Resonance-induced multimodal body-size distributions in ecosystems

Abstract

The size of an organism reflects its metabolic rate, growth rate, mortality, and other important characteristics; therefore, the distribution of body size is a major determinant of ecosystem structure and function. Body-size distributions often are multimodal, with several peaks of abundant sizes, and previous studies suggest that this is the outcome of niche separation: species from distinct peaks avoid competition by consuming different resources, which results in selection of different sizes in each niche. However, this cannot explain many ecosystems with several peaks competing over the same niche. Here, we suggest an alternative, generic mechanism underlying multimodal size distributions, by showing that the sizedependent tradeoff between reproduction and resource utilization entails an inherent resonance that may induce multiple peaks, all competing over the same niche. Our theory is well fitted to empirical data in various ecosystems, in which both model and measurements show a multimodal, periodically peaked distribution at larger sizes, followed by a smooth tail at smaller sizes. Moreover, we show a universal pattern of size distributions, manifested in the collapse of data from ecosystems of different scales: phytoplankton in a lake, metazoans in a stream, and arthropods in forests. The demonstrated resonance mechanism is generic, suggesting that multimodal distributions of numerous ecological characters emerge from the interplay between local competition and global migration.adaptive dynamics | evolutionary ecology | universal scaling | species packing | species assembly M ultimodal body-size distributions, in which a population exhibits several peaks of abundant sizes, characterize local ecosystems and have puzzled ecologists for many years (1-4). To resolve this puzzle, most previous studies concentrated on one important aspect of body size, namely resource partitioning or niche separation: larger individuals consume certain resources more efficiently than smaller ones, whereas smaller individuals have access to some resources that are unavailable to larger individuals. As a result, multimodal size distributions may originate from community-wide character displacement, in which natural selection pushes species to assort into packs, each of which fits to a particular niche (1,4,5).Another aspect of size, however, relates to a growth-vs.-efficiency tradeoff (6, 7). Smaller individuals generally invest less effort in their development and have a higher maximal growth rate when the resource is abundant (8-12), whereas larger individuals have more efficient metabolism and are more successful in direct competitive interferences (Fig. 1A) (6, 13, 14). As a result, smaller individuals are more likely to be the first to populate newly formed patches, and the larger individuals follow only later. In this study, we show how this tradeoff may promote multimodal body-size distributions, and we identify several characteristics of the underlying dynamics. Stochastic Subdivided Population ModelTo examine how the gr...

Sharp changes in resource availability may induce spatial nearly periodic population abundances

“…(4) comprises discrete steady state patterns that are similar to those of Eq. ((2) (Geritz et al, 1999;Kisdi, 1999;Lampert and Tlusty, 2013), with packs that are more apparent near the singularity at x = m (Lampert and Tlusty, 2013). We note, however, that transient dynamics of Eq.…”

“…(2) and comprise packs that propagate along the x-axis (Lampert and Tlusty, 2013). In contrast, the dynamics of Eq.…”

“…(2) since the unimodal solution, if exists, does not undergo instability even for rectangular kernels). Second, singular points appear also in asymmetric competitions, where an individual at x 0 interferes all individuals at larger x, but at the same time, a lower x implies lower growth rates (Geritz et al, 1999;Kisdi, 1999;Lampert and Tlusty, 2013) (e.g., this is realized where x is the body-size of an organism (Lampert and Tlusty, 2013) or the size of a seed (Geritz et al, 1999)). This has been modeled (Lampert and Tlusty, 2013) where GðxÞ ¼ R 1 À1 gðx À x 0 Þdx 0 and m is the mutation rate.…”

Scaling body size fluctuations

“…As such, they have long been attracting much interest in ecology because they hold important predictive power (e.g., fish stock projections from planktonic size spectra) (2,5). Because examples and counterexamples of scaling spectra abound (2,3,5,7,(21)(22)(23)(24), it is an unsettled issue as to whether scaling size spectra represent some central tendency of statistically stationary states of natural ecosystems. For instance, the operational computation of mean phytoplankton size was shown to depend on the sample size typically (7) and scaling relationships were documented for interspecific plant biomass (20,25,26), whereas some terrestrial ecosystems exhibit ubiquitous gaps in size and uneven relative abundances of organisms (1,21).…”

Where Two Are Fighting, the Third Wins: Stronger Selection Facilitates Greater Polymorphism in Traits Conferring Competition-Dispersal Tradeoffs

“…Polymorphism, however, may also occur when the competition is over a single resource. Competition is then asymmetric, namely, the trait provides some advantage while competing over the resource, but also bears some disadvantage such as greater mortality or less efficient dispersal [ 17 – 21 ]. Polymorphism in asymmetric traits is possible due to the ‘colonization-competition tardeoff’ [ 22 , 23 ].…”