It’s funny how complex outcomes can arise from simple realizations. For example, it is plausible that when there are differences among individuals of a species (like when local populations are adapted to the local environment), these could implications for function on the ecosystem scale. But while there is increasing evidence for the importance of intraspecific variation for ecological interactions within communities, the question of how intraspecific diversity scales up to ecosystem functioning is still ambiguous.
Sara Jackrel and Timothy Wootton explore this question in “Local adaptation of stream communities to intraspecific variation in a terrestrial ecosystem subsidy”. The basis for their study was simple: local adaptation is common, and populations/genotypes/ecotypes tend to be best adapted to the particular conditions of their locale. For example, “spatial variation in prey and predators can lead to a geographic mosaic of co-evolutionary interactions”. Further, these localized interactions can affect the greater ecosystem, if individuals or materials move between ecosystem boundaries.
In particular, the authors note that there is evidence that tree species composition riverside can alter the composition of the local aquatic community. This occurs via leaf litter fluxes into the river: the type and amount of leaf litter that falls into streams varies, and so the type of macroinvertebrates in the recipient stream also varies in response. These macroinvertebrates break down the leaf litter via shredding, collecting, and filtering, playing an important role in nutrient cycles. Leaf litter is carried from a given tree by wind or water and may decompose near or far away, creating a connection between ecosystems. The question then is whether macroinvertebrate compositional shifts will occur in response to intraspecific differences in leaf (i.e. trees), and what the implications might be for ecosystem functions such as leaf decomposition. To explore this question, Jackrel and Wootton performed reciprocal transplants of leaf litter material between eight sites along rivers in the Olympic Peninsula of Washington.
All eight of these sites were early successional forests dominated by red alder. The authors collected fresh leaves from alder trees, bagging leaves from each tree separately. These bags of leaves were either placed in the river adjacent to the trees they were taken from, or in a more distant site. Non-adjacent sites were either in the same river as the home site, or in different river all together. Leaf packs were weighed before and after spending 17-18 days in the river. This would allow comparison of how decomposition rates vary between home and away sites, and between home and away rivers.
Their results suggested a few interesting things. First, the identity of a tree affects the rate of decomposition of its leaves: individual alder trees’ leaves were highly variable in the rate of decomposition. Second, the combined identities of trees at a site seem to have affected the composition of the decomposer community at the home river site: put leaves from that site in another river with a new community of decomposers, and the decomposition rate drops significantly. In general, leaves decomposed significantly more rapidly when in their home river, regardless of whether at the home site or elsewhere along the river. But if they put leaves upstream from the home site, but in the same river, the rate of decomposition also dropped. Upstream decomposer communities were apparently much worse at breaking down leaves from novel communities of alders. However, if you put the leaves in sites downstream from home, the decomposition rates are not significantly different than in the home site. This is likely because of the directional movement of a river, such that downstream locations receive leaf litter from all upstream sites, and so downstream decomposer communities experience a greater variety of leaf litter than upstream sites. This might lead to upstream sites being more closely adapted to the individual trees in their neighbourhood than downstream sites, which receive inputs from a wide variety of trees. These results suggest that individual differences in trees at different spatial locations can matter, both locally, across trophic levels, and even across ecosystems.
Admittedly there is not a lot you can infer about the mechanisms at play from this preliminary experiment. One interesting follow up would be to measure compositional differences in aquatic macroinvertebrates at very fine scales in correspondence with differences in trees. Another important question is whether these communities differ via phenotypic plasticity, adaptation to local sites, or species sorting. But this paper does hint at one way in differences among individuals can shape local ecosystems and even structure distant ecosystems (e.g. downstream decomposer communities) through fluxes across boundaries. And that is a rather complicated implication from a logical and simple starting point.
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