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.
