When teaching ecology classes I frequently apply a method called concept-mapping. In concept-mapping assignments students are asked to connect different concepts on a sheet of paper with labeled arrows, thus outlining the interrelations among these concepts. I noticed that, if you wait long enough, every concept will be connected to every other. In addition to initially drawn solid arrows, over time, there are more and more dashed arrows drawn. Students like to expand the assignment to include dashed arrows that indicate indirect interactions and feedbacks among concepts.
This very much reminds me of food webs. While some arrows e.g. between predator and prey are obvious (quickly drawn) there are a lot of indirect interactions between non-adjacent components in the food web (dashed lines that require a second thought). Such indirect ecological interactions are increasingly recognized as being ubiquitous and ecologically relevant. The question then becomes: What are the evolutionary consequences of indirect trophic interactions?
|Lake Erken’s shoreline and all the other lakes in our survey are near-pristine except for the introduction of the zebra mussel (Photo credit: PE Hirsch).|
In a recent paper we addressed this topic. We asked how indirect effects of one species that cascade through the food web can contribute to the early stages of adaptive divergence. As a model species we used perch which show a clear phenotypic divergence between littoral and pelagic forms. The magnitude of this divergence is dependent on the availability of profitable habitat-specific resources. Feeding on these resources leads to a specialization of perch morphology to resource-acquisition, mainly through phenotypic plasticity. To study how this phenotypic divergence is affected by indirect interactions we compared perch from lakes in Sweden that were either pristine or invaded by the zebra mussels.
Perch do not feed on
zebra mussels. Zebra mussels affect both the benthic and pelagic food base of
lakes by filtering nutrients from the water column and providing a surplus of
structure and nutrients on the bottom. Their filtering activity also increases
water clarity. We therefore hypothesized that zebra mussels indirectly increase
the phenotypic divergence between littoral and pelagic perch by increasing the habitat-specific
food resources for perch.
|Zebra mussels are numerous in Lake Erken and in the other studied mussel-lakes (Photo credit: PE Hirsch)|
Indeed we found higher densities of large benthic prey items in the littoral and higher densities of large zooplankton in the pelagic zone of mussel-lakes. Water clarity was also higher in lakes with zebra mussels. Finally, the magnitude of divergence between littoral and pelagic perch was higher in lakes with zebra mussels.
|Phenotypic divergence between pelagic (top individuals) and littoral (individuals below) was higher in lakes with zebra mussels (Photo credit: PE Hirsch)|
The stronger plastic response in perch from lakes with zebra mussels plausibly results from the surplus of energy-rich invertebrates and zooplankton that specialized phenotypes can exploit in mussel-lakes. A higher growth rate in perch allows for a faster modulation of body shape to better fit the feeding mode. Costs of phenotypic plasticity should also be lower when profitable resources are plenty.
|Perch feeding on zebra mussels on the underside of a foot bridge in one of the study lakes (Photo credit: PE Hirsch)|
Here’s the link to our paper:
Hirsch P.E., Eklöv P. and Svanbäck R. Indirect trophic interactions with an invasive species affect phenotypic divergence in a top consumer. Oecologiahttp://link.springer.com/article/10.1007%2Fs00442-013-2611-1