A common garden food web experiment reveals local adaptation of consumers but not of prey

Background/Question/Methods

Leaves of Sarracenia purpurea form pitchers in which communities consisting of bacteria, protozoans and other inquiline species (e.g., mosquito larvae) decompose dead insects and provide nutrients to the plant. This plant originates from North America and was introduced to Switzerland a century ago, where similar types of communities, but with different evolutionary histories, have developed. To predict how whole communities will react to global changes, it is essential to understand how different communities will perform with regard to biotic and abiotic conditions. It can be hypothesized that communities perform better in their local environment (local adaptation at the community-level). However, most studies that test this hypothesis focus on one trophic level. It is not known if species at multiple trophic levels will respond to local conditions in the same way. We tested this question with a common garden experiment involving 4 Sarracenia food webs originating from Switzerland and North America. We manipulated temperature and origin of trophic levels, resulting in an experiment with 4 temperatures x 4 origins of prey x 4 origins of consumers (plus a control without consumers), replicated 4 times. We measured changes in densities of bacteria (prey) and different protozoan species (consumers) through time.

 

Results/Conclusions

We found that the bottom trophic level (bacteria) responded to temperature, with their densities being the highest in the warmest temperature treatment, regardless of consumer origin. On the other hand, their consumers (the protozoans) grew better in their own temperature and when they were with bacteria from their own site. The consumers were therefore adapted to their local conditions, while the prey were more affected by abiotic conditions and were not locally adapted. This difference in the response of the two trophic levels to changes in local conditions suggests that the effect of species to changing environmental conditions is dependent on what trophic level they inhabit. This suggests that a mismatch of predators and prey during global warming could occur. It is therefore important to take into account the interactions between species both within and between trophic levels to better understand what can be the consequence of climatic changes on communities.