Measuring Fitness and Importance of Species in Food Webs

Abstract: Ecosystems face intensifying threats from climate change, overexploitation, and other human pressures, emphasizing the urgent need to identify keystone species and vulnerable ones. While established network-based measures often rely on a single metric to quantify a species' relevance, they overlook how organisms can be both carbon providers and consumers, thus playing a dual role in food webs. Here, we introduce a novel approach that assigns each species two complementary scores -- an importance measure quantifying their centrality as carbon source and a fitness measure capturing their vulnerability. We show that species with high importance are more likely to trigger co-extinctions upon removal, while high-fitness species typically endure until later stages of collapse, in line with their broader prey ranges. On the other hand, low fitness species are the most vulnerable and susceptible to extinctions. Tested on multiple food webs, our method outperforms traditional degree-based analyses and competes effectively with eigenvector-based approaches, while also providing additional insights. Relying solely on interaction data, the approach is scalable and avoids reliance on expert-driven classifications, offering a cost-effective tool for prioritizing conservation efforts.