Resource competition is an indirect negative interaction between species that occurs with the exploitation of shared, limiting resources. Theoretical and empirical work shows that competition of two species for one limiting resource results in competitive exclusion (Tilman 1982; Chase & Leibold 2003). Multiple mechanisms can result in competitive coexistence, including trade-offs between competitive ability and predator avoidance (Chase & Leibold 2003), dispersal ability (Levins & Culver 1971; Tilman 1994) or tolerances to abiotic variation (Chesson 1986; Chesson & Huntly 1997). Another prominent mechanism for coexistence is multiple resources, which can yield coexistence depending on resource ratios and interspecific differences in use of resource types (Tilman 1982).
Current theory surrounding food web dynamics and species interactions often focuses on direct or indirect manifestations of photosynthetic primary productivity (Moore et al. 2004), even though the majority of energy in food webs passes through detrital pathways (O’Neill & Reichle 1980; Moore et al. 2004). The effect of detritus on community attributes can be large. Detritus inputs can support higher diversity (Hairston & Hairston 1993) or can stabilize otherwise unstable energy fluxes and the dynamics of consumer populations more so than energy derived directly from primary productivity (Moore et al. 2004). Although there appear to be strong effects of detritus in many systems, few studies have investigated whether resource competition theory and predictions about resource ratios are accurate for detritus-based food webs.
Mosquito larvae within detritus-based container systems (e.g. tree holes, bamboo stumps, discarded tyres) have proved to be useful model organisms for tests of hypotheses about processes structuring communities (e.g. predation: Lounibos, Nishimura & Escher 1993; Juliano & Gravel 2002; competition: Livdahl & Willey 1991; Juliano 1998; Teng & Apperson 2000). Natural containers (e.g. tree holes) are home to larvae of a variety of mosquito species, and interspecific competition can be strong (Kitching 2001). Tree holes are cavities in trees that receive inputs of terrestrial detritus, including senescent plant parts (primarily leaves, Kitching 2001), stem flow (water from precipitation flowing along tree trunks and containing organic matter, Carpenter 1982) and terrestrial invertebrate carcasses (Daugherty, Alto & Juliano 2000). Leaf inputs have variable effects on mosquito populations (Lounibos et al. 1993; Léonard & Juliano 1995; Walker et al. 1997; Juliano 1998), but animal-based detritus appears to have a greater effect on container mosquito performance (Yee & Juliano 2006) and can reduce the impact of competitive interactions (Barrera 1996; Daugherty et al. 2000). Mosquito larvae eat microorganisms growing on detritus surfaces (Merritt, Dadd & Walker 1992) and both animal and plant detritus support micro-organism growth (Yee & Juliano 2006). Because tree holes can contain different kinds of detritus resources, and these resources can affect performance of mosquitoes in different ways, we hypothesize that variation in the ratios of these detritus types will affect competitive coexistence of mosquitoes.
We tested for the effect of different detritus ratios on the competitive outcome between two container mosquitoes, Aedes albopictus (Skuse) (Asian tiger mosquito) and Ochlerotatus triseriatus (Say) (eastern tree hole mosquito), (formally in the genus Aedes, Reinert 2000). A. albopictus was introduced into North America in the mid-1980s from Asia, and it now occurs across most of the southern United States (Hawley et al. 1987; Moore 1999). O. triseriatus is typically the dominant consumer in tree holes in the eastern United States (Bradshaw & Holzapfel 1985). Currently, A. albopictus and O. triseriatus overlap considerably in their large-scale distributions within the United States, and co-occur frequently (Lounibos et al. 2001; Aliabadi & Juliano 2002). A. albopictus has negative effects on resident mosquito species and has been shown to be superior to O. triseriatus in competition under field and realistic laboratory conditions (Livdahl & Willey 1991; Edgerly, Willey & Livdahl 1993; Teng & Apperson 2000). These competition experiments have used only leaf detritus as a resource. When raised on animal vs. plant detritus, O. triseriatus larvae grew significantly faster, adults were significantly larger and estimated population growth was significantly higher, suggesting that animal detritus is a higher quality food source (Yee & Juliano 2006). Daugherty et al. (2000) showed that animal and plant detritus could increase the likelihood of coexistence between A. albopictus and another mosquito, A. aegypti, although leaf-only treatments resulted in exclusion of A. aegypti. Whether the outcome of competition between A. albopictus and O. triseriatus is affected by animal detritus or by the ratios of plant and animal detritus is unknown. Livdahl & Willey (1991) suggested that A. albopictus would out-compete O. triseriatus in discarded automobile tyres, whereas coexistence would occur in natural tree holes, and speculated that tyres contained resources that were absent in tree holes and better exploited by A. albopictus, but did not consider that ratios of different kinds of detritus resources may differ between these container types. Variation in micro-organism productivity from different detritus types also could explain patterns of coexistence or exclusion, because mosquito larvae consume microorganisms exploiting detritus. Thus, the mechanism by which A. albopictus excludes or coexists with O. triseriatus remains unknown.
We ask whether variation in the ratios of two detritus types could affect exclusion or coexistence between O. triseriatus and A. albopictus. We used nine ratios of two detritus types (leaves and invertebrate carcasses) in a factorial design with eight different combinations of densities of the two mosquitoes. We also quantified two measures of micro-organism activity (bacterial biomass production and metabolic rates) for each resource ratio, enabling us to quantify the direct food source for larvae. We used path analysis to determine the important direct and indirect casual links among detritus type, micro-organism productivity, mosquito densities and adult mosquito production and estimated population growth to test whether detritus ratios may be important for coexistence of A. albopictus and O. triseriatus. Finally, we measured detritus inputs into natural tree holes to characterize spatial and temporal patterns of these ratios in nature, and to infer whether exclusion or coexistence are likely under natural conditions.