Models in Ecology

The effect of life-history traits on resource competition outcomes is well understood in the context of a constant resource supply. However, almost all natural systems are subject to fluctuations of resources driven by cyclical processes such as seasonality and tidal hydrology. To understand community composition, it is therefore imperative to study the impact of resource fluctuations on interspecies competition. We adapted a well-established resource-competition model to show that fluctuations in inflow concentrations of two limiting resources lead to the survival of species in clumps along the trait axis, consistent with observations of “lumpy coexistence” [Scheffer M, van Nes EH (2006) Proc Natl Acad Sci USA 103:6230–6235]. A complex dynamic pattern in the available ambient resources arose very early in the self-organization process and dictated the locations of clumps along the trait axis by creating niches that promoted the growth of species with specific traits. This dynamic pattern emerged as the combined result of fluctuations in the inflow of resources and their consumption by the most competitive species that accumulated the bulk of biomass early in assemblage organization. Clumps emerged robustly across a range of periodicities, phase differences, and amplitudes. Given the ubiquity in the real world of asynchronous fluctuations of limiting resources, our findings imply that assemblage organization in clumps should be a common feature in nature.

https://www.pnas.org/content/pnas/115/4/738.full.pdf

Lumpy species coexistence arises robustly in fluctuating resource environments.

Ecologists have long agreed that to coexist species must be sufficiently different (1, 2). This worldview was challenged when Stephen Hubbell published his neutral theory of biodiversity showing that species that are essentially equal should also be able to evade competitive exclusion (3). Although this neutral theory inspired a fresh look at biodiversity, it also met resistance as it seemed to violate the deeply held belief that all species are fundamentally different. Interestingly, a harmonious combination of niche differentiation and neutrality can emerge naturally in simulated communities, thus suggesting a way of unifying the niche and neutral view (4). A peculiar prediction of this theory of emergent neutrality is that species should be distributed over niche space in a lumpy way, a pattern that is indeed often observed in nature (5⇓⇓⇓–9). In this view the lumps correspond to niches, while species within lumps are essentially more of the same, reflecting two alternative ways for species to coexist: being sufficiently different or being sufficiently similar (Fig. 1). A study in PNAS now lends important support to this unifying theory by showing its compatibility with Tilman’s widely embraced theory of competition for resources (10).



Fig. 1. 
Simulation of the evolutionary adaptation of species showing the emergence of regularly spaced lumps of near-neutral coexistence. Species move away from the gaps toward the self-organized niches reflected in the lumps. The distance between the lumps corresponds to the niche separation that is theoretically predicted from the classic theory of limiting similarity. Modified with permission from ref. 4; Copyright (2006), National Academy of Sciences, USA.



The theoretical prediction that coexistence of two species requires sufficient niche differentiation (2) has become one of the cornerstones of ecology. It also makes intuitive sense. The smaller the niche overlap, the weaker the competition … 

[↵][1]1To whom correspondence should be addressed. Email: Marten.Scheffer{at}wur.nl.

 [1]: #xref-corresp-1-1

https://www.pnas.org/content/pnas/115/4/639.full.pdf

Toward a unifying theory of biodiversity.

While species fulfill many different roles in ecosystems, it has been suggested that numerous species might actually share the same function in a near neutral way. So-far, however, it is unclear whether such functional redundancy really exists. We scrutinize this question using extensive data on the world’s 4168 species of diving beetles. We show that across the globe these animals have evolved towards a small number of regularly-spaced body sizes, and that locally co-existing species are either very similar in size or differ by at least 35%. Surprisingly, intermediate size differences (10–20%) are rare. As body-size strongly reflects functional aspects such as the food that these generalist predators can eat, these beetles thus form relatively distinct groups of functional look-a-likes. The striking global regularity of these patterns support the idea that a self-organizing process drives such species-rich groups to self-organize evolutionary into clusters where functional redundancy ensures resilience through an insurance effect.

/pdf/the-evolution-of-functionally-redundant-species-evidence-276b9byr7j.pdf

The Evolution of Functionally Redundant Species; Evidence from Beetles.

We found a segmented pattern, increasing for small sizes and decreasing for larger sizes, in the relationship between trophic position and body size. This pattern provides support for a recently de...

Gape and energy limitation determine a humped relationship between trophic position and body size

Understanding the mechanisms that maintain biodiversity is a fundamental problem in ecology. Competition is thought to reduce diversity, but hundreds of microbial aquatic primary producers species coexist and compete for a few essential resources (e.g., nutrients and light). Here, we show that resource competition is a plausible mechanism for explaining clumpy distribution on individual species volume (a proxy for the niche) of estuarine phytoplankton communities ranging from North America to South America and Europe, supporting the Emergent Neutrality hypothesis. Furthermore, such a clumpy distribution was also observed throughout the Holocene in diatoms from a sediment core. A Lotka-Volterra competition model predicted position in the niche axis and functional affiliation of dominant species within and among clumps. Results support the coexistence of functionally equivalent species in ecosystems and indicate that resource competition may be a key process to shape the size structure of estuarine phytoplankton, which in turn drives ecosystem functioning.

/pdf/competition-drives-clumpy-species-coexistence-in-estuarine-reunalat5c.pdf

Competition Drives Clumpy Species Coexistence in Estuarine Phytoplankton

Determining statistical patterns irrespective of interacting agents (i.e. macroecology) is useful to explore the mechanisms driving population fluctuations and extinctions in natural food webs. Here, we tested four predictions of a neutral model on the distribution of community fluctuations (CF) and the distributions of persistence times (APT). Novel predictions for the food web were generated by combining (1) body size–density scaling, (2) Taylor's law and (3) low efficiency of trophic transference. Predictions were evaluated on an exceptional data set of plankton with 15 years of weekly samples encompassing c. 250 planktonic species from three trophic levels, sampled in the western English Channel. Highly symmetric non-Gaussian distributions of CF support zero-sum dynamics. Variability in CF decreased while a change from an exponential to a power law distribution of APT from basal to upper trophic positions was detected. Results suggest a predictable but profound effect of trophic position on fluctuations and extinction in natural communities.

/pdf/community-fluctuations-and-local-extinction-in-a-planktonic-35jks5zc8h.pdf

C. E. Widdicombe

Papers

Competition Drives Clumpy Species Coexistence in Estuarine Phytoplankton

Community fluctuations and local extinction in a planktonic food web.