Influence of food availability on demography and local population dynamics in a long-lived seabird

Few studies have addressed the effects of food availability as a proximate factor affecting local adult survival in long-lived organisms and their consequences at local population dynamics. We used capture-recapture analysis of resightings of 10 birth cohorts of ringed Audouin's gulls, Larus audouinii, to estimate adult survival and dispersal (both emigration and immigration). For the first time, permanent emigration (the transient effect in capture-recapture analysis) was modelled for the whole population and not only for the newly marked birds. Gulls exploit to a large extent fishes discarded from trawlers, and a trawling moratorium established since 1991 has decreased food supply for the colony. This was used as a natural experiment of food availability to assess its effects on adult survival and emigration. These and other demographic parameters were used in a projection modelling to assess the probabilities of extinction of the colony under two scenarios of lower and higher food availability. Food availability (together with the age of individuals) influenced emigration probabilities, but not adult survival, which was estimated at 0.91 (s.e. = 0.02). When food was in shorter supply during the chick-rearing period, emigration was very high (ca. 65%) for younger breeders, although this rate decreased sharply with age. Probabilities of extinction were very high when food availability was low, and when environmental stochasticity was introduced, and only stochastic immigration from the outside seemed to prevent extinction. The results highlight the importance of dispersal processes in the population dynamics of long-lived organisms.     

The influence of predator--prey population dynamics on the long-term evolution of food web structure

We develop a set of equations to describe the population dynamics of many interacting species in food webs. Predator-prey interactions are nonlinear, and are based on ratio-dependent functional responses. The equations account for competition for resources between members of the same species, and between members of different species. Predators divide their total hunting/foraging effort between the available prey species according to an evolutionarily stable strategy (ESS). The ESS foraging behaviour does not correspond to the predictions of optimal foraging theory. We use the population dynamics equations in simulations of the Webworld model of evolving ecosystems. New species are added to an existing food web due to speciation events, whilst species become extinct due to coevolution and competition. We study the dynamics of species-diversity in Webworld on a macro-evolutionary time-scale. Coevolutionary interactions are strong enough to cause continuous overturn of species, in contrast to our previous Webworld simulations with simpler population dynamics. Although there are significant fluctuations in species diversity because of speciation and extinction, very large-scale extinction avalanches appear to be absent from the dynamics, and we find no evidence for self-organized criticality.     

The influence of habitat heterogeneity on host-pathogen population dynamics

The influence of spatial heterogeneity on the population dynamics of a naturally occurring invertebrate host-pathogen system was experimentally investigated. At ten week intervals over a two year period, I quantified the spatial distribution of natural populations of the terrestrial isopod crustacean Porcellio scaber infected with the isopod iridescent virus (IIV). During the seasonally dry periods of summer and early fall in central California, isopod populations were highly aggregated and the degree of patchiness and distance between inhabited patches was greatest. Coincident with increased patchiness and patch spacing was an increase in isopod density within patches. During the wet seasons of winter and spring, isopod population patchiness, inter-patch spacing, and within-patch density was low. Seasonal changes in virus prevalence were negatively correlated with within-patch density, patchiness, and inter-patch spacing. The influence of the spatial distribution of isopods on virus prevalence was also tested in field experiments. The virus was introduced into arrays of artificial habitat patches colonized by isopods in which interpatch distance was varied. The prevalence of resulting infections was monitored at weekly intervals. In addition, dispersal rates between artificial patches and natural patches were quantified and compared. The results showed that isopods in treatments with the smallest inter-patch spacing had the highest virus prevalence, with generally lower prevalence among isopods in more widely spaced patches. The spacing of experimental patches significantly affected virus prevalence, although the experiments did not resolve a clear relationship between patch spacing and virus prevalence. Rates of dispersal between patches decreased with increased patch spacing, and these rates did not differ significantly from dispersal between natural patches. The results suggest that rates of dispersal between isopod subpopulations may be an important component of the infection dynamics in this system. I discuss the consequences of these findings for host-pathogen dynamics in fragmented habitats, and for other ecological interactions in spatially heterogeneous habitats.     

The effects of stochastic population dynamics on food web structure

We develop a stochastic, individual-based model for food web simulation which in the large-population limit reduces to the well-studied Webworld model, which has been used to successfully construct model food webs with several realistic features. We demonstrate that an almost exact match is found between the population dynamics in fixed food webs, and that the demographic fluctuations have systematic effects when the new model is used to construct food webs due to the presence of species with small populations.     

Comparing shoot population dynamics methods on Posidonia oceanica meadows

Prediction capacity of three main shoot population dynamics methods (age structure, net shoot recruitment per plagiotropic rhizome and shoot census) have been tested for a period of four years (2002-2006) on a Posidonia oceanica meadow. Accuracy of each method was checked by comparing measured and predicted densities at the end of the study period. Predicted densities came from the evolution of initial densities (measured in 2002) by a basic exponential model of population growth. The exponential model used the different net shoot recruitment rate estimates by each population dynamics method on three depths (upper, medium and lower limit) and three localities at each depth. Predictions performed by shoot census and net shoot recruitment per plagiotropic rhizome methods matched with measured densities at the end of the study period. Conversely, age structure method underestimated shoot densities at each depth, indicating an unreal decrease of shoot population in the meadow.     

Competing pressures on populations: long-term dynamics of food availability,food quality,disease, stress and animal abundance

Despite strong links between sociality and fitness that ultimately affect the size of animal populations, the particular social and ecological factors that lead to endangerment are not well understood. Here, we synthesize approximately 25 years of data and present new analyses that highlight dynamics in forest composition, food availability, the nutritional quality of food, disease, physiological stress and population size of endangered folivorous red colobus monkeys (Procolobus rufomitratus). There is a decline in the quality of leaves 15 and 30 years following two previous studies in an undisturbed area of forest. The consumption of a low-quality diet in one month was associated with higher glucocorticoid levels in the subsequent month and stress levels in groups living in degraded forest fragments where diet was poor was more than twice those in forest groups. In contrast, forest composition has changed and when red colobus food availability was weighted by the protein-to-fibre ratio, which we have shown positively predicts folivore biomass, there was an increase in the availability of high-quality trees. Despite these changing social and ecological factors, the abundance of red colobus has remained stable, possibly through a combination of increasing group size and behavioural flexibility.     

The influence of larval density, food availability and habitat longevity on the life history and population growth rate of the midge Chironomus riparius

Despite long‐standing interest in the forms and mechanisms of density dependence, these are still imperfectly understood. However, in a constant environment an increase in density must reduce per capita resource availability, which in turn leads to reduced survival, fecundity and somatic growth rate. Here we report two population experiments examining the density dependent responses under controlled conditions of an important indicator species, Chironomus riparius. The first experiment was run for 35 weeks and was started at low density with replicate populations being fed three different rations. Increased ration reduced generation time and increased population growth rate (pgr) but had no effect on survival, fecundity and female body weight in the first generation. In the second generation there was a six‐fold increase in generation time, presumably due to the greatly reduced per capita resource availability as the estimated initial densities of the second generation were 300 times greater than the first. Juvenile survival to emergence, fecundity, adult body weight and pgr declined by 90%, 75%, 35% and 99%, respectively. These large between‐generation effects may have obscured the effects of the threefold variation in ration, as only survival to emergence significantly increased with ration in the second generation. These results suggest that some chironomid larvae survive a reduction in resource availability by growing more slowly. In the ephemeral habitats sometimes occupied by C. riparius, the effects of population density may depend crucially on the longevity of the environment. A second experiment was therefore performed to measure pgr from six different starting densities over an eight‐week period. The relationship between pgr and density was concave, viewed from above. At densities above 16 larvae per cm², less than 1% of the population emerged and no offspring were produced. Under the conditions of experiment 2 – an 8‐week habitat lifespan – carrying capacity was estimated as 8 larvae per cm².     

Dispersal and reproduction in Tribolium: The influence of food level

Cohorts of 100 newly eclosed adults of strains bI and bIV of T. confusum and strains cI and cIV of T. castaneum emigrated over a 15-day interval from experimental habitats which contained a constant volume of 8, 4, 1, and 0 g of flour diluted with powdered polyethylene. Food level affected the emigration of T. confusum adults only during the interval before the normal onset of reproduction. Thereafter the beetles exhibited virtually identical patterns of departure. Larval recruitment did not decline when food levels ranged from 8 to 1. T. castaneum delayed their emigration at progressively lower food levels, but also produced fewer progeny. In strain cI, where emigration was more strongly deferred, the decline in larval recruitment was less pronounced. Both species of Tribolium departed most rapidly when starved outright, but only after an initial delay.The results are interpreted from the point of view of the oögenesis-flight syndrome. It is suggested that the Tribolium system provides an opportunity to relate differences in colonizing ability among several closely related species to specific differences in the physiological mechanism of adaptive dispersal.     

Parasitic castration of a vertebrate: Effect of the cymothoid isopod, Anilocra apogonae, on the five-lined cardinalfish, Cheilodipterus quinquelineatus

Parasitic castration, the specific blocking of host reproductive output by an individual parasite, is a host-parasite interaction common to many invertebrates, particularly crustaceans, echinoderms and molluscs. It can reduce host density, alter host population dynamics and the evolution of host life history traits. Here we show that parasitisation by a single female cymothoid isopod, Anilocra apogonae, castrates its vertebrate host, the five-lined cardinalfish, Cheilodipterus quinquelineatus. Parasitised male fish fail to mouthbrood their young. The gonads of parasitised fish are smaller and parasitised female fish have substantially fewer and smaller ova than do the gonads of unparasitised fish. As for parasitic castrators of invertebrate hosts, A. apogonae on C. quinquelineatus are uniformly dispersed amongst infested hosts (one adult female isopod per host), are site specific, and their body size is highly correlated with that of their host. These isopods are large relative to the body size of their hosts, averaging 3.8% of the weight of the host. Parasitised fish also weigh less and are shorter than unparasitised fish of the same age. Despite the presence of other potential hosts, A. apogonae only infests C. quinquelineatus. The consistency of the ecological correlates amongst known parasitic castrators suggests that the parasitic castrator host-parasite relationship will be recognised for other parasites of vertebrates.     

Modeling the influence of nutrients, turbulence and grazing on Pfiesteria population dynamics

A semi-idealized marine ecosystem model, designed as a heuristic tool for exploring the population dynamics of non-inducible versus toxic forms of Pfiesteria is described. The model is based on empirical evidence suggesting that these differing functional types of Pfiesteria also differ substantially in terms of what they eat and how they utilize it to optimize their growth. Non-inducible strains are similar to other mixotrophic dinoflagellates, whereas toxic strains may consume organic matter and detritus, produce toxins and attack fish. In our model formulation we represent these differences in a simplified way: the non-inducible strain is kleptochloroplastidic and it can take up DIN, but it cannot utilize DON, whereas the toxic strain is heterotrophic, it cannot utilize DIN, but it can utilize DON directly. These differences give rise to very different impacts on prey and nutrient concentrations in our model. Under high DIN/DON ratio conditions, the non-inducible cells grew much faster and were therefore more likely to bloom, but this advantage is substantially mitigated when the DIN/DON ratio is low. A turbulence parameterization was also incorporated into our model. The effect of this was to reduce the grazing rate of Pfiesteria when turbulence levels are high. According to our model, increased turbulence is more detrimental to the toxic functional type because it grows more slowly. The further imposition of microzooplankton grazing in the model showed that top-down control effects can be very significant, which is consistent with both laboratory and field studies and the general idea that plankton blooms can only happen in the absence of substantial grazing control. In general, our model results suggest that non-toxic blooms are more likely to occur in more turbulent inorganic-nutrient rich conditions, which are often found in more open coastal and estuarine waters that are subject to high inorganic loading. In contrast, toxic blooms are more likely to occur in calm, organic-nutrient rich conditions, which are often found in shallow, protected tributaries that are subject to high organic nutrient loading. Our model results also support the idea that the absence of strong grazing pressure is a prerequisite to bloom formation for both non-inducible and toxic strains of Pfiesteria. These results are generally consistent with observed patterns of toxic Pfiesteria blooms in Chesapeake Bay, the Neuse River of North Carolina and many other coastal and estuarine environments.     

The influence of spatio-temporal resource fluctuations on insular rat population dynamics

Local spatio-temporal resource variations can strongly influence the population dynamics of small mammals. This is particularly true on islands which are bottom-up driven systems, lacking higher order predators and with high variability in resource subsidies. The influence of resource fluctuations on animal survival may be mediated by individual movement among habitat patches, but simultaneously analysing survival, resource availability and habitat selection requires sophisticated analytical methods. We use a Bayesian multi-state capture-recapture model to estimate survival and movement probabilities of non-native black rats (Rattus rattus) across three habitats seasonally varying in resource availability. We find that survival varies most strongly with temporal rainfall patterns, overwhelming minor spatial variation among habitats. Surprisingly for a generalist forager, movement between habitats was rare, suggesting individuals do not opportunistically respond to spatial resource subsidy variations. Climate is probably the main driver of rodent population dynamics on islands, and even substantial habitat and seasonal spatial subsidies are overwhelmed in magnitude by predictable annual patterns in resource pulses. Marked variation in survival and capture has important implications for the timing of rat control.     

The relation of food availability and food preferences to the field diet of an echinoid Evechinus chloroticus (Valenciennes)

Echinoids are important in causing disturbance in many marine communities yet little is known of their foraging behaviour in the field. This work investigates the field diet of an echinoid species, Evechinus chloroticus (Valenciennes), in relation to food availability and food preferences. E. chloroticus is a generalist grazer occurring abundantly in a variety of algal and encrusting communities dominated by sponges. In the field, urchins grazed the sponge species according to abundance. In the laboratory, food preferences were demonstrated for two populations of E. chloroticus when presented with 24 species of sponge. One population of urchins came from an intertidal reef where sponges were rarely encountered and the other from a sublittoral habitat where the encrusting community was dominated by sponges. The sponges used as food in the preference experiments came from two different areas: one where urchins were almost absent, the other (from which the second group of urchins were taken) where urchins were abundant and were observed to feed upon sponges. When presented with sponge species unfamiliar to both urchin groups, urchins ranked sponge species similarly. Food preferences did not relate to the diet of the urchins in the field. Reasons for the difference between experimental results and field observations are suggested.     

Resource availability and population dynamics of Nicrophorus investigator, an obligate carrion breeder

1. Food resources for rearing young may influence insect populations. This is particularly true for insects that breed obligately on rare, ephemeral resources such as dung, fungi, or carrion. 2. Beetles in the genus Nicrophorus bury small vertebrate carcasses for rearing their young. Studies reviewed by Scott (1998) have found a positive relationship between carcass mass and total brood size. It is likely that access to carcasses suitable for breeding, and not food or mates, limits reproduction in both male and female Nicrophorus. Thus, small mammal densities could determine Nicrophorus population sizes. 3. The work reported here examined the relationship between Nicrophorus investigator (Coleoptera: Silphidae) population size and small mammal abundance at two sites over a 4‐year period. 4. Nicrophorus investigator buried and reared young on all the native small rodent species trapped at two sites in south‐western Colorado, U.S.A. (Peromyscus maniculatus, Microtus montanus, Zapus princeps, Tamias minimus, Thomomys talpoides). They preferred to bury and reproduce on rodent carcasses weighing between 16 and 48 g; rodents of this size represented 82% of captures. 5. Population sizes of N. investigator and small rodents were estimated simultaneously using mark‐recapture censuses over a 4‐year period. Considering only rodents within the size range used by N. investigator, the estimated small mammal biomass per hectare in one year and the beetle population size in the following year were correlated significantly.     

The effect of different primary producers on Hydrobia ulvae population dynamics: a case study in a temperate intertidal estuary

The effect of macroalgal blooms and the consequent disappearance of Zostera noltii meadows on Hydrobia ulvae population dynamics and production was studied in the Mondego estuary based on data obtained from January 1993 to September 1995. Sampling was carried out at a non-eutrophicated area, covered with Z. noltii, and also at an eutrophicated area, where seasonal Enteromorpha spp. blooms occur.Stable populations represented by individuals of all age classes were found only at the Z. noltii meadows throughout the study period. On the contrary, at the eutrophicated area, during most of the time, solely juveniles were present, with adults appearing only during the macroalgal bloom (>1.5 mm width).During the algal bloom (e.g. 1993), H. ulvae population density was clearly higher in the eutrophicated area due to the combined effect of stronger benthic recruitments (99% of veliger larvae newly recruited) and dispersion of juveniles proceeding from the Z. noltii meadows to this area. On the other hand, in the absence of macroalgae (spring of 1994), 98.9% of veliger larvae was recruited in the Z. noltii meadows. Therefore, H. ulvae seems to respond rapidly to macroalgal dynamics and its presence at the eutrophicated area depends on the existence of green macroalgae.H. ulvae presented the same benthic recruitment pattern at the two sampling areas, with new cohorts being produced in March, June, July and September. Depending on the time of the year in which the recruitment took place, cohorts showed different growth rhythms. However, after 12 months they reached a similar size.A three-generation life cycle involving a short-lived (16 months), fast growing spring generation, a medium growing (17-19 months) summer generation and a longer-lived (20 months) slower growing generation that overwinters is identified.As a general trend, productivity and mean population standing biomass were higher at the Z. noltii meadows, during the entire study, except for a short period, during the macroalgal bloom, when production was higher at the eutrophicated area. On the contrary, P/B? ratios were higher at the eutrophicated area. According to our results, H. ulvae population structure and yearly productivity are clearly affected by eutrophication, namely by the dynamics of macroalgal blooms.In the long run, we may infer that, following the disappearance of the Z. noltii meadows, due to eutrophication, H. ulvae would also tend to disappear, since reproductive adults were almost exclusively found in this area.     

The influence of population density on fission and growth of Holothuria atra in natural mesocosms

Investigation of the population dynamics, asexual reproduction (fission) and growth of holothuroids has been impeded by the difficulty of tagging individuals. We conducted the first tests on the interactions between population density, fission and growth of holothuroids in experimental populations placed in natural mesocosms (microatolls) at One Tree Reef (OTR), Great Barrier Reef. Similarly sized Holothuria atra were translocated to the microatolls in low (LDT) and high (HDT) density treatments. We hypothesised that holothuroids in lower density treatments would have more resources per individual and that this would promote higher frequencies of asexual reproduction and smaller individuals. The seasonal pattern of fission was similar in natural (unmanipulated) and experimental populations, with the maximum number of fission products occurring in winter and spring. The overall density of the LDT (0.19 ind. m^- 2) and HDT (0.59 ind. m^- 2) did not vary over time. This ‘steady state’ suggested that some fission products died and that asexual reproduction compensated for overall mortality and emigration. There was no difference in sediment chlorophyll pigments between treatments indicating that the different densities of H. atra did not affect benthic microalgal biomass. The percentage of fission products was greater in the LDT than the HDT but this difference was not statistically significant, providing some support for the hypothesis that H. atra in the LDT exhibit a higher fission rates. At the end of the experiment H. atra in LDT were significantly longer and heavier than in HDT. H. atra surpassed their initial deployment weight and length after 13 months in the LDT by 115.2% and 45.2% respectively and in the HDT 86.9% and 24.6% respectively, changing from the small to the large phenotype known for this species. This differential growth may be linked to habitat stability and high benthic productivity and demonstrates the phenotypic plasticity of holothuroids and potential to achieve ‘Optimum Individual Size’ with respect to environmental conditions. Our results will assist in finetuning conceptual models on asexual reproduction and future experimental studies on the phenomena of fission and plastic growth in holothuroids.     

Impact of hypoxia on the survival, egg production and population dynamics of Acartia tonsa Dana

Concern for the increasing occurrence of coastal zone hypoxia has generally focused on the direct, short-term impact of reduced dissolved oxygen (DO) concentrations on the survival of commercially important species such as fish and crabs. Copepods, especially the naupliar stages, are important pelagic food web components, yet only a few studies have considered the effect of reduced DO concentrations on their survival and population dynamics. This study considers the impact of both lethal and sublethal DO concentrations on copepods. Acartia tonsa were reared at 25 °C at saturating DO (normoxic control) and reduced (hypoxic) DO concentrations of 1.5 or 0.7 ml l⁻¹. Oxygen concentrations were maintained in replicate flasks, by bubbling seawater with air (control), or mixtures of nitrogen and oxygen. Egg production, but not survival, was significantly higher in the controls compared to the 1.5 ml l⁻¹ DO treatment. Survival and egg production were significantly lower at 0.7 ml l⁻¹ DO compared to the control. The results suggest that the sublethal as well as the lethal effects of hypoxia may have important repercussions on population and community dynamics in coastal systems.     

Effects of food availability on larval development in the slipper limpet Crepidula onyx (Sowerby)

Effects of food availability on the larval survival and development of Crepidula onyx were studied in four experiments by feeding the larvae with different concentrations of the chrysophyte Isochrysis galbana and by starving the larvae for different periods of time. Food concentration had a clear impact on the survival, growth and development time of C. onyx veligers. Larval development occurred only at 10⁴ cells ml⁻¹ and higher algal concentrations. No shell increment was detected in the veligers cultured for 12 days at 10² cells ml⁻¹I. galbana or the blank control. At 10³ cells ml⁻¹, there was only a slight increase in shell length over 12 days. At 10⁴ cells ml⁻¹, about 40% of the larvae became competent in 18 days. At 10⁵ and 10⁶ cells ml⁻¹, more than 90% of the larvae reached competence in 7 days. Initial starvation negatively affected the larval development, but the sensitivity differed among parameters measured on day 5: lower survivorship was detected only for larvae that had suffered 3 days or longer initial starvation, whereas one-day initial starvation caused shorter shells and lower percentage of competent larvae. Three days of continuous feeding was required for 50% of the larvae to reach competence. After feeding for 3 days, most larvae could become competent to metamorphose even under starvation. The time of starvation was also critical: larvae that suffered 1-day food deprivation in the first 2 days of larval release had shorter shells and lowered percent competent larvae than those that suffered the same length of food deprivation in later stages of development. Our study thus indicates that both food concentration and short-term starvation have detrimental effects on the larval development of this species, and that once the larva has consumed certain amount of food, starvation may induce metamorphosis.     

The effects of food sources on Japanese monkey home range size and location, and population dynamics

The effects of supplemental feeding by tourists on wild Japanese monkey's home range size and location, and troop size and composition were studied for two monkey troops, Troop A and Troop B, living along the Irohazaka loop road, Nikko National Park, central Japan. Changes were documented based on data gathered from 1982 to 1996 by the use of radio telemetry. Troop A's home range size shrank and changed from separate winter and summer ranges to a single, year-round home range, with its core located in a high elevation area where supplemental feeding by tourists was heavy. Troop B's home range also shrank and shifted to a lower elevation where supplemental feeding by tourists was heavy. Troop A's population size increased between the winters of 1983–1984 and 1990–1991 in conjunction with an increase in human encounter rates, and then decreased. Troop B's size increased until the winter of 1993–1994, and then decreased. The instability of troop size between 1993 and 1996 may be explained by documented factors such as a decrease in the adult sex ratio, an increase in the infant-female ratio, and an increase in juvenile mortality and/or emigration, all of which may have been influenced by supplemental feeding by tourists.