Fleshing out facilitation – reframing interaction networks beyond top‐down versus bottom‐up

Abstract

Summary Rather than direct plant–plant interactions, research on the community‐scale influence of mistletoes reveals hitherto unappreciated roles of animals in mediating facilitation. Lacking roots and reliant upon animal vectors, mistletoes represent model systems with which to understand mechanisms underlying interaction networks. In addition to direct effects on nutrient dynamics via enriched litter‐fall, mistletoes are visited by pollinators, seed dispersers and natural enemies, complementing increased heterogeneity in nutrient returns reallocated from infected hosts with increased external inputs. These amplified bottom‐up effects are coupled with top‐down influences of insectivores attracted to infected hosts and stands by increased availability of favoured prey. Simultaneously influencing nutrient dynamics and plant–plant interactions from below and above, visiting animals help explain variation in the context dependence of facilitation.

Mistletoe, friend and foe: synthesizing ecosystem implications of mistletoe infection

“…Browsing herbivores may constrain mistletoe abundance, either by nipping off growing stems or, in the case of elephants and rhinoceros, removing entire plants (Watson 2001). Likewise, individual animals adjust their movements, diets, territories and breeding sites relative to mistletoe abundance and phenology, culminating in consistently close relationships between mistletoe occurrence and faunal diversity (Barea 2008, Bowen et al 2009, Watson 2016. Heavily-infected hosts may be actively defended, both as a food resource (Barea and Watson 2007) and also as a reliable source of water (Walsberg 1977), especially in arid areas where standing water may be unavailable seasonally.…”

“…As with other parasitic plants, mistletoe generate large amounts of nutrient-enriched litter, precipitating a cascading series of facilitative interactions beneath infected hosts. Convergent findings in Australian eucalypt woodlands, African acacia savannah and European pine forests suggest a generalized role of mistletoes as facilitators (reviewed by Watson 2016). In addition to re-allocating nutrients from infected hosts and shedding litter over a longer duration, the addition of mistletoe litter accelerates decomposition of recalcitrant host litter, thereby boosting nutrient availability.…”

Hemiparasites drive heterogeneity in litter arthropods: Implications for woodland insectivorous birds

“…In addition to altered litter inputs (in terms of quantity, seasonality, C:N ratio and micronutrient enrichment), trees infected by mistletoes are visited by more animals, leading to additional nutrient inputs over time (Mellado et al . ; Watson ). Hemiparasites also increase the abundance and diversity of fungal soil communities (Mueller & Gehring ), likely affecting arthropod fungivores.…”

“…In addition to boosting prey availability for bird insectivores, changes in abundance and activity of different arthropod groups contribute to key functions (e.g. decomposition, seed dispersal, soil bioturbation, herbivore control inter alia) which collectively underpin many aspects of ecosystem health (Watson ). Considering the worldwide distribution of hemiparasites, their facilitative influence on epigeic arthropods might be of more generalised significance, both for insectivores and for the proactive management of the multifunctional landscapes upon which many of them depend (Bennett & Watson , and references therein).…”

“…As well as modifying plant–plant interactions, nutrients mobilised by parasitic plants become available to visiting animals (Mellado & Zamora ; Watson ). Greater numbers of fleshy fruited understorey shrub seeds were found beneath infected pine trees than under otherwise comparable uninfected trees in southern Spain, reflecting differential habitat use by frugivorous birds (Mellado & Zamora ).…”