Little Tobago Island Survey
Environment TOBAGO first proposed a Biological Survey of Little Tobago Island in 1996. The idea was suggested by Dr. Stanley Temple (University of Wisconsin) at a meeting concerning the protected area network for Tobago as described in the draft Bill to Establish a National Parks and Wildlife Authority. We were discussing the need for a biological survey of the proposed protected areas that would provide a scientific basis for a management plan for these areas. Since the realisation of such a large scale project was still some time away, an interim/pilot project, A Biological Survey of Little Tobago Island, was proposed and adopted. The benefits deriving from this project include the following:
1. To uncover the ecological interactions of a seasonal deciduous forest. This information could be valuable in the conservation and restoration of this type of forest in Tobago. Only remnant patches of this forest type remain, having been almost completely removed by historic agricultural practices.
2. To provide experience in planning and conducting a biological survey for all of the participants - Environment TOBAGO, local residents and researchers. The experience would be a valuable training ground for the proposed biological survey of Tobago.
3. The opportunity to conduct an information campaign aimed at educating local residents regarding biological surveys and their importance to environmental conservation and to the quality of human life. This would provide a foundation of understanding that could be built upon as the larger biological survey commences.
Little Tobago was chosen as the site for the proposed pilot biological survey for the following reasons:
1. The island is relatively undisturbed so that species distributions are likely to be the result of ecological factors.
2. Identification of specimens should be fairly rapid and accurate since the species are relatively well known and low in variety.
3. The island is small enough to sample entirely on a systematic basis including all microenvironments.
4. Little Tobago has one of the few known remnants of the seasonal deciduous forest type. By inference, knowledge gained from this study can be applied to the restoration of this forest type on Tobago.
The project is to be a broad scale biological survey using systematic sampling methods. Sampling points have been set up on a 100 X 100 meters grid. In some cases difficulty of terrain may require variation from the grid. About 80 sampling points have been established. These are marked by steel rods and ribbons attached to nearby trees.
At each sampling point data will be gathered on mammals, reptiles, birds and invertebrates. Some taxa may not require sampling at each point.
The suggested number of sampling visits is four - two in the dry season and two in the rainy season.
Results from the survey will be analysed using a variety of techniques appropriate to the group being studied and the questions being pursued.
Progress to Date
The project commenced in April of 1997 with a survey of the natural vegetation of the island. Sampling points were located and abiotic data recorded (elevation, slope, aspect, canopy closure, soil nutrients, soil texture, soil drainage, leaf litter depth, and salt spray exposure). This phase ended in June of 1997.
The initial plant survey was done by Ms. Natalie Boodram (see below: Project Proposal for Natalie Boodram and Summary of Ms. Boodram's Study), a graduate student at the University of the West Indies (UWI). The results will be used to prepare her Masters thesis. Ms. Boodram will provide the raw data generated on her first visit and a map of the survey points to Environment TOBAGO. Her advisor is Michael Oatham, Lecturer of Plant Ecology at UWI.
Mr. Oatham conducted a small mammal survey in November of 1997 (see below: Little Tobago Small Mammal Survey).
Students of Santa Clara University in California, USA, conducted studies on birds and insects in 1998.
Natalie Boodram's Project Proposal
Ecology and Distribution of Plant Species on Little Tobago Island
This project is a broad scale ecological survey of the vegetation of Little Tobago using systematic sampling methods. Data gathered will be used to investigate the ecology of the distribution of the plant species on the island and to make proposals for future research work into the biotic-abiotic and biotic-biotic interactions which are uncovered.
Little Tobago Island was chosen as the site for the study of the deciduous seasonal forest on Tobago for a number of reasons. The island is relatively undisturbed by humans so that the distributions of the species on the island are likely to be the result of ecological factors other than interference by humans. The species of the island are relatively well known and low in species richness so that it is expected that identification of specimens will be fairly rapid and accurate. The island is small enough to sample entirely on a systematic basis so that all microenvironments will be sampled. Finally, the project has the support of the non-governmental organisation Environment Tobago who will provide logistical and political support and will benefit from the data gathered in the project. Environment Tobago will also continue surveys of the sample points set up in the future to provide longer term data on vegetation change.
The sample points set up in this study will be used by other groups to survey other taxa. The data collected by these groups may be referred to uncover any biotic-biotic ecological interactions.
Aim of the Project: To discover the ecological basis for distribution of plant species in deciduous seasonal forest using systematic sampling of vegetation and abiotic factors.
The Site: Little Tobago is a 100 ha island off the NE coast of Tobago. It is hilly with a variety of different microenvironments from littoral and exposed rock to closed canopy deciduous forest and stunted low evergreen scrub. Parts of the island have been disturbed by humans in the past but the majority of the vegetation remains in a pristine state. The flora of Little Tobago has been explored taxonomically and comprehensive species lists have been compiled.
Methods: Because the island is small enough, systematic sampling on a 100 x 100m grid will be used, generating approximately 100 sample points. Systematic sampling will be used so that all microenvironments on the island will be sampled.
At each sample point vegetation data will be gathered using three different methods for three different life forms:
- Point centred quarter method for trees >5cm DBH
- Inventory and percentage cover estimate of ground flora and trees <5cm in 2 x 2m quadrants
- Point centred quarter method adapted for epiphytes
To explain plant species distribution abiotic data will be gathered at each point. Parameters measured will be:
- soil nutrients, texture and depth
- salinity and pH of the soil
- slope and aspect
- biomass/percentage cover of dead organic matter
- height of tree canopy and percentage canopy closure
Sample points will be marked using steel rods and revisited several times. Total number of visits in this study will be four (two in the wet season and two in the dry).
Results from the vegetation survey will be analysed using ordination techniques. Observations will be used to construct a geographical information system (GIS) database to aid ecological interpretation.
Sample points will also be used by other groups conducting studies into other taxa (birds, invertebrates, mammals etc.). If data becomes available before the end of the study, biotic-biotic interactions between plants and other taxa could be investigated.
Protocol: The project will run for two years. The island will be visited four times for sampling purposes. The first survey will take the longest because all sample points will have to be located, marked and all abiotic data will be measured. This phase will occur April-May 1997 and last up to two months.
The next three visits to the island will require considerably less time as all sample points will be located and plant species will be identified. These surveys will be of approximately two to three weeks duration and occur in November-December of 1997, and March-April and August-September of 1998.
Summary of Ms. Boodram's Study
Plants on Little Tobago
For the past two years I have spent many happy days studying the plants on the gorgeous isle of Little Tobago. Well, maybe I wasn’t that happy. I did tumble down more slopes than I care to remember. My fingers were often riddled with cacti thorns. Then there were those days when I found myself dangling from a rope, along a cliff, trying to collect a piece of a plant. Ah well, according to my kind-hearted supervisor, what doesn’t kill you will make you stronger. Yeah right!
Okay I’ll stop whining. There were many perks to my research. The scenery on Little Tobago is spectacular, the seabirds are numerous, and as I will elaborate in this article, the plants are really amazing.
The vegetation on Little Tobago was classified by J.S. Beard in 1944 as the Bursera simaruba (Naked Boy) - Lonchocarpus domingensis (Dogroot), Deciduous Seasonal Forest Association. In other words, according to Beard, the two most abundant trees on the island were Naked Boy and Dogroot. He also noted a high abundance of the Silver Thatch palm (Coccothrinax barbadensis). The Naked Boy is rather unusual in that the bark is papery and peels off easily; (maybe that’s how it got its name). The Silver Thatch palm is quite beautiful as the leaves have a silvery sheen.
During my research, I also found that the Naked Boy and the Silver Thatch palm were abundant on Little Tobago. Interestingly, I did not find any Dogroot on the island. What I did find, was Piscidia carthagensis, which is quite similar in structure to the Dogroot. It is possible that P. carthagensis was misidentified as Dogroot in 1944. While this may not sound that exciting, this is rather important to botanists in Trinidad and Tobago. It means that for the past 50 years or more we may have been mislabeling an important vegetation type. In any event, P. carthagensis is currently not one of the more abundant trees on Little Tobago. Apart from the Naked Boy and the Silver thatch Palm, some of the more abundant trees are the Banana Wood (Pisonia fragrans) and Cleanteeth (Diospyros inconstans).
The ground flora on Little Tobago is peculiar, in that it is almost completely dominated by an aroid, Anthurium jenmanii. This plant has huge leaves, often more than one and a half metres in length. The Anthurium is so numerous on Little Tobago that walking through the forest is like swimming through an unending green Anthurium sea. Nowhere else in Trinidad and Tobago is this prominence of A. jenmanii reported.Imagine that! The plant is found sporadically in other forested areas but it occurs more commonly as an epiphyte, growing on the branches of trees. One of the reasons for the prevalence of the Anthurium on Little Tobago may be the deciduous nature of the forest. Deciduous forests normally have higher light levels under the tree canopy, which means that the Anthurium has enough light to flourish in the understorey. Apart from the Anthurium some other ground flora species include Aphelandra pulcherrima, and a vine Smilax cumanensis. S. cumanensis has nasty thorns, the kind that tear through your jeans. This vine is found deep within the forest however, where only the loony scientists venture. It is not common along the hiking trails.
Anthurium jenmanii is also the most common epiphyte on Little Tobago. There are a few others like the lovely white Virgin orchid (Caulathron bicornatum) and the epiphytic cactus, Hylocerus lemairei.
The vegetation on Little Tobago is relatively well protected, for the island is a wildlife sanctuary. This is important because the Deciduous Forest on Little Tobago is the last remaining pristine example of this type of forest in Trinidad and Tobago. Also some of the species like the Silver Thatch Palm and the Banana wood are rare in Trinidad and Tobago, whilst on LittleTobago they are quite abundant. Thus, the value of Little Tobago lies not only in its wonderful avifauna, but also in its rare plant species.
There is a threat to the vegetation of Little Tobago, namely the presence of Bamboo (Bambusa vulgaris) on the island. This is an exotic species on Little Tobago. In biological terms, an exotic species is a species introduced to an area outside of its normal range. On islands in particular, exotic plant species are often better competitors than the natural vegetation. Bamboo is abundant and widespread on Little Tobago. It may be expanding at the expense of the natural vegetation. This situation should be monitored, for it may be necessary to control the spread of this plant.
There are other exotic species on the island for example some fruit trees originally planted as a food source for the Birds of Paradise, which were once found on the island. However, these fruit trees do not appear to be a threat to the natural vegetation in that, they are limited to a few garden plots on the island.
So the next time you are on Little Tobago, think how unique it is and look out for some of the plants I have mentioned. I hope you have gained an appreciation of how special our natural heritage is. Enjoy!
Mike Oatham's Survey Report
Little Tobago Small Mammal Survey
As part of a larger study into the ecology of the plants and animals of Little Tobago Island, a survey was undertaken to identify what species of small mammals occurred on the island. Small mammals can be a very important part of the functioning of an ecosystem. Different species perform many different functions in natural ecosystems. They may be insectivores that control the number of insects which may otherwise breed out of control and decimate plant life. They may be frugivores or graminovores that help disperse the seeds of plants away from the competition or predation underneath the parent plant. They may be herbivores, which control highly competitive plants or weeds. They may be predators of the eggs of seabirds or other birds. Each of these functions in an ecosystem can have a profound effect on its structure and function.
It was decided therefore to carry out a trial survey on Little Tobago Island to determine if any of these important ecosystem components were present, this article reports on the survey and suggests further courses of action.
Reports of Mammals on Little Tobago in the Literature
Little has been written about the occurrence of small mammals on Little Tobago. Morris (1976) spent six months living on the island studying the Birds of Paradise. He also made observations of other wildlife on the island and could find no evidence of small mammals other than bats. Niddrie (1980) reported that there was at least one species of arboreal or tree rat on the island. Other islands similar in size, climate and vegetation cover around the world are known to support populations of small mammals particularly rodents. The Black Rat and the Norwegian Rat are cosmopolitan in their distribution and survive especially well in habitats disturbed by humans. These invasive rats are a big problem on islands with large colonies of ground nesting seabirds such as Little Tobago where they prey on the eggs and young birds and can decimate a seabird colony. Conservation agencies around the world invest large sums of money in rat eradication programs as a first step in restoring a stable ecosystem on island and preserving populations of rare and endangered flora and fauna. It appears from the literature that Little Tobago does not suffer from the problem of rats. There is no account in the literature however, of a survey being carried out exclusively for small, arboreal or terrestrial mammals.
Methodology of the Survey
So little is known about the occurrence of small mammals on Little Tobago that a survey could not be focused on one group, for example the frugivores with traps baited exclusively with fruit. Neither was any distribution of any potential mammal populations known. It was decided therefore, to use a variety of baits in the hope of attracting each of the different trophic types.
Sherman traps were used as they provided a standard trapping technique known to be effective in sampling populations of small mammals. The traps were placed at the vegetation survey sites set up by Natalie Boodram which were 100m apart along a transect running east-west across the whole island. The transect covered a variety of vegetation types, aspects, slopes, and seabird nesting sites. The advantage of using the vegetation sites already surveyed by Ms Boodram was that vegetation characters could be related to small mammal distribution so hypotheses could be put forward regarding the interaction of vegetation and small mammals.
Four different types of bait were used to try and attract any of the different types of small mammals that may be on the island. The baits used were:
1. Peanut butter on Crix crackers
2. Coconut meat
3. Goat meat
At each sample site along the transect four traps were set out at the four cardinal points of the compass. Each trap was baited with one of the four types of bait as described above. The traps were 5 to 10m apart and 5 to 10 m from the points established by Ms Boodram. Traps were
established at 10 points in the trap line for a total of forty traps. The traps were baited and set at 5-6 PM in the evening and visited between 7.30-8.30 am the next morning. The traps were baited thus for the first two nights and then baited only with Crix crackers as they seemed to be the only bait taken.
The survey took place in November in the middle of rainy season when few sea birds were nesting and the vegetation carried full foliage. No attempt was made to determine which plants were fruiting or flowering at the time.
The results were interesting in their negativity. No small mammals at all were caught in the Sherman traps during the three nights of trapping. The only animals that were caught by the trap were a Hermit crab and three feral chickens that got their heads caught in the traps. After the first
two nights it appeared that something was sampling the Crix crackers in the traps. None of the other baits appeared to have been touched. It was therefore decided to bait all the traps using Crix crackers during the third night. It was only the next morning that it was realised that it was the feral chickens which were taking the Crix crackers when three were found with their heads caught in the traps.
No small mammals were caught in this survey which indicates no small mammals were active on the island at the time or for some reason the traps were not effective in catching them. One section of the literature says that there is no small mammal life on Little Tobago and that seems
to be borne out by this survey. No small mammals were seen on the island at night when a brief spotlighting survey was carried out for an hour or so, and no small mammals were observed around the only human habitation on the island during the night. In addition the seabirds nest regularly on the ground on Little Tobago would be vulnerable to predation by rats if any were present. But they continue to nest which indicates rats are absent from the island.
Small mammals may not have been trapped through some fault of the trapping technique. The baits may have not been appropriate, the traps may not have been sufficiently sensitive to trap small light mammals (although they were sensitive enough to trap a medium sized hermit crab).
To completely banish the possibility of small mammals existing on the island at least one more survey should be undertaken on another part of the island using different baits, perhaps different fruits, oats or nuts. The implications of the absence of small mammals from the island are
interesting to contemplate. As mentioned previously, absence of rats on the island makes it ideal for seabird nesting. The only possible predators (apart from humans) of seabirds and their eggs are the various species of lizards when they grow large enough. The reptile population was noticeable in its abundance and it could be that reptiles fill the niches left empty by the absence of mammals. The feral chickens certainly appear to fill the niche of ground forager for seeds and fruits.
Implications for seed dispersal and other interactions with plants that small mammals play on the mainland need further research. Little Tobago gives an opportunity to test how small mammals may effect plants that they interact with by giving us a population that is protected from small
In conclusion, this survey indicates there are no small mammal populations on Little Tobago Island although this should be tested by at least one more trap survey and perhaps spotlight surveys. The ecosystem on the island could tentatively be thought of as free of the influence of small mammals, which has various implications for the other biota of the Island.
The work in this report was carried out as part of an ongoing biological survey of Little Tobago Island that is under the overall co-ordination of Environment Tobago. Wayne Rostant ably assisted in the field in setting and checking the traps. Howard Nelson lent the traps that made this survey possible. Pat Turpin provided accommodations for the researchers and Natalie Boodram provided transport to and from the island and found the appropriate trap sites that would have been much more difficult to find if she had not been there. The Forestry Department of Tobago gave permission for the work to be undertaken. The researchers wish to thank all these people for their kind assistance.
Niddrie D.L. 1980. Tobago. Litho Press Co. Cork
Morris R.D. 1976. The birds of Paradise Island. Trinidad Naturalist 3:29-36
Mike Oatham 3/3/98