A study on killifish in Trinidad has shown that the effects of predators on prey reaches far beyond the simple fact of just killing them.
The ecological impacts of predation have long been established but very few studies have looked at the evolutionary consequences.
Using populations of the Trinidadian killifish Rivulus hartii Matthew Walsh and David Reznick from the University of California showed that not only does predation result in an increase in mortality and direct evolutionary changes meaning that there are prey that are younger at maturity with more offspring.
The study also found indirect changes, which mean that the decline in prey numbers results in an increase in food availability and therefore a skew to fish both smaller and younger at maturity and producing larger numbers of young.
Living test tubesWalsh and Reznick looked at populations of killifish in ~living test tubes - areas of river found above and below waterfalls and as such varied in the levels of natural predators.
Populations above the waterfall had no predators- this species is one which has high dispersal capabilities and can therefore be found in areas where they are the only species present- whilst those below had large numbers of predators.
In addition to this natural experiment, the team also bred the grandchildren of these killifish in the lab simulating the natural changes in food availability to check that the changes observed were a result of genetic change rather than just environmental.
The study found that killifish from high predation areas breed at a younger age and produce 25% more offspring than those from areas with no predation. Fish with high levels of predation bred up to 17 days earlier than those from areas with no predation, resulting in a 15% reduction in the time to reproduce.
In turn the offspring were significantly smaller as eggs and hatchlings. Where food was limited in the lab experiment, the fish with high predation produced larger eggs and hatchlings whereas the non-predator fish produced large eggs irrespectively. These changes show that these fish have evolved to be much better adapted to converting increased food levels into babies.
Increased predationThe fact that increased predation can have indirect implications on evolution has far reaching consequences on ecosystems. Removing predators will not only affect the ecosystem but also cause a spectrum of evolutionary changes in the prey.
This may mean that reintroductions of conserved predator species may have further reaching effects than first thought and also explain why fish populations that have been overfished do not recover when quotas are dropped.
"Since predator-induced indirect increases in resource availability are common in both terrestrial and aquatic ecosystems, the evolutionary consequences of these interactions are potentially a very important component of evolutionary change in nature.
Moreover, biologists have observed evolutionary change occurring on short ecological timescales in nature, on the order of a few years to decades, suggesting that such interactions are contributing to overall ecosystem functioning and health." said Reznick.
For more information see: MR Walsh and DN. Reznick (2008) - Interactions between the direct and indirect effects of predators determine life history evolution in a killifish. PNAS January 15, 2008. vol. 105. no. 2 . 594-599