The morning I made a minor fool of myself in Dawn Chorus. Guest host Julie Waters posted a very nice warbler quiz. I misidentified one because of an assumption I made based on geographic location.
So I thought I'd talk a bit about biogeography, the study of the distribution of organisms. And I'll try and link that in with the size theme I introduced in my last Ancient World. So let's take a look at why organisms occur in some places and not others. We'll start with a bit of history and then jump around randomly talking about stuff until I get tired. Or maybe it'll be a carefully crafted treatise that will blow your socks off. You'll have to read to find out. Also to find out the identity of the wild beasties below.
The 18th and 19th Centuries were the 'golden age' of natural history. Much of the planet was accessible to European scientists for the first time and specimens streamed into the museums in London, Paris, and so on. It was rapidly becoming apparent that the plants and animals known 'at home' were a pretty small proportion of global biodiversity.
One of the men supplying the museums was Alfred Russell Wallace (1823-1913). Wallace should be of interest to kossacks for multiple reasons. Later in his life he was an advocate for many progressive causes including socialism and women's rights. From 1848 to 1862 he was a biological mercenary, roaming remote parts of the earth and collecting specimens. Wallace and his friend Henry Bates started out in the Amazon in 1848 working at first as a team and then separately. Wallace returned to England in 1852, losing almost all of his specimens in a shipboard fire. Undaunted he set out for southeast Asia two years later, the region of the world that would put him on the map (pun intended).
Wallace spent eight years, traveling the islands of southeast Asia, usually with just a local assistant. He collected specimens, mostly of insects and vertebrates, as he traveled and also took careful notes. His work resulted in the following.
1) On his return to England he wrote the book, 'The Malay Archipelago', one of the most popular books of popular science of the 19th century.
2) He independently came up with the idea of evolution by natural selection while suffering from malaria and mailed it off to Darwin with no idea Darwin had been working along similar lines for 20 years. Without Wallace, Darwin's caution may have meant that the Origin of Species would never have been published.
3) Most importantly for our diary tonight, Wallace invented the science of Zoogeography (back in the day Zoology and Botany were more separate disciplines than they are today and now we are much more likely to talk about Biogeography rather than Zoogeography and Phytogeography).
In 'The Malay Archipelago' Wallace spent quite a bit of time comparing the organisms found on the different islands. In particular he was quite struck by the differences between the more westerly Greater Sundas (Borneo, Sumatra, Java) which had a fauna similar to that of the Malay Peninsula on the mainland, and the more easterly islands which in many ways are more similar to New Guinea and northern Australia in their animal life. This was most striking in his comparison of the islands of Sulawesi and Borneo which are very close to one another but have very different animals. Wallace was able to draw a line that (imperfectly) divided the islands into two groups, one that had more affinity with mainland Asia and the other that had more affinity with Australia.
Note the pale blue ocean around Borneo and Sumatra compared to the darker blue further east. The color indicates ocean depth. The shallow seas in the west mean that those islands may have been attached to the mainland in the past, allowing easy movement of organisms among those areas. Further east the ocean is deeper and the islands more isolated.
The map also shows two other lines draw later by other scientists indicating some uncertainty about the best division. Sulawesi itself is a fascinating mixture of unique animals. It has a number of unique mammal species including several ungulates and primates. These would be derived from Asian ancestors but are now found nowhere else such as the Babirusa pigs pictured at the top. Below is a dwarf cuscus, a marsupial (Sulawesi being the furthest west that marsupials reach).
At the urging of Darwin and other colleagues Wallace went on to write a couple of additional books specifically on the distributions of animals. The first was a global summary which included extensive discussion of mechanisms that would influence distribution such as mountain ranges and land bridges. The second book focused on the peculiar faunas of islands.
Wallace's work divided the terrestrial world into a series of major geographical regions: the Australasian, the Oriental (now known as the Indomalayan or Indotropical), the Ethiopian (now known as the Afrotropical), the Neotropical, the Nearctic, and the Palearctic regions. A map showing the regions can below. Antarctica and associated islands is now considered an additional zone as are the oceanic islands of the Pacific. The Nearctic (green) and the Paleartic (red) are often considered a single region the Holarctic, reflecting the similarity of these two regions, particularly in the north.
Wallace's work in southeast Asia had determined the boundary between the Australasian (orange) and Indomalayan (tan) regions, now known as Wallace's line.
Biogeography represents an interesting intersection of ecology and evolutionary history. If we were to compare a rainforest in the Amazon to one in Borneo we would note a highly similar climate. We would also note similar ecotypes of plants and animals. For example, large numbers of broad leaf evergreen trees with simple leaves and smooth bark and butressed roots. The forests would look very similar. The treefrogs that a herpetologist might encounter walking the trails at night would be from different families. The large billed birds would be hornbills in Borneo and toucans in the amazon. And so on.
Biogeographical explanations for patterns of distribution became very complicated. Ancient land bridges between continents had to be proposed to explain various oddities of distribution. Then, in the early 20th century, the radical idea of plate tectonics and continental drift was proposed. I'm not going to delve into the details in this diary but it makes understanding the distribution of plants and animals a lot more straightforward.
Modern biogeography is a lot more than simply describing patterns of distribution - it often involves testing a hypothesis about how geography has influenced distribution by mapping evolutionary history (a phylogeny) onto the distribution of the species in the group.
Here's a relatively straightforward example showing the evolutionary relationships among different species of Monarchs (an old world group of songbirds that are something like flycatchers) that have colonized islands in the western Pacific.
The lines show not only the pattern of evolution (who is related to whom) but also the order in which the birds moved through the islands and where each set of colonists of a few island group came from.
On a much larger scale we can relate the distribution of whole groups of animals to plate tectonics. Here are the distributions of some different groups of freshwater fishes.
Note that the earliest branching group (the Osteoglossidae) is found in South America, Africa, and Australia. These areas were all once part of the great southern continent Gondwanaland. The more recently evolved groups such as the Characins and Cichlids are found in South America and Africa which remained joined after Australia split off.
These sorts of patterns can occur even in the absence of splitting continents or colonization of islands. This maps shows areas where rainforests persisted during the last ice age (green patches). The colored lines show the ranges of several related species of small toucan.
Here's another, more detailed example from the same region looking at fishes again.
In this case the figure shows how two geologic events influenced the evolution of two separate groups of knifefish (the red and black phylogenies (evolutionary trees) that overlay each other. The Michicola arch split both groups in two about 30 million years ago. The formation of the eastern Cordillera of the northern Andes caused a second series of splits. The Amazon region, which has the most species, sits in between these two geographic features and has a mixture of species that evolved in different areas.
Well so what? The species that live in a particular area are determined by geographic history and the ability (or lack thereof) of the species to disperse to new habitats.
In the Dawn Chorus I misidentified a Townsend's Warbler (a western species) as a Blackburnian Warbler (an eastern species) because I knew that the photo had been taken in New England. However migratory birds (as opposed to the tropical forest toucans) move great distances and sometimes end up in the wrong place. However such events probably result in breeding fairly rarely (I should note that the evidence does not support these two warblers as being close relatives). For example, molecular genetic evidence indicates that the Wandering Albatross is composed of five different distinct species. Each species has a specific island or set of islands on which it breeds and thus they evolve independently although members of the different species co-occur out at sea and may be impossible to tell apart.
The darters are a group of small fishes mostly found in the eastern drainages of the Mississippi river system and tributaries thereof (e.g. the Ohio). These guys are the warblers of the fish world. Males often have bright colors. And there are a lot of them - over 200 species, all found in eastern and central North America.
Why are there so many? Well they are small, meaning that they can't disperse very quickly. Also many (not all) are specialists in living in clean, rocky, small streams. These kinds of streams occur at the headwaters but not further downstream. So each little headwater stream is fairly isolated. So many species form, most of which have very small ranges. The situation is similar with salamanders in mountainous regions. The cool moist habitats that they favour are often isolated which leads to the formation of new species.
So complex habitats with small species that don't move around much lead to lots of species formation. Mountains, rivers, islands - all of these lead to lots of endemism (species that are not found elsewhere).
This has important conservation implications. Biodiversity hotspots are areas of high conservation priority, largely because of biogeography. The concept of a hotspot was created to identify areas where conservation dollars would get the most bang for their buck. There are three criteria.
1) High diversity = many species
2) High endemism = high proportion of species not found elsewhere
3) High threat = large amount of habitat gone
Note that if you look at the list the Amazon is not a hot spot. It doesn't have a high enough endemism rate and it is still too intact.
The hotspots that fall partly within the US are Polynesia-Micronesia (Hawaii and various US Pacific territories), Caribbean Islands (south Florida, Puerto Rico, US Virgin Islands), Madrean Pine Oak Woodlands (mountains of SE Arizona and SW New Mexico), and the California Floristic Province. These are typical of the hot spots being mountains, islands, or areas of unique climate (which are also islands of a sort).
These areas produce lots of endemic species. Most of these are small and live in restricted habitats. Big species need big ranges to have large enough populations to persist. The biodiversity hot spot model has been criticized for focus only on terrestrial habitats (oceans are not included) and for focusing on vertebrates and plants (data are lacking for most other groups). But for these groups the message is that islands and mountains and unique climate regions are the places to try and preserve biodiversity. Unfortunately these are also the regions most threatened by climate change. The specialized species of tropical mountains living in narrow climatic ranges face a grim future. Climate change is going to wipe out the ranges of many species. It is going to split up the ranges of other species and eventually generate new forms, as it has in the past. From an evolutionary stand point we are living in interesting times - and that is not a good thing for us - however it may end up for those that come later.
Only one hot spot is named for a person. Wallacea is the region that falls between Wallace's line and Lydekker's line on the map of the Malay Archipelago above.