Variation In Food Abundance Research Articles

Effects of depth and water flow on growth, survival and bioactivity of two temperate sponges cultured in different seasons

Successful in-sea aquaculture of sponges for metabolite production requires a sound understanding of the effects of environmental factors on explant growth, survival and metabolite biosynthesis. Using short-term experiments, we examined the effects of season, water flow and depth on the culture response of the New Zealand Demospongiae Latrunculia wellingtonensis (Alvarez, Bergquist and Battershill) and Polymastia croceus (Kelly–Borges and Bergquist). Sponge pieces were cultured seasonally for 2 months at depths of 5 and 10 m at three sites close to parent populations but characterised by differing water flow: high, medium and low. Survival of L. wellingtonensis and P. croceus was lowest in summer, possibly because of the relatively higher water temperature increasing stress during translocation. Growth of farmed sponges varied greatly among seasons, being greatest for L. wellingtonensis in winter and P. croceus in spring, and lowest for both species in autumn. Seasonal variation in growth may result from seasonal variation in food abundance and/or periodic investment of metabolic resources in reproduction. Within each season, growth of both species was greatest overall at the high-flow site, with increases up to 12.5% in weight per month. Culture depth influenced the final size of transplanted sponges by interacting with season and water flow. Bioactivity, a measure of metabolite biosynthesis, was high for both species regardless of the environmental condition.

Sex-Specific Recruitment and Brood Sex Ratios of Eurasian Kestrels in a Seasonally and Annually Fluctuating Northern Environment

Timing of birth and food availability may select for biased offspring sex ratios when they differentially affect the reproductive value of male and female young. Here we show that early hatching date enhances more the probability of male Eurasian kestrels (Falco tinnunculus) to breed as one-year-old than that of females in a Finnish population. This rarely documented phenomenon has been previously observed in a kestrel population in the Netherlands. As kestrels in the Finnish population are migratory, our results refute the hypothesis that early-fledged males would have an advantage for early breeding only in resident populations. Contrary to the predictions, the Finnish population showed no change in brood sex ratio during the breeding season in a long-term data from 8years. As far as we know, this is the first demonstration that biased sex allocation may not occur even when it would appear to be adaptive. This result is different from the Dutch kestrel population, in which the season began with a bias towards males and ended with a bias in favour of females. We suggest that high inter-annual variation in food abundance in Finland might reduce selection for a sex ratio trend.

Differential species‐specific ectoparasitic mite intensities in two intimately coexisting sibling bat species: resource‐mediated host attractiveness or parasite specialization?

Summary The mechanisms underlying host choice strategies by parasites remain poorly understood. We address two main questions: (i) do parasites prefer vulnerable or well‐fed hosts, and (ii) to what extent is a parasite species specialized towards a given host species? To answer these questions, we investigated, both in the field and in the lab, a host–parasite system comprising one ectoparasitic mite (Spinturnix myoti) and its major hosts, two sibling species of bats (Myotis myotis and M. blythii), which coexist intimately in colonial nursery roosts. We exploited the close physical associations between host species in colonial roosts as well as naturally occurring annual variation in food abundance to investigate the relationships between parasite intensities and (i) host species and (ii) individual nutritional status. Although horizontal transmission of parasites was facilitated by the intimate aggregation of bats within their colonial clusters, we found significant interspecific differences in degree of infestation throughout the 6 years of the study, with M. myotis always more heavily parasitized than M. blythii. This pattern was replicated in a laboratory experiment in which any species‐specific resistance induced by exploitation of different trophic niches in nature was removed. Within both host species, S. myoti showed a clear preference for individuals with higher nutritional status. In years with high resource abundance, both bat hosts harboured more parasites than in low‐resource years, although the relative difference in parasite burden across species was maintained. This pattern of host choice was also replicated in the laboratory. When offered a choice, parasites always colonized better‐fed individuals. These results show first that host specialization in our study system occurred. Second, immediate parasite choice clearly operated towards the selection of hosts in good nutritional state.

The effect of food availability on the semilunar courtship rhythm in the fiddler crab Uca lactea (de Haan) (Brachyura: Ocypodidae)

Semilunar or lunar reproductive rhythms that follow tidal cycles are common in marine animals. For brachyuran crabs, an adaptive explanation for the synchronous release of larvae in phase with the tidal cycle is that females release larvae when their offspring are best able to escape predators. As a corollary to this hypothesis, the synchronous male reproductive cycle is selected to be in phase with female receptivity. As an alternative hypothesis, we propose that variation in food abundance influences the onset and intensity of the semilunar courtship cycle. We tested this hypothesis in male fiddler crabs (Uca lactea) by experimentally manipulating food availability for 4 weeks. Food-supplemented males built more semidomes and waved for more days than did food-deprived males or untreated control males. Moreover, food-supplemented males began courtship earlier and the median courtship day advanced with an increase in food. Courtship intensity was not related to crab size. These results provide the first evidence in marine animals that changes in food abundance due to the tides, and hence in phase with the semilunar cycle, influence male reproductive rhythm.

Hatching asynchrony and brood reduction in Tengmalm's owl Aegolius funereus: the role of temporal and spatial variation in food abundance.

Hatching asynchrony is the consequence of birds initiating incubation before clutch completion. It has been suggested that variation in hatching asynchrony in owls is extensive, and therefore they should be excellent objects to study the effects of spatio-temporal variation in food abundance on this phenomenon. We examined how abundance and predictability of food affected hatching asynchrony in Tengmalm's owl Aegolius funereus (Linnaeus), which mainly feeds on voles which fluctuate in 3- to 4-year cycles in northern Europe. Hatching span averaged 6-7days (range 0-13days) and increased with clutch size. Food supply did not directly influence levels of hatching asynchrony but it influenced indirectly via marked among-year changes in clutch size. During the decrease phase of the vole cycle the proportion of hatchlings producing fledglings decreased with asynchrony, suggesting that chick mortality was most common among asynchronous broods when food became scarce. This finding is consistent with Lack's brood reduction hypothesis, i.e. that if food becomes scarce during the nestling period the youngest nestlings would die first without endangering the survival of the whole brood.

Population dynamics and chemical ecology of New Zealand Demospongiae Latrunculia sp. nov. and Polymastia croceus (Poecilosclerida: Latrunculiidae: Polymastiidae)

For 3 years aspects of the population dynamics, growth, and bioactivity (measure of biologically active metabolite biosynthesis) of the Demospongiae Latrunculia sp. nov. and Polymastia croceus (Kelly‐Borges & Bergquist) were examined on a subtidal reef on the Wellington south coast, New Zealand. For both species, survival of adult sponges was high in all seasons, whereas juvenile sponges had poor survival. Recruitment of Latrunculia sp. nov. occurred in all seasons indicating that this species is reproductively active throughout the year. P. croceus recruited mostly in autumn, supporting previous work that found the sponge to be reproductively active in summer and early autumn only. For both sponge species, growth rates varied greatly between individuals and were unaffected by initial sponge size within the range examined. Sponges generally grew during winter and spring as the water temperature rose, and shrank during summer and autumn as the water temperature fell. This growth pattern may relate to seasonal variation in food abundance, and for P. croceus it may result also from seasonal differences in reproductive investment. After 2 years, Latrunculia sp. nov. and P. croceus had on average, halved and doubled in size, respectively. Latrunculia sp. nov. showed a seasonal pattern of bioactivity, being most active in spring possibly to prevent the surface overgrowth of fouling organisms. P. croceus had no seasonal pattern of bioactivity, but individuals were either very active or inactive. The bioactive metabolites in both species possibly aid in competitive interactions and prevent predation and biofouling.

Intersexual niche differentiation in the blue tit (Parus caeruleus)

The intersexual niche differentiation hypothesis, according to which sexual size dimorphism (SSD) has evolved to facilitate niche separation between the sexes, has received equivocal support in studies of birds. One explanation for the inconsistency of the results is that other factors crucial for reproductive success, such as variation in food abundance among territories, may obscure food niche differentiation between parents. We studied the effects of SSD, food availability and food demands of broods on niche differentiation in the blue tit (Parus caeruleus). Parents attending experimentally enlarged broods delivered the same amount, but more diverse food items, to their offspring than parents attending reduced broods, although this effect was significant in only one of the two study years. The time-lag (deviation from the peak food abundance in habitat in days) and squared time-lag were the most important determinants of the diversity of food items delivered to young and their condition (fitness surrogate). SSD, as measured by principal component scores of morphological traits, did not explain any variation neither in the diversity or amount of food provided, nor in offspring condition. These results suggest that synchronization of breeding with peak food availability is the most important determinant of diversity of food delivered to young and, more importantly, Their condition. No evidence for intersexual food niche differentiation was found; at best, it is likely to constitute only a weak selection pressure on the maintenance or enhancement of SSD in this system.

Intersexual niche differentiation in the blue tit (Parus caeruleus)

The intersexual niche differentiation hypothesis, according to which sexual size dimorphism (SSD) has evolved to facilitate niche separation between the sexes, has received equivocal support in studies of birds. One explanation for the inconsistency of the results is that other factors crucial for reproductive success, such as variation in food abundance among territories, may obscure food niche differentiation between parents. We studied the effects of SSD, food availability and food demands of broods on niche differentiation in the blue tit (Parus caeruleus). Parents attending experimentally enlarged broods delivered the same amount, but more diverse food items, to their offspring than parents attending reduced broods, although this effect was significant in only one of the two study years. The time-lag (deviation from the peak food abundance in habitat in days) and squared time-lag were the most important determinants of the diversity of food items delivered to young and their condition (fitness surrogate). SSD, as measured by principal component scores of morphological traits, did not explain any variation neither in the diversity or amount of food provided, nor in offspring condition. These results suggest that synchronization of breeding with peak food availability is the most important determinant of diversity of food delivered to young and, more importantly, their condition. No evidence for intersexual food niche differentiation was found; at best, it is likely to constitute only a weak selection pressure on the maintenance or enhancement of SSD in this system.

Influence of annual variation in food supply on abundance of wintering common eiders Somateria mollissima

We investigated how the annual variation in the abundance of wintering common eiders Somateria mollissima was related to the abundance of benthos over 4 yr. The study was conducted at 2 sites, Tuno Knob and Ringebjerg Sand, Denmark, from 1994 to 1998. Benthos was sampled along permanent transects in November, and common eiders counted and localized from observation towers during the winter. The benthic community was dominated by the blue mussel Mytilus edulis, with sizes varying with depth. Small and intermediary-sized individuals were primarily found in 0 to 6 m depth, and large individuals exclusively in 6 to 12 m depth. The pattern of variation in abundance of common eiders matched that of the benthos in 0 to 6 m depth, where they were mostly found, but not in 6 to 12 m depth. Based on a bioenergetic model of food consumption, we estimated that com- mon eiders during the 3rd and 4th years exploited between 22 and 64% of the entire benthic com- munity biomass in 0 to 6 m depth, compared to  3% in 6 to 12 m depth. We conclude that common eiders tracked annual variation in food abundance, primarily related to the presence of small and intermediary-sized blue mussels.

Parental Effort and Blood Parasitism in Tengmalm's Owl: Effects of Natural and Experimental Variation in Food Abundance

We investigated the association between parental effort and susceptibility to haematozoan parasites in vole-eating Tengmalm's owls (Aegolius funereus) from 1993 to 1995. In a poor vole year (1993), almost all breeding Tengmalm's owls were infected with Trypanosoma avium, whereas in a good vole year (1994), only a few owls were infected. In a moderate vole year (1995), we found an intermediate prevalence of trypanosomes. In the moderate vole year, trypanosome-infected females were in poorer condition than were uninfected females. In the same moderate vole year, high parental effort was associated with increased susceptibility to haematozoan parasites for both genders, whereas in a good vole year no such association was found. In two breeding seasons (1996-1997) of relatively low vole abundance we tested whether supplementary food decreased parasite loads. In accordance with correlative data, trypanosome prevalence was lower among supplemented than control females. Our results support a hypothesis of a trade-off between parental effort and immunocompetence, and emphasize the importance of varying environmental conditions and physical condition of individuals on susceptibility to haematozoan infections.

Breeding Biology of Heermann's Gulls on Isla Rasa, Gulf of California, Mexico

During 1980 and 1981, I studied nest-site selection and reproductive success in Heermann's Gull (Larus heermanni) on Isla Rasa in the Gulf of California, Mexico. About 95% of the world population of this species nests on Isla Rasa. In 1980, about 130,240 nests were present within an area of 0.8 km 2. In the Midriff Island Region (which includes Isla Rasa), oceanic productivity is high even during E1 Nifio events because of tidal upwelling. My results indicate that the Heermann's Gull is characterized by high nesting density (up to 110 nests per 100 m 2) and high nesting synchrony. Massive synchronized communal flights to nesting areas occurred every evening during several weeks prior to egg laying. Nesting density in valleys (71 per 100 m ) was higher than on rocky hills (9.5 per 100 m2). Clutch size was higher in valleys in 1981 than in 1980 and was lowest on rocky substrates on hills. Hatch- ing success was independent of clutch size but varied with substrate and year. Fledging suc- cess among broods was independent of hatching order. Fledging success and reproductive success (i.e. the number of chicks fledged/number of eggs laid) were higher in valleys in 1981 and were lowest on rocky hills in 1980, and reproductive success was significantly high- er for three-egg clutches than for one-egg clutches. The observed patterns of reproductive performance may be correlated with annual variation in food abundance and with exposure to predators in different nesting habitats and among different nesting densities. Received 2 December 1996, accepted 30 October 1998.

Clutch Size of Kestrels: Seasonal Decline and Experimental Evidence for Food Limitation under Fluctuating Food Conditions

We studied the clutch size of Eurasian kestrels, Falco tinnunculus in western Finland during 1985-1996 (the total number of nests 546). Voles, the main food of kestrels, fluctuated in three-year population cycles. In low vole years, kestrels started to lay eggs in mid-May and on average 8 to 10 days later than in the increase and decrease years. Yearly clutch sizes were also smaller in low vole years (yearly mean 4.4 to 5.3) than in the increase and decrease years (5.4 to 5.9). Clutch size showed a similar seasonal decline (0.05 egg per day) in each phase of the vole cycle. After controlling for the effect of laying date, the phase of the vole cycle appeared to be the main determinant of clutch size; kestrels produced smaller clutches in the low than in the increase and decrease vole years. Supplementary feeding before and during the egg-laying periods in 1986-1988 did not advance the start of egg-laying and did not prevent the seasonal decline in clutch size, but resulted in larger clutches. These results show that both laying date and abundance of main food at the early stage of breeding are crucial determinants of clutch size in kestrels. Our result, that variation in food abundance can alter the relationship between clutch size and laying date, supports the quality hypothesis (that differences in quality between individual birds and/or their territories may result in the seasonal decline of clutch size). We suggest that food is limiting clutch size in kestrels both during the egg-laying and nestling periods, and not only at low but also at high levels of natural food abundance.

ASSESSING THE RISK OF EXTINCTION FOR THE BROWN BEAR (URSUS ARCTOS) IN THE CORDILLERA CANTABRICA, SPAIN

The status of the brown bear (Ursus arctos) in Spain has suffered a dramatic decline during the last centuries, both in area and numbers. Current relict populations are suspected to be under immediate risk of extinction. The aim of our model is to attain an understanding of the main processes and mechanisms determining population dynamics in the Cordillera Cantabrica. We compile the knowledge available about brown bears in the Cordillera Cantabrica, northern Spain, and perform a population viability analysis (PVA) to diagnose the current state of the population and to support current management. The specially constructed simulation model, based on long-term field investigations on the western brown bear population in the Cordillera Cantabrica, includes detailed life history data and information on environmental variations in food abundance. The method of individual-based modeling is employed to simulate the fate of individual bears. Reproduction, family breakup, and mortalities are modeled in annual time ...

Effects of food abundance and environmental parameters on foraging behavior of gadwalls and American coots in winter

Diurnal behaviors of wintering gadwalls (Anas strepera) and American coots (Fulica americana) were studied at Guntersville Reservoir, Alabama. Feeding accounted for a substantial amount of time spent by gadwalls (>50%) and coots (>62%). Feeding rate (pecks per minute) of female gadwalls was higher than that of males, and foraging speed (metres per minute) of unpaired gadwalls was higher than that of paired birds. For both gadwalls and coots, foraging speed increased and feeding rate decreased over time. Stepwise multiple regression was used to test the effects of food abundance, day length, ambient temperature, and mean daily wind speed on feeding rate and foraging speed. Foraging speed increased with declining food abundance for both gadwalls and coots. Additionally, day length accounted for a small but significant amount of variation in foraging speed of coots. Moving faster while feeding probably allowed gadwalls and coots to maintain adequate intake of nutritious food items from a declining food resource. Variation in feeding rate of gadwalls was not explained by food abundance or environmental variables. Feeding rate of coots decreased with declining food abundance and increased with decreasing ambient temperature. As food abundance declined, reduced density of suitable food items resulted in fewer surface pecks and more time-consuming dives. The higher feeding rate of coots at lower ambient temperatures probably resulted from increased thermoregulatory costs. Unlike coots, gadwalls typically do not dive, and adjusted to reduced food abundance at the surface by stealing food from diving coots. Because gadwalls exploited coots bringing food items to the surface in January and February, gadwall feeding rate during this time was somewhat uncoupled from variation in food abundance at the surface. Although diurnal foraging time is limiting for some waterfowl species, results from the present study indicate that variation in foraging behaviors of gadwalls and coots wintering in Alabama predominantly was a response to changes in food abundance.

Caterpillar abundance in the territory affects the breeding performance of great tit Parus major minor.

The effects of caterpillar food supply on the breeding performance of a population of the Japanese great tit Parus major minor were investigated. Since more than 90% of the food items in our study site were caterpillars living on trees, we estimated the food availability using 20 frass traps per hectare. The sampling error of this method was about 10% on average, which was accurate enough to detect differences between territories. Food abundance at laying in each territory affected the timing of egg laying. However, food amount after hatching was correlated with clutch size. No relationship was found between fledgling quality and food availability, probably because the effects of local variation in food abundance could be canceled out by parental effort such as extending the foraging area. There was a significant negative correlation between the length of the nestling period and food availability. We suggest that parent tits decide the timing of fledging at the point where two factors, predation risk before fledging and additional improvement of nestling quality, are balanced. Food availability just after fledging affected the length of post-fledging parental care; it seems that fledglings in "poor" territories would have had difficulty in finding food and hence needed to depend on their parents longer than those in "rich" territories.

Food selection by calanoid copepods in response to between‐lake variation in food abundance

SUMMARY1. Food selection experiments were conducted by acclimating calanoid copepods (Eudiaptomus spp.) in suspensions of natural seston and then adding pairs of dual‐labelled (14C/32P) algae. Each feeding trial measured selectivity between a small, high‐quality reference alga, Chlamydomonas reinhardii, and a test alga that differed in size and/or food quality. The influence of food concentration on food selection was tested by using seston from two lakes with contrasting food abundance and by including treatments with filtered lake water ('starved’) and seston diluted with filtered water or enriched with cultured algae.2. Copepods that had been starved or acclimated to natural seston with low food abundance preferred the larger of two labelled algae, regardless of the nutritional quality of the algae. In agreement with the predictions of an optimal diet model, however, copepods that were acclimated to high food conditions discriminated against low‐quality foods, including digestion‐resistant algae and dead algae.3. Selectivity coefficients showed excellent agreement with a previous study involving the same taxa of copepods and labelled algae but in which the copepods had been acclimated to pairs of cultured algae rather than natural seston. Thus, these comparisons emphasize the importance of food availability in modulating copepod selectivity for foods that differ in nutritional quality and suggest that such behaviour occurs in nature.

Kleptoparasitism of American Coots by Gadwalls and Its Relationship to Social Dominance and Food Abundance

Kleptoparasitism of American Coots (Fulica americana) by Gadwalls (Anas strep- era) was recorded from October 1991-February 1992. We used these data to test whether frequency and rate of kleptoparasitism were related to variation in food abundance and dominance status of Gadwalls. Overall, 30% (n = 423) of Gadwalls observed were kleptopar- asitic; the relative frequency and rate of kleptoparasitism varied monthly. The proportion of individuals engaged in food stealing and the rate of kleptoparasitism were greatest for subordinate sex/pair status classes of Gadwalls. When comparing diurnal time-activity bud- gets, kleptoparasitic Gadwalls spent less time feeding (56 vs. 68%) and more time searching for food (37 vs. 24%) than did individuals not participating in kleptoparasitism. watermilfoil (Myriophyllum spicatum) was the primary food of Gadwalls and American Coots. As this aquatic macrophyte declined in abundance over the winter as a result of the effects of herbivory and natural senescence, so did numbers of coots and Gadwalls. When coot and Gadwall numbers were held constant statistically, the rate of kleptoparasitism was inversely related to milfoil cover. Food-stealing behavior of Gadwalls was influenced by social domi- nance and food abundance; it was mediated by host/parasite numbers. Results suggest that kleptoparasitism is an alternative foraging strategy used by subordinate Gadwalls that do

Variations in food abundance for Japanese sardine larvae related to the Kuroshio meander

ABSTRACTLong‐term fluctuations of chlorophyll a concentration, and abundance of herbivorous or omnivorous small copepods during the spawning season of the Japanese sardine, Sardinops melanosticta, were examined in relation to the types of Kuroshio meander. The purpose was to clarify the influence of the meander on the production of food organisms for the sardine larvae and their survival in the Pacific coastal region of central Japan. During an A‐type meander of the Kuroshio, when the offshore and inshore movements of the Kuroshio path were small except at the beginning of the meander, the surface chlorophyll a concentration in the coastal region was lower than that during a non‐A‐type meander with frequent and prompt fluctuations of the Kuroshio path. The abundance of small copepods was also low during the A‐type meander except in the subsequent spring just after the beginning of an A‐type meander, but often high during the non‐A‐type meander. Prompt fluctuations of the Kuroshio path during the non‐A‐type meander probably cause local upwellings which stimulate primary and secondary production near the Kuroshio, and may enhance the survival of the sardine larvae. On the contrary, the quasi‐stationary state of an A‐type meander may be unfavourable for the production of phytoplankton and nauplii of herbivorous or omnivorous small copepods, and therefore the survival of the sardine larvae becomes poor.

Control of Phenotypic Plasticity Via Regulatory Genes

Control of Phenotypic Plasticity Via Regulatory Genes

Growth-rate dependent feeding rates in Daphnia pulicaria and Brachionus rubens: adaptation to intermediate time-scale variations in food abundance

Growth-rate dependent feeding rates in <i>Daphnia pulicaria</i> and <i>Brachionus rubens</i>: adaptation to intermediate time-scale variations in food abundance