Predator-Prey Refuges

A predator prey refuge refers to an ecological system where the predator is excluded and a large prey population is placed to physically hide from predation. Therefore the refuge strategies that decrease the risk of predation include prey aggregations, temporal refuges and a reduction in the search activities by prey (Heilmann, Sneppen and Krishna). In addition to that the presence of the refuges promote the coexistence of species and may lead to stabilization or destabilization of an ecosystem where equilibrium points change from attractor to repeller. A refuge is an ecological concept where organisms are protected from biological interaction and a predator feeds on prey by hiding in an area that is inaccessible. The ability of a specie to survive is dependent on its choice of ecotype and the population sizes of all the species in that refuge. Consequently there is indirect competition between prey species and that stable resident systems with each species belonging to the same ecotype cannot be destabilized by mutant ecotypes that are selectively neutral. However, for two indirectly competing species in a refuge, destabilizing occurs. For stabilization the population of prey in the refuge must either decrease with increasing prey density or increase with both increasing predator density and pressure. The review of data on bacteria and phage demonstrates the factors that promote coexistence of predator and prey.

Predator Prey Refuge

Introduction

Harvesting, refuge and dispersal directly affect the dynamics of the ecosystem including existence, coexistence equilibrium and periodic solution (Hare and Rebazaa). One of the factors influencing predator prey interactions is the hiding behavior of prey such that they are safe from their predator. Furthermore, safe refuges provide the prey with protection from predation and reduce their chances of extinction (Wibowo, Suryanto and Pusawidjayanti). A refuge is an ecological concept where organisms are protected from biological interaction and a predator feeds on prey by hiding in an area that is inaccessible.  The ability of a specie to survive is dependent on its choice of ecotype and the population sizes of all the species in that refuge. Consequently there is indirect competition between prey species and that stable resident systems with each species belonging to the same ecotype cannot be destabilized by mutant ecotypes that are selectively neutral. However, for two indirectly competing species in a refuge, destabilizing occurs. This paper focuses on bacteriophage species that are virulent predators that determine the shaping of bacterial life and have coexisted with virulent phages for eons stably.

Discussion

 The suitability of a habitat is affected by the changes in abiotic conditions and can be (Kar)triggered by different distribution. Prey diversity is influenced by an increase in predation intensity by a characteristic pattern. Further, there are three prey-diversity responses which include, bimodal, unimodal and negative (Cressman and Garay). To investigate the responses of predators and preys four scenarios are examined, one which gives a negative response in a simple habitat whereby diversity only increases with decrease in predation density. The second scenario is of a dominant competitor that persists over predation intensities and finally where the dominant is at low predation intensity but a unimodal prey diversity (Hixon and Menge).

 Assessessing bacterial ecosystems shows the coexistence of virulent phage which is a remarkable predator because for every bacterial infection they regenerate to 100 copies (Kutter and Sulakvelidze). Analysis of patterns provides insight into processes such as responses from individuals to population assemblage and the structural ecological processes.

An extensive study by Coberly Lc examined questions related to phage bacteria coexistence, evolution, coexistence and dynamics of population. Moreover, virulent phage avoid driving their bacterial prey to extinction because they are involved in continuous and balanced coevolutionary race where bacteria avoids being destroyed by evolving resistance to the phage which then counter attacks resistant bacteria (Coberly). Therefore, selective predation is suggested to promote bacterial life in a biofilm and govern structural function relationships. However, this argument also referred to as the ‘Red Queen Argument’ is criticized on the grounds that the evolutionary rates of phage and bacteria are not symmetric since measurements of phage in soil appear to be ahead of the bacteria (Heilmann, Sneppen and Krishna).

A basic model for the coexistence of bacteria and phage illustrates that they cannot coexist because phage are inefficient and die out quickly at high bacterial density (Coberly). Consequently, they drive out bacteria to extinction then die out. In addition to that, for coexistence to be possible the degradation rate must be high enough to ensure that the phage offspring die before they can find new host. For the stabilization of predator prey population analysis of nonevolutionary mechanisms is required thus focus is applied on heterogeneity as one of the mechanisms.

Soil and biofilms are examples of environments with high degree of spatial heterogeneity since such refuges aid in coexistence of such species. According to Schrage and Metzler coexistence between bacteria and virulent phage is feasible in a chemo stat rather than serial cultures due to formation of biofilms refuge. Kerr et Al reviewed the impact of spatial heterogeneity using a cellular automaton that models fragmented populations of both phage and bacteria. The result was the evidence that coexistence is easily achieved when phage migration pattern is induced with spatial heterogeneity. According to macro ecology, prey refuges may help stabilize predator prey interactions and formation of this refuge results in a boundary zone between two different environments where there is increased biodiversity between different types of habitats.

Applying a stochastic model to research on the effects of bacterial refuges on bacterial and phage coexistence further explores the density dependent mechanisms such as quorum sensing triggered biofilms formation which facilitates creation of refuges in an organized manner. In addition to that the coexistence of in the basic model occurs for a narrow range of parameters in a 2 dimensional phage-bacteria ecosystem model where neither of the two can go extinct for up to 1000 bacterial generations. The sample number is selected for the purposes of providing enough time for testing mechanisms to determine whether they can stabilize the population long enough for evolution and so that the simulations can be completed in suitable time with the available resources. To research on how bacterial refuges enhance coexistence a spatial refuge is introduced in the basic model where two regions are chosen with one being the nonexistent region where phage are too inefficient to coexist and the other region where they are too efficient to coexist (Hixon and Menge).

The observations made for this analysis are that coexistence. Bacterial refuges can also be created by density dependent mechanisms whereby the refuge is determined before the simulation and occupies a fixed position in space with the dynamic enhancement of coexistence (Brazier). An examination on whether the mechanisms create a condition that is unfriendly in areas of high bacterial density are sufficient to produce robust coexistence. Consequently, dense bacterial colonies hinder the survival of phage due to fast depletion of nutrients and the changes in physiological conditions which limit the cells and impair the regulation receptors for adsorption (Kar). Moreover, alteration of the cell kinetics by cross linking the cell wall with richer and covalently bounded lipo protein prevents infection. Therefore, cells with low growth rate and metabolic activity are observed to have reduced burst size, prolonged latent times and decreased infection rate.

Evidently, the high density of bacteria triggers production o biofilm which reduces diffusibility because of high density of exopolymers produced by bacteria (Brockhurst, Buckling and Rainey). In addition to that, the cell to cell binding is tight thus blocking phage receptors and thereafter reducing phage infection. The proteolytic enzymes present in biofilms can lead to the inactivation of phage.

Conclusion

The coexistence of phage and bacteria is dependent on density and becomes stable on boundaries between habitats which if separated are hostile to either bacterial or phage refuges. Spatial heterogeneity which is reflected in soil or biofilms and ocean data show a variance in the densities of both phage and bacteria since in oceans heterogeneity can be organized by cyanobacteria. The bacteria at high density creates s hostile environment for phage. For instance, the use of quorum sensing mechanisms to trigger formation of biofilms that is invisible to phage attack creates harsh environment for phage. For a predator prey refuge to maintain the richness of species it has to promote coexistence by developing mechanisms that will not lead to the extinction of the prey or the predator. Further, it has to support interspecific competition and interactive variation in predation.

References

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