The reproductive and larval ecology of Bufo marinus (Anura: Bufonidae)

The reproductive and larval ecology of the introduced species Bufo marinus was studied at two breeding sites in tropical northern Australia. Reproductive activity, recruitment patterns, larval growth and survival, and predation by conspecifics and by insect, crustacean and fish species were looked at in detail.\ud \ud Reproduction at both sites occurred from December to June. Recruitment from reproductive events was sporadic. It was not concentrated during any particular period of the breeding season. During breeding periods, individuals at oviposition sites were predominantly males, with a population sex ratio of 10:1. Less than 30 percent of the estimated male population and 5 percent of the estimated female population were active (found in transect searches) on any one night during the breeding period. The proportion of active individuals in amplexus at any time was low. Total numbers of active individuals declined by the end of the dry season and sex ratios approached 1:1. No breeding was observed during the late dry season although histological examination of gonads indicated that reproductively active males and females (with mature sperm and ova) were present all year round.\ud \ud Sampling of larval populations indicated that densities recorded at both sites varied temporally and spatially. High variance-mean ratios for sample densities indicated that larval distributions within transects were aggregated. Peak densities were typically between 600 to 800 per m². Larval density was used to determine population size but estimates showed no consistent association with recruitment of eggs from observed breeding.\ud \ud Experimental studies indicated that during the embryonic stages (Gosner stages 1 to 24) predator-free survival was approximately 70 percent (90 percent survival/day) over a mean developmental period of 72 hours. Highest mortality occurred between three days (hatching) and twelve days (stage 25) after laying. Mean larval periods of 22 to 54 days were recorded. Shorter larval periods were associated with lower larval density. Survival to metamorphosis ranged from 0.1 to 10 percent (97 percent mean survival/day). Survival and growth rates in most experiments were density dependent. Growth of pre-hatching stages was linear but growth was exponential or decreasing exponential during the tadpole stages, controlled either by temperature or density. Growth of larvae is food limited, with advantages to older larvae demonstrated in an artificial pond experiment with two overlapping cohorts of different age. Growth was reduced for both cohorts but the effect was greater for younger cohorts. Individuals that survived egg predation by older cohorts grew larger because of reduced competition.\ud \ud Egg predation by older cohorts of Bufo larvae appeared to be a major source of mortality. Intraspecific predation was restricted to pre-swimming embryo stages and greatly reduced pre-hatching survival in field and experimental populations (1.67 percent mean survival/day). No other predators of eggs were recorded.\ud \ud Few species were active predators of hatched tadpoles. Only two of eleven potential predators examined in feeding rate experiments consumed tadpoles. These were an adult dytiscid beetle, Homeodytes scutellaris (0.28 tadpoles/hour), and a freshwater crayfish, Cherax quadricarinatus (0.48 tadpoles/hour). None of the native fish species examined successfully consumed eggs or larvae.\ud \ud A predation experiment was conducted in experimental ponds with aeschnid dragonfly larvae, Hemianax papuensis, and high and low densities of Bufo larvae. Predation significantly reduced survival of larvae at both densities and growth was significantly lower for larvae in predator treatments at high density. Slower growth for larvae in predator treatments probably resulted from larvae altering their foraging behaviour in response to the presence of predators. Tadpoles appeared to be less conspicuous in predator treatments. In predator treatments with a low initial density of Bufo, initial predation decreased densities to a level where encounter rates with predators were low and tadpole food resources were probably not limiting. Final mass and larval period for tadpoles at low density in predator treatments were similar to controls.\ud \ud Larval population size is most likely to be influenced by differences in cohort overlap resulting from adult breeding phenology. A small overlap will result in generations with narrow size differences and will produce increased competition between cohorts. A large overlap may result in large size differences in cohorts and may lead to heavy predation of eggs by older cohorts. Invertebrate predators are likely to affect the survival of larval Bufo marinus populations during colonisation and periods of habitat deterioration