Table 1 Main rationale and hypotheses explaining the expected relationships between the seven environmental and habitat factors used in our study and the presence of protected areas on land and sea

Island

The average proportion of terrestrial and marine coverage by protected areas on islands is greater than on continents³³ since remoteness eases conservation efforts^21,22.

Positive

Atmospheric and Seawater Temperature

The latitudinal biodiversity gradient is related to air and seawater temperature with a greater diversity of species found in the tropics^66,67, where more biodiversity protection is expected.

Positive

Distance to the coast

The coverage by protected areas may increase with distance from the coast since locations close to coastlines are under intense human activities⁶⁸ while remoteness favours conservation efforts^21,22.

Positive

Primary Productivity (NDVI and Chla)

The Normalised Difference Vegetation Index (NDVI) and Chlorophyll-a (Chla) are proxies of primary productivity on land and sea, respectively. Species richness is generally higher in high productivity sites supporting more individuals and more viable populations⁶⁹, so where more conservation efforts are expected. On the other side, the most productive sites are the most suitable for agriculture or fisheries so less prone to protection.

Positive and negative

Precipitation

Precipitations are essential for forest and wetland habitats where most terrestrial biodiversity is found^70,71 so where more protection is expected.

Positive

Elevation

Protected areas tend to be found in locations with higher elevation often less suitable to human settlements and activities like agriculture²¹.

Positive

Freshwater

Freshwater is necessary for most terrestrial species⁷² and is critical for agriculture and human livelihoods. Locations with large areas covered by freshwater are likely to have a greater species diversity and thus expected to have more protection. However, such locations are also likely to have more productive land which will be used for agriculture, creating competition for space and thus expected to have less protection²¹.

Positive or Negative

Depth

Increasing depth is generally associated with decreasing light availability, decreasing temperature, increasing pressure and decreasing biological productivity and diversity^73,74, so deeper areas may receive less conservation efforts.

Negative

Distance to seamounts or knolls

Higher species abundance and richness is associated with seamounts compared to oceanic areas⁷⁵. Locations near seamounts or knolls are thus expected to receive greater protection.

Negative

Sea Surface Salinity

Species richness is expected to increase with salinity^76,77 while river mouths, estuaries or coastal lagoons with brackish waters are highly productive⁷⁸ so more prone to exploitation of resources.

Positive