Ideal free flows of optimal foragers: Vertical migrations in the ocean

We consider the collective motion of animals in time-varying environments, using as a case diel vertical migration in the ocean. The animals are distributed in space such that each animal moves optimally, seeking regions which offer high growth rates and low mortalities, subject to costs on excessive movements as well as being in regions with high densities of conspecifics. The model applies to repeated scenarios such as diel or seasonal patterns, where the animals are aware of both current and future environmental conditions. We show that this problem can be viewed as a differential game of mean field type, and that the evolutionary stable solution, i.e. the Nash equilibrium, is characterized by partial differential equations, which govern the distributions and migration velocities of animals. These equations have similarities to equations that appear in the fluid dynamics, specifically the Euler equations for compressible inviscid fluids. If the environment is constant, the ideal free distribution emerges as an equilibrium. We illustrate the theory with a numerical example of vertical animal movements in the ocean, where animals are attracted to nutrient-rich surface waters while repulsed from light during daytime due to the presence of visual predators, aiming to reduce both proximity to conspecifics and swimming efforts. For this case, we show that optimal movements are diel vertical migrations in qualitative agreement with observations.

The Old with a Fresh Outlook or the Issue of Protagonists’ Identity in Relation to Time

Roland Giorgadze regularly publishes his poetry and prose in Georgian literary journals and newspapers and thus humbly contributes to the development of Georgian literary processes. The bulk of his work is presented by the following publications: Dream President (2001), The Road (2011), The Cry (2017) and Diso, Diso (2020).This time we are dealing with the novel Diso, Diso. Here, the writer described life in Georgia in the 90-ies of the 20th century in gloomy, yet, very realistic colors without any embellishment. Some very accurate stresses are made upon the poor spiritual state of the country and upon the tragic reality it causes. The novel tries to show the deplorable reasons why a human being in this ideal-free society is doomed to become degraded even if they experience a rise (before a fall). The evil has a new beginning and life in every epoch and this new beginning is presented through the character of Elepter in the novel. The novel has a slightly documentary hue. The author openly discusses all the problems. His political flair is infallible and he has a good sense of judging and evaluating current events. All walks of life are branded with the mark of a country not properly attended. Human values and selfless love for one’s homeland are no longer priorities. Diso, Diso is full of didactic maxims that clearly show the life experience of the author, his adherence to lofty values. Roland Giorgadze understands well the purpose of a literary work and that is why all his words and phrases are loaded with meaning. The author manages to convey a meaningful message is a compressed, verbally laconic way. The writer uncovers the dramatic scenes of spiritual poverty marked with time, existential details of economical squalor, that cause the fear of national endangerment and the threat of looming genocide. The criticizing and uncovering the national and state faults should not be understood as self-flagellation. It aims at making readers think and also reminding them about the importance of moral life, being close to the creator, about love and the good of forgiving. The author sees the preserving national identity through religious faith. He also sets the ways of coping with these problems in this gloomy scene of national and religious degeneration.To our judgment, Diso, Diso is an interesting publication on the current literary scene and we hope it will get the attention of both general readers and literary critics.

The joint evolution of movement and competition strategies

Competition typically takes place in a spatial context, but eco-evolutionary models rarely address the the joint evolution of movement and competition strategies. Here we investigate a spatially explicit producer-scrounger model where consumers can either forage on a heterogeneous resource landscape or steal prey from conspecifics (kleptoparasitism). We compare different scenarios for the interaction of movement and competition strategies. In all cases, movement strategies evolve rapidly and consistently across replicate simulations. At equilibrium, foragers do not match the prey input rate, contrary to 'ideal free' predictions. We show that this is related to the intrinsic difficulty of moving effectively on a depleted landscape with few reliable cues for movement. The evolved movement strategies of kleptoparasites differ markedly from those of foragers. Even within each competition strategy, polymorphisms emerge, corresponding to individual differences in movement rules. Our study emphasises the advantages of a mechanistic approach when studying competition in a spatial context.

Ideal free eagles: Bald Eagle distribution in relation to salmon availability on four salmon spawning rivers

The movement of individuals according to the availability of resources has a fundamental effect on animal distributions. In the Pacific Northwest, Bald Eagles (Haliaeetus leucocephalus (Linneaus, 1766)) rely heavily on scavenging opportunities during the non-breeding period, and their distribution and movements are thought to be strongly influenced by the availability of post-spawning Pacific salmon (genus Oncorhynchus Suckley, 1861) carcasses. We surveyed the abundance of eagles and salmon on four adjacent rivers on Vancouver Island, British Columbia, Canada, during the 2017 fall spawning season. Salmon began to arrive in late September, peaked in abundance in mid-November, and were absent after early December. The seasonal progression of Bald Eagle abundance matched that of salmon carcass availability. The slope of proportional eagle – salmon relationship was significantly positive, though lower than the 1:1 match predicted by Ideal Free Distribution theory. The numerical response of Bald Eagles to salmon abundance was elevated on one of the rivers, potentially due to physical features such as sandbars and mudflats that increased the availability of carcasses and provided access points for eagles.

No neighbour-induced increase in root growth of soybean and sunflower in mesh divider experiments after controlling for nutrient concentration and soil volume

Root competition is a key factor determining plant performance, community structure and ecosystem productivity. To adequately estimate the extent of root proliferation of plants in response to neighbours independently of nutrient availability, one should use a setup that can simultaneously control for both nutrient concentration and soil volume at plant individual level. With a mesh-divider design, which was suggested as a promising solution for this problem, we conducted two intraspecific root competition experiments one with soybean (Glycine max) and the other with sunflower (Helianthus annuus). We found no response of root growth or biomass allocation to intraspecific neighbours, i.e. an ‘ideal free distribution’ (IDF) norm, in soybean; and even a reduced growth as a negative response in sunflower. These responses are all inconsistent with the hypothesis that plants should produce more roots even at the expense of reduced fitness in response to neighbours, i.e. root over-proliferation. Our results suggest that neighbour-induced root over-proliferation is not a ubiquitous feature in plants. By integrating the findings with results from other soybean studies, we conclude that for some species this response could be a genotype-dependent response as a result of natural or artificial selection, or a context-dependent response so that plants can switch from root over-proliferation to IDF depending on the environment of competition. We also critically discuss whether the mesh-driver design is the ideal solution for root competition experiments.

Breakdown of the ideal free distribution under conditions of severe and low competition

Under the ideal free distribution (IFD), the number of organisms competing for a resource at different sites is proportional to the resource distribution among sites. The ideal free distribution of competitors in a heterogeneous environment often predicts habitat matching, where the relative number of individuals using any two patches matches the relative availability of resources in those same two patches. If a resource is scarce, access might be restricted to individuals with high resource holding potential, resulting in deviation from the IFD. The distribution of animals may also deviate from the IFD in the case of resource abundance, when social attraction or preference for specific locations rather than competition may determine distribution. While it was originally developed to explain habitat choice, we apply the habitat matching rule to microscale foraging decisions. We show that chickens feeding from two nondepleting feeders distribute proportionally to feeder space under intermediate levels of competition. However, chicken distribution between the feeders deviates from the IFD when feeder space is limited and competition high. Further, despite decreasing aggression with increasing feeder space, deviation from IFD is also observed under an excess supply of feeder space, indicating different mechanisms responsible for deviations from the IFD. Besides demonstrating IFD sensitivity to competition, these findings highlight IFD’s potential as a biological basis for determining minimal resource requirements in animal housing. Significance statement The ideal free distribution (IFD) predicts how animals ought to distribute themselves within a habitat in order to maximize their payoff. Recent studies, however, have questioned the validity of the IFD concept following anomalous results. We studied the IFD in chickens by systematically varying the amount and distribution of space at two feed troughs. We show that when tested over a sufficiently large range, the distribution of birds depends on the overall resource availability. Furthermore, behavioral data suggest that distinctly different mechanisms account for deviations from the IFD at shortage and excess supply of feeder space, respectively.

Specialist Birds Replace Generalists in Grassland Remnants as Land Use Change Intensifies

The ideal free distribution theory predicts that mobile species distribute themselves among habitat patches so as to optimize their fitness. Changes in land use alter the quality of habitat patches and thereby affect the distribution of species. Following the loss of native habitat, habitat specialists are expected to move to patches where native habitat still remains in order to survive. Competition for resources in habitat remnants should consequently increase. As generalists are able to use other habitats, generalists are expected to gradually disappear in remnants in order to avoid increasing competition with specialists. Here, we test these predictions by studying the response of habitat specialist and generalist birds to land-use change in Brazil's southern grasslands. Using a space-for-time substitution approach, we surveyed bird communities in native grassland sites (~4 ha) in 31 regions (10 × 10 km) with differing levels of conversion to agriculture (1–94%). We found a higher abundance of specialists in native grassland patches with increasing agricultural cover in the region, while the total number of individuals in remnants remained constant. At the same time, the share of generalists in total abundance and total species richness decreased. To gain insights into whether these patterns could be driven by shifts in competition, we tested whether generalists that continued to co-occur with specialists in remnants, had less dietary overlap with specialists. As a consequence of community composition in remnants, a higher proportion of generalists were omnivorous and the average generalist species fed less on seeds, whereas the average specialist species fed more on seeds when agricultural cover was high in the region. Our results, therefore, support predictions of the ideal free distribution theory. Specialists that are assumed to have a low survivorship outside of their specialized habitat, distribute to remnants of this habitat when it is converted elsewhere, while generalists, being able to survive in other habitats, disappear gradually in remnants. Such a process could partly explain the segregation of habitat specialist and generalist birds observed in many agricultural landscapes. Finally, our results suggest that native habitat remnants can be important temporary refugia for specialists.