Energy flow between stream and forest

Submitted by editor on 2 October 2015.

Cross-system subsidies, or transfers of energy and nutrients from one ecosystem to another, have been studied extensively in many different types of environments. In particular, a great deal of research demonstrates that subsidies between streams and surrounding forests can be very important (e.g., Wallace et al. 1999[SC1] , Nakano & Murakami 2001[SC2] , Baxter et al. 2005[SC3] ).

Our new paper, The importance of terrestrial subsidies in stream food webs varies along a stream size gradient[SC4] , expands previous research on the role of terrestrial subsidies in both temperate and tropical stream ecosystems. This work started as part of an interdisciplinary project on guppy evolutionary ecology on the island of Trinidad [SC5] , when we began to conduct biological surveys in a gradient of stream sizes. Sites ranged from small streams that were high in the mountains and surrounded by dense forest to larger streams in the lowlands that had much more open canopies and much higher algal production. Predictive frameworks (especially the highly cited River Continuum Concept[SC6] , Vannote et al. 1980[SC7] ) suggest that communities of animals in streams will be dominated by species that can eat whatever food source is most prominent in that site, e.g., small, shaded streams with high inputs of leaf matter would be dominated by animals that specialize in shredding terrestrial leaf matter.

However, a common set of abundant insect and fish species occurred in most of our study sites even though resource availability was very different, with very little algal growth in shaded sites, but lots of visible algae in open sites. That observation led us to test whether there are within-species differences in the type of resources eaten by stream insects and fishes, i.e., are Functional Feeding Groups really more “flexible” than “functional”? Most broad ideas about stream food webs, including the River Continuum Concept, were developed in temperate systems, and there is some evidence in the literature that tropical and temperate streams might be fundamentally different[SC8] . We expanded our sample collection to temperate stream networks in the Adirondack Mountain region of New York so that we could test the idea of flexible feeding in both temperate and tropical systems. This contrast allowed us to address two main questions:

1. Are insects and fishes flexible or fixed in the amount of algae vs. terrestrial energy that they assimilate?

2. Are patterns consistent in both temperate and tropical sites?

We found that many aquatic insects and fishes are very flexible in what type of energy they assimilate, especially those that eat biofilms and fine organic matter from the bottom of streams. Some species can vary from eating 100% algae to 100% terrestrial material depending on whether they are living in an open-canopy stream or a closed-canopy stream. Leaf-shredders and predators were much less flexible, and consumed mostly terrestrial energy regardless of canopy cover and resource availability. Although there are some nuanced differences between patterns in Trinidad vs. New York, we found no major differences between tropical and temperate streams.

Sarah Collins

Photos:

1. Temperate stream site in the Adirondacks

2. Tropical stream site in Trinidad

[SC1]Link to: http://www.esajournals.org/doi/abs/10.1890/0012-9615%281999%29069%5B0409...

[SC2]Link to: http://www.pnas.org/content/98/1/166.full

[SC3]Link to: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2427.2004.01328.x/abst...

[SC4]Link to: http://onlinelibrary.wiley.com/doi/10.1111/oik.02713/abstract

[SC5]Link to: http://cnas.ucr.edu/guppy/

[SC6]Link to: http://www.lternet.edu/research/keyfindings/river-continuum

[SC7]Link to: http://www.nrcresearchpress.com/doi/abs/10.1139/f80-017?src=recsys#.Vg1s...

[SC8]Link to: http://www.bioone.org/doi/abs/10.1899/08-146.1