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Bat-Cactus interaction – 20 years bat research in Curacao!
Dr. Sophie Petit, the expert on the ecology of Curaçao’s bats, visited our island in August, for the first time again in five years. To examine the state of affairs regarding bats, cacti and the protection of both.
Curacao is very close to Petit’s heart, and the bats on the island are her passion in life. It was Dr. Petit, now working at the University of South Australia, who figured out pretty much everything we know about the interaction between bats and cacti, and the bat species that occur here on the island, using applied and systematic research which took over 20 years. She wants to share that knowledge with the people of Curacao. In this article an overview of the conversation we had on the research she has done, the results that have emerged and her thoughts on the state of nature and its conservation on Curacao.
PhD studies in Curaçao
A series of coincidences led Sophie to the island where she would work initially on research to the Curaçao white-tailed deer. Because of a wide variety of reasons she encountered on the island she switched her attention to the bats, and specifically to the pollination of cacti in relation to bats. ,,Pollination by bats has always interested me. After pilot research I wrote a research proposal which I send to my professors and which was accepted. I could start my PhD research on the island”. In practice it meant that the proposal for the research was greatly expanded and a series of additional research questions would come forward and were answered, all decisive and pioneering research to the island and resulting in guidelines for necessary management and protection measures which should be taken by relevant organizations.
First Sophie went to work on an inventory of the bat species that occur on the island. The available information was in fact outdated and nobody had a clue what species were actually present. Besides the types of bat species present, it was also important to know how many individuals of every species was living on the island. To find out, Sophie, together with assistant researchers Anna Rojer and Leon Pors, visited all locations where bats were known to occure – mainly caves – and the bats were counted according to a fixed method. These bat surveys were done periodically over the last 20 years to get an idea what the fluctuations in population size of the animals were (the number of animals per species of bats). These counts were also important to check if a decrease in numbers could be detected, an indication that there might be a problem.
The last survey was done in the third weekend in August of this year. The results were satisfactory for the scientists because the numbers even showed a slight increase. This was probably due to the rain-rich years that blessed the island in the past years.
All the bat species on the island were counted, the nectarivores, the insectivores and the carnivores or fish eaters.
During this study the three scientists even found a new species of bat, which was not known to the island, the insectivorous Pteronotus davyi.
Another result of the survey was the notion that only small populations of each species were present on Curacao, generally only a few hundred individuals per species. A big difference with other parts of the world where there are hundreds of thousands of individuals for a single species of bat to be found. ,, That was interesting in itself, to work with species where there were so little of, were so threatened,” said Petit. Two nectar eating bat species were found on the island, both of which already known to be present on the island and important for the study of Sophie: the Glossophaga elongata and Letonycterus longirostrus curasoae.
After discovering this, Petit wanted to know if the bats were indeed the main pollinators of the columnar cacti on the island. This suspicion arose from the observed behavior of both. Cacti flowers bloom mainly at night and bats are only active at night. Experiments showed that bats were 100% responsible for pollination of the columnar cacti species Datu and 90% of the Kadushi. The Kadushi di Pushi figures are not known because this species was not to be found in the test area around the Brievengat landhouse. This location, which used to be rich in huge and historically very important cacti and cactus hedges, a monument which belonged to the manor, can not be admired anymore. An owner of the mansion bulldozered the entire cactus fence and all other cacti in the area, an event which makes Petit still very sad.
The pollination rates by bats made it clear how important these animals are to the survival of the columnar cactus species on the island, and thus also for the fauna in total.
The next part of the study showed how important this relationship actually is. Within this study 50 individuals of Datu, 50 of Kadushi and 33 Kadushi di Pushi were monitored every 2 weeks and subjected to a complete survey. In addition, on each individual cactus the numbers of present flower buds, flowers and fruits were checked. In this study it was shown that most of the flower and fruit production of these cacti occurs in the dry season. There are few other water rich food resources available in the dry season in Curacao and so columnar cacti are extremely important for the fauna of Curaçao. The bats are crucial in this process. ,,Something nobody knew at that time, and nobody cared about the bats of the island. The largest cacti produce more flowers and fruits than the small cacti. These large cacti are also very important to bridge the period between two years. During the transitional period in the Christmas period, it is these large cacti that still bear fruit and thus form an essential link for the survival of the bats. At this time of the year, there are other plants that are productive and produce flowers and fruits as well, and the rest of the fauna of the island has a reasonable amount of food, but its the very large cacti that are important for the survival of the bats”, says Petit.
Subsequently, it was important to see if the cacti could fertilize themselves with pollen from flowers of the same plant. That was totally not the case. The columnar cacti on the island have an embedded system which ensures that pollen can not fertilize their own flowers, thereby preventing inbreeding. During the effectiveness study which followed afterwards, Petit discovered that bats have a preference for the type of cactus which they visited. The Glossophaga’s visited the Datu more often and Leptonycterus more often the Kadushi’s. A flower can be visited hundreds of times by several bats on one night. ,,I watched the cacti competing for the attention of the bats as they all obviously want to have bats visit them for pollination. And in this research I found very interesting things, in fact, a scientific first. All three cactus species on the island react differently to of rainfall. Kadushi di Pushi responds very quickly to rain and produces fruit and flowers very fast. The Datu makes no new flowers when it rains, the already fertilized flowers and fruits that are there already will be finished but they spend no energy producing new flower buds. The Kadushi doesn’t change its behaviour at all. So all three react differently. That was something nobody knew, “explains Petit.
In Part 2 of this interview we will talk about further discoveries about the relationship between bats and cacti on Curaçao, the work that Petit is now doing in Australia, the work she has done during her stay in Curacao last August, and what the future holds.