Effects of provisioning in rhesus macaques and ecosystems
Have you fed a monkey when you are at a place of worship, or at a bus stand? This act of kindness towards a fellow primate can cause changes to entire forest food webs and ecosystem processes, finds a recent study. For starters, the amount of fruits monkeys ate decreased by almost half when they had access to human food.
Asmita Sengupta from the National Institute of Advanced Studies and her advisors have recently conducted a study to understand the effects of 'provisioning' – providing of extra food' – by humans on the rhesus macaque. Although the rhesus macaque (Macaca mulatta) is not recognized to be a keystone species, a recent study shows thatit is an important seed disperser for many fruit species in the forests. Interestingly, these monkeys are also highly tolerant to human disturbances and are often provisioned with human food even in protected areas.
Provisioning of wild animals have been shown to have several ill effects at both the individual level and at the population level. This study, conducted at the Buxa Tiger Reserve, aims to understand the effects that such provisioning has on the seed dispersal and frugivory behaviour of rhesus macaques.
The study group of monkeys was one that was provisioned with human food for part of the year and dependant on natural sources for the rest. This group, consisting of 64 individuals was called Troop D, and was studied from October 2013 to September 2014. Troop D’s home range was an amalgamation of natural forest patches as well as human settlements and even national highways. The researchers chose this group because it provided them with a natural experimental set up to see how much of an effect provisioning had. Their previous study on a troop (Troop C) which was completely dependent on natural food sources showed that rhesus macaques were efficient seed dispersers. In the present study, May to September was the non-provisioning period, while October to April was the period in which the macaques were provisioned. During provisioning months, the macaques ate a variety of human food including cookies, bread, boiled chickpeas, cakes and kitchen waste.
Observations were carried out by arduously following the study group from their waking site to their sleeping tree for 10 days each month. This amounted to about 12 hours a day. “I will have to admit that I did find it tough the first couple of days but then of course, I got used to it”, said Sengupta.
Sengupta noted what the macaques ate and the proportion of human food and fruits consumed every month, to find the degree of provisioning and frugivory. They also checked the state of the seeds that were excreted or spat out, to see if they were damaged or viable. Using GPS TrackMaker, they were also able to estimate the average daily range for each month of the year. Other observations they made included the fruit availability in each month, and the locations in which the seeds were dispersed.
The results of the study showed that the macaques had a high (94%) frugivory index during September, the peak of the non-provisioning period. This was in sharp contrast to 0% frugivory in April, showing clearly that the degree of frugivory was related to the degree of provisioning. Consequently, they examined faecal samples to find that no seeds were found during the months of highest provisioning. They also found that the macaques’ daily range during the provisioning period was only a half of what it was during the non-provisioning period. Accordingly, 67% of faecal seeds and all seeds that were spat out were deposited on roads in January, making them unviable for germination.
When asked if these dramatic results surprised them, Asmita replied saying, “Well, given that the macaques largely fed on anthropogenic food resources when they were available, I cannot say these numbers surprised me!”
The researchers also compared the seed dispersal activities of Troop C and Troop D and found that provisioning adversely affected both frugivory and seed dispersal. To be more precise, provisioning reduced the degree of frugivory by almost half (42%). Moreover, due to the smaller daily ranges of macaques during the provisioned period, seeds did not get dispersed as far as they would have if there had been no provisioning. Moreover, a large number of seeds were deposited on motorable roads which were not conducive for germination. This can bring about changes in plant dynamics as well as gene flow in forests.
The researchers suggest a complete banning of all provisioning activities as well as afforestation programmes in order to prevent further damage to the ecosystem.
About the paper
This paper appeared in PLoS ONE in October 2015.
About the authors
Asmita Sengupta, Kim R. McConkey and Sindhu Radhakrishna are with the School of Natural and Engineering Sciences, National Institute of Advanced Studies, Indian Institute of Science, Bangalore.
Asmita Sengupta: asmita[dot]sengupta[at]gmail[dot]com