Predator detection and avoidance by starlings under differing scenarios of predation risk

Practically all animals must find food while avoiding predators.An individual's perception of predation risk may depend on manyfactors, such as distance to refuge and group size, but it isunclear whether individuals respond to different factors ina similar manner. We tested whether flocks of foraging starlingsresponded in the same way to an increased perception of predationrisk by assessing three factors: (1) neighbor distances, (2)habitat obstruction, and (3) recent exposure to a predator.We found that in all three scenarios of increased risk, starlingsreduced their interscan intervals (food-searching bouts), whichincreased the frequency of their vigilance periods. We thenexamined how one of these factors, habitat obstruction, affectedescape speed by simulating an attack with a model predator.Starlings were slower to respond in visually obstructed habitats(long grass swards) and slower when they had their head downin obstructed habitats than when they had their head down inopen habitats. In addition, reaction times were quicker whenstarlings could employ their peripheral fields of vision. Ourresults demonstrate that different sources of increased riskcan generate similar behavioral responses within a species.The degree of visibility in the physical and social environmentaffects both the actual and perceived risk of predation.     

The influence of fire frequency on arbuscular mycorrhizal colonization in the shrub Dillwynia retorta (Wendland) Druce (Fabaceae)

 Fire regimes have three inter-related components that can affect population dynamics: frequency, intensity and season. However, there has been little effort to study the effects of any of these components on arbuscular mycorrhizas (AM). In order to examine the long-term effects of fire frequency on AM colonization, the roots of Dillwynia retorta were examined at 32 sites supporting Hawkesbury Sandstone vegetation in the Sydney region of southeastern Australia. These sites were representative of the broad-scale variability in fire frequencies with respect to the length and timing of inter-fire intervals found in the Sydney region during the previous 30 years. The length of the shortest inter-fire interval was significantly correlated with total AM colonization, and the length of the longest inter-fire interval was related to the arbuscular colonization. The length of the most-recent inter-fire interval and the time since the shortest inter-fire interval were not related to AM colonization in D. retorta. Furthermore, AM colonization was directly related to the local abundance of the host plant, indicating that the effects of fire frequency on AM colonization are likely to occur indirectly via direct effects on the host plant. Canonical correspondence analysis demonstrated that the presence and abundance of alternate potential hosts had no influence on mycotrophy in D. retorta. Thus, the impact of fire on D. retorta was probably the main factor influencing its mycorrhizal status in relation to fire history. Accepted: 16 October 1998     

Growth and reproduction of the mosquitofish,Gambusia affinis,in relation to temperature and ration level: consequences for life history

The allocation of energy to growth and reproduction, in relation to temperature and food availability, was investigated in laboratory experiments with the mosquitofish,Gambusia affinis. At constant temperature of 20, 25 and 30°C and ad libitum feeding, specific growth rates increased with increasing temperature at 1.7, 3.1 and 3.4% dry mass day⁻¹, respectively. Growth rates in a cycling temperature regime (20–30°C, ) were faster than in a 25°C constant temperature. As temperature increased from 20 to 30°C, mean age at first reproduction decreased from 191 to 56 days and brood size and mass of offspring increased significantly. Interbrood interval was also temperature dependent; estimates at 25 and 30°C for females >1000 mg were 22.6 and 18.6 days, respectively. Interbrood interval could not be calculated at 20°C. Although fitness was highest at 30°C, females at 25°C invested a greater proportion of surplus energy (growth and reproduction) to reproduction (38%) than at 20 (17%) or 30°C (36%) during the 32-week study. Fish at cooler temperatures began reproduction at a smaller size. Where rations were controlled at low, medium, and ad libitum levels, somatic and gonadal growth increased with increasing temperatures and food availability. The proportion of energy invested in reproduction was highest at 25°C for each comparable ration level. Calculated energy budgets indicated that over the 10-week study, 17–22% of the food energy was invested in growth, 0–7% in reproduction, and 75–83% in respiration and excretory losses, depending on feeding and temperature conditions.     

Confidence Intervals on Ratios of Variance Components for the Unbalanced Two Factor Nested Model

The model considered in this article is the two-factor nested unbalanced variance component model: for p = 1, 2, …, P; q = 1, 2, …, Q_p; and r = 1, 2, …, R_pq. The random variables Y_pqr are observable. The constant μ is an unknown parameter, and A_p, B_pq and C_pqr are (unobservable) normal and independently distributed random variables with zero means and finite variances σ²_A, σ²_B, and σ²_C, respectively. Approximate confidence intervals on ?_A and ?_B using unweighted means are derived, where The performance of these approximate confidence intervals are evaluated using computer simulation. The simulated results indicate that these proposed confidence intervals perform satisfactorily and can be used in applied problems.     

Effects of fire frequency on plant species composition of sandstone communities in the Sydney region: Inter-fire interval and time-since-fire

Abstract Fire frequency is the number of fires experienced by a particular community within a given time period. This concept can potentially be resolved into a number of interacting variables, including: time since the most recent fire, the length of the inter-fire intervals, and the variability of the length of the inter-fire intervals. We estimated the effects of these three variables on the floristic composition of 65 samples from dry sclerophyll vegetation with different fire histories in Brisbane Water, Ku-ring-gai Chase and Royal National Parks near Sydney. Our analyses suggest that fire frequency may account for about 60% of the floristic variation among our samples. They confirm the hypothesis that the recent (<30 years) fire frequency produces effects on floristic composition of fire-prone communities that can recognizably be attributed both to the time since the most recent fire and to the length of the intervals between fires. These effects are equal in magnitude but are different in the nature of the floristic variation they are associated with. Increasing time-since-fire is associated with a decline in the evenness of fire-tolerant species, indicating that fewer of these species come to dominate the community in the prolonged absence of fire. Herbs and small shrubs decrease in abundance, while larger shrubs increase in abundance. Inter-fire intervals of decreasing length are associated with a decrease in the evenness of the fire-sensitive species, particularly those large Proteaceae shrubs that often dominate the community biomass in dry sclerophyll shrublands of southeastern Australia. Furthermore, the variation associated with inter-fire intervals is not necessarily solely related to the shortest inter-fire interval, but is related to combinations of inter-fire intervals through time. Thus, increasing variability of the length of the inter-fire intervals is associated with an increase in the species richness of both fire-sensitive and fire-tolerant species, implying that it may be variation of the inter-fire intervals through time that is primarily responsible for maintaining the presence of a wide variety of plant species in a particular community. Our results also suggest that the floristic variation associated with different inter-fire intervals decreases with increasing time-since-fire.     

THE STABILITY OF INTERVAL MATRICES

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Are daughters more costly to produce for Japanese macaque mothers?: Sex of the offspring and subsequent interbirth intervals

Based on a sample of 237 live births recorded over a period of 30 years, a tendency for longer interbirth intervals following the birth of daughters than sons was recognized, in the provisioned Arashiyama troop of Japanese macaques. This may indicate that female infants were more costly to produce than male infants. This tendency seemed to be independent of a mother’s rank.     

Ovarian development, spawning frequency and batch fecundity in Encrasicholina heteroloba (Ruppell, 1858)

A histological and gravimetric analysis of oocyte development in Encrasicholina heteroloba (Ruppell, 1858) indicated that this species spawns serially and has a group-synchronous mode of ovarian development. A six stage maturity scale, based on both external morphology and oocyte composition, was proposed to classify ovarian development in E. heteroloba . The incidence of females with hydrated oocytes and post-ovulatory follicles in samples from two regions; the south Java Sea and Roviana lagoon, in the Solomon Islands were used to estimate spawning frequency. Estimates of mean inter-spawning intervals ranged from around 2 days in fish from Roviana lagoon to up to 16.7 days in fish from the south Java Sea. Batch fecundity was determined from the number of oocytes in the largest oocyte size class in ripe stage ovaries. Batch fecundity was related to size and was significantly greater for a given size in the Roviana lagoon population ( F = 0.081 × L ^4.89, Roviana population; F = 1.682 × L ^2.83, Jepara population).