Review: Barber PH, Palumbi SR, Erdmann MV, Moosa MK (2000). Biogeography: A marine Wallace's line? Nature, 406:692-693.

Feature Paper: Barber PH, Palumbi SR, Erdmann MV, Moosa MK (2000). Biogeography: A marine Wallace's line? Nature, 406:692-693. 


Author Abstract: As most coral reef organisms with a pelagic larval phase are presumed to be readily dispersed between distant populations, sea-surface current patterns should be crucial for predicting ecological and genetic connections among threatened reef populations. Here we investigate this idea by examining variations in the genetic structuring of populations of the mantis shrimp Haptosquilla pulchellataken from 11 reef systems in Indonesia, in which a series of 36 protected areas are presumed to be connected by strong ocean currents. Our results reveal instead that there is a strong regional genetic differentiation that mirrors the separation of ocean basins during the Pleistocene low-sea-level stands, indicating that ecological connections are rare across distances as short as 300–400 km and that biogeographic history also influences contemporary connectivity between reef ecosystems.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: The paper we'll review this week addresses a concept known on on land in Australasia called Wallace's line, which among other things, is the biogeographical boundary that separates marsupial and mammalian faunas (with the exception of some bat species) in Australia and parts of Melanesia with Indonesia and the rest of Asia (see figure below from Wikipedia that shows Wallace's line terrestrially, which is demarcated by deep ocean "passes" between islands from Indonesia to Australia).

 

Wallace's line is named after the esteemed 19th century naturalist (and contemporary of Charles Darwin), Alfred Russel Wallace. The "line" varies in distance between land masses but at its shortest (between Bali and Lombok islands in Indonesia) it is only 35 km wide, yet many plants and animals (including birds) fairly consistently show abrupt distribution changes according to the "line." The figure above also shows a few other "lines" that were noted, depending on geomorphology or specific other organism groups, but the main observation is that at some point in the vicinity of central-east Indonesia, the plants and animals of Asia are no longer found, while the plants and animals of Australia and New Guinea (and a few other Indonesian islands) replace them.


The purpose of this week's paper is to show that even though many marine organisms are "presumed to be readily dispersed between distant populations," in fact many boundaries exist (often based on sea-surface and sub-surface oceanic current patterns) that keep populations from dispersing over seemingly invisible boundaries. The entire basis of the field of biogeography is to try and determine why such demarcations and boundaries exist. Sometimes the answer is simple, with deserts or mountains or vast oceans creating natural barriers to dispersal. But sometimes organisms are found in one location but not a nearby and seemingly suitable location, with the reasons less clear.


As the authors point out, "genetic connections among threatened reef populations" must be understood to effectively manage marine protected areas. The authors looked at mantis shrimp population genetics from 11 Indonesian reef systems (encompassing 36 marine protected areas, or MPAs) straddling Wallace's line. They found "a strong regional genetic differentiation that mirrors the separation of ocean basins during the Pleistocene low-sea-level stands, indicating that ecological connections are rare across distances as short as 300-400 km and that biogeographic history also influences contemporary connectivity between reef ecosystems."


What is interesting about the authors' results is that oceanographic drifters (instruments released into the ocean at a given spot and recovered later, allowing a map of known drift) released in the study area "traversed 1,500 km through the Celebes Sea and Makassar Strait in four weeks, indicating that planktonic larvae may travel great distances, yielding high connectivity between distant populations."


In other words, if the mantis shrimp larvae (in this example) could cross Wallace's line, why didn't they? The authors found "a broad genetic break perpendicular to Wallace's line" between marine populations (of a single mantis shrimp species) as close as 300 km despite a predicted larval dispersal distance of 600 km based on benthic reef crustacean larval periods.


The "answer" to the authors' "why" turns out to be "the greater isolation of ocean basins during Pleistocene low-sea-level stands" that preserves separate genetic populations to this day, despite "6,000 - 10,000 years of modern oceanographic conditions" that should have presumably "erased these historical boundaries." 


Because of the clear "break" in genetic connectivity of mantis shrimp populations in the study area over such a short geographic distance corresponding roughly to the historic area of Wallacea (the area between Wallace's line and the Australian continental shelf that consisted of islands during the Pleistocene low-sea-level stand), the authors "suggest the presence of a marine equivalent of Wallace's line."


The ramifications of the authors' finding are that biogeographers must consider historic oceanographic and biogeographic patterns and not just generalized ocean currents.
One might ask whether the authors' results were biased by their case organism, but they found "genetically homogenous populations" in various parts of Indonesia across distances from 10 - 400 km. The authors also found genetic connectivity at low levels (through the occurrence of rare haplotypes) between distant marine populations, even across Wallace's line. However, even terrestrial fauna and flora shows exceptions to Wallace's line and the important point is that most genetic connectivity is effectively stopped by the boundary the authors' examined.


It is also important to point out that the authors looked at a single species that did in fact distribute across the whole of the study area in central Indonesia. Yet, despite being a single species, the authors found divergence in the genetic code of examined specimens. That divergence is the basis for eventual speciation. Therefore, it is important also for biogeographers to consider genetic breaks that can reveal both cryptic species (distinct species that look morphologically the same through conservation of phenotypes but are distinct genetically and do not share genetic connections today) as well as distinct populations of the same species. This is important to know because when scientists and managers create MPAs they want to ensure that healthy populations within MPA boundaries can distribute to regions outside of MPAs.


The authors found that "the association of stomatopod populations with old ocean basins suggests that reef populations throughout Indonesia cannot simply be assumed to be interconnected units; marine reserves need to be designed that also take biogeography and historical oceanography into account." 


The study also shows that animal distribution patterns do not always conform to political boundaries. As a result, greater cooperation is needed to perform surveys between countries as well as to ensure that uniform data are collected within a given country's borders. Even in Indonesia, a country in the center of the coral triangle and a region of peak coral reef diversity for many organisms, significant variation can exist between islands less than 300 km apart.