For those that have been paying attention, you will be aware of our recent post on copepod evolution in high food and low food regimes. Despite the ongoing lockdowns, Alex Blake returned to the lab to follow individuals in a third generation of ‘common gardens’ throughout their whole life history. He measured growth, survival, probability of egg production, clutch size and egg size across the entire lifespan of individuals and used that data to compile population models.

Alongside his supervisor from Oxford University, Professor Tim Coulson, Alex wanted to know how food supply would affect population measures such as population growth rates, population size, age structure and size structure within populations. Alex and Tim compiled population models called Integral Projection Models; a mathematical modelling technique that essentially combines regression models of how traits change across the lifespan of individuals.

As with Alex’s previous work they did find differences between the food regimes. Copepods cultured in high food environments grew slightly faster to a smaller adult size, reached peak egg production younger and produced smaller eggs compared to copepods from low food environments. The high food copepods were also longer lived and had shorter generation times.

But Alex and Tim found these discrepancies didn’t translate to differences at the population level. Population growth rates, and age and size distributions within populations are projected to be similar at the different food supply regimes.

This is potentially good news for marine food webs; ocean productivity is expected to decline with climate change but Alex And Tim’s work suggests that even under lower food supply copepods will be able to evolve to deal with these harsher environments. But there could be costs. Changes in body size and reproductive outputs may have knock on effects for copepod consumers. Fish fry in particular, have been shown to be meticulous about the size of copepods they eat.

Alex and Tim emphasise that there is more work to be done. They were unable to untangle the effects of food supply from the effects of density; crowded environments could impact copepod life histories in a number of ways separate to the effects of food. They also suggest future work looks at whether life history evolution to food regimes is actually adaptive and not simply a result of non-adaptive forces such as drift. Reciprocal transplant experiments are the gold standard test for this question and Alex and Tim are keen to see them implemented.

This research is published in the journal Oikos.