A Consumer-Resource Approach to Community Structure

Because all species are consumers and all, eventually, are consumedby other species, consumer-resource interaction is one of themost fundamental processes of ecology. Simple models that includethe direct mechanisms of consumer-resource interactions maythus be the fundamental building-block for models of communitystructure. These models are easily extended to include suchcomplexity as the effects of physical limiting factors, spatialheterogeneity in resource supply, fluctuating resource supply,and multiple trophic levels. Each such modification places constraintson the traits of species that can persist. Consumer-resourcemodels make predictions about many aspects of community structure,including species richness, species composition, species dominance,population dynamics, morphological or physiological traits ofspecies, and patterns of phenotypic variation within species.Thus, each model affords numerous opportunities to test andmodify or reject it. A review of a variety of communities suggeststhat much of the structure of each community can be explainedby a relatively simple consumer-resource model, but that differentelements of complexity may be important in different communities.     

Electroantennogram responses of tsetse flies (Glossina pallidipes) to host odours in an open field and riverine woodland

The present study was initiated to gain insight into the way in which tsetse flies ( Glossina spp.) sense odours at different locations in odour plumes in both an open field and a wooded area.
We recorded the antennal responses (EAGs) from stationary living female G. pallidipes 15 m upwind and at various (60, 40, 20, 10, 5 and 1 m) distances downwind from a synthetic host odour source (containing 1-octen-3-ol, acetone and two phenols), in the natural habitat of the fly (Zimbabwe) using a portable electrophysiological device. Experiments were performed in a flat open area (an airstrip) and in riverine woodland. Differences between responses in different environments were determined by comparing various parameters of the EAGs (intermittency, frequency, amplitude, duration and rate of depolarization).
We found that a fly senses odours as puffs that, further downwind, contain less odour and pass less frequently. In an open field downwind from the source, tsetse perceive more olfactory information than upwind for only 10–20 m, whereas in woodland, olfactory responses remain higher and more frequent than upwind up to at least 60 m. In an open field, olfactory information rapidly increases when approaching the odour source from 20 m and in woodland from 5 m onwards.
It is proposed that averaging odour information over time may be of minor importance in long-range location of odour sources. The results suggest that tsetse may smell odour-baited targets from at least 60 m downwind and that the number of flies responding to and being caught by these baits may be higher in woodland than in an open field.     

Variation in social rank acquisition influences lifetime reproductive success in black-capped chickadees

Dominance relationships structure many animal societies, yet the process of rank attainment is poorly understood. We investigated acquisition of social dominance in winter flocks and its fitness consequences in male black-capped chickadees (Poecile atricapillus) over a 10-year period. Age was the best predictor of rank, and paired comparisons showed high-ranked males to be older than their low-ranked flock-mates. When controlling for age, morphological variables did not predict male social rank, but high-ranked males were heavier, had lower fat scores and were in leaner condition than low-ranked males. Males that survived between years tended to increase in rank over time; however, the rate of rank advancement varied individually. Rank reversals between familiar contestants were rare, and changes in male social rank were associated with changes in flock membership. Average lifetime reproductive success (LRS) of males and females was variable and best predicted by lifespan. Male rank history also influenced realized reproductive success. Birds with higher average rank over their lifespan were more likely to reproduce successfully. However, among successful birds, average rank did not significantly predict LRS. Thus, birds that lived longer and attained high social rank earlier had higher fitness, but this effect was not manifested as fine-scale differences among successful individuals. Taken together, these findings demonstrate the importance of social factors influencing individual fitness.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 85–95.     

Effects of elevated CO2, nitrogen deposition, and decreased species diversity on foliar fungal plant disease

Three components of global change, elevated CO₂, nitrogen addition, and decreased plant species richness (‘diversity’), increased the percent leaf area infected by fungi (pathogen load) for much to all of the plant community in one year of a factorial grassland experiment. Decreased plant diversity had the broadest effect, increasing pathogen load across the plant community. Decreased diversity increased pathogen load primarily by allowing remaining plant species to increase in abundance, facilitating spread of foliar fungal pathogens specific to each plant species. Changes in plant species composition also strongly influenced community pathogen load, with communities that lost less disease prone plant species increasing more in pathogen load. Elevated CO₂ increased pathogen load of C₃ grasses, perhaps by decreasing water stress, increasing leaf longevity, and increasing photosynthetic rate, all of which can promote foliar fungal disease. Decreased plant diversity further magnified the increase in C₃ grass pathogen load under elevated CO₂. Nitrogen addition increased pathogen load of C₄ grasses by increasing foliar nitrogen concentration, which can enhance pathogen infection, growth, and reproduction. Because changes in foliar fungal pathogen load can strongly influence grassland ecosystem processes, our study suggests that increased pathogen load can be an important mechanism by which global change affects grassland ecosystems.     

Amino acids as taste stimuli for tsetse flies

Abstract.This paper reports the responses of taste cells on the legs of the blood-feeding tsetse fly Glossina fuscipes fuscipes Newstead 1910 (Diptera: Glossinidae) to twenty protein amino acids and to their mixture as it is present in human sweat. It is investigated whether the mixture is sensed differently than the amino acids singly. The taste cells are most sensitive to phenylalanine (K≈ 1 μm ) and tyrosine; and they respond in a lesser degree to methionine, valine, isoleucine, cysteine, tryptophan, histidine, alanine, and threonine. The amino acids serine, proline, asparagine, arginine, glutamine, lysine, aspartic acid, glutamic acid, and glycine give little or no response even at 10 mm . As the succession of effectiveness of the amino acids appears to be the same for all cells, it is deduced that the flies are unable to discriminate the amino acids by comparing responses across sensory cells. A temporal coding of quality does not seem feasible either. Thus, the properties of the taste cells limit the sense to assessing the intensity of an amino acid stimulus and not its identity. Although several parameters in the response adaptation curves are concentration-dependent, it is suggested that the flies judge intensity of a stimulus only from the first 50 or so milliseconds. Although other studies and these indicate that a multiplicity of binding sites may be responsible for the reception of amino acids, the response to the mixture can be predicted from a no-interaction model, whereby each ligand's access to the binding sites is proportional to its mole fraction. It is argued that this may be the case for more of the naturally occurring mixtures which comprise structurally similar ligands. The responses to the mixture and to phenylalanine alone are equally susceptible to inhibition by sodium chloride. It is suggested that, although discrimination of hosts probably requires another sense, the sense of taste is an excellent tool to detect a host underfoot during the local search for a feeding site.     

Single cell recordings from tsetse (Glossina m.morsitans) antennae reveal olfactory, mechano - and cold receptors

Abstract. Action potentials from individual cells and receptor potentials were recorded from antennae (funiculi) of living tsetse flies, Glossina m.morsitans Westwood, using a 'surface-contact' recording technique. Stimuli were pressure of the electrode, changes in temperature and the vapours of l-octen-3–ol, acetone, 3–methylphenol, dichloromethane and CO₂. Two types of mechano-receptive cells were found. One type only fired action potentials when pressure was increased, the other type continued firing when pressure was maintained. Of the 182 cells tested for their sensitivity to temperature change and the odour stimuli, 19% did not respond to any of the stimuli, suggesting that we are still unaware of cues which may be of import to the flies, and 4% responded to temperature only, increasing and decreasing their activity with a decrement and increment in temperature, respectively. Of the 141 cells which were olfactory receptors, 52% responded to 1-octen-3–ol and 40% were exclusively sensitive to this substance. For 3–methylphenol these percentages were 23 and 18, for acetone 13 and 11, for dichloromethane 12 and 7, for CO₂ 13 and 13. No clustering of cell types in certain areas of the funiculi was found, nor was a difference apparent between the sexes. The majority of the olfactory cells responded by increasing their activities on odour stimulation. Inhibition was found in three cells only, in which spontaneous activity was suppressed on stimulation with 3–methylphenol. Spike responses were phasic-tonic and varied between two extremes: Cells showing relatively rapid cessation of spike activity after the end of stimulation, and cells which continued firing for several seconds or minutes after stimulation. Possible behavioural effects of the activities of the various cell types are discussed.     

Effects of age, sex and hunger on the antennal olfactory sensitivity of tsetse flies

Abstract The effects of age on electroantennogram (EAG) responses were investigated in male and female Glossina morsitans morsitans and comparative studies on the effects of starvation and sex on the EAG in G.m. morsitans, G.austeni, G.tachinoides and G.fuscipes fuscipes were made. Stimuli were the vapours of l-octen-3-ol, 4-heptanone, 3-nonanone and acetone. EAG decreased with age in both sexes of G.m.morsitans , responses in 5-day-old flies already being significantly lower than those in 1-day-old flies. In G.m.morsitans and G.tachinoides , EAG responses of males were higher than those of females. In G.austeni and G.f.fuscipes , however, the reverse was found. With increasing starvation EAG sensitivity increased in both sexes of G.m.morsitans and G.tachinoides. In G.austeni and in G.f.fuscipes no clear effects of starvation were observed. Response spectra of the individual species to the four odour substances did not change with increasing hunger. It is concluded that receptor sensitivity may be modulated depending on the insect's needs. Possible mechanisms of regulation and significance of this modulation are discussed.     

Measurement of leaf longevity of 14 species of grasses and forbs using a novel approach

Grasses and forbs are often classified into separate functional types, although systematic differences between the types have only been verified for a few functional traits. Since leaf longevity has been shown to be a key trait linking plant ecophysiology, whole-plant growth and ecosystem resource cycling, we compared the leaf longevity of 14 species to determine if there were consistent differences between grasses and forbs or other functional classifications, such as persistence of leaves into winter. Leaf longevity was assessed in 6-yr-old monoculture plots in central North America by tagging and sequentially monitoring the phenological states of whole forb leaves and sections of grass leaves. This new approach enables a calculation of leaf longevity unbiased by the manner in which grass leaves grow and provides a more accurate comparison between grasses and forbs. Lupinus perennis had the shortest leaf longevity (4 wk) and Koeleria cristata , Poa pratensis , and Solidago rigida the longest (13–14 wk). Average leaf longevity for the 14 species was c . 9 wk, with no significant differences between grasses and forbs nor between current alternative functional classifications.