Trophic flows across ecosystem boundaries : an examination of the strength and consequences of linkages between stream and forest food webs

Extreme climatic events can trigger sudden but often long-lasting and irreversible changes in ecosystems by causing mortality of foundation (habitat-forming) species. The magnitude and frequency of such events are likely to increase due to human-induced climate change, but the dynamics of such extreme biological events remain poorly understood, with only a limited number of case studies reported in the past. In many cases, assessing the impact of rare, extreme biological events can be challenging because these events can be unexpected and sudden, often making pre-event sampling not achievable. In late 2015 to early 2016, an extensive area of mangrove forest along ~ 1,000 km of coastline in the Gulf of Carpentaria, Australia, experienced severe dieback as a result of a climatic extreme event that included extreme temperatures, drought conditions and lower than average sea levels. My research aimed to address a knowledge gap in the effects of extreme climatic events on intertidal coastal ecosystems by assessing the ecological impacts of mass mortality of mangrove trees on the intertidal ecosystem. This aim was achieved through the use of a comparative experiment of an impacted forest and an adjacent unimpacted forest using traditional ecological survey techniques combined with conventional bulk stable isotope analyses and a more novel compound-specific isotope analysis of amino acids. My research also offers significant insights into the use of the more novel compound-specific isotope analysis of amino acids to complement the conventional bulk stable isotope analysis in mangrove ecosystem trophic analyses. Firstly, I used a combination of traditional ecological survey techniques and bulk stable isotope analysis of carbon (C) and nitrogen (N) to measure the effects of mangrove forest mortality on benthic faunal communities, with a focus on functional aspects of food web dynamics. I tested if changes in benthic faunal assemblages would be evident due to mangrove mortality, and if food web structure was impacted by the mangrove mortality and alternations in available food resources. I found that in the forest that experienced tree mortality, there were fewer crabs that relied on mangrove litter as a food source but more crabs that fed on the microphytobenthos. As the microphytobenthos was largely unaffected by the die-back event, they provided a buffer to the food-web responses. The infauna, e.g. burrowing crabs, was also largely unaffected by the mortality effect. However, overall, the habitat value for mangrove ecosystem services could be decreased due to lower physical habitat complexity following tree losses. Secondly, the initial dieback and recovery of the impacted mangrove ecosystem were evaluated using a combination of bulk stable C, N and S isotopes and a more novel amino acid compound-specific isotope method. I tested if tree mortality changed the overall circulation of C, N and S elements, and if this change would be reflected in δ13C, δ15N and δ34S values of mangrove ecosystem components such as mangrove plants, soil and associated animals. I also tested if/how these isotopic compositions change over time with the recovery of mangrove vegetation. Stable isotope analyses confirmed significant changes to the circulation of C, N and S elements following tree mortality. Recovery of the mangrove vegetation was evident from increased numbers of mangrove seedlings and saplings in the impacted forest over the two-year survey, but recovery of CNS cycling was not evident even after 30 months, suggesting a long-lasting effect of the mortality event. Finally, the use of bulk stable CNS isotopes and more novel compound-specific stable C and N isotopes of amino acids were compared to evaluate which isotopic compositions are more conservative tracers of mangrove organic matter and suited for analyses of mangrove food webs. I tested if stable isotopic compositions in essential amino acids that cannot be synthesised by animals would be more conservative in food web links between consumers and mangrove organic matter. Isotopic compositions in essential amino acids effectively separated mangrove organic matter from the microphytobenthos and helped to trace mangrove organic matter in a mangrove food web. These more sophisticated tracing techniques complemented traditional bulk stable isotope analyses by providing improved resolving power in mangrove trophic analyses. The outcome of this research will be an important contribution to the emerging global body of case studies that show significant ecological impacts driven by extreme climatic events, and how changes in habitat forming species result in significant impacts on ecosystem community dynamics as well as biogeochemical processes including C, N and S cycling. My research also provides a framework for combining the use of conventional and novel stable isotope measurements with traditional ecological survey techniques in reporting difficult to measure impacts of extreme biological events. The results of this research may also be used for planning and future-proofing coastal wetlands from future impacts of climatic extreme events, and support wetland conservation and restoration efforts.

Nearly all ecosystems are contaminated with highly toxic methylmercury (MeHg), but the specific sources and pathways leading to the uptake of MeHg within and among food webs are not well understood. In this study, we report stable mercury (Hg) isotope compositions in food webs in a river and an adjacent forest in northern California and demonstrate the utility of Hg isotopes for studying MeHg sources and cross-habitat transfers. We observed large differences in both δ(202)Hg (mass-dependent fractionation) and Δ(199)Hg (mass-independent fractionation) within both food webs. The majority of isotopic variation within each food web could be accounted for by differing proportions of inorganic Hg [Hg(II)] and MeHg along food chains. We estimated mean isotope values of Hg(II) and MeHg in each habitat and found a large difference in δ(202)Hg between Hg(II) and MeHg (∼2.7‰) in the forest but not in the river (∼0.25‰). This is consistent with in situ Hg(II) methylation in the study river but suggests Hg(II) methylation may not be important in the forest. In fact, the similarity in δ(202)Hg between MeHg in forest food webs and Hg(II) in precipitation suggests that MeHg in forest food webs may be derived from atmospheric sources (e.g., rainfall, fog). Utilizing contrasting δ(202)Hg values between MeHg in river food webs (-1.0‰) and MeHg in forest food webs (+0.7‰), we estimate with a two-source mixing model that ∼55% of MeHg in two riparian spiders is derived from riverine sources while ∼45% of MeHg originates from terrestrial sources. Thus, stable Hg isotopes can provide new information on subtle differences in sources of MeHg and trace MeHg transfers within and among food webs in natural ecosystems.

Land-use responses of dung beetle communities and their ecosystem services

Shigeru Nakano was a Japanese ecologist whose work crossed boundaries among subdisciplines in ecology, between aquatic and terrestrial habitats, and between different languages and cultures. He published his first paper in 1985 while still an undergraduate, and is well known for his early research on the individual behavior of stream salmonids in dominance hierarchies. Shortly after completing his Master’s degree in 1987 he began collaborating with many graduate students and other scientists, including those from the US, and expanded his research to include factors controlling stream salmonid distribution and abundance across spatial scales ranging from local to landscape levels. In 1995 he moved to a research station in southwestern Hokkaido and began new collaborative research on interactions between forest and stream food webs. Nakano pioneered large-scale field experiments using greenhouses to sever the reciprocal fluxes of invertebrate prey between stream and riparian food webs. The strong direct and indirect effects of isolating these food webs from each other on organisms ranging from stream algae to fish, riparian spiders, and bats have revealed new linkages and explained phenomena that were previously unexplained. When combined with similar results from other investigators, they have created a paradigm shift in ecology. Shigeru Nakano was lost at sea in Baja California on March 27, 2000 at the age of 37, but key papers from his 15-year career set new standards for rigor, detail, and synthesis. They continue to be highly cited and inspire new research, and to foster new collaborations among Japanese and western scientists.

INTRODUCTION Historically, rural villagers in northern Thailand had a stable coexistence with forest ecosystems due to the intrinsic value attached to forests in Thai culture as well as to customary common property management systems.(1) In the contemporary scene the state management of forests has gradually undermined these customary laws and the authority of traditional institutions.(2) This has created considerable tension between the traditional villagers' view of forests as common property resources, and official government policy. Rural communities dependent on forest resources are fighting for their rights to benefit from forest land they have long considered as their own. The case of Huey Kaew represents a bottom-up push for reestablishing common property rights. It is also a case where the Royal Forestry Department is attempting to integrate common property rights to forest lands into the framework of national policy. The purpose of this case study in Huey Kaew is to explore the dynamics involved in reestablishing common property rights. SIGNIFICANCE OF STUDY The contemporary situation in Thailand suggests that environmental degradation and natural resource allocation will continue to be major sources of conflict between the government and rural communities.(3) The community forest program in Huey Kaew therefore has the potential to provide a model for other forest communities that face such conflict.(4) The successful implementation of the Huey Kaew community forest is a goal of the Royal Forestry Department and the Ministry of Agriculture and Agricultural Cooperatives.(5) If it is successful, this program will provide benefits for both the villagers and the nation. The villagers will secure their ability to satisfy their own needs and the right to protect and manage forest resources, while the government will find a viable alternative to state control for managing Forest Reserve Land. DESCRIPTION OF HUEY KAEW The village of Huey Kaew is located in an upper valley floor in the northeast corner of Kaew sub-district in Chiang Mai Province. The upper part of the village is located within a Forest Reserve which contains a watershed covering 3000 rai (480 hectares) that supplies water to 10,000 rai (1,600 ha) of paddy land. The whole village is surrounded by mountain ranges and forests which are primarily secondary growth as a result of logging concessions in the early part of the century. The forest has been classified by the Royal Forestry Department as dry deciduous dipterocarp at the lower elevations and mixed deciduous at the higher elevations. Mixed Deciduous forests have great economic value because many commercial species are abundant. It is the primary habitat of teak (Tectona grandis).(7) The Dry Deciduous Dipterocarp forest has few trees of commercial importance but because trees in these forests are able to coppice freely they assure a continual source of fuelwood and various species of bamboo.(8) The watershed is the source of four streams, one of which flows through the village of Huey Kaew. This stream supplies sufficient water for cultivation and home consumption throughout the year. Two small traditional irrigation canals draw water to feed rice fields on both sides of the river. Huey Kaew is an oval shaped village surrounded by rice paddy and forest. Most houses are built along the main road. The houses of the wealthier families are generally made of teak with tile roofs, while the houses of the poor are generally made from bamboo materials with roofs made from leaves. The population of Huey Kaew is 513 people,(9) living in 126 households. Ninety-two of the households are landless.(10) The villagers of Huey Kaew understand the importance of conserving the forest in the watershed in order to maintain the source of water which irrigates their rice fields. Villagers depend on the forest as a source of building materials for their homes, fuelwood, grazing land for domestic animals such as water buffalo and cattle, natural foods, herbs and medicines, and as a hunting ground for small birds, animals and insects. …

Freshwater and terrestrial biodiversity are linked through spatial resource flows. Key examples are detrital subsidies from the riparian vegetation that form the base of food webs in small streams. Despite the central role of detritivores in these food webs, the consequences of altered riparian vegetation type and resource availability on their trophic strategies are less known. Therefore, we experimentally tested direct and indirect effects of riparian vegetation type (i.e. riparian forests present vs. absent) on trophic interactions and dietary imbalances of aquatic detritivores. To characterise trophic strategies, we used stoichiometric and isotopic differences between consumers and resources as functional measures of trophic link strength in leaf litter bags naturally colonised by stream decomposers and detritivores. Our results show that an absence of riparian forests affected stoichiometric and isotopic properties in stream detrital food webs. Where local riparian forests were absent, leaf litter showed lower C:N ratios, which only trichopteran detritivores mirrored, whereas plecopteran and crustacean detritivores showed differences in their isotopic signatures. Diverging patterns between resources and consumers could lead to a rewiring of energy flow paths and weaken the coupling between aquatic detritivores and terrestrial detritus. Consequently, our findings demonstrate that riparian forests are essential for aquatic food webs by not only influencing organisms themselves but also their trophic interactions and energy flow paths. Aquatic detritivores depending on terrestrial subsidies can be highly sensitive to local changes in their resource environment. Therefore, functionally divergent patterns between resources and consumers in recipient systems highlight how habitat properties of the donor system can affect food webs connected across ecosystem boundaries. Read the free Plain Language Summary for this article on the Journal blog.

The ecology of aquatic invertebrates with specific reference to tropical peat swamps are poorly known. This thesis is a first attempt which examines the temporal variation in aquatic invertebrate community composition and changes in the invertebrate composition under different land uses in North Selangor Peat Swamp Forest (NSPSF). The thesis also examines the differences between food webs of natural intact peat swamp forest site versus human-modified peat swamp habitats. This study is a first attempt to describe the energy sources of peat swamp forest food web dynamics as well as the impact of disturbances on food webs in human -modified peat swamp sites.

The gypsy moth is considered one of the most harmful invasive forest insects in North America. It has been suggested that gypsy moth may indirectly impact native caterpillar communities via shared parasitoids. However, the impact of gypsy moth on forest insect food webs in general remains unstudied. Here we assess such potential impacts by surveying forest insect food webs in Ontario, Canada. We systematically collected caterpillars using burlap bands at sites with and without histories of gypsy moth outbreak, and then reared these caterpillars until potential parasitoid emergence. This procedure allowed us to generate quantitative food webs describing caterpillar-parasitoid interactions. We estimated the degree of parasitoid sharing between gypsy moth and native caterpillars. We also statistically modeled the effect of gypsy moth outbreak history and current gypsy moth abundance on standard indices of quantitative food web structure and the diversity of parasitoid communities. Rates of gypsy moth parasitism were very low and gypsy moth shared very few parasitoids with native caterpillars, suggesting limited potential for indirect interactions. We did not detect any significant effects of gypsy moth on either food web structure or parasitoid diversity, and the small amount of parasitoid sharing strongly implies that this lack of significance is not merely due to low statistical power. Our study suggests that gypsy moth has limited impact on native host-parasitoid food webs, at least for species that use burlap bands. Our results emphasize that extrapolations of theoretical and experimental conclusions on the impacts of invasive species should be tested in natural settings.

Faculty Opinions recommendation of Fish invasion restructures stream and forest food webs by interrupting reciprocal prey subsidies.

Faculty Opinions recommendation of Fish invasion restructures stream and forest food webs by interrupting reciprocal prey subsidies.

Biodiversity is the term given to the variety of life on Earth and the natural patterns it forms. The biodiversity we see today is the fruit of billions of years of evolution, shaped by natural processes and, increasingly, by the influence of humans. It forms the web of life of which we are an integral part and upon which we so fully depend. Biological resources are the pillars upon which we build civilizations. Nature's products support such diverse industries as agriculture, cosmetics, pharmaceuticals, pulp and paper, horticulture, construction and waste treatment. The loss of biodiversity threatens our food supplies, opportunities for recreation and tourism, and sources of wood, medicines and energy. It also interferes with essential ecological functions. While the loss of individual species catches our attention, it is the fragmentation, degradation, and outright loss of forests, wetlands, coral reefs, and other ecosystems that poses the gravest threat to biological diversity. While loss of species has always occurred as a natural phenomenon, the pace of extinction has accelerated dramatically as a result of human activity. Ecosystems are being fragmented or eliminated, and innumerable species are in decline or already extinct. In this context this study has tried to bring out an assessment of the biodiversity in the Ratapani Forests block of Dungarpur range. Pure stand of Tectona Grandis can be seen in Dungarpur district where it dominates the vegetation but in varied degree of degradation due to biotic influence. Associated trees seen in the area are Diospyros melanoxylon, Aegle marmelos, Anogeissus latifolia(which is the most common), Bauhinia racemosa, Soymida febrifuga, Mitragyna parvifolia and Terminalia tomentosa. Undergrowth plant varieties cover Nyctanthes arbor-tristis, Carissa opaca etc. The present study found that the increasing pressure of both human and livestock population is taking a heavy toll on the biodiversity of the area particularly in terms of rapid falling of trees and excessive grazing of livestock. On the flat plateau and ridges of the hills most of the fertile soil has been washed away due to serious erosion and these areas are not capable for good teak growth. It is therefore suggested that as the soil of hilly and plateau tracks is fragile and has a thin horizon so these areas must be monitored very closely so that the soil erosion due to removal of vegetation cover can be checked by planting of new saplings which can bind the soil in short term and then these areas too can be made viable to support the teak vegetation as they were supporting prior to the deterioration conditions were set in. The study also suggests various ways and means to arrest the degradation of biodiversity in the area and to regenerate the forest cover on the patches which are rendered barren due to manmade practices.

Stream ecosystems form an active component of the carbon (C) cycle, and are identified as “hotspots” for carbon dioxide (CO2) emissions. However, the mechanisms driving CO2 emissions from streams are not completely understood. Beside the input of C in the form of CO2 from groundwater, streams receive organic matter from aquatic and terrestrial origins which is partly mineralized to inorganic nutrients and CO2. Future predictions suggest enhanced input of terrestrial organic matter into streams. As such, surrounding land use may highly influence dissolved organic matter (DOM) composition and turnover in streams. The quality, i.e. bioavailability or lability, of aquatic and terrestrial organic matter, as well as which quality feature provides which bioavailability, is controversially discussed and the research is still in its infancy. Thus, the main goal of my thesis is to enhance the understanding of the role of organic matter quality as a potential driver for organic matter turnover in stream ecosystems. A further goal is to shed light on C dynamics with main focus on CO2 of streams surrounded by different land use. The presented work is based on an experimental approach in the laboratory, supported by seasonal field studies and a developed model in order to explore C dynamics and the corresponding drivers in stream ecosystems. The underlying mechanisms and the importance of DOM quality as a main driver was assessed on the small scale in laboratory experiments. The C emissions from streams were quantified and the influence of DOM quality was examined on a stream reach scale by investigating two stream types with different organic matter quality inputs. By developing a process-based model, the understanding of the daily and seasonal scale of C turnover in stream ecosystems was amplified. The results from the experiment under controlled conditions demonstrate that DOM quality governs microbial metabolism (i.e. respiration and bacterial protein production). Moreover, I revealed significant quality differences between two terrestrial DOM sources, while respiration and bacterial protein production increased with the available proportion of the labile DOM source. The molecular weight of DOM was the strongest predictor of bacterial protein production and respiration, while among others, the concentration of low molecular weight substances was another highly influential predictor. The importance of molecular size/weight and DOM quality for microbial metabolism was further confirmed on the stream reach scale where we demonstrated among others a significant linkage between molecular size of DOM and pCO2 across agricultural and forest streams. Moreover, agricultural streams contained significantly higher pCO2 compared to forest streams during all seasons. However, CO2 emissions measured with the powerful drifting chamber method were not significantly different between the stream types. Modeled dissolved oxygen (O2) and CO2 dynamics calibrated with field data resulted in respiratory quotients (RQ = mole of CO2 produced per mole of O2 consumed), which are intimately linked to the elemental composition of the respired compounds across four seasons and two stream types. RQ values were not related to adjacent land use or season. Nevertheless, I found significant relationships between RQ values and DOM quality indicators, such as fluorescing component characteristic for higher plant material and molecule size of DOM in agricultural streams. In conclusion, this thesis demonstrates that DOM quality is an important driver for organic matter turnover in streams. Consequently, my results indicate that ongoing and future land use change and enhanced terrestrial DOM input into streams may influence CO2 emissions, and underline the status of streams as C turnover “hotspots”. Thus, my thesis contributes to the mechanistic understanding of organic matter cycling in stream ecosystems and their role in the regional and global C cycle. Therefore, organic matter quality should be considered in future models and studies with respect to C cycling.

Coral reef mesopredator trophodynamics in response to reef condition

Trophic shifts of generalist consumers can have broad food-web and biodiversity consequences through altered trophic flows and vertical diversity. Previous studies have used trophic shifts as indicators of food-web responses to perturbations, such as species invasion, and spatial or temporal subsidies. Resource pulses, as a form of temporal subsidies, have been found to be quite common among various ecosystems, affecting organisms at multiple trophic levels. Although diet switching of generalist consumers in response to resource pulses is well documented, few studies have examined if the switch involves trophic shifts, and if so, the directions and magnitudes of the shifts. In this study, we used stable carbon and nitrogen isotopes with a Bayesian multi-source mixing model to estimate proportional contributions of three trophic groups (i.e. producer, consumer, and fungus-detritivore) to the diets of the White-footed mouse (Peromyscus leucopus) receiving an artificial seed pulse or a naturally-occurring cicadas pulse. Our results demonstrated that resource pulses can drive trophic shifts in the mice. Specifically, the producer contribution to the mouse diets was increased by 32% with the seed pulse at both sites examined. The consumer contribution to the mouse diets was also increased by 29% with the cicadas pulse in one of the two grids examined. However, the pattern was reversed in the second grid, with a 13% decrease in the consumer contribution with the cicadas pulse. These findings suggest that generalist consumers may play different functional roles in food webs under perturbations of resource pulses. This study provides one of the few highly quantitative descriptions on dietary and trophic shifts of a key consumer in forest food webs, which may help future studies to form specific predictions on changes in trophic interactions following resource pulses.

Sexual differences in parental investment, predation pressure, and foraging efforts are common in nature and affect the trophic flow in food webs. Specifically, the sexual differences in predator and prey behavior change in trophic inflow and outflow, respectively, while those in parental investment alter the reproductive allocation of acquired resources in the population. Consequently, these factors may play an important role in determining the system structure and persistence. However, few studies have examined how sexual differences in trophic flow affect food web dynamics. In this study, I show the ecological role of sex by explicitly incorporating sexual differences in trophic flow into a three‐species food web model. The results demonstrated that the ecological waste of males, that is, the amount of trophic inflow into males with less parental investment, plays an important role in system persistence and structure. In particular, the synergy between sexual differences in parental investment and trophic inflows and outflows is important in determining web persistence: Significant impacts of male‐biased trophic flows require the condition of anisogamy. In addition, the dynamic effects of the ecological waste of males differ with trophic level: The coexistence of a food web occurs more frequently with biased inflows into predator males, but occurs less frequently with biased inflows into consumer males. The model analysis indicates that investigating the pattern of sexual differences among trophic positions can enrich our understanding of food web persistence and structure in the real world.