Abstract
BackgroundSpecies or clades may retain or shift their environmental niche space over evolutionary time. Understanding these processes offers insights into the environmental processes fuelling lineage diversification and might also provide information on past range dynamics of ecosystems. However, little is known about the relative contributions of niche conservatism versus niche divergence to species diversification in the tropics. Here, we examined broad-scale patterns of niche evolution within a Pliocene–Pleistocene clade of epiphytic Bulbophyllum orchids (30 spp.) whose collective distribution covers the northwest and eastern forest ecosystems of Madagascar.ResultsUsing species occurrence data, ecological niche models, and multivariate analyses of contributing variables, we identified a three-state niche distribution character for the entire clade, coinciding with three major forest biomes viz. phytogeographical provinces in Madagascar: A, Northwest ‘Sambirano’; B, ‘Eastern Lowlands’; and C, ‘Central Highlands’. A time-calibrated phylogeny and Bayesian models of niche evolution were then used to detect general trends in the direction of niche change over the clade’s history (≤5.3 Ma). We found highest transitions rates between lowlands (A and B) and (mostly from B) into the highland (C), with extremely low rates out of the latter. Lowland-to-highland transitions occurred frequently during the Quaternary, suggesting that climate-induced vegetational shifts promoted niche transitions and ecological speciation at this time.ConclusionsOur results reveal that niche transitions occurred frequently and asymmetrically within this Madagascan orchid clade, and in particular over Quaternary time scales. Intrinsic features germane to Bulbophyllum (e.g., high dispersal ability, drought tolerance, multiple photosynthetic pathways) as well as extrinsic factors (ecological, historical) likely interacted to generate the niche transition patterns observed. In sum, our results support the emerging idea of dramatic environmental and climatic fluctuations in Madagascar during the recent geological past, which overturns the long-held paradigm of long-term stability in tropical forest settings. The generality of the patterns and timings reported here awaits the availability of additional comparative studies in other Madagascan endemics.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0586-3) contains supplementary material, which is available to authorized users.
Highlights
Species or clades may retain or shift their environmental niche space over evolutionary time
Macroecological niche identification and characterization Our results of the fuzzy C-means (FCM) cluster analysis offered more than one optimal solution for partitioning the 604 occurrence data of Madagascan Bulbophyllum clade C in environmental space (Additional file 1)
Samples of A and B were largely separated along PC2 (27 %), which was mainly positively correlated with temperature seasonality but negatively with both precipitation seasonality and isothermality; (Additional file 2)
Summary
Species or clades may retain or shift their environmental niche space over evolutionary time. At the phylogenetic level, geographical patterns of niche shifts versus niche conservatism can offer insights into the environmental processes fuelling lineage diversification and may provide important information on the range dynamics of species and ecosystems over geological time (e.g., [7, 9, 11,12,13]). Using palaeoclimatic modelling, Rakotoarinivo et al [29] have shown that Quaternary changes in vegetation-precipitation relationships in Madagascar exerted a strong influence on plant species richness, as exemplified by palms (Arecaceae) of the island’s eastern rainforest biomes It is feasible, that Madagascan forest ecosystems underwent repeated range expansion/contraction cycles due to Quaternary palaeoclimatic change, which in turn promoted phylogenetic niche shifts along steep environmental gradients [21,22,23, 39,40,41,42]. Mutually non-exclusive hypothesis is that niche conservatism had likewise a role in facilitating speciation and lineage divergence through the maintenance of long-term allopatry in proposed glacial refuge locations [21], notably habitat isolation caused by aridification of low-elevation/montane river catchments [23, 29]
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