Abstract
In many natural systems, the physical structure of the landscape dictates the flow of resources. Despite mounting evidence that communities’ dynamics can be indirectly coupled by reciprocal among ecosystem resource flows, our understanding of how directional resource flows might indirectly link biological communities is limited. We here propose that differences in community structure upstream should lead to different downstream dynamics, even in the absence of dispersal of organisms. We report an experimental test of the effect of upstream community structure on downstream community dynamics in a simplified but highly controlled setting, using protist microcosms. We implemented directional flows of resources, without dispersal, from a standard resource pool into upstream communities of contrasting interaction structure and then to further downstream communities of either one or two trophic levels. Our results demonstrate that different types of species interactions in upstream habitats may lead to different population sizes and levels of biomass in these upstream habitats. This, in turn, leads to varying levels of detritus transfer (dead biomass) to the downstream communities, thus influencing their population densities and trophic interactions in predictable ways. Our results suggest that the structure of species interactions in directionally structured ecosystems can be a key mediator of alterations to downstream habitats. Alterations to upstream habitats can thus cascade down to downstream communities, even without dispersal.
Highlights
In many natural systems, the physical structure of the landscape dictates the flow of organisms and resources
Many of the systems exhibiting this directionality are strongly dependent on external resource inputs, yet, attempts to look at their effects on community dynamics are scarce, and contrary to research on reciprocal exchanges (Gounand et al, 2014; Gravel, Guichard, Loreau, & Mouquet, 2010; Leibold et al, 2004), there is no general understanding of how directional resource flows might indirectly link biological communities
We experimentally showed that upstream community structure affects downstream community dynamics through resource flows only
Summary
Directionally biased movement of resources is a special case of metaecosystems where spatial feedbacks are only possible in one direction. Such directional flows are especially relevant in ecosystems where the geomorphic structure of the landscapes and physical processes (erosion, gravity, currents) are inherently driving biased movements of resources, such as in rivers, hillslope erosion, and along coastlines or ocean currents. All else being equal, the same amount of resources assimilated by different upstream communities may lead to the production of qualitatively very different subsidies (Gounand, Harvey, Ganesanandamoorthy, & Altermatt, 2017). We implemented directional flows of nutrients moving from a standard resource pool into upstream communities of contrasting protist interaction structure (“Monoculture,” “Competition,” “Predation,” “Facilitation,” “Bacteria alone,” see Figure 1b),
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