Ecosystems depend on the maintenance of multiple processes and functions across space and time under environmental-change scenarios. We therefore require indices that allow us to address the multifunctionality offered by natural systems. Functional metrics are seen as descriptors of ecosystem function, providing a more direct and mechanistic link to ecosystem processes, and by extension to ecological resilience.
Here we employ two functional indices: functional dispersion and functional redundancy, which relate to a systems’ capacity to respond to disturbance and its insurance from species loss respectively. We combine these indices into ‘functional integrity’ which we define as ‘the resilience afforded by the range and distribution of functions in a system, represented by organismal traits’. We quantify functional integrity (functional dispersion and functional redundancy) for the 825 terrestrial ecoregions across the globe, utilising the occurrence and life-history traits of 5,232 mammals and 10,253 birds. Six traits were systematically selected (based on data availability and multicollinearity) that together capture the essence of mammal and bird form and function: body mass, litter/clutch size, diel activity, volancy (flight capability), habitat breadth and diet category. These traits dictate how species respond to change and determine their effect on function.
Functional integrity was found to be highest in the Neotropics (especially the Amazon and surrounding forest), indicating that this region has the highest ecological resilience. This was largely due to higher functional redundancy than expected at random. Low function systems are widespread at high latitudes including eastern North America, Eurasia, Australia and New Zealand. Where ecoregions often had no functional redundancy (i.e. each species performed a unique function), these systems are particularly vulnerable to collapse or degradation if species (and therefore functions) are lost. On average birds showed greater functional redundancy than mammals, providing insurance against the loss or decline of species. Whereas mammals contributed to areas of higher functional dispersion, which may help to buffer communities from various environmental changes, as a diversity of responses should lead to some species successfully responding to a particular environmental change.
Overall we identify limited areas of high functional integrity and therefore many areas where function may be impaired, making these systems less resilient to disturbance. In these low function systems key ecosystem processes, such as productivity, nutrient cycling, and decomposition may be/become degraded, with undisputed consequences on ecosystem services, which humanity depends upon.