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Bat-Cactus interaction – 20 years of bat research in Curacao!
This article is the second part of the interview we had with Dr. Sophie Petit (click here for the first) and describes further discoveries surrounding the relationship between bats and cacti on Curaçao, the work she is doing in Australia, her work during her stay in Curacao last August, and what the future brings.
Without cacti no bats, without bats no cacti
The flowers of cacti consist of a large number of ovulary bodies, each of which may develop into a seed, provided that fertilization occurs. The flesh of the fruit is actually the connective tissue between the seed and the plant. The more bats visit the flowers, the more ovulary organs are being fertilized, the more seeds will develop and the larger the fruits will become. When the rainy season starts, the fruits will be smaller because of the fact that they are less frequently visited by bats.
Bats are essential for the reproduction of cacti, but how essential are the cacti for the survival of bats? To answer this question Petit did a study of their diet, in which she took pollen samples from the fur of bats and looked at the contents of the excretions of the animals. This study showed that a huge part of the Leptonycterus bats only had cactus products including pollen and fruits on their menu. Glossophaga forages on multiple flowers and fruit bearing plants species, does not fly as far as the other species of nectar eating bat, and remains mostly in the vicinity of where the animal sleeps at daytime. The conclusion was clear, however: cacti are essential for the survival of bats on the island.
Bearing young the limiting factor
The numbers for all bat species on the island are limited. The small size of the island combined with little available food resources ensures that, in regard to both nectar-eating bat species, there can be no more than about 1200 individuals per species. For Glossophaga the number is slightly higher than for Leptonycterus.
Bats are mammals. They give birth to live young while hanging upside down, catching the pups within their wings. They feed their young with milk. The production of milk is an activity which takes a lot of energy and to comply with this they need to find more food. The research of Dr. Petit showed that the peak of bat births takes place exactly at the moment when there is a peak in the production of cactus flowers. This also shows clearly the strong interdependence between the two bat species with the cacti on the island. The two groups have a strong mutualistic relationship, whereby they mutually need each other and can not survive on the island without each other.
From the research Petit did with Drs. Leon Pors it became clear what the ‘carrying capacity’ for the bats was, which basically stands for the number of bats that can survive with the available resources on the island throughout the year. To answer this it was necessary to find out what time of the year represented the critical factor for the survival of the nectarivorous bats. It was proven to be the month of July, the exact month during which the bats were observed to give birth to their young and raised them.
Both researchers looked at several things, including the amount of cacti that were on the island, through the use of aerial photographs, and the amount of food these produced. These data were combined with the amount of energy that the bats needed. In July, usually the driest month of the year, there is almost no other food resource to be found for the animals than cactus flowers and fruits. With the use of an ecological model Petit calculated how many bats could be maintained with the existing food sources on the island and that proved to be exactly the number of bats which were counted during the surveys which were held periodically. In reality the bats have little access to the fruits, because for the most part they are already being eaten by other animals before they are even ripe.
A cactus fruit must be ripe for a bat to be able to eat it. An anecdote from Petit illustrates that bats love cactus fruits and will eat them whenever they can. During the bat research some bats were caught for measurements but were placed in cotton pouches first, in order for them to ease down a little. After opening one of the pouches it seemed as if the animal was so severely injured that it was completely covered in blood. Nothing was further from the truth. The animal had found itself some ripe datu fruits and stuffed itself with the fruits. ,,They love it, and if they will always eat these fruits when they find them”, Petit said indicating that it was a relief to realize that the animal was not hurt after all.
With the matching numbers and figures it was soon clear that the ecological system was at carrying capacity, the island had the maximum number of bats that nature could maintain through the cactus species.
There were several other parts to this research, about the fruits and the number of fruits that got the chance to mature. This was done by Drs. Anna Rojer, who also looked at the germination effectiveness of the seeds. It very soon became clear that the seeds from unripe fruits do not germinate.
Native plants are part of the identity
Petit’s visit to the Island last August was no vacation at all. She worked with Pors and Rojer on new scientific publications, conducted a full bat survey to determine the 2012 population sizes and to compare the numbers with those of the last 20 years, gave a free lecture for the general public about the carrying capacity of bats on Curacao, worked on a new book and recorded HD video with all information about cacti and bats to be edited into a new educational program by the Curaçao Footprint Foundation.
The surveys yielded a positive result, the numbers of bats have remained mostly the same and for some species are even slightly better than 5 years ago. ,,There are many food sources at the moment because of the many consecutive wet years the island had over the past years. Because of this, there are many more insects to eat for insectivorous bats and beside the cacti there are many other food sources for nectar-eating bats”, said Petit. But she indicated still not to know enough about the insectivorous bats on the island, something she wishes to address with more research in the future while specifically focusing on management and protection
She observed many changes occurring on the island over the last five years, with advancing development disregarding the native flora and fauna being a huge thorn in the eye. ,,We do not know what the effects are of these far reaching developments on nectar-eating bats. The destruction of columnar cacti will affect the bats and all other aspects of nature. Now that I have come back to the island after 5 years, I am dismayed that there are so many new developments without paying attention to the native vegetation. There is so little soil on the island, so we need to keep the native vegetation, which is well adapted to the climate and needs little water, in place. By removing these plants you cause the small amount of soil to be washed away as well. If you want new plants for subsequent landscaping you need huge amounts of water to make them grow. There are plenty of beautiful local plants that are adapted to the climate, native plants you have to be proud of, they are great plants. But I have to stay positive, because what strikes me is that people are now much more aware of nature than 20 years ago. There is much more interest and people want to know more, have an awareness of the development of their own identity and find that in the culture and nature of the island. It is something you don’t see that often”, Petit said.
Fiji and Australia
The role of small mammals in the fertilization of plants is a passion of Petit. Now she works for the University of South Australia, and her work includes studies of certain bat species in Fiji and the role these play in the maintenance of the rain forest there and the importance of these animals for the health of the forest.
In addition, she does research on the Western Pygmy Possum, a very small marsupial that plays a very important pollination role to the native Australian plants including eucalyptus trees. ,,It’s a bat without wings”, jokes Petit on the animal which is the epiphany of cuteness.
Sophie Petit is not finished with Curacao and will continue research on the island whenever possible. Everything depends on the possibility of taking a sabbatical at the University. ,,I hope to come back, but it’s getting increasingly difficult to get a sabbatical. ”
Petit has traveled on to the United States and Mexico, among others, since being on the island in August.