Experimental dispersal reveals characteristic scales of biodiversity in a natural landscape

Abstract

Ecological theory posits that dispersal among habitat patches links local communities and is a key "regional" process that maintains biological diversity. However, manipulations required to experimentally test regional processes are infeasible for most systems, and thus more work is needed to detect the scales at which regional processes manifest and their overall effect on diversity. In a Californian grassland, a hotspot for global biodiversity, we used a seed vacuum to increase dispersal at spatial scales varying from 1 m to 10 km while maintaining a realistic spatial structure of species pools and environmental conditions. We found that dispersal limitation has a profound influence on diversity; species richness increased with the spatial scale of seed mixing, doubling in plots that received seed from large (≥5 km) compared with small (≤5 m) scales. This increase in diversity corresponded to an increase in how well species distributions were explained by environmental conditions, from modest at small scales (R 2 = 0.34) to strong at large scales (R 2 = 0.52). Responses to the spatial scale of seed mixing were nonlinear, with no differences below 5 m or above 5 km. Nonlinearities were explained by homogeneity of environmental conditions below 5 m and by a lack of additional variation in the species pool above 5 km. Our approach of manipulating natural communities at different spatial scales reveals (i) nonlinear transitions in the importance of environmental sorting and dispersal, and (ii) the negative effects of dispersal limitation on local diversity, consistent with previous research suggesting that large numbers of species are headed toward regional extinction.The problem of pattern and scale is the central problem in ecology, unifying population biology and ecosystem science, and marrying basic and applied ecology. S. A. Levin (1992) T he processes that structure ecological populations, biodiversity, and ecosystem properties transition in importance across spatial scales (1, 2). As the spatial scale of observation increases, the range of environments sampled (1, 3) and the geographic distance separating localities (4) become increasingly important in shaping species distributions. As a consequence, the relative spatial scaling of different ecological processes is thought to underlie some of the most important patterns in ecology, such as species-area (5) and biodiversity-ecosystem function relationships (6). Because identifying the important scales is challenging, ecologists often compare ecological patterns among local and regional scales to simplify theoretical (7-9) and empirical research (10, 11). However, how closely local and regional delineations match up with the actual scaling of ecological processes is rarely known (2, 12). Quantifying the spatial scaling of these processes promises to enrich our understanding of the mechanisms that maintain diversity, yet remains elusive even in biodiversity hotspots that require this information for conservation decisions (13).A major challenge to testing how ...

Using functional trait diversity patterns to disentangle the scale‐dependent ecological processes in a subtropical forest

“…The spatial‐scale dependence of community assembly observed in this 20‐ha fully mapped species‐rich subtropical forest plot is consistent with previous studies (de Bello et al., ; Condit et al., ; Germain, Strauss, & Gilbert, ; Talbot et al., ), which have argued that abiotic filtering and dispersal limitation dominate community assembly at small and large scales, respectively. However, contrasting patterns of community assembly across spatial scales are also reported in other forest communities.…”

Cryptic dispersal networks shape biodiversity in an invaded landscape

“…Recent experimental work demonstrates that dispersal limits plant diversity at our study site (Germain et al. ), yet as we will discuss, we find no evidence of spatial distance as a proxy for dispersal limitation through our observational data; this contradiction motivates our examination of other landscape features relevant to dispersal. Specifically, the absence of tall vegetation in serpentine grasslands allows landscape features, such as hydrological networks and animal paths, to be captured via satellite imagery (Fig.…”

“…The data presented here are subsetted from a larger data set (Germain et al. ) to include only sites within the same region as our current survey, and only plots from two treatments: (1) unmanipulated control plots (local dispersal) and (2) treatments that mixed species pools among sites within the regional extent of our survey (100‐m spatial scale, regional dispersal).…”

“…A 2014 field experiment at McLaughlin Natural reserve found strong evidence of dispersal limitation in serpentine plant communities (Germain et al. ). For the current paper, we consider data from 5 of the 30 serpentine patches included in the original study that overlap with and are of the same spatial extent as our current observational research to facilitate comparisons; here, we describe methods relevant to that subset of sites.…”

“…First, the presence of dispersal limitation can cause local species richness to fall short of what a given environment can support (Foster and Tilman , Germain et al. ), and second, the failure of propagules to reach suitable sites can generate spatial turnover in species composition that contributes to regional biodiversity (Hurtt and Pacala , Mouquet and Loreau ). Numerous statistical tools have been developed to isolate the relative explanatory power of dispersal vs. environment from field data (e.g., Peres‐Neto et al.…”