Wikipedia:Reference desk/Archives/Science/2018 July 23

= July 23 =

How much water is needed to block various species from reaching an island they would've thrived on if there was a landbridge?
Thus if you find them on the isle they were either introduced or came when there was less water.

How much is needed to allow an average population of at least 0.0000001 but much less than if there was a landbridge? (i.e. pedigree collapse or they can't find each other from too low of a mobility to population density ratio)

This is often prevailing wind and/or current dependent right? Wind and current going towards the island would lengthen this distance. Sagittarian Milky Way (talk) 21:05, 23 July 2018 (UTC)
 * Some birds can cover 1000's of km of ocean without landing. But there are other birds that don't like to go more than 10m from tree to tree. Over time humans have developed means to colonise land further away, so I suspect that now 20000 km of sea would not stop a human colonisation. But 100,000 km may be too much! Graeme Bartlett (talk) 00:07, 24 July 2018 (UTC)
 * This is a topic with connections to why Australia has marsupials (kangaroos, koalas, etc) and most of the rest of the world has placental mammals, and there is very little overlap. The standard explanation is the existence of the Wallace Line between Bali and Lombok in Indonesia. It's currently 35 kilometres (22 miles) wide and very deep, but would have been narrower during various ice ages. It has existed for a very long time. The Wallace Line article addresses the question fairly directly, stating "many birds do not cross even the shortest stretches of open ocean water". HiLo48 (talk) 00:19, 24 July 2018 (UTC)
 * Aren't birds and especially flying insects sometimes blown where they don't want to go by storms? Also I've heard that in Polynesia land birds aren't normally found more than 20 miles from land, or out of sight or something like that. Sagittarian Milky Way (talk) 02:00, 24 July 2018 (UTC)
 * It would be difficult to give a meaningful figure, because in addition to Island hopping there is thought to be evidence that some terrestrial species have (inadvertantly) spread from one landmass to another by rafting on floating conglomerations of vegetation – even trans-Atlantic dispersal by this mechanism is considered a likely explanation for the spread of some genera. Because such events will have been rare, and successful only by chance, it's difficult to put numerical constraints on the limits to their possible success. {The poster formerly known as 87.81.230.195} 2.219.34.253 (talk) 05:09, 24 July 2018 (UTC)
 * What about big animals like deer, elephants and bear? Wouldn't they not want to get on cause there's not enough food and freshwater and it might not be able to support their weight? And there must be other animals that wouldn't have their range extended by vegetation rafts, i.e. perigrine falcons maybe? Would they have an interest in making a long voyage on a raft where they can't hunt by slamming animals into the ground? Sagittarian Milky Way (talk) 19:08, 24 July 2018 (UTC)
 * Peregrine falcons can be blown off course and like any other bird - there are several records of them on Tenerife for instance as vagrants. Mikenorton (talk) 20:16, 24 July 2018 (UTC)


 * SMW, you are attributing deliberate forward planning to non-sapient creatures, in a scenario where even H. sapiens would not be able to forecast an outcome. Animal dispersion by rafting is purely accidental, with animals inadvertently stranded on such rafts (which can be quite large) very occasionally being lucky enough to survive long enough to come within reach of land again – only one "success" in a million years might be enough to trans-oceanically disperse a given species.
 * Keep in mind that many large animals can swim surprisingly long distances if they choose, or are forced, to: Elephants have been seen swimming in the Indian ocean miles from land (see also Elephantidae); Polar bears routinely swim tens of miles, (and other bears can probably swim quite well); while Scandinavian Elk (aka Moose) are now known to dive several meters in order to feed on lake-bottom vegetation and coastal seaweed. Even sloths have been observed swimming! {The poster formerly known as 87.81.239.195} 90.218.111.216 (talk) 09:54, 25 July 2018 (UTC)