Emergence asynchrony between herbivores leads to apparent competition in the field

Abstract

Abstract. It has been established that herbivore populations can be structured by apparent competition, even if they do not compete directly for resources. But we lack evidence on the mechanisms behind such indirect competition. This study shows that temporal asynchronies in emergence time lead to apparent competition via shared natural enemies in a leafminer-parasitoid community. We present three kinds of evidence on mechanisms driving apparent competition. First, we conducted a two-year population census of Liriomyza helianthi and Calycomyza platyptera, along with all associated parasitoids, at seven sites in the Californian Central Valley, USA. We then assessed C. platyptera parasitism on 16 vegetation islands, half with experimental removal of early-season L. helianthi populations. Finally, we examined parasitoid host preference between leafminer species. We found that Liriomyza helianthi populations emerged approximately one month before C. platyptera. Experimental removal of L. helianthi populations in the early summer led to a 60% reduction in parasitism of C. platyptera. We found no evidence of differential parasitoid preference for host species. The findings suggest that temporal asynchrony can lead to negative effects on later-emerging species and that such indirect competition may be a major structuring force in herbivore communities.

Related herbivore species show similar temporal dynamics

“…Despite an overall imprint of phylogenetic relatedness, species with more similar parasitoid communities did not exhibit any more similar (or dissimilar) temporal dynamics than species attacked by distinct parasitoid assemblages. This is consistent with earlier work at the Finnish study site, where a previous multi-year experiment (Tack et al, 2011), as well as a natural experiment (Kaartinen & Roslin, 2013), failed to reveal any detectable impact of increased densities of herbivore species i in year t − 1 on the abundance of herbivore species j (sharing parasitoids with herbivore species i) in year t. This contrasts with empirical field studies showing apparent competition in plant-feeding insects (Blitzer & Welter, 2011;Frost et al, 2016;Morris, Lewis, & Godfray, 2004). Indeed, while parasitism usually causes high mortality in plant feeding insects (Hawkins, Cornell, & Hochberg, 1997), studies have generally failed to find an impact of parasitoids on the pattern of herbivore cycling and outbreak dynamics (Hagen, Jepsen, Schott, & Ims, 2010;Schott, Hagen, Ims, & Yoccoz, 2010).…”

Species range expansion constrains the ecological niches of resident butterflies

“…The population dynamics of these parasites is likely to have been positively a↵ected by the increase in potential larval hosts (A. levana) between the two time-periods (Morris, Lewis & Godfray, 2004). Moreover, the di↵erences in phenology between A. urticae, A. io, and A. levana may form a breeding ground to stimulate parasites population dynamics as it may also correlate with a prolonged temporal niche for parasites to achieve their reproductive cycle (Blitzer & Welter, 2011). In a scenario of parasite-driven apparent competition, the phenologically late species is expected to be the more vulnerable as its larval growth will coincide with an increase in parasite population size (Blitzer & Welter, 2011).…”

Extended leaf phenology may drive plant invasion through direct and apparent competition

“…For example, ELP invaders may subsidize herbivore density when native plants are scarce, essentially preventing some herbivores from starving. ELP-supported herbivore density, then, could more strongly suppress native species (Settle et al 1996, Blitzer andWelter 2011). However, the importance of ELP-mediated apparent competition may depend strongly on the relative palatability of invaders ( Fig.…”

“…For example, ELP invaders may subsidize herbivore density when native plants are scarce, essentially preventing some herbivores from starving. ELP‐supported herbivore density, then, could more strongly suppress native species (Settle et al , Blitzer and Welter ).…”