The spatial and temporal distributions of arthropods in forest canopies: uniting disparate patterns with hypotheses for specialisation

Abstract

Arguably the majority of species on Earth utilise tropical rainforest canopies, and much progress has been made in describing arboreal assemblages, especially for arthropods. The most commonly described patterns for tropical rainforest insect communities are host specificity, spatial specialisation (predominantly vertical stratification), and temporal changes in abundance (seasonality and circadian rhythms). Here I review the recurrent results with respect to each of these patterns and discuss the evolutionary selective forces that have generated them in an attempt to unite these patterns in a holistic evolutionary framework. I propose that species can be quantified along a generalist-specialist scale not only with respect to host specificity, but also other spatial and temporal distribution patterns, where specialisation is a function of the extent of activity across space and time for particular species. When all of these distribution patterns are viewed through the paradigm of specialisation, hypotheses that have been proposed to explain the evolution of host specificity can also be applied to explain the generation and maintenance of other spatial and temporal distribution patterns. The main driver for most spatial and temporal distribution patterns is resource availability. Generally, the distribution of insects follows that of the resources they exploit, which are spatially stratified and vary temporally in availability. Physiological adaptations are primarily important for host specificity, where nutritional and chemical variation among host plants in particular, but also certain prey species and fungi, influence host range. Physiological tolerances of abiotic conditions are also important for explaining the spatial and temporal distributions of some insect species, especially in drier forest environments where desiccation is an ever-present threat. However, it is likely that for most species in moist tropical rainforests, abiotic conditions are valuable indicators of resource availability, rather than physiologically limiting factors. Overall, each distribution pattern is influenced by the same evolutionary forces, but at differing intensities. Consequently, each pattern is linked and not mutually exclusive of the other distribution patterns. Most studies have examined each of these patterns in isolation. Future work should focus on examining the evolutionary drivers of these patterns in concert. Only then can the relative strength of resource availability and distribution, host defensive phenotypes, and biotic and abiotic interactions on insect distribution patterns be determined.

Diptera species and functional diversity across tropical Australian countryside landscapes

“…Given that populations of tropical insects can show substantial temporal variability (Wardhaugh, 2014), future studies will be enhanced by sampling more widely across seasons and years. Though limited in scope, our results provide important insights into the link between this important insect group and the amount and type of fragmentation occurring across much of the tropics.…”

The value of targeted reforestations for local insect diversity: a case study from the Ecuadorian Andes

“…For various reasons outlined in Wardhaugh (2014) insect activity can vary throughout the day. By restricting sampling to the hours between 8:00 am and 3:00 pm, we might have systematically overlooked some species.…”

The advantage of the extremes: tree seedlings at intermediate abundance in a tropical forest have the highest richness of above‐ground enemies and suffer the most damage

“…This suggests that generalist enemies might be at the origin of the hump-shape patterns observed between the richness of enemies and species abundance, and between the amount of foliar damage and species abundance. The overall similar richness of specialist enemies in rare and common tree species was however surprising given the anticipated higher resource and evolutionary costs required to specialize on rare hosts (Jaenike 1990;Barrett & Heil 2012;Forister et al 2012;Wardhaugh 2014). Enemies might have evolved specialized attributes to enable them to detect and overcome the defences developed by rare hosts, as it is the case in some Lepidoptera species (Courtney & Courtney 1982), particularly in highly diverse ecosystems that exhibit high levels of enemy specialization (Novotny et al 2004;Forister et al 2015 but see Morris et al 2014).…”

“…1b). The rationale behind this hypothesis is that endophages (enemies that penetrate in the host) tend to be more specialized than ectophages (enemies that remain outside the host) (Gaston, Reavey & Valladares 1992) and high host abundance has often been shown to promote enemy specialization (Jaenike 1990;Barrett & Heil 2012;Forister et al 2012;Wardhaugh 2014).…”