Variance in Landscape Connectivity Shifts Microbial Population Scaling

Abstract

Understanding mechanisms shaping distributions and interactions of soil microbes is essential for determining their impact on large scale ecosystem services, such as carbon sequestration, climate regulation, waste decomposition, and nutrient cycling. As the functional unit of soil ecosystems, we focus our attention on the spatial structure of soil macroaggregates. Emulating this complex physico-chemical environment as a patchy habitat landscape we investigate on-chip the effect of changing the connectivity features of this landscape as Escherichia coli forms a metapopulation. We analyze the distributions of E. coli occupancy using Taylor's law, an empirical law in ecology which asserts that the fluctuations in populations is a power law function of the mean. We provide experimental evidence that bacterial metapopulations in patchy habitat landscapes on microchips follow this law. Furthermore, we find that increased variance of patch-corridor connectivity leads to a qualitative transition in the fluctuation scaling. We discuss these results in the context of the spatial ecology of microbes in soil.

Keywords: Taylor's Law; landscape ecology; metapopulations; microfluidics; scaling laws; spatial microbial ecology.

Figure 1

Microfluidics device. (A) A sketch of the microfluidics device with single inlets on each side leading into four parallel habitat landscapes. (B) Zoom-in view of patch-corridor structure in each of the four parallel landscapes. For type 1 corridor width is kept constant at 10μm. Types 2–4 the average width is also 10 μm with variance around the mean increasing from type 1 to 4 as σ² = 0, 4, 9, 16 μm². This can be appreciated in (C) where we show the pattern of corridor widths used for each landscape type. Grey shading indicates location of zoom-in.

Figure 2

(A) Semi-log plot of Rank-Size Distribution for all landscape replicates (n = 30) in light color and the ensemble average in bold. Occupancy ranges from 0 to 100%. (B) Occupancy frequency histogram for individual patches (n = 2,550).

Figure 3

Landscape dependency of Taylor's Law in a bacterial metapopulation. (A) Log-log plot showing the relationship between the average occupancy of a patch, 〈ϕ〉, and its variance, ${\sigma }_{\varphi }^2$, over (n = 30) replicates and across all patches (85) generating the unique Taylor's Law (TL) for each landscape type. Inset displays a box plot of corridor widths around the mean (10 μm). Four variances were used in this study, (σ² = 0, 4, 9, 16μm²). (B) The slope of TL, α, is plotted against the variance of corridor width (randomness) defining landscape types.