established seedlings. It has been further suggested that the typically elephant-dispersed fruits may survive even after the extirpation of the mega herbivore due to the dispersal activities of smaller, common frugivores like macaques, rodents, and bovids. The extirpation or even a decline in the number of such common animals may curb tropical forest regeneration and recruitment. Hence, the roles of these species as seed dispersers clearly require further investigation.
An effective disperser should disperse the seeds of many plant species, but the number of species which qualifies as ‘many’ is a function of plant community composition and fruit availability. Thus, further research is required to investigate how dietary diversity and, consequently, seed dispersal effectiveness of frugivores may vary with resource availability across different spatiotemporal scales.
As fruits of a certain species may be dispersed by many frugivores and a single frugivore may disperse the seeds of many plant species, seed dispersal networks are established within communities. Certain plant-frugivore interactions may be more specialised and stronger than others within such assemblages. Thus, a complete understanding of the importance of any frugivore as a seed disperser would require examining the different dispersal interactions that it is a part of. Such studies would also help us determine how vulnerable different plant species are to changes in frugivore abundance.
Species that are not classified as frugivores may also act as seed dispersers. For example, macaques are usually known to be omnivorous. Nevertheless, the Japanese, northern pig-tailed, and rhesus macaques have been found to be important seed dispersers. Similarly, animals that are predominantly carnivorous may also act as seed dispersers. For example, in an undisturbed area of south-eastern Spain, seeds of 27 plant species, accounting for 40 % of all the fleshy-fruited plants of the region, were found in carnivore faeces. Animals that are labelled seed predators may also be effective seed dispersal agents. While rodents are generally considered to be seed predators, many species such as the Korean field mouse (Apodemus peninsulae), agouti (Dasyprocta punctata), and red acouchi (Myoprocta exilis) are important dispersal agents of Prunus armeniaca, Virola nobilis, and Vouacapoua americana seeds, respectively. It is thus important to identify such species to gain a better understanding of their ecological functions and thereby facilitate protection measures for their future survival.
And lastly, research on rhesus macaques shows that irrespective of fruit availability, macaque frugivory (and, therefore, seed dispersal) declines when humans feed them. The daily ranges of macaques also decrease during provisioning resulting in shorter dispersal distances. Additionally, during provisioning periods, macaque-handled seeds are usually deposited on tarmac roads, thereby precluding seed germination. If in the future, most of the forests are lost and there is a scarcity of natural resources, it is likely that some ecologically-resilient animals would still survive. But whether these animals will have any seeds to disperse depends on whether we choose to keep our forests pristine and stop interfering with their diets by providing food subsidies. While this is easier said than done in the Anthropocene, spreading awareness about the ill-effects of anthropogenic activities, and being more responsible about our own actions may be the only way ahead.
The following is an excerpt from the article – The Tree Travelogues by Asmita Sengupta published in Resonance.
Read the article here: Link to PDF