Linking Metacommunity Paradigms to Spatial Coexistence Mechanisms

Shoemaker, Lauren G, and Brett A Melbourne. 2016. “Linking Metacommunity Paradigms to Spatial Coexistence Mechanisms.” Ecology, May, 1–32. doi:10.1002/ecy.1454.

Metacommunity theory provides a foundation to expand community dynamics to a larger spatial-scale, and incorporate the effects of spatially-dependent phenomenon (e.g. habitat heterogeneity) into community structure, however, it is somewhat limited because most studies rely on using four metacommunity paradigms:

– neutral: all species respond to environmental heterogeneity the same
– species sorting: allows species to be adapted to certain types of patches, coupled with low and equal dispersal among species
– mass effects: similar to species sorting, but with higher levels of disperal, creating phenomena such as source-sink dynamics
– patch dynamics: based on metapopulation models, relies on a trade-off between dispersal and competition amonst species

These paradigms result in very different types of models, making comparisons amongst them difficult. Shoemaker and Melbourne (2016) use spatial coexistence theory to develop a mathematical model that can fit all four of the above paradigms, allowing them to analyze coexistence potential in scenarios that contain characteristics of multiple paradigms, which are more biologically realistic interpretations of metacommunities.

The model consists of parameters for competition, dispersal, patch heterogeneity, and environmental disturbance, which are then varied to approximate the characteristics of the above paradigms. In addition to the four above, they also create two additional scenarios with a mixture of species sorting and mass effects and with a mixture of species sorting and patch dynamics. Spatial coexistence is defined as mutual invasibility, or the ability of both species to increase when at low densities. Coexistence mechanisms are either equalizing (non-spatial fitness) or stabilizing (storage effect and fitness-density covariance), which are combined in a final equation to equal the overall coexistence strength. These mechanisms are represented as covariances, and are not solvable analytically, although the overall spatial coexistence is, and so the result is a hybrid analytical-simulation approach that allows for the calculation of the relative contribution of each mechanism to coexistence in each of the metacommunity scenarios.

As expected, the neutral paradigm had no stable coexistence, and only co-occurence. The other five scenarios, however, did exhibit coexistence, although the mechanisms causing this differed across scenarios. Species sorting and mass effect scenarios had stable coexistence that was primarily due to fitness-density covariance, with the storage effect influence coexistence to a lesser amount, although the coexistence strength was lower in mass effect scenarios than species sorting. The patch dynamics model exhibited trait tradeoffs. While there was no spatial storage effect, fitness-density covariance and differed across species, negating each other to allow for overall co-existence. Additionally, non-spatial effects seemed to “balance” the spatial effect of fitness-density covariance, mirroring the dispersal-competition trade-off expected in a patch dynamic paradigm.

This paper expanded on multi-scale modeling theory by taking a type of multi-scale model (the metacommunity) that is limited in its representation of systems, and expanding it to incorporate a range of scenarios through their development of a model of spatial coexistence mechanisms. Shoemaker and Melbourne demonstrate its usefulness in this paper, but it could also be applied to other metacommunity questions that have previously been restricted to the four paradigms.