Erosion of a pollination mutualism along an environmental gradient in a south Andean treelet, Embothrium coccineum (Proteaceae)

Abstract

Th e limit of a species ' distribution can be determined biotically if an environmental gradient causes the loss of critical mutualists such as pollinators. We assessed this hypothesis for Embothrium coccineum , a self-incompatible red-fl owered treelet growing along a strong west-east precipitation gradient from rainforest to forest-steppe ecotone in the rain shadow of the southern Andes in northwestern Patagonia. For 16 populations along this gradient, we quantifi ed composition of the pollinator assemblage, pollination effi ciency and limitation, and reproductive output. Th e treelet has a generalized pollination system, but the hummingbird Sephanoides sephaniodes was the most eff ective pollinator. Th e relative importance of this hummingbird as a fl ower visitor within populations infl uenced pollen transfer and fruit set more strongly than local precipitation. As hummingbirds and other pollinators, including passerine birds and nemestrinid fl ies, were replaced by bees towards the dry eastern range limit, pollen limitation increased and reproduction eventually failed. Th ese results support the hypothesis that pollinators can act as important biotic fi lters infl uencing plant distribution, and warn against predictions of geographical range shifts based solely on climatic variables.

El papel de aves no nectarívoras como polinizadoras de Anarthrophyllum desideratum en la estepa patagónica: una aproximación geográfica

“…In line with this idea, we found lower reproductive success under open pollination in populations located in the south-western (i.e., Escarchados) and north-eastern (i.e., Río Chico) edges of the species distribution (Paiaro et al 2012a) in relation to the two central populations that were studied (i.e., Monte León and Tecka). In addition, these populations had different visitor assemblages (Figure 2) and visitation frequencies (Table 3), supporting the idea that dependence on pollinators for setting fruits varies among sites differing in abundance and composition of floral visitors (Moeller 2006;Cosacov et al 2008;Chalcoff et al 2012). In this sense, it is interesting to notice that reproductive success under open pollination was higher in those populations where Phrygilus gayi was present in a high proportion (>30%) and had a higher visitation rate (i.e., Monte León and Tecka), suggesting the potential role of this bird species as pollinator of A. desideratum in these populations.…”

“…It is possible that idiosyncratic responses of these species explain the spatial changes in their abundances along the geographic range of A. desideratum, as it was suggested for granivorous birds of the Monte desert (Blendinger and Ojeda 2001). Alternatively, shifts in pollinator abundance and composition may respond to ecological factors varying over large spatial scales, such as climate (e.g., Moeller 2005;Devoto et al 2006;Chalcoff et al 2012). However, shifts in bird assemblages did not show a geographical structure across the A. desideratum range and spatial variations in floral visitor composition were only weakly associated with precipitation and maximum temperature of the flowering period.…”

“…However, it is important to notice that plants at this population display a really large number of flowers (Table 3), which might limit the chances of each flower of being pollinated by birds in the open pollination treatment, even with the highest visitation rate (Table 3). On the other hand, our results of pollinator dependence are based on two populations located near the southern limit of the species distribution (Paiaro et al 2012a), where pollen limitation may increase and reproduction eventually fail (Chalcoff et al 2012). In line with this idea, we found lower reproductive success under open pollination in populations located in the south-western (i.e., Escarchados) and north-eastern (i.e., Río Chico) edges of the species distribution (Paiaro et al 2012a) in relation to the two central populations that were studied (i.e., Monte León and Tecka).…”

“…Climatic gradients along which plant populations occur may also determine geographical changes in the abundance and composition of pollinators, especially under adverse weather conditions (Moeller 2005;Devoto et al 2006;Cosacov et al 2008;Chalcoff et al 2012). In arid ecosystems such as the Patagonian steppe, where birds are exposed to harsh environmental conditions, pronounced daily and seasonal thermal fluctuations and largely unpredictable food and water availability, bird assemblage composition can be highly variable in both space and time (Schodde 1982;Dean 1997;Blendinger 2005).…”

Restabelecimento das interações entre plantas e visitantes florais em áreas restauradas de floresta estacional semidecidual

“…A perda de recursos florais devido ao desaparecimento de populações vegetais representa uma séria ameaça aos polinizadores , cujas populações tem diminuído devido à modificação contínua dos habitats (SteffanDewenter & Tscharntke 1999;. Por outro lado, existem também indícios de que a perda de polinizadores pode levar à extinção local de espécies de plantas ) e de que a ruptura das interações planta-polinizador pode agir como filtro biológico influenciando os limites espaciais da distribuição geográfica das espécies vegetais (Chalcoff et al 2012).…”