J.S. Bradley

Long-term population studies of seabirds

Long-term studies of seabirds, some now 30-40 years old, have begun to reveal significant age-related changes in the survival and reproduction o f these long-lived animals. Evidence for density-dependent regulation of seabird numbers, however, remains sparse whereas unpredictable, disastrous breeding years may be an important influence. Critical evaluation will require better data on (1) the extent of movements of seabirds between colonies, (2) the characteristics of those individuals that contribute disproportionately to the next generation, and (3) the importance of year and/or cohort effects on population processes.

The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. II. A field experiment

The hypothesis that predation by feral cats and introduced foxes reduces population sizes of small, native vertebrates was supported by results of a predator-removal experiment at Heirisson Prong, a semi-arid site in Western Australia. The methods of control used against cats and foxes to protect native mammals reintroduced to Heirisson Prong produced three broad ‘predator zones’: a low-cat and low-fox zone, where foxes were eradicated and spotlight counts of cats declined after intensive cat control; a high-cat and low-fox zone where spotlight counts of cats increased three-fold after foxes were controlled; and a zone where numbers of cats and foxes were not manipulated. Small mammals and reptiles were monitored for one year before and three years after predator control began. Captures of small mammals increased in the low-cat and low-fox zone, but where only foxes were controlled captures of small mammals declined by 80%. In the absence of cat and fox control, captures of small mammals were variable over the sampling period, lower than where both cats and foxes were controlled, yet higher than where only foxes were controlled. The capture success of reptiles did not appear to be related to changes in predator counts. This study presents the first experimental evidence from mainland Australia that feral cats can have a negative impact on populations of small mammals.

Seasonal and spatial changes in the larval fish fauna within a large temperate Australian estuary

A total of 66814 fish larvae, representing 37 families and 74 species, were collected in samples taken monthly between January 1986 and April 1987 from 13 sites located at frequent intervals throughout the large Swan Estuary in south-western Australia. The Gobiidae was the most abundant family, comprising 88.2% of the total number of larvae, followed by the Clupeidae (3.4%), Engraulididae (2.9%) and Blenniidae (1.0%). The most abundant species were Pseudogobius olorum (53.3%), Arenigobius bifrenatus (31.2%) and Engraulis australis (2.9%). Abundance of fish larvae in the lower, middle and upper regions of the estuary each reached a maximum between mid-spring and early summer, 2 to 4 mo before the attainment of maximum temperatures. Larvae of species such as Nematalosa vlaminghi and Apogon rueppellii were collected only between November and February, whereas those of others such as P. olorum, E. australis and Leptatherina wallacei were present over many months. The times and locations of capture of larvae have been related to the distribution and breeding periods of the adults of these species. The mean monthly number of species was far greater in the lower than upper estuary (14.7 vs 2.7), whereas the reverse was true for mean monthly concentration (42 vs 197 larvae per 100 m3). Classification, using the abundance of each of the 74 species recorded at the different sites, showed that the composition of the larval fish fauna in the lower, middle and upper estuary differed markedly from each other. Most larvae caught in the lower estuary belonged to marine species, whereas those in the upper estuary almost exclusively represented species that spawn within the estuary. The fact that the larvae of the 59 species of marine teleosts recorded during this study were restricted mainly to the lower estuary, and yet contributed only 6.2% to the total numbers for the whole estuary, helps to account for the relatively high species diversity in this region. The lack of penetration of many of these larvae beyond the first 12.5 km of the estuary presumably reflects the weak tidal effect in the wide basins of the middle estuary and saline regions of the tributary rivers. The larvae of the 13 teleosts that typically spawn within the estuary contributed 93.8% to the total numbers of larvae. Most of these estuarine-spawned larvae belong to teleosts that deposit demersal eggs and/or exhibit parental care (egg-guarding and oral and pouch-brooding), characteristics which would maximize their chances of retention within the estuary.

Age-Dependent Survival of Breeding Short-Tailed Shearwaters Puffinus tenuirostris

(1) A small population of short-tailed shearwaters has been monitored each year since 1947. Survival rates have been calculated from the disappearance of marked individuals and related to breeding experience. (2) Shearwaters breeding for the first time had a relatively high rate of mortality. This rate then decreased considerably and remained low for about ten years before increasing substantially in older birds. (3) The sex of a shearwater, the period when it was active in the colony and whether or not it had been born in the colony had no major impact on the length of its breeding lifetime. (4) Increasing offspring production was associated with a decreasing mortality rate early in a shearwaters breeding career but not in mid-life. Later in its career, higher offspring production was associated with an increasing rate of mortality. (5) The log proportional hazard regression model and GLIM algorithm used attempted to avoid some of the problems inherent in analysing age-dependent survival in long-lived species.

The influence of sampling method on the classification of wetland macroinvertebrate communities

Macroinvertebrate communities sampled by a corer, plankton net and sweep net from five wetlands on the Swan Coastal Plain were compared. The composition of the fauna collected in sweeps and tows was generally similar and differed from that collected in the cores. Cores caught fewer species than tows and sweeps at all wetlands and did not capture fast swimming hemipterans or less abundant taxa. The highest species richness was recorded in sweep samples in four out of the five wetlands. Classification (TWIN-SPAN) and ordination (SSH) of the samples collected in sweeps and tows gave good separation of the wetlands, whereas classification of core samples did not. Coring appeared to be the least suitable sampling method for describing the major components of the macroinvertebrate communities of these wetlands. Plankton tows were useful if the time available for sorting was limited as these samples were free of sediments and generally gave similar results to those obtained with sweeps. Sweeps appeared to be the most useful method for a large classification study as they collected more species and resulted in the best discrimination amongst wetlands.

The effect of parental condition on egg-size and reproductive success in short-tailed shearwaters Puffinus tenuirostris

Eggs were exchanged between 50 pairs of shorttailed shearwaters Puffinus tenuirostris on Great Dog Island, Bass Strait, Australia, in an attempt to distinguish the intrinsic effects of egg-size from any effects stemming from differential quality of parental care. At 64 “experimental” nests, large and small eggs were exchanged whereas at 36 “control” nests, eggs of equivalent, medium, size were exchanged. Egg-size appeared independent of maternal effects. In both groups, hatching and fledging success were independent both of eggsize and of the body condition of the attending parents. This suggests that breeding success in these birds may be more closely related to the behavioural traits of parents than to physiological factors.

Digging and soil turnover by a mycophagous marsupial

The woylie Bettongia penicillata is a small (1 kg) kangaroo-like marsupial that digs to obtain the fruiting bodies of fungi. The number of woylies in a 60 ha area of sclerophyll woodland in south-western Australia was estimated using mark-recapture at 3 month intervals over 3 successive years. The number of new diggings by woylies, determined at the same intervals, allowed an assessment of the rate of digging per individual. This varied three-fold from 38 to 114 diggings per individual per night, with no consistent seasonality. On average, each woylie displaced 4.8 tonnes of soil annually.

The Relationship of Pair-Bond Formation and Duration to Reproductive Success in Short-Tailed Shearwaters Puffinus tenuirostris

A substantial correlation exists between the breeding ages of short-tailed shear-waters in pair-bonds. Much of this correlation can be explained by prolonged pair-bonds between the same mates, and the predominant availability of unpaired, inexperienced birds. Using Monte Carlo simulation, a comparison of the differences in age observed between pair members with those produced assuming random pairing of available birds, indicated some weak assortative mating on the basis of breeding age. The probability of breeding success depended both upon the breeding ages of the partners and upon the length of time they had bred together; the latter appeared the stronger effect. In the first year of a pair-bond, the breeding age of the female had a much stronger effect than that of the male in determining the probability of reproductive success.

The influence of environmental variables on the density of larval lampreys in different seasons

SummaryThe objective of the study was to identify a subset of a set of twenty environmental variables which could explain variations in the density of larval lampreys (Geotria australis) in a south-western Australian stream. Generalised linear modelling, assuming Poisson distributions for the larval counts, led to a different model for each of the four seasons, with variations in larval density being explained in each season by a combination of between five and eight environmental variables. The influence of stream region also had to be taken into account in the model for winter.Four environmental variables (substrate organic material and chlorophyll a, macrophyte roots and low-angle shading) were present in three of the four seasonal models. A further six variables (water depth, substrate depth and profile, medium-sized sands, light intensity, and the presence of an eddy) were each found useful for two models. Two variables (current velocity and substrate profile) were each retained in one model. Eight of the twenty variables were not required for any of the seasonal models. The importance of organic material, shade, eddies, current velocity substrate particle size and a sufficient depth of substrate in our models agree with the largely subjective assessments of larval lamprey habitats made in the field by many previous workers for other lamprey species in diverse geographical localities.Our finding that larval density increased with increases in organic material and unicellular algae in the substrate and with shade, contrasts with the results of a different model based on data collected in a northern European stream. These differences can be related to our use of a more rigorous and comprehensive sampling regime and a more appropriate form of statistical analysis.

The relationship between animal foraging and nutrient patchiness in south-west Australian woodland soils

The woylie (Bettongia penicillata) was once common and abundant over the southern third of the Australian continent. Since European settlement the range of this rat-kangaroo has become reduced by more than 97%, and until the early 1990s, only 3 small natural populations remained, all in south-western Australia. These medium-sized (c. 1 kg) marsupials create a large number of diggings as they forage for the hypogeous fruiting bodies of ectomycorrhizal fungi upon which they feed. The effect of such foraging activity on the availability of plant nutrients in the vicinity of such diggings was evaluated in simulated digging experiments. Available nitrate, ammonium, and sulfur decreased significantly 3 years after diggings were constructed and had filled in, whereas phosphorus, potassium, iron, and organic carbon remained unchanged. The results suggest that preferential water infiltration via woylie diggings leads to a decrease in those soil nutrients that are susceptible to leaching and indicates that digging vertebrates may influence the distribution of surface soil nutrients. SR at nd s di n M rk et Additional keywords: ectomycorrhizal fungi, biopedturbation, mycophagy, soil heterogeneity, woodlands, soil water repellency.