Towards a mechanistic understanding of variation in aquatic food chain length.

Ecologists have long sought to understand variation in food chain length (FCL) among natural ecosystems. Various drivers of FCL, including ecosystem size, resource productivity and disturbance, have been hypothesised. However, when results are aggregated across existing empirical studies from aquatic ecosystems, we observe mixed FCL responses to these drivers. To understand this variability, we develop a unified competition-colonisation framework for complex food webs incorporating all of these drivers. With competition-colonisation tradeoffs among basal species, our model predicts that increasing ecosystem size generally results in a monotonic increase in FCL, while FCL displays non-linear, oscillatory responses to resource productivity or disturbance in large ecosystems featuring little disturbance or high productivity. Interestingly, such complex responses mirror patterns in empirical data. Therefore, this study offers a novel mechanistic explanation for observed variations in aquatic FCL driven by multiple environmental factors.

Shifting stream planform state decreases stream productivity yet increases riparian animal production.

In the Colorado Front Range (USA), disturbance history dictates stream planform. Undisturbed, old-growth streams have multiple channels and large amounts of wood and depositional habitat. Disturbed streams (wildfires and logging < 200 years ago) are single-channeled with mostly erosional habitat. We tested how these opposing stream states influenced organic matter, benthic macroinvertebrate secondary production, emerging aquatic insect flux, and riparian spider biomass. Organic matter and macroinvertebrate production did not differ among sites per unit area (m-2), but values were 2 ×-21 × higher in undisturbed reaches per unit of stream valley (m-1 valley) because total stream area was higher in undisturbed reaches. Insect emergence was similar among streams at the per unit area and per unit of stream valley. However, rescaling insect emergence to per meter of stream bank showed that the emerging insect biomass reaching the stream bank was lower in undisturbed sites because multi-channel reaches had 3 × more stream bank than single-channel reaches. Riparian spider biomass followed the same pattern as emerging aquatic insects, and we attribute this to bottom-up limitation caused by the multi-channeled undisturbed sites diluting prey quantity (emerging insects) reaching the stream bank (riparian spider habitat). These results show that historic landscape disturbances continue to influence stream and riparian communities in the Colorado Front Range. However, these legacy effects are only weakly influencing habitat-specific function and instead are primarily influencing stream-riparian community productivity by dictating both stream planform (total stream area, total stream bank length) and the proportional distribution of specific habitat types (pools vs riffles).

Size-dependence of food intake and mortality interact with temperature and seasonality to drive diversity in fish life histories.

Understanding how growth and reproduction will adapt to changing environmental conditions is a fundamental question in evolutionary ecology, but predicting the responses of specific taxa is challenging. Analyses of the physiological effects of climate change upon life history evolution rarely consider alternative hypothesized mechanisms, such as size-dependent foraging and the risk of predation, simultaneously shaping optimal growth patterns. To test for interactions between these mechanisms, we embedded a state-dependent energetic model in an ecosystem size-spectrum to ask whether prey availability (foraging) and risk of predation experienced by individual fish can explain observed diversity in life histories of fishes. We found that asymptotic growth emerged from size-based foraging and reproductive and mortality patterns in the context of ecosystem food web interactions. While more productive ecosystems led to larger body sizes, the effects of temperature on metabolic costs had only small effects on size. To validate our model, we ran it for abiotic scenarios corresponding to the ecological lifestyles of three tuna species, considering environments that included seasonal variation in temperature. We successfully predicted realistic patterns of growth, reproduction, and mortality of all three tuna species. We found that individuals grew larger when environmental conditions varied seasonally, and spawning was restricted to part of the year (corresponding to their migration from temperate to tropical waters). Growing larger was advantageous because foraging and spawning opportunities were seasonally constrained. This mechanism could explain the evolution of gigantism in temperate tunas. Our approach addresses variation in food availability and individual risk as well as metabolic processes and offers a promising approach to understand fish life-history responses to changing ocean conditions.

Predicting the stability of multitrophic communities in a variable world.

Identifying the factors that destabilize communities is critical for predicting and mitigating the ecological impacts of environmental change. Although theory has shown that local ecosystem size and regional dispersal can determine biodiversity, less is known about the direct and indirect effects of these factors on community stability. Here we show that multitrophic community instability of invertebrates and fishes in coastal ponds is negatively related to local pond size and positively related to distance to the ocean, a proxy for dispersal limitation. Importantly, the effects of pond size and distance on instability were direct rather than indirectly mediated by species richness. This suggests that the diversity-stability relationship is an epiphenomenon whose resolution is neither necessary nor sufficient to understand the stability of these multitrophic communities. Instead, well-established and easily measured local and regional factors historically linked to species richness can be used to predict multitrophic community stability in a variable world.

Environmental correlates of food-chain length, mean trophic level and trophic level variance in invaded riverine fish assemblages.

Examining how the trophic structure of biotic assemblages is affected by human impacts, such as habitat degradation and the introduction of alien species, is important for understanding the consequences of such impacts on ecosystem functioning. We used general linear mixed models and hierarchical partitioning analyses of variance to examine for the first time the applicability of three hypotheses (ecosystem-size, productivity and disturbance) for explaining food-chain length (FCL) in invaded fish assemblages. We used Fishbase trophic level (TL) estimates for 16 native and 18 alien fish species in an extensive riverine system in north-eastern Spain (99,700 km2, 15 catchments, 530 sites). The FCL of assemblages ranged from 2.7 to 4.42. Ecosystem size-related variables (Strahler stream order, physical habitat diversity) and human-disturbance (conductivity) made the largest contribution to the explained variance in the FCL model after accounting for spatial confounding factors and collinearity among predictors. Within-assemblage TL also was positively associated with Strahler stream order, suggesting that large rivers have the highest trophic diversity. High conductivity was negatively associated with FCL, as did with the mean TL of fish assemblages. However, an inverse association was found between mean TL and Strahler stream order, possibly because the presence of fish species of high TL may be offset by larger numbers of alien species of lower TL in large rivers. Given that there may be trophic replacements among native and alien species, this inference needs to be addressed with detailed trophic studies. However, reducing water conductivity by improved wastewater treatment and better agricultural practices probably would help to conserve the fish species on the apices of aquatic food-webs.

ECOSYSTEM SIZE, BUT NOT DISTURBANCE, DETERMINES FOOD-CHAIN LENGTH ON ISLANDS OF THE BAHAMAS.

Ecologists have long struggled to explain variation in food-chain length among natural ecosystems. Food-chain length is predicted to be shorter in ecosystems subjected to greater disturbance because longer chains are theoretically less resilient to perturbation. Moreover, food-chain length is expected to be longer in larger ecosystems because increasing ecosystem size increases species richness and stabilizes predator-prey interactions, or increases total resource availability. Here we test the roles of disturbance and ecosystem size in determining the food-chain length of terrestrial food webs on Bahamian islands. We found that disturbance affected the identity of top predators, but did not change food-chain length because alternative top predators occupied similar trophic positions. On the other hand, a 106 -fold increase in ecosystem size elevated food-chain length by one trophic level. We suggest that the effect of disturbance on food-chain length is weak when alternate top predators are trophic omnivores and have similar trophic positions. This and previous work in lakes suggest that ecosystem size may be a strong determinant of food-chain length in both aquatic and terrestrial ecosystems.