Space-size relative to prey width (Sp/Py) and total cover in an area (Ct/At) influence the habitat choices of desert beetles in the field and laboratory freshwater angelfish Pterophyllum scalare in a similar manner

Background/Question/Methods

We present a model for prey survivorship varying with average space size / prey width (Sp/Py) and total cover within an area (Ct/At), which are indices of habitat complexity. The model predicts that prey survivorship is maximized at intermediate Sp/Py values. At high Sp/Py values, both predators and prey can move through the spaces, and prey are vulnerable. If space size / predator width (Sp/Pr) < 1 but Sp/Py > 1 then prey survival is maximized (assuming Pr > Py). If Sp/Py < 1, prey are excluded from using the habitat as a refuge.  The model also predicts that prey survivorship increases rapidly from zero to low cover, but increases at a slower rate as cover increases further. This is because as Ct/At increases, the amount of habitat obstructed from sensory detection by predators increases, but with diminishing returns. We used artificial structures to test the habitat choices of small and large desert beetles in the field and freshwater angelfish Pterophyllum scalare in the lab against our model’s predictions, assuming that prey chose habitats providing better refuge. 

Results/Conclusions

Large beetles significantly preferred medium complexity artificial structures over low and high complexity structures. Small beetles exhibited no preference. P. scalare exhibited different choice patterns for Sp/Py depending upon cover level. At constant, low cover, fish did not exhibit any significant preferences for space size, at constant, medium cover, fish exhibited a significant preference for a narrower-space structure over a wider-space structure, and at constant, high cover, fish exhibited significant preferences for a wider-space structure over narrower-space structures. At constant Sp/Py,  P. scalare exhibited a marked preference for a higher cover structure at low cover levels. P. scalare also preferred the higher cover structures at higher cover levels, but these preferences were not as pronounced. Our results, except for the small beetle results, support our model. Sp/Py and Ct/At may both influence habitat choices of different prey organisms in different habitats.