Of birthday cakes and interactions of density-dependence

Submitted by editor on 30 August 2016.Get the paper!

There are few things that we perceive as exclusively good or bad.

Whether something is good or bad often depends on conditions and circumstances. Take, for example, a birthday party. Most of us like to be surrounded by many dear friends to celebrate. It’s just more fun. But only as long as there is enough cake for everybody (pre-maturely running out of cake is detrimental). Therefore, whether or not it’s good to invite lots of friends to the birthday party depends on the amount of cake available.

In statistics, this is referred to as an interactive effect and in ecology, we are becoming more and more aware of the role of such interactive effects in nature. Our study “Interactive effects of exogenous and endogenous factors on demographic rates of an African rodent” provides an exciting example of just how important interactions among different components of the environment can be.

Our study species, the African striped mouse, lives in an unpredictable semi-arid habitat, and temperature and food plant abundance determine its ecology. Density feedbacks also play an important – and extremely fascinating - role. Effects of particularly negative density-dependence have been found in many species, but for the African striped mouse, density-dependence can shift from negative to positive and vice versa depending on interacting food availability! When little food is available, survival gets lower when there are more mice around (similarly to the detrimental scenario of too little cake for too many people). But when there is enough food, the density effect works in reverse, and striped mice actually profit from a large numbers of conspecifics.


These (and several other) results from our study highlight how important it can be to consider interactive environmental effects in ecological studies. 

The authors through Chloé Nater

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