Density-dependent Response Research Articles

Response of the shot-hole borer of tea, Xyleborus fornicatus (Coleoptera: Scolytidae) to conspecifics and plant semiochemicals

The responses of the tea shot-hole borer beetle, Xyleborus fornicatus, to conspecific and host plant odours were tested in an olfactometer. Males showed greater attraction to unmated females than to mated females and mated females showed a negative density dependent response with extremely significant repulsion shown when 10 mated females were present. Plants of the susceptible tea cultivar TRI2025 were significantly more attractive to females than those of the less susceptible cultivar TRI2023, although stem extracts of the susceptible cultivar were not found to be attractive to beetles. Beetles were found to be attracted to ethanol and to the known tea volatiles eugenol, hexanol, α- and β-pinene, geraniol, and methyl salicylate. The attraction of the tea volatiles was found to be enhanced when they were mixed with ethanol, indicating that ethanol has a broad synergistic effect. In a study on phagostimulant effects of tea extracts, sugars, and caffeine, the methanol extract of cultivar TRI2025 and the sugars glucose and sucrose were found to be attractive, and caffeine repulsive, to beetles. Pellets containing the extract were found to be preferred over those containing glucose. Beetles were attracted to pellets containing glucose/inositol mixture with glucose:inositol in the 3:1 ratio found in the susceptible cultivar TRI2025, being preferred to the mixture in 5:1 ratio found in stems of the less susceptible cultivar TRI2025. Both volatile and nonvolatile constituents of tea appear to play a role in attracting the female beetle to the plant.

Experimental evidence for density-dependent responses to mortality of snake-necked turtles

Density-dependent compensation has rarely been demonstrated in long-lived vertebrates in highly variable environments, such as the wet-dry tropics, where complex factors impact on vital rates. We used an experimental manipulation of population density in six replicated wild populations of the northern snake-necked turtle (Chelodina rugosa). We show that this species can rebound rapidly following reductions in density, and so is resilient to harvest and predation by pigs. Remarkably, in some populations, turtle abundance took as little as 1 year to recover from a strong negative perturbation (>50% experimental population reduction) in adult density. This was achieved through an increase in hatchling recruitment and survival into larger size classes. Our manipulative experiments, viewed concomitantly with previous experimental and correlative research, challenge the general perceptions that freshwater turtles universally are highly susceptible to any form of off-take and that high sub-adult and adult survival is crucial for achieving long-term population stability in freshwater turtles generally. In the case of C. rugosa, such generalities would produce overly cautious prescriptions for sustainable management.

Life‐History Variation in Contrasting Habitats: Flowering Decisions in a Clonal Perennial Herb (Veratrum album)

Quantifying intraspecific demographic variation provides a powerful tool for exploring the diversity and evolution of life histories. We investigate how habitat-specific demographic variation and the production of multiple offspring types affect the population dynamics and evolution of delayed reproduction in a clonal perennial herb with monocarpic ramets (white hellebore). In this species, flowering ramets produce both seeds and asexual offspring. Data on ramet demography are used to parameterize integral projection models, which allow the effects of habitat-specific demographic variation and reproductive mode on population dynamics to be quantified. We then use the evolutionarily stable strategy (ESS) approach to predict the flowering strategy-the relationship between flowering probability and size. This approach is extended to allow offspring types to have different demographies and density-dependent responses. Our results demonstrate that the evolutionarily stable flowering strategies differ substantially among habitats and are in excellent agreement with the observed strategies. Reproductive mode, however, has little effect on the ESSs. Using analytical approximations, we show that flowering decisions are predominantly determined by the asymptotic size of individuals rather than variation in survival or size-fecundity relationships. We conclude that habitat is an important aspect of the selective environment and a significant factor in predicting the ESSs.

Detritivorous earthworms directly modify the structure, thus altering the functioning of a microdecomposer food web

Epigeic earthworms are key organisms in fresh organic matter mounds and other hotspots of heterotrophic activity. They turn and ingest the substrate intensively interacting with microorganisms and other soil fauna. By ingesting, digesting and assimilating the surrounding substrate, earthworms could directly modify the microdecomposer community, yet little is known of such direct effects. Here we investigate the direct effects of detritivore epigeic earthworms on the structure and function of the decomposer community. We characterized changes in the microfauna, microflora and the biochemical properties of the organic substrate over a short time (72 h) exposure to 4 different densities of the earthworm Eisenia fetida using replicate mesocosms (500 ml). We observed a strong and linear density-dependent response of the C and N mineralization to the detritivore earthworm density. Earthworm density also linearly increased CO 2 efflux and pools of labile C and inorganic N. This effect on the function was likely a direct consequence of earthworm activity. Furthermore, earthworms affected the microbial metabolic activity, but this response was not linearly related to the earthworm density, possibly because of indirect effects through the microbial community. Earthworms also had strong effects on the structure of the two trophic levels examined; they enhanced the fungal populations and reduced the numbers of bacterivore nematodes. The effect on the fungi was clearly dependent on the earthworm density, and the reduction of bacterivorous nematodes was also related to the earthworm density, but only marginally. In contrast, earthworms did not have significant effects on microbial biomass carbon, flagellate protozoa or ciliate protozoa. A meaningful part of the short term changes in microflora and microfaunal communities after some hours might be attributable to the earthworm gut associated processes. Hence, detritivore earthworms can directly and quickly modulate the decomposer community altering the decomposition rates of organic matter.

Analytic form for a nonlocal kinetic energy functional with a density-dependent kernel for orbital-free density functional theory under periodic and Dirichlet boundary conditions

We derive an analytic form of the Wang-Govind-Carter (WGC) [Wang et al., Phys. Rev. B 60, 16350 (1999)] kinetic energy density functional (KEDF) with the density-dependent response kernel. A real-space aperiodic implementation of the WGC KEDF is then described and used in linear scaling orbital-free density functional theory (OF-DFT) calculations.

Density-dependent effects of a new invasive false mussel, Mytilopsis trautwineana (Tryon 1866), on shrimp, Litopenaeus vannamei (Boone 1931), aquaculture in Colombia

Dreissenid and mytilid bivalve molluscs have become some of the most important invasive pests in the world's marine, brackish and fresh water ecosystems. We report on a newly invasive dressenid false mussel, Mytilopsis trautwineana, within a shrimp farm on the Caribbean coast of Colombia. The impacts of the mussel and the response of the shrimps can be explained through density-dependent interactions. A low density of filter feeding mussels has a positive effect on aquaculture through the removal of suspended particulate matter. However, at high densities mussels reduce shrimp survival from 76.0 ± 2.1% to 61.2 ± 2.3%. The economic impact of reduced shrimp survival is somewhat mediated through density-dependent responses in the form of compensatory growth, with shrimps attaining a higher mean weight and higher food conversion ratio. Despite these responses, it is estimated that fouling by M. trautwineana costs US$9000 ha − 1 yr − 1 . The causes of reduced shrimp productivity are both direct and indirect. Mussels displace the shrimps from the food-rich pond bed. Mussel filtration increases water clarity, which in turn exposes shrimps to enhanced predation from birds. Shrimps respond to clearer water by reducing pigmentation, which reduces their commercial value. Mussels also foul aeration devices, significantly reducing their ability to maintain favourable DO 2 levels. The apparent salinity tolerance of M. trautwineana suggests the species will become a more widespread biofouling nuisance. Possible control strategies include changes to management procedures, chemical treatments, microencapsulated BioBullets, polyculture with omnivorous fishes, or encouragement of indigenous mangrove-dwelling grapsid crabs.

Temporal variability in the life history and reproductive biology of female dugongs in Torres Strait: The likely role of sea grass dieback

The extensive sea grass meadows in Torres Strait enable it to be a globally important habitat for the dugong, Dugong dugon, a marine mammal of cultural and dietary significance to Torres Strait Islanders and the basis for the substantial island-based fishery in the Torres Strait Protected Zone. Torres Strait sea grass communities are subjected to episodic diebacks which are now believed to be largely natural events. Information on dugong life history was obtained from specimens obtained from female dugongs as they were butchered for food by Indigenous hunters at two major dugong hunting communities in Torres Strait: Daru (9.04°S, 143.21°E) in 1978–1982 (a time of sea grass dieback and recovery) and Mabuiag Island (9.95°S, 142.15°E) in 1997–1999 (when sea grasses were abundant). Dugongs sampled in 1997–1999 had their first calf at younger ages (minimum of 6 cf. 10 years), and more frequently (interbirth interval based on all possible pregnancies 2.6±0.4 (S.E.) yr cf. 5.8±1.0 yr) than the dugongs sampled in 1978–1982. Pregnancy rates increased monotonically during 1978–1982, coincident with sea grass recovery. The age distribution of the female dugongs collected in 1997–1999 also suggested a low birth rate between 1973 and 1983 and/or or a high level of mortality for animals born during this period. These results add to the evidence from other regions that the life history and reproductive rate of female dugongs are adversely affected by sea grass loss, the effect of which cannot be separated from a possible density-dependent response to changes in dugong population size. Many green turtles in Torres Strait were also in poor body condition coincident with the 1970s sea grass dieback. The impacts of future sea grass diebacks need to be anticipated when management options for the traditional Torres Strait fisheries for dugongs and green turtles are evaluated.

Fecundity of the autumnal moth depends on pooled geometrid abundance without a time lag: implications for cyclic population dynamics

1. The abundance and fecundity-related body size variation of the cyclic autumnal moth Epirrita autumnata were monitored from the early increase phase and throughout the outbreak to the end of the density decline in northernmost Norway during 1999-2006. Another geometrid, the winter moth Operophtera brumata, did not increase in density until the autumnal moth had its post-peak in 2004, and was at its own peak concurrent with the steeply declining autumnal moth abundance in 2005-06. 2. The body size variables measured (forewing lengths of males and females and hind femur lengths of males) of the autumnal moth showed a similar density-dependent response, i.e. increasing density was associated with decreasing body size and fecundity. Nevertheless, regression analyses clearly ranked the pooled geometrid abundance without a time lag as the best predictor for the body size variation, ahead of the abundance of the autumnal moth or past abundance of all geometrids. 3. Nondelayed effects of lowered food quality and absolute shortage of foliage under congested conditions are the most plausible reasons for reduced body size. 4. Two most commonly proposed causal factors of the autumnal moth population cycle, i.e. delayed inducible resistance of the host plant (mountain birch Betula pubescens czerepanovii) and delayed density-dependent parasitism by specialized hymenopteran parasitoids, cannot easily explain the diverging population trends between the autumnal and winter moths. 5. We suggest that either the inducible resistance of the host tree or the host utilization of the most important parasitoids and/or pathogens have to be strictly species-specific between these closely related moth species to produce the population dynamics observed. That fecundity of the autumnal moth was best related to the pooled geometrid abundance weakens support for the former hypothesis, while our lack of host-specific information limits conclusions about the role of natural enemies.

Precipitation, Litterfall, and the Dynamics of Density and Biomass in the Black-Eared Opossum,Didelphis aurita

Density and biomass may reveal different aspects of the dynamics of populations, but most studies have focused on density or relative abundance. Density and biomass also may behave differently in parts of the population composed of males and females because of differences in vagility and parental care between sexes. Herein, we explore seasonal and multiannual variation in density and biomass in a population of black-eared opossums (Didelphis aurita). Females and males were analyzed separately for associations with precipitation and litterfall, indicators of resource availability. Litterfall, density, and biomass were estimated from 1997 to 2003 on three 0.64-ha trapping grids in an area of Atlantic Forest in Rio de Janeiro, Brazil. Density of females exhibited a significant increase during the study. Density and biomass of males were more variable, without any significant positive trend or correlation with precipitation or litterfall. Seasonal and biannual patterns of precipitation were detected, followed by the density and biomass of females with time lags varying from 0 to 1, and from 9 to 12 months. The relative stability of biomass of females, and its association with precipitation and litterfall, suggests that resource availability combined with density-dependent responses regulated the local population of females. The local population of males may be more influenced by the stochasticity resulting from the higher vagility of males.

Aggregation of parasitism risk in an aphid-parasitoid system: Effects of plant patch size and aphid density

Few studies have linked density dependence of parasitism and the tritrophic environment within which a parasitoid forages. In the non-crop plant-aphid, Centaurea nigra – Uroleucon jaceae system, mixed patterns of density-dependent parasitism by the parasitoids Aphidius funebris and Trioxys centaureae were observed in a survey of a natural population. Breakdown of density-dependent parasitism revealed that density dependence was inverse in smaller colonies but direct in larger colonies (>20 aphids), suggesting there is a threshold effect in parasitoid response to aphid density. The CV 2 of searching parasitoids was estimated from parasitism data using a hierarchical generalized linear model, and CV 2 >1 for A. funebris between plant patches, while for T. centaureae CV 2 >1 within plant patches. In both cases, density independent heterogeneity was more important than density-dependent heterogeneity in parasitism. Parasitism by T. centaureae increased with increasing plant patch size. Manipulation of aphid colony size and plant patch size revealed that parasitism by A. funebris was directly density dependent at the range of colony sizes tested (50–200 initial aphids), and had a strong positive relationship with plant patch size. The effects of plant patch size detected for both species indicate that the tritrophic environment provides a source of host density independent heterogeneity in parasitism, and can modify density-dependent responses. Wenige Studien haben eine Verbindung zwischen dichteabhängigem Parasitismus und der tritrophischen Umwelt hergestellt, in der ein Parasitoid nach Wirten sucht. In einem System aus Nicht-Nutzpflanze und Blattlaus, dem Centaurea nigra-Uroleucon jaceae -System, wurden gemischte Muster von dichteabhängigem Parasitismus durch die Parasitoiden Aphidius funebris und Trioxys centaureae bei einer Untersuchung einer natürlichen Population beobachtet. Der Zusammenbruch der dichteabhängigen Parasitierung zeigte, dass die Dichteabhängigkeit in kleinen Kolonien invers und in größeren Kolonien (>20 Blattläuse) direkt war, und lässt vermuten, dass es einen Schwelleneffekt in der Reaktion der Parasitoide auf die Blattlausdichte gibt. Der CV 2 der suchenden Parasitoide wurde abgeschätzt, indem hierarchisch generalisierte, lineare Modelle verwendet wurden. Für A. funebris wurde ein CV 2 >1 zwischen den Pflanzenvorkommen festgestellt, während T. centaureae einen CV 2 >1 innerhalb der Pflanzenvorkommen zeigte. In beiden Fällen war die dichteunabhängige wichtiger als die dichteabhängige Heterogenität der Parasitierung. Die Parasitierung durch T. centaureae nahm mit der Größe der Pflanzenvorkommen zu. Die Manipulation der Koloniegrößen der Blattläuse und der Größe der Pflanzenvorkommen zeigte, dass die Parasitierung durch A. funebris direkt dichteabhängig im Bereich der untersuchten Koloniegrößen (50-200 initiale Blattläuse) war und einen starken positiven Zusammenhang mit der Größe des Pflanzenvorkommens zeigte. Die Effekte der Größe der Pflanzenvorkommen, die bei beiden Arten festgestellt wurden, weisen darauf hin, dass eine tritrophische Umwelt eine Quelle für eine von der Wirtsdichte unabhängige Heterogenität in der Parasitierung darstellt und dichteabhängige Reaktionen modifizieren kann.

Fish Assemblage, Density, and Growth in Lateral Habitats within Natural and Regulated Sections of Washington's Elwha River Prior to Dam Removal

We characterized seasonal fish assemblage, relative density, and growth in river margins above and between two Elwha River dams scheduled for removal. Fish assemblage and relative density differed in the lateral habitats of the middle-regulated and upper-unregulated sections of the Elwha River. Rainbow trout was the numerically dominant salmonid in both sections, with bull trout present in low numbers. Sculpin were common in the middle section, but not detected in the upper section. In 2004, mean length and biomass of age-0 rainbow trout were significantly smaller in the middle section than in the upper section by the end of the growing season (September). In 2005, an earlier emergence of rainbow trout in the middle section (July) compared to the upper section (August) corresponded with warmer water temperatures in the middle section. Despite lower growth, the margins of mainstem units in the middle section supported higher mean areal densities and biomass of age-0 rainbow trout than the upper section. These results suggest that growth performance of age-0 rainbow trout was lower in the middle section than in the upper section, which could have been a density-dependent response, or a result of poor food production in the sediment-starved regulated section, or both. Based on our findings, we believe that seasonal sampling of river margins within reference reaches is a cost effective and repeatable method for detection of biologically important short- and long-term changes in emergence timing, density, and growth of rainbow trout before and after dam removals in the Elwha River.

Controlling Mouflon Sheep at the Kahuku Unit of Hawai`i Volcanoes National Park

Hawai`i Volcanoes National Park acquired the 115,000-acre Kahuku Ranch Unit in 2003. The Kahuku Unit contains numerous exceptional natural resources including endemic plants and birds, many of which are federally listed threatened and endangered species. Eleven European mouflon sheep were introduced to Kahuku from 1968-1974 for private trophy hunting. Because Hawaiian plants did not evolve with mammalian herbivores, managers began to control the large mouflon population with a closely directed volunteer program. Since 2004, more than 1,900 mouflon have been removed through this program. We estimated that there were 2,586 ± 705 (90% CI) mouflon at Kahuku in November 2004, but by December 2006 the population decreased 30%. Although the ram:ewe ratio did not change significantly after this population decrease, the mean (±95% CI) overall ratio in 2004 was 1:2.4 (1:2.1-1:2.7) and 1:2.7 (1:2.4-1:3.1) in 2006. We found that 82.6% of adult ewes (n = 26) were pregnant with a single fetus in early 2007, and there was a significant increase in the number of lambs per ewe from 2005 to 2007 consistent with a density-dependent response. The maximum (±95% CI) number of lambs per ewe was 0.484 (0.412-0.558) in 2005 and 0.667 (0.587-0.750) in 2007. The directed volunteer program has been more successful in reducing mouflon abundance at Kahuku than species such as feral pigs elsewhere on Hawai`i Island, but some population-level responses such as increased reproduction could result from density decreases.

Density-dependent body condition and recruitment in a tropical ungulate

In tropical areas, annual rainfall and predation have been reported to drive population dynamics of most species of large herbivores studied so far, with little direct empirical evidence for density-dependent responses of life-history traits. We here investigated in a game ranch in Zimbabwe density-dependent responses of body condition and recruitment in an impala ( Aepyceros melampus (Lichtenstein, 1812)) population that underwent an experimental drastic reduction of density within 2 years under similar rainfall and predation pressure. Body condition of all sex and age classes was lower during the high-density year than during the low-density year, suggesting increased competition for restricted resources at high density. In addition, we observed a significant increase in population recruitment (from 0.47 to 0.80 juveniles/female) as population density declined. Our study provides a rare example of a direct density-dependent response of body condition at the individual level in a tropical ungulate species, and indicates that food resource variation controls population dynamics of impala under constant and moderate predation pressure as is commonly reported in temperate populations of large herbivores.

Plant Chemical Defense Induced by a Seed-Eating Pollinator Mutualist

Plant-seed parasite pollination mutualisms involve a specific pollinator whose larvae develop by consuming a fraction of the host plant seeds. These mutualisms are stable only if the plant can control seed destruction by the larvae. Here, we studied the chemical response of the European globeflower Trollius europaeus to infestation by an increasing number of Chiastocheta fly larvae. We used liquid chromatographic analysis to compare the content of phenolic compounds in unparasitized and parasitized fruits collected in two natural populations of the French Alps, and mass spectrometry and nuclear magnetic resonance to elucidate the structure of adonivernith, a C-glycosyl-flavone. This compound is present in many of the organs of T. europaeus, but not found in other Trollius species. Furthermore, it is overproduced in the carpel walls of parasitized fruits, and this induced response to infestation by fly larvae is density-dependent (increases with larval number), and site-dependent (more pronounced in the high-altitude site). Mechanical damage did not induce adonivernith production. This tissue-specific and density-dependent response of T. europaeus to infestation by Chiastocheta larvae might be an efficient regulation mechanism of seed-predator mutualist population growth if it decreases survival or growth of the larvae.

Demographic response of snake‐necked turtles correlates with indigenous harvest and feral pig predation in tropical northern Australia

Species that mature late, experience high levels of survival and have long generation times are more vulnerable to chronic increases in mortality than species with higher fecundity and more rapid turnover of generations. Many chelonians have low hatchling survival, slow growth, delayed sexual maturity and high subadult and adult survival. This constrains their ability to respond quickly to increases in adult mortality from harvesting or habitat alteration. In contrast, the northern snake-necked turtle Chelodina rugosa (Ogilby 1890) is fast-growing, early maturing and highly fecund relative to other turtles, and may be resilient to increased mortality. Here we provide correlative evidence spanning six study sites and three field seasons, indicating that C. rugosa is able to compensate demographically to conditions of relatively low subadult and adult survival, caused by pig Sus scrofa (Linnaeus 1758) predation and customary harvesting by humans. Recruitment and age specific fecundity tended to be greater in sites with low adult and subadult survival (and thus reduced densities of large turtles), owing to higher juvenile survival, a smaller size at onset of maturity and faster post-maturity growth. These patterns are consistent with compensatory density-dependent responses, and as such challenge the generality that high subadult and adult survival is crucial for achieving long-term population stability in long-lived vertebrates such as chelonians. We posit that long-lived species with 'fast' recruitment and a capacity for a compensatory demographic response, similar to C. rugosa, may be able to persist in the face of occasional or sustained adult harvest without inevitably threatening population viability.

Density-dependent optical response of gold nanoparticle monolayers on silicon substrates

We experimentally and theoretically study the density-dependent optical properties of 20 nm gold nanoparticle monolayers on silicon at 530 nm. Both results indicate a density-dependent resonance behavior. The density-dependent resonance suggests that the optical properties of the gold nanoparticle monolayers can be drastically modulated by changing the volume ratio V(r). It also implies, in this composite system, that the total surface plasmon resonance is maximized at a certain V(r).

Density-dependent responses of soybean aphid (Aphis glycines Matsumura) populations to generalist predators in mid to late season soybean fields

Abstract Since the arrival of the soybean aphid, Aphis glycines, to the U.S. in 2000, year to year abundances have varied substantially. Although, field studies have suggested that several resident generalist predators have the ability to suppress A. glycines populations, factors responsible for its population dynamics remain unclear. For natural enemies to regulate herbivore populations, predation rates must increase as prey population densities increase (i.e., density dependence). Experiments were conducted in 2004–2006 to assess the ability of the existing natural enemy community to exert density-dependent effects on A. glycines populations. Aphid densities were manipulated on individual plants (transplanted to the field) or in small plots to contain a range of aphid densities from low (10 per plant) to high (>1000 per plant). Populations were then monitored weekly. Caged controls were included to measure population growth in the absence of predators. In 2006, an additional treatment was included to quantify the proportion of aphids emigrating from plants. In all experiments, a strong density-dependent decline in population abundance was observed as aphid populations on plants with low initial densities ( 100 aphids per plant) decreased by as much as two orders of magnitude over a 1–2 week period. We estimate that fewer than 5% of aphids emigrated from plants when aphid densities were below 4000 per plant. This suggests that the extreme declines in aphid populations over these brief experiments were not due to aphids leaving plants, but rather, due to an aggregative response of predators to high density prey populations. Given the rapidity and degree of density-dependent population decline in three consecutive years of study, we suggest the resident community of natural enemies in Midwestern soybean may have great potential to regulate soybean aphid populations.

Density-dependent immune responses against the gastrointestinal nematode Strongyloides ratti

Negative density-dependent effects on the fitness of parasite populations are an important force in their population dynamics. For the parasitic nematode Strongyloides ratti, density-dependent fitness effects require the rat host immune response. By analysis of both measurements of components of parasite fitness and of the host immune response to different doses of S. ratti infection, we have identified specific parts of the host immune response underlying the negative density-dependent effects on the fitness of S. ratti. The host immune response changes both qualitatively from an inflammatory Th1- to a Th2-type immune profile and the Th2-type response increases quantitatively, as the density of S. ratti infection increases. Parasite survivorship was significantly negatively related to the concentration of parasite-specific IgG1 and IgA, whereas parasite fecundity was significantly negatively related to the concentration of IgA only.

An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus)

Abstract Crawford, R. J. M., Underhill, L. G., Upfold, L., and Dyer, B. M. 2007. An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus). – ICES Journal of Marine Science, 64: 570–576. The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5–3.0 million adult birds in the 1920s to just 10–20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability.

Antler Size of Alaskan Moose Alces Alces Gigas: Effects of Population Density, Hunter Harvest and Use of Guides

Moose Alces alces gigas in Alaska, USA, exhibit extreme sexual dimorphism, with adult males possessing large, elaborate antlers. Antler size and conformation are influenced by age, nutrition and genetics, and these bony structures serve to establish social rank and affect mating success. Population density, combined with anthropogenic effects such as harvest, is thought to influence antler size. Antler size increased as densities of moose decreased, ostensibly a density-dependent response related to enhanced nutrition at low densities. The vegetation type where moose were harvested also affected antler size, with the largest-antlered males occupying more open habitats. Hunts with guides occurred in areas with low moose density, minimized hunter interference and increased rates of success. Such hunts harvested moose with larger antler spreads than did non-guided hunts. Knowledge and abilities allowed guides to satisfy demands of trophy hunters, who are an integral part of the Alaskan economy. Heavy harvest by humans was also associated with decreased antler size of moose, probably via a downward shift in the age structure of the population resulting in younger males with smaller antlers. Nevertheless, density-dependence was more influential than effects of harvest on age structure in determining antler size of male moose. Indeed, antlers are likely under strong sexual selection, but we demonstrate that resource availability influenced the distribution of these sexually selected characters across the landscape. We argue that understanding population density in relation to carrying capacity (K) and the age structure of males is necessary to interpret potential consequences of harvest on the genetics of moose and other large herbivores. Our results provide researchers and managers with a better understanding of variables that affect the physical condition, antler size, and perhaps the genetic composition of populations, which may be useful in managing and modelling moose populations.