World’s greatest concentration of unique mammal species are all on 1 Island

An interesting little paper has popped up via Science Daily and concerns the publication of the results of a 15 year study of the biodiversity of one Island in the Philippines.

In summary …

Luzon, the largest island in the Philippines, is home to the world’s greatest concentration of unique mammal species — 93 percent of the land mammals there are found nowhere else. A new paper announces that 52 of the island’s 56 non-flying mammals live nowhere else in the world. Of these 56 species, 28 were discovered during the course of the 15-year project.

Wow, fascinating, and it is also interesting to perhaps understand why this Island is special, but first let’s meet some of the islands inhabitants …

Photographs of (A) Carpomys melanurus, (B) Batomys gran , (C) Phloeomys pallidus, (D) Carpomys phaeurus, (E) Crateromys schadenbergi, and (F) Musseromys gulantang.
Photographs of (A) Carpomys melanurus, (B) Batomys gran , (C) Phloeomys pallidus, (D) Carpomys phaeurus, (E) Crateromys schadenbergi, and (F) Musseromys gulantang.

So what was driving the 15-year study?

If you want to have a viable and verified model that describes the richness and diversity of life in an island ecosystem then you need to start by measuring what is actually there, so that is what they set out to do. The biodiversity of Luzon Island, (which is quite large, roughly 103,000 km2), was assumed to be well known, and that perhaps they might discover a few new species. The big surprise is that quite a lot was not known at all. As explained within their abstract …

Prior to the start of our study in 2000, 28 native, non-flying mammals had been documented, and extrapolation from prior discoveries indicated that the rate of discovery of new species was steady but low. From 2000 to 2012, we surveyed non-flying mammals at 17 locations and discovered at least 28 additional species, doubling the number known. Nearly all of the new species are restricted to a single mountain or mountain range, most of which had not been sampled previously, thus also doubling the number of local centers of endemism within Luzon from four to eight. The number of species on a mountain is strongly correlated with the elevation of the peak, and the number of endemic species on a mountain range is strongly correlated with the maximum elevation of the range. All 28 of the new species, and 20 of the species discovered prior to 2000, are members of two morphologically and ecologically diverse endemic clades (“cloud rats” and “earthworm mice”), which strongly implies that species richness has primarily been the product of speciation within the island. We reject the general assumption that mammals on tropical oceanic islands are sufficiently well known that analysis and modeling of the dynamics of species richness may be conducted with precision. In the development of conceptual biogeographic models and implementation of effective conservation strategies, existing estimates of species richness, levels of endemism, and the number of subcenters of endemism should be actively reassessed and verified through robust field, museum, and laboratory studies.

Why is the Biodiversity so rich here?

What appears to be going on is that you have a lot of isolated communities of small mammals that inhabit elevations on the various mountain ranges and are in effect isolated from each other by the lowlands and so over time, without any disruption from predators, they have evolved and diversified into separate species …

The increase in diversity of native small mammals documented in the preceding section was intimately related to the distribution of small mammals on a local scale, specially along elevational gradients. For example, on small mountain ranges such as Mt. Isarog and Mt. Mingan that are isolated by extensive lowlands from other high- elevation areas, only three or four native species were known below about 1000 m, and only rarely was more than one of these a local endemic species (Fig. 4). With increasing elevation up to about 1600 m on these mountains, the total number of native species, the number of local endemics, and the percentage of local endemics increased steadily, reaching ve species of local endemics that accounted for 60% or more of the total small mammal fauna. Above this elevation, where the terrain became increasingly steep and rocky, total species richness and representation of local endemics decreased slightly

… and so this is the lightbulb moment. It was perhaps suspected since about the 1990s, but is now confirmed by this study …

biological diversity is not always highest in tropical lowlands; rather, for many groups of organisms, diversity increases with increasing elevation up to some point along the elevational gradient

Why is this discovery important?

Well basically because you can now use this information to work out the actual biodiversity …

the number of species endemic to a given range is correlated with the elevation of that mountain range

In essence, a mountain on an island acts as a sky island where species can both thrive, flourish, and rapidly diversify and adapt. The secret sauce here is that not only is Luzon itself isolated and huge (it is about about 40,000 square miles, the size of Indiana), but is is also covered in mountains that in turn act as pockets of isolation, hence a huge diversity have evolved over time.

As explained in Science Daily ..

Among the 28 new species discovered by the team are four species of tiny tree-mice with whiskers so long they reach nearly to their ankles, and five species of mice that look like shrews and feed primarily on earthworms. Most of the new species live in tropical cloud forest high in the mountains, where frequent typhoons can drop four or five meters (12 to 15 feet) of rain per year.

“All 28 of the species we discovered during the project are members of two branches on the tree of life that are confined to the Philippines,” according to Eric Rickart, a team member who is based at the Natural History Museum of Utah. “There are individual mountains on Luzon that have five species of mammals that live nowhere else. That’s more unique species on one mountain than live in any country in continental Europe. The concentration of unique biodiversity in the Philippines is really staggering.”

One other rather important point arises here concerning conversation

“The Philippines is one of the most heavily deforested countries in the tropics; only about seven percent of the old-growth tropical forest is left. We learned that quite a few of the species are seriously threatened by habitat loss and over-hunting, but none are yet extinct.” Luzon has a human population of about 50 million, including about 23 million in greater Manila, the country’s capital. “Protecting all of these species from extinction is going to be a big challenge. The good news is that when the native forest is allowed to regenerate, the native mammals move back in, and the pest rats get kicked out.”

– Danny Balete, a Research Associate at the Field Museum who is based in the Philippines


The PDF for the actual paper entitled “Doubling diversity: a cautionary tale of previously unsuspected mammalian diversity on a tropical oceanic island” is published within Frontiers of Biogeography and is freely downloadable.

There are also additional notes via Field Museum in Chicago.

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