Tropical Elevational Gradient Research Articles

Functional structure and specialization in three tropical plant–hummingbird interaction networks across an elevational gradient in Costa Rica

Understanding causes of variation in multispecies assemblages along spatial environmental gradients is a long‐standing research topic in ecology and biogeography. Ecological networks comprising interacting species of plants and pollinators are particularly suitable for testing effects of environmental gradients on the functional structure and specialization in multispecies assemblages. In this study, we investigated patterns in functional assemblage structure and specialization of hummingbirds at the individual and species level along a tropical elevational gradient. We mist‐netted hummingbirds at three elevations in Costa Rica in seven temporally distinct sampling periods and used the pollen carried by hummingbird individuals to construct plant–hummingbird networks at each elevation. We measured four functional traits of hummingbird species and quantified different metrics of functional community structure. We tested the effect of elevation on functional metrics of hummingbird assemblages and specialization within the networks, employing the variability across sampling periods and hummingbird species to compare the respective metrics among elevations. Hummingbird species and individuals were more specialized at low and mid elevations than at the highest elevation. This pattern corresponded to a more even and over‐dispersed assemblage structure at the lower elevations throughout the year and suggests a high level of floral resource partitioning in functionally diversified communities. In contrast, an uneven and clustered functional structure of the highland assemblage across all sampling periods suggests that this assemblage was structured by environmental filtering and by niche expansion of hummingbird individuals and species at this elevation. We conclude that high degrees of specialization on specific floral resources might be crucial for the coexistence of hummingbird species in diversified lowland communities. Spatial variation in animal resource use may be an important crucial driver of spatial patterns in the functional structure of diversified species assemblages also in other types of ecological networks.

Taxonomic, functional, and phylogenetic dimensions of rodent biodiversity along an extensive tropical elevational gradient

Relationships among taxonomic, functional, and phylogenetic dimensions of biodiversity provide insight about the relative contributions of ecological and evolutionary processes in structuring local assemblages. We used data for rodent species distributions from an extensive tropical elevational gradient to 1) describe elevational gradients for each of three dimensions of biodiversity, 2) evaluate the sufficiency of species richness as a surrogate for other dimensions, and 3) quantify the relative support for mechanisms that increase or decrease phylogenetic or functional dispersion. Taxonomic biodiversity was quantified by species richness, as well as by richness, evenness, diversity, dominance, and rarity at generic and familial levels. Morphological and categorical traits were used to estimate functional biodiversity, and an ultrametric mammalian supertree was used as the basis for estimating phylogenetic biodiversity. Elevational gradients of each dimension of biodiversity were strong, with significant linear and non‐linear components based on orthogonal polynomial regression. Empirical linear and non‐linear regression components were consistently different than those expected based on species richness for generic, familial, and phylogenetic biodiversity, but not for functional biodiversity. Nevertheless, the congruence of dimensions of biodiversity based on correlation analyses indicated that any one dimension is a useful surrogate for the other dimensions for rodents at Manu. Given variation in species richness, assemblages from lowland rainforests comprised more biodiversity than expected, whereas assemblages from cloud and elfin forests represented less biodiversity than expected. Warm temperatures, vertical complexity of the vegetation, and high productivity likely facilitate niche differentiation in rainforests, whereas cricetid rodents are competitively superior to other clades in the less structurally complex, less productive, and colder, high elevation habitats.

Temperature is the major driver of distribution patterns for C4 and C3 BEP grasses along tropical elevation gradients in Hawai‘i, and comparison with worldwide patterns

The distribution patterns of C4 and C3 grasses in relation to climate have attracted much attention, but few studies have examined grass distributions along tropical elevation gradients. Previous studies identified either temperature, precipitation, or both variables as the major climatic factor(s) driving these distributions. Here we investigated relative dominance of C4 grasses in relation to climate along five elevation gradients in Hawai‘i. The transition temperature between C4 and C3 BEP (Bambusoideae, Ehrhartoideae, and Pooideae) grasses (where their relative dominance is equal) was determined; in our study, the subfamily Bambusoideae was not included. A worldwide synthesis of previous studies testing climatic factors and transition temperatures associated with C4 and C3 grass distributions was also carried out. Mean July maximum temperature was significantly correlated with C4 dominance along all elevation transects in Hawai‘i, while precipitation was only correlated along three transects when precipitation was correlated with temperature. A spatially explicit multiple regression model indicated that C4 relative cover was best explained by temperature. Temperature appears to be the major climatic factor shaping distribution patterns of C4 and C3 BEP grasses in Hawai‘i. According to the worldwide analysis, temperature primarily influenced grass distribution patterns more often in temperate studies (70%) than in tropical studies (45%). Degree of correlation or covariance between temperature and precipitation was rarely reported in previous studies, although this can strongly affect conclusions. C4-C3 BEP transition temperatures (mean July maximum) ranged from 18 to 21 °C in Hawai‘i; these transition temperatures are lower than those reported in temperate localities (26–31 °C), but similar to transition temperatures for other localities at tropical latitudes (21–22 °C). A warming climate is likely to shift C4 grass dominance upward in elevation, threatening higher elevation native communities by perpetuating a grass–fire cycle.

Lattice-work corridors for climate change: a conceptual framework for biodiversity conservation and social-ecological resilience in a tropical elevational gradient

Rapid climate change poses complex challenges for conservation, especially in tropical developing countries where biodiversity is high while financial and technical resources are limited. The complexity is heightened by uncertainty in predicted effects, both for ecological systems and human communities that depend heavily on natural resource extraction and use. Effective conservation plans and measures must be inexpensive, fast-acting, and able to increase the resilience of both the ecosystem and the social-ecological system. We present conservation practitioners with a framework that strategically integrates climate change planning into connectivity measures for tropical mountain ecosystems in Costa Rica. We propose a strategy for doubling the amount of habitat currently protected in riparian corridors using measures that are relatively low cost and fast-acting, and will employ and expand human capital. We argue that habitat connectivity must be enhanced along latitudinal gradients, but also within the same elevational bands, via a lattice-work corridor system. This is needed to facilitate range shifts for mobile species and evolutionary adaptation for less mobile species. We think that conservation measures within the elevational bands must include conservation-friendly land uses that improve current and future human livelihoods under dynamic conditions. Key components include community involvement, habitat priority-setting, forest landscape restoration, and environmental services payments. Our approach is fundamentally adaptive in that the conservation measures employed are informed by on-the-ground successes and failures and modified accordingly, but are relatively low risk and fast-acting. Our proposal, if implemented, would satisfy tenets of climate-smart conservation, improve the resilience of human and ecological communities, and be a model for other locations facing similar challenges.

Microbial community composition explains soil respiration responses to changing carbon inputs along an Andes-to-Amazon elevation gradient.

1. The Andes are predicted to warm by 3–5 °C this century with the potential to alter the processes regulating carbon (C) cycling in these tropical forest soils. This rapid warming is expected to stimulate soil microbial respiration and change plant species distributions, thereby affecting the quantity and quality of C inputs to the soil and influencing the quantity of soil-derived CO2 released to the atmosphere.2. We studied tropical lowland, premontane and montane forest soils taken from along a 3200-m elevation gradient located in south-east Andean Peru. We determined how soil microbial communities and abiotic soil properties differed with elevation. We then examined how these differences in microbial composition and soil abiotic properties affected soil C-cycling processes, by amending soils with C substrates varying in complexity and measuring soil heterotrophic respiration (RH).3. Our results show that there were consistent patterns of change in soil biotic and abiotic properties with elevation. Microbial biomass and the abundance of fungi relative to bacteria increased significantly with elevation, and these differences in microbial community composition were strongly correlated with greater soil C content and C:N (nitrogen) ratios. We also found that RH increased with added C substrate quality and quantity and was positively related to microbial biomass and fungal abundance.4. Statistical modelling revealed that RH responses to changing C inputs were best predicted by soil pH and microbial community composition, with the abundance of fungi relative to bacteria, and abundance of gram-positive relative to gram-negative bacteria explaining much of the model variance.5. Synthesis. Our results show that the relative abundance of microbial functional groups is an important determinant of RH responses to changing C inputs along an extensive tropical elevation gradient in Andean Peru. Although we do not make an experimental test of the effects of climate change on soil, these results challenge the assumption that different soil microbial communities will be ‘functionally equivalent’ as climate change progresses, and they emphasize the need for better ecological metrics of soil microbial communities to help predict C cycle responses to climate change in tropical biomes.

Impact of mycorrhization on the abundance, growth and leaf nutrient status of ferns along a tropical elevational gradient

Mycorrhizal fungi are crucial for the ecological success of land plants, providing their hosts with nutrients in exchange for organic C. However, not all plants are mycorrhizal, especially ferns, of which about one-third of the species lack this symbiosis. Because the mycorrhizal status is evolutionarily ancestral, this lack of mycorrhizae must have ecological advantages, but what these advantages are and how they affect the competitive ability of non-mycorrhizal plants under natural conditions is currently unknown. To address this uncertainty, we studied terrestrial fern assemblages and species abundances as well as their mycorrhization status, leaf nutrient concentration and relative annual growth along an elevational gradient in the Ecuadorian Andes (500-4,000 m). We surveyed the mycorrhizal status of 375 root samples belonging to 85 species, and found mycorrhizae in 89% of the samples. The degree of mycorrhization decreased with elevation but was unrelated to soil nutrients. Species with mycorrhizae were significantly more abundant than non-mycorrhizal species, but non-mycorrhizal species had significantly higher relative growth and concentrations of leaf N, P, Mg, and Ca. Our study thus shows that despite lower abundances, non-mycorrhizal fern species did not appear to be limited in their growth or nutrient supply relative to mycorrhizal ones. As a basis for future studies, we hypothesize that non-mycorrhizal fern species may be favoured in special microhabitats of the forest understory with high soil nutrient or water availability, or that the ecological benefit of mycorrhizae is not related to nutrient uptake but rather to, for example, pathogen resistance.

Multiple dimensions of bat biodiversity along an extensive tropical elevational gradient

Research concerning spatial dynamics of biodiversity generally has been limited to considerations of the taxonomic dimension, which is insensitive to interspecific variation in ecological or evolutionary characteristics that play important roles in species assembly and provide linkages to ecosystem services. Consequently, the assumption that the taxonomic dimension is a good surrogate for other dimensions remains unconfirmed. We assessed variation in taxonomic (species richness) as well as phylogenetic and functional (Rao's quadratic entropy, a measurement of dispersion) dimensions of bat biodiversity along an elevational gradient in the Manu Biosphere Reserve of Peru. Phylogenetic dispersion was based on relatedness of species derived from a mammalian supertree. Functional dispersion was estimated separately for each of six functional components that reflect particular niche axes (e.g. diet, foraging strategy, body size) and for all functional components combined. Species richness declined nonlinearly with elevation, whereas phylogenetic dispersion and functional dispersion based on all functional components were not significantly associated with elevation (orthogonal polynomial regression). Moreover, considerable heterogeneity in the form of elevational relationships existed among functional components. After accounting for variation in species richness, dispersion of phylogenetic, diet and foraging strategy attributes were significantly greater than expected at high elevations, whereas dispersion of body size was significantly less than expected at high elevations. Species richness was a poor surrogate for phylogenetic or functional dispersion. Functional dispersion based on multiple components obscured patterns detected by particular components and hindered identification of mechanistic explanations for elevational variation in biodiversity. Variation in phylogenetic dispersion effectively captured the composite variation represented by all functional components, suggesting a phylogenetic signal in functional attributes. Mechanisms that give rise to variation in richness do not fully account for variation in phylogenetic or functional characteristics of assemblages. Greater than expected phylogenetic, diet and foraging strategy dispersion at high elevations were associated with the loss of phylogenetically or functionally redundant species, suggesting that increasing interspecific competition with decreasing productivity resulted in competitive exclusion. In contrast, low dispersion of size attributes at high elevations suggests the importance of abiotic filtering that favours small-sized species that can more easily enter torpor.

Commonness patterns and the size of the species pool along a tropical elevational gradient: insights using a new quantitative tool

The goal of this study was a quantitative assessment of two‐dimensional commonness in the lowland, dry, and montane tropical forests of the Madidi region (Bolivia). This region spans a large elevational and environmental gradient, with great diversity differences among sites. We aimed to correlate commonness patterns at different scales with elevation and with the size of the species pool. We also developed 1) a measure of ecological commonness and 2) a criterion to separate common from uncommon species; both based on the h index of academic productivity. With this approach, we calculated the 1) mean commonness of all species of the community; 2) proportion of common species of the community; and 3) mean commonness of the common species. The results showed that the commonness patterns in the Madidi region are strongly linked to the size of the species pool, independently of the environmental heterogeneity involved and the type of forest and spatial scale considered. Although these factors do not affect the general quality of the community, they do influence differences in commonness among species. Overall, we identified strong support for the oligarchy hypothesis, regardless of the strength of the pattern, and conclude that a quantitative approach to commonness could lead to great insights into community structure.

The body temperature of active amphibians along a tropical elevation gradient: patterns of mean and variance and inference from environmental data

Summary Tropical montane amphibians have been the focus of recent and crucial conservation efforts. These initiatives require understanding on how elevation influences amphibian body temperature beyond the simplistic assumption of a monotonical decrease with elevation. This study addresses patterns and potential for inference in this context. As elevation increases, mean body temperature (BT) of tropical montane amphibians decreases linearly, but intrapopulation variation (VAR) in BT increases exponentially. These relationships are influenced by biome structure, but display both local nuances and species‐specific remarks. Substrate temperature (ST) and BT hold a close relationship across elevation. The noise around this relationship is lowest in mid‐elevation cloud forests and maximum in the paramo, a biome above the tree line. The relationships between BT and ST, and between elevation and either BT or VAR, are valuable to infer general patterns of thermal ecology for amphibians and to highlight species‐specific exceptional cases. The BT of montane tropical amphibians can be estimated from temperature data collected at a scale compatible with the size and microhabitat of individual frogs. Estimates from elevation are valid as general trends that can be enhanced if natural history is taken into account. Worldclim data allow for rough inference only and have limited predictive power. A framework is proposed to study how the BT and VAR of amphibians change with elevation. This framework encompasses information on biome structure and natural history.

Upward expansion of fire‐adapted grasses along a warming tropical elevation gradient

High mountain regions in the tropics have thus far been impacted relatively little by anthropogenic activity or plant invasions, however, they are unlikely to be immune to impacts of global change, including climate change and other anthropogenic disturbances. Changes in fire regimes are known to accelerate the spread of invasive C4 grasses and interactions between changes in fire and climate can alter species distributions. The aim of this study was to compare grass distributions along an elevational gradient in Hawai‘i between 1966–1967 and 2008 to determine whether C4 and C3 grass distributions are shifting upward in response to alterations in fire and climate patterns. Field plots at Hawai‘i Volcanoes National Park were surveyed for grass species and cover at ˜150 m elevation intervals and compared to previous surveys done in 1966–1967. We found that the transition elevation, marking a shift in dominance between C4 and C3 grasses based on relative cover, shifted upward over 40 yr (95% confidence interval = 1476 m ± 130 m in 2008 versus 1200 m ± 106 m in 1966–1967). On the other hand, maximum elevations of all C4 or C3 grasses as a group were not significantly greater than 1966–1967 elevations; however, a subset of C4 (and fewer C3) grasses moved to substantially higher elevations, and these were the species adapted to fire. 100% of fire‐adapted grasses moved up in elevation compared to 29% of non‐fire adapted species, and the change in elevation of those species (=+454 m) was significantly greater than the change in elevation of non‐fire adapted species (p = 0.003). Our study documents an upward expansion of fire‐adapted grasses at high elevations in the tropics as an important threat that seems to be compounded by warming trends.

Measuring and modelling above-ground carbon and tree allometry along a tropical elevation gradient

Emerging international policy aimed at reducing carbon emissions from deforestation and forest degradation (REDD+) in developing countries, has resulted in numerous studies on above-ground live carbon (AGC) in tropical forests. However, few studies have addressed the relative importance of disturbance, topography, climate, soil and methods for stem measurement, on the estimation of AGC, or the costs of improving AGC estimates by altering sample regimes. We established 18 one hectare plots containing 7201 stems, stratified along forested elevation gradients in Tanzania. We recorded a broad set of physical, climatic and edaphic predictors of AGC and tree stature. AGC estimates using stem diameter, height and wood density, gave a mean value of 174.6tha−1, compared with 229.6tha−1 when height was excluded. Regression models revealed that stems were tallest for a given diameter at mid-elevation (1000–1250m), on south-facing slopes, and without past logging. High AGC was strongly associated with shallow slopes, followed by intermediate elevation, elephant absence, low potential evapotranspiration and low soil pH. Further regression models to investigate the structural habitat features associated with AGC, revealed significant positive influence of basal area, stem density, and height:diameter ratio, rather than the mean wood density of species present. Large stems (⩾70cmdbh; 4.6% of stems) contained 52% of AGC in all plots, declining to 36% in lowland plots. We discuss the cost:benefit of different measurements and recommend a tiered approach to AGC monitoring, depending on available resources. AGC assessments in African forests could exclude small stems, but should aim to record disturbance, topography and species. Stem height is vital for AGC estimation and valuation; when excluding height our 55tha−1 over-estimation of AGC would have over-valued the carbon resource by 24% (US$3300ha−1).

Symposium summary: Marine biogeography

news and update ISSN 1948‐6596 References Brown, J.H. (1995) Macroecology. Chicago University press, Chicago. Cardillo, M. (2011) Phylogenetic structure of mammal assemblages at large geographic scales: linking phylogenetic community ecology with mac‐ roecology. Philosophical Transactions of the Royal Society B, (in press). Colwell, R.K. and T.F. Rangel (2010) A stochastic, evolu‐ tionary model for range shifts and richness on tropical elevational gradients under Quaternary glacial cycles. Philosophical Transactions of the Royal Society B, 365, 3695–3707. Gaston, K.J. & Blackburn, T.M. (2000) Pattern and proc‐ ess in macroecology. Blackwell, London. Griffith D.A. & Peres‐Neto, P.R. (2006) Spatial modeling in ecology: the flexibility of eigenfunction spatial analysis. Ecology, 87, 2603‐2613. Losos, J.B. (2011) Seing the forest for the trees: the limitations of phylogenies in comparative biol‐ ogy. American Naturalist, 177, 709‐727. Olalla‐Tarraga, M.A., McInnes, L., Bini, L.M., Diniz‐Filho, J.A.F., Fritz, S.A., Hawkins, B.A., Hortal, J., Orme, C.D.L., Rahbek, C., Rodriguez, M.A. & Purvis, A. (in press) Climatic niche conservatism and the evolutionary dynamics in species’ range bounda‐ ries: global congruence across mammals and amphibians. Journal of Biogeography. Phillimore, A.B., Hadfield, J.D., Jones, O.R. & Smithers, R.J. (2010) Differences in spawning date be‐ tween populations of common frog reveal local adaptation. Proceedings of the National Acad‐ emy of Sciences of the USA, 107, 8292‐8297. Ree, R.H. & Sanmartin, I. (2009) Prospects and chal‐ lenges for parametric models in historical bio‐ geographical inference. Journal of Biogeogra‐ phy, 36, 1211‐1220. Edited by Joaquin Hortal symposium summary Marine biogeography A contributed session at the 5th International Biogeography Society Conference – Heraklion, Greece, 7–11 January 2011 How does life in the sea compare or contrast with life in or above its land, lakes, rivers, and streams? By asking questions about geographic patterns of marine speciation (Renema, Tellier et al., Tyberg‐ hein et al.), how well landscape metrics describe saltwater systems (Wedding et al.), and how pat‐ terns of human use impact the marine biota (Davidson et al., Tittensor and Worm), one might begin to intuit some answers. Willem Renema’s study of the distinctive Cycloclypeus, the largest living benthic foraminif‐ era (10‐12 cm diameter), which occupies a very well characterized niche on tropical coral reefs, provides a well‐constrained fossil series, of a qual‐ ity rare in any realm, for examining patterns and rates of speciation. He found that long‐distance dispersal events from the Indo‐West Pacific culmi‐ nate in extinction in the Mediterranean, and that morphological disparity increases regionally but not globally during the Middle Miocene following range contraction and re‐expansion. The region of speciation may vary among taxa, even if speci‐ ation is contemporaneous. These results suggest patterns of marine species diversity can be influ‐ enced by local ‘species pumps’ (that is, vacuum pumps drawing species in, rather than pressure pumps pushing species outward) causing regional dispersal and diversification, and that evolution‐ ary ecological dynamics of at least some taxa may play out as a series of taxon cycles. Florence Tellier and colleagues also found evidence of geographically variable evolutionary dynamics along the intertidal rocky shore of the southeastern Pacific. Their comparative phy‐ logeographic study of two low‐dispersal co‐ distributed species, an isopod (Excirolana hirsuti‐ cauda) and a kelp (Lessonia nigrescens), revealed genetic discontinuities in both taxa at 30 o S, with sub‐groups—possibly cryptic species—of one or the other species either side of this well‐known biogeographic filter. Their inference is that con‐ cordant phylogeographic patterns may arise from significant environmental factors such as vicari‐ ance mediated by oceanography. Yet, secondary discontinuities that are geographically incongru‐ ent exist in these taxa: at 27°S for the kelp and 33° S for the isopod. Transplant and laboratory experi‐ ments with the kelp suggested at least some of frontiers of biogeography 3.1, 2011 — © 2011 the authors; journal compilation © 2011 The International Biogeography Society

Variation in Morphological Characters of Two Invasive Leafminers,Liriomyza huidobrensisandL. sativae,across a Tropical Elevation Gradient

Changes in morphological traits along elevation and latitudinal gradients in ectotherms are often interpreted in terms of the temperature-size rule, which states that the body size of organisms increases under low temperatures, and is therefore expected to increase with elevation and latitude. However other factors like host plant might contribute to spatial patterns in size as well, particularly for polyphagous insects. Here elevation patterns for trait size and shape in two leafminer species are examined, Liriomyza huidobrensis (Blanchard) (Diptera: Agromyzidae) and L. sativae Blanchard, along a tropical elevation gradient in Java, Indonesia. Adult leafminers were trapped from different locations in the mountainous area of Dieng in the province of Central Java. To separate environmental versus genetic effects, L. huidobrensis originating from 1378 m and 2129 m ASL were reared in the laboratory for five generations. Size variation along the elevation gradient was only found in L. huidobrensis and this followed expectations based on the temperature-size rule. There were also complex changes in wing shape along the gradient. Morphological differences were influenced by genetic and environmental effects. Findings are discussed within the context of adaptation to different elevations in the two species.

The impact of sterile populations on the perception of elevational richness patterns in ferns

Dispersal may influence the spatial distribution of species richness through mass or source-sink effects, but the extent of sink populations at the community level remains largely unknown due to difficulties of identifying such populations. We compared the richness patterns of ferns in 333 plots along six tropical elevational gradients in America, the Mascarenes, and southeast Asia, using sterile populations as an indication of sink populations. First, we tested whether sterile fern records were more common towards the elevational range limits of the individual species, but found this pattern in only one out of ten cases. It is therefore uncertain if sterile records correspond to marginal sink populations. Second, we compared the elevational richness patterns of sterile and fertile species. In several cases, elevational trends for sterile and fertile records were quite similar, but in others they differed distinctly. The percentage of sterile records per plot decreased with elevation among epiphytic ferns along all six transects, whereas terrestrials showed mixed results (decrease, increase, and U-shaped patterns). The percentage of sterile species records per plot relative to the number of species per plot recovered four significant patterns among the twelve cases analysed: higher percentages at higher species numbers among terrestrial ferns on two transects and lower percentages among epiphytes on two others. Despite the problems with equating sterile records to sink populations, we thus found distinct elevational patterns of sterile records that clearly affected our perception of the overall richness patterns. Ignoring the impact of population dynamics on diversity patterns is thus liable to result in misinterpretations of the diversity patterns.

A stochastic, evolutionary model for range shifts and richness on tropical elevational gradients under Quaternary glacial cycles

Quaternary glacial-interglacial cycles repeatedly forced thermal zones up and down the slopes of mountains, at all latitudes. Although no one doubts that these temperature cycles have left their signature on contemporary patterns of geography and phylogeny, the relative roles of ecology and evolution are not well understood, especially for the tropics. To explore key mechanisms and their interactions in the context of chance events, we constructed a geographical range-based, stochastic simulation model that incorporates speciation, anagenetic evolution, niche conservatism, range shifts and extinctions under late Quaternary temperature cycles along tropical elevational gradients. In the model, elevational patterns of species richness arise from the differential survival of founder lineages, consolidated by speciation and the inheritance of thermal niche characteristics. The model yields a surprisingly rich variety of realistic patterns of phylogeny and biogeography, including close matches to a variety of contemporary elevational richness profiles from an elevational transect in Costa Rica. Mountaintop extinctions during interglacials and lowland extinctions at glacial maxima favour mid-elevation lineages, especially under the constraints of niche conservatism. Asymmetry in temperature (greater duration of glacial than of interglacial episodes) and in lateral area (greater land area at low than at high elevations) have opposing effects on lowland extinctions and the elevational pattern of species richness in the model--and perhaps in nature, as well.

Phylogenetic diversity, trait diversity and niches: species assembly of ferns along a tropical elevational gradient

Aim To analyse the structure of pteridophyte assemblages, based on phylogenetic relatedness and trait properties, along an elevational gradient. Ecological theory predicts that co-occurring species may be: randomly selected from a regional pool; ecologically sorted so that they are functionally different hence resulting in reduced competition (overdispersion); or functionally similar as an adaptation to specific ecological conditions (clustering). Location Braulio Carrillo National Park and Cerro de la Muerte, Costa Rica, Central America. Methods We used an empirical dataset of the quantitative pattern of species occurrences and individual numbers of ferns within 156 plots along a tropical elevational gradient to test whether directed ecological sorting might cause deviations in patterns of trait and phylogenetic diversity. Mean pairwise distances of species based on phylogenetic and trait properties were compared with two different sets of null assemblages, one maintaining species frequency distributions (constrained) and one not (unconstrained). Results Applying different null models resulted in varying degrees of overdispersion and clustering, but overall patterns of deviation from random expectations remained the same. Contrary to theoretical predictions, phylogenetic and trait diversity were relatively independent from one another. Phylogenetic diversity showed no patterns along the elevational gradient, whereas trait diversity showed significant trends for epiphytes. Main conclusions Under stressful environmental conditions (drought at low elevations and frost at high elevations), epiphytic fern assemblages tended to be clustered with respect to trait characteristics, which suggests environmental filtering. Conversely, under less extreme environmental conditions (middle of the transect), the sorting was biased towards high differentiation (overdispersion), presumably because of interspecific competition and trait shifts among closely related species (character displacement).

Weighing Defensive and Nutritive Roles of Ant Mutualists Across a Tropical Altitudinal Gradient

The diversity of mutualistic interactions influences many ecological components of community structure, including biodiversity and ecosystem stability. However, mutualistic interactions are not well resolved because of a historical bias toward examining antagonistic interactions. Here we examine both antagonistic and facilitative interactions between tropical plants and arthropods by characterizing the biotic interactions between a common myrmecophytic shrub, Piper immutatum Trel. (Piperaceae), the ants hosted by this plant, Pheidole sp. (Formicidae: Myrmicinae), and their associated communities of herbivorous and predatory arthropods. To determine if ant mutualists affect the altitudinal distribution of Neotropical myrmecophytes, P. immutatum interactions with arthropods were quantified across a tropical elevational gradient. Piper immutatum was most abundant in lower montane forests (1000‐1600masl) and disappeared above 1600masl, and colonies of Pheidole sp. inhabited 90 percent of the sampled plants. The myrmecophyte was then transplanted within and beyond its altitudinal range, excluding ants from half of the transplanted plants. Plant survival was affected primarily by elevation, with only 20 percent surviving above 1600masl. Ant exclusion did not significantly affect plant mortality. Nevertheless, ant colony size did affect both herbivory and nutrient availability for surviving P. immutatum, with nutrient availability having a stronger effect than antiherbivore defense on growth and biomass. This approach of studying the contributions of ant mutualisms across the myrmecohpyte’s habitat range yields an improved picture of the role of mutualistic interactions in determining community structure.

Life history variation in a temperate plant invader, Verbascum thapsus along a tropical elevational gradient in Hawaii

Few studies have examined the life history of temperate plant invaders in the tropics. Temperate invaders that utilize seasonal cues to influence their life histories may be expected to behave differently in the tropics. This study examined variation in life history in an invading temperate weed, Verbascum thapsus, across an elevation gradient (1,690–2,720 m) along the montane and subalpine slopes of Mauna Kea, Hawaii. Over 7,000 seedlings were marked and monitored over a period of 3 years. Germination, survival, growth, and reproduction in V. thapsus varied among sites along the elevational gradient. Compared to plants at lower elevations, those at higher elevation sites (>2,000 m) had lower early seedling survival, higher established rosette survival, higher vegetative growth rates, higher threshold sizes for flowering, and commonly lived more than 3 years before flowering. The abundance of competing vegetation generally decreased with elevation, and this may drive variation in V. thapsus survival and growth. Size-dependent survival appears to play a major role in the selection for smaller size at first flowering and shorter generation time at lower elevations. This pattern is opposite to that reported in temperate mountains where high elevation plants flower sooner and at smaller size, but both patterns appear consistent with general life history theory for biennials. Due to novel biotic and climatic interactions in the tropics, predictions of growth patterns and invasion dynamics for temperate weeds in the tropics can be misleading when based on the plant’s behavior in temperate systems.

The determinants of land snail diversity along a tropical elevational gradient: insularity, geometry and niches

AbstractAim We investigated the patterns of species richness in land snails and slugs along a tropical elevational gradient and whether these patterns correlate with area, elevation, geographic constraints, and productivity. We did so both at the scale at which land snail population processes take place and at the coarser scale of elevational zones.Location Mount Kinabalu (4096 m) and the adjacent Mount Tambuyukon (2588 m) in Kinabalu Park, Sabah, Malaysian Borneo.Methods We used an effort‐controlled sampling protocol to determine land snail and slug species richness in 142 plots of 0.04 ha at elevations ranging from 570 to 4096 m. Extents of elevational ranges were determined by interpolation, extended where appropriate at the lower end with data from lowlands outside the study area. We used regression analysis to study the relationships between species density and richness on the one hand and elevation and area on the other. This was done for point data as well as for data combined into 300‐m elevational intervals.Results Species density (based on the individual samples) showed a decline with elevation. Elevational range length profiles revealed that range lengths are reduced at greater elevations and that a Rapoport effect is absent. Diversity showed a mild mid‐domain effect on Kinabalu, but not on Tambuyukon. When the data were combined into 300‐m elevational intervals, richness correlated more strongly with elevation than with area. Ecomorphospace was seen to shrink with increasing elevation.Main conclusions The elevational species richness patterns show the combined effects of (1) reduced niche diversity at elevations with lower productivity and (2) historical events in which the upward migration of lowland species as well as the speciation of highland endemics took place.

Fungal growth on a common wood substrate across a tropical elevation gradient: Temperature sensitivity, community composition, and potential for above-ground decomposition

Fungal breakdown of plant material rich in lignin and cellulose (i.e. lignocellulose) is of central importance to terrestrial carbon (C) cycling due to the abundance of lignocellulose above and below-ground. Fungal growth on lignocellulose is particularly influential in tropical forests, as woody debris and plant litter contain between 50% and 75% lignocellulose by weight, and can account for 20% of the C stored in these ecosystems. In this study, we evaluated factors affecting fungal growth on a common wood substrate along a wet tropical elevation gradient in the Peruvian Andes. We had three objectives: 1) to determine the temperature sensitivity of fungal growth – i.e. Q 10, the factor by which fungal biomass increases given a 10 °C temperature increase; 2) to assess the potential for above-ground fungal colonization and growth on lignocellulose in a wet tropical forest; and 3) to characterize the community composition of fungal wood decomposers across the elevation gradient. We found that fungal growth had a Q 10 of 3.93 (95% CI of 2.76–5.61), indicating that fungal biomass accumulation on the wood substrate nearly quadrupled with a 10 °C increase in temperature. The Q 10 for fungal growth on wood at our site is higher than Q 10 values reported for litter decomposition in other tropical forests. Moreover, we found that above-ground fungal growth on the wood substrate ranged between 37% and 50% of that measured in the soil, suggesting above-ground breakdown of lignocellulose represents an unexplored component of the C cycle in wet tropical forests. Fungal community composition also changed significantly along the elevation gradient, and Ascomycota were the dominant wood decomposers at all elevations. Fungal richness did not change significantly with elevation, directly contrasting with diversity patterns observed for plant and animal taxa across this gradient. Significant variation in fungal community composition across the gradient suggests that the characteristics of fungal decomposer communities are, directly or indirectly, influenced by temperature.