Evidence for the Persistence of Food Web Structure After Amphibian Extirpation in a Neotropical Stream

Species losses are predicted to simplify food web structure, and disease‐driven amphibian declines in Central America offer an opportunity to test this prediction. Assessment of insect community composition, combined with gut content analyses, was used to generate periphyton–insect food webs for a Panamanian stream, both pre‐ and post‐amphibian decline. We then used network analysis to assess the effects of amphibian declines on food web structure. Although 48% of consumer taxa, including many insect taxa, were lost between pre‐ and post‐amphibian decline sampling dates, connectance declined by less than 3%. We then quantified the resilience of food web structure by calculating the number of expected cascading extirpations from the loss of tadpoles. This analysis showed the expected effects of species loss on connectance and linkage density to be more than 60% and 40%, respectively, than were actually observed. Instead, new trophic linkages in the post‐decline food web reorganized the food web topology, changing the identity of “hub” taxa, and consequently reducing the effects of amphibian declines on many food web attributes. Resilience of food web attributes was driven by a combination of changes in consumer diets, particularly those of insect predators, as well as the appearance of generalist insect consumers, suggesting that food web structure is maintained by factors independent of the original trophic linkages

Evaluating ecosystem effects of climate change on tropical island streams using high spatial and temporal resolution sampling regimes

Climate change is expected to alter precipitation patterns worldwide, which will affect streamflow in riverine ecosystems. It is vital to understand the impacts of projected flow variations, especially in tropical regions where the effects of climate change are expected to be one of the earliest to emerge. Space-for-time substitutions have been successful at predicting effects of climate change in terrestrial systems by using a spatial gradient to mimic the projected temporal change. However, concerns have been raised that the spatial variability in these models might not reflect the temporal variability. We utilized a well-constrained rainfall gradient on Hawaii Island to determine (a) how predicted decreases in flow and increases in flow variability affect stream food resources and consumers and (b) if using a high temporal (monthly, four streams) or a high spatial (annual, eight streams) resolution sampling scheme would alter the results of a space-for-time substitution. Declines in benthic and suspended resource quantity (10- to 40-fold) and quality (shift from macrophyte to leaf litter dominated) contributed to 35-fold decreases in macroinvertebrate biomass with predicted changes in the magnitude and variability in the flow. Invertebrate composition switched from caddisflies and damselflies to taxa with faster turnover rates (mosquitoes, copepods). Changes in resource and consumer composition patterns were stronger with high temporal resolution sampling. However, trends and ranges of results did not differ between the two sampling regimes, indicating that a suitable, well-constrained spatial gradient is an appropriate tool for examining temporal change. Our study is the first to investigate resource to community wide effects of climate change on tropical streams on a spatial and temporal scale. We determined that predicted flow alterations would decrease stream resource and consumer quantity and quality, which can alter stream function, as well as biomass and habitat for freshwater, marine, and terrestrial consumers dependent on these resources

Energy Flow and the Trophic Basis of Macroinvertebrate and Amphibian Production in a Neotropical Stream Food Web

1. Despite the typically high taxonomic and functional diversity of tropical habitats, little is known about the roles of individual consumers in their ecosystem structure and function. We studied the trophic basis of production in a tropical headwater stream by identifying major sources of energy, measuring energy flow through consumers and characterising interactions among trophic levels and functional groups. 2. We examined gut contents of 18 dominant macroinvertebrate and two tadpole taxa and used these data, along with previously published estimates of secondary production, to quantify food‐web structure and energy flow pathways. We also examined the prevalence of omnivory and patterns of resource consumption across seasons and habitats. 3. Non‐algal biofilm, a heterogeneous polysaccharidic matrix, was the most utilised food resource in the stream. Contrary to some studies of Old World tropical stream food webs, detrital energy sources were consumed at relatively high rates and contributed significantly to overall energy flow, although much of this was attributable to a single shredder taxon. Algal consumption rates were similar to values reported for temperate streams and were highest during the dry season. 4. Omnivory was prevalent across all functional groups, particularly predators, suggesting traditional functional and trophic assignments based on temperate regions may not be appropriate for tropical systems. Seasonal patterns of resource consumption appeared linked to hydrological disturbance. 5. This is the first study to provide quantitative estimates of energy flow through a neotropical stream food web. Extirpation and extinction rates in tropical freshwater habitats are high; our study provides baseline information for conservation and management of remaining systems, and for quantifying the consequences of further losses of biodiversity such as ongoing amphibian declines

Animal legacies lost and found in river ecosystems

Animals can impact freshwater ecosystem structure and function in ways that persist well beyond the animal’s active presence. These legacy effects can last for months, even decades, and often increase spatial and temporal heterogeneity within a system. Herein, we review examples of structural, biogeochemical, and trophic legacies from animals in stream and river ecosystems with a focus on large vertebrates. We examine how the decline or disappearance of many native animal populations has led to the loss of their legacy effects. We also demonstrate how anthropogenically altered animal populations, such as livestock and invasive species, provide new legacy effects that may partially replace lost animal legacies. However, these new effects often have important functional differences, including stronger, more widespread and homogenizing effects. Understanding the influence of animal legacy effects is particularly important as native animal populations continue to decline and disappear from many ecosystems, because they illustrate the long-term and often unanticipated consequences of biodiversity loss. We encourage the conservation and restoration of native species to ensure that both animal populations and their legacy effects continue to support the structure and function of river ecosystems

Consequences of Catastrophic Amphibian Declines on the Food Web Attributes of a Neotropical Stream

Understanding the effects of species losses on food web structure can help bridge significant knowledge gaps of how declining biodiversity affects ecosystems. Empirical assessments of the influence of species losses on the structure of species-rich food webs have not been performed. We use gut content analyses to empirically assess fourteen food web attributes of a periphyton-insect food web before and five years after a disease-driven amphibian declines in a highland Panamanian stream. The connectance and linkage density from pre- to post-decline food webs in stream pools was reduced from 0.11 to 0.10 and 6.86 to 4.76, respectively. However, other food web attributes such as consumer generalism, omnivory, and average path length did not change despite the loss of 25% of taxa (including tadpoles, insects, and diatoms). Nearly 50% of the trophic linkages present after the decline were not present before, reflecting a reorganization of food web topology, possibly mitigating the effect of species loss and suggesting that food web attributes may be more resistant to species loss than previously predicted

Food Web Properties Persist Following Amphibian Extirpations in a Neotropical Stream

Background/Question/Methods As species are extirpated, ecosystem productivity will likely decline with concomitant declines in food web structure as linkages are removed. Catastrophic amphibian extirpations in Central America can affect ecosystem function, but have unknown consequences on food web structure. We empirically assessed periphyton-insect food webs, both before and after (5 yrs) a disease-driven amphibian extirpation in a highland Panamanian stream. Food webs were constructed using gut content analysis of 891 tadpoles and insects to identify 1793 linkages from four food webs: pools and riffles with and without tadpoles. For each food web, fourteen structural properties were estimated and the effect size was measured between pre- and post-decline food webs. Simulations using pre-decline food webs that reflected the loss of tadpoles and cascading extirpations of macroinvertebrates, were then compared with the observed post-extirpation food web to quantify the robustness of food web structure with respect to these perturbations. Results/Conclusions Amphibian extirpations resulted in the loss of 52% of macroinvertebrate taxa, but connectance was reduced by an average of only 3%. The mean difference in the values of the fourteen food web properties from pre- to post-decline wa

Examining Why Grazing Mayflies Do Not Functionally Compensate for the Top-Down Control of Algal Communities Following Disease-Driven Tadpole Declines in a Neotropical Stream

Background/Question/Methods Quantifying functional trait diversity in an assemblage is essential to understanding the potential for functional redundancy following an extinction event. We examined an apparent lack of functional redundancy by algae grazing mayflies following the disease-driven loss of stream-dwelling tadpoles from a Panamanian montane stream. Prior to their extirpation, four species of grazing tadpoles reduced algal biomass and we predicted that insect grazers would functionally compensate for tadpoles given that 9 genera of mayflies (Insecta: Ephemeroptera) in our study stream also consume algae. However, algal biomass increased 4-fold following tadpole declines, maintaining high levels for 4 yrs post-extirpation and was accompanied by shifts in diatom assemblages from small- to large-sized taxa. We then hypothesized that mayflies are gape-limited and unable to consume large-bodied diatom taxa. We compared mayfly abundances and diatom community composition by collecting monthly pre-extirpation samples (N=3) in 2004 and monthly post-extirpation samples (N=3) in 2009. We also assessed pre-extirpation tadpole diets and pre- and post-extirpation mayfly diets via gut analysis. Effects of tadpoles and mayflies on the algal assemblage were estimated using a structural equation model (SEM), and tadpole/mayfly preferences for different diatom size classes were calculated via an electivity index. Results/Conclusions The abundance of mayflies with the smallest gape size (200-300 μm) declined by 90% from 2004 to 2009 while total mayfly abundance declined by 83% from 2004 to 2009. Populations of small diatom taxa doubled while large diatom taxa increased 5-fold from 2004 to 2009. The SEM showed that tadpoles had negative effects on both small- and large diatom taxa, but no effect on medium-sized diatom taxa, while mayflies had a weak negative effect on large diatoms. The electivity index indicated that tadpoles preferred medium-sized diatoms, but avoided the largest diatoms, while mayflies preferred the smallest diatoms. Following the amphibian decline, mayflies switched to medium-sized diatom taxa. Consumer functional trait richness also declined from 2.71 in 2004 to 0.007 in 2009. Findings suggest tadpoles had direct consumptive effects on small- and medium-sized diatoms, and non-consumptive effects on large diatoms. Non-consumptive effects by tadpoles may have been due to bioturbation associated with tadpole foraging. The decline in functional trait richness suggests tadpoles possessed traits that were lost from the algal-grazing community, preventing functional compensation by mayflies after the decline. Our results also suggest that the loss of consumer trait diversity can be linked to loss of ecosystem function

Occurrence of maize detritus and a transgenic insecticidal protein (Cry1Ab) within the stream network of an agricultural landscape

Widespread planting of maize throughout the agricultural Midwest may result in detritus entering adjacent stream ecosystems, and 63% of the 2009 US maize crop was genetically modified to express insecticidal Cry proteins derived from Bacillus thuringiensis. Six months after harvest, we conducted a synoptic survey of 217 stream sites in Indiana to determine the extent of maize detritus and presence of Cry1Ab protein in the stream network. We found that 86% of stream sites contained maize leaves, cobs, husks, and/or stalks in the active stream channel. We also detected Cry1Ab protein in stream-channel maize at 13% of sites and in the water column at 23% of sites. We found that 82% of stream sites were adjacent to maize fields, and Geographical Information Systems analyses indicated that 100% of sites containing Cry1Ab-positive detritus in the active stream channel had maize planted within 500 m during the previous crop year. Maize detritus likely enters streams throughout the Corn Belt; using US Department of Agriculture land cover data, we estimate that 91% of the 256,446 km of streams/rivers in Iowa, Illinois, and Indiana are located within 500 m of a maize field. Maize detritus is common in low-gradient stream channels in northwestern Indiana, and Cry1Ab proteins persist in maize leaves and can be measured in the water column even 6 mo after harvest. Hence, maize detritus, and associated Cry1Ab proteins, are widely distributed and persistent in the headwater streams of a Corn Belt landscape

Mechanisms Underlying Lack of Functional Compensation by Insect Grazers After Tadpole Declines in a Neotropical Stream

Resilience of ecosystems to the sudden decline of large-bodied species is dependent on characteristics of surviving guild members. However, that response may also be mediated by local habitat conditions. Here, we examine the mechanisms behind the observed lack of functional compensation in the algal-grazing guild by insect grazers following the decline of tadpole grazers in a forested Panamanian stream. We examined: (1) shifts to the individual size distribution of insect grazers between pre- and post-tadpole declines in pool and riffle habitats; (2) tadpole and insect preferences for small-, medium-, and large-sized diatoms; and (3) a causal explanation for why insects did not functionally compensate for tadpole declines. The size distribution of insect grazers following tadpole declines differed between habitats, becoming uniform in pools and more right skewed toward a smaller size class in riffles. In both habitats, tadpoles selectively consumed medium-sized diatoms but avoided the largest-sized diatoms. In contrast, grazing insects selectively consumed small-sized diatoms, but switched to medium-sized diatoms after tadpole declines. Tadpole declines led to the loss of the strongest interactions between consumers and diatoms. Smaller-bodied grazing insects could not duplicate these interactions, even with a shift in resource use, providing an explanation for the lack of functional compensation. Furthermore, tadpole declines led to different community structures in each habitat indicating that local habitat conditions mediated the response of surviving guild members. This suggests that the sudden decline of a large-bodied species does not lead to a singular outcome for the surviving community

Appendix A. Macroinvertebrate community biomass.

Macroinvertebrate community biomass