Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Tradeoffs of food and safety in contrasting environments : from the deserts of the Mojave and the Negev to the Coral Reefs of Eilat

עקרון ההקצאה בסביבות מנוגדות : מהמדבריות של מוהאבי והנגב אל שוניות האלמוגים של אילת.

Auteur : Dixon, Austin K.,

Etablissement de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2017

Résumé partiel
Coexistence can occur when a tradeoff exists between two species along an axis of heterogeneity. In many cases, the tradeoff incorporates a forager that is more efficient in the absence of risk and one that is more efficient in the presence of risk, thus hinging on tradeoffs of food and safety. Studying coexistence and tradeoffs of food and safety have helped in understanding the mechanisms operating within communities and addressing many of the questions relating to their structure. In this dissertation, I examine tradeoffs of food and safety in contrasting environments ; this includes damselfish on Red Sea coral reefs and granivorous rodents from the Mojave and Negev Deserts. Examining these environments, I use tradeoffs of food and safety and foraging ecology to further the understanding of particular questions related to spatial structuring, community structure, and personalities. But I also discuss possible commonalities between such contrasting environments which may help ecologists to work towards more generalized theories of ecology.

The first part of this dissertation is a theoretical extension of Brown’s (1988) optimal patch use model, which predicts that individuals should forage in a patch until their harvest rates become equal to their energetic, predation, and missed opportunity foraging costs. Its use, combined with the technique of giving-up densities, has been used extensively in many ecological disciplines, including foraging, behavioral, conservation, and community ecology. Despite this, the model ignores the spatial environment and direct interactions of foragers, which limits the understanding of animal distributions and patch use across time. We extend the original model to include a forager’s spatial environment and interactions while maintaining the potential for spatial changes across time. Using this new model and planktivorous damselfishes as an example, we make predictions regarding mechanisms producing site attachment (e.g. Dascyllus sp., Chromis sp.) and pelagic (e.g. Caesio sp.) lifestyles observed in planktivorous fishes. The model suggests that site attachment occurs when energetic and/or predation costs are high and push foragers closer to refuge. When these costs are low, foragers should instead move higher into the water column and spread out horizontally. We also use the new model to predict group distributions, showing that when energetic gains from forager interactions are greater than their energetic costs and/or the cost of isolation increases, then fish form schools. When the opposite is true, individuals should be solitary. Further discussion of these results and the model’s relevance for other systems (e.g. fission-fusion social dynamics) provide a simple, but more complete picture of patch use across a range of disciplines and environments.

In the second part of the dissertation, the theoretical framework is used to begin building a mechanistic understanding of spatial structure on coral reefs. Space is a limiting resource on coral reefs that has resulted in many hypotheses for explaining the observed spatial structure. Despite these many hypotheses over decades of research, a mechanistic understanding explaining why these spatial structures develop is lacking. One of the most abundant spatial structures on coral reefs is the site-attachment of planktivorous fishes. Here we utilize acoustic and optical technology in a new method to study the foraging behavior of a site-attached damselfish (Dascyllus marginatus) and its use of space. Acoustics measure of prey density, current magnitude, and overall prey flux, while optics in a stereo array provide the X, Y, and Z coordinates of each fish in a group. Combined with a theoretical spatial patch use equation, the positions of fish are compared with patch richness along different spatial dimensions to determine how foraging costs are mitigated. Results show that D. marginatus used its vertical dimension to control energy and predation costs and the horizontal plane to handle competition costs. These results suggest site-attachment occurs because 1) the optimal behavior is to move vertically above a refuge, thereby controlling energy and safety, and 2) prey must be replenished at a high enough rate to prevent starvation at the site of attachment.

Présentation et version intégrale (PRIMO)