Environmental Science
How Islands Exhibit Idiosyncratic Species-Area Relationships in Biodiversity Hotspots
Quick fact
On the Galápagos Islands, a single island can hold species found nowhere else on Earth, yet the species-area relationship across these islands is much steeper than on mainland ecosystems—meaning a small loss of area leads to a disproportionately large loss of species.
Why this is interesting
You might expect larger islands to always hold more species, but some tiny islands host more unique species than vast continents. Why do islands seem to break the rules of species-area relationships?
Read the full explanation
Understanding How Islands Exhibit Idiosyncratic Species-Area Relationships in Biodiversity Hotspots
Imagine you have a big box and a small box. If you put random stuff in them, the big box likely holds more items—that's the general species-area relationship (SAR). Now imagine the boxes are islands in the ocean. The small box might be home to a unique item that only exists there, like a special bird that evolved on that tiny island. So, the relationship between box size and number of items isn't simple—each island is special. In biodiversity hotspots, islands are often 'idiosyncratic' because their unique evolutionary history, isolation, and limited space create patterns that don't follow the usual curve. For example, the number of species on an island might not increase smoothly with area; it might jump up because a new habitat type appears, or stay flat because the island is too isolated for colonisation. These exceptions are not random—they reflect the island's individual history and geographical context.
A deeper explanation
The idiosyncratic species-area relationships on islands stem from the interplay of three factors: immigration, extinction, and speciation. Islands in biodiversity hotspots are often volcanic or continental fragments with distinct geological histories. Their isolation filters immigrant species, leading to unique evolutionary radiations. The SAR assumes a simple power law (S = cA^z), but on islands, the exponent 'z' can vary dramatically. For instance, in the Hawaiian archipelago, the slope is steeper because new species arise via in-situ speciation, and extinction risk is higher on smaller islands. Additionally, habitat heterogeneity within an island can create microhabitats that support endemic species, influencing the shape of the curve. The 'small island effect' is another idiosyncrasy: below a certain area, species number is nearly constant because the island is too small to support many species, but it may still host a few highly specialised ones. Conservation implications are profound: if an island's SAR is steeper than expected, then reducing habitat area by even 10% could cause many more extinctions than mainland ecosystems. Therefore, tailoring conservation plans to island-specific SARs is essential to protect biodiversity in hotspots.