Toward a mechanistic understanding and prediction of biotic homogenization

The widespread replacement of native species with cosmopolitan, nonnative species is homogenizing the global fauna and flora. While the empirical study of biotic homogenization is substantial and growing, theoretical aspects have yet to be explored. Consequently, the breadth of possible ecological mechanisms that can shape current and future patterns and rates of homogenization remain largely unknown. Here, we develop a conceptual model that describes 14 potential scenarios by which species invasions and/or extinctions can lead to various trajectories of biotic homogenization (increased community similarity) or differentiation (decreased community similarity); we then use a simulation approach to explore the model's predictions. We found changes in community similarity to vary with the type and number of nonnative and native species, the historical degree of similarity among the communities, and, to a lesser degree, the richness of the recipient communities. Homogenization is greatest when similar species invade communities, causing either no extinction or differential extinction of native species. The model predictions are consistent with current empirical data for fish, bird, and plant communities and therefore may represent the dominant mechanisms of contemporary homogenization. We present a unifying model illustrating how the balance between invading and extinct species dictates the outcome of biotic homogenization. We conclude by discussing a number of critical but largely unrecognized issues that bear on the empirical study of biotic homogenization, including the importance of spatial scale, temporal scale, and data resolution. We argue that the study of biotic homogenization needs to be placed in a more mechanistic and predictive framework in order for studies to provide adequate guidance in conservation efforts to maintain regional distinctness of the global biota.     

The matrix-tolerance hypothesis: an empirical test with frogs in the Atlantic Forest

The matrix-tolerance hypothesis suggests that the most abundant species in the inter-habitat matrix would be less vulnerable to their habitat fragmentation. This model was tested with leaf-litter frogs in the Atlantic Forest where the fragmentation process is older and more severe than in the Amazon, where the model was first developed. Frog abundance data from the agricultural matrix, forest fragments and continuous forest localities were used. We found an expected negative correlation between the abundance of frogs in the matrix and their vulnerability to fragmentation, however, results varied with fragment size and species traits. Smaller fragments exhibited stronger matrix-vulnerability correlation than intermediate fragments, while no significant relation was observed for large fragments. Moreover, some species that avoid the matrix were not sensitive to a decrease in the patch size, and the opposite was also true, indicating significant differences with that expected from the model. Most of the species that use the matrix were forest species with aquatic larvae development, but those species do not necessarily respond to fragmentation or fragment size, and thus affect more intensively the strengthen of the expected relationship. Therefore, the main relationship expected by the matrix-tolerance hypothesis was observed in the Atlantic Forest; however we noted that the prediction of this hypothesis can be substantially affected by the size of the fragments, and by species traits. We propose that matrix-tolerance model should be broadened to become a more effective model, including other patch characteristics, particularly fragment size, and individual species traits (e.g., reproductive mode and habitat preference).     

Intraspecific variability in rhynchonellid brachiopods: test of a competition hypothesis

The variability in the plicae of the central fold of eight rhynchonellid brachiopods – Lepidocyclus capax (Ordovician), Stegerhynchus whitii (Silurian), Megalopterorhynchus baldwini (Devonian), 'Camarotoechia' purduei (Mississippian), Wellerella osagensis (Pennsylvanian), Leiorhynchus weeksi (Permian), 'Pugnoides'; Iriassicus (Triassic) and Tetrarhynchia sp. (Jurassic) – is inversely related to the diversity of brachiopods within the faunal assemblage. Explanations for the phenotypic variability due to taxonomic splitting, sexual polymorphism or ontogenetic development are precluded or unsupported by the morphologic analysis, although a small percentage of the morphologic variability can be ascribed to ecophenotypic differentiation by environmental stimuli.
After considerations of the abiotic influences of time, geographic location and isolation, and environmental stability and homogeneity, most of the morphologic variability in these brachiopods is attributed to the biotic influence, namely competition. Other proposed relationships, i.e. population abundance, sample size, shell size or ribbing pattern and intraspecific variability are not statistically significant.     

Foliar damage, parasitoids and indirect competition: a test using herbivores of birch

ABSTRACT.

• 1 The hypothesis that leaf-chewing caterpillars and leaf miners indirectly compete via the influence of foliar damage on parasitoids was experimentally tested using the leaf miner Coleophora serratella (L.) and a complex of folivorous caterpillars on birch.
• 2 Separate experiments tested the effects of manipulating artificial or caterpillar-induced leaf damage on parasitism of leaf miners at several spatial scales.
• 3 Parasitism of C. serratella by specialist and generalist parasitoids was independent of the extent of either collateral or leaf-miner damage, whether experimentally or naturally induced.
• 4 Support in the literature for the hypothesis is restricted to a single case, and only on a limited spatial scale; thus, the idea that collateral damage directs parasitoid-mediated competition among herbivores remains highly speculative.

     

Interference competition between coyotes and raccoons: a test of the mesopredator release hypothesis

Some predator species appear to conform to the mesopredatorrelease hypothesis (MRH), in which larger predators help limitpopulations of smaller predators. This hypothesis has been usedto explain the possible relationship between coyotes, mesopredators,and resultant cascades involving nonpredators. However, relationshipsbetween coyotes and noncanid mesopredators are poorly understood,and predictions from the MRH have rarely been rigorously tested.We monitored sympatric raccoon and coyote populations to assess2 predictions derived from the MRH: coyote predation is an importantcause of mortality in raccoon populations or raccoons avoidareas used by coyotes. Between March 2000 and September 2001,we recorded 3553 locations for 27 radio-collared raccoons and1393 locations for 13 coyotes captured on the Max McGraw WildlifeFoundation in Illinois, USA. No raccoon mortality from coyotepredation was observed during the study, and raccoon survivalwas >0.7 each season. All raccoon 95% home ranges exhibitedoverlap with 95% coyote home ranges in each season. The meanproportion of raccoon locations within 95% coyote home rangesdid not vary by sex but did vary by season. Raccoon overlapof coyote core areas varied considerably among individuals withinseasons, ranging from 0% to 83%. However, 45% of raccoons had<10% overlap with coyote core areas, whereas only 14% ofraccoons exhibited >50% overlap. Mean overlap with core areasdid not vary by season or sex. For those raccoons with homeranges overlapping coyote core areas, mean proportion of observedraccoon locations within coyote core areas was generally greaterthan the mean proportion of random locations. Scent-stationexperiments failed to document raccoon avoidance of specificsites that had been marked with coyote urine. We did not findsupport for a mortality prediction or avoidance prediction tosupport MRH with regard to raccoons and coyotes. These resultssuggest that relationships among mammalian predators may notbe simply dictated by body size, particularly for species outsidethe Canidae.     

Social competition and plasma testosterone profile in domesticated canaries: an experimental test of the challenge hypothesis

The challenge hypothesis predicts that plasma testosterone (T) concentration is high when male-male competitions are high and decreases when males are engaged in paternal care. In monogamous species, T concentration increases at the beginning of the breeding period and decreases after egg laying. According to the challenge hypothesis, increasing competition should also lead to T increase. The aim of our study was to test this hypothesis. In a first experiment, we measured the T profile of domesticated canaries housed with their mate in separated cages without competition. In a second one, we created a competition by housing male and female domestic canaries together (in an aviary) and emphasized this competition by limiting food access. We also studied social status effect. Our results showed no effect of social status in both sexes and no differences in female's T concentration. Concerning males, we obtained a clear monogamous T profile from the ones housed in a low competition situation and a polygamous profile from the others housed in high competition situation. Thus, our results support the hypothesis of the plasticity of the mechanisms controlling T concentration according to environmental conditions.     

Comparative demographic impacts of `info-chemicals' and exploitative competition: an empirical test using Daphnia magna

1. Crowding `info-chemicals', metabolites released into the environment that act as signals to conspecifics as well as other organisms, have often been shown to cause phenotypic plasticity in cladoceran life-history traits including morphology, reproductive strategy and sex induction. Effects on population demography and diversity, however, have not been examined directly.
2. Replicate laboratory populations of the zooplankter, Daphnia magna, were started with 250 juveniles and allowed to develop for 8 weeks in 10 L flow-through aquaria with food levels of 1 mg CL^–1. Two crowded water treatments from different clonal sources were compared with untreated water as a control.
3. The presence of crowding chemicals affected initial population structure, through reduction in parthenogenetic female body lengths, brood sizes and juvenile densities. However, the timing of population density maxima and subsequent population structure, clonal composition and diversity were similar among all treatments.
4. Clonal richness remained slightly higher in untreated populations because of increased prevalence of rare clones.
5. Exploitation competition had greater impact on population structure than the relative concentration of crowding chemicals, indicating that density dependent effects were mediated primarily by competition for common resources.
6. Crowding info-chemicals may play a greater role in community structure than in single species population structure because of taxonomic diversity in species' response that may be unrelated to the source species density.     

Movement of modulator in maize: a test of an hypothesis

A model of Modulator movement (Greenblatt 1968) from one chromosomal site to another requires that all movements produce potential twin mutations within the affected cell lineage. From this model the prediction would be that (1) untwinned red and untwinned light-variegated sectors within the pericarp of medium-variegated maize must occur in equal frequency even though partners become lost, during ear morphogenesis, to the final pericarp tissue, and that (2) among the backcross progeny of a homozygous P(rr)Mp individual mated with P(wr)/P(wr) pollen, red offspring and light-variegated offspring would occur in equal frequency. Both expectations have been realized and herein reported.     

Resource competition between two rotifer species (Brachionus rubens and B.calyciflorus): an experimental test of a mechanistic model

A mechanistic model of competition on a single resource wastested experimentally with two freshwater rotifers, Brachionusrubens and B.calyciflorus, both grown on the alga Chlorellapyrenoidosa. Using open culture systems for each species wemeasured: (i) the resource-saturated exponential growth rate,µ_max, and (ii) the relationship between specific growthrate, µ, at steady-state and the residual algal concentrationover a range of system turnover rates, or dilution rates, D.The µ_max of B.calyciflorus was {small tilde}60% higherthan B. rubens. These results were then used to construct agraphical model for predicting the victor in interspecific competitionbetween the two rotifers. Since the two resource-dependent growthrates crossed, one species, B.calyciflorus, was predicted tobe the victor at a high D while B. rubens was predicted as thevictor at low D. Finally, the outcome of competition was determinedfor two turnover rates. As predicted by the graphical competitionmodels, B.calyciflorus was the dominant species at rapid D (0.029h^–1) and B.rubens was dominant at slow D (0.0044 h^–1).These studies support recent conclusions that mechanistic competitionmodels may be applied to predict dominant species from a prioriinformation on growth potential and resource levels, which isnot possible with traditional Lotka-Volterra models.     

Mechanical advantages of the Neanderthal thumb in flexion: a test of an hypothesis

It has been proposed that the pollical phalangeal length proportions of the Neanderthals provided them with a greater mechanical advantage relative to recent humans for their pollical flexor muscles in power grips across the interphalangeal (IP) joint at the expense of the mechanical advantage of those pollical flexor muscles in precision grips at the finger tip. To test these related hypotheses, we compared the pollical load arm dimensions (phalanx lengths) to power arm dimensions (dorsopalmar articular heights) for the European and Near Eastern Neanderthals and for European and Amerindian samples of recent humans. It was found, initially, that the proximal articular height of the pollical distal phalanx is a poor predictor of the power arm at the IP articulation, even though the proximal articular height of the pollical proximal phalanx was an adequate indicator of the power arm size at the metacarpophalangeal (MCP) joint. In addition, differences in distal pollical ulnar deviation at the IP joint appeared to make little difference in the mechanical advantage comparisons. More importantly, the relative shortness of Neanderthal proximal pollical phalanges and the relative lengthening of their distal pollical phalanges was confirmed, and it was determined that, despite some minor differences in articular dimensions between Neanderthals and recent humans, these pollical phalangeal length contrasts translated into significant differences in mechanical advantages for the flexor muscles across the MCP and IP articulations.     

An empirical test of bet-hedging polyandry hypothesis in the field cricket Gryllus bimaculatus

Journal of Ethology - Theory shows that polyandry (mating with multiple males within a reproductive season) works as bet-hedging to increase the geometric mean fitness (GMF) of polyandrous genotype...     

An empirical test of the `predictability' hypothesis for the evolution of viviparity in reptiles

Abstract Viviparity (live‐bearing) has evolved from oviparity (egg‐laying) >100 times in reptile phylogeny, but the selective forces responsible remain unclear. Tinkle & Gibbons (1977) proposed that prolonged uterine retention of eggs (leading ultimately to viviparity) is favoured by natural selection when it allows the reproducing female to better predict the incubation conditions that will occur in alternative potential nest‐sites, and hence select the optimal site in which to deposit her eggs. This ingenious hypothesis has never been tested empirically. Over a 7‐year period, I monitored temperatures inside 124 natural nests of egg‐laying scincid lizards at three different elevations in the Brindabella Range of south‐eastern Australia. As a measure of thermal predictability, I used correlation coefficients from comparisons of temperatures early vs. later in incubation among nests within each site. Both the mean and standard deviation of nest temperatures were examined in this way for each week through incubation. I performed these calculations under two models: one where the female assesses nest temperatures at the time of oviposition only, and one where she monitors temperatures constantly from the usual oviposition date until the actual time of laying. These analyses falsified two major assumptions of the ‘predictability’ hypothesis. First, nest temperatures at higher elevations were no less predictable than were those at lower elevations; instead, predictability was high in all situations. Secondly, a longer delay before oviposition decreased rather than increased the predictability of thermal conditions during subsequent incubation. I conclude that critical assumptions of the ‘predictability’ hypothesis are not supported in this study system.     

Toward an understanding of the structural basis of translation

The recently solved X-ray crystal structures of the ribosome have provided opportunities for studying the molecular basis of translation with a variety of methods including cryo-electron microscopy - where maps give the first glimpses of ribosomal evolution - and fluorescence spectroscopy techniques.     

Correction to: An empirical test of bet-hedging polyandry hypothesis in the field cricket Gryllus bimaculatus

Journal of Ethology -     

Toward an individual-level understanding of vigilance: the role of social information

A gregarious lifestyle affords the benefit of collective detectionof predators through the many-eyes effect. Studies of vigilanceare generally concerned with exploring the relationship betweenvigilance rates and group size. However, a mechanistic understandingof the rules individual animals use to achieve this group-levelbehavior is lacking. Building on a previous modeling approach,we suggest that individuals reconcile their own private informationagainst the social information they receive from their groupmates in order to decide whether to feed or be vigilant at anyone time. We present a novel modeling approach utilizing a Markovchain Monte Carlo process to describe the transition betweenvigilant and nonvigilant states. Many of our assumptions arebased qualitatively on recently published experimental observations.We vary the amount of social information and the fidelity withwhich individuals process this information and show that thishas a profound effect on the individual vigilance rate, theindividual vigilant bout length, and the proportion of vigilantindividuals at any one time. A wide range of group-level vigilancepatterns can be obtained by varying simple behavioral characteristicsof individual animals. We find that generally, increasing theamount of, and sensitivity to, social information generatesa more cooperative vigilance behavior. This model potentiallyprovides a theoretical and conceptual framework for examiningspecific real-life systems. We propose analyzing individual-baseddata from real animals by considering their group to be a connectednetwork of individuals, with information transfer between them.     

Sexual interference in the Golden snub-nosed monkey (Rhinopithecus roxellana): a test of the sexual competition hypothesis in a polygynous species

Sexual interference (SI), which is defined as any disturbance directed to a mating pair by other individuals, has been reported in several primate species. It is widely suggested that successful harassers experience improved mating success by increasing their access to reproductive partners as well as by reducing the mating success of rivals. Although theories of primate sexual conflict highlight male intra-sexual mating competition, females also are reported to actively disrupt copulations between mating partners. In this study, we investigated SI in a multilevel troop of Golden snub-nosed monkeys (Rhinopithecus roxellana) inhabiting the Qinling Mountains, China. Behavioral observations of 11 one-male units (OMU) that comprised the multilevel troop were conducted from September 2007 to May 2008. During this period 17.1% of 652 documented intra-OMU sexual encounters were characterized either by mild or aggressive forms of harassment. Sexual harassment was typically performed by a single individual (91.9%), and in 75.7% of cases the harasser was an adult or sub-adult female. The frequency of female harassment was positively correlated with the number of adult and sub-adult females residing in an OMU, and resulted in a significant decrease in matings ending in ejaculation. We found that the amount of SI a female received was not a significant predictor of her reproductive success. However, females who conceived during the mating season directed higher levels of harassment at other females than females who did not conceive. We evaluate the strength of the sexual competition hypothesis and the hormonally modulated aggression hypothesis in explaining patterns of SI in female Golden snub-nosed monkeys.