Table 1 Main rationale and hypotheses explaining the expected relationships between the 16 factors used in our study and the presence or the 30% coverage of protected areas on islands for both terrestrial and marine areas.
From: Global correlates of terrestrial and marine coverage by protected areas on islands
Factor (abbreviation) | Rationale and hypotheses | Expected relationship with protection coverage |
|---|---|---|
Maximum temperature (Max temp) | The latitudinal biodiversity gradient is related to air and seawater temperature with a greater diversity of species found in the tropics47,48,60. Islands with high temperature are likely to host more species and thus expected to have greater protection | Positive |
Minimum temperature (Min temp) | Many species that originated from the tropics cannot cope with freezing conditions given climate niche conservatism82. So, islands with low minimum temperatures are likely to host fewer species and thus set lower levels of protection | Positive |
Maximum precipitation (Max prec) | Precipitation is essential for forest and wetland habitats where most of terrestrial biodiversity is found83,84. Islands with more precipitation are likely to host more species and thus expected to have greater protection | Positive |
Minimum precipitation (Min prec) | Drought severely limits life on terrestrial ecosystems85,86. Islands with dry conditions are likely to host fewer species and thus expected to have lower levels of protection | Positive |
Freshwater area | Freshwater is necessary for most terrestrial species and is critical for agriculture and human livelihoods87. Islands with large areas covered by freshwater are likely to have a greater diversity of species and thus expected to have more protection. However, such islands are also likely to have more productive land available for agriculture creating competition for space and thus the expectation of lower levels of protection | Positive or negative |
Surface area (Surf area) | Large islands host more varied habitats, and consequently more species86,88, but also have more space for human activities (cities, agriculture, fisheries) and thus less competition for space. Large islands are expected to have more protection and to achieve minimum protection coverage more easily. However, small islands are expected to be more likely to reach ambitious conservation targets because their areal coverage requirements are lower | Positive for the least demanding conservation target (presence of protection) but negative for the most demanding (>30% coverage) |
Distance to the mainland (Dist to mainland) | According to the theory of biogeography89, distance to the mainland is the main driver of species richness on islands, so isolated islands may host less species and thus receive lower levels of protection. Besides, islands located far from the mainland have greater resource management requirements (e.g., cost of establishing and running protected areas). These islands are thus expected to have less protection90. An “Islandness” feeling is likely to increase with distance from the continent so isolated islands may reject restrictions more strongly than connected islands and such conflicts may decrease protection area acceptation and protection coverage64. Yet, we cannot preclude some exceptions if biodiversity is the main richness of small and isolated islands and a critical source of economic incomes (tourism) that need protection | Negative |
Maximum altitude (Max alt) | Altitude can create a wide variety of habitats and environmental niches but also isolated populations91. Mountainous islands are likely to host more species, particularly endemics, and thus expected to receive more protection | Positive |
Mean altitude (Mean alt) | A high mean altitude can be detrimental to agriculture and urbanization (e.g., colder conditions)92. High altitude islands are likely to host fewer people and activities reducing competition for space and are thus expected to have more protection51 | Positive |
Fractal dimension (Fractal dim) | A tortuous coastal line may provide more terrestrial and marine habitats and are thus likely to host more species93. Furthermore, tortuous islands have less favorable conditions to host large cities and agriculture, reducing competition for space, and are thus expected to have more protection | Positive |
Population density (Pop density) | High human population densities are associated with overexploitation of natural resources7 or more degraded habitats94, and create competition for space51. Densely populated islands are thus expected to have lower levels of protection | Negative |
Cropland area | Under demographic pressure, many islands have undergone a transition from food surpluses to deficits as land was converted for cash crops by a land-owning elite27. Moreover, cropland area is in direct competition with other uses, such as protected areas, for space72. Islands with extensive cropland area are expected to have lower levels of protection | Negative |
Number of languages (No. of languages) | The number of languages spoken in a country or island is associated with the diversity of landscapes and cultures. The number of languages is also positively associated with species diversity55,56. Islands with a greater number of languages are expected to host more species and are thus expected to have more protection | Positive |
Sovereignty | Islands that are territories, and not states, can have access to additional resources from the sovereign country (e.g., human and economic capital). Independent islands are therefore expected to have lower resource capacity and lower levels of protection | Negative |
Nature of institutional languages (Inst languages) | Social capital, which tends to be greater where languages are common, can influence a countries institutional performance62. The nature of institutional languages (i.e., indigenous, non-indigenous, or both) reflects the historical colonization and its footprint. Social capital is therefore likely to be lower in islands with a mix of institutional languages (i.e., both indigenous and non-indigenous). Islands with a mix of institutional languages are expected to have lower capacity to reach shared agreement on management, and are expected to have less protection | Negative for a mix between indigenous and non-indigenous languages |
Human development index (HDI) | Countries with a higher human development index (measure of health, education, and economy) are likely to have a greater capacity to manage their environment43,60, and consequently these islands are expected to have more protection | Positive |
