“Windows of opportunity” for dinoflagellate blooms: Reduced microzooplankton net growth coupled to eutrophication

Links between eutrophication, plankton community structure, microzooplankton grazing and dinoflagellate abundance were investigated in two tributaries of the Chesapeake Bay, the Choptank and Patuxent Rivers (MD, USA). Sampling and experiments were conducted during the spring of consecutive dry (below average freshwater flow) and wet (above average freshwater flow) years. During the wet year (2003), dissolved inorganic nitrogen, phytoplankton, and copepod biomass, but not microzooplankton abundance, were greater than in the dry year. In 2003, but not 2002, small cell size photosynthetic dinoflagellates were abundant and blooms occurred in both rivers. Average potential microzooplankton grazing pressure on small dinoflagellates was spatially and temporally variable, but was not significantly different between years. Our data suggest that the variability in microzooplankton grazing pressure provided “windows of opportunity” for net growth of dinoflagellates in response to nutrient loading. The lack of net population growth of micrograzers in response to increases in dinoflagellate prey allowed dinoflagellate blooms to reach relatively high densities, however grazing also appeared to be important in limitation or demise of some blooms. We hypothesize that uncoupling of micrograzer–prey dynamics was partly due to strong top-down control by copepods of microzooplankton in the proportionately more eutrophic year, and perhaps also due to inhibition of microzooplankton grazing/growth once dinoflagellate densities are high.     

Microzooplankton grazing and the control of phytoplankton biomass in the Suwannee River estuary,USA

Microzooplankton grazing can have significant impacts on the distribution and abundance of phytoplankton, thereby influencing the frequency and duration of algae blooms. Observations of high ciliate abundances in the Suwannee River estuary, Florida, suggest a significant potential for top-down pressure on the phytoplankton community by microzooplankton. We examined the composition of the microzooplankton and determined grazing mortality losses for phytoplankton within the Suwannee River estuary from 2001 to 2002. Our results indicated grazing mortality rates of 1.4 d⁻¹, equivalent to a loss of up to 76% of phytoplankton standing crop and up to 83% of total daily primary production. The microzooplankton community was primarily composed of ciliates, dinoflagellates, and copepod nauplii. The densities of ciliates in the estuary were comparable to densities reported in highly eutrophic ecosystems (9,400–72,800 ciliates l⁻¹). Grazing pressure on small phytoplankton may be further enhanced because ciliates and small dinoflagellates have growth rates similar to those of phytoplankton, and therefore can keep up with surges in abundance. Handling editor: Judit Padisak     

Growth and mortality rates of phytoplankton in the northwestern North Pacific estimated by the dilution method and HPLC pigment analysis

Pigment-based growth rates of phytoplankton and mortality rates due to microzooplankton grazing were estimated using a dilution method combined with high-performance liquid chromatography (HPLC) pigment analysis in the northwestern North Pacific in autumn 1998. The dilution experiments were conducted at different hydrographic stations in both colder and warmer water masses. No significant difference was found between the growth rate of the phytoplankton community (0.38–0.70 day⁻¹; estimated by chlorophyll a) at the colder and warmer water stations, while the mortality rate (0.15–0.88 day⁻¹; estimated by chlorophyll a) tended to be higher at warmer water stations. The combination of estimates of daily chlorophyll a production and particulate organic carbon (POC) production enabled us to assess the carbon to chlorophyll a ratio (C/chl a) of “new” organic matter produced by living phytoplankton. The method provided an implicit value of the C/chl a of in situ living phytoplankton. The rate estimates from taxon-specific pigments suggested a possibility that chlorophyll b-containing green algae were grazed preferentially by microzooplankton during their active growth, and the standing stock of green algae was more strictly controlled by micrograzer than other algal groups such as diatoms. This result is one possible explanation for the fact that blooms of green algae have not been reported in the open ocean, in contrast with diatoms.     

东海春季水华期浮游植物生长与微型浮游动物摄食

2005年4～6月在东海有害水华频发区14个站位采样,通过现场稀释法实验对春季东海水域浮游植物比生长率和微型浮游动物比摄食率进行了研究.结果表明东海有害水华频发区浮游植物群落以甲藻为优势.浮游植物比生长率在水华爆发前相对较低,平均为1.18 d~(-1);进入水华期后比生长率明显升高,但在水华站位随现存量增加而降低;非水华区比生长率近岸高、远岸低.微型浮游动物主要以急游虫和桡足类幼体为主,而种类上以砂壳纤毛虫居多.微型浮游动物比摄食率在水华爆发前波动较大,介于0.53～1.73 d~(-1),平均为0.90 d~(-1);在水华区比摄食率较为稳定,浮游植物比生长率的降低导致群落净生长率持续下降;在非水华区,比摄食率整体较高,近岸低而远岸高.微型浮游动物的摄食对浮游植物群落的生长有一定的控制作用,但在水华爆发后这种控制作用将减弱.     

Novel interactions between phytoplankton and microzooplankton: their influence on the coupling between growth and grazing rates in the sea

Understanding the processes that regulate phytoplankton biomass and growth rate remains one of the central issues for biological oceanography. While the role of resources in phytoplankton regulation (`bottom up' control) has been explored extensively, the role of grazing (`top down' control) is less well understood. This paper seeks to apply the approach pioneered by Frost and others, i.e. exploring consequences of individual grazer behavior for whole ecosystems, to questions about microzooplankton–phytoplankton interactions. Given the diversity and paucity of phytoplankton prey in much of the sea, there should be strong pressure for microzooplankton, the primary grazers of most phytoplankton, to evolve strategies that maximize prey encounter and utilization while allowing for survival in times of scarcity. These strategies include higher grazing rates on faster-growing phytoplankton cells, the direct use of light for enhancement of protist digestion rates, nutritional plasticity, rapid population growth combined with formation of resting stages, and defenses against predatory zooplankton. Most of these phenomena should increase community-level coupling (i.e. the degree of instantaneous and time-dependent similarity) between rates of phytoplankton growth and microzooplankton grazing, tending to stabilize planktonic ecosystems. Conversely, phytoplankton, whose mortality in the sea is overwhelmingly due to microzooplankton grazing, should experience strong pressure to evolve grazing resistence. Strategies may include chemical, morphological, and `nutrient deficit' defenses. Successful deployment of these defenses should lead to uncoupling between rates of phytoplankton growth and microzooplankton grazing, promoting instability in ecosystem structure. Understanding the comparative ecosystem dynamics of various ocean regions will require an appreciation of how protist grazer behavior and physiology influence the coupling between rates of phytoplankton growth and microzooplankton grazing.     

Fast microzooplankton grazing on fast-growing,low-biomass phytoplankton: a case study in spring in Chesapeake Bay,Delaware Inland Bays and Delaware Bay

Dilution experiments were performed to examine the growth and grazing mortality rates of picophytoplankton (<2 μm), nanophytoplankton (2–20 μm), and microphytoplankton (>20 μm) at stations in the Chesapeake Bay (CB), the Delaware Inland Bays (DIB) and the Delaware Bay (DB), in early spring 2005. At station CB microphytoplankton, including chain-forming diatoms were dominant, and the microzooplankton assemblage was mainly composed of the tintinnid Tintinnopsis beroidea. At station DIB, the dominant species were microphytoplanktonic dinoflagellates, while the microzooplankton community was mainly composed of copepod nauplii and the oligotrich ciliate Strombidium sp. At station DB, nanophytoplankton were dominant components, and Strombidium and Tintinnopsis beroidea were the co-dominant microzooplankton. The growth rate and grazing mortality rate were 0.13–3.43 and 0.09–1.92 d⁻¹ for the different size fractionated phytoplankton. The microzooplankton ingested 73, 171, and 49% of standing stocks, and 95, 70, and 48% of potential primary productivity for total phytoplankton at station CB, DIB, and DB respectively. The carbon flux for total phytoplankton consumed by microzooplankton was 1224.11, 100.76, and 85.85 μg C l⁻¹ d⁻¹ at station CB, DIB, and DB, respectively. According to the grazing mortality rate, carbon consumption rate and carbon flux turn over rates, microzooplankton in study area mostly preferred to graze on picophytoplankton, which was faster growing but was lowest biomass component of the phytoplankton. The faster grazing on Fast-Growing-Low-Biomass (FGLB) phenomenon in coastal regions is explained as a resource partitioning strategy. This quite likely argues that although microzooplankton grazes strongly on phytoplankton in these regions, these microzooplankton grazers are passive. Handling editor: K. Martens     

The effect of predator limitation on the dynamics of simple food chains

We investigate the influence of competition between predators on the dynamics of bitrophic predator–prey systems and of tritrophic food chains. Competition between predators is implemented either as interference competition, or as a density-dependent mortality rate. With interference competition, the paradox of enrichment is reduced or completely suppressed, but otherwise, the dynamical behavior of the systems is not fundamentally different from that of the Rosenzweig–MacArthur model, which contains no predator competition and shows only continuous transitions between fixed points or periodic oscillations. In contrast, with density-dependent predator mortality, the system shows a surprisingly rich dynamical behavior. In particular, decreasing the density regulation of the predator can induce catastrophic shifts from a stable fixed point to a large oscillation where the predator chases the prey through a cycle that brings both species close to the threshold of extinction. Other catastrophic bifurcations, such as subcritical Hopf bifurcations and saddle-node bifurcations of limit cycles, do also occur. In tritrophic food chains, we find again that fixed points in the model with predator interference become unstable only through Hopf bifurcations, which can also be subcritical, in contrast to the bitrophic situation. The model with a density limitation shows again catastrophic destabilization of fixed points and various nonlocal bifurcations. In addition, chaos occurs for both models in appropriate parameter ranges.     

Density-dependent effects in a parasitic nematode,Elaphostrongylus rangiferi,in the snnail intermediate host

Summary Density-dependent effects in Elaphostrongylus rangiferi, a parasitic nematode in the CNS and muscular system of reindeer, were studied in a laboratory population of the snail intermediate host, Arianta arbustorum. The rates in parasite growth, development and mortality were all affected by parasite density. The effects on growth and development were, however, much more marked, than the effect on mortality.All density-dependent rates were intensified by decreasing snail size, and by snail starvation. The snail host showed marked tissue reactions against infection, and the intensity of these reactions increased with increasing parasite density. The mechanism behind the observed density-dependent rates is discussed, and is tentatively concluded to be competition for nutritive substances in the host tissue.The importance of a density-dependent developmental rate in natural populations of this parasite is discussed, and it is hypothesized that this effect may counteract the strong temperature-dependent developmental rate of E. rangiferi In a more general context it is pointed out that density-dependent developmental rates, although common amongst animal populations, has been neglected in models of population dynamics. Developmental rates are usually represented by a constant time lag in such models, but should be treated as a density-dependent variable.     

Selection-delayed population dynamics in baleen whales and beyond

While it is known that population cycles are driven by delayed density-dependent feedbacks, the search for a common feedback mechanism in natural populations with cyclic dynamics has remained unresolved for almost a century. To identify the existence and cause of delayed feedbacks I apply six age- and sex-structured population dynamics models to seven species of baleen whales (suborder Mysticeti) that were heavily depleted by past commercial whaling. The six models include a predator–prey model with killer whale (Orcinus orca) as the predator, and five singe-species models based on (1) exponential growth, (2) density-regulated growth, (3) density-regulated growth with depensation, (4) delayed density-regulated growth and (5) selection-delayed dynamics. The latter model has a density-regulated growth rate that is accelerated and decelerated by the intra-specific natural selection that arises from the density-dependent competitive interactions between the individuals in the population. Essential parameters are estimated by a Bayesian statistical framework, and it is shown that baleen whales have a delayed recovery relative to density-regulated growth. The time-lag is not explained by depensation, or by interactions with prey or predators. It is instead resolved by a selection-delayed acceleration of the intrinsic growth rate. The results are discussed in relation to the literature on cyclic dynamics, and it is noted (1) that selection-delayed dynamics is both theoretically and empirically sufficient for cyclic population dynamics, (2) that it is widespread in natural populations owing to the widespread occurrence of otherwise unexplained phenotypic cycles in populations with cyclic dynamics, and (3) that there is a lack of empirical evidence showing that predator–prey interactions is a sufficient cause for the cyclic dynamics of natural populations. The conclusion stresses the importance of intra-specific delays in cyclic dynamics, and suggests that it is the acceleration of the growth rate, and not the growth rate itself, that is determined by the density-dependent environment.     

Evaluation of alternate harvesting strategies using experimental microcosms

Experimental evidence to evaluate alternate conservation policies for harvested populations is currently meager. We used populations of the ciliate Tetrahymena thermophila growing in test tube microcosms to experimentally evaluate the effects of alternate harvesting policies in a controlled, replicable setting. Simple density-dependent models were effective in predicting patterns of ciliate population growth in the microcosms. We evaluated several univariate models, finding that a Ricker logistic model was a better predictor of ciliate population dynamics than Gompertz logistic, non-linear logistic, or random walk models. Using the Ricker logistic model as a demographic skeleton, we modeled ciliate population dynamics with respect to three alternate harvesting policies (fixed quota, fixed proportion, and fixed escapement), each conducted at four comparable levels of harvest intensity. The parameterized demographic models predicted that fixed quota harvesting would lead to lower mean ciliate abundance and higher temporal variability in ciliate abundance than fixed proportion or fixed escapement policies, with an appreciable risk of extinction, even under the controlled environmental conditions of our experimental system. For each harvesting policy, the intensity of harvest had demonstrable effects on population density. Population variability was higher for fixed quota harvesting than the other policies. The stochastic demographic model successfully predicted heightened extinction risk in the fixed quota system, relative to the other management treatments. Our experimental evidence lends support to the theoretical prediction that fixed quota harvesting is riskier than fixed proportion or fixed escapement policies.     

What makes a species common? No evidence of density-dependent recruitment or mortality of the sea urchin Diadema antillarum after the 1983–1984 mass mortality

A potential consequence of individuals compensating for density-dependent processes is that rare or infrequent events can produce profound and long-term shifts in species abundance. In 1983–1984 a mass mortality event reduced the numbers of the abundant sea urchin Diadema antillarum by 95–99 % throughout the Caribbean and western Atlantic. Following this event, the abundance of macroalgae increased and the few surviving D. antillarum responded by increasing in body size and fecundity. These initial observations suggested that populations of D. antillarum could recover rapidly following release from food limitation. In contrast, published studies of field manipulations indicate that this species had traits making it resistant to density-dependent effects on offspring production and adult mortality; this evidence raises the possibility that density-independent processes might keep populations at a diminished level. Decadal-scale (1983–2011) monitoring of recruitment, mortality, population density and size structure of D. antillarum from St John, US Virgin Islands, indicates that population density has remained relatively stable and more than an order of magnitude lower than that before the mortality event of 1983–1984. We detected no evidence of density-dependent mortality or recruitment since this mortality event. In this location, model estimates of equilibrium population density, assuming density-independent processes and based on parameters generated over the first decade following the mortality event, accurately predict the low population density 20 years later (2011). We find no evidence to support the notion that this historically dominant species will rebound from this temporally brief, but spatially widespread, perturbation.     

An empirical model of carbon flow through marine viruses and microzooplankton grazers

Viruses and microzooplankton grazers represent major sources of mortality for marine phytoplankton and bacteria, redirecting the flow of organic material throughout the world's oceans. Here, we investigate the use of nonlinear population models of interactions between phytoplankton, viruses and grazers as a means to quantitatively constrain the flow of carbon through marine microbial ecosystems. We augment population models with a synthesis of laboratory-based estimates of prey, predator and viral life history traits that constrain transfer efficiencies. We then apply the model framework to estimate loss rates in the California Current Ecosystem (CCE). With our empirically parameterized model, we estimate that, of the total losses mediated by viruses and microzooplankton grazing at the focal CCE site, 22 ± 3%, 46 ± 27%, 3 ± 2% and 29 ± 20% were directed to grazers, sloppy feeding (as well as excretion and respiration), viruses and viral lysate respectively. We identify opportunities to leverage ecosystem models and conventional mortality assays to further constrain the quantitative rates of critical ecosystem processes.     

A new demographic function maximized by life-history evolution

A goal of life-history theory has been to understand what combination of demographic traits is maximized by natural selection. In practice, researchers usually choose either density-independent population growth rate, lambda, or lifetime reproductive success, R0 (expected number of offspring produced in a lifetime). Others have shown that the maxima of density-independent lambda and R0 are evolutionarily stable strategies under specific density-dependent conditions: population regulation by equal density dependence among all age classes for lambda and by density dependence on a single age class for R0. Here I extend these connections between density-independent optimization models and density-dependent invasion function models in two ways. First, I derive a new demographic function for which a maximum corresponds to attainability of the equilibrium strategy or stability of the mean rather than stability of the variance of the strategy distribution. Second, I show explicitly a continuous range of cases with maxima between those for the lambda and R0. Graphical and biological interpretations are given for an example model. Finally, exceptions to a putative life-history generality (from lambda and R0 models), that high early-life mortality selects for high iteroparity, are shown.     

Comparing diatom and Alexandrium catenella/tamarense blooms in Thau lagoon: Importance of dissolved organic nitrogen in seasonally N-limited systems

Diatom blooms in Thau lagoon are always related to rain events leading to inputs of inorganic nutrients such as phosphate, ammonium and nitrate through the watershed with time lags of about 1 week. In contrast, blooms of Alexandrium catenella/tamarense can occur following periods of 3 weeks without precipitation and no significant input of conventional nutrients such as nitrate and phosphate. Field results also indicate a significant drop (from 22–25 to 15–16 μM over 3 days) in dissolved organic nitrogen (DON) at the bloom peak, as well as a significant inverse relationship between A. catenella/tamarense cell density and DON concentrations that is not apparent for diatom blooms. Such dinoflagellate blooms are also associated with elevated (6–9 μM) ammonium concentrations, a curious feature also observed by other investigators, possibly the results of ammonium excretion by this organism during urea or other organic nitrogen assimilation.The potential use of DON by this organism represents short cuts in the nitrogen cycle between plants and nutrients and requires a new model for phytoplankton growth that is different from the classical diatom bloom model. In contrast to such diatom blooms that are due to conventional (nitrate, phosphate) nutrient pulses, Alexandrium catenella/tamarense blooms on the monthly time scale are due to organic nutrient enrichment, a feature that allows net growth rates of about 1.3 d⁻¹, a value higher than that generally attributed to such organisms.     

Size symmetry of competition alters biomass-density relationships

As crowded populations of plants develop, the growth of some plants is accompanied by the death of others, a process called density-dependent mortality or 'self-thinning'. During the course of density-dependent mortality, the relationship between total population biomass (B) and surviving plant density (N) is allometric: B = aN(b). Essentially, increasing population biomass can be achieved only through decreasing population density. Variation in the allometric coefficient a among species has been recognized for many years and is important for management, assessment of productivity and carbon budgets, but the causes of this variation have not been elucidated. Individual-based models predict that size-dependent competition causes variation in the allometric coefficient. Using transgenic Arabidopsis with decreased plasticity, we provide experimental evidence that morphological plasticity of wild-type populations decreases the size asymmetry of competition for light and thereby decreases density-dependent mortality. This decrease in density-dependent mortality results in more biomass at a given density under size-symmetric compared with size-asymmetric competition.     

A brief summary of the physiology and ecology of Karenia brevis Davis (G. Hansen and Moestrup comb. nov.) red tides on the West Florida Shelf and of hypotheses posed for their initiation,growth, maintenance,and termination

This contribution represents a review of the historical and recent literature describing the environmental factors that relate to the distribution, growth, primary production, nutrient requirements and utilization along with hypotheses that are extant for the initiation, growth, maintenance and termination of Karenia brevis blooms on the West Florida Shelf. Potential nutrient sources that support blooms and relate to recent questions on the duration, frequency, and intensity of WFS blooms are summarized and some thoughts are presented which relate to the question of why K. brevis, a slow growing dinoflagellate, becomes dominant in a nearshore shelf region that is typically dominated by diatoms.There is no single hypothesis that can account for blooms of K. brevis along the west coast of Florida. Of the approximately 24 thoughts and hypotheses described herein (including the 1880s speculation), seven are related to rainfall and/or riverine flux, six invoke the benthos or bottom flux in one form or another, seven involve water column hydrodynamics or are unrelated to the benthos or land sources, and four are primarily chemical/allelopathy based. Nutrient sources for growth and maintenance range from atmospheric deposition, N-fixation, riverine and benthic flux, and zooplankton excretion to decaying fish killed by the toxic dinoflagellate with no one source being conclusively identified as a primary contributor to prolonged bloom maintenance. Insufficient information is available to delimit specific mechanisms that may play a role in the termination of K. brevis blooms. However, general processes such as macro- and microzooplankton grazing, bacterial and viral cell lysis, and dispersal by physical advection and the break down of fronts, that originally may have acted as concentrating mechanisms, are reviewed.     

Dynamics of stage-structure predator-prey systems under density-dependent effect and mortality

We develop a four dimensional predator-prey system in continuous time with stage-structure for both the communities. The reproduction rate of the prey and the transition rate for the predator, in our model, are assumed to be density-dependent. The stability results for the coexisting equilibrium are obtained by making use of Routh–Hurwitz criteria. Because of the density-dependent effects, numerical simulations are applied in complex situations. We observe that increasing values of the coefficients linked with density-dependent term promote the stability of the coexisting steady state. Our main focus is to understand the variation of stocks when mortality rates on different stage classes are increased. We verified that stable stock on mature predator increases with its increasing mortality rate in three different modeling frameworks. However, no such positive effect on the biomass of the immature predator occurs when immature predators are removed, culled or harvested. Therefore, we could conclude that the appearance of hydra effect on many unstructured predator-prey models is due to the mortality of the mature predator only. No hydra effect is also detected when mature prey is removed in several situations we discussed. Overall, the obtained results are new and could be interesting contribution in theoretical ecology.     

Revisiting Beverton–Holt recruitment in the presence of variation in food availability

Understanding density-dependent changes in juvenile survival and growth rates is of great importance because these rates determine recovery rates for imperiled populations and/or sustainable harvest rates. Unfortunately, the mechanisms leading to density dependent survival and growth are among the least understood process in biology and fisheries. Previous work has shown that small fish may vary foraging times to achieve a target growth rate, resulting in the well-known Beverton–Holt recruitment function with variation in food availability affected the initial slope of the recruitment curve. We amend their derivation to show that incorporating fish growth under a variety of evolutionary strategies for balancing foraging time and predation risk still leads to recruitment approximately as expected under the Beverton–Holt recruitment model but that changing food availability affects both the initial slope and maximum recruitment level. We demonstrate that when food availability is known to vary over time, these models often result in a more parsimonious alternative than the standard Beverton–Holt function. Further, Beverton–Holt recruitment is expected when foraging times are adjusted to balance fitness gains from growth against mortality risk. Finally, linking recruitment success to food availability warns that species with high scope for density dependent survival (high compensation ratio or steepness) may be extremely sensitive to changes in available food densities. This work emphasizes the sensitivity of stock-recruitment parameters to food availability and strongly suggests a need to carefully monitor lower trophic levels to better understand and predict dramatic changes in juvenile recruitment and carrying capacity.     

Population responses of a conifer-dwelling aphid to seasonal changes in its host

Abstract.  1. Current evidence suggests that seasonal changes in spruce needle sap nutrients have a decisive influence on green spruce aphid ( Elatobium abietinum ) population density, but the mechanisms of population change, the roles of development rate, fertility and mortality, and the existence of density-dependent processes, are not clearly understood.
2. Experimental studies of aphid populations were conducted in controlled environments to estimate seasonal patterns in aphid mean relative growth rate, prenatal development, fertility, and mortality. Studies were also made of the effect of aphid crowding on vital rates.
3. Independent of the degree of aphid crowding, seasonal changes in the amino acid concentration of needle sap were tracked by aphid growth rate, fertility (and adult size), but not by rates of aphid mortality. The most pronounced change in vital rates, and the one most likely to drive seasonal population change, was in fertility. Prenatal development time actually became shorter in periods when nutrients were scarce, but the resulting adult aphids were smaller and less fertile than during periods of improved nutrition.
4. Density dependence of vital rates was only observed during mid-summer when nutrients were least available. Mortality, growth rate, and prenatal development were the most strongly density-dependent processes. In contrast, there was no evidence that fertility rates were likely to respond to crowding.
5. There were no important differences between populations reared on small, potted spruce trees and those on plantation trees aged 25 years. This gives confidence that demographic data from a variety of field and laboratory sources could be used to compile data appropriate for population models.     

Social and demographic effects of anthropogenic mortality: a test of the compensatory mortality hypothesis in the red wolf

Whether anthropogenic mortality is additive or compensatory to natural mortality in animal populations has long been a question of theoretical and practical importance. Theoretically, under density-dependent conditions populations compensate for anthropogenic mortality through decreases in natural mortality and/or increases in productivity, but recent studies of large carnivores suggest that anthropogenic mortality can be fully additive to natural mortality and thereby constrain annual survival and population growth rate. Nevertheless, mechanisms underlying either compensatory or additive effects continue to be poorly understood. Using long-term data on a reintroduced population of the red wolf, we tested for evidence of additive vs. compensatory effects of anthropogenic mortality on annual survival and population growth rates, and the preservation and reproductive success of breeding pairs. We found that anthropogenic mortality had a strong additive effect on annual survival and population growth rate at low population density, though there was evidence for compensation in population growth at high density. When involving the death of a breeder, anthropogenic mortality was also additive to natural rates of breeding pair dissolution, resulting in a net decrease in the annual preservation of existing breeding pairs. However, though the disbanding of a pack following death of a breeder resulted in fewer recruits per litter relative to stable packs, there was no relationship between natural rates of pair dissolution and population growth rate at either high or low density. Thus we propose that short-term additive effects of anthropogenic mortality on population growth in the red wolf population at low density were primarily a result of direct mortality of adults rather than indirect socially-mediated effects resulting in reduced recruitment. Finally, we also demonstrate that per capita recruitment and the proportion of adults that became reproductive declined steeply with increasing population density, suggesting that there is potential for density-dependent compensation of anthropogenically-mediated population regulation.