Combat and communication in the Everglades pygmy sunfish

Changes in population density and resource patterning affect the aggressive behaviour of pygmy sunfish (Elassoma everglaidei) in many ways. Increases in density significantly reduced the proportion of fights directly over clumped prey, the likelihood of initiators winning contests or acquiring clumped prey, and the length of contests. Increases in prey dispersion also reduce the proportion of resource fights in low-and high-density populations, the greater tendency for subordinates to initiate fights directly over resources, the likelihood of initiators acquiring a contested prey item in lowdensity populations, and the length of contests. Such increases, however, increase the likelihood of initiators winning contests at moderate competitive levels, and increase the effectiveness of rapid sequential communication in populations that abandon territoriality. In addition, some of these findings, such as the inverse relationship between contest length and both prey dispersion and rank differential, are consistent with predictions of cost-benefit models of fighting behaviour, namely, that escalated contests become more likely as asymmetries in fighting ability decrease and asymmetries in resource valuation simultaneously increase.     

Population diversity of the pygmy shrew

A morphogenetic comparison of pygmy shrews (Sorex minutus Linnaeus, 1766) from two geographical regions in the North of European Russia, located 1000 km apart, showed a relatively high stability in all of their parameters.     

Constant density and stable territoriality in some tropical insectivorous birds

Summary Four species of understory antbirds (Formicariidae: Myrmotherula fulviventris, M. axillaris, Microrhopias quixensis, and Thamnophilus punctatus) had stable populations over eight rainy seasons on Barro Colorado Island, Panama. The co-defended territories of M. fulviventris and Microrhopias quixensis, were essentially identical from year to year on our intensive study site, despite a moderate turnover of territory owners. The location of the territories of T. punctatus was also similar between years. This stability occurred in the face of considerable annual variation in the survivorship of adult M. fulviventris and T. punctatus. This variation was not significantly correlated with patterns of rainfall. Stable territoriality has rarely been reported from relatively-short-lived insectivorous birds. The annual production of young was significantly variable only in M. axillaris. Because BCI is an island comprised of one habitat (tropical forest) and so supports a closed population of antbirds, and because it is unlikely that natality equaled mortality on our study site during the entire eight years of the study, we suggest that these breeding populations are socially regulated at a constant level below the limits directly set by food supply.     

Variation in resource abundance affects diet and feeding morphology in the pumpkinseed sunfish (Lepomis gibbosus)

Summary Growth responses and accumulation of N and P were studied in two pygmy south-west Australian species of Drosera following supplementary feeding of arthropods (collembolans, Hypogastrura vernalis and fruit flies, Drosophila melanogaster) and/or a balanced mineral nutrient supplement (N as nitrate) via the roots. One feeding experiment used glasshouse-raised germlings from vegetative propagules (gemmae) of the perennial Drosera closterostigma, the other three (two on D. closterostigma and one on the annual D. glanduligera) involved natural populations engaging in natural captures of indigenous prey. All experiments recorded highly significant increases in plant dry matter, N and P (all plant age groups) and in reproductive performance (adult plants only) from artificial feeding of arthropods, but no apparent benefits from minerals alone or additive effects of minerals above that due to insects. Unresponsiveness to mineral nutrients was suggested to relate to inability of the species to use nitrate, while up to three-fold growth and nutrient uptake response to insects indicated that growth of natural populations might be severely limited by inadequate catches of prey. It is concluded that the highly nutrient-poor conditions typical of the habitat of pygmy species of Drosera may have promoted marked specialization towards carnivory and an attendant decline in ability to utilize soil-derived sources of nutrients.     

Population density, phenotype and mortality in the grasshopper Chorthippus brunneus

ABSTRACT.

• 1 The patterns of mortality in laboratory populations of the grasshopper Chorthippus brunneus Thunberg are examined at a range of densities and at different stages of nymphal development with particular reference to the phenotypes of nymphs that die or survive.
• 2 Total mortality during nymphal development rises from density-independence at the lowest densities to approximately compensating density-dependence at the highest densities.
• 3 The greatest proportion of total mortality occurs in the first two instars. The stage broadly covering the second instar is the ‘key-stage’ determining adult density. The mortality during these stages is largely density-independent. There is relatively little mortality during stages broadly covering the final instar, but it is density-dependent and also plays a significant part in determining final adult density.
• 4 As the mortality becomes more density-dependent, either with density itself or with cohort age, the smallest individuals become increasingly disproportionately prone to mortality.
• 5 Males, which are smaller, are more susceptible to mortality than females, but their susceptibility relative to their size is less than that of females.

     

Defence, intrusion and the evolutionary stability of territoriality

Territorial behaviour can only be adaptive if its costs are outweighed by its benefits. Territorial individuals incur costs by defending their territories against intruders. Usually these intruders are assumed to be non-territorial floaters attempting to take over the whole territory or neighbours trying to extend the borders of their own territory. We instead investigate how costs and benefits of territorial behaviour are affected by neighbours which invade to steal resources on a territory.We show analytically that in the absence of defence intrusion into neighbouring territories always pays and that even if territories are defended intrusion levels can still be high. Using a more detailed simulation model we find that territory defence usually disappears from the population even if owners have a strong advantage over intruders in terms of fighting costs or foraging efficiency. Defence and thus territoriality can only be evolutionarily stable if fighting costs for the intruder relative to the productivity of the territory are very high or if crossing the borders between territories carries additional costs.Our results show that stealing of resources by neighbours can have a considerable effect on the evolutionary stability of territory defence and thus territoriality itself. A more mechanistic model of territorial behaviour is needed to incorporate these kinds of mechanisms into a general theory on the evolution of territoriality.     

Lateral root initiation in Arabidopsis: developmental window, spatial patterning, density and predictability

BACKGROUND AND AIMS: The basic regulatory mechanisms that control lateral root (LR) initiation are still poorly understood. An attempt is made to characterize the pattern and timing of LR initiation, to define a developmental window in which LR initiation takes place and to address the question of whether LR initiation is predictable. METHODS: The spatial patterning of LRs and LR primordia (LRPs) on cleared root preparations were characterized. New measures of LR and LRP densities (number of LRs and/or LRPs divided by the length of the root portions where they are present) were introduced and illustrate the shortcomings of the more customarily used measure through a comparative analysis of the mutant aux1-7. The enhancer trap line J0121 was used to monitor LR initiation in time-lapse experiments and a plasmolysis-based method was developed to determine the number of pericycle cells between successive LRPs. KEY RESULTS: LRP initiation occurred strictly acropetally and no de novo initiation events were found between already developed LRs or LRPs. However, LRPs did not become LRs in a similar pattern. The longitudinal spacing of lateral organs was variable and the distance between lateral organs was proportional to the number of cells and the time between initiations of successive LRPs. There was a strong tendency towards alternation in LR initiation between the two pericycle cell files adjacent to the protoxylem poles. LR density increased with time due to the emergence of slowly developing LRPs and appears to be unique for individual Arabidopsis accessions. CONCLUSIONS: In Arabidopsis there is a narrow developmental window for LR initiation, and no specific cell-count or distance-measuring mechanisms have been found that determine the site of successive initiation events. Nevertheless, the branching density and lateral organ density (density of LRs and LRPs) are accession-specific, and based on the latter density the average distance between successive LRs can be predicted.     

Population density, phenotype and reproductive output in the grasshopper Chorthippus brunneus

ABSTRACT. 1. The effects of population density on the reproductive output of individual female grasshoppers ( Chorthippus brunneus Thunberg) and the relationships between reproductive output and component elements of each female's phenotype, were investigated.
2. Reproductive output was primarily determined by the rate of egg-pod production. Increased density led to significant reductions in the rate of egg-pod production and reproductive output.
3. Exoskeleton size (reflecting nymphal experience) was positively correlated with clutch size at both high and low densities, but condition (reflecting adult experience) showed no such correlation. At high density, exoskeleton size was more strongly correlated with the rate of egg-pod production and reproductive output than condition. It is concluded that the nymphal aspects of size are more important than the adult aspects.
4. At low density, females with small exoskeletons compensated for their smaller clutches by producing egg-pods at a faster rate. Thus, there was no overall relationship between reproductive output and any of the phenotypic characteristics.
5. The effects of competition were weakly asymmetric in high density populations. The weakness of the asymmetry suggests scramble-like interactions for resources. It is concluded that not only are the effects of competition influenced by individual differences, but also that competition may reveal differences that would not otherwise be apparent.     

Pair territoriality in the longnose filefish,oxymonacanthus longirostris

The longnose filefish,Oxymonacanthus longirostris, usually lives in heterosexual pairs, the male and female swimming together and sharing the same territory. Pair territoriality in the species was examined in detail in relation to sexual differences in territorial defense activities. Rigorous pair territoriality was maintained only during the breeding season, although pairs used their home ranges exclusively to a certain extent, during the non-breeding season. The frequency of aggression against other conspecific pairs in the breeding season was higher than in the non-breeding season. Agonistic interactions appear to be over both mates and food resources, the strict pair territoriality in the breeding season possibly being due to mutual mate guarding. In intraspecific aggressive interactions, males usually led their partner females when attacking intruders. The feeding frequency of males was much lower than that of females in the breeding season. Mate removal experiments indicated that females could not defend their original territories solitarily and their feeding frequency decreased. Conversely, males could defend territories solitarily without a decrease in feeding frequency. These results suggest that males contribute most to the defense of the pair territory, with females benefiting from territorial pair-swimming with their partner males.     

ESS analysis of mechanistic models for territoriality: the value of scent marks in spatial resource partitioning

In this paper elements of game theory are used to analyse a spatially explicit home range model for interacting wolf packs. The model consists of a system of nonlinear partial differential equations whose parameters reflect the movement behavior of individuals within each pack and whose solutions describe the patterns of space-use by each pack. By modifying the behavioral parameters, packs adjust their patterns of movement so as to maximize their reproductive output. This involves a tradeoff between maximizing prey intake and minimizing conflict with neighbors. Evolutionarily stable choices of the behavioral parameters yields territories that are immune to invasion by groups with alternate behaviors.     

Variation in plant stem density and its effects on foraging success of juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish, Lepomis macrochirus, are known to use beds of aquatic vegetation as a refuge from predators. This study examines the effects of increasing plant stem density on juvenile bluegill foraging. Three stem densities (100, 250 and 500 stems m⁻²), varying in their refuge potential for bluegills from predators, were tested. Results demonstrate that stem densities chosen as a refuge from predation (i.e. 500 stems m⁻²) significantly reduced bluegill foraging success and increased time required to capture prey. Therefore, juvenile bluegills seeking safety in vegetation may be faced with a trade-off between foraging success and effective refuge from predation when choosing among plant stem densities.     

Foraging performance of diet-induced morphotypes in pumpkinseed sunfish (Lepomis gibbosus) favours resource polymorphism

Morphological plasticity can influence adaptive divergence when it affects fitness components such as foraging performance. We induced morphological variation in pumpkinseed sunfish (Lepomis gibbosus) ecomorphs and tested for effects on foraging performance. Young-of-year pumpkinseed sunfish from littoral and pelagic lake habitats were reared each on a 'specialist diet' representing their native habitat-specific prey, or a 'generalist diet' reflecting a combination of native and non-native prey. Specialist and generalist diets, respectively, induced divergent and intermediate body forms. Specialists had the highest capture success on their native prey whereas generalist forms were inferior. Specialists faced trade-offs across prey types. However, pelagic specialists also had the highest intake rate on both prey types suggesting that foraging trade-offs are relaxed when prey are abundant. This increases the likelihood of a resource polymorphism because the specialized pelagic form can be favoured by directional selection when prey are abundant and by diversifying selection when prey resources are restricted.