Food‐web dynamics in a large river discontinuum

Abstract

Abstract. Nearly all ecosystems have been altered by human activities, and most communities are now composed of interacting species that have not co-evolved. These changes may modify species interactions, energy and material flows, and food-web stability. Although structural changes to ecosystems have been widely reported, few studies have linked such changes to dynamic food-web attributes and patterns of energy flow. Moreover, there have been few tests of food-web stability theory in highly disturbed and intensely managed freshwater ecosystems. Such synthetic approaches are needed for predicting the future trajectory of ecosystems, including how they may respond to natural or anthropogenic perturbations.We constructed flow food webs at six locations along a 386-km segment of the Colorado River in Grand Canyon (Arizona, USA) for three years. We characterized food-web structure and production, trophic basis of production, energy efficiencies, and interaction-strength distributions across a spatial gradient of perturbation (i.e., distance from Glen Canyon Dam), as well as before and after an experimental flood. We found strong longitudinal patterns in food-web characteristics that strongly correlated with the spatial position of large tributaries. Above tributaries, food webs were dominated by nonnative New Zealand mudsnails (62% of production) and nonnative rainbow trout (100% of fish production). The simple structure of these food webs led to few dominant energy pathways (diatoms to few invertebrate taxa to rainbow trout), large energy inefficiencies (i.e., ,20% of invertebrate production consumed by fishes), and right-skewed interaction-strength distributions, consistent with theoretical instability.Below large tributaries, invertebrate production declined ;18-fold, while fish production remained similar to upstream sites and comprised predominately native taxa (80-100% of production). Sites below large tributaries had increasingly reticulate and detritus-based food webs with a higher prevalence of omnivory, as well as interaction strength distributions more typical of theoretically stable food webs (i.e., nearly twofold higher proportion of weak interactions). Consistent with theory, downstream food webs were less responsive to the experimental flood than sites closest to the dam. We show how human-induced shifts to foodweb structure can affect energy flow and interaction strengths, and we show that these changes have consequences for food-web function and response to perturbations.

Annual mass drownings of the Serengeti wildebeest migration influence nutrient cycling and storage in the Mara River

“…More detailed crocodile surveys are needed to estimate the total number of wildebeest that may be consumed by crocodiles in the Mara River during the migration period. These data suggest that consumption by fish and other aquatic animals may be an important pathway for wildebeest carcasses, although further analysis of secondary production is needed to quantify this pathway (38). Moreover, pulsed inputs of wildebeest carcasses may influence the river food web at decadal time scales through the contribution of bone to basal resources.…”

Body Condition of Endangered Humpback Chub in Relation to Temperature and Discharge in the Lower Colorado River

“…Similarly, within the Colorado River near the confluence with the Little Colorado River, Finch et al (2015) estimated juvenile Humpback Chub abundance during 2009-2011 and found the highest abundance in 2011, despite finding no changes in condition. Cross et al (2011) documented that prey consumption rates by fishes in the Colorado River nearly equaled or exceeded invertebrate production, which may cause food limitation that could lead to lower condition. We were only able to estimate length-weight relationships for 1 y (2011) during the time period Van Haverbeke et al (2013) noted increasing abundance for Humpback Chub; but in this one year Humpback Chub showed the most robust size for a given length of all the years we examined (Figures 2 and 3).…”

Biodiversity and Distribution Patterns of Free-Living Nematodes Communities in Balai River, Bentre Province

“…Addition, dam construction and operation can make a substantial contribution to ecosystem hazards, things like loss of forests and wildlife habitats, loss of aquatic diversity, including upstream and downstream fisheries, interruption of fish migration and loss of the services of downstream wetlands, riverine and estuarine ecosystems [40,41]. Ecologically, dams have the ability to change river's physicochemical characteristics that are the major reasons for change in the aquatic communities structure [42].…”