Seasonality affects specialisation of a temperate forest herbivore community

Abstract

Understanding spatiotemporal trends on insect-plant interaction networks is essential to unveil the ecological and evolutionary processes driving herbivore specialisation. However, community studies accounting for temporal dynamics in host-plant specialisation of herbivorous insects are surprisingly scarce. Here, we investigated how seasonality affects specialisation of a temperate forest herbivore community. A substantial body of literature suggests that young plant foliage tends to be more protected by defence mechanisms effective against generalist insect herbivores than mature leaves. We thus hypothesised that herbivore specialisation would be highest in the early season, when young leaves are available, and that the degree of dietary specialisation would decline with leaf maturation. To test this hypothesis, we sampled more than 4700 folivorous caterpillars associated with 16 tree species within a 0.2 ha deciduous forest stand in eastern North America. We further examined the dietary specialisation of exposed feeders versus shelter builders (e.g. leaf rollers, leaf tiers, webbers). Contrary to our prediction, we observed a significantly less specialised herbivore fauna in the early season than in the two subsequent summer seasons. We further found that the seasonal increase in specialisation was driven by a remarkable turnover in species composition rather than by shifts in guild structure or intraspecific changes in diet breadth of the herbivores. These findings run counter to the widespread belief that young leaves are less susceptible to generalist insect herbivores. Our study underscores the need to account for temporal dynamics when contrasting herbivore specialisation among sites with pronounced seasonality.

What Drives Caterpillar Guilds on a Tree: Enemy Pressure, Leaf or Tree Growth, Genetic Traits, or Phylogenetic Neighbourhood?

“…As leaves of a host tree develop, their quality for herbivores decreases [1,2], reducing the abundance and diversity of herbivorous insects within a few weeks [3,4]. Trees may thus differ in insect communities at a given time due to variation in the timing of budburst ( [5][6][7][8][9][10][11]; Figure 1, arrow 1).…”

Ecological specialisation and range size determine intraspecific body size variation in a speciose clade of insect herbivores

“…For most univoltine species, by contrast, caterpillar occurrence is restricted to a very specific time window within the vegetation period, e.g. when host plant quality provides optimal conditions for larval development, such as the case for many generalist spring feeders (Feeny 1970, van Asch and Visser 2007, Forkner et al 2008, Seifert et al 2021). Strictly univoltine species thus usually develop under less variable conditions, which likely canalises intraspecific variation in adult body size.…”

Pollinator individual‐based networks reveal the specialized plant–pollinator mutualism in two biodiverse communities

“…In our study, we collected 2 months of data on alpine meadows in order to structure the pollination network, which may encompass some species with non-overlapping phenology and introduce temporally forbidden links into the networks. For example, a pol- season can consequently prevent the establishment of a highly connected network core and thus limit the specialization of the entire network and nodes (Schwarz et al, 2020;Seifert et al, 2021). For example, species can appear as specialists in networks that are combined over narrow time scales, such as 1 day or week.…”

“…The pollination network across the entire growing season, however, encompasses both early, medium, and late flowering plants as well as pollinators that only occur at certain times within the season. A high temporal variation of species and the rewiring of links over the entire growing season can consequently prevent the establishment of a highly connected network core and thus limit the specialization of the entire network and nodes (Schwarz et al, 2020; Seifert et al, 2021). For example, species can appear as specialists in networks that are combined over narrow time scales, such as 1 day or week.…”