Although only covering around 2% of the Earth’s surface, tropical rainforests are home to more than half of all life forms on our planet. Its biodiversity is truly immense, but the answer on why so many different taxonomic groups have evolved in this biome is rather complex.
When thinking about the biodiversity of the Bornean forests the name of Alfred Russel Wallace automatically comes into my mind. As collaborator of Charles Darwin and co-author of the famous 1858 paper On the Tendency of Species to form Varieties; and on the perpetuation of Varieties and Species by Natural Means of Selection Wallace is one of the founding fathers of the evolution theory – but where Darwin’s fame got firmly cemented by his book On the Origin of Species, Wallace’s contribution to what is now known as ‘Darwinism’ became almost forgotten. However, with his skills as an animal collector, storyteller and founder of biogeography, Wallace has left behind his own legacy; especially his observation that the islands of the Malayan Archipelago represented a frontier between two faunal provinces (the Indo-Malayan to the west and Australian to the east, separated by an imaginary line running between the Philippines, Borneo and Java, and Celebes, the Moluccas, Timor and New Guinea, also known as the Wallace Line) has been a major contribution to biological science to this day.
Rainforests are ancient biomes that have been aged 130 to 180 million years old – a very, very long time for speciation, longer than in any other biome according to Wallace. In the Bornean forests species have evolved very different again from their counterparts on the mainland when the island separated from the Sunda landmass around 1.4 million years ago, and speciation through geographical isolation was an almost common observation Wallace made during his eight-year exploration of the Malayan archipelago. The warm, humid and rather predictable climate further provides a relative constant food supply, beneficial for living organisms while enabling them to specialise in one or more of them. As the process of specialisation started a very long time ago too, animals in the rainforests have found and protected their own niches against intruders, explaining both the high number of endemic species as well as the occurrence and heterogeneity of life forms in the forests layers or strata. Although most activity takes place in the sunlit canopy, the forest floor is the most accessible and therefore the most explored by humans, and the mesmerizing diversity of shapes and colours we witnessed there were highlights in itself.
When reading Wallace’s annotated diary On the Organic Law of Change the passages on Sarawak’s Orang Utans and Aru’s birds of paradise are amongst the most captivating. However, while trying to saturate the era’s naturalists hunger for exotic species – and generate sufficient funding for his monumental travels – Wallace collected an impressive collection of animals which not only contained the big and beautiful, but also a whopping 110,000 insects including a large number of beetles – the majority of this impressive collection which nowadays is kept at the Natural History Museum in London. It were exactly those insects and other bugs we kept stumbling upon when trekking through the jungle, and the discovery of a longhorn beetle (Batucera rubus) on a fallen tree log (a preferred habitat for beetles as mentioned by Wallace in his notes) made natural history come alive again.