Switching up your pollinator can have unexpected side effects

A study pub­lished in Evol­u­tion Let­ters shows that hum­ming­bird pol­lin­a­tion is a sur­pris­ingly rare trait across the per­en­ni­al wild­flower genus Pen­ste­mon, des­pite a large num­ber of Pen­ste­mon lin­eages hav­ing switched dur­ing their evol­u­tion­ary his­tory to attract hum­ming­bird pol­lin­at­ors instead of bees. Here, the study’s author Dr Car­o­lyn Wes­sing­er tells us more about her findings.

Many spe­cies of flower­ing plants have evolved flowers that attract a cer­tain type of anim­al pol­lin­at­or. For example, spe­cies that attract moth pol­lin­at­ors have white flowers that emit a strong and sweet flor­al scent, spe­cies that attract bees have blue/purple or yel­low open flowers, and spe­cies that attract hum­ming­birds have bright red tubu­lar flowers with lots of nectar.

Over evol­u­tion­ary time – thou­sands to mil­lions of years – a spe­cies might aban­don its cur­rent pol­lin­at­or and begin to attract a new pol­lin­at­or, if it is prof­it­able to do so. (Per­haps the old pol­lin­at­or has become a less abund­ant or reli­able pol­lin­at­or). This should be bene­fi­cial in the short term, but it turns out there can be unex­pec­ted long-term con­sequences of switch­ing pollinators!

In our new paper pub­lished in Evol­u­tion Let­ters, we study the effects of switch­ing pol­lin­at­ors on plant spe­cies per­sist­ence and diver­si­fic­a­tion using the per­en­ni­al wild­flower genus Pen­ste­mon. Most of the nearly 300 Pen­ste­mon spe­cies in North Amer­ica are pol­lin­ated by bees or wasps – cer­tainly has been a suc­cess­ful part­ner­ship in the long term. But roughly 15–20 lin­eages with­in Pen­ste­mon have switched to attract hum­ming­bird pol­lin­at­ors instead of bees. Most likely, these spe­cies live in places where hum­ming­birds are loc­ally abund­ant or effi­cient pol­lin­at­ors, mak­ing this a good short-term decision.

Because Pen­ste­mon has ditched bees for hum­ming­birds so many times, it is an ideal group of plants to exam­ine the long-term con­sequences of switch­ing to hum­ming­bird pol­lin­a­tion in North Amer­ica. In a sense, nature has provided us with a large num­ber of “rep­lic­ated exper­i­ments” for us to study.

In our study, we recon­struc­ted spe­cies rela­tion­ships for a large branch of the Pen­ste­mon fam­ily tree, allow­ing us to trace the evol­u­tion­ary his­tory of trans­itions from bee to hum­ming­bird pol­lin­a­tion. Each ori­gin of hum­ming­bird pol­lin­a­tion usu­ally con­sists of just one or (rarely) two hum­ming­bird-pol­lin­ated spe­cies, nev­er lead­ing to a large group of related hum­ming­bird-pol­lin­ated spe­cies. This makes hum­ming­bird pol­lin­a­tion a sur­pris­ingly rare trait across the sampled Pen­ste­mon spe­cies, giv­en the large num­ber of times this pol­lin­a­tion switch has occurred (17 switches to hum­ming­bird pol­lin­a­tion in our sample!).

What explains this rar­ity of hum­ming­bird-pol­lin­ated spe­cies in Pen­ste­mon? Per­haps switches to hum­ming­birds occurred so recently that there just has­n’t been enough time for hum­ming­bird-pol­lin­ated spe­cies to become more com­mon. Anoth­er pos­sib­il­ity is that there is a side effect of the hum­ming­bird strategy that lim­its spe­cies diversification.

To exam­ine these pos­sib­il­it­ies, we modeled spe­ci­ation and extinc­tion rates of each pol­lin­a­tion strategies in Pen­ste­mon. We found that the net diver­si­fic­a­tion rate (spe­ci­ation minus extinc­tion) for hum­ming­bird-pol­lin­ated spe­cies is much lower, com­pared to the diver­si­fic­a­tion rate for bee-pol­lin­ated spe­cies. So, a low diver­si­fic­a­tion rate is appar­ently a side effect of hum­ming­bird pol­lin­a­tion in Pen­ste­mon.

Next we cal­cu­lated how the rel­at­ive num­bers of bee- vs. hum­ming­bird-pol­lin­ated spe­cies are expec­ted to change over time, giv­en the diver­si­fic­a­tion rate dif­fer­ence that we have detec­ted. We wanted to know: would this lower diver­si­fic­a­tion rate be enough to keep hum­ming­bird-pol­lin­ated spe­cies rare in the long term? The answer was yes: we expect hum­ming­bird-pol­lin­ated spe­cies to be rarer than bee-pol­lin­ated spe­cies in Pen­ste­mon over the long term. So, it seems that switch­ing to hum­ming­bird pol­lin­a­tion may be a great short-term strategy, but has an unex­pec­ted con­sequence of redu­cing diver­si­fic­a­tion rates, rel­at­ive to the bee pol­lin­a­tion strategy.

A major unanswered ques­tion is why the hum­ming­bird strategy leads to low diver­si­fic­a­tion rates. Our study was not equipped to answer this ques­tion, but there are sev­er­al pos­sib­il­it­ies. I’ll describe one of our favor­ites here:

A key fea­ture of spe­ci­ation is isol­a­tion. When a loc­al pop­u­la­tion is isol­ated from oth­er pop­u­la­tions, it may begin to diverge and, giv­en enough time, accu­mu­late enough genet­ic dif­fer­ences that it con­sti­tutes a sep­ar­ate spe­cies. Per­haps hum­ming­birds move pol­len great dis­tances, at least occa­sion­ally, in the course of their sea­son­al migra­tions. Such long-dis­tance pol­lin­a­tion could help main­tain genet­ic con­nec­tions between geo­graph­ic­ally dis­tant pop­u­la­tions, hinder­ing the spe­ci­ation pro­cess. That’s not to say that hum­ming­bird-pol­lin­ated spe­cies in Pen­ste­mon aren’t suc­cess­ful, since there are sev­er­al spe­cies with very large range sizes in North America.

There is a lot to learn in the future about wheth­er pol­lin­a­tion strategy in Pen­ste­mon affects the organ­iz­a­tion of genet­ic vari­ation with­in and between pop­u­la­tions. This is where we are mov­ing next with this research, so please stay tuned!

Dr Car­o­lyn Wes­sing­er is a postdoc­tor­al fel­low in the Hile­man Lab at the Uni­ver­sity of Kan­sas. The ori­gin­al art­icle is freely avail­able to read and down­load from Evol­u­tion Let­ters.