1611Ecology,85(6), 2004, pp. 1611–1618q2004 by the Ecological Society of AmericaDENSITY REGULATION IN TADPOLES OFRANA TEMPORARIA:A FULL POND FIELD EXPERIMENTJONLOMAN1Department of Animal Ecology, University of Lund, SwedenAbstract.Tadpole density dependence has repeatedly been studied in laboratories andmesocosms, but rarely in natural ponds. The present study manipulated density in threeponds; each pond was divided into one low density section and one high density section.The experiment was run for eight years, switching density treatments within ponds amongyears. This experimental design cannot definitively separate the experimental density effectsfrom certain lag effects. This problem is discussed. Nonetheless, in all ponds tadpolesurvival was affected by the density manipulation. The reduced survival of larvae at highdensity was most pronounced in the pond with the overall highest density. In this pond,the density-dependent mortality almost offset the original, experimental, density differencebetween sections. Effects on tadpole and metamorph size and development rate were foundin those cases in which density-dependent mortality was weak and treatment effects ondensity persisted to the later stages of tadpole development. However, even if significant,these were generally of small magnitude compared to the variation among ponds and years.I suggest that different modes and degrees of regulation among populations of one speciesof frog are to be expected, not only due to variation in pond quality but also due to variationin the surrounding terrestrial habitat. This may determine the balance between tadpole andadult population regulation.Key words: anura; complex life cycle; density regulation; growth rate; intraspecific competition;metamorphosis; population regulation;Rana temporaria;tadpole.INTRODUCTIONThe size of animal populations is determined byfeedback and nonfeedback mechanisms. Negativefeed-back mechanisms tend to stabilize a population aroundsome mean number. Identifying these mechanismshelps us understand why population sizes in nature arethose that we observe and is thus a key task in animalecology. However, the task to identify such factors isassociated with several complications. I will mentionsome of those that relate to the study of frogs.Most frogs have complex life cycles. This means thatthe tadpole and adult face completely different situa-tions, and the regulating mechanisms, if at all presentin both stages, are not likely to be the same (Wilbur1980). So far, most studies have been concerned withthe study of factors that may operate on the tadpolestage (Loman 2001).Studies of tadpole regulation span a range from sim-ple laboratory setups (Wilbur 1976, Cummins 1989,Loman 1999) to studies in almost natural settings (vanBuskirk and Smith 1991, Beebee et al. 1996, Loman2001). The two approaches complement each other(Scott 1990). The former gives an opportunity to iden-tify the mechanisms that are responsible for density-dependent effects, but there is little guarantee that theeffects that are found play an important part in theManuscript received 17 March 2003; revised 31 July 2003;accepted 19 September 2003; final version received 3 November2003. Corresponding Editor: B. Sinervo.1E-mail: [email protected] regulation of populations (Morin 1998, Polis etal. 1998). Studies manipulating natural ponds are moredifficult to interpret, but effects found are more directlyrelated to processes actually influencing natural pop-ulations. However, such studies are almost completelylacking (Skelly 2001). Among the few examples arethe studies of van Buskirk and Smith (1991), Beebeeet al. (1996), and Loman (2001). Other less obviousdifferences among experimental settings may also leadto conflicting conclusions (Skelly 2002).Population regulation is ultimately concerned witheffects on reproduction and survival. However, thismay be difficult to measure and other parameters maywell give important information. For example, in frogs,large adults have large clutches (Ho¨nig 1966, Berven1988, Joly 1991) and large tadpoles have been foundto more easily escape from size-limited predators(Richards and Bull 1990, Semlitsch 1990, Tejedo 1993,Lardner and Loman 1995) than small ones. Thus, thestudy of effects on growth has direct implication forpopulation regulation.Density regulating factors may well vary geograph-ically, at different scales. The number of predators anddensity of tadpoles vary by magnitudes among pondseven in restricted areas, creating different scopes forregulating effects (Loman 2002a).With these considerations in mind, the present studyoccupies the following niche. It addresses the effect ofdensity regulation on the survival, growth, and devel-opment of tadpoles. Through carry-over effects on the1612JON LOMANEcology, Vol. 85, No. 6adult stage, it also has implications for an understand-ing of the population dynamics of the species. Thisapproach is not terribly original but is common to moststudies of frog population regulation. The study is how-ever unusual because it is completely carried out onfree-swimming tadpoles in natural ponds, using the en-tire ponds and an (almost) natural range of tadpoledensities. This means that any effects found can con-fidently be said to be part of the natural history of realpopulations of the study species. However, unlike theresults of mesocosm experiments, the exact mecha-nisms cannot be ascertained because of uncontrolledvariation among the natural ponds used in the study.Effects on survival, growth, and development are an-alyzed. The study is carried out in three different ponds,involving three separate populations, and spread outover a period of six to seven years of manipulations.This is too many to expect a consistent result and toofew to make any generalizations. The three sites aretherefore considered three separate case studies and theevaluation of the results is made accordingly.The basic approach of the study is to subdivide nat-ural ponds into two sections and redistribute naturallylaid spawn so as to create low and high density sections.This is done for a succession of years, switching lowand high density sections. Number and size of tadpolesand metamorphs is monitored.In approach and methods used, this study is similarto a previous study (Loman 2001). There are howeverthree differences. The present study is concerned withthe common frogRana temporaria, a close relative tothe moor frogR. arvalisthat was the