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Separating Two Worlds: Faunal Boundary Lines

  • Alexander Boenjamin
  • Oct 13, 2023
  • 3 min read

Boundaries and borders have separated nations for millennia. They have caused the separation of people, and cultures and have at times caused inter-state conflicts. Most importantly though, such boundaries have caused the development of knowledge and technology on both sides to differ greatly over time. The Romans had countless intellectual renaissances incomparable to those of their neighbouring tribes. The British, French, Germans and Italians had vastly different lifestyles during the Middle Ages. Even during modern history, the Iron Curtain separated a continent into two contrasting worlds. Although animals certainly can't wage war against one another (it would be interesting if we could see a conflict between bears and lions wearing ghillie suits and helmets), there are certain invisible boundary lines across the world that separate utterly different biodiversity landscapes on either side, with similar qualities as nation boundaries.


The term faunal boundary line or biogeographic boundary may not ring a bell in many people’s ears, and rightly so. It is a concept not widely discussed, but nonetheless interesting. So what is it? Simply put, they are the geographic and sometimes imaginary lines that separate two areas with distinct biotas. Scratch that, it wasn't simply put. Anyways, the fauna on both sides of the lines developed independently of each other over millennia and had acquired their own, unique genetic traits. The most well-known example of a faunal boundary line is the Wallace line, splitting Indonesia in two. On one side, you could find tigers, elephants and orangutans, while on the other the only thing you would find when searching are small rodents and marsupials.


History

Scientists have contemplated the existence of biological boundaries since the early days of mankind; the Greeks and Romans had understood different distributions of animals. Nonetheless, most innovations in the field of faunal boundary lines began with the birth of Alfred Wallace. His birth itself wasn't significant in any way, but the discoveries that he unearthed were. Over the course of several expeditions in the Malay archipelago, he observed the striking contrasts between the fauna of the west and east. In 1863, he mapped out the first draft of the Wallace Line, becoming the first ‘imaginary’ faunal boundary, which has since been perfected. This discovery coincided with his contemplation of the theory of evolution, worked on and stolen by Charles Darwin.


For further reading, Asienreisender* provides a complete and in-depth explanation of Wallace’s life and journeys.



Science behind Faunal Boundary Lines

As Wallace travelled the 20km distance between Bali to Lombok and documented different landscapes of flora and fauna on both ends, he suspected that geography was not the only factor separating the two biogeographic areas. In fact, the geography of the past and tectonic plate movements serve as a leading cause of the formation of faunal boundaries. Taking the Wallace Line as an example, the islands situated on the west of the line had been connected to the Asian mainland in the past. Thus, animals from the Asian mainland were able to travel to these islands, until a recent rise in sea levels. The islands east of the line were also connected to form their own large landmass, prior to tectonic plate shifting which brought it closer to the Asian mainland. Sharp changes in climate conditions, orographic barriers, and others are additional factors behind biogeographic boundaries.



The Wallace Line

Now, back to the aforementioned Wallace Line. A faunal boundary that divides the Indonesian archipelago in two between the islands of Bali and Lombok, and the islands of Kalimantan and Sulawesi, the Wallace Line is also the only search result that pops up when one searches faunal boundary lines. This line separates two greatly different biological scenes on either end; on its west the Indomalayan zoologic region, and on its east the Australasian zoologic region. Major differences on the two sides include mammal species, such as tigers, elephants, rhinos and others on the Wallace Line’s west, and smaller mammals like marsupials and monotremes on its east. Bird species and reptile species also greatly differ between the two sides, the former being the observation that led to the discovery of the Wallace Line.


 
 
 

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