Abstract
The issues of food quality and food quantity are crucial for trophic interactions. Although most research has focussed on the primary producer – herbivore link, recent studies have shown that quality effects at the bottom of the food web propagate to higher trophic levels. Negative effects of poor food quality have almost exclusively been demonstrated at higher food quantities. Whether these negative effects have the same impact at low food availability in situations where the majority if not all of the resources are channelled into routine metabolism, is under debate. In this study a tri-trophic food chain was designed, consisting of the algae Rhodomonas salina, the copepod Acartia tonsa and freshly hatched larvae of the European lobster Homarus gammarus. The lobster larvae were presented with food of two different qualities (C∶P ratios) and four different quantities to investigate the combined effects of food quality and quantity. Our results show that the quality of food has an impact on the condition of lobster larvae even at very low food quantities. Food with a lower C∶P content resulted in higher condition of the lobster larvae regardless of the quantity of food. These interacting effects of food quality and food quantity can have far reaching consequences for ecosystem productivity.
Highlights
The quality and quantity of food available to consumers influences development, growth and reproduction and thereby shapes trophic interactions and food web dynamics
An Analysis of Covariance (ANCOVA) was run showing statistically highly significant effects for food quantity (p,0.001) and quality (p,0.001). These results indicate that the condition (RNA:DNA ratio) of the lobster larvae is affected by both the quantity and the quality of their food
Differences in the behaviour of the fed and non-fed larvae were observed, with the fed animals displaying sustained bursts of swimming activity during their hunt for the prey items, while the starved animals remained inert and did not show a great deal of activity. This is the first study on the combined effect of food quality and food quantity on a secondary consumer that we are aware of, building on the previous work of Boersma and Kreutzer [13]
Summary
The quality and quantity of food available to consumers influences development, growth and reproduction and thereby shapes trophic interactions and food web dynamics. Herbivorous consumers are often faced with low quality food as plants contain high amounts of C and relatively low amounts of P and N, making the latter nutrients potentially limiting [1,3]. Herbivores regularly face imbalanced food in terms of energy and nutrients, which negatively affects their growth, development and reproduction [1]. Under low quality food conditions (high C:nutrient content) animals are faced with an excess of carbon in their food. Under low nutrient conditions growth and reproduction can be nutrient limited, as both N and P are required for production [8,9,10]. Low algal food quality will result in reduced population growth rates of herbivores, resulting in a decrease of ecosystem productivity
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