Re: Evidence for bat predation on birds

[Michael Habib <mhabib5@jhmi.edu>]

> Googling around on stuff like "chiroptera, altitude" gleans 3000m 
> foraging for t. braziliensis... I was mainly wondering about maximum 
> observed reproductive altitude.

That's the figure I found as well, and seems to be what Dann Pigdon 
came up with.

> As usual the most interesting stuff is accidental-- e.g., it seems 
> species richness in birds and bats "declines smoothly" w/ increasing 
> altitude (in the Andes) whereas species richness in local mice does 
> not. Further, the mouse species are more 'discretely zonated' than the 
> volants. Encouraging and unsurprising to a functionalist like myself, 
> who assumes that the gradual change in flight medium density has 
> shaped the bird/bat species profile... in all the ways you might 
> theoretically expect. I hasten to add that as near as I can tell, the 
> authors listed below don't necessarily share my locomotively based 
> assumption. Hard to tell until I get the whole article.

I just took a very quick look through the paper; it's pretty 
interesting.  One thing I note is that the species richness decline 
isn't really that smooth in birds, though it is in bats.  Given the 
spike around 2500 m for bird diversity, and the high mid-range 
amplitude, it might be that changes in arboreal habitat are more at 
work than air density for the species richness change.  The evidence 
for density effects looks (at first glance) to be a bit better for bats 
than birds.  One bit of help would be to include some larger-bodied 
volant species; they should be less affected by density changes (though 
they would tend to be less affected by habitat gradients, as well).

> This is a little surprising if you just paste it into a black/white 
> cartoon world-- "Highland bat species occupy broader elevational 
> ranges than lowland bat
> species, but for both birds and mice, species at intermediate 
> elevations
> had the broadest amplitudes."

It is a bit counter-intuitive.  Maybe intermediate elevation bird 
species have wider habitat preferences, or maybe they are adapted to 
flight at both highlands and lowlands, and just happen to roost at 
intermediate elevations.  Hard to say, but just a couple of ideas to 
toss out there.  Thanks for posting the reference.

Cheers,

--Mike H.