UNEP Islands Web Site

General comments on indicators
- Coastal Index
- Sea level rise risk Indicator
- Isolation Indicator
- Threat Indicator
- Natural Protection Indicator
- Ecosystem Richness Indicator
- Species Richness Indicator
- Endemism Indicator
- Special Features Indicator
- Invasive Species Indicator
- Urbanization Indicator
- Human Threat Indicator
- Economic Pressure Indicator
- Protected Area Coverage Indicator
- Reliability of Data Indicator
- Human Impact Index
- Terrestrial Conservation Importance Index
- Marine Conservation Importance Index

General comments on indicators
One aim of this island directory is to classify islands by various criteria, and to make comparisons between different islands or areas as a basis for facilitating inter-island cooperation and sharing of solutions, and identifying conservation importance.  Given the large number of islands, the diversity of their characteristics, and the inadequacies in the data, it seems important to show as clearly as possible the bases on which indicators have been calculated, and the weightings that are given to different objective and subjective factors.  To do this, the series of simple numerical indicators developed for the Review of the Protected Areas System in Oceania (Dahl, 1986) have been refined and expanded to adapt them to the needs of a world island list. These include indicators for the nature and isolation of the island, for features of conservation interest, for risks to that conservation interest, and for the feasibility of conservation action, and are incorporated in the island entries and the various tables.  They make it possible to reduce, or at least to make more consistent and explicit, the subjectivity of judgements as to relative island significance and conservation importance.  They also can help to identify islands with particular characteristics.

While this approach using numerical values reduces the chances of personal bias or the drift in judgement that can occur between the beginning and the end of a long analysis, it does have its weaknesses.  First among these is the tendency to see the numbers as having more accuracy or weight than is actually the case.  The indicators only reflect present information, and can be expected to change as more data become available.  A low overall indicator including a low reliability of data rating can easily reflect a lack of information rather than a lack of conservation interest. At the same time, the system of indicators can be updated rapidly when appropriate.

Users of this directory are cautioned not to place too much weight on these indicators without first studying their derivation to ensure that they are appropriate to the intended use of the information.  Small differences between indicators should not be considered important, since there are too many uncertainties in the database, and some indicators are derived from what are essentially subjective judgements.  Where particular uses require other selection criteria, it is not difficult to modify the content and weighting of the indicators for such other uses.

The indicators of conservation importance are explained in broad catagories, since the numerical ratings in the island entries are based on data that are still too incomplete for fair inter-island comparisons.  Rankings in the tables, however, are based on the calculated numerical values.

Despite these weaknesses in detail, the indicators do give a good overall picture of conservation needs and relative importance of islands, and should prove a useful tool in sustainable development planning if used wisely.

Coastal Index
A coastal index as one measure of insularity is calculated by dividing the length of the shoreline by the land area.

Sea level rise risk
The risk to the island from sea level rise due to climate change is evaluated as the percentage of the land area less than 5 metres above sea level, divided by 10 to give a single digit.

To measure the isolation of the island from potential sources of colonization, the square roots of the distances to the nearest equivalent or larger island, the nearest island group or archipelago and the nearest continent are added to give an index of isolation.  Where one of these does not exist, the next higher distance is repeated, except in the case of small satellite islands close to much larger land masses.  See table of more isolated islands.

One point is given for each of the major categories of large scale catastrophic threats to the island environment:
---  cyclones (hurricanes or typhoons);
---  volcanic eruptions;
---  earthquakes, tsunamis (tidal waves), landslides, etc.;
---  severe drought;
---  susceptibility to major fires;
---  high risk of oil spills.
This is a measure of the risk of natural or human catastrophes that could threaten human welfare, seriously damage the economy, and endanger endemic species or protected areas, thus increasing the importance of adequate conservation action.
(Scale: 0 to 6)

Natural Protection (NP)
The natural protection afforded to the island by its condition or situation is rated by giving one point each for:
a)   remoteness (at least 200 km) from the nearest island or other land area;
b)   not presently inhabited;
c)   few or no introductions of predatory or competitor species such as feral animals, european rats and aggressive weeds.
(Scale: 0 to 3)

Ecosystem Richness (ER)
The ecosystem richness is measured as the number of terrestrial or marine ecosystem types or biomes, based where possible on an existing classification or estimated from the island description and structure.  For the terrestrial measure, less than 5 ecosystems or biomes is typical of impoverished low coral islands, 5 to 10 ecosystems indicates some distinct vegetation types, 10 to 25 ecosystem types would be found on high islands with some habitat diversity and differentiation of biomes with altitude, 25 to 40 ecosystem types shows considerable diversity of habitats, and more than 40 biomes and ecosystems approaches continental areas in richness.

A separate marine rating is based on a count of the marine ecosystems in the coastal zone to 100 metres depth.

Species Richness (SR)
The numbers of species of different categories of organisms that occur on an island are an important measure of its biological diversity.  The figures most frequently available are for terrestrial plants and/or land birds, and these are used as the basis for the terrestrial indicator scale.  Where individual island data are not available, estimates have been made based on figures for the country or island group.
     0 = few or no species (less than 24 plants and/or 8 land birds);
     1 = poor in species (25-224 plants; 9-23 land birds);
     2 = moderate species richness (225-624 plants; 24-39 birds);
     3 = good species richness (625-1224 plants; 40-55 birds);
     4 = high species richness (1225-2024 plants; 56-71 birds);
     5 = very rich in species (over 2025 plants and/or 72 birds).

An equivalent indicator of  marine species richness follows the same scale as that used for terrestrial plants, based on the few categories of organisms such as fish, corals or molluscs for which species data are available.

The endemism is rated both for island groups (GE) and individual islands (IE) on the basis of the number of endemic species and sub-species recorded.  For terrestrial endemism, the scale for the indicator is as follows:
        0 = no endemic species;             1 = 1 to 5 endemic species;
        2 = 6 to 38 endemic species;        3 = 39 to 149 endemic species;
        4 = 150 to 410 endemic species;     5 = 411 to 915 endemic species;
        6 = over 916 endemic species.
For marine endemism, the following scale is used:
        0 = no endemic species;             1 = 1-2 endemic species;
        2 = 3-6 endemic species;            3 = 7-12 endemic species;
        4 = 13-20 endemic species;          5 = 21-30 endemic species;
        6 = over 31 endemic species
The two indicators permit identifying both individual islands with endemic species restricted to that island, and islands in a group which may share endemic species with other nearby islands.

Special Features (SpFe)
Many islands have special features of conservation or tourism importance that need to be highlighted in any evaluation, such as seabird rookeries, turtle nesting beaches, marine mammal breeding areas, lakes, active volcanos, scenic mushroom islets, caves, etc.  These are listed and counted separately for marine and terrestrial environments to produce two special features indicators.

Invasive Species (INV)
The threat represented by invasive introduced species is rated both on the number of such species and their aggressiveness in island situations.  (scale 0-5)
      0 = few or no introductions
      1 = some introductions (i.e. rats, common weeds)
      2 = common domestic introductions (dogs, cats, pigs)
      3 = some problems with invasive species
      4 = major problems with invasive species
      5 = devastated by invasive species

Urbanization (UR)
The urbanization indicator is based on the proportion of the island population living in urban areas.  (scale: 0-9)

Human Threat (HT)
This indicator estimates the pressure of the local population on the land and resources.  Statistics for the percentage of the population in agriculture, mining and fishing are divided by 30, giving a scale of 0 to 3.  Some adjustments must be made for specific islands (such as remote or uninhabited islands) where the situation is known to be different from the country average.

Economic Pressure (EP)
This measures the level of economic development and thus of modern development impact on the environment.  The indicator is generally calculated for each country, based on the Gross Domestic Product, GNP or income per capita (in US$) divided by 2000, giving a scale of 0 to 9.  There are at present no statistics which would allow these average indicators for each country to be adjusted for rural islands with less development or for islands with urban centres or major development projects.

Protected Area Coverage (PA)
The rating for the coverage of the land and marine areas of the island by protected areas is rated according to the following scale based on the percentage of surface protected:
     0 = <0.5% protected              4 = 25-40% protected
     1 = 0.5-4% protected             5 = 41-60% protected
     2 = 5-12% protected              6 = 61-84% protected
     3 = 13-24% protected             7 = >85% protected
See table of islands with protected areas.

Reliability of Data (DA)
It is important to know whether an island situation or local conservation problem is well documented and clearly understood, or only suspected on the basis of inadequate data.  The following scale is used:
          0 = no reliable data;
          1 = poor data (both partial and out of date);
          2 = data only partial or out of date
          3 = good recent data (within the last 10 years).
This scale favours islands with problems that are well understood, and where conservation action can be clearly defined, over those that closer examination may show not to have problems, or to be irremediable.  A low indicator does not mean that conservation action is not needed, but that is should be preceded by further studies to determine the present situation.  This indicator can also be used to judge the overall accuracy of the combined ratings.


The data for each island are summarized in three aggregated indices to give an overall evaluation and to permit comparisons and rankings.

Human Impact (HI)
The Human Impact index measures the overall human pressure or impact on the island and therefore the potential threat to remaining natural areas or endemic species.  Since it is based on
the present situation and its potential for future change, it does not measure past changes (such as caused by former inhabitants or abandoned mines) which no longer represent a current threat, nor
does it reflect the cumulative human impact of such past changes.

The HI index is calculated as follows: the population density (for the island if known, or else for the country), divided by 50, is multiplied by a population trend factor based on the growth rate that produces a reduced figure for a declining population and a larger value for a fast-growing population.  To this figure for the demographic pressure on resources is added the Human Threat indicator based on the percent of the population in agriculture, mining and fishing), the Economic Pressure indicator (based on GDP), the Urbanization indicator (based on the percentage of the population in urban areas), the number of tourists divided by the population multiplied by 5, the invasive species rating, and the percentage of developed plus degraded land divided by 10.  Range: 0 to 99.

      HI = (density/50) x (trend/2) + HT + EP + URB

         + 5*tourism/population + INV + (devland+degraded)/10

See table of Islands by Human Impact Index

Terrestrial Conservation Importance (CI-T)
The Terrestrial Conservation Importance index is intended to give an overall numerical evaluation of the significance of the land area of the island for the conservation of nature.  It
consists of the sum of a series of measures of conservation interest weighted for their relative importance.  Both measures of biological importance and measures of their natural conservation
status have been included, since both are important for successful conservation action, although biological factors are given the heavier weighting.  In a sense the formula tries to reflect the
kind of evaluation process used by a conservation planner or a protected area manager in selecting a protected area.  The elements of the CI index are:

- the Ecosystem Richness (ER) indicator divided by 10 and the Species Richness (SR) indicator, both multiplied by 2 (scale: 0 to 20 points) as measures of the richness of natural communities;

- the percentage of the land area covered by Forest divided by 10 (up to 10 points);

- the Island Endemism (IE) and Group Endemism (GE) indicators based on numbers of endemic species (maximum 12 points), and the percent endemism of plants and of land birds, divided by 10 (up to 10 points each);

- measures of threatened species, including the number of Endangered, Vulnerable, Rare, Indeterminate or K (EVRI) plant and bird species divided by 5 (up to 31 points in exceptional cases such as New Caledonia);

- one point each for Special Features (SpFe) of conservation interest, such as seabird rookeries, sea turtle nesting areas or other critical habitats, lakes, unusual geological formations or other features deserving specific protection (generally 0 to 4 per island);

- the Vulnerability (Vu) indicator (up to 6 points); and

- the Natural Protection (NP) indicator (0 to 3 points).

       CI-T = 2(ERT/10+SRT)+forest/10+IENDT+GENDT+percent endemism


The theoretical maximum value of the terrestrial CI (for present numbers of endangered and threatened species) approaches 100 points.

This index of Conservation Importance will thus favour islands with greater ecological complexity and species diversity, with more species endemism, with larger numbers of endangered and threatened species, with more special features of conservation interest, with greater vulnerability to natural disasters, and with better natural protection.

The terrestrial CI does not favour pristine but simple islands except for their natural conservation status, even though such highly simplified ecosystems may be of great scientific and conservation interest.  It also does not single out islands with critical habitats of importance to sea birds, turtles or marine mammals.  Conservation priorities for these should be identified using other measures.

See table of Islands by Conservation Importance Index

Marine Conservation Importance (CI-M)
The index for Marine Conservation Importance is intended to provide an equivalent measure to the terrestrial Conservation Importance (CI-T) index, but adapted to the special characteristics of the island marine environment down to 100 metres depth.

Since even less data are available for coastal and marine areas of islands than for terrestrial areas, it may not be possible at present to calculate a viable marine indicator for many islands, but it is included here to highlight the information needed.  The index consists of:

- the marine Ecosystem Richness (ERM) indicator, divided by 3 because of the relative importance of coastal marine ecosystem diversity, and the marine Species Richness (SRM) indicator, both multiplied by 2;

- the marine Island Endemism (IENDM) and Group Endemism (GENDM) indicators based on the numbers of marine endemic species multiplied by 3 because of the relative rarity of marine endemism;

- one point for each marine Special Feature (SpFe) of conservation interest;

- the Vulnerability (Vu) indicator (up to 6 points); and

- the Natural Protection (NP) indicator (0 to 4 points).

       CI-M = 2(ERM/3+SRM)+(IENDM*3)+(GENDM*3)+SpFeM+VU+NP

As with the CI-T, the CI-M favors rich and complex coastal areas with good prospects for conservation action.  It is not a good measure to identify simple undisturbed coastal areas which maybe unique in their own way, nor does it include any measure of the importance of marine protected areas for fisheries management.

There can be many reasons for taking special action to protect an island, and the above choice of measures and weightings may be debatable in particular instances.  Overall, however, broadly-based aggregate indices such as these should help to identify and rank the different islands in terms of priorities for terrestrial and marine conservation and sustainable development action. Indices can also be combined, as in the table of Islands most at risk, combining HI and CI indices.

Return to Island Directory Home Page

Updated 23 July 2003