Mechanisms Of Population Regulation Research Articles

DEMOGRAPHY OF FRONDS OF SARGASSUM LAPAZEANUM FROM SOUTHERN BAJA CALIFORNIA, MEXICO, DURING EARLY STAGES

In terms of biomass, Sargassum lapazeanum (Phaeophyceae, Fucales) is one of the most important seaweeds in La Paz Bay, on the southeastern coast of the Baja California Peninsula, Mexico. This species can be found year‐round in shallow subtidal sites. Standing biomass is lowest between fall and winter and highest between spring and summer. We are currently studying the annual demography of fronds as a necessary step to understanding the mechanisms of population regulation. Work is being done on a bed of about 90 m in length and three m in width. At this stage of abstract submission, we can report results for winter only. Random samples were collected in February and in March 2000 to estimate stand biomass, frond density, and size (frond length) structure. In addition, we labeled fronds with numbered plastic tags to estimate their rates of growth and of mortality during this period. Rates of recruitment were estimated from a combined analysis of the above. In February, fronds were all shorter than 6.5 cm. Between February and March, mean total frond density increased from 122 to 776 fronds m−2 (n = 30 quadrats). The mean recruitment rate was 667 fronds m−2, the mean growth rate was 0.5 cm day−1 (n = 60 fronds), and the mean mortality rate was 43 fronds m−2. From last year's preliminary observations, we expect to observe peaks of biomass and of reproduction in late spring, followed by negative growth rates and high mortality rates during summer.

Spatial synchronization of vole population dynamics by predatory birds.

Northern vole populations exhibit large-scale, spatially synchronous population dynamics. Such cases of population synchrony provide excellent opportunities for distinguishing between local intrinsic and regional extrinsic mechanisms of population regulation. Analyses of large-scale survey data and theoretical modelling have indicated several plausible synchronizing mechanisms. It is difficult, however, to determine the most important one without detailed data on local demographic processes. Here we combine results from two field studies in southeastern Norway--one identifies local demographic mechanisms and landscape-level annual synchrony among 28 enclosed experimental populations and the other examines region-level multi-annual synchrony in open natural populations. Despite fences eliminating predatory mammals and vole dispersal, the growth rates of the experimental populations were synchronized and moreover, perfectly linked with vole abundance in the region. The fates of 481 radio-marked voles showed that bird predation was the synchronizing mechanism. A higher frequency of risky dispersal movements in slowly growing populations appeared to accelerate predation rate. Thus, dispersal may induce a feedback-loop between predation and population growth that enhances synchrony.

Population viability analyses on a cycling population: a cautionary tale

Population viability analysis (PVA) packages do not always allow realistic simulation of particular life cycle features, so they may produce unrealistic predictions of extinction risk. This was suspected for a cycling Soay sheep population, Ovis aries L., that grows, overeats its habitat and then suffers high winter mortality. We compared projections of PVA models for the sheep that incorporated either an unrealistic ceiling carrying capacity using INMAT (the only choice) and VORTEX (the default), or realistic density dependence for survival (VORTEX). At year 50, the ceiling models predicted extinction probabilities of 60.4% (INMAT) and 87.4% (VORTEX), compared to only 4.6% for the density-dependent model. Small populations were equally likely to increase or decrease with the ceiling models, while they had high probabilities of increase with the density-dependent model, as found in reality. PVA cannot be relied upon to produce realistic projections if inappropriate mechanisms of population regulation are used.

VARIATION IN THE PRESENCE AND CAUSE OF DENSITY-DEPENDENT MORTALITY IN THREE SPECIES OF REEF FISHES

Determining the mechanisms by which natural populations are regulated is a key issue in ecology. Identifying the biological causes of density dependence has, however, proved difficult in many systems. In this study we tested whether adults of three species of reef fish (all gobies) suffered density-dependent mortality, and whether the density-dependent component of mortality was caused by predation. We used field experiments to test for density dependence in each prey species, manipulating the presence of predators and prey density in a factorial design. Prey were stocked on replicate patches of reef constructed of natural materials, with each reef receiving a different density of gobies. Predatory fishes were excluded from half of the reefs using a combination of removals and exclusion cages. Survival of the first species, Lythrypnus dalli, was high and density-independent on reefs free of predators, but declined rapidly with increasing density on reefs to which predators had access. Density dependence in L. dalli was thus a result of mortality inflicted by predatory fishes. In the second species, Coryphopterus nicholsii, predators caused a large reduction in the survival in one experiment but had a negligible effect in a second experiment. More importantly, though, survival of C. nicholsii was always independent of its density, regardless of predator presence. In the final species, Coryphopterus glaucofraenum, two separate experiments showed that natural changes in adult abundance (experiment 1) and survival of stocked adults (experiment 2) were density-dependent regardless of predator presence. Both experiments thus indicated that the density-dependent component of loss in C. glaucofraenum was caused, at least in part, by an interaction other than predation. The presence, intensity, and biological cause of density-dependent survival were thus strikingly different for each of these three fishes, despite the fact that they are taxonomically closely related and ecologically similar. These findings suggest a need for further studies aimed at predicting under what circumstances different mechanisms of population regulation will operate.

Variation in the Presence and Cause of Density-Dependent Mortality in Three Species of Reef Fishes

Determining the mechanisms by which natural populations are regulated is a key issue in ecology. Identifying the biological causes of density dependence has, however, proved difficult in many systems. In this study we tested whether adults of three species of reef fish (all gobies) suffered density-dependent mortality, and whether the density-dependent component of mortality was caused by predation. We used field experiments to test for density dependence in each prey species, manipulating the presence of predators and prey density in a factorial design. Prey were stocked on replicate patches of reef constructed of natural materials, with each reef receiving a different density of gobies. Predatory fishes were excluded from half of the reefs using a combination of removals and exclusion cages. Survival of the first species, Lythrypnus dalli, was high and density-independent on reefs free of predators, but declined rapidly with increasing density on reefs to which predators had access. Density dependence in L. dalli was thus a result of mortality inflicted by predatory fishes. In the second species, Coryphopterus nicholsii, predators caused a large reduction in the survival in one experiment but had a negligible effect in a second experiment. More importantly, though, survival of C. nicholsii was always independent of its density, regardless of predator presence. In the final species, Coryphopterus glaucofraenum, two separate experiments showed that natural changes in adult abundance (experiment 1) and survival of stocked adults (experiment 2) were density-dependent regardless of predator presence. Both experiments thus indicated that the density-dependent component of loss in C. glaucofraenum was caused, at least in part, by an interaction other than predation. The presence, intensity, and biological cause of density-dependent survival were thus strikingly different for each of these three fishes, despite the fact that they are taxonomically closely related and ecologically similar. These findings suggest a need for further studies aimed at predicting under what circumstances different mechanisms of population regulation will operate.

Life history plasticity and population regulation in sea otters

We contrasted body condition, and age‐specific reproduction and mortality between a growing population of sea otters (Enhydralutris) at Kodiak Island and a high‐density near‐equilibrium population at Amchitka Island, Alaska. We obtained data from marked individuals, population surveys, and collections of beach‐cast carcasses. Mass:length ratios indicated that females (but not males) captured in 1992 at Amchitka were in poorer condition than those captured at Kodiak in 1986–1987. In 1993, the condition of females at Amchitka improved in apparent response to two factors: (1) an episodic influx of Pacific smooth lumpsuckers, Aptocyclus ventricocus, from the epi‐pelagic zone, which otters consumed; and (2) an increase in the otters’ benthic invertebrate prey resulting from declining otter numbers. Reproductive rates varied with age (0.37 [CI=0.21 to 0.53] births female−1 yr−1 for 2–3‐yr‐olds, and 0.83 [CI=0.69 to 0.90] for females ≥4 yr old), and were similar at both areas. Weaning success (pups surviving to ≥120 d), in contrast, was almost 50% lower at Amchitka than at Kodiak and for females ≥4 yr of age was 0.52 (CI=0.38 to 0.66) vs 0.94 (CI=0.75 to 0.99), respectively. Sixty‐two percent of the preweaning pup losses at Amchitka occurred within a month of parturition and 79% within two months. Postweaning survival was also low at Amchitka as only 18% of instrumented pups were known to be alive one year after mother‐pup separation. Adult survival rates appeared similar at Amchitka and Kodiak. Factors affecting survival early in life thus are a primary demographic mechanism of population regulation in sea otters. By maintaining uniformly high reproductive rates over time and limiting investment in any particular reproductive event, sea otters can take advantage of unpredictable environmental changes favorable to pup survival. This strategy is consistent with predictions of “bet‐hedging” life history models.

Crowding‐induced changes in growth, reproduction and morphology of Daphnia

SUMMARY Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life‐history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow‐through system in the presence of abundant food. Water from Daphnia at 85 L‐1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small‐bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species. Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D. lumholtzi and D. ambigua. Chemicals released by crowded conspecifics may provide an additional, density‐dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.

Hymenolepis diminuta: Praziquantel Removal of Adult Tapeworms Is Followed by Apoptotic Down-Regulation of Mucosal Mastocytosis

Starke, W. A., and Oaks, J. A. 1999. Hymenolepis diminuta: Praziquantel removal of adult tapeworms is followed by apoptotic downregulation of mucosal mastocytosis. Experimental Parasitology92, 171–181. The rat tapeworm, Hymenolepis diminuta, induces mastocytosis, hypertrophy of enteric smooth muscle, alteration of enteric myoelectric activity, and slowed enteric transit of the rat host's intestine. This report examines the resolution of both tapeworm-induced mastocytosis and tissue changes during the period following removal of the tapeworm with Praziquantel (PZQ). The dynamics of the mucosal mast cell (MMC) population following removal of the tapeworms was assessed by histochemical identification of MMC and morphometric techniques. As a possible mechanism of MMC population regulation, MMC apoptosis was examined over the same experimental period using the in situ nick end labeling of fragmented DNA (TUNEL). Shifts in MMC numbers were correlated with functional and morphological changes of the intestine following removal of the adult-stage tapeworm. Ileal tissues from rats infected 32 days with H. diminuta (the beginning of plateau phase of tapeworm-induced chronic mastocytosis) were harvested 1, 2, 3, and 4 weeks after the PZQ treatment. Control ilea were obtained either from rats which were never infected and never treated with PZQ or from rats infected with H. diminuta for 32 days but not treated with PZQ. In order to detect MMC and apoptosis, tissue sections of ileum were doubled stained sequentially with Astra blue for MMC granules followed by a modification of the TUNEL technique. No alteration in MMC numbers were observed in PZQ-treated animals until 3 weeks after the removal of the tapeworms. The decline of MMC occurred in the mucosa and submucosa. MMC numbers first approached uninfected control levels at 4 weeks posttreatment. Coincident with the decline in mucosal MMC numbers, the rate of MMC entering apoptosis also declined. Simultaneously, ileal smooth muscle layers, hypertrophied by infection, and mucosal structures began the process of involution and atrophy. Apoptosis of MMC in the submucosa and muscularis mucosa was not detected. In conclusion, H. diminuta-elicited mastocytosis and increased thickness of both mucosa and muscularis externa do not begin a decline toward control values until 3 weeks after the parasites are gone and normal intestinal motility is restored. These data are consistent with the lack of MMC mediation of altered motility, and the decline in the rate of MMC apoptosis at 3 weeks post-PZQ suggests that apoptosis may play an important role in the involution of tapeworm-induced mastocytosis.

Life history of the sandhopper Pseudorchestoidea brasiliensis (Amphipoda) in sandy beaches with contrasting morphodynamics

Exposed marine beaches are physically rigorous habitats in which macrofauna community patterns are related to physical factors such as sedimentary parameters and wave/swash processes. In this context, a consistent increase in species richness, abundance and biomass from reflective to dissipative conditions has been widely reported, and proposed as a paradigm of sandy beach ecology. Here, we examine this hypothesis on the demography and life history characteristics of the sandhopper Pseudorchestoidea brasiliensis of Uruguay. Abundance, population structure by sex and size, individual growth, natural mortality, fecundity, female maturity and size at maturity, and the length-weight relationship were compared between populations of 2 microtidal exposed sandy beaches that differed widely in physical characteristics (i.e. grain size; slope, penetrability and water content), during 20 consecutive months. Contrasting with the predictions of 'the sandy beach ecological paradigm, the population of P. brasiliensis at the reflective beach presented (1) higher abundance both for males and females; (2) higher egg production potential and recruitment levels; (3) lower natural mortality; and (4) no major differences in individual growth and estimated life span to those of the dissipative beach population. On the contrary, growth in weight, individual fecundity and average size at maturity were higher for the dissipative beach population. We conclude that population level responses to variation in sandy beach morphodynamics may markedly differ from community level responses, and thus macroscale, world-wide community patterns could not necessarily characterise life history and demographic variations of individual species in a similar manner. We suggest that the paradigm of the forces generating patterns in sandy beach communities has underestimated the importance of population regulation mechanisms in these communities.

Variability of Immune Response to Heterologous Erythrocytes during Population Cycles of Red (Clethrionomys rutilus) and Bank (C. glareolus) Voles

Immunocompetence is regarded to be one of the principal links in mechanisms of population regulation in small mammals. We assessed immune responsiveness to injection of sheep red blood cells (SRBC) in sympatric populations of red and bank voles in southern West Siberia, Russia, during the cycle of density from 1991 to 1995. We used a splenic plaque-forming cells (PFC) technique to measure the immune response to the antigen. In an attempt to establish relationships of immunity with intrinsic factors involved in population regulation we monitored the following parameters: population numbers, recruitment rate, and adrenocortical activity. The analysis of covariance showed a significant effect of reproductive status of voles and independent significant effects of month and year of capture on immune response to SRBC. In general, immature young was significantly higher responders as compared to mature (breeding) adults. The seasonal and long-term variations in numbers of PFC were similar in both species. Seasonal minimum of the immune response was observed within the period of intensive breeding in June and July. Among the years the immune responsiveness was maximal in the phase of population depression. The increase in maturation rate as density increased coincided with a decrease in immune responsiveness. The correlation analysis showed little or no evidence of links between the immunity and adrenocortical activity in the populations under study. Based on the patterns of seasonal and long-term variations of surveyed variables it is likely that physiological mechanisms governing the processes of sexual maturation and reproduction of red and bank voles are involved in the regulation of immunity.

Frequency distribution of parasites in host populations: review

A review on investigations of parasite frequency distribution in host population is presented. An application of theoretical distribution models for approximation of empirical data, the causes predetermining pattern of parasite distribution, the connection among distribution parameters, as well as the connection between distribution patterns and mechanisms of population regulation are analysed.

Effects of Recreational Shooting on Prairie Dog Colonies

The Bureau of Land Management (BLM) and the U.S. Fish and Wildlife Service (USFWS) manage lands in central Montana under a multiple-use mandate. These agencies are expected to provide forage for livestock and recreation for the public while maintaining the ecological integrity of biological communities. When conflicts among land uses occur, such as actual or perceived competition between livestock and wildlife for forage, agency personnel need options to resolve problems. We measured the effects of recreational shooting, a potential mechanism for population regulation, on black-tailed prairie dog (Cynomys ludovicianus) population dynamics and activity patterns in 18 prairie dog towns in central Montana during 1994-95. The mean number of prairie dogs killed in colonies open to hunting was estimated at 23 in 1994 and 58 in 1995. Prairie dog population size declined 35% in hunted colonies and 15% in nonhunted colonies from early to late summer 1995. Age and sex ratios were similar in hunted and nonhunted colonies. Prairie dogs spent more time in alert postures and less time foraging in hunted than nonhunted towns. Prairie dogs could also be approached more closely in nonhunted colonies than hunted colonies. Recreational shooting has potential as a management tool to limit rather than eliminate populations of black-tailed prairie dogs.

The Effects of Density on the Zoanthid Palythoa caesia

1. Few studies have examined density dependence or the mechanisms of population regulation in clonal marine organisms. Clonal organisms living on coral reefs have been particularly neglected. I examine the response of the tropical intertidal zoanthid Palythoa caesia to a density manipulation, where I decreased density from natural levels of 80-90% cover to a range of values down to 0% cover. 2. I found major differences in fission and fusion rates with density, with fission decreasing and fusion increasing as density increased. The growth rate of large colonies decreased substantially as density increased, although small colonies were unaffected. Mortality, on the other hand, was independent of density. Recruitment rates peaked at intermediate densities, but were low overall. 3. While density accounted for less than 10% of the variation in the demographic rates measured, it explained 62% of the variation in changes in cover within a given aggregation. Change in cover was greatest (30% in 2 years) for quadrats with 23-33% cover, and became negative for quadrats with over 80% cover. The highest density quadrat (90% cover) experienced an 18% decrease in cover over 2 years. 4. When the data on colony level dynamics for each density were integrated into population dynamics models, I found a steady decrease in population growth rates with density, along with an increase in the proportion of individuals in the largest size class. However, the proportional contribution of each size class to future generations (reproductive value) did not change with density. 5. These results suggest that during the study period the population size of Palythoa was being regulated, probably primarily by fission and/or fusion, with all quadrats approaching an equilibrium cover of 80%.

SITE-DEPENDENT REGULATION OF POPULATION SIZE:A NEW SYNTHESIS

The nature and extent of population regulation remains a principal unanswered question for many types of organisms, despite extensive research. In this paper, we provide a new synthesis of theoretical and empirical evidence that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability. The sites may be territories, refuges from predation, oviposition sites, etc. The mechanism, which we call site dependence, is not an alternative to density dependence; rather, site dependence is one of several mechanisms that potentially generate the negative feedback required for regulation. Site dependence has two major features: (1) environmentally caused heterogeneity among sites in suitability for reproduction and/or survival; and (2) preemptive site occupancy, with the tendency for individuals to move to sites of higher quality as they become available. Simulation modeling shows that these two features, acting in concert, generate negative feedback when progressively less suitable sites are used as population size increases, reducing average demographic rates for the population as a whole. Further, when population size decreases, only sites of high suitability are occupied, resulting in higher average demographic rates and, thus, population growth. The modeling results demonstrate that this site-dependent mechanism can generate negative feedback at all population sizes in the absence of local crowding effects, and that this feedback is capable of regulating population size tightly. Operation of site dependence does not rely on the particular type of environmental factor(s) ultimately limiting population size, e.g., food, nest sites, predators, parasites, abiotic factors, or a combination of these. Furthermore, site dependence operates in saturated or unsaturated habitats and over a broad range of spatial scales for species that disperse widely relative to site diameter. A review of relevant field studies assessing the assumptions of the mechanism and its regulatory potential suggests that site dependence may provide a general explanation for population regulation in a wide variety of species.

Population Persistence Time: Estimates, Models, and Mechanisms

The conservation and management of populations require some insight into mechanisms of population regulation, yet unambiguous identification of density dependence in vital rates is notoriously difficult. Through the reanalysis of data published by Stacey and Taper for a semi-isolated sub-population of the Acorn Woodpecker we demonstrate the utility of an approach in which a single data set is “attacked” using a suite of models with different assumptions. Tests for density dependence of the reproductive rate and of juvenile survival are inconclusive, but there is evidence for density-dependent adult survival. Since “survival” rates estimated from banded birds confound the effects of mortality and immigration/emigration, we construct and analyze three models: (1) a closed population with density-independent vital rates and an upper limit to population size, (2) a regulated closed population, (3) a regulated sub-population with immigration and emigration. Very small interchanges with other sub-populations can have a very large effect on population persistence; thus our results highlight the importance of distinguishing the effects of interchange from density dependencies in vital rates.

POPULATION PERSISTENCE TIME: ESTIMATES, MODELS, AND MECHANISMS

The conservation and management of populations require some insight into mechanisms of population regulation, yet unambiguous identification of density dependence in vital rates is notoriously difficult. Through the reanalysis of data published by Stacey and Taper for a semi-isolated sub-population of the Acorn Woodpecker we demonstrate the utility of an approach in which a single data set is “attacked” using a suite of models with different assumptions. Tests for density dependence of the reproductive rate and of juvenile survival are inconclusive, but there is evidence for density-dependent adult survival. Since “survival” rates estimated from banded birds confound the effects of mortality and immigration/emigration, we construct and analyze three models: (1) a closed population with density-independent vital rates and an upper limit to population size, (2) a regulated closed population, (3) a regulated sub-population with immigration and emigration. Very small interchanges with other sub-populations can have a very large effect on population persistence; thus our results highlight the importance of distinguishing the effects of interchange from density dependencies in vital rates.

The relationship between food intake and predation risk in migratory caribou and implications to caribou and wolf population dynamics

We examined the hypothesis that spring migration in barren-ground caribou (Rangifer tarandus) enhances access to high quality food, reduces predation risks or both. We related our findings to the hypothesis that one of the consequences of migration is that prey populations cannot be regulated by predation because predators are unable to respond numerically to changes in abundance of migratory prey. In the Northwest Territories, migration to calving grounds by pregnant cows reduced the risk of predation on neonates. Wolf (Canis lupus) densities on calving grounds averaged only 22% of winter range densities because most wolves denned near tree line. The quality and quantity of food that was available to cows that migrated to calving grounds was lower than for bulls and other caribou that lagged far behind the pregnant cows during spring migration. Fecal nitrogen levels were higher in bulls than in cows in late May and early June but there were no differences in mid or late June. Areas occupied by bulls in late May had a greater biomass of live sedges than on the calving ground in early June. It appears that although food in July is abundant and nutritious, insect harassment prevents efficient feeding. Body fat reserves in both sexes declined to almost zero by mid-July, the lowest level of the year. Insect numbers declined in August and body fat levels increased to the highest level of the year by early September. Because the timing of caribou's return to the hunting ranges of tree line denning wolves was related to caribou density, our data were inconsistent with the suggested consequence of migration. Tree line denning by wolves and density-dependent changes in caribou migration suggests a mechanism for population regulation in caribou and wolves. We suggest that the process is as follows; when caribou numbers increase, some density-dependent factor causes range expansion in August (e.g., competition for food) causing caribou to return earlier to the hunting ranges of tree line denning wolves, more denning wolves have access to caribou, wolf pup survival increases and wolf numbers increase. The effect on caribou population growth will depend on the timing and magnitude of the wolf numerical response.

Identifying Density-Dependent Processes: A Comment on the Regulation of Winter Moth

Long-term population studies provide the foundation against which ecological hypotheses on mechanisms of population regulation can be accepted or refuted. One of the classic long-term population studies has focused on the winter moth, Operophtera brumata (L.) (Lepidoptera: Geometridae) and its associated natural enemies. Detailed population studies have been carried out in a native area at Wytham Wood, Oxfordshire (Varley & Gradwell 1963, 1968; Varley, Gradwell & Hassell 1973) and in regions where the moth has been introduced into Canada (Embree 1965, 1991; Roland 1986, 1988). Different regulatory mechanisms have emerged from these studies. At Wytham Wood, mortality occurring in the soil, mainly due to pupal predators, acts in a density-dependent manner and seems to be the primary regulatory process (Varley et al. 1973). In Canada, however, pupal predation seemed much less important and an introduced tachinid parasitoid, Cyzenis albicans (Fall.) (Diptera: Tachinidae), was identified as the main density-dependent process (Embree 1966). More recently, Roland (1986, 1988 1990, 1994) has presented yet further interesting interpretations of the winter moth population dynamics based on his studies at Mount Tolmie Park, British Columbia, Canada. In his most recent paper (Roland 1994), data are presented on the relationships between mortality and the estimated density at different stages in the life cycle between 1982 and 1990 (Fig. 1). During this period the winter moth population declined from initial high levels to relatively low levels (for the final 7 years). In order to determine the importance of these mortalities to the winter moth dynamics, Roland selected only the final six 'equilibrium' years of data for his analysis (Fig. 2). On the basis of this, he concluded that egg and larval mortality (k fec and klarv) were not densitydependent and therefore not important in regulating the winter moth population. Pupal predation by ground beetles, however, did appear to be strongly density-dependent and was thus inferred as the primary cause of stability in the British Columbia winter moth population. In this note, we question this conclusion. In particular, we caution against looking for specific densitydependent processes from relatively stationary populations. By considering all the available data (198290), we show that egg and larval mortality (k fec and klarv), in contrast to pupal mortality (kpred), are densitydependent and could have a major impact on the winter moth population regulation.

Plant Competition and Herbivory in Relation to Vegetation Biomass

We tested whether effects of competition and herbivory on plant growth depend on the aboveground biomass of vegetation. Our objective was to test theoretical predictions about the site—dependence of biotic mechanisms of plant population regulation. Biomass ranged from a mean of 64 to 776 g/m2 at the eight study sites, which included six old fields and two herbaceous plant communities near water. To measure effects of competition, 4—5 wk old plants of a perennial grass (Poa compressa) were transplanted into plots where neighboring plants were either left intact or removed and after 4 mo the shoot mass of transplants was compared in the two treatments. Transplants were caged in both treatments to exclude herbivores. Removing neighbors increased transplant shoot mass significantly at all sites, indicating that transplants experienced less competition for resources when their neighbors were removed. Regression analysis indicated that the increase in shoot mass due to neighbor removal was significantly greater at sites with higher biomass. This result supports theories that predict competition should increase with increasing biomass. To measure effects of herbivory, the growth of caged and uncaged plants was compared. Caging transplants did not increase their shoot mass significantly at the two sites with the lowest biomass. However, shoot mass was significantly greater for caged transplants than for uncaged transplants at all other sites. Caged transplants lost less tissue to herbivores such as small mammals and molluscs. Regression analysis indicated that the increase in shoot mass due to caging was significantly greater at sites with higher biomass. The combined effect of herbivory and competition was measured by comparing transplant growth in plots where transplants were caged and neighbors were removed with the growth of transplants in plots where transplants were not caged and neighbors were left intact. Transplant shoot mass was significantly greater where herbivores were excluded and neighbors were removed, especially at sites with high biomass. Our results indicate that competition and herbivory each have a greater effect on plant growth at sites with higher biomass and that herbivory has less effect than competition on plant growth at sites with relatively low biomass.

Spatial and temporal patterns of predation of simulated sage grouse nests at high and low nest densities: an experimental study

We examined patterns of predation on 252 simulated sage grouse (Centrocercus urophasianus) nests placed at two densities around six active leks in southeastern Wyoming, U.S.A. Predation intensity, as measured by the frequency of multiple-nest predation events, was significantly greater at high-density sites, implying enhanced prey capture (functional and (or) numerical response) by predators. Significant spatial aggregation of nest predation further implies enhanced prey capture by predators at high prey densities. Predation varied significantly among sites, but there were no significant first-order differences in predation between densities. Predation was also significantly affected by year–density and site–year–density interactions. Several factors, including nest cover, prey defense mechanisms, study site location, nest location, year, search methods of predators, number of predators, and random encounter may inhibit or confound density-dependent nest predation. Enhanced prey capture provides a mechanism for density-dependent population regulation.