Multitrophic diversity effects of network degradation

Abstract

Predicting the functional consequences of biodiversity loss in realistic, multitrophic communities remains a challenge. No existing biodiversity–ecosystem function study to date has simultaneously incorporated information on species traits, network topology, and extinction across multiple trophic levels, while all three factors are independently understood as critical drivers of post‐extinction network structure and function. We fill this gap by comparing the functional consequences of simulated species loss both within (monotrophic) and across (bitrophic) trophic levels, in an ecological interaction network estimated from spatially explicit field data on tropical fecal detritus producer and consumers (mammals and dung beetles). We simulated trait‐ordered beetle and mammal extinction separately (monotrophic extinction) and the coextinction of beetles following mammal loss (bitrophic extinction), according to network structure. We also compared the diversity effects of bitrophic extinction models using a standard monotrophic function (the daily production or consumption of fecal detritus) and a unique bitrophic functional metric (the proportion of daily detritus production that is consumed). We found similar mono‐ and bitrophic diversity effects, regardless of which species traits were used to drive extinctions, yet divergent predictions when different measures of function were used. The inclusion of information on network structure had little apparent effect on the qualitative relationship between diversity and function. These results contribute to our growing understanding of the functional consequences of biodiversity from real systems and underscore the importance of species traits and realistic functional metrics to assessments of the ecosystem impacts of network degradation through species loss.

Ecological functions provided by dung beetles are interlinked across space and time: evidence from 15N isotope tracing

“…The single functions approach helps to shed light on which and how many processes drive trends in multifunctionality, but additional evidence provided by stable isotope tracing allowed us to evaluate spatial and temporal trends in the processes controlling the fate of dung-derived N. The univariate perspective that addresses single time and space scales has the potential to generate erroneous conclusions about the functional role of biodiversity (Rosenfeld 2002, Duffy et al 2005, 2007, Srivastava and Vellend 2005, Nichols et al 2016. For example, mineralization processes were enhanced by beetles after 1 month, but the influence on nutrient uptake and herbage growth were detected only later.…”

Extinctions of interactions: quantifying a dung beetle–mammal network

“…As the total biomass of dung beetles is closely related to dung removal (Nichols et al. ), these extinction scenarios indicate the potential for the loss of the associated ecosystem functions provided by dung beetles, such as declines in seed dispersal (Culot et al. ), seed burial (Lugon et al.…”

“…, Emmons and Feer ) and raw proportional occupancy ( a j ), following Nichols et al. (): . The interaction frequency between dung beetle species i and mammal species j was based on dung beetle numbers recorded, defined for each trap type as: where p j is the total volume of dung of mammal species j in one trap type (bag or pitfall), and B ij represents the number of beetles captured through trapping of dung beetle species i on dung from mammal species j in one trap type.…”

“…, Nichols et al. ) and the role of secondary seed dispersers, such as ants and dung beetles, remains largely unexplored.…”

“…, Nichols et al. ), yet beyond estimating interactions from co‐occurrence data (Nichols et al. ), there are no dung beetle–mammal association networks and there is a lack of data demonstrating this association directly through trapping with native mammal dung (Nichols et al.…”