Scale-dependent responses of plant biodiversity to nitrogen enrichment.
Experimental studies demonstrating that nitrogen (N) enrichment reduces plant diversity within individual plots have led to the conclusion that anthropogenic N enrichment is a threat to global biodiversity. These conclusions overlook the influence of spatial scale, however, as N enrichment may alter beta diversity (i.e., how similar plots are in their species composition), which would likely alter the degree to which N-induced changes in diversity within localities translate to changes in diversity at larger scales that are relevant to policy and management. Currently, it is unclear how N enrichment affects biodiversity at scales larger than a small plot. We synthesized data from 18 N-enrichment experiments across North America to examine the effects of N enrichment on plant species diversity at three spatial scales: small (within plots), intermediate (among plots), and large (within and among plots). We found that N enrichment reduced plant diversity within plots by an average of 25% (ranging from a reduction of 61% to an increase of 5%) and frequently enhanced beta diversity. The extent to which N enrichment altered beta diversity, however, varied substantially among sites (from a 22% increase to an 18% reduction) and was contingent on site productivity. Specifically, N enrichment enhanced beta diversity at low-productivity sites but reduced beta diversity at high-productivity sites. N-induced changes in beta diversity generally reduced the extent of species loss at larger scales to an average of 22% (ranging from a reduction of 54% to an increase of 18%). Our results demonstrate that N enrichment often reduces biodiversity at both local and regional scales, but that a focus on the effects of N enrichment on biodiversity at small spatial scales may often overestimate (and sometimes underestimate) declines in regional biodiversity by failing to recognize the effects of N on beta diversity.
Chalcraft
David R
DR
Department of Biology, East Carolina University, Greenville, North Carolina 27858, USA. chalcraftd@ecu.edu
Cox
Stephen B
SB
Clark
Christopher
C
Cleland
Elsa E
EE
Suding
Katharine N
KN
Weiher
Evan
E
Pennington
Deana
D
D044822:Biodiversity; D018533:Biomass; D005308:Fertilizers / Q000032:analysis; D009584:Nitrogen / Q000737:chemistry / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*
D005308:Fertilizers; D009584:Nitrogen