The migratory and reproductive behaviour ofBufo calamita was studied at four neighbouring breeding sites in the northern Rhineland, Germany, from 1986 to 1991. Radio telemetry and marking systems based on toe-clipping and on microchips were used to follow the tracks of toads and for individual recognition. Emphasis lay on estimates of (1) the exchange of reproductive individuals between neighbouring sites, and (2) the reproductive success at each site. Allozyme electrophoresis served to assess the genetic diversity of local populations. More than 90% of all reproductive males showed a lifelong fidelity to the site of first breeding, whereas females did not prefer certain breeding sites. Due to the female-biased exchange of individuals among neighbouring sites the genetic distance between local populations was generally low but increased with geographical distance. This pattern of spatial relations is consistent with the structure of a metapopulation. Morever, up to three mass immigrations of males per breeding period, replacing previously reproductive individuals, suggested the existence of temporal populations successively reproducing at the same locality. Genetic distances were considerably greater between temporal populations than between local ones, indicating partial reproductive isolation. In fact, an exchange of reproductive individuals between the temporal populations at each site was not detected, but gene flow due to the recruitment of first-breeders originating from offspring other than their own seems probable. Thus, natterjack metapopulations consist of interacting local and temporal populations. The reproductive success differed considerably among the four sites and also between the temporal populations. Three out of four local populations had low reproductive success as well as the latest temporal population. The persistence of these populations depended entirely on the recruitment of juveniles from the only self-sustaining local population. This “rescue-effect” impeded local extinction. The discussion focuses on the modifications required to fit the classical metapopulation concept to the empirical findings and their consequences for the dynamics of amphibian metapopulations.

Structure and dynamic of a natterjack toad metapopulation (Bufo calamita).

We censused pond-breeding hylid frogs in northeastern Costa Rica weekly for 15 months to deseribe patterns of reproduction, habitat use, and to establish baseline data on relative abundance for members of the assemblage. Reproduction in the Costa Rican assemblage was seasonal and occurred only during wet months. Some species called, but none reproduced, during the dry season. Three species (Agalychnis callidryas, Hyla ebraccata, and Scinax elaeochroa) accounted for more than 75% of the observations made during the study. The species overlapped broadly in time and space, but differed in substrate use and phenology. Two species of leaf-breeding frogs (A. callidryas and A. saltator) used perches that were significantly higher than those used by the other species. Some phenological differences were associated with different mating strategies. Explosive breeders (Scinax elaeochroa and Smilisca baudinii) were most common early in the wet seasons. Prolonged breeders (A. callidryas and H. ebraccata) were the most persistent members of this assemblage. Predation affects early and late life history stages of these hylids. Predation on arboreal egg masses by two snake species was observed. Ctenid spiders preyed on recently metamorphosed frogs and small adults. Our weekly samples were pooled into 21-day periods so that we could compare our results with those obtained for two communities of breeding anurans from South America. The patterns observed in the Costa Rican assemblage differed from those reported for South American pond-breeding frogs, but in all three assemblages reproduction was associated with wet periods.

Patterns of reproduction and habitat use in an assemblage of Neotropical hylid frogs.

Although anuran amphibians are diverse and conspicuous in many vertebrate communities, worldwide population declines have been observed. Climatic change is a global factor that has been implicated in some of these declines. In this paper, we speculate on how Neotropical anurans might respond to changes in climate predicted by Hulme and Viner (1998). We focus on two distinct groups of Neotropical anurans: frogs that live and oviposit in leaf litter and frogs that congregate at ponds to breed. Increased temperature, increased length of dry season, decreased soil moisture, and increased inter-annual rainfall variability will affect Neotropical frogs strongly. We expect that these changes will directly affect frogs by changing reproductive success and breeding periodicity, and indirectly by altering the invertebrate prey base. The individual effects will likely translate into changes at the population and community levels. We also speculate on how climatic change will affect Neotropical amphibians that are restricted ecologically and/or geographically. We suggest directions for future research that will increase our ability to predict how amphibians in the New World tropics will respond to climatic change.

Potential Effects of Climate Change on Two Neotropical Amphibian Assemblages

Mark-recapture studies indicate that Bufo marinus populations introduced into Australia tend to attain densities much higher than those reported in native habitats in South America. Moreover, its high rate of spread into northern Australia is raising concerns about the local fauna. The major ecological factors contributing to the observed differences in density are unknown. A population model based on empirical data from both continents was derived to explore the relationship between the life history parameters and population densities of this species, and to identify those parameters most likely to explain the differences in densities observed between continents. Model predictions seem consistent with the empirical data. The model predicted equilibrium densities in Australia one order of magnitude higher than those reported in South America. Moreover, a sensitivity analysis showed that adult equilibrium densities were much more sensitive to variations in adult survival than to variations in any of the other parameters at high densities. Coincidentally, it is adult survival for which the biggest differences between continents have been detected, suggesting that the higher toad densities reported in Australia are likely to be the result of higher adult survival in that continent. On the other hand, the high sensitivity of adult equilibrium densities to adult survival also has important implications for control of this species in Australia.

The invasion ecology of the toad bufo marinus:from south america to australia

Summary
1. Measuring the costs imposed by specific environmental challenges is difficult, because organisms adapt in ways that reduce those costs. Invasive species provide an opportunity to quantify environmental impacts before organisms can make adaptive changes.

2. The native range of cane toads (Rhinella marina) lies within the wet neotropics; although part of this range experiences seasonal drought, many of the places recently invaded by this large anuran species are much more arid.

3. Five years’ fieldwork from a seasonally arid site in the Australian wet–dry tropics shows strong seasonal shifts in the toads’ (i) population structure, reflecting seasonality in breeding and recruitment; (ii) adult morphology (secondary sexual characteristics in males); (iii) growth rates; (iv) energy balance; (v) spatial ecology (philopatry, dispersal rates) and (vi) adult mortality rates.

4. Some of these patterns accord with intuition: for example, wet-season conditions resulted in higher rates of growth, reproduction and movement, better body condition and more pronounced secondary sexual characteristics. However, seasonal patterns for other traits were non-intuitive: for example, neither hematocrit levels (reflecting hydration state) nor corticosterone levels (reflecting stress) showed significant seasonal variation, and mortality rates were higher in the wet season than the dry season.

5. The toads’ ability to flexibly adjust their behaviour and ecology to local hydric conditions has allowed them to thrive even under climatic conditions that preclude activity, feeding and reproduction for most of the year.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2011.01888.x

Effects of seasonal aridity on the ecology and behaviour of invasive cane toads in the Australian wet-dry tropics

In the Bangalore population of Bufo melanostictus the ovarian cycles in individual toads are characterized by synchronous growth of the vittellogenic oocytes and by asynchrony with respect to stage in the ovarian cycle among the toads. However, late ovarian cycles predominated during the period of August-October. The incidence of atresia, as observed in the ovaries sampled throughout the year, suggests that about two thirds of the toads preparing for spawning are primiparous, leaving only one third to represent iteroparity within the population. Body size varied greatly at first maturation, when somatic growth declined or stopped. The greatly varying body size is consistent with a seasonal timing of sexual maturation preparing for the main breeding season, which coincides with the height of the monsoon rains during August-October.

https://www.jstor.org/stable/info/3565838

Katre Shakuntala

Papers

Body size, reproduction and growth in a tropical toad, Bufo melanostictus, with a comparison of ovarian cycles in tropical and temperate zone anurans