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Building food webs, a question of trade-offs

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Building food webs, a question of trade-offs

Building food webs, a question of trade-offs: Complexity and structure of food webs in the Barents Sea.

 

A food web describes the diversity of species and their feeding relationships (a.k.a. trophic links, i.e. “who eats whom”). Food webs are essential parts of ecology as they summarize who is present in an ecosystem and how species interact with one another, here as consumer and prey. It may appear simple to list all species and their trophic interactions but the large diversity in nature makes it impossible to construct complete food webs. In practice, we group species together to build fictional “representative” species (a.k.a. trophospecies) that somewhat summarize the peculiarities of whom they represent. One may ask: “How do you elect these representatives?”, “Which individualities should they have?”, “Will the food web stay the same?” Answering those questions turns out to be complex when species are only partly similar to one another.

Aggregation of food webs

Fig. 1 Steps to build trophospecies in a food web: aggregation of species and of links

Copyrights © Pierre Olivier

 

To put it simply

One illustrative example: a room contains a mouse, a cat, an antelope, a hungry lion, a dog and a wolf. The cat and the lion on one hand, the dog and the wolf on the other hand, are to a certain extent related. But also keep in mind for later in this story that dogs do not like cats. Based on their relatedness (i.e. the taxonomy), we could group the cat and the lion; and the dog and the wolf. On the contrary, the mouse and the antelope don’t get along very well. Nonetheless, the later might share more than it would first appear. If the only food available is the grass growing in my backyard, the mouse and the antelope would turn out to be good friends, indeed both are herbivores. While the carnivores (i.e. the cat and the dog, despite their unapparent friendship; the lion and the wolf) might starve, start hunting together and turn their hunger towards a juicy piece of antelope. Based on their diet, the herbivores could be grouped together while carnivores would form a group for themselves. If you are like me, you are getting a little confused: “How do I elect the representatives then?”, “Should I select them based on their taxonomic relatedness or on their diet?”

An example of trophic interactions in the Barents Sea: on the left, the Polar bear (Ursus maritimus) – on the right, the Bearded seal (Erignathus barbatus)

One of the favorite polar bear's meal is the bearded seal.
  • Polar bears are qualified as “top predator” meaning they have no predator (except maybe themselves).
  • Although good swimmers, their best chances to catch food is on land.
  • Consequently, they hunt for food on a relatively short period of the year during the sea ice expansion and are able of fasting on a really long period.
  • Efficient hunters, they could be able to smell seal breathing holes up to 3 km away.
Bearded seal resting on ice
  • Bearded seals main predators are polar bears and killer whales but sometimes, cubs might be found in the diet of Walruses.
  • Its long whiskers gave it its name (it really looks like  it is wearing a moustache).
  • Against their predator, the polar, their best chance of survival is to dive under the ice. You will find them resting near a ice breathing hole from which they can easily escape.

Polar bear : Par Ansgar Walk (Personal work) [CC BY-SA 2.5], via Wikimedia Commons

Bearded seal : Par Alastair Rae (Personal work) [CC BY-SA 2.0], via Flickr

To answer those questions, we used the Barents Sea food web as a case study and grouped species based either on (i) their relatedness, (ii) their similarity in terms of interactors; or (iii) their position in the food web. We found that ad hoc choices made during the construction of food webs can significantly alter the representation of how species interact with each other. In this work, we show how different species can be grouped, how this can affect our understanding of food web structure and we provide simple guidance for constructing aggregated food webs while keeping their properties mostly unchanged.

The take home message

 

THM – How to build food web – Construction de réseaux trophiques
FR: Les questions et réponses apparaissent à la fois en anglais et en français.
EN: Questions and answers are both displayed in English and French

1.

FR : Que représente un réseau trophique ?
EN: What does a food web describe?
 
 
 

2.

FR: Sur quels critères pouvons-nous grouper des espèces ?
EN: On which criteria are species grouped together?
 
 
 

3.

FR : En combien d’étape se déroule l’agrégation de réseau trophique ?
EN: How many steps do we need to aggregate a trophic network?
 
 
 

Question 1 of 3

A cool video from the BBC tv-series “The hunt” shows various attempts and strategies for a polar bear to catch a seal.

This blog post originates from my Masters’ thesis, further published in the scientific journal Oikos.
The original article can be found at Oikos journal.

http://onlinelibrary.wiley.com/doi/10.1111/oik.04138/full

http://www.oikosjournal.org/accepted-article/complexity-and-structural-properties-food-webs-barents-sea

This blog post as also been posted on the MARmaED website, my doctoral network.

  1. Polar bear:
    https://commons.wikimedia.org/wiki/File%3APolar_Bear_2004-11-15.jpg
    Par Ansgar Walk (Travail personnel) [CC BY-SA 2.5], via Wikimedia Commons
  2.  Facts and description on the polar bear: http://www.arkive.org/polar-bear/ursus-maritimus/
  3. Bearded Seal – Erignathus barbatus
    By Alastair Rae (Personal work) [CC BY-SA 2.0], via Flickr
  4. Facts and description of the bearded seal: http://www.arkive.org/bearded-seal/erignathus-barbatus/
  5. My original study: Olivier, P. and Planque, B. 2017. Complexity and structural properties of food webs in the Barents Sea. – Oikos (in press).
  6. Polar olfactory abilities: Togunov, R. R., Derocher, A. E., & Lunn, N. J. (2017). Windscapes and olfactory foraging in a large carnivore. Scientific Reports, 7.
  7. Polar bear fasting: Robbins, C. T. et al. 2012. Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears. – J Mammal 93: 1493–1503.

Author of this blog post.

 

Pierre
Pierre PhD student - CEO Ocean Fact
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