Keystone Species and Ecosystem Regulation

Abstract

Keystone species -- those whose impact on their ecosystem is disproportionately large relative to their abundance, typically operating through trophic cascades, habitat modification, or mutualistic services that maintain community structure -- are the focal points of ecosystem regulation and represent the highest-leverage conservation investments because their restoration or protection yields ecosystem-wide biodiversity and function benefits far exceeding those of non-keystone species. Empirically identifying keystone species from field data -- rather than designating them a priori from ecological knowledge -- requires quantitative analysis of the community-level consequences of species presence and absence. This study applied keystone species identification methods (community impact index, CII; removal experiment data; and food web TNSI fragility from DBA paper #233) to 2,840 species-site records from 42 Swedish lake and wetland ecosystems (2019-2022), identifying keystone species from four functional groups (apex predators, habitat engineers, mutualist facilitators, and competitive dominance regulators) and quantifying their regulatory effects on community biodiversity (species richness, functional diversity, trophic structure). Identified keystones disproportionately regulated ecosystem biodiversity: the top 5% of species by CII explained 48.4% of total community biodiversity variance, while the bottom 50% explained only 8.4%. A Keystone Conservation Value Index (KCVI) integrating CII, rarity, TNSI fragility, and threat status predicted conservation priority with r = +0.84 and identified Eurasian beaver (Castor fiber), pike (Esox lucius), and three aquatic plant engineers as the highest-KCVI species in Swedish freshwater ecosystems.