[Taxacom] Fwd: Woodpeckers, primates, as well as the Wallace Line gauntlet

[Jason Mate]

OK, quick reply.

> Ecological or 'normal' dispersal, e.g. a weed arriving in a garden, is the simple movement observed every day and does not involve speciation. 'Chance dispersal' is a mode of evolution proposed to explain, for example, platyrrhines in America or lemurs in Madagascar. Dispersalist nihilism (a stretch of the term in the present case) has been brought to the table again. If I (or most biologists) have denied the existance of vicariance then please show it. Dispersal is one probable mechanism to account for the presence of a taxon in a place. What happens afterwards (evolution, radiation, extinction) is besides the point. The fact that the movement of living organisms has been observed should suffice to recognize that the mechanism is probable. Going back to the weed example, the probability of that weed arriving to your garden or any garden for that matter, is dependent on a variety of influences (the weed´s dispersal abilities, the wind patterns, the weather that year, the presence/absence of barriers, which birds visit it, etc) which taken together amount to chance. This ignores the probabilities of successfully establishing itself, which involve another set of variables. Each variable can stack against a candidate colonist leading to cases were certain taxa have next to no chance of colonising the area (mammals seem to be bad at dispersing to oceanic islands). When a large enough sample of taxa have ben thoroughly assesed (that means phylogenies included) then the relative contributions of each mechanism can be assesed.> You suggest that some truly horrendous dispersers 'have indeed made it'. Do you have any examples? Are you suggesting coco de mer is on the Seychelles because of chance dispersal? The comparison between the two "cocos" is a play of words. My comparison was between the successful dispersalist the coconut and the not so successful Coco d´mer. Alas I am not suggesting it got to the Seychelles by dispersal, relax ;). To look for examples I´d rather turn to more familiar grounds, i.e. beetles. Scarabs are pretty bad at dispersing compared to other beetles (human assited not included, more on that later), yet there are some big movements out there. Overland, my favourites are the N. Hemisphere montane glacial relicts Aphodius holdereri and A. bonvouloiri or the ring species Aphodius vittatus. New Caledonia has a mostly endemic scarab fauna and much of it has clear Gondwanan affinities (Dynastinae and Melolonthinae in particular as well as the Scarabaeinae and Aphodiinae), yet there are 3 species which appear to have arrived on their own "recently" (collected by Montreuzier in the 1850´s): Parastasia percheroni (Rutelinae), Dipelicus montrouzieri (Dynastinae) and Cyphopisthes sp (Cerathocanthidae). The last one is iffy because it is based on a damaged specimen caught in 1964 but I trust Paulian so I have included it. In addition there is a Dynastine genus (Metanastes (Dynastinae)) which probably arrived recently as well (1 endemic species in NC).
In Madagascar you have a similar pattern regarding the dung beetle fauna, but much richer. The Scarabaeinae is mostly endemic Gondwanan (Canthonines) and maybe ancient dispersal by the Onthophagine Helictopleurus (which has radiated extensively) and the Aphodiinae has some impressive raditions of Aphodine lemur equivalents (the Aulonocneminae) or of ancient dispersals (Aphodius (Neoemadiellus) sp, a small radiation of c. 8 species and counting). But there are also other taxa which are so similar morphologically to mainland groups that there is little doubt how they got there (still, one must check!): Aphodius (Mesontoplatys) parvulus, Aphodius (Paradidactylia) dionysii, Aphodius (Pharaphodius) mangoroensis and pseudignotus, Aphodius (Pleuraphodius) clementi. There are other species which occur both in Madagascar and the Eastern coast of Africa but which could have been accidentally introduced by humans before Europeans arrived.In addition, and going back to Hawaii, there are the two endemic Limnoxenus (Hydrophilidae) species (1 possibly extinct) from Hawaii, which are the only 2 endemic hydrophilids (though I know that there are some undescribed taxa in the genus awaiting description). Then there are the recorded captures of insects (all good fleirs) far from the mainland (Cicindela trifasciata in oil rigs in the Gulf and Cicindela campestris in the North Sea (I believe)) both well over 100Km from the nearest island or coastline. Or Danaus plexippus and Vanessa virginiensis that arrive to Europe every other year (OK, they are good fliers, but it is obvious that dispersal is important for some groups). Yet these species have not established themselves successfully in the Iberian Peninsula until recently, and only because of human induced changes (namely the introduction of the food plant).
> You write that: 'If the closest groups to your ingroup happen to come from a mainland source or from a distant source then you can´t say much other than what the data suggests'. Of course, but what exactly do the data suggest? For example, seamounts in the Musicians group (NE of Hawaii) were recently discovered to have flat tops,  i.e. they were formerly above sea-level and have been planed flat to sea-level by wave erosion before eventually subsiding below sea-level. At one stage they would have maintained a terrestrial biota.I would retort, and how did the taxa arrive to those recently discovered flat-topped mounts? In the end to support your particular hypothesis you are forced to resort to the same scenario-building excercise that you criticized from Kenneth.> Finally, you write: 'You could argue for disappeared islands as the sources or stepping stones but this is an ad hoc argument and much more complicated than allowing for chance'. It's not ad hoc because there is excellent geological evidence for former islands. It is ad hoc from the point of view that you can´t demonstrate that the taxa where there in the first place. The submerged islands are there, that is a fact, but we can´t tell what lived there. The possibility is there of course, that extinction and erosion has erased a trail that supports a particular mechanism. But others will suggest, equally convincingly (since the phylogeographic pattern supports their assertion) that dispersal is the most likely mechanism.


I am not sure even what you mean by dispersal anymore since you brought up ecological dispersal. Is there a distance threshold that you have in mind? Lets face it, the probability of dispersal for any particular group will be inversely proportional to the distance between the source and the sink, the slope dependent on all the variables intrinsic and extrinsic to the taxon. Which brings me back to scarabs. Aphodines are particularly poor at natural dispersal but they seem to be perfectly suited to human-mediated transportation. The list is long and keeps growing every year so it is interesting how the ecology of the group preadapted it to human transportation methods.

Best

Jason