Environmental determinants of macroinvertebrate diversity in small water bodies: insights from tank-bromeliads

The interlocking leaves of tank-forming bromeliads (Bromeliaceae) collect rainwater and detritus, thus creating a freshwater habitat for specialized organisms. Their abundance and the possibility of quantifying communities with accuracy give us unparalleled insight into how changes in local to regional environments influence community diversity in small water bodies. We sampled 365 bromeliads (365 invertebrate communities) along a southeastern to northwestern range in French Guiana. Geographic locality determined the species pool for bromeliad invertebrates, and local environments determined the abundance patterns through the selection of traits that are best adapted to the bromeliad habitats. Patterns in community structure mostly emerged from patterns of predator species occurrence and abundance across local–regional environments, while the set of detritivores remained constant. Water volume had a strong positive correlation with invertebrate diversity, making it a biologically relevant measure of the pools’ carrying capacity. The significant effects of incoming detritus and incident light show that changes in local environments (e.g., the conversion of forest to cropping systems) strongly influence freshwater communities. Because changes in local environments do not affect detritivores and predators equally, one may expect functional shifts as sets of invertebrates with particular traits are replaced or complemented by other sets with different traits.     

Structural Degradation in Mediterranean Sea Food Webs: Testing Ecological Hypotheses Using Stochastic and Mass-Balance Modelling

Human-mediated disturbances such as fishing, habitat modification, and pollution have resulted in significant shifts in species composition and abundance in marine ecosystems which translate into degradation of food-web structure. Here, we used a comparative ecological modelling approach and data from two food webs (North-Central Adriatic and South Catalan Sea) and two time periods (mid-late 1970s and 1990s) in the Mediterranean Sea to evaluate how changes in species composition and biomass have affected food-web properties and the extent of ecosystem degradation. We assembled species lists and ecological information for both regions and time periods into stochastic structural and mass-balance food-web models, and compared the outcomes of 22 food-web properties. Our results show strong similarities in structural food-web properties between the North-Central Adriatic and South Catalan Seas indicating similar ecosystem structure and levels of ecological degradation between regions and time periods. In contrast, a comparison with other published marine food webs (Caribbean, Benguela, and US continental shelf) suggested that Mediterranean webs are in an advanced state of ecological degradation. This was reflected by lower trophic height, linkage density, connectance, omnivory, species involved in looping, trophic chain length and fraction of biomass at higher trophic levels, as well as higher generality and fraction of biomass at lower trophic levels. An analysis of robustness to simulated species extinction revealed lower robustness to species removals in Mediterranean webs and corroborated their advanced state of degradation. Importantly, the two modelling approaches used delivered comparable results suggesting that they both capture fundamental information about how food webs are structured. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Biodiversity maintenance in food webs with regulatory environmental feedbacks

Although the food web is one of the most fundamental and oldest concepts in ecology, elucidating the strategies and structures by which natural communities of species persist remains a challenge to empirical and theoretical ecologists. We show that simple regulatory feedbacks between autotrophs and their environment when embedded within complex and realistic food-web models enhance biodiversity. The food webs are generated through the niche-model algorithm and coupled with predator-prey dynamics, with and without environmental feedbacks at the autotroph level. With high probability and especially at lower, more realistic connectance levels, regulatory environmental feedbacks result in fewer species extinctions, that is, in increased species persistence. These same feedback couplings, however, also sensitize food webs to environmental stresses leading to abrupt collapses in biodiversity with increased forcing. Feedback interactions between species and their material environments anchor food-web persistence, adding another dimension to biodiversity conservation. We suggest that the regulatory features of two natural systems, deep-sea tubeworms with their microbial consortia and a soil ecosystem manifesting adaptive homeostatic changes, can be embedded within niche-model food-web dynamics.     

Food web structure changes with elevation but not rainforest stratum

Changes in species richness along elevational gradients are well documented. However, little is known about how trophic interactions between species and, in particular, the food webs that these interactions comprise, change with elevation. Here we present results for the first comparison of quantitative food webs in forest understorey and canopy along an elevational gradient. Replicate quantitative food webs were constructed for assemblages involving 23 species of cavity‐nesting Hymenoptera and 12 species of their parasitoids and kleptoparasites in subtropical rainforest in Australia. A total of 1589 insects were collected using trap nests across 20 plots distributed at sites ranging from 300 to 1100 m a.s.l. Insect abundance, insect diversity and parasitism rate generally decreased with increasing elevation. Food web structure significantly changed with elevation. In particular, weighted quantitative measures of linkage density, interaction evenness, nestedness (weighted NODF) and potential for enemy mediated interactions (PAC) decreased with increasing elevation, and network specialisation (H₂′) increased with increasing elevation, even after controlling for matrix size; but there was no change in weighted connectance. Changes in forest type and temperature along the elevational gradient are likely to be, at least partly, responsible for the patterns observed. We found no significant differences in insect abundance, insect diversity or parasitism rate between canopy and understorey. Furthermore, there were no differences in food web structure between strata. These results contribute further evidence to studies revealing changes in food web structure along natural environmental gradients and provide information that can potentially be used for predicting how communities may respond to climate change.     

Estimating morphologically determined connectance and structure for food webs of freshwater invertebrates

1. Connectance is a parameter of central importance in determining food-web structure, but the processes determining its value remain unclear. In evaluating possible explanations it is useful to know what patterns, and values, of connectance occur in food webs assembled at random from a set of species in a regional species pool; i.e. where the number of links is determined by the morphological features of the species present, not by the immediate effects of energetics or stability on the particular web. 2. This study examines, by means of laboratory experiments, the occurrence of potential feeding interactions among a set of freshwater invertebrate species randomly selected from different freshwater sites in a geographical region. The results from pairwise feeding trials are used to construct two ‘theoretical’ food webs, in which the patterns and values of connectance are examined. 3. Analyses of these webs indicate that their structure is consistent with the observed values in previously documented ‘real’ webs. Directed connectance values of 0.12–0.16 (or less) suggest that the assembled webs are no more connected than many freshwater webs from natural systems. The number of links per species increases curvilinearly with the number of species, during web assembly, consistent with recent hypotheses. 4. These results also indicate that quantifying, and understanding the determinants of, trophic generalism or specialism does have implications for understanding how connectance is constrained in real webs. Freshwater invertebrates seem to be relatively generalist, and freshwater food webs perhaps correspondingly highly connected. Such arguments have implications for interpreting other aspects of food-web structure in these systems, and for parameterizing models that are based on connectance.     

Understorey environments influence functional diversity in tank‐bromeliad ecosystems

1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad food webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad food webs in samples taken from 171 tank‐bromeliads. 3. We observed a gradient of detritus‐based to algal‐based food webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad food webs in exposed areas, whereas filter‐feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun‐exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank‐bromeliads, which grow in a broad range of ecological conditions, promote aquatic food web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus‐based systems in the literature, we show that algal production can support a non‐detrital food web in these systems.     

Food web structure in Mediterranean streams: exploring stabilizing forces in these ecosystems

We constructed the food webs of six Mediterranean streams in order to determine ecological generalities derived from analysis of their structure and to explore stabilizing forces within these ecosystems. Fish, macroinvertebrates, primary producers and detritus are the components of the studied food webs. Analysis focused on a suite of food web properties that describe species’ trophic habits, linkage complexity and food chains. A great structural similarity was found in analyzed food webs; we therefore suggest average values for the structural properties of Mediterranean stream food webs. Percentage of omnivorous species was positively correlated with connectance, and there was a predominance of intermediate trophic level species that had established simple links with detritus. In short, our results suggest that omnivory and the weak interactions of detritivores have a stabilizing role in these food webs.     

Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems

In the study of food webs, the existence and explanation of recurring patterns, such as the scale invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing issues. Our study focused on litter-associated food webs and explored the influence of detritivore and predator niche width (as δ¹³C range) on web topological structure. To compare patterns within and between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed 42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and cornfield), using an experimental approach with litterbags. The results suggest that although web differences exist between ecosystems, patterns are more similar within than between aquatic and terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in the community. The tendency was more marked in terrestrial ecosystems and can be explained by a lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance with these results, the number of links increased with the number of species but with a significantly sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found to be directly correlated to niche width, increased with the total number of species in terrestrial webs, whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the total number of species. In both types of ecosystem, web robustness to rare species removal increased with connectance and the niche width of predators. In conclusion, although limited to litter-associated macroinvertebrate assemblages, this study highlights structural differences and similarities between aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in determining the structure of detritus-based food webs and posing foraging optimisation constraints as a general mechanistic explanation of food web complexity differences within and between ecosystem types.     

Historical Changes in Marine Resources,Food-web Structure and Ecosystem Functioning in the Adriatic Sea,Mediterranean

The Mediterranean Sea has been strongly influenced by human activities for millennia. Although the environmental history of its surrounding terrestrial ecosystems has received considerable study, historical changes in its marine realm are less known. We used a multidisciplinary approach combining paleontological, archeological, historical, fisheries, and ecological data to reconstruct past changes in marine populations, habitats, and water quality in the Adriatic Sea. Then, we constructed binary food webs for different historical periods to analyze possible changes in food-web structure and functioning over time. Our results indicate that human activities have influenced marine resource abundance since at least Roman times and accelerated in the nineteenth and twentieth centuries. Today, 98% of traditional marine resources are depleted to less than 50% of former abundance, with large (>1 m) predators and consumers being most affected. With 37% of investigated species rare and 11% extirpated, diversity has shifted towards smaller, lower trophic-level species, further aggravated by more than 40 species invasions. Species providing habitat and filter functions have been reduced by 75%, contributing to the degradation of water quality and increased eutrophication. Increased exploitation and functional extinctions have altered and simplified food-web structure over time, especially by changing the proportions of top predators, intermediate consumers, and basal species. Moreover, simulations of species losses indicate that today’s ecosystems may be less robust to species extinctions than in the past. Our results illustrate the long-term and far-reaching consequences human activities can have on marine food webs and ecosystems.     

Parasites alter the topology of a stream food web across seasons

Relatively few published food webs have included parasites, and in this study we examined the animal community in a stream across eight contiguous seasons to test how inclusion of helminth parasites alters the topology or structure of the food web. Food webs constructed for each season and analyzed using common binary matrix measures show that species richness, linkage density, and the number of observed and possible links increased when parasites were included as individual species nodes. With parasite–parasite and predator–parasite links omitted, measures of community complexity, such as connectance (C), generally increased over multiple seasons. However, relative nestedness (n*) decreased when parasites were included, which may be a result of low resolution of basal resources inflating specialist-to-specialist links. Overall, adding parasites resulted in moderate changes in food web measures when compared to those of four other published food webs representing different ecosystems. In addition, including parasites in the food web revealed consistent pathways of energy flow, and the association of parasite life histories along these pathways suggest stable evolutionary groups of interacting species within the community. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Network structure and biodiversity loss in food webs: robustness increases with connectance

Food-web structure mediates dramatic effects of biodiversity loss including secondary and `cascading' extinctions. We studied these effects by simulating primary species loss in 16 food webs from terrestrial and aquatic ecosystems and measuring robustness in terms of the secondary extinctions that followed. As observed in other networks, food webs are more robust to random removal of species than to selective removal of species with the most trophic links to other species. More surprisingly, robustness increases with food-web connectance but appears independent of species richness and omnivory. In particular, food webs experience `rivet-like' thresholds past which they display extreme sensitivity to removal of highly connected species. Higher connectance delays the onset of this threshold. Removing species with few trophic connections generally has little effect though there are several striking exceptions. These findings emphasize how the number of species removed affects ecosystems differently depending on the trophic functions of species removed.     

Effects of heterogeneous interaction strengths on food web complexity

Using a bioenergetic model we show that the pattern of foraging preferences greatly determines the complexity of the resulting food webs. By complexity we refer to the degree of richness of food-web architecture, measured in terms of some topological indicators (number of persistent species and links, connectance, link density, number of trophic levels, and frequency of weak links). The poorest food-web architecture is found for a mean-field scenario where all foraging preferences are assumed to be the same. Richer food webs appear when foraging preferences depend on the trophic position of species. Food-web complexity increases with the number of basal species. We also find a strong correlation between the complexity of a trophic module and the complexity of entire food webs with the same pattern of foraging preferences.     

Geographical variation in food web structure in Nepenthes pitcher plants

Abstract. 1. Relative to Nepenthes species in West Malaysia near the evolutionary centre of the genus, outlying species of Nepenthes in the Seychelles, Sri Lanka and Madagascar have fewer species of both prey and predator living in them, fewer and smaller guilds of species, much apparently empty niche space, less complex food webs, and a greater connectance. The ratios of prey to predators, and of connectance (C₁) to the total number of trophic types present remain approximately constant.
2. Differences between the food webs appear to be related in a complex way to the size of the country and its degree of spatial and temporal isolation, the size of the local species pool capable of colonizing the pitchers, and the number of Nepenthes species present. However, the maximal length of food chains in the richest and most complex food webs is probably limited by energetic constraints or environmental predictability.
3. The data may illustrate how food webs change to become more complex, both by the addition of new guilds of species and the addition of species to existing guilds, while at the same time certain properties of the food web are kept approximately constant.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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The predatory Chironomidae of an iron-rich stream: feeding ecology and food web structure

Abstract. 1. Three species of Tanypodinae (Chironomidae) were found in an acid and iron-rich stream in southern England. Maximum abundance was achieved in summer and they were sparse at other times. Individuals were aggregated on the stream bed and were overrepresented in accumulations of leaf litter.
2. The diets of all three species consisted of a mixture of prey (prominently detritivorous chironomid larvae) and detritus. More detritus and fewer prey were taken in winter than in summer.
3. When comparing large tanypod species with small and, intraspecifically, late instars with early, the proportion of guts containing prey increased with increasing body size.
4. Stonefly larvae were more prominent in the diet of Zavrelimyia barbatipes (Kieffer) in summer than in winter but for the other two species the reverse was true. A bigger proportion of Trissopelopia longimana (Staeger) guts contained prey in early summer than in August whereas more Macropelopia goetghebueri (Kieffer) guts contained prey in August. This was apparently a consequence of seasonal differences in the distribution of body size among the populations of these two species.
5. The stream contains two further common predators, Plectrocnemia conspersa (Curtis) and Sialis fuliginosa Pict. These are important predators of tanypod larvae but might also compete with them since they severely deplete populations of prey taken in common.
6. Analysis of the food-web in Broadstone Stream reveals remarkably high values of connectance (C and C_max) and of species richness times connectance (SC_max). Such characteristics are theoretically associated with fragile and dynamically unstable food webs, and may be found in 'constant' environments. There is also an apparently unusual prevalence of omnivory in the community.     

Food-web structure of seagrass communities across different spatial scales and human impacts

Seagrass beds provide important habitat for a wide range of marine species but are threatened by multiple human impacts in coastal waters. Although seagrass communities have been well-studied in the field, a quantification of their food-web structure and functioning, and how these change across space and human impacts has been lacking. Motivated by extensive field surveys and literature information, we analyzed the structural features of food webs associated with Zostera marina across 16 study sites in 3 provinces in Atlantic Canada. Our goals were to (i) quantify differences in food-web structure across local and regional scales and human impacts, (ii) assess the robustness of seagrass webs to simulated species loss, and (iii) compare food-web structure in temperate Atlantic seagrass beds with those of other aquatic ecosystems. We constructed individual food webs for each study site and cumulative webs for each province and the entire region based on presence/absence of species, and calculated 16 structural properties for each web. Our results indicate that food-web structure was similar among low impact sites across regions. With increasing human impacts associated with eutrophication, however, food-web structure show evidence of degradation as indicated by fewer trophic groups, lower maximum trophic level of the highest top predator, fewer trophic links connecting top to basal species, higher fractions of herbivores and intermediate consumers, and higher number of prey per species. These structural changes translate into functional changes with impacted sites being less robust to simulated species loss. Temperate Atlantic seagrass webs are similar to a tropical seagrass web, yet differed from other aquatic webs, suggesting consistent food-web characteristics across seagrass ecosystems in different regions. Our study illustrates that food-web structure and functioning of seagrass habitats change with human impacts and that the spatial scale of food-web analysis is critical for determining results.     

Using network connectance and autonomy analyses to uncover patterns of photosynthetic responses in tropical woody species

Daily courses of leaf gas exchange and chlorophyll fluorescence in forest gap and understorey environments were used to build photosynthetic networks in two pioneers and two late-successional species. Photochemical and gas exchange networks were linked to each other by the relationship between electron transport rate and net CO₂ assimilation. Global network connectance (Cg), which represents the mean strength of connections within a given network, was calculated in the photochemical and gas exchange networks for both functional groups and environments. Autonomy in relation to environmental fluctuations was estimated considering the mean correlation between environmental and physiological data. Cg was consistently higher in plants under gap condition. High daily-amplitude of environmental variables in the gap induced strong connectance in photochemical and gas exchange networks regardless of functional group. Gap scenario demands network modulation with higher level of control than understorey, which would be attained by strong connections among components of photochemical and gas exchange networks. This would allow fine and fast tuning adjustments when facing highly variable and demanding environmental conditions throughout a day. As a consequence of this highly variable environment, both functional groups showed lower autonomy in the gap, where higher coupling between leaf physiology and environmental fluctuations was evident. Our results suggest that high plant–environment coupling demands high network connectance. Contrastingly, Cg was lower (especially in photochemical network) under forest understorey, promoting autonomy in a more stable environment. Our results indicate that there is a conservative pattern of photosynthesis control based on network modulation and environmental coupling. This suggests that changes in network connectance may not be specific of a functional group but rather a more general response to environmental fluctuations, strongly related to system stability. We consider this information crucial in understanding how complex adaptive systems deal with environmental fluctuations.     

Shrinking by numbers: landscape context affects the species composition but not the quantitative structure of local food webs

1. With habitat fragmentation spreading around the world, there is a pressing need to understand its impacts on local food webs. To date, few studies have examined the effects of landscape context on multiple local communities in a quantitative, spatially realistic setting. 2. To examine how the isolation of a food web affects its structure, we construct local food webs of specialist herbivores and their natural enemies on 82 individual oaks (Quercus robur) growing in different landscape contexts. 3. Across this set of webs, we find that communities in isolated habitat patches not only contained fewer species than did well-connected ones, but also differed in species composition. 4. Surprisingly, the effects observed in terms of species composition were not reflected in the quantitative interaction structure of local food webs: landscape context had no detectable effect on either the interaction evenness, linkage density, connectance, generality or vulnerability of local webs. 5. We conclude that the quantitative structure of food webs may be stable in the face of habitat fragmentation, despite clear-cut impacts on individual species. This finding offers hope-inspiring news for conservation, but should clearly be verified by empirical studies across both naturally and more recently fragmented systems.