A theoretical framework for understanding the ecology and conservation of bamboo-specialist birds

ARETA JI; COCKLE K

Journal of Ornithology

Springer

Año: 2012 vol. 153 p. 163 - 170

2193-7192

Worldwide, populations of a diverse array ofbamboo-specialist birds must respond to the life cycles oftypical woody bamboos, which include a long vegetativephase, a short period in sexual reproduction (seeding), anda short period dead. While some bamboo species flowercontinuously in time, but scattered in space, over differentparts of their distribution, others exhibit a single, synchronizedmass-seeding event, followed by death overhundreds of square kilometers. Thus, bamboo-specialistbirds experience extreme variability in their habitat andfood sources, and may pass from a period of overabundanthabitat and food resources to a period of extreme resourcescarcity. At a community level, bamboo species are out ofphase with each other. Masting events of several bamboospecies may coincide one cycle, but not the next. Theresulting long-term dynamics of bamboo cycles at acommunity scale will result in a temporal and geographicmosaic of resource availability over very long periods oftime, where the initial conditions that any bird experiencedmay occur only sporadically over centuries and very largeregions. We aim to propose a theoretical framework forunderstanding the population ecology of bamboo-specialistbirds and discuss implications for their conservation. Threemain diet and life-history strategies of bamboo-specialistbirds can be identified based on their natural history. First,insectivorous bamboo specialists that inhabit woody bamboosmay be relatively sedentary during the long vegetativegrowth phase, but they must experience populationdeclines, switch to alternative habitat, or travel widelyduring bamboo die-off events. They benefit from long-termhabitat stability but must face short-term reductions inhabitat that do not affect all generations of birds. Second,bamboo seed specialists must rely on an ephemeral sourceof food available only after large intervals of time at mostlocations. Since bamboo cycles are typically much longerthan the lives of individual birds, there is no chance forseed-specialist birds to learn when and where bambooseeds will be available in the future. Thus, although bambooseed availability is ecologically predictable (i.e., seedsare always available after the same length of time at agiven place), seed availability is unpredictable to individualbirds. During their lifetime, most individual bambooseed-specialists must face a lack of their preferred foodsupply, at which time they must wander to survive. Andthird, mixed strategists usually feed on bamboo shoots,leaves, and insects, but consume bamboo seeds whenavailable. Their life history combines elements of bothprevious categories: they can benefit from long-term habitatstability like insectivores, and take advantage of mastingevents like seed specialists. Niche width (degree ofspecialization) in conjugation with the long-term dynamicsof bamboo cycles should determine the degree of nomadismand amplitude of bamboo-specialist population cycles.Nomadism appears to be distributed along a continuumfrom the temporarily nomad insectivores and mixed strategiststo the local and regional nomadic seed specialists. Aprotected area cannot shelter healthy populations of themost specialized bamboo-seed specialists because theytrack temporally and spatially unpredictable resources overlarge temporal and spatial scales. Overall, to effectivelyprotect this ecological system, it is crucial to conserve anetwork of natural areas capable of providing bamboo indifferent phases and at different times and places, as wellas subsidiary food resources that will allow birds to moveaway from bamboo patches as they become unsuitable tonew, suitable patches.