Changes in function and temporal variation in a guild of gall-parasitoids across a temperature gradient in Australian subtropical rainforest CASEY R. HALL, 1, * CHRIS J. BURWELL 1,2 AND ROGER L. KITCHING 1 1 Environmental Futures Research Institute (EFRI), Griffith School of Environment, Griffith University, 170 Kessels Road, 4111 (Email: casey.hall@griffithuni.edu.au), and 2 Natural Environments Program, Queensland Museum, Brisbane, Queensland, Australia Abstract Parasitoids play an important role in ecosystem functioning through their influence on herbivorous insect populations. Theoretical and experimental evidence suggest that increased species richness can enhance and stabilize ecosystem function. It is important to understand how richness-driven functional relationships change across environmental gradients. We investigated how temperature affected the relationship between parasitoid richness and parasitism rate in a guild of gall-parasitoids along an elevational gradient.We collected galls at 15 sites along five elevational gradients (between 762 m and 1145 m asl) on six occasions over a year. A total of 1902 insects, including 1593 parasitoids, were reared from 12 402 galls. Parasitism rate increased significantly with temperature on all sampling occasions, except December and February.We found a significant, positive richness– parasitism relationship. This relationship, however, was weaker at higher elevations which may be linked to decreased functional efficiency of parasitoids at lower temperatures. Temporal variability in parasitism rate and parasitoid richness were significantly related, regardless of temperature. A stable functional guild of this kind may provide a more reliable ecosystem service under environmental changes. Key words: Cecidomyiidae, elevation gradients, parasitism, rainforest, trophic interactions. INTRODUCTION Environmental gradients, by selecting for certain func- tional traits, affect species in a non-random manner and can determine how species interact with each other (Hodkinson 2005). Elevational gradients, in particular, create pronounced patterns in species dis- tributions due to the contrasting environmental con- ditions found over short geographical distances (Körner 2007). In general, decreasing temperatures with increasing elevation may constrain insect traits, such as thermal tolerance and phenology, leading to reduced species richness, turnover and functional diversity (Hodkinson 2005). Usually studies of elevational patterns in insect richness and assemblage turnover focus on a single taxon or trophic level (e.g. moths, Ashton et al. 2011; ants, Burwell & Nakamura 2011; dung beetles, Escobar et al. 2005), and elevational effects on species interactions are seldom taken into account (but see Maunsell et al. 2014). Parasitoids provide an interesting case study as they play significant roles in ecosystem functioning (LaSalle & Gauld 1993), yet are constrained in their responses to environmental gradients by changes in host traits. Few studies have looked explicitly at how elevation affects host–parasitoid interactions. Those that have, have found that the decrease in parasitism rates with increasing elevation is disproportionately greater than that of their host populations (Virtanen & Neuvonen 1999; Maunsell et al. 2014). Such asymmetrical responses may explain why most studies report a decline in parasitism rates with increasing elevation (Hodkinson 2005; Péré et al. 2013). In some cases, parasitism rates reach zero before the upper elevational limits of the host species, thereby creating high eleva- tion refuges from parasitism (Randall 1982; Maunsell et al. 2014). Even when present, the efficacy of parasitoids may be reduced at higher elevations.This is supported by laboratory experiments demonstrating decreased parasitoid functional efficiency at lower temperatures (Enkegaard 1994; Menon et al. 2002). Other factors, however, may also be important determinants of parasitism rates. Complementary use of host resources by a guild of parasitoids may lead to increased rates of parasitism (Snyder et al. 2008). Dif- ferent parasitoid species may be both temporally (e.g. egg vs. larval parasitoids) and spatially (e.g. ecto- vs. endoparasitoids) separated on the same host species (Bonsall et al. 2002; Hackett-Jones et al. 2009). In addition, the relative abundances of different parasitoids change seasonally (Sorribas et al. 2010). Such complementary host use may lead to a positive relationship between parasitoid richness and host *Corresponding author. Accepted for publication May 2015. Austral Ecology (2015) ••, ••–•• © 2015 Ecological Society of Australia doi:10.1111/aec.12283