Growth is a critical aspect of life for all organisms, and understanding what can and cannot affect it allows us to predict what effect climate change may have on organisms like these zebrafish (Image Credit: Lynn Ketchum, CC BY-SA 2.0).

Warming increases the cost of growth in a model vertebrate (2019) Barneche et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13348

The Crux

In ecology, how organisms grow is relevant across all levels of life. Growing faster than others can be selected for as an evolutionary advantage, if being bigger earlier means that you have a competitive advantage over other members of your species.

Because growth is so critical to life, it is important to understand what may affect the ability of an organism to grow. The only way an organism can grow is by converting energy it acquires from food to its own body mass, but outside influences, like temperature, can affect how efficient an organism is at this energy conversion. The authors of today’s paper wanted to investigate if this efficiency and the cost of growth itself changed across a range of projected temperatures.

What They Did

The authors designed an experimental approach based on mathematical theory using juvenile zebrafish (Danio rerio) to understand the the cost of growth, which is defined as the use of chemical energy from the resting metabolism to grow, the more energy invested in growth the higher the cost. They measured oxygen consumption and growth rates weekly for individuals kept at different temperatures. Afterwards, they then measured energetic efficiency (see our Did You Know section below) in these same individuals at the end of the experiment.

The authors tested five hypotheses: 1) The cost of growth does not change as the fish grows to its final size; 2) The cost of growth depends on the temperature; 3) Fish will, on average, be smaller when raised at higher temperatures; 4) Energy efficiency is temperature dependent; and 5) Energy efficiency is mass independent.

Did You Know: P/O Ratios

Many different disciplines within ecology use resting metabolic rate to understand what can affect an organism’s energy use. In order to do that, researchers will measure how much energy (in the form of ATP) is produced by mitochondria per molecule of oxygen used by the mitochondria, known as P/O ratios. Think of it like a car using fuel to move, the more efficient the mitochondria (car) is the more energy (movement) it can produce with a given amount of oxygen (fuel).

What They Found

The authors found that the cost of growth did, in fact, change as the fish grew, with the highest cost of growth occurring during the juvenile phase of life. Not surprisingly, the cost of growth was temperature dependent, with the highest temperature costing twice as much as the lowest temperature. The size of fish did not vary across the temperature treatments, but there may have been a reason for that (see Problems).

In contrast with their predictions, energy efficiency was not affected by temperature, but instead was most dependent on the mass of the individual fish. This helps to explain why the cost of growth changed as the fish grew, because larger fish produced more energy using less oxygen than the smaller fish (they were more efficient).

Various projections for the rise in average temperature due to global warming over the next century. (Image credit: Wikicommons, CC BY-SA 3.0).

Problems

The fish used in this study are native to tropical regions, and as such all fish were held at relatively warm temperatures prior to them being split up into the four different temperature groups. The authors state that by first holding the fish before moving them to colder waters (in the lowest temperature treatment) they may have precluded any ability of the fish to tolerate the lower temperatures, affecting how the fish could grow and possibly being the reason why the fish from all of the treatments had such variation in size and didn’t vary across temperature treatments.

So What?

This showed that the cost of growth is a variable trait, and not a constant value as it was previously thought to be. This variation in the cost of growth should be the focus of many future studies in order understand why this variation happens in a biological sense, as well as the ecological variables and different life‐history strategies that can cause it to vary.

The authors emphasize that there are higher costs to growing in warmer environments. This means that, all else being equal, higher costs of growth lead to less efficient energy transfer between trophic levels of a food web. These results are yet another example of how rising temperatures due to climate change can alter both individual-level growth and ecosystem dynamics as a whole.