Less is worse than none: ineffective adaptive foraging can destabilise food webs

Consumers can potentially adjust their diet in response to changing resource abundances, thereby achieving better foraging payoffs. Although previous work has explored how such adaptive foraging scales up to determine the structure and dynamics of food webs, consumers may not be able to perform perfect diet adjustment due to sensory or cognitive limitations. Whether the effectiveness of consumers' diet adjustment alters food-web consequences remains unclear. Here, we study how adaptive foraging, specifically the effectiveness (i.e. rate) with which consumers adjust their diet, influences the structure, dynamics, and overall species persistence in synthetic food webs. We model metabolically-constrained optimal foraging as the mechanistic basis of adaptive diet adjustment and ensuing population dynamics within food webs. We compare food-web dynamical outcomes among simulations sharing initial states but differing in the effectiveness of diet adjustment. We show that adaptive diet adjustment generally makes food-web structure resilient to species loss. Effective diet adjustment that maintains optimal foraging in the face of changing resource abundances facilitates species persistence in the community, particularly reducing the extinction of top consumers. However, a greater proportion of intermediate consumers goes extinct as optimal foraging becomes less-effective and, unexpectedly, slow diet adjustment leads to higher extinction rates than no diet adjustment at all. Therefore, food-web responses cannot be predicted from species' responses in isolation, as even less-effective adaptive foraging benefits individual species (better than non-adaptive) but can harm species' persistence in the food web as a whole (worse than non-adaptive). Whether adaptive foraging helps or harms species coexistence has been contradictory in literature. Our finding that it can stabilise or destabilise the food web depending on how effectively it is performed help reconcile this conflict. Inspired by our simulations, we deduce that there may exist a positive association between consumers' body size and adaptive-foraging effectiveness in the real world. We also infer that such effectiveness may be higher when consumers cognise complete information about their resources, or when trophic interactions are driven more by general traits than by specific trait-matching. We thereby suggest testable hypotheses on species persistence and food-web structure for future research, in both theoretical and empirical systems.

Adaptive foraging behaviour increases vulnerability to climate change

Adaptative foraging behavior should promote species coexistence and biodiversity under climate change as predators are expected to maximize their energy intake, according to principles of optimal foraging theory. We test these assumptions using a dataset comprising 22,185 stomach contents of fish species across functional groups, feeding strategies, and prey availability in the environment over 12 years. Our results show that foraging shifts from trait-dependent prey selectivity to density dependence in warmer and more productive environments. This behavioral change leads to lower consumption efficiency as species shift away from their optimal trophic niche, undermining species persistence and biodiversity. By integrating this adaptive foraging behavior into dynamic models, our study reveals higher risk profiles for ecosystems under global warming.

Top-down effects of foraging decisions on local, landscape and regional biodiversity of resources (DivGUD)

Foraging by consumers acts as a biotic filtering mechanism for biodiversity at the trophic level of resources. Variation in foraging behaviour have cascading effects on abundance, diversity, and functional trait composition of the community of resource species. Here we propose diversity at giving-up density (DivGUD), when foragers quit exploring a patch, as a novel concept and simple measure to quantify these effects at multiple spatial scales. In experimental landscapes, patch residency of wild rodents decreased local α-DivGUD (via elevated mortality of species with large seeds) and regional γ-DivGUD, while dissimilarity among patches in a landscape (ß-DivGUD) increased. Thus, DivGUD provides a framework linking theories of adaptive foraging behaviour with community ecology allowing to investigate cascading indirect predation effects across multiple trophic levels e.g. the ecology-of-fear framework; feedbacks between functional trait composition of resource species and consumer communities; and effects of inter-individual differences among foragers on the biodiversity of resource communities.

Adaptive foraging strategy of an insular snake (Lycodon semicarinatus, Colubridae) feeding on patchily distributed nests of sea turtles

Foraging tactics of predators generally include two major modes, active searching and ambushing. A colubrid snake, Lycodon semicarinatus, is a typical example of a predator, which uses both tactics to forage on sea turtles on islands of the Kerama Group in the Central Ryukyu Archipelago, Japan. To investigate factors that determine the foraging mode of this snake, we conducted a four-year field survey on its foraging behaviour on sea turtles on another island, Okinawa Island. We found that the snake performs only active searching at our study site. Snakes visited a small area exactly above the nest of sea turtles and attempted to burrow a tunnel to feed on eggs and hatchlings in the sand. Tunnels leading from the surface of the beach to the inside of the nest were formed only by large snakes. Many other snakes used the already made tunnels to capture eggs and hatchlings in the nest. When the snakes caught a hatchling, they brought the hatchling away into the nearby bush area without swallowing it above the nest (taking-away behaviour). When snakes failed to find food on a nest, they terminated the intensive search above the nest in approximately 5 minutes irrespective of snake body size, season, and the condition of the nest. Subsequently, they left the nest and resumed extensive searching for other nests. Our findings showed that L. semicarinatus has a different foraging strategy depending on populations. Two environmental traits, diversity of available prey animals other than sea turtles and characteristics of sand that beaches consist of, were considered as factors that might cause the difference in the foraging strategy. The fine sand of our study site enables snakes to form a sturdy tunnel in nests. We presume that such an environment facilitates the use of active searching by the snakes to find the nest with tunnels suitable for exploitation. The taking-away behaviour may be effective to reduce excessive contact with other conspecifics under the situation that the nest with tunnels attracts many visitors. Furthermore, the observation that the snake left the nest site after a consistent duration of unprofitable searching supports the giving-up time rule, which has been predicted by a theoretical model concerning the optimal time for predators to leave a patch.

Giving-up diversity (GUDiv): top-down effects of foraging decisions on local, landscape and regional biodiversity of resources

Foraging by consumers has direct effects on the community of their resource species, and may serve as a biotic filtering mechanism of diversity. Determinants of foraging behaviour may thus have cascading effects on abundance, diversity, and functional trait composition of the resource community. Here we propose giving-up diversity (GUDiv) as a novel concept and simple measure to quantify community effects of foraging at multiple spatial diversity scales. GUDiv provides a framework linking theories of adaptive foraging behaviour with community ecology. In experimental resource landscapes we showcase effects of patch residency of foraging wild rodents on α-GUDiv, ß-GUDiv and γ- GUDiv, and on functional trait composition of resources. Using GUDiv allows for prediction-based investigation of cascading indirect predation effects (ecology of fear) across multiple trophic levels, of feedbacks between functional trait composition of resource and consumer communities, and of effects of inter-individual differences among foragers on the diversity of resource communities.

Food restriction engages prefrontal corticostriatal cells and local microcircuitry to drive the decision to run vs conserve energy

ABSTRACTFood restriction (FR) evokes running, which may promote adaptive foraging in times of food scarcity, but can become lethal if energy expenditure exceeds caloric availability. Here, we demonstrate that chemogenetic activation of either the general medial prefrontal cortex (mPFC) pyramidal cell population, or the subpopulation projecting to dorsal striatum (DS) drives running specifically during hours preceding limited food availability, and not during ad libitum food availability. Conversely, suppression of mPFC pyramidal cells generally, or targeting mPFC-to-DS cells, reduced wheel running specifically during FR and not during ad libitum food access. Post-mortem c-Fos analysis and electron microscopy of mPFC layer 5 revealed distinguishing characteristics of mPFC-to-DS cells, when compared to neighboring non-DS projecting pyramidal cells: 1) greater recruitment of GABAergic activity and 2) less axo-somatic GABAergic innervation. Together, these attributes position the mPFC-to-DS subset of pyramidal cells to dominate mPFC excitatory outflow, particularly during FR, revealing a specific and causal role for mPFC-to-DS control of the decision to run during food scarcity. Individual differences in GABAergic activity correlate with running response to further support this interpretation. FR enhancement of PFC-to-DS activity may influence neural circuits both in studies using FR to motivate animal behavior and in human conditions hallmarked by FR.

Phenology and flowering overlap drive specialization in pollinator networks

Variation in diet breadth and specialization stems from fundamental interactions species have with their environment1-3. Consequently, understanding the drivers of this variation is key to understanding ecological and evolutionary processes, and will facilitate the development of predictive tools as ecological networks respond to environmental change4,5. Diet breadth in wild bees has been an area of focus due to both their close mutualistic dependence on plants, and because both groups are under threat from global biodiversity loss6. Though many of the principles governing specialization for pollinators have been identified7,8, they remain largely unvalidated. Using mechanistic models of adaptive foraging in pollinators9,10, we show that while temporal resource overlap has little impact on specialization in pollinators with extended flight periods, reduced overlap increases specialization as pollinator flight periods decrease. These results are corroborated empirically using pollen load data taken from bees with shorter (genus Andrena) and longer (genus Lasioglossum) flight periods across environments with both high and low temporal resource overlap. This approach reveals how interacting phenologies structure plant-pollinator networks and drive pollinator diet breadth via the temporal overlap of floral resources.