Testing a Simple Rule for Dominance in Resource Competition

Enrichment (increasing K) destabilizes simple consumer Á/resource interactions, but increasing food web complexity in various ways can remove this paradox of enrichment. We varied resources and number of omnivorous predators (mosquitoes) and tested for effects on the stability (persistence and temporal variability) of microfaunal populations living in pitcher plants. Top-down (omnivorous) effects were destabilizing, decreasing the persistence time of a rotifer, Habrotrocha rosa, and perhaps a microflagellate, Bodo sp. Enrichment effects were more complex, in part due to effects of shredding midges on resource availability, and in part due to interactions with predation. The persistence of Bodo increased with resource availability (more bacteria due to shredding by midges; no paradox of enrichment). Increasing resources by adding ants decreased persistence of H. rosa when mosquitoes were rare (paradox of enrichment), but the effect was reversed in leaves with significant colonization by mosquitoes. Thus, in the microfaunal community of pitcher plants, omnivorous predation tends to be destabilizing, and also tends to remove the paradox of enrichment.

Section: Discussionmentioning

confidence: 90%

Stability of pitcher‐plant microfaunal populations depends on food web structure

Trzcinski¹,

Walde²,

Taylor³

2005

“…Many studies have found support for the R* principle, including microcosm studies that investigated competitive outcomes with heterotrophic protists (Fox 2002). Given that resource consumption is the basis for metabolic power, the R* principle and the MPP may be invoking essentially the same processes.…”

Section: Discussionmentioning

confidence: 99%

The maximum power principle predicts the outcomes of two‐species competition experiments

DeLong¹

2008

The maximum power principle (MPP) states that biological systems organize to increase power whenever the system constraints allow. The MPP has the potential to explain a variety of ecological patterns because biological power (metabolism) is a component of all ecological interactions. I empirically tested the MPP by reanalyzing three two-species competition experiments by Gause, Vandermeer, and Fox and Morin. These experiments investigated competitive outcomes in microcosms of heterotrophic protists. I introduce metabolic state-space graphs to portray the metabolic trajectories of the communities and show that the steady-state outcomes of these experiments are consistent with the MPP. Winning species were successfully predicted a priori from their status as the species with the highest power when alone. In addition, periods of coexistence, although not predictable a priori, were consistent with the MPP because coexistence states had community-level power that was higher than either species could achieve alone. Thus, the outcomes of all ten trials were the maximum power states, given the options. The results suggest that the maximum power principle may represent a useful energetic organizing principle for communities.Ecologists have long sought a general principle that explains both the directional change in non-equilibrium systems and the characteristics of steady-state systems. To this end, several thermodynamic goal functions have been offered that cast organization in biological systems as a maximization or minimization process for energetic quantities (see review of the topic and comparison of goal functions in Fath et al. 2001). The maximum power principle (MPP) is one of these principles. Originally formulated by Lotka (1922a) and further developed by Odum and Pinkerton (1955), the MPP states that biological systems capture and use energy to build and maintain structures and gradients that allow additional capture and utilization of energy. Non-biological systems such as the Atwood machine and Bernard cells have been used to explain the principle, but these analogies can make it hard to visualize what power maximization really looks like, or how to find it, in nature (Hall 1995, Odum 1995. The MPP was formulated more than 80 years ago, but to date very few ecological studies have sought to empirically evaluate the MPP concept (Hall 2004; but see Cai et al. 2006). Despite this, the MPP has been hotly debated in the ecological literature (Månsson and McGlade 1993, Patten 1993). In this paper, I strive to bring the MPP back to its biological roots and show that we can generate testable predictions about real biological phenomenon through the lens of the MPP.Power has units of energy per time. Metabolic rate, usually expressed in watts (J s (1 ; i.e. energy per time), can be thought of as biological power and can be used as the quantity of interest for studies of MPP in ecology. One of the great strengths of the MPP is that it directly relates energetics to fitness; organisms maximize fitness by maximizing pow...

“…The four productivity levels respectively comprised 0.01, 0.1, 0.55 or 1 g PP l −1 . These productivity levels span a range known to strongly affect protist densities (Kaunzinger and Morin 1998, Fox 2002, Fukami and Morin 2003). The two lowest productivity levels had 1/4 wheat seed dish −1 .…”

Section: Methodsmentioning

confidence: 99%

“…Further, in many systems the predictability of secondary succession and the consistency with which certain species dominate in certain environments indicates that initial conditions are of little importance (Tuomisto et al 2003). Instead, final community composition in these systems is a predictable reflection of species’ traits, such as competitive ability for different limiting resources (Tilman 1982, Fox 2002, Adams et al 2007).…”

mentioning

confidence: 99%

Testing whether productivity mediates the occurrence of alternate stable states and assembly cycles in a model microcosm system

Fox¹

2008

When do initial conditions, which reflect the assembly history of a community, affect the final community state? Comparative field studies and recent theory suggest that initial conditions matter at high productivity, because uninvasible alternate stable states and assembly cycles are more likely at high productivity. However, this prediction and the mechanisms behind it have not been tested experimentally. An alternative hypothesis is that initial conditions are relatively unimportant, and that communities generally are comprised of species with appropriate traits, which might vary with productivity. I assembled communities of protists and rotifers in laboratory microcosms from a species-rich, trophically-diverse species pool using all possible combinations of two initial conditions and four productivity levels. After communities approached their final states, invasions by the species that initially failed to persist tested the invasibility of those final states and tested for assembly cycles. I also examined how local (within-microcosm) diversity and regional diversity (total species richness of all microcosms of a given productivity level) varied with productivity. Comparative field work has used such scale-dependent diversityÁproductivity relationships as evidence for effects of assembly history. Productivity had a modest effect on final pre-invasion species composition, while initial conditions had a very weak effect. Most invasions failed, and the frequency of successful invasions and of post-invasion extinctions did not vary with productivity. Instead, species that were present most frequently pre-invasion were also the most successful invaders, and the least-likely species to go extinct post-invasion. Local and regional richness did not vary substantially with productivity. Overall, the results suggest that final communities are predictably comprised of species with appropriate traits, and are not an unpredictable outcome of initial conditions.