Frog Density Research Articles

Functional response and area‐restricted search in a predator: seasonal exploitation of anurans by the European polecat, Mustela putorius

AbstractA study of the feeding habits and movements of 11 radiotracked polecats Mustela putorius in western France revealed that seasonal predation upon agile frogs, Rana dalmatina, was directly influenced by prey abundance and distribution. Although dietary structure showed the importance of mammalian prey (71.5%), polecats exploited nocturnal, terrestrial anurans in spring (31.6%). The periodic activity of anurans at spawning sites led both to a maximum density in spring and to a patchy distribution. The monthly variations in anuran dietary occurrences were associated with changes in frog availability. The functional response of polecats to frog density was sigmoidal shaped (type 3 response). Frog consumption rate increased more slowly than prey density but frogs were actively removed at higher density. It is therefore suggested that frog populations were moderately affected by the predator and this density dependent effect tends to stabilise anuran populations. Predation upon anurans was also correlated with a prey dispersion index as revealed by a polynomial regression. Polecats concentrated their predation on spawning congregations of the breeding adult frogs. Movements were smallest in spring and polecats changed their track length by increasing the difference between a succession of small movements and of longer journeys towards profitable sites. Changes in movements correlated with the anuran dispersion index and the response was sigmoidal (polynomial regression) revealing an area‐restricted search. This response may be regarded as an ‘aggregative response’ according to the first part of the definition of Begon et al. (1996) . Functional and area‐restricted search responses to the frog abundance and dispersion constitute an original example of predator‐prey coexistence strategies among vertebrates. I suggest that such predation could be favoured by the individualistic habits of the mustelid.

Diversity and Abundance of Litter Frogs in a Montane Forest of Southeastern Brazil: Seasonal and Altitudinal Changes1

ABSTRACTWe sampled litter frogs in an 1800‐ha mid‐elevation seasonal forest in southeastern Brazil. One hundred 8 * 8‐m plots were sampled during the dry/cold season and wet/warm season (unburned areas); we also examined the effects of fire in recently burned areas. A total of 267 frogs (305 g), belonging to 16 species (4 families) were caught. A single species comprised 78.5 percent of the individuals in the dry/cold season and 54.3 percent in the wet/warm season. The density of individuals did not change significantly with season, biomass did. Density and biomass of frogs were positively correlated with altitude. A Mantel test indicated that biological data (species and their abundance) were significantly associated with environmental parameters. The burned areas showed low values in richness, density and biomass of frogs. Harsh seasonal climate and a history of human disturbance may produce the low observed diversity values. The greater densities of frogs in sites of higher elevation may primarily result from mist‐generated humidity, which diminishes the harshness of the dry/cold season in relation to lower sites.

Relationships between Population Density of Frogs and Environmental Conditions in <I>Yatsu</I>-Habitat

千葉県印旛沼流域の谷津環境に生息するカエル類について, 種および個体数と生息地の環境要因の関係について調べた。調査は, 1997, 98年5月から7月にかけて行なった。畦の上を歩き, 目撃したカエルの数と, 歩いた畦の距離, 周辺の環境要素などを同時に記録した。目撃種ごとの個体密度と環境要因との関係について分析した結果, 個体密度に影響を与えた要因として, 水田の暗渠排水施設の整備斜面の土地利用, 用水の取水方法, 水路の配置などがあげられた。ニホンアカガエル・トウキョウダルマガエルの生息は圃場整備による乾田化によって負の影響を受けやすいが, ニホンアマガエルは乾田化の影響を受けにくいことが明らかとなった。

Anuran Development, Density and Diversity in Relation to Agricultural Activity in the Holland River Watershed, Ontario, Canada (1990–1992)

Significantly lower anuran species diversity and density was recorded in a vegetable growing area relative to upstream and downstream sites in the Holland River watershed, Ontario, Canada. Egg hatching success and tadpole deformity rates of American toads (Bufo americanus americanus), green frogs (Rana clamitans melanota) and northern leopard frogs (Rana pipiens) eggs in water from field sites and control water were assessed. Compared to the control and the upstream site, the total abnormality rate (unhatched eggs plus deformed tadpoles) was higher for American toads in water from the agricultural and downstream sites. Total abnormality was higher in green frog eggs in water from the agricultural site and a downstream site. Trace concentrations of organophosphorus pesticides were detected most often in agricultural zone water and sediments than in upstream and downstream sites. Organochlorine pesticide residues, especially in agricultural zone samples, exceeded the no effect level guidelines for the protection of aquatic life. Ammonia, phosphorus, particulates, BOD and TKN were highest in the agricultural zone. Significant correlations between these parameters and anuran development suggest nutrient run-off as a causal or contributing factor in lower anuran diversity, density and reproductive success of American toads and green frogs in the site dominated by agriculture.

Biparental care in the tadpole-feeding Amazonian treefrog Osteocephalus oophagus

AbstractThe Central Amazonian treefrog Osteocephalus oophagus breeds in water-filled bromeliad or palm tree leaf axils or in treeholes. The larvae feed on eggs provided by their parents; larvae not provided with eggs die. Survival of the larvae is ensured by the fact that the pair always spawns at the same site. They return at intervals of 5 to 7 days. Pair bonding is the rule in areas with low frog densities. The first eggs develop into tadpoles and later clutches of fertilized eggs serve as food. There is no apparent communication system between larvae and mother. After metamorphosis of the larvae the pair continues to lay eggs into the same leaf axils and these eggs also develop into tadpoles. This behavior is compared to that of other frogs that feed their tadpoles on eggs. It seems to be the least advanced mode of parental care involving tadpole feeding and demonstrates one of the initial steps that has led to more complex parental care behaviors in frogs.

The Role of Androgens in Opportunistic Breeding, Tropical Frogs

In this paper we present the results of an interspecific study on the androgen levels of several species of Old World tropical frogs. These data are used to test three hypotheses: (1) in unpredictable, aseasonal environments with opportunistic breeding, gonadal hormones have a permissive rather than an activating role in the expression of mating behavior (Crews and Moore, 1986), (2) a behaviorally induced androgen response is present in opportunistic aseasonal species as well as in temperate seasonal breeders (Crews and Moore, 1986; Wingfieldet al.,1990), and (3) because maintenance of high plasma testosterone levels has an associated energetic and/or fitness cost (Moore and Marler, 1987; Wingfieldet al.,1990), continuously breeding frogs have lower testosterone levels than seasonal, discontinuous breeders. Androgen and corticosterone levels were found to vary significantly among species exhibiting the same mating behavior, providing support for the hypothesis that hormones have a permissive role. Differences in androgen level were correlated with (1) variation in level of male–male aggression, (2) relative density of male frogs, and (3) call rate and power output of male vocalization. These results indicate that a behaviorally induced androgen response is present in opportunistic, aseasonal breeders. Furthermore, tropical, aseasonal, opportunistic breeding frogs appeared to have lower androgen levels, generally, than confamilial temperate, seasonal breeders. This difference is consistent with the hypothesis that maintenance of high plasma testosterone levels is energetically expensive.

Competition and the distribution of spring peeper larvae.

Studies of tadpole distributions have shown that despite overlapping affinities for semipermanent and permanent ponds, distributions of the spring peeper (Pseudacris crucifer) and the green frog (Rana clamitans) tend to be nonoverlapping. Because spring peepers are believed to be poor competitors, I hypothesized that competition from green frog larvae limits the distribution of spring peeper larvae. I stocked field enclosures with a constant density of spring peeper larvae, and one of four densities of green frog larvae (a "target-neighbor" design). Increased green frog density had a small effect on metamorphic size and no effects on survivorship, larval period or growth rates of spring peepers. In contrast to these small interspecific effects, green frogs had a large effect on their own performance. Intraspecific competition resulted in a 50% decline in growth rate and an 11% decline in metamorphic size. These results suggest that the species are segregated in resource use, or that compared with green frogs, spring peepers are better able to cope with depressed resource densities. In either case, this field experiment provides no evidence that interspecific competition from green frogs limits distributions of spring peepers. Other factors such as predation and breeding site choice by adults may contribute to the absence of spring peeper larvae from many semipermanent and permanent ponds.

Mine-drainage treatment wetland as habitat for herptofaunal wildlife

Land reclamation techniques that incorporate habitat features for herptofaunal wildlife have received little attention. We assessed the suitability of a wetland, constructed for the treatment of mine-water drainage, for supporting herptofaunal wildlife from 1988 through 1990 using diurnal and nocturnal surveys. Natural wetlands within the surrounding watershed were also monitored for comparison. The treatment wetland supported the greatest abundance and species richness of herptofauna among the sites surveyed. Abundance was a function of the frog density, particularly green frogs (Rana clamitans) and pickerel frogs (R. palustris), while species richness was due to the number of snake species found. The rich mix of snake species present at the treatment wetland was believed due to a combination of an abundant frog prey base and an amply supply of den sites in rock debris left behind from earlier surface-mining activities. Nocturnal surveys of breeding male frogs demonstrated highest breeding activity at the treatment wetland, particularly for spring peepers (Hyla crucifer). Whole-body assays of green frog and bullfrog (R. catesbeiana) tissues showed no differences among sites in uptake of iron, aluminum, and zinc; managanese levels in samples from the treatment wetland were significantly lower than those from natural wetlands. These results suggest that wetlands established for water quality improvement can provide habitat for reptiles and amphibians, with the species composition dependent on the construction design, the proximity to source populations, and the degree of acidity and heavy-metal concentrations in drainage waters.

Reproductive Phenology, Population Structure, and Habitat Use of the Frog Eleutherodactylus johnstonei in Barbados, West Indies

-I report on population structure, growth, reproduction, and use of retreats by an abundant but poorly-known terrestrial frog, Eleutherodactylus johnstonei, in two contrasting habitats in Barbados, West Indies. A population inhabiting a forest site exhibited early maturity, small adult body size, short residency time, high juvenile to adult ratio, and low density, when compared to a population inhabiting an untended flower bed. At both sites, reproduction peaked in the early part of the wet season (JuneAugust), but males called about a month later in the flower bed site. Most growth took place during the wet season, and growth rates did not differ between sites. I suggest that both quality and quantity of retreat sites and the length of time available for activity are key factors in shaping life history characteristics of E. johnstonei at these sites. urnal of Herpetology, Vol. 25, No. 4, pp. 424-430, 1991 yright 1991 Society for the Study of Amphibians and Reptiles roductive Phenology, Population St ucture, and Habitat Use of e Frog Eleutherodactylus joh stonei in Barbados, W t Indies Eleutherodactylus johnstonei is a widespread and abundant neotropical species, native to several islands in the Lesser Antilles (Schwartz, 1967, 1978; Hardy and Harris, 1979). It commonly inhabits disturbed areas but also occurs in mesic, mature forests (Schwartz, 1967). Stewart (1979) and Stewart and Martin (1980) examined the role of introduced Eleutherodactylus (including E. johnstonei) in anuran communities in Jamaica, but life history of E. johnstonei in the Lesser Antilles is poorly known. Bayley (1950) studied its natural history, and Lemon (1971) investigated spacing of calling males in Barbados. Here I report on population structure, reproductive phenology, growth, and habitat use by 'Present Address: Department of Zoology, The University of British Columbia, Vancouver, B.C., Canada V6T 1W5. leutherodactylus johnstonei is a widespread and ndant neotropical species, na iv to several lands in the L sser Antilles (Schwartz 1967, 78; Hardy and Harris, 1979). It c mmonly inE. johnstonei in two habitats in Barbados, where the species is probably indigenous (Marsh, 1982; but see Schwartz, 1967). My objective was to examine intraspecific variation between populations occupying contrasting habitats. One study site was in a flower bed, and the other site was in a mature forest along a seasonallyactive stream. Based on the abundance of potential retreat sites due to the dense ground vegetation in the flower bed, I predicted that it would support a greater density of frogs with higher individual growth rates and a longer breeding season than the forested site.

Harlequin Frogs Along a Tropical Montane Stream: Aggregation and the Risk of Predation by Frog-Eating Flies

We observed larvae of the fly Notochaeta bufonivora (Sarcophagidae) feeding on harlequin frogs, Atelopus varius (Bufonidae), along a gallery forest stream in montane Costa Rica during the dry season. Harlequin frogs tended to aggregate in areas containing waterfall spray, and in these areas an individual's probability of attack by Notochaeta was greater than in areas lacking waterfall spray zones. To ascertain whether this increased risk to a frog resulted from its choosing an area with a waterfall spray zone or from being in an area of high frog density, we manipulated the density of frogs in a series of quadrats. The results of this experiment suggest that the increased risk of attack in the environs of waterfalls was a hazard associated with patch choice by the frogs rather than with aggregation per se. AGGREGATIONS OF ANIMALS have often been viewed in the context of predator avoidance (e.g., Hamilton 1971, Bertram 1978). However, because predation events in nature are usually brief and unpredictable in space and time, one can rarely assess the relationship between an individual's risk of predation and its proximity to conspecifics. We studied an extraordinary predator-prey interaction between the harlequin frog, A. varius (Bufonidae), and a frogeating fly, N. bufonivoraz (Sarcophagidae), along a montane stream in Costa Rica. Harlequin frogs, which are brightly variegated with yellow and black, contain the potent nerve poison tetrodotoxin in their integument (Kim et al. 1975). Although tetrodotoxin probably discourages most predators, or at least most predaceous vertebrates, Notochaeta larvae, which feed internally, appear unaffected by this chemical defense (Crump & Pounds 1985). Because a frog lives several days after attack by Notochaeta, the probability of detecting predation events is much higher than is usually the case for predator-prey systems involving anuran prey. This paper focuses on the relationship between aggregation behavior of harlequin frogs and risk of predation by these frog-eating flies. Male frogs and toads commonly exhibit a clumped pattern of dispersion within the breeding habitat (Whitney & Krebs 1975, Wells 1977), and this pattern may reflect a patchy distribution of resources such as suitable oviposition sites. On the other hand, a clumped pattern of dispersion within the habitat may also be attributable to the advantages of aggregation itself. In theory, being in a group may dilute an individual's probability of being eaten should a predator arrive (Hamilton 1971); this may be especially important for aposematic prey, as a naive predator may avoid the rest of the group after sampling one individual. Also, if females are more attracted to groups of males than to solitary displaying males, the average mating success may be higher for males in groups (Wells

Population Density of Tropical Forest Frogs: Relation to Retreat Sites

The forest frog Eleutherodactylus coqui defends specific sites for retreats and nests in the Luquillo Forest, Puerto Rico. The hypothesis that shortages of nest and retreat sites limit population size was tested by placing 100 bamboo frog houses in plots measuring 100 square meters in areas of high frog density. These new sites were readily adopted by adult frogs. After one year, experimental plots had significantly more nests and frogs of all sizes than did control plots.

Densities of Floor-Dwelling Frogs and Lizards in Lowland Forests of Southeast Asia and Central America

Two large samples of amphibians and reptiles from lowland primary rain forests in Borneo come from communities that appear to have densities similar to that of a Bornean community previously reported (Lloyd et al. 1968). To this group may be added a much smaller sample from a primary rain forest in peninsular Malaysia. Densities of nonriparian terrestrial frogs and lizards in these Indo-Malayan communities are about an order of magnitude smaller than those reported from lowland forests in Central America (Scott 1976). Indo-Malayan forests are peculiar among rain forests because of supraannual synchronized mast fruiting of dipterocarp trees that dominate them (Wood 1956; Ashton 1969). I suggest that this habit reduces the number of seed-eating insects and associated arthropod predators on the floor of these forests, thereby reducing the food supply of floor-dwelling frogs and lizards and acting to limit the populations of these animals below levels achieved in neotropical forests. Large samples of frogs an...

Spacing and calling in Pacific tree frogs, Hyla regilla

Calling male Pacific tree frogs space out more than if they were distributed at random on the available calling sites. When new frogs are added to an area that is already occupied by some calling males, fewer additional frogs call than in a control situation where frogs are added to an empty area. This shows that behavioral interactions between frogs limit the number that call in a local area. A further experiment showed that the mating call played through loudspeakers is sufficient to inhibit additional frogs from calling. We conclude that calling leads to spacing out, which in turn limits the density of calling frogs in a local area.