Comparative species composition and relative abundance of decapod crustaceans in marine habitats of Panamá

The community structure of the decapod crustacean fauna of 7 tropical, shallowwater, marine habitats (sandy beaches, mangrove swamps and rocky intertidal habitats on both the Pacific and Caribbean coasts of Panama, and Pocillopora damicornis coral habitat of the Bay of Panama) were examined and analyzed for species composition and relative abundances. Collections from the 7 habitats yielded 4361 individuals, representing 236 species. The number of species per habitat was (Pacific, Caribbean): sandy beach (16, 7); mangrove (20, 17); P. damicornis (53); rocky intertidal (78, 67). There were more species represented by more individuals in the Pacific habitats. An index of faunal similarity was calculated for each pair (Pacific-Caribbean) of habitats. This index is the number of ecologically similar congeneric species which occurred in both habitats expressed as a percentage of the total number of species present in the pair of habitats. For the sandy beach communities there were three Pacific species which were similar to three Caribbean species, a similarity of 6/23 or 26%. The index of similarity for the mangrove communities is 54% and for the rocky intertidal communities it is 37%. The P. damicornis community has affinities with the Pacific rocky intertidal community (18%), with that of the Caribbean rocky intertidal (16%) and with that of Indo-West Pacific pocilloporid corals (20%). A few specialized species dominated each of the communities. The habitats and the number of species accounting for over half of the individuals present are (Pacific, Caribbean): sandy beach (1, 1); mangrove (4, 4); P. damicornis (5); and rocky intertidal (3, 6). Most of the species in each community were represented by one or a few individuals.     

Radiation induced effects on viability and antioxidant enzymes of crustaceans from different habitats

The paper describes differential tolerance of two fresh water crustaceans Mesocyclops hyalinus and Allodiaptomus satanus to 60Co gamma radiation. Mesocyclops hyalinus is dominant species at site 1, near a Thermal Power Plant at Kolaghat East Midnapore where fly ash deposition is a regular phenomenon. Allodiaptomus satanus is dominant species at site 2 at Kolkata, Ballygung where anthroponotic activities are more pronounced. M. hyalinus is naturally exposed to more stressful situation than A. satanus as revealed by comparing the hydrological parameters of the two habitats. Experimental exposure to ionizing radiation resulted in differential changes in viability morphology and antioxidant enzyme activities in the two selected species. Survival experiments showed greater tolerance of M. hyalinus compared to A. satanus up to 8Gy (absorbed dose) after which if showed drastic fail in survival. More pronounced morphological changes were observed in A. satanus as compared to that in M. hyalinus. The pattern of changes in antioxidant enzyme activity is distinctly opposite in the two radiation exposed species. While in M. hyalinus stimulation in activity of both CAT (excepting at 10Gy absorbed dose) and SOD was observed A. satanus showed decrease in activity of both the enzymes when compared to their unirradiated counterparts.     

Observations on the embryonic development of some deep-living decapod crustaceans,with particular reference to species of Acanthephyra

Eggs of meso- and bathypelagic decapod crustaceans were removed from gravid females and allowed to develop in vitro. The time course of embryonic development, as indicated by the appearance of certain morphological and physiological features, was determined for most species at a constant temperature of 12°C. The embryonic period, and the time between the appearance of certain key features such as naupliar and compound eyes and heartbeats, showed little variation in any one species. Significant differences were observed in the development times of different species. In general, species with large eggs have a longer embryonic period than those with small eggs, but there also variations between species with eggs of similar size. During the development of Acanthephyra eggs there is an approximate doubling of the egg volume, correlated with a decrease in egg density and an increase in the water content. The increase in the water content and egg volume is similar to that of other decapod eggs. It is concluded that those species with small eggs could have several broods in 1 year, whereas those species with large eggs are likely to be restricted to one, or in some cases possibly two, broods.     

Extremes, plasticity, and invariance in vertebrate life history traits: insights from coral reef fishes

Life history theory predicts a range of directional generic responses in life history traits with increasing organism size. Among these are the relationships between size and longevity, mortality, growth rate, timing of maturity, and lifetime reproductive output. Spanning three orders of magnitude in size, coral reef fishes provide an ecologically diverse and species-rich vertebrate assemblage in which to test these generic responses. Here we examined these relationships by quantifying the life cycles of three miniature species of coral reef fish from the genus Eviota (Gobiidae) and compared their life history characteristics with other reef fish species. We found that all three species of Eviota have life spans of < 100 days, suffer high daily mortality rates of 7-8%, exhibit rapid linear growth, and matured at an earlier than expected size. Although lifetime reproductive output was low, consistent with their small body sizes, short generation times of 47-74 days help overcome low individual fecundity and appear to be a critical feature in maintaining Eviota populations. Comparisons with other coral reef fish species showed that Eviota species live on the evolutionary margins of life history possibilities for vertebrate animals. This addition of demographic information on these smallest size classes of coral reef fishes greatly extends our knowledge to encompass the full size spectrum and highlights the potential for coral reef fishes to contribute to vertebrate life history studies.     

Seasonal polyphenism in life history traits: time costs of direct development in a butterfly

Insects with two or more generations per year will generally experience different selection regimes depending on the season, and accordingly show seasonal polyphenisms. In butterflies, seasonal polyphenism has been shown with respect to morphology, life history characteristics and behaviour. In temperate bivoltine species, the directly developing generation is more time-constrained than the diapause generation, and this may affect various life history traits such as mating propensity (time from eclosion to mating). Here, we test whether mating propensity differs between generations in Pieris napi, along with several physiological parameters, i.e. male sex pheromone synthesis, and female ovigeny index and fecundity. As predicted, individuals of the directly developing generation—who have shorter time for pupal development—are more immature at eclosion; males take longer to synthesise the male sex pheromone after eclosion and take longer to mate than diapause generation males. Females show the same physiological pattern; the directly developing females lay fewer eggs than diapausing females during the first days of their life. Nevertheless, the directly developing females mate faster after eclosion than diapausing females, indicating substantial adult time stress in this generation and possibly an adaptive value of shortening the pre-reproductive period. Our study highlights how time stress can be predictably different between generations, affecting both life history and behaviour. By analysing several life history traits simultaneously, we adopt a multi-trait approach to examining how adaptations and developmental constraints likely interplay to shape these seasonal polyphenisms.     

Observations on the burrows and burrowing behaviour of two mud-dwelling decapod crustaceans,Nephrops norvegicus and Goneplax rhomboides

The burrows of the Norway lobster Nephrops norvegicus (L.) and of the crab Goneplax rhomboides (L.) were studied in Loch Torridon, Scotland. Polyester resin casts of burrows in the sea were made by divers to reveal their subsurface form. Tunnels made by N. norvegicus were usually simple, with two or more openings on the mud surface, and penetrated to a depth of about 30 cm. G. rhomboides burrows did not descend more than about 15 cm beneath the surface, but were usually more complex than the lobster burrows and had several openings. The methods of burrow construction used by the two crustaceans are described from aquarium observations. Neither N. norvegicus nor G. rhomboides show obvious morphological adaptations for burrowing, and it is suggested that the fossorial habit was adopted very early by decapods. The burrows of N. norvegicus do not seem to have assumed any functions in addition to the original one of providing refuge from predators. There is not sufficient known of the biology of the crab to indicate whether the same is true in its case.     

Prediction of extinction in plants: interaction of extrinsic threats and life history traits

The global extinction of species proceeds through the erosion of local populations. Using a 60-year time series of annual sighting records of plant species, we studied the correlates of local extinction risk associated with a risk of species extinction in the Park Grass Experiment where plants received long-term exposure to nutrient enrichment, soil acidification, and reductions in habitat size. We used multivariate linear models to assess how extrinsic threats and life history traits influence extinction risk. We investigated effects of four extrinsic threats (nitrogen enrichment, productivity, acidification, and plot size) as well as 11 life history traits (month of earliest flowering, flowering duration, stress tolerance, ruderalness [plant species' ability to cope with habitat disturbance], plant height, diaspore mass, seed bank, life form, dispersal mode, apomixis [the ability for a species to reproduce asexuall through seeds], and mating system). Extinction risk was not influenced by plant family. All of the 11 life history traits except life form and all threat variables influenced extinction risk but always via interactions which typically involved one threat variable and one life history trait. We detected comparatively few significant interactions between life history traits, and the interacting traits compensated for each other. These results suggest that simple predictions about extinction risk based on species' traits alone will often fail. In contrast, understanding the interactions between extrinsic threats and life history traits will allow us to make more accurate predictions of extinctions.     

Natural variation in the concentrations of haemocyanin from three decapod crustaceans, Nephrops norvegicus, Liocarcinus depurator and Hyas aranaeus

Natural variation in the concentrations of haemocyanin ([Hc]) is examined for three decapod species collected from two different locations (Gullmarsfjord and Kattegat) off the west coast of Sweden (August to September 1998). Only one of the frequency distributions for [Hc] is normally distributed, and median values differ both between and within species. Differences in [Hc] cannot be attributed to sex, reproductive condition or moulting condition (over the limited range of moult stages examined) for any of the species. While body size did not scale with [Hc] for the Norway lobster Nephrops norvegicus or the swimming crab Liocarcinus depurator, it did for the spider crab Hyas araneus. To our knowledge this is the first time a negative relationship between body size and [Hc] has been reported for any crustacean. Thus the right-skewed frequency distribution of spider crab [Hc] could be accounted for by the right-skewed body size distribution recorded. The shape of the frequency distributions for [Hc] from the other two species could not be accounted for through differences in the factors examined here, although it is suggested that the amount of food consumed (or not) may be important. Received: 19 July 1999 / Accepted: 4 October 1999     

Trade-offs between time, predation risk and life history, and their implications for biogeography: A systems modelling approach with a primate case study

Group sizes are often considered to be the result of a trade-off between predation risk and the costs of feeding competition. We develop a model to explore the interaction between different ecological constraints on group sizes, using a primate (baboons) case study. The model uses climatic correlates of time budgets to predict maximum ecologically tolerable group size, and climatic predictors of predation risk (reflected mainly in predator density and female body mass) to predict minimum tolerable group size for any given habitat. As well as defining the range of sustainable group sizes for a given habitat, the model also allows us to reliably predict our exemplar taxon's biogeographical distribution across Africa. We also explore the life history implications of the model to ask whether baboons form group sizes which maximise survival or fecundity in the classic trade off between these two key life history variables. Our results indicate that, within the range of study sites in our sample, baboons prefer to maximise fecundity. However, the data indicate that in higher predation risk habitats they would switch to maximising survival at the expense of fecundity. We argue that this is due to the fact that interbirth interval and developmental rates have a ceiling that cannot be breached. Thus, while females can shorten interbirth intervals to compensate for increased predation risk, there is a limit to how much these life history variables can be altered, and when this is reached the best strategy is to maximise survivorship.     

Molt cycle alterations in behavior,feeding and diel rhythms of a decapod crustacean,the spiny lobster Panulirus argus

The crustacean molt cycle manifests extensive behavioral changes in addition to physiological and integumentary modifications. The paucity of quantitative studies led us to characterize molt stage dependent alterations in rhythmic locomotor, feeding, and agonistic behavior of subadult spiny lobster Panulirus argus held grouped and solitarily in simulated natural conditions. Non-disruptively determined molt stages were defined as proportions of intermolt duration. Significant nocturnal rhythmicity persisted through the full molt cycle, though daily form varied as a function of activity level and molt stage. A distinct early evening peak signaled initiation of foraging and walking behavior. Thereafter, rhythm amplitude either declined progressively (low activity: unimodal pattern), remained high (high activity: nocturnal plateau), or exhibited a secondary early morning peak (intermediate activity: bimodal and polymodal patterns). Activity ceased at or immediately prior to sunrise. Ecdysis was predominantly nocturnal, probably reflecting species specific spatiotemporal movement patterns and social behavior. A selective advantage of nocturnal ecdysis was postulated concerning avoidance of cannibalism and diurnal predators. Locomotor activity and feeding rates were not equivalent through the molt cycle, though both peaked in stages B₂-C₁. Locomotor activity remained high in metecdysis, decreased sharply at proecdysis initiation (D₀), reaching lowest levels in D₁-D₃, then increased in D₃ through B₁. Activity dropped steeply at ecdysis, though lobsters were capable of intense and coordinated activity. Feeding decreased slowly through metecdysis after the B₂-C₁ maximum, then increased temporarily in C₄ and D₀, indicating heightened feeding motivation. This contrasts with the locomotor activity decrease at proecdysis. Food consumption declined rapidly in D₁ and D₂ and ceased at the D₂-D₃ transition. Feeding resumed in B₁ or B₂, intensifying to maximum in late B₂. Feeding remained relatively constant within stages, whereas locomotor activity varied greatly, though both correlated with metabolic needs. Grouped and solitary lobsters displayed similar patterns of foraging and walking, equivalent to those of locomotor activity and feeding of solitary individuals. Frequency of agonistic interactions (not aggression per se) remained relatively constant through the cycle, peaking in metecdysis, though the highest relative proportion occurred near ecdysis. Lobsters then were submissive and avoided physical contact with conspecifics. Clearly, locomotor activity, feeding, and social behavior of P. argus are not simply determined. Indeed, behavior is distinctly phase coordinated with varying metabolic requirements dependent on the proximity to ecdysis and ecological pressures.     

Habitat harshness and morphodynamics: life history traits of the mole crab Emerita brasiliensis in Uruguayan sandy beaches

The habitat harshness hypothesis (HHH) postulates that in reflective beaches the harsh environment forces organisms to divert more energy towards maintenance and they therefore have lower abundance, fecundity, growth and survival rates than in dissipative beaches. Recent investigations have tested this hypothesis through single comparisons of only two beaches, and thus the observed trends in population level variables cannot be attributed incontestably to the beach state, but only to location. Here, abundance, reproduction, recruitment, population structure and body size of the intertidal mole crab Emerita brasiliensis were compared between populations from eight microtidal exposed sandy beaches with contrasting morphodynamics, sampled bimonthly during 22 months throughout the 180 km Uruguayan Atlantic coast. Physical variables and compound indices of the beach state were used to categorize sandy beaches. The results of this bi-annual large-scale analysis were fully consistent with the predictions of the HHH: abundance (total and population components), duration of the reproduction and recruitment seasons and the individual size of megalops and females of the mole crab E. brasiliensis decreased from dissipative to reflective beaches. This was reflected by linear or, mostly, nonlinear relationships between biological and both physical variables and compound indices of beach state. In conclusion, this multi-beach sampling provides compelling evidence of a consistent response of demographic and life history traits of an intertidal beach species to morphodynamic characteristics.     

From metamorphosis to maturity in complex life cycles: equal performance of different juvenile life history pathways

Performance in one stage of a complex life cycle may affect performance in the subsequent stage. Animals that start a new stage at a smaller size than conspecifics may either always remain smaller or they may be able to "catch up" through plasticity, usually elevated growth rates. We study how size at and date of metamorphosis affected subsequent performance in the terrestrial juvenile stage and lifetime fitness of spadefoot toads (Pelobates fuscus). We analyzed capture-recapture data of > 3000 individuals sampled during nine years with mark-recapture models to estimate first-year juvenile survival probabilities and age-specific first-time breeding probabilities of toads, followed by model selection to assess whether these probabilities were correlated with size at and date of metamorphosis. Males attained maturity after two years, whereas females reached maturity 2-4 years after metamorphosis. Age at maturity was weakly correlated with metamorphic traits. In both sexes, first-year juvenile survival depended positively on date of metamorphosis and, in males, also negatively on size at metamorphosis. In males, toads that metamorphosed early at a small size had the highest probability to reach maturity. However, because very few toadlets metamorphosed early, the vast majority of male metamorphs had a very similar probability to reach maturity. A matrix projection model constructed for females showed that different juvenile life history pathways resulted in similar lifetime fitness. We found that the effects of date of and size at metamorphosis on different juvenile traits cancelled each other out such that toads that were small or large at metamorphosis had equal performance. Because the costs and benefits of juvenile life history pathways may also depend on population fluctuations, ample phenotypic variation in life history traits may be maintained.     

The effects of size-dependent predation risk on the interaction between behavioral and life history traits in a stream-dwelling isopod

In field surveys, laboratory observations and field-based assays of behavior, I examined the effects of size-dependent predation risk on the interaction between size at reproductive maturity and maternal care behavior in the stream-dwelling isopod, Lirceus fontinalis. L. fontinalis exhibit population-specific sizes at reproductive maturity which result in population differences in predation risk during the adult phase. Females from streams containing salamander larvae (that prefer small prey) mature at large sizes and then become relatively safe from predation. Females from streams containing fish (that consume all size classes of prey equally) mature at small sizes and remain at risk. I tested whether these differences in expected survival were reflected in the behavior of females during the maternal phase (i.e., the period during which females exhibit maternal care). Female L. fontinalis carry developing juveniles inside a brood pouch. I simulated predatory attacks on gravid female L. fontinalis from the different population types and found that female behavior correlated with population differences in risk. When “attacked”, females from streams with predatory fish (that experience high risk to adult females) released juveniles from the brood pouch, whereas females from populations with predatory salamander larvae (that pose relatively little risk to adult females) did not release juveniles. I discuss the results with reference to the joint evolution of behavioral and life history traits. Received: 6 March 1996 /Accepted after revision: 12 August 1996     

Early life history traits of the gold-eye rockfish, Sebastes thompsoni, in relation to successful utilization of drifting seaweed

The gold-eye rockfish Sebastes thompsoni lives in association with drifting seaweed during the early developmental period. Early life history traits of the gold-eye rockfish were studied by means of otolith analysis of larvae and juveniles associated and unassociated with drifting seaweed in the waters off southern Sanriku, Tohoku, northern Japan, from 1993 to 1995.␣Daily growth increments and the existence of an extrusion check were confirmed by both a series of tetracycline marking experiments and an analysis of the relation between the number of increments and the number of days after extrusion. Larvae and juveniles associated with drifting seaweed occurred from late May to late July, with a peak in late June, showing the same seasonal fluctuation in abundance as drifting seaweed. Larvae seemed to associate with drifting seaweed after aggregation to surface slick areas. Body size at the commencement and the end of the seaweed-associated period was estimated at 18 and 50 mm total length, respectively. Growth rates were low until the beginning of the association with seaweed; thereafter they became high but declined towards the end of seaweed-associated life. Larvae and juveniles born later grew faster than those born earlier. This tendency was detected both before and during the association with drifting seaweed. Although the season of abundant drifting seaweed was about 2␣months shorter, and occurred 2 months later, than the extrusion season of the gold-eye rockfish from early February to early June, this seemed to be compensated by the growth variation dependent on birth month. Early life history traits of the gold-eye rockfish are discussed in relation to the successful utilization of drifting seaweed. Received: 3 August 1997 / Accepted: 24 March 1998     

Diets of four decapod crustaceans (Linuparus trigonus,Metanephrops andamanicus,M. australiensis and M. boschmai) from the continental shelf around Australia

Linuparus trigonus (Palinuridae) was collected from northeast Australia at depths of 220 to 300 m in the austral summer of 1985–1986 and its diet was analysed for diel feeding-periodicity and composition. The sex and size of the lobsters and the depth at which they were caught were recorded. About 35% of the foreguts were less than 10% full. L. trigonus appears to be a predator of slow-moving or nearsessile benthic invertebrates; its diet is mainly bivalves, gastropods, ophiuroids, crustaceans, polychaetes, and foraminiferans. Most dietary items differed significantly between the size classes of lobster. The frequency of crustacean remains increased with depth and more gastropods were consumed by lobsters caught at 260 m than at other depths. No significant periodicity in feeding was found for L. trigonus. The foreguts of three Metanephrops spp. (Nephropidae) collected from north-west Australia in August 1983 (M. andamanicus) and January/February 1984 (M. australiensis and M. boschmai) were nearly empty, and nearly half were less than 10% full. Fish, crustaceans and squids were the most common food items eaten. The main food items varied between the species: M. andamanicus ate nearly equal amounts of fish and crustaceans; M. australiensis ate chiefly crustaceans; M. boschmai mainly ate fish and seldom fed on squid. Metanephrops spp. appear to feed by attacking mobile animals and tearing off appendages or by scavenging.     

Live fast, die young: flexibility of life history traits in the fat-tailed dwarf lemur (Cheirogaleus medius)

The fat-tailed dwarf lemur, Cheirogaleus medius, occurs in ecologically very different habitat types (rainforest and dry forest) across Madagascar. Its extraordinary biological characteristics, such as monogamy and long-term hibernation, allow us to investigate behavioral, ecological, and physiological flexibility of this species in populations across different ecological environments. This study aims to determine whether different life history and physiological traits show variation in adaptation to the differing ambient conditions or are conservative and influenced more by the organism's evolutionary history. We compared body masses, life history traits, social organization, and hibernation duration of two populations of C. medius, one from a littoral rainforest and one from the dry deciduous forest. We revealed clear geographical differences in the length of hibernation duration as well as, more surprisingly, in life history traits. We found that this species' reproductive strategies seem to be highly flexible. Animals in the rainforest can spend more time in the active state due to a shorter hibernation period, but have, in general, a shorter life expectancy due to higher mortality rates. Hence, they seem to maximize their total reproductive output with higher reproductive rates (larger litter sizes, greater number of litters). Home ranges and social organization, on the other hand, did not vary between habitats, suggesting that the general requirements of this species are independent of environmental conditions. In conclusion, some life history traits, formerly assumed to be genetically fixed parameters of primate species, prove in fact to be highly flexible. Different populations of the same species show distinct adaptations according to the prevailing conditions in order to maximize individual reproductive output.     

Nymphal density, behavioral development, and life history in a field cricket

Population density regulates the strength of intraspecific competition and may thereby be reflected in life-history variables, such as development time, growth rate, or investment in immune defense. However, population density may also affect the fitness payoffs of different behaviors and consequently shape the development of personality. Here we studied if population density during nymphal development (one, four, or ten individuals raised together) affects the level of boldness, measured as the latency time to recover from freezing and emerge from a shelter, aggressiveness towards conspecifics or their correlation at the adult stage in the field crickets, Gryllus integer. In addition, we tested if individuals invest more resources in immune function or speed up their development in response to a high conspecifics density during ontogeny. Nymphal density did not affect adult boldness or aggressiveness towards conspecific males per se, but these variables showed a negative association, i.e., indicated an unconventional behavioral syndrome in the highest density treatment. Supporting the effectiveness of density treatments in inducing plastic responses, individuals reached maturity sooner and invested more resources in immune function in the highest nymphal density group compared to groups consisting of one or four individuals. Our results suggest that population density may play an important role in shaping both the realized life history and development of behavioral syndromes.     

Tissue type matters: selective herbivory on different life history stages of an isomorphic alga

Selective grazing by herbivores can have large effects on the population dynamics and community structure of primary producers. However, the ecological impacts of within-species herbivore preference for tissues of different phases (e.g., ploidy levels) or reproductive status remain relatively poorly known, especially among algae and other species with free-living haploid (gametophyte) and diploid (sporophyte) phases. We tested for herbivore selectivity among tissue types of the isomorphic (identical haploid and diploid free-living stages) red alga Mazzaella flaccida. Laboratory feeding assays demonstrated that the snail Tegula funebralis exhibited more than a threefold preference for gametophyte reproductive tissue over other tissue types, due to morphological differences. In contrast, the urchin Strongylocentrotus purpuratus did not distinguish as clearly between gametophytes and sporophytes; but it did prefer sporophyte reproductive to nonreproductive tissue, due to differences in water-soluble chemicals. Field surveys of grazer damage on M. flaccida blades were consistent with these laboratory preferences, with more damage found on gametophytes than sporophytes and reproductive than nonreproductive tissues. Differential fecundity can contribute to a skew in relative frequencies of phases in the field, and our results suggest that differential grazing by snails may contribute to this pattern and thus play a role in algal population biology.     

Pre-migratory life history stages of juvenile Arctic birds: costs, constraints, and trade-offs

Many young birds on the Arctic tundra are confronted by a challenging task: they must molt their feathers and accumulate fat stores for the autumn migration before climatic conditions deteriorate. Our understanding of the costs and constraints associated with these stages is extremely limited. We investigated post-juvenal molt and premigratory fattening in free-ranging juvenile White-crowned Sparrows (Zonotrichia leucophrys gambelii) on the Arctic tundra. We found evidence for trade-offs between premigratory fat accumulation and molt: heavily molting birds had significantly less fat. Birds increased the rate of fat accumulation as the season progressed, but we found no evidence of a similar increase in rate of molt. Using a controlled captive study to isolate the energetic costs of body feather replacement, we found no difference in fat or size-corrected mass of birds actively growing body feathers as compared to controls. Molting birds, however, consumed 17% more food than controls, suggesting a significant cost of body feather growth. Our results provide evidence of significant costs, constraints, and trade-offs associated with post-juvenal molt and premigratory fat accumulation in young Arctic birds.     

A model for a species with two life history stages and added mortality

A differential equation model of a species with two life stages is constructed, and the behavior of the equilibria investigated when density-independent mortality is continuously imposed. With density dependence largely in recruitment, an increase in either life stage is possible even though mortality is being imposed on both life stages. The equilibrium of a given life stage can increase if density dependence is mainly in that stage.The most effective stage to attack is that which is numerically dominant. With species having a high reproductive potential the most effective way of reducing either stage is to attack the stage with the lesser natural density-independent mortality.Certain parameter values allow multiple positive steady states. Data from Lucilia cuprina appear to conform to some of the predictions of the model.