Wednesday, April 27, 2016

Intraspecific Variation, Community Ecology, and Ecosystem Function

Should intraspecific trait variation be ignored as noise, or can such variation have the capacity to alter ecosystem function?  In my dissertation research, I have been documenting an example of community-level local adaptation to intraspecific variation.  Adaptation can arise due to changes in communities that make organisms more suited to living in their environment.  We often imagine examples of genetic changes occurring within species, however species sorting in a community could also be considered a form of adaptation.  While working in the rivers of the Olympic Peninsula of Washington, I have found that aquatic decomposer and riparian soil communities rapidly consume terrestrial leaf litter from immediately local red alder trees, resulting in greater energy capture with a potential range of further food web implications. 

My venture into local adaptation started with a curiosity about the potential cascading implications of plant defenses on aquatic food webs.  I found that a plant’s defense response against terrestrial herbivores also strongly inhibits aquatic decomposition.  However, my first attempts using comparative data led me down a different path.  I noticed that the degree of natural herbivory damage provided minimal predictive power of aquatic decomposition.  Instead, leaves that I had collected from local trees tended to show rapid decomposition, while leaves from other regions (including other riparian zones) tended to be less favored food sources.  I then aimed to directly test whether there was a general local adaptation/preference pattern among aquatic communities.  I found this pattern was quite evident throughout rivers in this region, and indeed often occurred on very small scales.  I then wanted to investigate the possible drivers of this adaptation pattern.  Spatial variation in secondary metabolites seemed to be a logical starting point given the diversity and complexity of compounds found within and among plant species.  Our recent paper reports that red alder trees produce an array of these secondary metabolites, many of which have been shown elsewhere to play important roles in defense against terrestrial herbivores.  These trees were highly geographically structured in their secondary metabolite composition, and using artificial diet experiments, we found that this geographic divergence in chemistry alone was sufficient to drive the same local adaptation patterns that we had originally documented with intact leaves.  The precise mechanism driving this adaptation pattern is unknown, and could indeed be another example of genetic changes within species, or species sorting of a range of taxa, such as stream macroinvertebrates, free living microbial taxa, or even the gut-associated microbes of these macroinvertebrates.  

I concluded from this work that variation among conspecifics can dramatically alter ecosystem functions.  We may overlook key processes regulating ecosystems if we ignore such intraspecific variation as noise in favor of more immediately obvious interspecific differences.  These findings inspire a number of theoretical and applied questions.  How pervasive is the importance of the individual in driving ecosystem function?  What are the mechanisms driving these patterns?  Can we predict where these local adaptation patterns may occur?  What factors drive the tempo of these local adaptation processes and do natural and anthropogenic disturbances disrupt the development of adaptation?  Are there cascading implications of adaptation (or lack thereof) on other trophic levels (such as fish feeding on macroinvertebrate decomposers) and nutrient cycles (including carbon cycling and sequestration in aquatic systems)?  Overall, our results indicated that individual differences can have far reaching implications on ecosystems and emphasize the potential utility in integrating population-level variation when considering community and ecosystem structure.      

Literature Cited

Jackrel, S.L., T.C. Morton and J.T. Wootton. 2016. Intraspecific leaf chemistry drives locally accelerated
ecosystem function in aquatic and terrestrial communities. Ecology: In Press.    

Jackrel, S.L. and J.T. Wootton. 2015. Diversity of riparian plants among and within species shapes river
communities. PLOS ONE 10: e0142362.

Jackrel, S.L. and J.T. Wootton. 2015. Cascading effects of induced terrestrial plant defences on aquatic
and terrestrial ecosystem function. Proceedings of the Royal Society B 1805: 20142522.

Jackrel, S.L. and J.T. Wootton. 2014. Local adaptation of stream communities to intraspecific variation in  
a terrestrial ecosystem subsidy. Ecology 95:37-43.