Ground-dwelling arthropods and environmental factors in two semi-arid habitats: data from 1992-2004

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Amaris Swann
Sandra L. Brantley
David Lightfoot
Julieta Bettinelli

Ground-dwelling arthropods, primarily predators and detritivores, form a large part of the energy flow through ecosystems, but there are few long-term studies looking at many taxa. These animals have been monitored at the Sevilleta National Wildlife Refuge in central New Mexico, a Long-Term Ecological Research (LTER) site since 1990. We report on patterns in relative abundance from 2 study sites: desert grassland and creosotebush shrubland. Arthropods were collected in pitfall traps, operated year-round and collected every 2 months. Our questions focused on factors that might influence the abundance of several family groups and what that might mean for their ecological functions as climate changes. 1) Do the ground-dwelling arthropods respond to interannual changes in precipitation or other environmental factors such as temperature extremes and net primary production (aNPP)? 2) Because a number of the target taxa are long-lived, are there correlations between arthropod abundance and environmental conditions from the previous season or year (time lags)? We grouped the data into 2 seasons: winter (Nov-May) and summer (Jun-Oct), the period when the area receives most of its precipitation. Precipitation during the dry period is greatly influenced by the El Nino Southern Oscillation (ENSO).

We used correlations and ANOVAs of log-transformed abundance data to determine the most influential factors from among temperature, precipitation and aNPP. To examine changes in abundance over time we used an ordination based on Euclidean distance and a cluster analysis to produce dendrograms for the sites and seasons. We also looked for changes in family ranks as a measure of the stability of the communities.

Results from the analyses showed the most important environmental factors affecting arthropod abundance were minimum winter temperature and aNPP. There was no trend associating abundance with ENSO events. Similarly there were no large changes in family ranks: those in the top five tended to remain there, and lower ranked families rarely broke into the top five. Analysis of trophic groups was less informative than analysis of taxonomic families.

The community of ground-dwelling arthropods is largely stable over long periods. While they are relatively insensitive to climate changes lasting a year or two, such as ENSO events, they provide a reliable base for continued ecological functions. The families respond independently. Our results showed that minimum winter temperatures and aNPP were strongly correlated with arthropod abundance, so that these taxa may be sensitive over the longer term to the extreme events associated with climate change.