Abstract
In the face of global change it is important to understand how changes in species abundance and richness can affect ecosystem functions. Here we modelled seed dispersal by animals in a fragmented secondary forest of the Cantabrian Range (northwestern Iberian Peninsula), simulating the activity of six frugivorous bird species when dispersing three species of fleshy‐fruited trees. We calculated the density and richness of seeds deposited across a forested landscape, as well as the density of seeds arriving to open areas. We 1) study the complementarity of functional traits of each species with frugivore assemblages varying in species compositions (i.e. abundance and richness of bird assemblages), 2) identify those bird species whose functional roles are not redundant, and 3) explore the response of seed dispersal to random losses and to two non‐random bird loss scenarios (i.e. overhunting and fewer individuals from migrant species). We found that simulations with the avian composition observed in the field (i.e. with uneven abundances of six bird species) led to values of seed dispersal higher to those emerging from four bird species equally abundant. The selective removal of dominant bird species led to significant decays in seed dispersal, suggesting non‐redundant roles of abundant bird species. Seed dispersal decays were stronger under non‐random than random scenarios of bird loss. In terms of seed density, the functional decays also differed between the scenarios of overhunting and reduced arrival of migrant birds, notably beyond 50% changes in bird species composition. Our results illustrate the need to integrate species composition (controlling for bird abundance and richness) and their sensitivity to disturbances when predicting the impact of global change on ecosystem functions.
Highlights
The decline of ecological functions resulting from global change depends on how extinction drivers alter species communities (Duffy, 2003; Elmqvist, et al, 2003)
Along an increasing gradient of bird richness, we found that the observed bird composition generated non-linear responses in seed density and richness, linear trends in seed density in open microhabitats
Detailed analyses showed that the values of all seed dispersal components generated by the observed bird composition tended to be higher than those predicted by the combination of fourspecies evenly abundant
Summary
The decline of ecological functions resulting from global change depends on how extinction drivers alter species communities (Duffy, 2003; Elmqvist, et al, 2003). The magnitude and the stability of ecosystem functions depend on the composition of biotic communities, as species may differ in their abundance, and in morphological and behavioural traits (Wardle et al 2011; Luck et al 2012), and on the interactions betweenspecies (Naeem& Wright, 2003;Balvanera et al 2006; Schleuning et al 2015). Complementarity in species traits and interactions within communities leads to an increased magnitude of ecosystem functions (Díaz et al 2013; Schleuning et al 2015). Species with common traits may provide stability to ecosystem functions due to redundancy and the loss of one of these species would not lead to strong ecological disruptions. Predicting changes in an ecosystem under specific drivers of global change requires forecasting of the variations in functional mechanisms surrogate to a given species composition, but alsounderstanding how the alteration of species assemblages compromise stability and resilience to particular ecosystem functions (Larsen et al 2005; Hillebrand and Matthiessen 2009)
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