Deviation from strict homeostasis across multiple trophic levels in an invertebrate consumer assemblage exposed to high chronic phosphorus enrichment in a Neotropical stream

A central tenet of ecological stoichiometry is that consumer elemental composition is relatively independent of food resource nutrient content. Although the P content of some invertebrate consumer taxa can increase as a consequence of P-enriched food resources, little is known about how ecosystem nutrient loading can affect the elemental composition of entire consumer assemblages. Here we examine the potential for P enrichment across invertebrate consumer assemblages in response to chronic high P loading. We measured elemental ratios in invertebrate consumers and basal food resources in a series of streams in lowland Costa Rica that range widely in P levels (2-135 μg l^?1 soluble reactive P). Streams with high P levels receive natural long-term (over millennia) inputs of solute-rich groundwater while low-P streams do not receive these solute-rich groundwater inputs. P content of leaf litter and epilithon increased fourfold across the natural P gradient, exceeding basal resource P content values reported in the literature from other nutrient-rich streams. Invertebrate consumers from the high-P study stream were elevated twofold in P content across multiple taxonomic and functional feeding groups, including predators. Our results strongly support the hypothesis that elevated P content in consumers feeding on P-enriched food resources is a consequence of deviation from strict homeostasis. In contrast to prior studies, we found that between-stream variation in P content of a given taxon greatly exceeded within-stream variation among different taxa, suggesting that environment may be as important as phylogeny in controlling consumer stoichiometry. Relaxing the assumption of strict homeostasis presents challenges and opportunities for advancing our understanding of how nutrient limitation affects consumer growth. Moreover, our findings may provide a window into the future of how chronic anthropogenic nutrient loading can alter stoichiometric relationships in food webs.     

Species sorting and stoichiometric plasticity control community C:P ratio of first‐order aquatic consumers

Ecological stoichiometry has proven to be invaluable for understanding consumer response to changes in resource quality. Although interactions between trophic levels occur at the community level, most studies focus on single consumer species. In contrast to individual species, communities may deal with trophic mismatch not only through elemental plasticity but also through changes in species composition. Here, we show that a community of first‐order consumers (e.g. zooplankton) is able to adjust its stoichiometry (C:P) in response to experimentally induced changes in resource quality, but only to a limited extent. Furthermore, using the Price equation framework we show the importance of both elemental plasticity and species sorting. These results illustrate the need for a community perspective in ecological stoichiometry, requiring consideration of species‐specific elemental composition, intraspecific elemental plasticity and species turnover.     

The impact of nesting cormorants on plant and arthropod diversity

Seabirds can strongly affect several major factors correlated with species diversity by concentrating marine nutrients on their nesting islands and by physically disturbing island vegetation. In this study, we investigated the effects of nesting cormorants on the abundance, species richness, and composition of plants and arthropods (Coleoptera, Heteroptera, Araneae, and Chironomidae) on islands in Stockholm archipelago, Sweden. Nesting cormorants negatively affected plant species richness and vegetation cover and that changed plant species composition. The effect of nesting cormorants on island arthropods varied between feeding groups and sampling methods. Most orders did not change in abundance or species richness but some, such as coleopterans and spiders changed in species composition. Herbivorous coleopterans were generally negatively affected by cormorants whereas fungivorous species and scavengers were generally positively affected. In structural equation modeling we found that the effect of cormorants was sometimes direct, such as on scavengers, but many effects on island consumers were mediated by changes in vegetation caused by cormorant presence. Overall, arthropod communities were highly dissimilar between cormorant and reference islands, and we therefore conclude that nesting cormorants not only affect the diversity of their nesting islands but also of the archipelago as a whole. The total diversity in the archipelago may increase through regional increased habitat heterogeneity and by adding species which are favored by seabirds (e.g. scavenging and fungivorous coleopterans).     

Nutrient Dynamics of Estuarine Invertebrates Are Shaped by Feeding Guild Rather than Seasonal River Flow

This study aimed to determine the variability of carbon and nitrogen elemental content, stoichiometry and diet proportions of invertebrates in two sub-tropical estuaries in South Africa experiencing seasonal changes in rainfall and river inflow. The elemental ratios and stable isotopes of abiotic sources, zooplankton and macrozoobenthos taxa were analyzed over a dry/wet seasonal cycle. Nutrient content (C, N) and stoichiometry of suspended particulate matter exhibited significant spatio-temporal variations in both estuaries, which were explained by the variability in river inflow. Sediment particulate matter (%C, %N and C:N) was also influenced by the variability in river flow but to a lesser extent. The nutrient content and ratios of the analyzed invertebrates did not significantly vary among seasons with the exception of the copepod Pseudodiaptomus spp. (C:N) and the tanaid Apseudes digitalis (%N, C:N). These changes did not track the seasonal variations of the suspended or sediment particulate matter. Our results suggest that invertebrates managed to maintain their stoichiometry independent of the seasonality in river flow. A significant variability in nitrogen content among estuarine invertebrates was recorded, with highest % N recorded from predators and lowest %N from detritivores. Due to the otherwise general lack of seasonal differences in elemental content and stoichiometry, feeding guild was a major factor shaping the nutrient dynamics of the estuarine invertebrates. The nutrient richer suspended particulate matter was the preferred food source over sediment particulate matter for most invertebrate consumers in many, but not all seasons. The most distinct preference for suspended POM as a food source was apparent from the temporarily open/closed system after the estuary had breached, highlighting the importance of river flow as a driver of invertebrate nutrient dynamics under extreme events conditions. Moreover, our data showed that estuarine invertebrates concentrated C and N between 10–100 fold from trophic level I (POM) to trophic level II (detritivores/deposit feeders) and thus highlighted their importance not only as links to higher trophic level organisms in the food web, but also as providers of a stoichiometrically homeostatic food source for such consumers. As climate change scenarios for the east coast of South Africa predict increased rainfall as a higher number of rainy days and days with higher rainfall, our results suggest that future changes in rainfall and river inflow will have measurable effects on the nutrient content and stoichiometry of food sources and possibly also in estuarine consumers.     

The Impact of Cormorants on Plant–Arthropod Food Webs on Their Nesting Islands

This study investigated the effects of cormorant colonies on plant–arthropod island food webs, the consequences of nutrient-rich runoff on marine communities, and feedback loops from marine to terrestrial ecosystems. Terrestrial plant responses were as expected, with the highest plant biomass on islands with low nest density and the highest nitrogen (N) content on islands with high nest density. In contrast to our hypothesis, we found no uniform density response across guilds. Among herbivores, the variable responses may depend on the relative importance of plant quality or quantity. As expected, nutrient-rich runoff entered water bodies surrounding cormorant nesting islands, but only at high nest density, and increased the density of emerging insects. This created a potential feed-back loop to spiders (major terrestrial predators), where stable isotope analyses suggested great use of chironomids. Contrary to our expectation, this potential feed-back did not result in the highest spider density on islands with a high cormorant nest density. Web spiders showed no changes in density on active cormorant islands, and lycosids were actually less abundant on active cormorant islands compared to reference islands. The variable response of spiders despite increased dipteran densities, and also in other consumer groups, may be due to direct negative effects of cormorants on soil chemistry, vegetation cover, and other density regulating forces (for example, top–down forces) not studied here. This study highlights the importance of including processes in the surrounding marine ecosystem to understand the impacts of seabirds on the food web structures of their nesting islands.     

Resource quality and stoichiometric constraints on stream ecosystem functioning

1. Resource quality and stoichiometric imbalances in carbon : nutrient ratios between consumers and resources can influence key ecosystem processes. In many streams, this has important implications for food webs that are based largely upon the utilization of terrestrial leaf‐litter, which varies widely among litter types in its value as a food source for detritivores and as a substrate for microbial decomposers. 2. We measured breakdown rates and macroinvertebrate colonization of leaf‐litter from a range of native and exotic plants of differing resource quality and palatability to consumers [e.g. carbon : nitrogen : phosphorus (C : N : P) ratios, lignin and cellulose content], in a field experiment. We also measured C : N : P ratios of the principal leaf‐shredding invertebrates, which revealed strong stoichiometric imbalances across trophic levels: C : N and C : P ratios typically differed by at least one order of magnitude between consumers and resources, whereas N : P imbalances were less marked. Application of the threshold elemental ratio approach, which integrates animal bioenergetics and body elemental composition in examining nutrient deficiency between consumers and resources, revealed less marked C : P imbalances than those based on the simpler arithmetic differences described above. 3. Litter breakdown rates declined as nutrient imbalances widened and resource quality fell, but they were independent of whether resources were exotic or native. The principal drivers of total, microbial and invertebrate‐mediated breakdown rates were lignin : N, lignin : P and fungal biomass, respectively. However, multiple regression using orthogonal predictors yielded even more efficient models of litter breakdown, as consumers responded to more than one aspect of resource quality. For example, fungal biomass and litter C : N both influenced invertebrate‐mediated breakdown. 4. Large stoichiometric imbalances and changes in resource quality are likely to have serious consequences for stream ecosystem functioning, especially when riparian zones have been invaded by exotic plant species whose chemical composition differs markedly from that of the native flora. Consequently, the magnitude and direction of change in breakdown rates and, thus, resource depletion, will be driven to a large extent by the biochemical traits (rather than taxonomic identity per se) of the resident and invading flora.     

The evolution of ecosystem processes: growth rate and elemental stoichiometry of a key herbivore in temperate and arctic habitats

Understanding the reciprocal interactions between the evolved characteristics of species and the environment in which each species is embedded is a major priority for evolutionary ecology. Here we use the perspective of ecological stoichiometry to test the hypothesis that natural selection on body growth rate affects consumer body stoichiometry. As body elemental composition (nitrogen, phosphorus) of consumers influences nutrient cycling and trophic dynamics in food webs, such differences should also affect biogeochemical processes and trophic dynamics. Consistent with the growth rate hypothesis, body growth rate and phosphorus content of individuals of the Daphnia pulex species complex were lower in Wisconsin compared to Alaska, where the brevity of the growing season places a premium on growth rate. Consistent with stoichiometric theory, we also show that, relative to animals sampled in Wisconsin, animals sampled in Alaska were poor recyclers of P and suffered greater declines in growth when fed low‐quality, P‐deficient food. These results highlight the importance of evolutionary context in establishing the reciprocal relationships between single species and ecosystem processes such as trophic dynamics and consumer‐driven nutrient recycling.     

Exploring patterns and mechanisms of interspecific and intraspecific variation in body elemental composition of desert consumers

Key processes such as trophic interactions and nutrient cycling are often influenced by the element content of organisms. Previous analyses have led to some preliminary understanding of the relative importance of evolutionary and ecological factors determining animal stoichiometry. However, to date, the patterns and underlying mechanisms of consumer stoichiometry at interspecific and intraspecific levels within natural ecosystems remain poorly investigated. Here, we examine the association between phylogeny, trophic level, body size, and ontogeny and the elemental composition of 22 arthropod as well as two lizard species from the coastal zone of the Atacama Desert in Chile. We found that, in general, whole‐body P content was more variable than body N content both among and within species. Body P content showed a significant phylogenetic signal; however, phylogeny explained only 4% of the variation in body P content across arthropod species. We also found a significant association between trophic level and the element content of arthropods, with carnivores having 15% greater N and 70% greater P contents than herbivores. Elemental scaling relationships across species were only significant for body P content, and even the P content scaling relationship was not significant after controlling for phylogeny. P content did decrease significantly with body size within most arthropod species, which may reflect the size dependence of RNA content in invertebrates. In contrast, larger lizards had higher P contents and lower N:P ratios than smaller lizards, which may be explained by size‐associated differences in bone and scale investments. Our results suggests that structural differences in material allocation, trophic level and phylogeny can all contribute to variation in the stoichiometry of desert consumers, and they indicate that the elemental composition of animals can be useful information for identifying broad‐scale linkages between nutrient cycling and trophic interactions in terrestrial food webs.     

Taxonomic and regional patterns in benthic macroinvertebrate elemental composition in streams

1. Ecological stoichiometry has been used to better understand dynamics in consumer growth and the role of consumer‐recycled nutrients because it focuses on more than one element. Most research has focused on pelagic rather than benthic consumers. Variation in elemental composition among benthic consumer taxa would suggest that taxa differ in their susceptibility to nutrient limitation or in their role in recycling nutrients. 2. We collected benthic macroinvertebrates from streams in two regions (Indiana–Michigan and Wisconsin, U.S.A.) to examine taxonomic and regional variation in benthic macroinvertebrate body carbon (C), nitrogen (N), and phosphorus (P) concentrations and ratios. 3. Elemental composition varied little within taxa common to both regions. In contrast, elemental composition differed greatly among taxa and appeared to be related to phylogeny. The elemental composition of macroinvertebrates clustered into three distinct groups: insects, mollusks, and crustaceans. To a lesser extent, insects and mollusks also differed in elemental composition among genera. 4. Functional feeding groups (FFGs) differed in elemental composition, with predators having a higher N content than other groups. Substantial elemental imbalances between C and N were found between most primary consumers and their likely food sources, and the magnitude of the imbalance depended in part on the FFG. 5. Our results support an assumption of most ecological stoichiometry models that, within a species, the elemental composition of aquatic invertebrates is relatively constant. Variation in elemental composition among taxa at various higher taxonomic levels suggests that susceptibility of stream invertebrates to nutrient limitation and their role in nutrient cycling will strongly depend on phylogeny.     

Biotic invasions can alter nutritional composition of zooplankton communities

Ecologists and ecosystem managers often base their understanding of trophic dynamics on consumer and resource biomass. However, the factors that alter the relative nutritional value of resources are often poorly understood, despite their potential to decouple trophic interactions. Recent population declines in pelagic fishes of the upper San Francisco Estuary were not accompanied by an equivalent decrease in zooplankton biomass, which are the main resource for the fish and their larvae. It was hypothesized that changes in zooplankton nutritional conditions following the establishment of invasive species caused food‐quality related limitations for these higher‐order consumers. Using stable isotopes, elemental stoichiometry and fatty acid analyses for all dominant invasive and native zooplankton taxa and seston, we characterized the plankton community structure in the estuary and demonstrated taxon‐specific differences in their nutritional value. We then quantified the temporal dynamics in meso‐zooplankton proportions of docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), polyunsaturated fatty acids (PUFA), and ratio of n3:n6 fatty acids. We found temporal increase in the community‐level DHA, n3 to n6 fatty acid ratio, decrease in the community‐level EPA and PUFA in the brackish water region, but no change in the bulk PUFA proportions in the freshwater region of the estuary. These changes were caused mainly by declines of native cladocerans that are rich in EPA and by an increase in the dominance of invasive taxa with high DHA concentrations, similar to that of native taxa. Although we showed temporal shifts in individual fatty acid classes, the proportion of the essential fatty acids remained relatively high, suggesting that nutritional prey availability for fish remained unchanged with the shift in species composition. We argue that the nutritional content of resource communities should be considered when analyzing the long‐term trophic dynamics and designing effective management and restoration strategies.     

浮游动物化学计量学稳态性特征研究进展

稳态性是有机体的基本属性,也是生态化学计量学理论成立的前提和基础。一般来讲,浮游植物的元素组成变化较大,而浮游动物具有明显的稳态性特征。浮游动物稳态性特征的研究不仅有助于了解水生生态系统的能量流动和物质循环,同时也对研究营养元素如何调节生物生长、繁殖和代谢起到促进作用。在综述生态化学计量学研究的基础上,主要介绍了稳态性的概念和浮游动物稳态性特征的基本框架及变化规律,以期为促进国内相关研究工作的开展提供参考。     

Threshold elemental ratios of carbon and phosphorus in aquatic consumers

Inadequate supply of one or more mineral elements can slow the growth of animal consumers and alter their physiology, life history and behaviour. A key concept for understanding nutrient deficiency in animals is the threshold elemental ratio (TER), at which growth limitation switches from one element to another. We used a stoichiometric model that coupled animal bioenergetics and body elemental composition to estimate TER of carbon and phosphorus (TER_C:P) for 41 aquatic consumer taxa. We found a wide range in TER_C:P (77–3086, ratio by atoms), which was generated by interspecific differences in body C : P ratios and gross growth efficiencies of C. TER_C:P also varied among aquatic invertebrates having different feeding strategies, such that detritivores had significantly higher threshold ratios than grazers and predators. The higher TER_C:P in detritivores resulted not only from lower gross growth efficiencies of carbon but also reflected lower body P content in these consumers. Supporting previous stoichiometric theory, we found TER_C:P to be negatively correlated with the maximum growth rate of invertebrate consumers. By coupling bioenergetics and stoichiometry, this analysis revealed strong linkages among the physiology, ecology and evolution of nutritional demands for animal growth.     

Nutrient transfer from sea to land: the case of gulls and cormorants in the Gulf of Maine

1. The structure of communities is influenced by the transport of resources across ecosystem boundaries. Seabirds are capable of introducing large amounts of marine-derived nutrients to land, thereby modifying resource availability to terrestrial species. 2. In this study we investigated the hypothesis that variation in nesting densities of great black-backed gulls Larus marinus and double-crested cormorants Phalacrocorax auritus would modify the effect of these species on soil nutrients and plant species composition on offshore islands in the Gulf of Maine, USA. 3. Our results showed a significant positive correlation between nest density and concentrations of ammonia and nitrate in soils, but no significant relationship between nest density and phosphate. Ammonia and phosphate concentrations were good predictors of plant species composition; there were more annual forbs than perennial grasses in the abandoned cormorant colony compared with the gull colonies. Extremely high concentrations of ammonia in the highest density colony (active cormorant) may have been the main factor inhibiting plant germination at this site. All of the plant species in gull and cormorant colonies showed enriched delta(15)N signatures, indicating substantial input of marine-derived nitrogen from seabirds. 4. Our study demonstrated that gulls and cormorants are effective vectors for the transport of marine nutrients to terrestrial ecosystems. However, transported nutrients occurred in particularly high concentrations in areas with nesting cormorants. Nesting densities and species-specific variation in resource transport should be considered when predicting the effects of seabirds and other biogenic vectors of allochthonous resources.     

Widespread natural intraspecific variation in tissue stoichiometry of two freshwater molluscs: Effect of nutrient enrichment

A central premise of ecological stoichiometry is that consumers maintain relatively fixed elemental composition in their bodies, a process known as elemental homoeostasis. Although nutrient enrichment is a ubiquitous problem facing many freshwater lakes around the world, intraspecific variation in elemental composition of freshwater invertebrates and its relationship with nutrient loading have not been well addressed. Here, we examined carbon:nitrogen:phosphorus (C:N:P) stoichiometry of two widely distributed molluscs, Corbicula fluminea and Bellamya aeruginosa, from several subtropical shallow lakes across a nutrient gradient. Our results showed that these two species exhibited substantial natural intraspecific variation in tissue stoichiometry which can reach or even exceed the values among different freshwater taxa investigated before. Our results suggest that tissue P content presents the greatest variations, followed by N content, and lowest in C content. Tissue P content ranged about three-fold (0.56%–1.65%) and five-fold (0.41%–2.28%) for B. aeruginosa and C. fluminea, respectively. Correspondingly, N content ranged from 5.16% to 12.06% and from 6.47 to 11.36%, respectively. Tissue %P, C:P and N:P ratios were strongly correlated with PO₄³⁻-P and/or chlorophyll-a in the water column. Tissue N and P contents of B. aeruginosa and P content of C. fluminea increased with increasing lake trophic levels (mesotrophic to eutrophic to hypertrophic). These results suggest that the two molluscs can adjust their tissue stoichiometry in relation to nutrient enrichment. Relaxing the assumption of strict homeostasis may help them cope with potential stoichiometric constraints. The results provide additional clues to why these two species are successful invaders and widely distributed.     

Allometric and trait‐based patterns in parasite stoichiometry

We measured the elemental content (%C, N and P) and ratios (C:N, C:P, N:P) of a diverse assemblage of parasitic helminths to ask whether taxonomy or traits were related to stoichiometric variation among species. We sampled 27 macroparasite taxa, spanning four phyla, infecting vertebrate and invertebrate hosts from freshwater ecosystems in New Jersey. Macroparasites varied widely in elemental content, exhibiting 4.7‐fold variation in %N, 4.6‐fold variation in %P, and 11.5‐fold variation in N:P. Across all species, parasite %P scaled negatively and C:P scaled positively with body size. Similar relationships between parasite P content and body size occurred at the phylum level and within individual species. The allometric scaling of P across species supports the growth rate hypothesis, which predicts that smaller taxa require more P to support relatively higher growth rates. Life cycle stage was related to %N and C:N, with non‐reproductive parasite stages lower in %N and higher in C:N than actively reproducing parasites. Parasite phylum, functional feeding group, and trophic level did not explain elemental variation among species. Organismal stoichiometry is linked to ecological function, and wide variation in macroparasite stoichiometry likely generates diverse patterns in host–parasite nutrient dynamics and variable relationships between parasitism and nutrient cycling.     

Are nesting seabirds important determinants of invertebrate community composition on subantarctic Adams Island?

Nesting seabirds import marine-derived nutrients into terrestrial food webs, affecting invertebrate abundance and community composition directly, through provision of decaying animal matter as a food source, and indirectly through effects on vegetation and prey abundance. Invertebrates have shown strong responses to seabird presence in some, but not all, ecosystems previously studied. In contrast to mainland range contractions, New Zealand’s subantarctic islands retain abundant seabird populations. We sampled ground invertebrates on mammal-free Adams Island, using pitfall traps. We surveyed sites in two vegetation types (tussock and forest) with either no nesting seabirds or nesting colonies of Gibson’s wandering albatross, sooty shearwaters or white-headed petrels. We collected 11 invertebrate orders and identified 20 Coleoptera species or higher taxa. The carrion beetle, Paracatops antipoda comprised over 50 % of Coleoptera individuals collected. P. antipoda was more abundant in forest than tussock and was positively associated with sooty shearwaters and negatively associated with white-headed petrels when compared with bird-free sites using a Poisson generalized linear model. Sooty shearwaters were also associated with elevated abundance of several herbivorous and invertebrate decomposer taxa. Nesting seabirds do appear to influence invertebrate community composition on Adams Island, but the direction of this effect appears to be taxa-specific. Further sampling with spatial replication of colonies is required to determine the extent to which these apparent taxa-specific responses are consistent across colonies and habitats.     

Extending the growth rate hypothesis to species development: Can stoichiometric traits help to explain the composition of macroinvertebrate communities?

Ecological stoichiometry seeks to understand the ecological consequences of elemental imbalances between consumers and their resources. Therein, the well-accepted growth rate hypothesis (GRH) states that organisms exhibiting rapid growth have higher phosphorus (P) demand – and thus lower C:P and N:P ratios – than slow growing ones, due to a higher allocation to P-rich rRNA. However, GRH has rarely been extended to other biological traits than growth, especially at the community level. In this study, we investigated whether macroinvertebrate stoichiometric traits (e.g. C:P and N:P ratios) can be linked to their development traits, and whether these stoichiometric traits are related to macroinvertebrate community assemblage under different nutrient conditions. We allocated more than 400 European taxa to different groups, defined using available information about three development-related traits: ‘life span', ‘voltinism' and ‘number of reproductive cycles per individual'. We sampled 18 invertebrate taxa in six streams exhibiting different levels of nutrient concentration and measured their stoichiometric traits. Further, we quantified invertebrate taxon abundances in these streams during an annual survey. Based on these data, we tested whether community composition regarding the developmental groups differs, depending on nutrient concentration. We found significant differences in the proportions of the developmental groups along a gradient of water N:P, in relation to their stoichiometric traits. Taxa with low C:P and N:P ratios were generally associated with faster development groups, and these taxa tended to occur at higher proportions in streams exhibiting low dissolved N:P ratios. In contrast, communities from P-poor, high dissolved N:P streams, were dominated by slowly developing taxa with high N:P ratios. Our results highlight that extending the GRH to species development rate might give some insights about the mechanisms by which nutrient concentrations in ecosystems influence consumers' community composition.     

When microbes and consumers determine the limiting nutrient of autotrophs: a theoretical analysis

Ecological stoichiometry postulates that differential nutrient recycling of elements such as nitrogen and phosphorus by consumers can shift the element that limits plant growth. However, this hypothesis has so far considered the effect of consumers, mostly herbivores, out of their food-web context. Microbial decomposers are important components of food webs, and might prove as important as consumers in changing the availability of elements for plants. In this theoretical study, we investigate how decomposers determine the nutrient that limits plants, both by feeding on nutrients and organic carbon released by plants and consumers, and by being fed upon by omnivorous consumers. We show that decomposers can greatly alter the relative availability of nutrients for plants. The type of limiting nutrient promoted by decomposers depends on their own elemental composition and, when applicable, on their ingestion by consumers. Our results highlight the limitations of previous stoichiometric theories of plant nutrient limitation control, which often ignored trophic levels other than plants and herbivores. They also suggest that detrital chains play an important role in determining plant nutrient limitation in many ecosystems.     

Effects of management on double-crested cormorant nesting colony fidelity

The increase of double-crested cormorant (Phalacrocorax auritus; hereafter, cormorant) populations during the last 2 decades has impacted many stakeholder groups. The negative effects of nesting cormorants on trees and other vegetation have motivated private organizations and government agencies to manage nesting colonies and reduce their impacts to private property and public resources. Management-induced reproductive failure has been shown to influence cormorant inter-annual nesting colony fidelity, but not complete abandonment from a nesting colony site. We attached very high frequency (VHF) transmitters and Global Positioning System (GPS) transmitters to nesting cormorants to monitor their movement response on a managed site (Young Island, VT [YI]) and an unmanaged site (Four Brothers Islands, NY [FB]). Additionally, we monitored these sites to determine the influence of management activities on subsequent-year colonization. On YI, management consisted of egg-oiling all cormorant nests (some nests had been oiled in previous years) and culling approximately 20% of adults. Annual dispersal rates did not differ between managed and unmanaged sites, but a nesting period interaction occurred with greater dispersal on the managed site following the incubation period. After 4 years of both egg oiling and culling, cormorant nesting on YI declined to zero. Simultaneously, cormorant numbers increased on the nearby unmanaged FB. We propose either the cumulative effect of partial or complete reproductive failure (8 yr) or simply the inclusion of adult culling (4 yr) caused the abandonment. From a colony-specific management perspective, the rapid decline was beneficial to the goal of restoring the vegetative community on YI. The effects of adult culling at nesting colonies, prior-year reproductive failure caused by egg oiling, or the combination of these factors may be required for complete and rapid nesting site abandonment. The use of culling adult breeders reduced nesting and likely limits the cost and logistics of control and allows more rapid initiation of mitigation measures and island habitat restoration. © 2011 The Wildlife Society.