Evidence for resource homogenization in 50 trophic ecosystem networks

Background/Question/Methods   Connectivity patterns of ecological elements are often the core concern of ecologists working at multiple levels of organization (e.g., communities, ecosystems, and landscapes) because these patterns often reflect the forces shaping the system's development as well as constraining their operation or function. One reason these patterns of direct connections are critical is that they establish the pathways through which the elements influence each other indirectly. Here, we test a hypothesized consequence of connectivity in ecosystems: the homogenization of resource distributions in flow networks. Specifically, we test the generality of the systems ecology hypothesis of resource homogenization in 50 empirically derived trophic ecosystem models representing 35 distinct ecosystems. We applied Network Environ Analysis (NEA) to calculate the resource homogenization for these models, where homogenization is defined as the ratio of the coefficient of variation of the direct flow intensity matrix (CV(G)) to the covariance of the integral flow intensity matrix (CV(N)). A ratio greater than unity indicates the presence of homogenization. We evaluated the robustness of our results by conducting a Monte Carlo based uncertainty analysis to determine the robustness of our results to +/-5% error introduced into the original flow matrices for each model.

Results/Conclusions   Our results show that resource homogenization occurs universally in the 50 ecosystem models tested, with values ranging from 1.04 to 1.97 and a median of 1.61. However, our results do not support the hypothesized relationship between network homogenization and system size and connectance, as the results of the linear regressions are insignificant. Further, there is only weak support for the positive relationship between homogenization and cycling. Finally, we found that our results are robust to +/-5% error in the flow matrices. The error in the homogenization values was less than the error introduced into the models. The error ranged from a minimum of 0.24% to a maximum of 1.5% with a median value of 0.58%. The error also did not change the qualitative interpretation of the homogenization values. In conclusion, we find strong support for the resource homogenization hypothesis in 50 empirically derived ecosystem models.