In seasonal wet Neotropical forests, many studies have suggested that species-rich terrestrial frog assemblages are regulated bottom-up by the abundance of leaf litter. However, terrestrial frogs are prey to a diverse community of predators, and no studies have tested for top-down effects of predators on this or other anuran assemblages. Here, we used an extensive field dataset to model the relative contribution of food resources, microhabitat resources and predators towards the occupancy and detection of two frog species (Craugastor bransfordii and Oophaga pumilio) at La Selva, Costa Rica. Frog occupancy was most strongly influenced by predatory spiders and secondarily influenced by the abundance of leaf litter. Predators exerted stronger effects on frogs than food resources, and frogs avoided predators more as leaf litter decreased. Detection probability was elevated when predators were present. We found support for bottom-up effects of leaf litter on the terrestrial frog assemblage, but top-down effects by predators exerted stronger effects on frog occupancy and detection. Because predator avoidance varied along a resource gradient, predator and resource effects appear to be dependent, supporting interactions between top-down and bottom-up mechanisms. Climate-driven decreases in leaf litter may drive decreased availability of frog refugia and increased interactions between frogs and predators.

Predation can regulate populations and strongly affect invasion success of novel prey. The deer ked (Lipoptena cervi; Linnaeus 1758) is an invasive ectoparasite of cervids that spends a long period of its life cycle outside the host. Prior to this study, virtually nothing was known about natural summer time predation on the deer ked. We aimed to evaluate the magnitude of summer time predation on L. cervi pupae in different habitats and to identify potential predators. We conducted a set of field experiments, where we exposed L. cervi pupae to various ground-dwelling vertebrate and invertebrate predators. The loss of pupae was monitored for different predator guilds. Three habitats of the moose, the main host species, were studied: (1) moist heath forest; (2) dry, logged heath forest; and (3) moist meadow. The results indicate notable summer time predation on L. cervi pupae, and the pupal predation varied within and between habitats, being lowest in the meadow habitat. We found a positive correlation between pupal loss and abundance of the common lizard (Zootoca vivipara), harvestmen (Opiliones), ground spiders (Gnaphosidae) and Formicinae-ants. We conclude that summer time predation during the pupal phase can have a notable local importance for the L. cervi abundance.

We sampled 27 month-old mussel populations grown on collector ropes in Cascapédia Bay,Quebec, to test whether density-dependent growth was present concomitantly toself-thinning, a process which was previously shown to occur in this system and thought tobe driven by spatial constraints. Biomass-density curves of raw samples were curvilinear,suggesting density-dependent growth. However, at least two cohorts were present.Fractionating the samples on the basis of age yielded a linear relationship for the main,2 year-old cohort. This implies density-independent growth and rules out food regulationin these populations. Therefore, our results are consistent with inferences drawnpreviously from the values of the self-thinning exponent, that is, space-regulatedself-thinning. Our results suggest that curvilinearity of the raw biomass-density curvesresulted from a bias caused by including the 1 year-old cohort and spat of the year in theanalysis. This conclusion is supported by a model showing that samples with mixed cohortscan yield linear, concave or convex biomass-density curves without density-dependentgrowth. The shape of the curves depends on the scaling relationships between cohortabundances. It appears that the shape of biomass-density curves may be a usefulcomplementary criterion – in addition to the value of self-thinning exponents – for theidentification of food or space as factors regulating cultured populations.

This review considers three case studies based on macroparasites of anurans: (a) natural infections in the permanently-aquatic Xenopus laevis which represent the worm burdens acquired, and the implications for pathology, when hosts are exposed to continuous, year-round, transmission; (b) the desert toad, Scaphiopus couchii, which experiences invasion very briefly each year and provides a simplified system involving only a single significant infection (Pseudodiplorchis americanus); (c) the mesic Bufo bufo which has been the subject of experimental laboratory studies designed to measure the effects of Rhabdias bufonis infection on host growth, physical performance and survival. Experimental manipulation of both Scaphiopus and Bufo provide quantitative data on disease effects of macroparasites, including precise measurements of parasite-induced host mortality. Field data for Xenopus and Scaphiopus show that, despite high initial worm burdens from efficient transmission, infection levels at parasite maturity are modulated below those leading to significant disease. Experimental data for Scaphiopus and Bufo have documented the time-course and magnitude of this decline in intensities, and there is circumstantial evidence for Scaphiopus that this regulation is host-mediated. Immunological studies on Xenopus show that disease effects of the pathogenic Pseudocapillaroides xenopodis are exacerbated in thymectomised hosts and reversed by implantation of thymuses from MHC-compatible donors. Thus, whilst factorial experiments can demonstrate the potential of helminths to cause significant disease and mortality in anuran host-macroparasite interactions, powerful post-invasion regulation of worm burdens appears to exert a strong control of parasite-induced disease in natural host populations.

Whether the size of an animal population is environmentally limited or regulatedby density dependent negative feedback mechanisms is of ecological interest. Proponents oflimitation theory have issued a set of specific challenges which are addressed in this paperusing field data for the insect Tipula paludosa. This species is known to be subject to population crashes caused by adverse environmental conditions and assumed to be limited.We re-examine published data in support of this hypothesis and show that there is alsoevidence of negative feedback between generations. Meta-analyses of annual count data fromindividual fields, and regional surveys provide further evidence of negative feedback and itis concluded that population regulation occurs at both local and regional scales. Evidencefrom other studies is used to propose that cannibalism is the causative mechanism. Sincesimilar negative feedback responses were apparent under both normal and environmentallylimiting conditions when populations crash, a single population model that could simulatethe differing regional dynamics observed from the annual surveys was produced. We concludethat the range of annual fluctuations observed in T. paludosa populations is the outcome ofboth limiting and regulating processes, confirming from field observations that these are notexclusive mechanisms.

Density dependence of the Addo Elephant National Park (South Africa) elephants Loxodonta africana was assessed using a long-term data set. Estimated carrying capacity is 0.1–0.5 elephants km−2 but stocking rates have been up to 4 elephants km−2. Population growth rate was found to be positively correlated with increasing density. There was no relationship between birth rate, the age of first calving or calf sex ratio and elephant density but there was a positive relationship between birth rate and rainfall during conception year. Mortality rates, particularly for juveniles, were low, and mean inter-calf interval was 3.3 years. There is no evidence of density dependent regulation in this population, despite the population being consistently above the estimated sustainable carrying capacity and a loss of phytomass and biodiversity. This is interpreted in light of the characteristics of the aseasonal habitat, succulent thicket vegetation and the ability of elephants to utilize accumulated vegetation biomass. These findings indicate that density dependence should not be considered as an option in the control of elephant numbers in this Park, or where elephant resources are not seasonally limited.

We studied diet choice by a generalist herbivore, the red howler monkey (Alouatta seniculus) under conditions of high and normal population density. Densities equivalent to 800–1000 km−2 (roughly 20–40 times normal) occurred in populations trapped on small, predator-free islands in Lago Guri, Venezuela. For three successive years, we studied one such population, a group of six animals living on a 0.6-ha island and compared its feeding ecology to that of two groups living at normal densities on a 190-ha island. The 0.6-ha island supported a total of 351 trees >10 cm dbh of 46 species, whereas >100 species probably occurred within the 16- and 23-ha home ranges of the two large-island howler troops. Small-island howlers were thus predicted to consume fewer resources, in particular less fruit, and to be less selective in diet choice than large-island howlers. As predicted, small-island howlers consumed fewer resources and obtained a smaller fraction of their intake from rare tree species (those contributing <1% of basal area). Small-island howlers consumed less fruit (2% of feeding time vs. 22%) and more foliage (73% vs. 55%) than the large-island groups. Diet breadth of small-island howlers was markedly less than that of their large-island counterparts. Tree species not present on the small island contributed ≥60% of leaf consumption by large-island howlers. Foliage sources preferred by large-island howlers were different in each of 3 years, whereas foliage of the same species of tree consistently ranked first on the small island. Long-term persistence (17 y) of self-perpetuating howler groups on Lago Guri islets at >20 times normal density strongly suggests that food availability does not limit mainland populations.

The idea that the size of animal populations may be regulated by factors intrinsic to them, such as behaviour, has a long history in ecology. Although this idea is now rejected as a general mechanism, it may apply in some species where females are damaged during courtship attempts, such as the Mediterranean tortoise Testudo hermanni. The sex ratios (adult males : adult females) of most dense Greek populations were more extreme (over 1.5:1) than could be accounted for by the earlier age at maturity of males, and some ranged to over 6:1. The sex ratio was correlated with the population density of adult males and the frequency of courtship attempts on females, and negatively correlated with the longevity of females. A high population density of males limits the density of adult females. This intrinsic regulation of population size is a consequence, rather than a goal, of a particular type of courtship behaviour (persistent thrusts by the male using a tail with a sharp terminal spur) in T. hermanni. Existing population densities at many Greek sites are probably unnaturally high, owing to the reduction of natural predators.

In 1981 an attempt was made to eradicate a 300 km2 fly-belt of Glossina pallidipes (Austen) in the Lambwe Valley of western Kenya by means, principally, of sequential aerial application of endosulfan aerosol spray. The progress of spraying operations was monitored entomologically by surveying for fly with biconical traps sited in main habitats of thicket, Acacia woodland and coniferous plantation, and by age-grading females caught between spray cycles.

Although nine spray applications of relatively heavy dosages of insecticide were carried out, the campaign ultimately failed to accomplish its objective. Females which had clearly survived and bred in the intervals between spray cycles were found throughout the area. By the end of operations, the tsetse population appeared to have been reduced by 99.9% in thicket and woodland, and by 99% in coniferous plantation. Nevertheless, on the basis of a crude estimate of total fly numbers in the Lambwe Valley, several thousand flies possibly remained after spraying.

Monthly post-spray monitoring showed that the population in thicket fully recovered in about 12 months, but to only 20–30% of its original level in woodland and plantation two years after spraying. The population in thicket maintained an approximate equilibrium density after recovery, indicating that density-dependent processes had re-established a regulatory effect. There was some evidence that this was brought about in part by fly movement. Calculations of the population growth rate gave a value for r, the instantaneous rate of increase, of 0.015 per day; equivalent to a finite rate of increase of 1.610 per month, and a population doubling time of 45 days. Some possible ecological reasons for failure to achieve eradication are discussed.