Effects of partitioning allochthonous and autochthonous resources on food web stability

The flux of energetic and nutrient resources across habitat boundaries can exert major impacts on the dynamics of the recipient food web. Competition for these resources can be a key factor structuring many ecological communities. Competition theory suggests that competing species should exhibit some partitioning to minimize competitive interactions. Species should partition both in situ (autochthonous) resources and (allochthonous) resources that enter the food web from outside sources. Allochthonous resources are important sources of energy and nutrients in many low productivity systems and can significantly influence community structure. The focus of this paper is on: (i) the influence of resource partitioning on food web stability, but concurrently we examine the compound effects of; (ii) the trophic level(s) that has access to allochthonous resources; (iii) the amount of allochthonous resource input; and (iv) the strength of the consumer–resource interactions. We start with a three trophic level food chain model (resource–consumer–predator) and separate the higher two trophic levels into two trophospecies. In the model, allochthonous resources are either one type available to both consumers and predators or two distinct types, one for consumers and one for predators. The feeding preferences of the consumer and predator trophospecies were varied so that they could either be generalists or specialists on allochthonous and/or autochthonous resources. The degree of specialization influenced system persistence by altering the structure and, therefore, the indirect effects of the food web. With regard to the trophic level(s) that has access to allochthonous resources, we found that a single allochthonous resource available to both consumers and predators is more unstable than two allochthonous resources. The results demonstrate that species populating food webs that experience low to moderate allochthonous resources are more persistent. The results also support the notion that strong links destabilize food web dynamics, but that weak to moderate strength links stabilize food web dynamics. These results are consistent with the idea that the particular structure, resource availability, and relative strength of links of food webs (such as degree of specialization) can influence the stability of communities. Given that allochthonous resources are important resources in many ecosystems, we argue that the influence of such resources on species and community persistence needs to be examined more thoroughly to provide a clearer understanding of food web dynamics.     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

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Critical indirect effects of climate change on sub‐Antarctic ecosystem functioning

Sub‐Antarctic islands represent critical breeding habitats for land‐based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub‐Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; “island mass effect”; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long‐term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long‐term decline in enhanced phytoplankton productivity at the islands in response to a climate‐driven shift in the position of the sub‐Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore‐feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of offshore‐feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.     

Patch dynamics in a landscape modified by ecosystem engineers

Ecosystem engineers, organisms that modify the environment, have the potential to dramatically alter ecosystem structure and function at large spatial scales. The degree to which ecosystem engineering produces large-scale effects is, in part, dependent on the dynamics of the patches that engineers create. Here we develop a set of models that links the population dynamics of ecosystem engineers to the dynamics of the patches that they create. We show that the relative abundance of different patch types in an engineered landscape is dependent upon the production of successful colonists from engineered patches and the rate at which critical resources are depleted by engineers and then renewed. We also consider the effects of immigration from either outside the system or from engineers that are present in non-engineered patches, and the effects of engineers that can recolonize patches before they are fully recovered on the steady state distribution of different patch types. We use data collected on the population dynamics of a model engineer, the beaver, to estimate the per-patch production rate of new colonists, the decay rate of engineered patches, and the recovery rate of abandoned patches. We use these estimated parameters as a baseline to determine the effects of varying parameters on the distribution of different patch types. We suggest a number of hypotheses that derive from model predictions and that could serve as tests of the model.     

The functional trophic role of lotic primary consumers: generalist versus specialist strategies

1. The relative extent of generalist or specialist resource use strategies is an important question in ecology. A community dominated by specialist strategies suggests a high level of interspecific competition for resources, resulting in the evolutionary development of isolating mechanisms between species (e.g. resource specialization to avoid and/or outcompete other species). A community dominated by generalist strategies suggests less interspecific competition for resources, allowing many taxa to utilize the same resources. In stream systems, generalist food habits are a common strategy among primary consumers, but little is known about resource assimilation strategies (resources incorporated into tissue growth). Published data indicate that generalist resource assimilation strategies may prevail in lotic systems as well.
2. Functional feeding groups (FFGs) are often used to infer resource assimilation among lotic macroinvertebrates (e.g. shredder-detritivore, scraper-herbivore). While these groupings are aptly used to describe invertebrate feeding modes and community structure, the use of FFGs to describe resource assimilation among lotic consumers is not appropriate. Sufficient data now exist to seriously question how accurately FFG assignments describe the processes of energy flow and material transfer between trophic levels in stream ecosystems.
3. Because FFGs may not accurately describe functional attributes in lotic systems, an alternative approach is needed. One approach is to determine the amount of secondary production that is derived from autochthonous (e.g. periphyton and algae) and allochthonous (e.g. detritus) resources directly. A simple model of community function based on this approach is presented. The model incorporates trophic generalists into measurement of consumer–resource energetics in lotic systems.     

The functional trophic role of lotic primary consumers: generalist versus specialist strategies

1. The relative extent of generalist or specialist resource use strategies is an important question in ecology. A community dominated by specialist strategies suggests a high level of interspecific competition for resources, resulting in the evolutionary development of isolating mechanisms between species (e.g. resource specialization to avoid and/or outcompete other species). A community dominated by generalist strategies suggests less interspecific competition for resources, allowing many taxa to utilize the same resources. In stream systems, generalist food habits are a common strategy among primary consumers, but little is known about resource assimilation strategies (resources incorporated into tissue growth). Published data indicate that generalist resource assimilation strategies may prevail in lotic systems as well.
2. Functional feeding groups (FFGs) are often used to infer resource assimilation among lotic macroinvertebrates (e.g. shredder-detritivore, scraper-herbivore). While these groupings are aptly used to describe invertebrate feeding modes and community structure, the use of FFGs to describe resource assimilation among lotic consumers is not appropriate. Sufficient data now exist to seriously question how accurately FFG assignments describe the processes of energy flow and material transfer between trophic levels in stream ecosystems.
3. Because FFGs may not accurately describe functional attributes in lotic systems, an alternative approach is needed. One approach is to determine the amount of secondary production that is derived from autochthonous (e.g. periphyton and algae) and allochthonous (e.g. detritus) resources directly. A simple model of community function based on this approach is presented. The model incorporates trophic generalists into measurement of consumer–resource energetics in lotic systems.     

Food web stability: the influence of trophic flows across habitats

In nature, fluxes across habitats often bring both nutrient and energetic resources into areas of low productivity from areas of higher productivity. These inputs can alter consumption rates of consumer and predator species in the recipient food webs, thereby influencing food web stability. Starting from a well-studied tritrophic food chain model, we investigated the impact of allochthonous inputs on the stability of a simple food web model. We considered the effects of allochthonous inputs on stability of the model using four sets of biologically plausible parameters that represent different dynamical outcomes. We found that low levels of allochthonous inputs stabilize food web dynamics when species preferentially feed on the autochthonous sources, while either increasing the input level or changing the feeding preference to favor allochthonous inputs, or both, led to a decoupling of the food chain that could result in the loss of one or all species. We argue that allochthonous inputs are important sources of productivity in many food webs and their influence needs to be studied further. This is especially important in the various systems, such as caves, headwater streams, and some small marine islands, in which more energy enters the food web from allochthonous inputs than from autochthonous inputs.     

Combined stable‐isotope and fatty‐acid analyses demonstrate that large wood increases the autochthonous trophic base of a macroinvertebrate assemblage

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Altitudinal patterns of diversity and functional traits of metabolically active microorganisms in stream biofilms

Resources structure ecological communities and potentially link biodiversity to energy flow. It is commonly believed that functional traits (generalists versus specialists) involved in the exploitation of resources depend on resource availability and environmental fluctuations. The longitudinal nature of stream ecosystems provides changing resources to stream biota with yet unknown effects on microbial functional traits and community structure. We investigated the impact of autochthonous (algal extract) and allochthonous (spruce extract) resources, as they change along alpine streams from above to below the treeline, on microbial diversity, community composition and functions of benthic biofilms. Combining bromodeoxyuridine labelling and 454 pyrosequencing, we showed that diversity was lower upstream than downstream of the treeline and that community composition changed along the altitudinal gradient. We also found that, especially for allochthonous resources, specialisation by biofilm bacteria increased along that same gradient. Our results suggest that in streams below the treeline biofilm diversity, specialisation and functioning are associated with increasing niche differentiation as potentially modulated by divers allochthonous and autochthonous constituents contributing to resources. These findings expand our current understanding on biofilm structure and function in alpine streams.     

Allochthonous inputs: integrating population changes and food-web dynamics

Most ecosystems are recipients of allochthonous materials that enhance in situ productivity. Recent theoretical and empirical studies suggest that low to moderate inputs can stabilize food webs. However, depending on the trophic levels that use the resource, food webs can become unstable as inputs increase. Where large amounts of agricultural resources are transferred to natural habitats, trophic dynamics change: trophic cascades can occur and rare or uncommon species can become invasive. Rates of change in species abundances can also be amplified by the effects of changes in legislation and management practices on subsidized consumers.     

Death, detritus, and energy flow in aquatic ecosystems

1. Pelagic trophic structure and energy fluxes are evaluated predominantly on the basis of ingestion of particulate organic matter by living organisms and the effects of consumption on the population dynamics of trophic levels. 2. Population fluxes are not representative of the material and energy fluxes of either the composite pelagic region or the lake ecosystem. Metabolism of particulate and especially dissolved organic detritus from many pelagic and non-pelagic autochthonous and from allochthonous sources dominates both material and energy fluxes. Because of the very large magnitudes and relative chemical recalcitrance of these detrital sources, the large but slow metabolism of detritus provides an inherent ecosystem stability that energetically dampens the ephemeral, volatile fluctuations of higher trophic levels. 3. The annual time period is the only meaningful interval in comparative quantitative analyses of material and energy fluxes at population, community, and ecosystem levels. 4. Non-predatory death and metabolism by prokaryotic and protistian heterotrophs dominate. Continued application of animal-orientated relationships to the integrated, process-driven couplings of the aquatic ecosystems impedes understanding of quantitative ecosystem pathways and control mechanisms.     

Detecting Multiple States of Trophic Connectivity Between Mangroves and Salt Marshes

The productivity gradient between adjacent habitats can fluctuate over time due to seasonal cycles and lead to both habitats being alternately subsidized. Although this process is well known for prey subsidies in stream-riparian forest ecotones, few studies are available for other systems or subsidy types. Moreover, the effects of transport intensity on this expected alternate subsidy exchange are still poorly understood. We assessed whether subsidy input and allochthonous carbon assimilation by resident benthic invertebrates alternated between adjacent mangroves and salt marshes during peaks of detritus productivity (summer and winter, respectively) in a subtropical estuary, by using detritus trapping techniques and stable isotope ratios. Sampling was performed simultaneously in the sheltered (inner sector) and exposed (outer sector) regions of the estuary to assess the influence of different physical conditions on the intensity of subsidy flow. Transport of mangrove litter into the salt marsh occurred mainly in the summer in both sectors; however, most of the litter remained trapped in the marsh boundary. The mixing model also showed that there was little influence of allochthonous carbon in the diet of salt marsh benthic invertebrates. Marsh litter supply to mangroves did not vary significantly between seasons but was significantly higher in the outer than in the inner sector. Likewise, the mixing model showed great contribution of salt marsh carbon to the diet of benthic invertebrates from the outer-sector mangroves, whereas autochthonous carbon predominated in those from the inner mangroves. Our findings reinforce the model that trophic connectivity relies on the relative proportion of allochthonous (subsidy) and autochthonous resources rather than only on asymmetric productivity between habitats. Differences in the proportion of resources result from interaction among productivity, permeability, and transport vectors that lead to many states of trophic connectivity.     

Trophic interactions in the Zoige Alpine wetland on the eastern edge of the Qinghai–Tibetan Plateau inferred by stable isotopes

The Zoige wetland ecosystem of the eastern edge of the Qinghai–Tibetan Plateau is the largest plateau peat bog worldwide and an important biodiversity conservation centre in China. Nevertheless, limited information is available about this Alpine wetland aquatic ecosystem, including its food-web structure and complexity. In this study, we analysed the trophic interactions among dominant organisms in the food web of this Alpine wetland aquatic ecosystem and assessed the importance of allochthonous and autochthonous carbon sources during the wet season using δ¹³C and δ¹⁵N analysis. The results supported the hypothesis that the food web of the Zoige Alpine wetland is largely maintained by allochthonous carbon sources rather than autochthonous food sources during the wet season when the allochthonous resource availability is relatively high. The food web contains approximately three trophic levels, and Schizopygopsis pylzovi (a genus of cyprinid fish) is the top predator. Intermediate trophic positions, including zooplankton and macroinvertebrates, represent the primary animal community in the food web. The allochthonous carbon sources are transferred to the food web via these macroinvertebrates (e.g., Ecdyonurus sp. and Sigara substriata). Although our findings show a low biodiversity in the Zoige wetland during the wet season, the trophic redundancy in the food-web structure could support the stability of the Zoige Alpine wetland aquatic ecosystem.     

Resource Availability and Spatial Heterogeneity Control Bacterial Community Response to Nutrient Enrichment in Lakes

The diversity and composition of ecological communities often co-vary with ecosystem productivity. However, the relative importance of productivity, or resource abundance, versus the spatial distribution of resources in shaping those ecological patterns is not well understood, particularly for the bacterial communities that underlie most important ecosystem functions. Increasing ecosystem productivity in lakes has been shown to influence the composition and ecology of bacterial communities, but existing work has only evaluated the effect of increasing resource supply and not heterogeneity in how those resources are distributed. We quantified how bacterial communities varied with the trophic status of lakes and whether community responses differed in surface and deep habitats in response to heterogeneity in nutrient resources. Using ARISA fingerprinting, we found that bacterial communities were more abundant, richer, and more distinct among habitats as lake trophic state and vertical heterogeneity in nutrients increased, and that spatial resource variation produced habitat specific responses of bacteria in response to increased productivity. Furthermore, changes in communities in high nutrient lakes were not produced by turnover in community composition but from additional taxa augmenting core bacterial communities found in lower productivity lakes. These data suggests that bacterial community responses to nutrient enrichment in lakes vary spatially and are likely influenced disproportionately by rare taxa.     

Ecosystem engineering,environmental decay and environmental states of landscapes

Although environmental modification by ecosystem engineers influences species distributions and abundances and ecological process rates, general determinants of the environmental states of engineered landscapes are not well understood. Here we develop a general, spatially implicit model of engineered landscapes that includes parameters driving engineer populations (demographics, environmental modification) and environmental decay. We show that average environmental states and heterogeneities of landscapes are the result of a balance between parameters determining engineering rates and decay rates that can be expressed as a net engineering ratio (NER). This ratio highlights the need to include environmental decay in ecosystem engineering studies. Moreover, it defines a significant engineer as one that can alter the environment despite decay and generates expectations for different kinds of effects on the engineer, other species and ecological processes depending on ratio values. Finally, it suggests that, in general, decay places limits as to what can be inferred about engineer population dynamics from environmental dynamics and vice versa.     

Ecosystem engineering moving away from just-so stories

The concept of ecosystem engineering has been proposed recently to account for key processes between organisms and their environment which are not directly trophic or competitive, and which result in the modification, maintenance and/or creation of habitats. Since the initial reporting of the idea, little work has been undertaken to apply the proposed concept to potential ecosystem engineers in the marine environment. Biological and ecological data for the burrowing ghost shrimp Callianassa filholi (Decapoda: Thalassinidea) allowed for a formal assessment of this species as an ecosystem engineer, in direct accordance with published criteria. Despite a low population density and the short durability of its burrow structures, Callianassa filholi affected a number of resource flows by its large lifetime per capita activity. Ecosystem effects were evident in significant changes in macrofauna community composition over small spatial and temporal scales. Seasonal variation in the effects of ghost shrimp activity were associated with changes in seagrass (Zostera novazelandica) biomass, which revealed the probability of interactions between antagonistic ecosystem engineers. The formal assessment of Callianassa filholi provides the opportunity to aid discussion pertaining to the development of the ecosystem engineering concept.     

Are Primates Ecosystem Engineers?

Animals can play important roles in structuring the plant communities in which they live. Some species are particularly influential in that they modify the physical environment by changing, maintaining, and/or creating new habitats; the term ecosystem engineer has been used to describe such species. We here assess the two major foraging strategies of primates, frugivory and folivory, in terms of the potential for primates to function as ecosystem engineers. We argue that whereas the role of primates as seed dispersers has received a great deal of attention, the potential role that folivorous primates play in structuring their environment through herbivory has received much less attention. Further, while quantifying if frugivorous primates are ecosystem engineers through their seed dispersal has proved very difficult, it is not as difficult to ascertain whether folivorous primates are ecosystem engineers. We document situations in which folivorous primates act as ecosystem engineers by 1) eating the leaves and/or bark of trees to the extent that they kill trees, 2) feeding on trees to the degree that they slow their growth relative to nonpreferred tree species, 3) eating the flowers of species to the extent that it does not set fruit, or 4) feeding on plants in such a way as to increase their productivity and abundance. Because evidence from the literature is very limited, where possible we present new evidence of these processes from the colobus monkeys at our long-term field site in Kibale National Park, Uganda. We conclude by discussing promising research programs that could be established to refine our understanding of the role primates play in shaping the structure of plant communities, especially tropical forests.     

Shoreline urbanization interrupts allochthonous subsidies to a benthic consumer over a gradient of lake size

The role of resource subsidies across ecosystem boundaries has emerged as an important concept in contemporary ecology. For lake ecosystems, this has led to interest in quantifying the contribution of terrestrial allochthonous carbon to aquatic secondary production. An inverse relationship between habitat area and the role of allochthonous subsidies has been documented on marine islands and assumed for lakes, yet there have been no tests of this pattern among benthic (lake bottom) consumers. Here, we used carbon stable isotopes to trace terrestrial allochthonous and benthic autochthonous carbon use by the crayfish Pacifastacus leniusculus over a gradient of lake area, productivity and urbanization. Consistent with findings from terrestrial islands, habitat size dictated the importance of allochthonous subsidies, as P. leniusculus transitioned from using predominantly terrestrial carbon in small lakes to an increased reliance on autochthonous production in larger lakes. However, shoreline urbanization interacted with this pattern, particularly for small lakes where greater urbanization resulted in reduced use of allochthonous resources. As such, we provide, to our knowledge, the first confirmation of the predicted relationship between habitat size and importance of allochthonous subsidies to lake benthic consumers, but found that urbanization can interfere with this pattern.     

Shifts in the trophic base of intermittent stream food webs

Understanding spatial and temporal variation in the trophic base of stream food webs is critical for predicting population and community stability, and ecosystem function. We used stable isotope ratios (¹³C/¹²C, and ¹⁵N/¹⁴N) to characterize the trophic base of two streams in the Ozark Mountains of northwest Arkansas, U.S.A. We predicted that autochthonous resources would be more important during the spring and summer and allochthonous resources would be more important in the winter due to increased detritus inputs from the riparian zone during autumn leaf drop. We predicted that stream communities would demonstrate increased reliance on autochthonous resources at sites with larger watersheds and greater canopy openness. The study was conducted at three low-order sites in the Mulberry River Drainage (watershed area range: 81–232 km²) seasonally in 2006 and 2007. We used circular statistics to examine community-wide shifts in isotope space among fish and invertebrate consumers in relation to basal resources, including detritus and periphyton. Mixing models were used to quantify the relative contribution of autochthonous and allochthonous energy sources to individual invertebrate consumers. Significant isotopic shifts occurred but results varied by season and site indicating substantial variation in the trophic base of stream food webs. In terms of temporal variation, consumers shifted toward periphyton in the summer during periods of low discharge, but results varied during the interval between summer and winter. Our results did not demonstrate increased reliance on periphyton with increasing watershed area or canopy openness, and detritus was important at all the sites. In our study, riffle–pool geomorphology likely disrupted the expected spatial pattern and stream drying likely impacted the availability and distribution of basal resources.     

Integrating Ecosystem Engineering and Food Web Ecology: Testing the Effect of Biogenic Reefs on the Food Web of a Soft-Bottom Intertidal Area

The potential of ecosystem engineers to modify the structure and dynamics of food webs has recently been hypothesised from a conceptual point of view. Empirical data on the integration of ecosystem engineers and food webs is however largely lacking. This paper investigates the hypothesised link based on a field sampling approach of intertidal biogenic aggregations created by the ecosystem engineer Lanice conchilega (Polychaeta, Terebellidae). The aggregations are known to have a considerable impact on the physical and biogeochemical characteristics of their environment and subsequently on the abundance and biomass of primary food sources and the macrofaunal (i.e. the macro-, hyper- and epibenthos) community. Therefore, we hypothesise that L. conchilega aggregations affect the structure, stability and isotopic niche of the consumer assemblage of a soft-bottom intertidal food web. Primary food sources and the bentho-pelagic consumer assemblage of a L. conchilega aggregation and a control area were sampled on two soft-bottom intertidal areas along the French coast and analysed for their stable isotopes. Despite the structural impacts of the ecosystem engineer on the associated macrofaunal community, the presence of L. conchilega aggregations only has a minor effect on the food web structure of soft-bottom intertidal areas. The isotopic niche width of the consumer communities of the L. conchilega aggregations and control areas are highly similar, implying that consumer taxa do not shift their diet when feeding in a L. conchilega aggregation. Besides, species packing and hence trophic redundancy were not affected, pointing to an unaltered stability of the food web in the presence of L. conchilega.