Lack of Impact of Posidonia oceanica Leaf Nutrient Enrichment on Sarpa salpa Herbivory: Additional Evidence for the Generalist Consumer Behavior of This Cornerstone Mediterranean Herbivore

Abstract

The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean. Through direct and indirect mechanisms, this herbivore can exert significant control on the structure and functional dynamics of seagrass beds and macroalgae. Past research has suggested nutritional quality of their diet influences S. salpa herbivory, with the fish feeding more intensively and exerting greater top down control on macrophytes with higher internal nutrient contents. However recent findings have questioned this notion and shown that herbivores do not preferentially feed on macrophytes with higher nutrient contents, but rather feed on a wide variety of them with no apparent selectivity. To contribute to this debate, we conducted a field fertilization experiment where we enriched leaves of the seagrass Posidonia oceanica, a staple diet for S. salpa, and examined the response by the herbivore. These responses included quantification of leaf consumption in fertilized and non-fertilized/control plots within the bed, and food choice assays where fertilized and non-fertilized/control leaves were simultaneously offered to the herbivore. Despite the duration of leaf exposure to herbivores (30 days) and abundant schools of S. salpa observed around the plots, leaf consumption was generally low in the plots examined. Consumption was not higher on fertilized than on non-fertilized leaves. Food choice experiments did not show strong evidence for selectivity of enriched leaves. These results add to a recent body of work reporting a broad generalist feeding behavior by S. salpa with no clear selectivity for seagrass with higher nutrient content. In concert, this and other studies suggest S. salpa is often generalist consumers not only dictated by diet nutrient content but by complex interactions between other traits of nutritional quality, habitat heterogeneity within their ample foraging area, and responses to predation risk.

Stable isotopes reveal an invasive plant contributes more than native sources to anuran larvae diets

“…Organisms are faced with choices of dietary composition, where to search for the optimal sources, how much time to spend foraging, and how far to move between nutrient sources when attempting to maximize their individual fitness (Pyke 1984;Searle et al 2005;Bartumeus & Catalan 2009). Organisms selectively choose food sources to meet their preferred and balanced dietary needs (Bryant & Kuropat 1980;Sedinger & Raveling 1984;Felton et al 2016), but selection can be influenced by other factors, such as predation risk and abundant food choices within their diverse habitat (Marco-M endez et al 2016), altering the effect the choices have on their fitness and survival. To achieve a balanced diet, organisms choose to consume sources that maximize energy (Felton et al 2016), are nutrient rich (Felton et al 2009;Rothman et al 2011), and are void of toxins (Bryant & Kuropat 1980).…”

“…In cases where these novel resources have nutritive and toxic qualities that are unfamiliar to herbivores, it is unclear how (or if) foraging preferences will shift in invaded areas (Pyke 1984;Ehrenfeld et al 2003;Marco-M endez et al 2016). The impact of invaders to wetlands is vigorously debated.…”