The common frog is one of the amphibians with the
highest distribution in the Iberian Peninsula. It reproduces preferably in
permanent areas of water where it comes into contact with the red swamp
crayfish, which preys on its larvae. Research carried out by the Spaniard
Germán Orizaola from the University of Uppsala (Sweden) confirms that the
larvae of these frogs have developed a defensive response to the invasive
species. They also have deeper tails and larger bodies if they co-exist with
the crayfish.
Numerous invasive organisms are currently spreading
outside of their natural habitat at an unprecedented rate, mainly due to human
actions. As a result of this spread, various ecosystems have been altered and
this fact seriously affects the balance of biodiversity.
More specifically, the spread of exotic predators is
considered to be one of the major causes of population decline and species
extinction on a global scale.
This is the case of the red swamp crayfish (Procambarus
clarkii), a species native to the south of the USA and north Mexico,
deliberately introduced by humans in several areas on all continents, including
the Iberian Peninsula. "The first records of this invasive species were
located in Extremadura and Andalusia in the seventies. From this date on, they
have spread due to the number of wetland areas, to practically the whole of the
Peninsula. P. clarkii is an active predator of numerous aquatic
organisms, including amphibian larvae," the Spanish researcher Germán
Orizaola from the University of Uppsala (Sweden) said, having published a study
in the journal Ecology on the interaction between the two species.
The aim of his study was to examine whether the
period of co-existence with the red swamp crayfish could influence in the type
of defensive response developed by the larvae of the common frog (Pelophylax perezi).
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An adult and larva of the Common Frog, Pelophylax perezi Photo credit: David Perez.
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"The study involved collecting recently laid P.
perezi frogspawn in five locations in the south of Portugal. In two of
them P. perezi has been co-existing with the red swamp crayfish for
more than 30 years (populations of long-term coexistence), in another they have
been sharing a habitat for 20 years (population of short-term coexistence),
while the last two populations are found in an area which has not yet been colonized
by the exotic predator (populations without coexistence)," adds the
scientist.
The study consisted of an experiment developed in
the Centre for Environmental Biology in Grândola (Portugal), where the larvae
of the five populations were raised from tadpoles to metamorphosis in the
presence or absence of the red swamp crayfish.
"We held them in aquariums with a compartment
into which a red swamp crayfish was introduced, or it was left predator-less.
We also fed the predators in these compartments with larvae so that they produced
chemical signals indicative of predation which could be detected by the larvae
in the experiment," Orizaola explains. Using this design of experiment,
they analyzed the response, both of the behavior and the morphology of the P.
perezi frogspawn in the presence and absence of the predator.
According to the scientist, the results reveal that
the populations of these amphibians with a tradition of coexisting with the
predator for 30 years (between 10 and 15 generations of frogs) present a
pattern of activity that is totally different from the populations with less
coexistence or no coexistence with the red swamp crayfish.
"A long coexistence with the predator generated
extraordinarily reduced patterns of activity, even in the absence of the
predator. This would help the larvae to go unnoticed, which would help to
increase its chances of survival," Orizaola points out. On the other hand,
the other populations were five times more active.
Bigger tails and bodies In terms of their
morphology, the larvae of populations which have coexisted with the red swamp
crayfish had deeper tails and bigger body volumes than the rest.
The researcher emphasizes that "these two
responses would allow them on the one hand to direct the attacks of the
predator to less vulnerable areas of their body like the tail fin and, at the
same time, maintain a larger digestive area to counteract the potential
negative effects of lowered activity."
This indicates that the presence of exotic predators
can induce a rapid process of evolutionary change in invaded ecosystems. Until
recently, it was considered that such evolutionary processes would only happen
through slow accumulation of changes over exceptionally long periods of time.
Lastly, not all amphibians behave in the same way.
Recent research reveals that various endemic species to the Iberian Peninsula,
such as the Iberian Parsley Frog (Pelodytes ibericus), are incapable of
responding to the presence of the red swamp crayfish, which increases its risk
of extinction.
"Better knowledge of the magnitude and time
scale of the evolutionary changes is the key to properly understanding the
biological processes and for correct development of efficient conservation
measures," concludes Orizaola.
Citation
Nunes
AL, Orizaola G, Laurila A, Rebelo R. Rapid evolution of constitutive
and inducible defences against an invasive predator. Ecology,
2014, 95 (5) DOI: 10.180/13-1380.1