Are my enemies my future friends? They could be for this tiny fly

A new exper­i­ment­al study, pub­lished in Evol­u­tion Let­ters, reveals that the loss of pred­at­ors from com­munit­ies can make it more dif­fi­cult for prey spe­cies to adapt to uncer­tain future envir­on­ments. Lead author Dr Matt Bar­bour tells us more.

We are in the midst of the Earth’s sixth mass extinc­tion. As an eco­lo­gist, I find this troub­ling because we know that dif­fer­ent spe­cies play dif­fer­ent roles in main­tain­ing healthy eco­sys­tems — eco­sys­tems that not only inspire us with their nat­ur­al beauty, but that we also crit­ic­ally depend upon for the air we breathe, the water we drink, and the food we eat. At a cer­tain point, the extinc­tion of spe­cies and loc­al pop­u­la­tions can cause dra­mat­ic changes in eco­sys­tems — often degrad­ing their nat­ur­al beauty and func­tion and, as a res­ult, our own phys­ic­al and men­tal health.

I’m also an evol­u­tion­ary bio­lo­gist, and I became curi­ous to know how the extinc­tion of spe­cies from an eco­sys­tem would affect the abil­ity of the remain­ing spe­cies to evolve and adapt to their future envir­on­ment. On one hand, the extinc­tion of spe­cies may make it easi­er for the remain­ing spe­cies to evolve, which could buf­fer the neg­at­ive effects of extinc­tion on the eco­sys­tem. On the oth­er hand, being part of a diverse eco­sys­tem may provide essen­tial “fuel” for spe­cies to adapt and per­sist in an uncer­tain and chan­ging world. In this scen­ario, the extinc­tion of spe­cies would make it more dif­fi­cult for the remain­ing spe­cies to evolve and adapt, put­ting eco­sys­tems at even great­er risk.

As a first step toward solv­ing this puzzle, myself and oth­ers con­duc­ted a field exper­i­ment that sim­u­lated the extinc­tion of a group of wasps who para­sit­ize a tiny fly. This mini­ature eco­sys­tem of the fly and wasps is fas­cin­at­ing, and I want to take a minute to explain it before I describe the experiment.

The Gall Fly Ecosystem

Fly babies are called lar­vae, and when lar­vae of this fly feed on a plant, they cause the plant to grow these tooth-shaped struc­tures called galls. The lar­vae live inside these galls, where they are pro­tec­ted from being attacked by pred­at­ors, such as ants or spiders. How­ever, a spe­cial group of wasps called para­sit­oids have figured out ways to cir­cum­vent the pro­tect­ive shield of the gall.

gall-midge-Iteomyia-biology
Bio­logy of the gall fly Iteo­myia salicis­ver­ruca. On the left, a female fly is lay­ing an egg inside the leaf of the wil­low Salix hook­eri­ana. After hatch­ing, the fly’s lar­vae feed on the plant, which causes the plant to devel­op a gall (cen­ter photo). Each fly larva lives in a cham­ber inside the gall (right photo).

For example, one type of wasp has figured out how to find the fly’s eggs before they hatch and form the gall. This wasp is called an egg para­sit­oid, because it lays its egg inside the fly’s egg (!!!), and the wasp’s egg waits until the fly larva is older to hatch and devour it from the inside out (ever see the movie Ali­en? … that’s basic­ally what happens).

parasitoid-platygaster-ovipositing
The egg para­sit­oid finds its host. The egg para­sit­oid Platy­gaster sp. is able to find fly eggs before they hatch and devel­op into a gall. The para­sit­oid in this photo is lay­ing its egg inside the egg of a fly.

Anoth­er group of wasps attack the fly at the lar­val stage and are called lar­val para­sit­oids. They find the gall after it has already developed and use a long needle-like part of their body to pierce through the gall, stun the larva inside, and lay their egg … which even­tu­ally hatches and devours the larva. Often, lar­val para­sit­oids will para­sit­ize a fly larva that has already been para­sit­ized by an egg para­sit­oid (!!!), in which case their young eat both.

parasitoids-fly-larva-compilation
The gall fly eco­sys­tem. Gall fly larva (cen­ter) are attacked by an egg para­sit­oid (left) and three dif­fer­ent types of lar­val para­sit­oids (right). Note the long needle-like body part (its ovi­pos­it­or) of the lar­val para­sit­oid in the middle (Torymus sp.). The ovi­pos­it­or of the oth­er para­sit­oids is hid­den under­neath their abdomen.

The Experiment & Our Results

In our exper­i­ment, we used fine-mesh bags to pre­vent the lar­val para­sit­oids from attack­ing the fly larva. We could not exclude the para­sit­oid that attacks the fly’s egg, because it’s vir­tu­ally impossible for human eyes to find these tiny eggs on the plant. As a con­trol, we allowed both kinds of para­sit­oids to attack oth­er galls. We flagged these con­trol galls with tape so we could find them again later.

parasitoid-exclusion-compilation
Manip­u­lat­ing the gall fly eco­sys­tem. We used fine-mesh bags to pre­vent lar­val para­sit­oids from attack­ing the fly (left and top right). We marked oth­er galls that could be attacked by both types of para­sit­oids with a piece of tape (lower right).

At the end of the sum­mer, we col­lec­ted these galls, brought them back to the lab, and dis­sec­ted them to fig­ure out which fly larva sur­vived and which ones were killed by a para­sit­oid. We also took data on each gall’s traits, such as the size of the gall, the num­ber of lar­vae with­in a gall, and the fly’s pref­er­ence to cre­ate galls on par­tic­u­lar plants. We meas­ured these traits because we knew from pre­vi­ous work that each of these traits influ­enced the like­li­hood of a fly sur­viv­ing from para­sit­oid attack.

We found that a unique com­bin­a­tion of traits helped fly larva sur­vive when the lar­val para­sit­oids were removed. Fly lar­vae were more likely to sur­vive if they were by them­selves inside a large gall and if there wer­en’t many neigh­bor­ing galls on the same plant. This sug­gests that, if the lar­val para­sit­oids went extinct, the fly would evolve to have this unique com­bin­a­tion of traits — a unique com­bin­a­tion which helps them sur­vive. In oth­er words, there is only one optim­al solu­tion for a fly’s sur­viv­al when lar­val para­sit­oids are gone.

In con­trast, when both types of para­sit­oids were present, we found that dif­fer­ent com­bin­a­tions of traits equally helped flies sur­vive. For example, it didn’t affect its sur­viv­al if the fly larva was by itself or with many lar­vae inside the same gall. Also, lar­vae were more likely to sur­vive if they had either a few or a lot neigh­bors (but an inter­me­di­ate amount of neigh­bors was bad news). In oth­er words, when both types of para­sit­oids were present in the fly’s eco­sys­tem, the fly had more poten­tial solu­tions (i.e. trait com­bin­a­tions) for survival.

complex_simple_foodwebs_revised2_promotion
Sum­mary of the experiment’s res­ults. You can view the full paper here: https://doi.org/10.1002/evl3.170.

This diversity of poten­tial solu­tions for sur­viv­al pre­serves genet­ic vari­ab­il­ity in the traits of the fly’s galls. And since we know that genet­ic vari­ation is the essen­tial ingredi­ent that enables a spe­cies to evolve and adapt, our find­ings sug­gest that the extinc­tion of the fly’s para­sit­oids may make it more dif­fi­cult for it to adapt to a chan­ging environment.

Although we only stud­ied a spe­cif­ic mini­ature eco­sys­tem, we know that vir­tu­ally all spe­cies on Earth have one or more “nat­ur­al enemies”. Being attacked by a diverse group of nat­ur­al enemies may pre­serve a spe­cies’ genet­ic diversity, which (indir­ectly) helps it adapt to an uncer­tain future. This also means that the extinc­tion of nat­ur­al enemies could make it more dif­fi­cult for the remain­ing spe­cies to adapt and per­sist. If this is true, this would put many eco­sys­tems at even great­er risk than we cur­rently realize.

 

Dr Matt Bar­bour is a Postdoc­tor­al Research Asso­ci­ate at the Uni­ver­sity of Zurich. The ori­gin­al study is freely avail­able to read and down­load from Evol­u­tion Let­ters.