1. WALLACE RESOURCE LIBRARY
Module 02 – Survey Techniques:
D01 – Population estimates of Hog Island Boa Constrictors

2. Why Survey Biodiversity?
Global decline in biodiversity
But how do we know?
In order to assess biodiversity we must apply appropriate survey techniques
This provides the information we need to understand what is happening to biodiversity on a local as well as a global scale
We are all familiar with the notion that global biodiversity is at increasing risk of extinction due to a variety of reasons. These reasons include, but are not limited to; habitat destruction and degradation, environmental pollution, over-harvesting of wildlife for human consumption, wildlife diseases and climate change. But how do we know that biodiversity is threatened on a global scale?
In order to understand fully what is happening to global biodiversity, we must find sensible ways of monitoring it. How we go about this enormous task depends very much firstly on at what level we are trying to assess biodiversity levels. It is impossible to look at all biological diversity at the same time so we must break it down into more manageable sections in some way.
The survey techniques that we use will depend on the size of the area that we are interested in studying. For example, satellite images can be taken of large areas of forest to calculate the proportion of forested, degraded or deforested areas, whereas a microscope might be necessary to study the diversity of the invertebrate community living on a single tree within that same forest. So the first thing that we have to do is identify our study area.
The next decision to make is are we interested in all biological diversity in that area, or are we actually only interested in a small number of key species? Normally researchers are only interested in a small proportion of the overall diversity present, because even within a small area it can be very difficult to survey and identify everything present. This is why it is common to have different researchers working in the same location all carrying out different survey techniques in order to monitor the particular species or communities that they are interested in. We can then combine this information to get a clearer picture of the biodiversity contained within the study area.
However, it is impractical to survey and identify 100% of species within any given area. Therefore, we often monitor species that are easier to detect and then use information on these species to provide us with information on what is happening within the ecosystem in general.

3. Why Survey Biodiversity?
IUNC Red list of Threatened Species
Extinct (EX)
Extinct in the Wild (EW)
Critically Endangered (CR)
Endangered (EN)
Vulnerable (VU)
Near Threatened (NT)
Least Concern (LC)
Perhaps the most well known way that biodiversity survey data is used is for the classification of species under the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. This system categorises species into the above levels of conservation concern based on all of the data that is available. Although many species have been assessed under the IUCN Red List, there are still a very large number of species that have not yet been formally classified due to a lack of data. These species currently fall into another category called “Data Deficient (DD)” and yet many other species have not even been assessed at all (they fall into a Not Evaluated category).
Therefore, one very important use of survey data is to identify which species are of the highest conservation concern. The IUCN red list provides a mechanism for doing this on a global scale, but many other databases exist that assess the conservation status of species on a national or local level.
It is the collection of this important data that allows us to piece together the puzzle of what is actually happening to wildlife populations around the globe.

4. How do we Survey Biodiversity?
Global or National scale
- Satellite images
Local scale
Capture-Mark-Recapture
Transect and Distance sampling
Passive trapping
Camera trapping
Direct counts e.g. Helicopter surveys
We can get a broad understanding of the rate of loss of biological diversity and even predict its future decline by looking at satellite images and measuring habitat change in particular ecosystems. However, satellites are only useful if we first have an idea of the amount of biodiversity contained within that ecosystem. In order to get that information we need researchers to actually study those systems using a variety of different survey techniques.
The methods that we use will depend on what we want to know. For example we may simply be interested in the number of different species present in a given area. Alternatively, we might want to know the population size for a particular species within that area, or perhaps we may want to know the population size of all large mammal species in that area.
As conservationists we have a wide variety of survey techniques available to us and which one we use will depend on the question we want to answer. Below are some examples of common techniques used (there are many more). How we actually carry out these techniques will depend on the species we are trying to survey:
Capture-Mark-Recapture – suitable for estimating the population size of locally abundant species or intensively studied populations
Transect sampling - suitable for estimating species diversity and relative abundance
Distance sampling – suitable for estimating population size of multiple species encountered along a transect
Passive trapping – suitable for assessing local species diversity and estimating local abundance
Camera trapping - suitable for assessing local species diversity and estimating local abundance of elusive animals such as large mammals
Direct counts e.g. Helicopter surveys – suitable for obtaining accurate population data for large animals in open areas such as large mammals in Africa

5. Capture-Mark-Recapture
Population at Time 1
Population at Time 2
Capture-Mark-Recapture is a very useful survey method to use when we want to get an accurate estimate of the population size of a particular species of interest. For example we might want to know what the population size is for an endangered species within a protected area of forest or on an isolated island.
The basic principle behind any capture-mark-recapture study is the same. You first capture as many individuals as you can from the population within a set period of time. You then mark all of those captured individuals in some way before releasing them back into the population.
You then revisit the population at a later time and sample for a second time. Within that second sample there will likely be a mixture of marked and unmarked individuals. It is the ratio of marked-unmarked individuals in the second sample that allows you to estimate the total population size.
It is very important that the methods that you use when you capture and mark the animals does not have a negative impact on their ability to survive or make them more difficult to capture in the future.
Capture-Mark-Recapture estimates can be done using very basic designs such as that above or can be much more complex, using multiple sampling occasions, but all are based on the same basic theory above.
Sample 1
Sample 2
Total Marked = 8
Total Captured = 6
Total Recaptures = 3

6. The Hog Island Boa
Exists on only two small islands called the Cayos Cochinos off the northern coast of Honduras
Population devastated in the 1980s due to intensive collection for the pet trade
The Hog Island Boa constrictor (Boa constrictor imperator) is an island dwarfed race of snake from the Cayos Cochinos archipelago, Honduras. This population reportedly experienced severe decline as a result of intense collection for the pet trade throughout the late 1970s and 1980s, during which time thousands of snakes were removed from the islands. Just a decade after collection began, a herpetological expedition to the Cayos Cochinos was unable to find a single specimen of this previously abundant snake, and local residents involved in the trade confirmed virtually all adult boas had been removed from the islands.
Fortunately, in 1993 the Cayos Cochinos was declared a protected area and in 1994 the Honduran Coral Reef Foundation (HCRF) was established to protect the area. Since this time, the removal of boas has been dramatically reduced and the population appears to be recovering.
In 2004, a long-term mark-recapture study was initiated by Operation Wallacea on Cayo Cochino Pequeño, the smaller of the two islands, to estimate current population size in order to determine the level of population recovery and, thus, the effectiveness of current conservation management. The small size and isolation of the study site, as well as high density of boas provided a rare opportunity to conduct such a study and to gather data which will help inform future management of the population.

7. The Hog Island Boa
Thousands of boas illegally removed from the islands caused population to crash
Increased protection of the islands after 1993
But how many Boas (if any) were left in the wild?
Mark-Recapture Population estimate started by Operation Wallacea in 2004
The Hog Island Boa constrictor (Boa constrictor imperator) is an island dwarfed race of snake from the Cayos Cochinos archipelago, Honduras. This population reportedly experienced severe decline as a result of intense collection for the pet trade throughout the late 1970s and 1980s, during which time thousands of snakes were removed from the islands. Just a decade after collection began, a herpetological expedition to the Cayos Cochinos was unable to find a single specimen of this previously abundant snake, and local residents involved in the trade confirmed virtually all adult boas had been removed from the islands.
Fortunately, in 1993 the Cayos Cochinos was declared a protected area and in 1994 the Honduran Coral Reef Foundation (HCRF) was established to protect the area. Since this time, the removal of boas has been dramatically reduced and the population appears to be recovering.
In 2004, a long-term mark-recapture study was initiated by Operation Wallacea on Cayo Cochino Pequeño, the smaller of the two islands, to estimate current population size in order to determine the level of population recovery and, thus, the effectiveness of current conservation management. The small size and isolation of the study site, as well as high density of boas provided a rare opportunity to conduct such a study and to gather data which will help inform future management of the population.

8. Tasks for this dataset
You are going to estimate the current population size of Boas on the island of Cayo Menor in the Cayos Cochinos, Honduras
You will do this by analysing mark- recapture data
There is a two year dataset that you will analyse using the Lincoln-Peterson method
There is also a three year dataset that can be analysed using the Schnabel method
Estimating the population size of endangered species using capture-mark-recapture is very useful for conservation managers. Having an idea of the size of population that you are trying to protect is very important. In the following exercises we will estimate the population size of Boa constrictor on the island of Cayo Menor; first using a two year data set (Lincoln-Peterson method) and then using a three year data set (Schnabel method).
Both the Lincoln Peterson and the Schnabel methods assume the we are looking at a 'closed' population, which means that we would not expect there to be any animals entering or leaving the population between our sampling events. What we mean by animals entering or leaving the population would be any animals that emigrate out of or immigrate into the population as well as any births or deaths that occur in the population between sampling occasions. For the purposes of this study, because we are looking at a small, isolated island population, we can assume that no animals entered or left the island during the study period. We will also assume for the purposes of this exercise that no births or deaths took place (although in reality births and deaths are frequent events in populations).
Populations estimates can be done for 'open' populations, that take into account animals entering or leaving the population during the study period (either through migration, birth or death), however, these are more complex and will not be considered here.
We will start with the Lincoln-Peterson method using a two year dataset. Then, if you wish, you can try the slightly more complicated Schnabel method using the three year data set.

9. Research questions:
1. Using the mark recapture data provided, estimate the population size of Boa constrictor on Cayo Menor.
2. By how much do the estimates produced using the 2 year dataset and 3 year data set differ and what might this tell us about the possible benefits of using multiple years of data when conducting population estimates?
In order to answer question 1. students will need to complete just the Lincoln-Peterson method using the two year dataset (although they could also complete the Schnabel method using the three year dataset if they wish).
To answer the second question students will need to complete both the Lincoln-Peterson and Schnabel methods.

10. Summary Conclusions
1. The Lincoln-Peterson method, using the two year data set, gives a population estimate of 938 boas.
2. The Schnabel estimate, using the three year dataset, gives a population estimate of 1,062 boas. Therefore, the difference between the two estimates is 124 boas.
Answer and discussion to the research questions:
1. The Lincoln-Peterson method, using the two year data set, gives a population estimate of 938 boas.
2. The Schnabel estimate, using the three year dataset, gives a population estimate of 1,062 boas. Therefore, the difference between the two estimates is 124 boas. This may suggest that the Lincoln-Peterson estimate, based on only two sampling occasions, has underestimated the population size slightly. The advantage of having multiple years data (more than two) is that the population estimate is using more data to reach a final answer and should, in theory, be more accurate. The more accurate the information we have about a population, the better it is for conservation managers when they make decisions about the best way of managing the population and what, if any, action may be required.
Due to increased conservation effort the population appears to be recovering !