Rats dying for mice: Modelling the competitor release effect

Abstract Introduced vertebrate predators are one of the most important threats to endemic species throughout a range of ecosystems, in particular on islands in biodiversity hot spots. Consequently, the reduction of predator numbers is considered a key conservation action in the management of many native vertebrates vulnerable to predators. It is now established that control attempts may affect non‐target species through trophic interactions, but little is known concerning their consequences on competitive relationships. We study a mathematical model mimicking the effects of controlling introduced species in the presence of their competitors. We used two competing rodents to illustrate our study: black rats, Rattus rattus, and mice, Mus musculus. Analyses of the model show that control of only one introduced species logically results in the dramatic increase of the overlooked competitor. We present empirical data that confirm our theoretical predictions. Less intuitively, this process, which we term ‘the competitor release effect’, may also occur when both introduced competitors are simultaneously controlled. In our setting, controlling both predators can promote their coexistence. This occurs as soon as the inferior competitor benefits from the differential effect of the simultaneous control of both competitors, that is, when the indirect positive effect of control (the removal of their competitors) exceeds its direct negative effect (their own removal). Both control levels and target specificity have a direct influence on the extent of this process: counter‐intuitively, the stronger and more specific the control, the greater the effect. The theoretical validation of the competitor release effect has important implications in conservation, especially for control management.     

Stomatal Movements, Frequencies, and Resistances in Two Maize Varieties Differing in Photosynthetic Capacity

The Stomatal characteristics of two maize varieties previouslyfound to differ in rates of net photosynthesis were examinedin a controlled environment. Measurements with a ventilateddiffusion porometer showed that one variety exhibited a pronouncedand the other a weak periodicity in stomatal resistance of theadaxial epidermis. At equal illumination the stomatal resistanceof the adaxial epidermis decreased from upper to lower leaves,while the resistance of the abaxial epidermis changed in theopposite manner. Stomata on the adaxial and abaxial surfacesof maize leaves exhibited random not compensatory, movementsin a constant environment. The variety with the lesser stomatalfrequency and higher total leaf resistance to water loss hadnevertheless faster net photosynthesis than the variety withthe greater stomatal frequency, demonstrating the importanceof the so-called mesophyll resistance.     

MODELING OF SWEET, BITTER AND IRRITANT SENSATIONS AND THEIR INTERACTIONS ELICITED BY MODEL ICE WINES

Interactions between taste and irritant sensations elicited by model ice wine solutions were investigated, including the use of U and Γ′ models for predicting the perceived intensity of these sensory interactions. Fifteen solutions of varying ethanol and sugar concentrations representative of commercial ice wine values were evaluated in two trials by a trained sensory panel (n = 12) for perceived sweetness, bitterness and heat intensities. Sweetness perception of lower sugar‐concentration level in ice wine model solution was affected by ethanol concentration. The sweetness intensities of the sugar and ethanol mixtures are higher than the sweetness intensities of sugar solutions. The Γ′ index indicates a slight synergy between ethanol and sugar on sweetness perception. The bitterness intensities elicited by ethanol–sugar mixtures are lower than those elicited by unmixed ethanol solutions. The Γ′ index indicates inhibition of ethanol and sugar perception on bitterness perception. Suppression of heat sensation was found in model base wine solutions across sugar and ethanol concentrations.