The "Hybrid Bridge" hypothesis: host shifting via plant hybrid swarms

Article Abstract:

The hybrid bridge hypothesis, which argues that plant hybrids facilitate shifting of herbivores to new plant hosts, was discussed. Current explanations of the mechanism for host shifting is based on preadaptation hypothesis, which involves preadaptation by an herbivore before it can make the shift to a new plant host, or at least a key mutation has to occur soon after shifting to ensure survival in the new host. The hybrid bridge hypothesis, on the other hand, considers the role of naturally occurring hybrid plants that morphologically, genetically and spatially bridge the gap between parental species and allow herbivores to shift hosts gradually.

Group selection (Evolution), Group selection (Natural selection)

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Interactions between Abert's squirrel and ponderosa pine: the relationship between selective herbivory and host plant fitness

Article Abstract:

The dependence of Abert's squirrels, a highly selective and specialized herbivore, on its host ponderosa pine leads to deteriorating fitness levels and seed quality. Abert's squirrels also show a marked preference for certain specific individual ponderosa trees to use as sources of inner bark, and thus aid in their natural selection. The reductions in host fitness result from energetic constraints caused by the squirrel's feeding activities. Squirrels consume most of the upper crown of target trees, which are rich in photosynthetic tissues, and this retards growth and reproduction.

Environmental aspects, Food and nutrition, Squirrels, Ponderosa pine

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Assessing functional explanations of host-specificity

Article Abstract:

A modification of some models for the evolution of host range or habitat specificity in parasite species is proposed. Mathematical equations expressing the conditions for specificity in terms of the proportion of dispersing parasites that find hosts were derived. The specificity conditions were simplified by assuming equilibrium state for the parasite population, and are then used to explain host-specificity. The proposed modification eliminates the need for difficult-to-measure parameters such as 'rate of encounter' and 'instantaneous mortality.'

Models, Parasitism

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