Linking heart size and feeding habits in wild fish: A possible mechanism for ecosystem stability

August 1, 2018

Fish heart ventricle and walleye

The stability and persistence of species assemblages that exist in ecosystems rely in part on balanced population densities, in other words, the absence of any one species occurring at very high or very low numbers. Ecosystem stability risks disruption in either of these scenarios due to extinctions, whether it be on a local or larger scale.

The relationships and interactions that comprise food webs are key components of ecosystems. Organisms that occupy higher levels of a food web can contribute to stability because they can adapt to changing conditions in their environment by consuming different types of organisms. For example, a predatory fish such as northern pike consumes waterfowl as well as various fish and invertebrates, whereas an organism low in the food web may only eat one or two different types of food, as in the case of daphnia (aka common water flea), which are restricted to eating mostly algae and bacteria. The flexible feeding habits demonstrated by organisms higher in the food web such as northern pike, can help prevent extremes in population densities of their prey, thereby decreasing extinction rates within an ecosystem. Since human impacts on ecosystems tends to be biased towards organisms at high levels in food webs, such as predators, it is imperative to better understand the ecosystem stabilizing effect that these organisms provide.

Edmunds et al. (2018) examined the physiological mechanisms that may be contributing to the more developed foraging ability and flexibility observed in organisms as you move up a food web. Flexible foraging could be due to higher mobility, allowing an organism to explore and exploit food available in different habitats. Mobility in turn reflects efficiency of the circulatory system which must deliver oxygen, energy, and nutrients to muscles and the nervous system to support movement. In fish, the capacity of the circulatory system can be assessed by looking at the size of the heart ventricle which powers blood pumping. Edmunds et al. (2018) studied whether a relationship exists between heart ventricle size, food web position, and foraging flexibility between littoral and pelagic lake habitats, by examining these factors in 16 different species of freshwater fish captured from Lake Huron, Ontario, Canada.

They found that, across species, heart ventricle size did increase with food web position and use of the pelagic habitat. Foraging flexibility between lake habitats was not associated with heart ventricle size. The authors suggest that relatively large heart size could benefit fish higher in the food web by allowing them the mobility needed to capture large prey and to exploit the pelagic area of lakes. The lack of an association between foraging flexibility and heart size observed in this study could point to the importance of behavioural flexibility in influencing ecosystem stability, because previous work on the same sample of fish showed that foraging flexibility was associated with larger brains (Edmunds et al. 2016).

Read the full paper here (journal subscription required):
http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2017-0412#.W2IRJfZFw5h

References:
Edmunds NB, McCann KS, Laberge F (2018) Relative heart size and fish foraging ecology in a lake food web. Canadian Journal of Fisheries and Aquatic Sciences.

Edmunds NB, Laberge F, McCann KS (2016) A role for brain size and cognition in food webs. Ecology Letters. 19(8):948-955.