Resource Acquisition Rates Research Articles

Traffic Dynamics of the Leaf‐Cutting Ant,Atta cephalotes

Colonies of Atta cephalotes (Myrmicinae: Formicidae) construct cleared paths between their nest and the vegetation sources at which they harvest leaf tissue. Here, we employ ideas from traffic engineering to study streams of laden and unladen ants on these paths. The relationship between average traffic speed and the concentration of workers on the road surface follows a relationship similar to what is expected by analogy to fluid dynamics. Although the traffic is composed of eusocial organisms with a common interest in group success, the coarse-grained behavior of Atta traffic displays little more coordination than a moving fluid. The relationship between speed and concentration implies that maximum flow rates (which are likely to be closely tied to colony-level rates of resource acquisition) occur at a relatively high concentration that keeps individual speeds well below their "free flow" maximum. We predict that this optimal concentration will characterize peak traffic throughout a trail network, and we propose a simple behavioral mechanism that would allow trails to be cleared to the correct width to provide the optimal concentration. Collisions (including encounters for antennation) are common in leaf-cutting ant traffic because traffic is not segregated into unidirectional streams. Nonetheless, we find a counterintuitive suggestion that flow rates (with concentration differences statistically removed) are higher when traffic is near a 50:50 mix of outbound and returning ants than when it contains majority flows in a single direction. Mixed-direction traffic may help disperse laden ants with reduced agility, thereby preventing inhomogeneities in the traffic stream that could clog the trail.

Traffic Dynamics of the Leaf-Cutting Ant, Atta cephalotes

Colonies of Atta cephalotes (Myrmicinae: Formicidae) construct cleared paths between their nest and the vegetation sources at which they harvest leaf tissue. Here, we employ ideas from traffic engineering to study streams of laden and unladen ants on these paths. The relationship between average traffic speed and the concentration of workers on the road surface follows a relationship similar to what is expected by analogy to fluid dynamics. Although the traffic is composed of eusocial organisms with a common interest in group success, the coarse‐grained behavior of Atta traffic displays little more coordination than a moving fluid. The relationship between speed and concentration implies that maximum flow rates (which are likely to be closely tied to colony‐level rates of resource acquisition) occur at a relatively high concentration that keeps individual speeds well below their “free flow” maximum. We predict that this optimal concentration will characterize peak traffic throughout a trail network, and we propose a simple behavioral mechanism that would allow trails to be cleared to the correct width to provide the optimal concentration. Collisions (including encounters for antennation) are common in leaf‐cutting ant traffic because traffic is not segregated into unidirectional streams. Nonetheless, we find a counterintuitive suggestion that flow rates (with concentration differences statistically removed) are higher when traffic is near a 50:50 mix of outbound and returning ants than when it contains majority flows in a single direction. Mixed‐direction traffic may help disperse laden ants with reduced agility, thereby preventing inhomogeneities in the traffic stream that could clog the trail.

The Evolution of Traits Affecting Resource Acquisition and Predator Vulnerability: Character Displacement under Real and Apparent Competition

This article investigates some simple models of the evolutionary interaction between two prey species that share a common resource and a common predator. Each prey species is characterized by a trait that determines both the rate of resource capture and vulnerability to a predator. In a simple model of a three‐species food chain, such traits usually increase in response to an imposed reduction in resource density. When the per capita growth rates of each of two prey species depend linearly on resource density, such traits will change in opposite directions when the two prey come into sympatry. In addition, the ratio of the effect of the predator on prey fitness to the effect of the resource on prey fitness will diverge from the corresponding ratio in a second prey species when those species coexist in sympatry. These simple predictions need not hold under several alternative assumptions, which may be more common in biological systems. Parallel changes in sympatry may occur if the relationship between resource consumption and prey growth is nonlinear, if the prey species have partial overlap in the set of resources used or in the set of predators that consume them, or if prey experience direct intraspecific competition. The responses to a second prey can also differ significantly from those predicted by the simplest model if separate traits affect vulnerability to predators and resource acquisition rate. It is important to determine whether examples of character displacement previously interpreted as responses to competition for resources might also reflect responses to altered predation risks in sympatry.

The contribution of trait-mediated indirect effects to the net effects of a predator.

Many prey modify traits in response to predation risk and this modification of traits can influence the prey's resource acquisition rate. A predator thus can have a "nonlethal" impact on prey that can lead to indirect effects on other community members. Such indirect interactions are termed trait-mediated indirect interactions because they arise from a predator's influence on prey traits, rather than prey density. Because such nonlethal predator effects are immediate, can influence the entire prey population, and can occur over the entire prey lifetime, we argue that nonlethal predator effects are likely to contribute strongly to the net indirect effects of predators (i.e., nonlethal effects may be comparable in magnitude to those resulting from killing prey). This prediction was supported by an experiment in which the indirect effects of a larval dragonfly (Anax sp.) predator on large bullfrog tadpoles (Rana catesbeiana), through nonlethal effects on competing small bullfrog tadpoles, were large relative to indirect effects caused by density reduction of the small tadpoles (the lethal effect). Treatments in which lethal and nonlethal effects of Anax were manipulated independently indicated that this result was robust for a large range of different combinations of lethal and nonlethal effects. Because many, if not most, prey modify traits in response to predators, our results suggest that the magnitude of interaction coefficients between two species may often be dynamically related to changes in other community members, and that many indirect effects previously attributed to the lethal effects of predators may instead be due to shifts in traits of surviving prey.

Cheatgrass and Yellow Starthistle Growth at 3 Soil Depths

Community dynamics and dominance on cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) infested rangeland appear to be influenced by resource acquisition rates and duration of growth. Objectives were to determine the effects of densities, proportions, and soil depth on the growth rate and duration of growth of these species. In 6 field experiments isolated individuals, monocultures (100, 1,000, 10,000 plants m-2), and mixtures (same densities arranged factorially) were grown with unrestricted and restricted (0.2- and 0.5-m) soil depths. Shoot weights were determined on 12-day intervals beginning on day 24 and ending on day 72 for plants grown with restricted soil depth and day 96 (cheatgrass) and day 108 yellow starthistle) for plants grown in unrestricted soil. Quadratic growth curves were fit for each replication for plants grown in isolation. Linear and quadratic models season. We tested whether blue grama [Bouteloua gracilis (HBK.) Lag.] could be established in crested wheatgrass fields to increase plant diversity and grazing season length. Three seeding methods (unseeded, seed broadcast after rototilling, or seed drilled) and 2 herbicide treatments (none or glyphosate [N (phosphonomethyl)-glycine] sprayed at 1.1 kg a.i. ha-1) were applied in a complete factorial design to 3 X 10 m plots in 1994 in two 50 year-old crested wheatgrass stands. Blue grama established in seeded plots, but the effectiveness of seeding methods varied between sites. At both sites, herbicide control of crested wheatgrass greatly promoted the establishment of blue grama. Crested wheatgrass biomass and cover were reduced by rototilling and by spraying. The results suggest that the establishment of blue grama in crested wheatgrass stands is possible if seed is added and competition from crested wheatgrass is controlled.

Dynamic Ideal Free Distribution: Effects of Optimal Patch Choice on Predator-Prey Dynamics

This study examines the influence of individual behavior on predator-prey dynamics in a two-patch environment. I assume that individuals behave to maximize their fitness measured by the instantaneous per capita growth rate. The population dynamics in each patch are de- scribed by the Lotka-Volterra continuous model. Two cases are studied in detail. The first case assumes only predators are free to move between patches, whereas the second assumes that both predators and prey move freely between patches. The study shows that the optimal behavior of animals leads to persistence of predator-prey systems and reduction of oscillations in population densities. In this article, I develop a dynamic framework for modeling the spatial distribu- tion of predator and prey populations, assuming animals behave optimally (i.e., they maximize their fitness). I follow the idea behind the ideal free distribution (Fretwell and Lucas 1970; Kacelnik et al. 1992), which describes the equilibrium distribution of a population of predators among several habitat patches. The Fret- well and Lucas model assumes that predators are free to settle in any patch, travel time between patches is zero, predators are omniscient, and resource densities in patches remain constant through time (Kacelnik et al. 1992). If each predator settles in the patch where its rate of resource acquisition is maximized, the model predicts that at equilibrium, predators in all patches will experience the same intake rate. The original Fretwell and Lucas model was modified in several direc- tions to include factors such as interference (Sutherland 1983), differences in competitive ability (Sutherland and Parker 1985; Parker and Sutherland 1986), perceptual constraints (Abrahams 1986), learning (Bernstein et al. 1988), and resource dynamics (Lessells 1995). However, the original Fretwell and Lucas model and these generalizations do not consider predator-prey dynamics that in turn will influence the distribution of predators and prey among patches. More- over, the optimality criterion based on maximization of energy intake should also be replaced by a more general criterion that includes the consequences of varia- tion in predator mortality rates among all patches. In natural communities, the richer habitat is often also the more dangerous one, owing to a higher predation

PLEIOTROPY CAUSES LONG‐TERM GENETIC CONSTRAINTS ON LIFE‐HISTORY EVOLUTION INBRASSICA RAPA

Fundamental, long-term genetic trade-offs constrain life-history evolution in wild crucifer populations. I studied patterns of genetic constraint in Brassica rapa by estimating genetic correlations among life-history components by quantitative genetic analyses among ten wild populations, and within four populations. Genetic correlations between age and size at first reproduction were always greater than +0.8 within and among all populations studied. Although quantitative genetics might provide insight about genetic constraints if genetic parameters remain approximately constant, little evidence has been available to determine the constancy of genetic correlations. I found strong and consistent estimates of genetic correlations between life-history components, which were very similar within four natural populations. Population differentiation also showed these same trade-offs, resulting from long-term genetic constraint. For some traits, evolutionary changes among populations were incompatible with a model of genetic drift. Historical patterns of natural selection were inferred from population differentiation, suggesting that correlated response to selection has caused some traits to evolve opposite to the direct forces of natural selection. Comparison with Arabidopsis suggests that these life-history trade-offs are caused by genes that regulate patterns of resource allocation to different components of fitness. Ecological and energetic models may correctly predict these trade-offs because there is little additive genetic variation for rates of resource acquisition, but resource allocation is genetically variable.

An Experimental Demonstration of Exploitation Competition in an Ongoing Invasion

A native asexual gecko, Lepidodactylus lugubris, declines numerically when the sexual gecko Hemidactylus frenatus invades urban/suburban habitats throughout the Pacific. Previous studies showed that the competitive displacement occurs rapidly and is facilitated by clumped insect resources. Five lines of evidence suggest that the mechanism of displacement is primarily due to differences in the ability of each species to exploit insect resources. (1) These species show nearly complete diet overlap. (2) Insects are a limiting resource for both geckos as evidenced by positive demographic effects with increased insect abundance. (3) Hemidactylus frenatus depletes insect resources to lower levels than L. lugubris, which results in reduced rates of resource acquisition in L. lugubris. (4) This reduced resource acquisition translates into significant reductions in the body condition, fecundity, and survivorship of L. lugubris individuals. (5) Evidence for interference (and other) mechanisms does not account for these negative demographic effects on L. lugubris. Interspecific competition is stronger than intraspecific competition for L. lugubris, with increasing L. lugubris density having negligible effect on H. frenatus, mirroring the asymmetry of the large—scale displacement. The superior harvesting ability of H. frenatus is most pronounced when insects are clumped spatially and temporally, and is attributable to a variety of species—specific traits such as their larger body size, faster running speed, and reduced intraspecific interference while foraging. We conclude that clumped resources can increase interspecific exploitation competition, and this mechanism may contribute to species turnover when human environmental alterations redistribute resources.

Observation: Comparative Live-History of Cheat-Grass and Yellow Starthistle

The objective of this research was to characterize the life-histories of cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) growing in association. Biweekly demographic attributes were monitored during 1991 (moist spring) and 1992 (dry spring). Data were arranged into life-history tables, and sensitivity analysis was performed to determine key transition phases. The entire cheatgrass seed crop reached the soil surface, 41% of yellow starthistle's seed output was lost during seed rain. Frost heaving reduced winter seedling populations of cheatgrass (53%) more than yellow starthistle (40%). All cheatgrass seedlings surviving the frost heaving period became adults. Yellow starthistle density was reduced by 75% during the juvenile phase. Cheatgrass adults appeared about 6 weeks before yellow starthistle adults. Cheatgrass seed output remained about 7,000 m2 with moist and dry spring conditions. Yellow starthistle seed output was about 21,600 m2 and 5,200 m2 with moist and dry spring conditions, respectively. Reduction of yellow starthistle seed output with dry spring conditions suggest oscillatory community dynamics. Key processes associated with life-history transitions were interference (competition), resource acquisition rates and duration, and reproductive allocation.

THE ROLE OF VACUOLES

THE ROLE OF VACUOLES

Primary sex‐ratio and differential progeny survivorship in solitary haplo‐diploid parasitoids

ABSTRACT. A method of partitioning emergence sex‐ratio to give estimates of primary sex‐ratio and male and female offspring survival from oviposition to emergence in solitary haplo‐diploid parasitoids is presented. This method is applied to sex‐ratio data from a larval parasitoid, Aphidius ervi Haliday, and a pupal parasitoid, Coccygomimus turionella L., parasitizing a range of host types. There was no evidence of facultative control of primary sex‐ratio in either species. Parasitoid emergence sex‐ratios were similar for all host sizes attacked by C. turionella. In contrast, emergence sex‐ratios of A.ervi showed a significant male bias in smaller hosts. This shift in emergence sex‐ratio was attributable to differential progeny survival. Pre‐emergence mortality in both species was a function of host size, with few offspring surviving from small hosts. This suggests that host size may be an important component in the dynamics of host‐parasitoid interactions. The evolution of sex‐ratio regulatory mechanisms in solitary haplo‐diploid parasitoids is discussed in the context of parasitoid life‐history. We suggest that there is a constraint to the evolution of the facultative control of primary sex‐ratio in parasitoids attacking larval stages as a result of the uncertainty of future host resource acquisition rates.

Direct and indirect effects of temperature on arctic plants

Most physiological processes of arctic plants are less temperature sensitive than those of temperate counterparts and consequently are most strongly limited by factors other than temperature. Annual carbon gains is limited primarily by length of growing season and secondarily by light, temperature, and nitrogen. Nutrient absorption is limited much more strongly by nutrient availability than by temperature. Nutrient availability in turn is restricted by a variety of factors stemming indirectly from low temperature. The in situ growth rate of arctic plants is comparable to or higher than that of temperate plants despite a 15–20°C difference in average air temperature. I conclude that temperature limits in several ways the rate at which resources become available to arctic plants and thereby the rates of resource acquisition and growth that can be maintained in an arctic environment, but that temperature is not a strong direct limitation to plant growth in the Arctic.