[Show abstract][Hide abstract] ABSTRACT: Aims: Across tropical regions, large forest areas have been converted to different agricultural land uses. These uses impose ecological disturbances affecting forest regeneration potential after field abandonment. Finding ways to identify those agricultural land uses limiting forest regeneration is a critical issue for conserving biodiversity in human-modified landscapes. Here, we developed a fast and inexpensive index, useful for quantifying ecological disturbance regimes associated with agricultural land uses, and tested its power to predict forest regeneration potential.
Location: Municipality of Marques de Comillas, southeast Mexico.
Methods: Interviews were conducted with local farmers to quantify disturbance components (size, duration and severity) associated with agricultural land uses. The scaled values of these disturbance components were added in a simple ecological disturbance index (EDI). In each one of nine recently abandoned
fields representing a wide range of EDI values, two 10-m2 plots, one close to and one far from nearby forest remnants, were established. On each plot, all woody plants of 10–100 m in height were counted, identified and measured in four 1-m2 subplots, at the time of field abandonment and 2 yr later. In addition, at each
plot, 18 site condition (microclimate and soil) attributes were quantified at the time of abandonment. Plant density, biomass, species richness and species diversity were used as regeneration variables, and EDI and site condition attributes as independent ones.
Results: Two years after abandonment, most regeneration variables declined exponentially with EDI. Biomass was not explained by EDI but changed positively with light availability. EDI was strongly correlated to vapour pressure deficit, which also predicted regeneration potential (except biomass).
Conclusions: EDI is a cheap and easy tool for quantifying the ecological disturbance produced by a wide range of agricultural land uses. The index predicted several regeneration variables as well as or better than direct measurements of the site condition at the time of abandonment. EDI can be used to identify biodiversity-friendly agricultural land uses in human-modified landscapes.
[Show abstract][Hide abstract] ABSTRACT: Old-growth tropical forests are being extensively deforested and fragmented worldwide. Yet forest recovery through succession has led to an expansion of secondary forests in human-modified tropical landscapes (HMTLs). Secondary forests thus emerge as a potential repository for tropical biodiversity, and also as a source of essential ecosystem functions and services in HMTLs. Such critical roles are controversial, however, as they depend on successional, landscape and socio-economic dynamics, which can vary widely within and across landscapes and regions. Understanding the main drivers of successional pathways of disturbed tropical forests is critically needed for improving management, conservation, and restoration strategies. Here, we combine emerging knowledge from tropical forest succession, forest fragmentation and landscape ecology research to identify the main driving forces shaping successional pathways at different spatial scales. We also explore causal connections between land-use dynamics and the level of predictability of successional pathways, and examine potential implications of such connections to determine the importance of secondary forests for biodiversity conservation in HMTLs. We show that secondary succession (SS) in tropical landscapes is a multifactorial phenomenon affected by a myriad of forces operating at multiple spatio-temporal scales. SS is relatively fast and more predictable in recently modified landscapes and where well-preserved biodiversity-rich native forests are still present in the landscape. Yet the increasing variation in landscape spatial configuration and matrix heterogeneity in landscapes with intermediate levels of disturbance increases the uncertainty of successional pathways. In landscapes that have suffered extensive and intensive human disturbances, however, succession can be slow or arrested, with impoverished assemblages and reduced potential to deliver ecosystem functions and services. We conclude that: (i) succession must be examined using more comprehensive explanatory models, providing information about the forces affecting not only the presence but also the persistence of species and ecological groups, particularly of those taxa expected to be extirpated from HMTLs; (ii) SS research should integrate new aspects from forest fragmentation and landscape ecology research to address accurately the potential of secondary forests to serve as biodiversity repositories; and (iii) secondary forest stands, as a dynamic component of HMTLs, must be incorporated as key elements of conservation planning; i.e. secondary forest stands must be actively managed (e.g. using assisted forest restoration) according to conservation goals at broad spatial scales.
[Show abstract][Hide abstract] ABSTRACT: Aim Tropical forests store 25% of global carbon and harbour 96% of the world's tree species, but it is not clear whether this high biodiversity matters for carbon storage. Few studies have teased apart the relative importance of forest attributes and environmental drivers for ecosystem functioning, and no such study exists for the tropics. Location Neotropics. Methods We relate aboveground biomass (AGB) to forest attributes (diversity and structure) and environmental drivers (annual rainfall and soil fertility) using data from 144,000 trees, 2050 forest plots and 59 forest sites. The sites span the complete latitudinal and climatic gradients in the lowland Neotropics, with rainfall ranging from 750 to 4350 mm year −1. Relationships were analysed within forest sites at scales of 0.1 and 1 ha and across forest sites along large-scale environmental gradients. We used a structural equation model to test the hypothesis that species richness, forest structural attributes and environmental drivers have independent, positive effects on AGB. Results Across sites, AGB was most strongly driven by rainfall, followed by average tree stem diameter and rarefied species richness, which all had positive effects on AGB. Our indicator of soil fertility (cation exchange capacity) had a negligible effect on AGB, perhaps because we used a global soil database. Taxo-nomic forest attributes (i.e. species richness, rarefied richness and Shannon diversity) had the strongest relationships with AGB at small spatial scales, where an additional species can still make a difference in terms of niche complementarity, while structural forest attributes (i.e. tree density and tree size) had strong relationships with AGB at all spatial scales. Main conclusions Biodiversity has an independent, positive effect on AGB and ecosystem functioning, not only in relatively simple temperate systems but also in structurally complex hyperdiverse tropical forests. Biodiversity conservation should therefore be a key component of the UN Reducing Emissions from Deforestation and Degradation strategy.
[Show abstract][Hide abstract] ABSTRACT: 1.Successional gradients are ubiquitous in nature, yet few studies have systematically examined the evolutionary origins of taxa that specialize at different successional stages. Here we quantify successional habitat specialization in Neotropical forest trees and evaluate its evolutionary lability along a precipitation gradient. Theoretically, successional habitat specialization should be more evolutionarily conserved in wet forests than in dry forests due to more extreme microenvironmental differentiation between early and late successional stages in wet forest.
2.We applied a robust multinomial classification model to samples of primary and secondary forest trees from 14 Neotropical lowland forest sites spanning a precipitation gradient from 788 to 4000 mm annual rainfall, identifying species that are old growth specialists and secondary forest specialists in each site. We constructed phylogenies for the classified taxa at each site and for the entire set of classified taxa, and tested whether successional habitat specialization is phylogenetically conserved. We further investigated differences in the functional traits of species specializing in secondary vs. old-growth forest along the precipitation gradient, expecting different trait associations with secondary forest specialists in wet vs. dry forests since water availability is more limiting in dry forests and light availability more limiting in wet forests.
3.Successional habitat specialization is non-randomly distributed in the angiosperm phylogeny, with a tendency towards phylogenetic conservatism overall and a trend toward stronger conservatism in wet forests than in dry forests. However, the specialists come from all the major branches of the angiosperm phylogeny, and very few functional traits showed any consistent relationships with successional habitat specialization in either wet or dry forests.
4.Synthesis: The niche conservatism evident in the habitat specialization of Neotropical trees suggests a role for radiation into different successional habitats in the evolution of species-rich genera, though the diversity of functional traits that lead to success in different successional habitats complicates analyses at the community scale. Examining the distribution of particular lineages with respect to successional gradients may provide more insight into the role of successional habitat specialization in the evolution of species-rich taxa.
Journal of Ecology 06/2015; DOI:10.1111/1365-2745.12435 · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Lacandona rainforest represents one of the most diverse Mexican tropical wet forests. Although some studies have described the amphibians and reptiles of the region, most herpetological lists come from the northern part of the Lacandona, and there are no confirmed records for many of the expected species. We reviewed databases of scientific collections, taxonomy, and published herpetological lists to produce the most recent updated list of amphibian and reptile species in the region (35 amphibians and 90 reptiles). Furthermore, based on recent inventories (2007–2013) we establish 40 range extensions of 8 amphibians and 32 reptiles for the southeastern part of the Lacandona rainforest. Four out of these 40 records confirmed the occurrence of Dermophis mexicanus, Eleutherodactylus leprus, Pantherophis flavirufus, and Bothriechis schlegelii in the region.
REVISTA MEXICANA DE BIODIVERSIDAD 05/2015; 28(2). DOI:10.1016/j.rmb.2015.04.005 · 0.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Over half of the world's forests are disturbed and the rate at which ecosystem processes recover after disturbance is important for the services these forests can provide. We analyse the drivers underlying changes in rates of key ecosystem processes (biomass productivity, litter productivity, actual litter decomposition and potential litter decomposition) during secondary succession after shifting cultivation in wet tropical forest of Mexico.
We test the importance of three alternative drivers of ecosystem processes: vegetation biomass (vegetation quantity hypothesis), community-weighted trait mean (mass ratio hypothesis) and functional diversity (niche complementarity hypothesis) using structural equation modelling. This allows to infer the relative importance of different mechanisms underlying ecosystem process recovery.
Ecosystem process rates changed during succession, and the strongest driver was aboveground biomass for each of the processes. Productivity of aboveground stem biomass and leaf litter as well as actual litter decomposition increased with initial standing vegetation biomass, whereas potential litter decomposition decreased with standing biomass. Additionally, biomass productivity was positively affected by community-weighted mean of specific leaf area and potential decomposition was positively affected by functional divergence, and negatively by community-weighted mean of leaf dry matter content.
Our empirical results show that functional diversity and community-weighted means are of secondary importance for explaining changes in ecosystem process rates during tropical forest succession. Instead, simply the amount of vegetation in a site is the major driver of changes, perhaps because there is a steep biomass build-up during succession that overrides more subtle effects of community functional properties on ecosystem processes. We recommend future studies in the field of biodiversity and ecosystem functioning to separate the effects of vegetation quality (community-weighted mean trait values and functional diversity) from those of vegetation quantity (biomass) on ecosystem processes and services.
Read More: http://www.esajournals.org/doi/abs/10.1890/14-0472.1
[Show abstract][Hide abstract] ABSTRACT: Conversion of tropical forests to agriculture affects vertebrate assemblages, but we do not know how fast or to what extent these assemblages recover after field abandonment. We addressed this question by examining amphibians and reptiles in secondary forests in southeastern Mexico. We used chronosequence data (12 secondary forests fallow for 1–23 yr and 3 old-growth forest sites) to analyze successional trajectories and estimate recovery times of assemblage attributes for amphibians and reptiles. We conducted 6 surveys at each site over 14 mo (1200 person-hours) and recorded 1552 individuals, including 25 species of amphibians and 36 of reptiles, representing 96 and 74 percent of the expected regional number of species, respectively. Abundance, species richness, and species diversity of amphibians increased rapidly with successional age, approaching old-growth forest values in < 30 yr. Species richness and species diversity of reptiles reached old-growth forest values in < 20 yr. By contrast, the abundance of reptiles and the assemblage composition of amphibians and reptiles recovered more slowly. Along the chronosequence, we observed more species replacement in reptile assemblages than in amphibian assemblages. Several species in the old-growth forest were absent from secondary forests. Dispersal limitation and harsh conditions prevailing in open sites and early successional environments appear to preclude colonization by old-growth forest species. Furthermore, short fallow periods and isolation of forest remnants lead to the formation of new assemblages dominated by species favored by human disturbances. Abstract in Spanish is available in the online version of this article.
[Show abstract][Hide abstract] ABSTRACT: Global plant trait studies have revealed fundamental trade-offs in plant resource economics. We evaluated such trait trade-offs during secondary succession in two species-rich tropical ecosystems that contrast in precipitation: dry deciduous and wet evergreen forests of Mexico. Species turnover with succession in dry forest largely relates to increasing water availability and in wet forest to decreasing light availability. We hypothesized that while functional trait trade-offs are similar in the two forest systems, the successful plant strategies in these communities will be different, as contrasting filters affect species turnover. Research was carried out in 15 dry secondary forest sites (5-63 years after abandonment) and in 17 wet secondary forest sites (
PLoS ONE 04/2015; 10(4). DOI:10.1371/journal.pone.0123741 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Population Dynamics and Sustainable Management of Mescal Agaves in Central Mexico:
in the Tehuacán-Cuicatlán Valley. A total of 37 agave species are extracted from forests of Mexico for producing mescal. This activity has caused decline of numerous populations, and their sustainable management is indispensable for preventing species extinctions. Our study analyzed demographic information about Agave potatorum in the Tehuacán Valley with the goal of developing proposals for sustainable use for agaves in general. We studied protected populations in two contrasting environments, and through prospective analyses and real data about extraction and reforestation rates, we simulated different scenarios of actions. Our analyses indicate that the populations’ growth rates (λ) in conserved populations are 0.9903 ± 0.062 and 1.021 ± 0.062, but viability analyses suggest that even those unmanaged populations would decrease 30% to 90% in 30 years. Survival and growth of early agave plant stages contribute most to λ; adult stages and fecundity have low contribution but their conservation is crucial for population recovery. Based on a successful management experience with A. cupreata, we suggest that at least 30% of reproductive plants should be left to ensure seed provision for natural and assisted populations’ recovery. The reintroduction of plants at two early stages of growth is recommended, particularly 1–2-year-old plants, the size categories with the highest contribution to λ. Current efforts by local people to promote cattle exclusion from forest areas, seed collection, and their propagation in nurseries, and actions for recovery and conservation of populations are strategies of high value. Our research contributes to optimizing the effectiveness of such actions and aids in the conservation of other agave species.
[Show abstract][Hide abstract] ABSTRACT: Tropical forests around the world have been lost, mainly because of agricultural activities. Linear elements like riparian vegetation in fragmented tropical landscapes help maintain the native flora and fauna. Information about the role of riparian corridors as a reservoir of bat species, however, is scanty. We assessed the value of riparian corridors on the conservation of phyllostomid bat assemblage in an agricultural landscape of southern Mexico. For 2 years (2011–2013), mist-netting at ground level was carried out twice during the dry season (December to May) and twice during the wet season (June to November) in different habitats: (1) riparian corridors in mature forest, (2) riparian corridors in pasture, (3) continuous forest away from riparian vegetation, and (4) open pastures. Each habitat was replicated three times. To determine the influence of vegetation structure on bat assemblages, all trees (≥10 cm dbh) were sampled in all habitats. Overall, 1752 individuals belonging to 28 species of Phyllostomi-dae were captured with Sternodermatinae being the most rich and abundant subfamily. Riparian corridors in mature forest and pastures had the greatest species richness and shared 65% of all species. Open pastures had the lowest richness and abundance of bats with no Phyllostominae species recorded. Six of the 18 species recorded could be considered as habitat indicators. There was a positive relationship between bat species composition and tree basal area. Our findings suggest that contrary to our expectations, bats with generalist habits and naturally abundant could be useful detector taxa of habitat modification, rather than bats strongly associated with undisturbed forest. Also in human-dominated landscapes, the maintenance of habitat elements such as large trees in riparian corridors can serve as reservoirs for bat species, especially for those that are strongly associated with undisturbed forest.
Ecology and Evolution 02/2015; 5(4):903-913. DOI:10.1002/ece3.1375 · 2.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Population Dynamics and Sustainable Management of Mescal Agaves in Central Mexico: Agave potatorum in the Tehuacán-Cuicatlán Valley. A total of 37 agave species are extracted from forests of Mexico for producing mescal. This activity has caused decline of numerous populations, and their sustainable management is indispensable for preventing species extinctions. Our study analyzed demographic information about Agave potatorum in the Tehuacán Valley with the goal of developing proposals for sustainable use for agaves in general. We studied protected populations in two contrasting environments, and through prospective analyses and real data about extraction and reforestation rates, we simulated different scenarios of actions. Our analyses indicate that the populations' growth rates (λ) in conserved populations are 0.9903 ± 0.062 and 1.021 ± 0.062, but viability analyses suggest that even those unmanaged populations would decrease 30% to 90% in 30 years. Survival and growth of early agave plant stages contribute most to λ; adult stages and fecundity have low contribution but their conservation is crucial for population recovery. Based on a successful management experience with A. cupreata, we suggest that at least 30% of reproductive plants should be left to ensure seed provision for natural and assisted populations' recovery. The reintroduction of plants at two early stages of growth is recommended, particularly 1–2-year-old plants, the size categories with the highest contribution to λ. Current efforts by local people to promote cattle exclusion from forest areas, seed collection, and their propagation in nurseries, and actions for recovery and conservation of populations are strategies of high value. Our research contributes to optimizing the effectiveness of such actions and aids in the conservation of other agave species. Dinámica de poblaciones y manejo sustentable de agaves mezcaleros en la región central de México: Agave potatorum en el Valle de Tehuacán-Cuicatlán. En total, 37 especies de agaves se extraen de los bosques de México para producir mezcal, lo que ha determinado la degradación o extinción de numerosas poblaciones. Su manejo sustentable es indispensable para evitar la extinción de poblaciones y aún especies. Nuestro estudio analizó información demográfica de poblaciones de Agave potatorum del Valle de Tehuacán con el fin de desarrollar propuestas generales para el uso sustentable de agaves. Estudiamos poblaciones conservadas de agave en dos ambientes contrastantes y a través de análisis prospectivos e información real sobre tasas de extracción y reforestación simulamos las consecuencias de diferentes escenarios de acciones para la conservación. Nuestros análisis indican que las tasas de crecimiento (λ) en poblaciones conservadas son 0.9903 ± 0.062 y 1.021 ± 0.062, respectivamente, pero los análisis de viabilidad poblacional sugieren que aún tales poblaciones decrecerían de 30 a 90%, respectivamente en 30 años. La sobrevivencia y crecimiento de las etapas tempranas del ciclo de vida contribuyen mayormente al valor de λ. Las etapas adultas y la fecundidad tienen baja contribución, pero el manejo sustentable y el manejo de restauración sustentable de las poblaciones requieren que éstas se mantengan. Con base en una experiencia exitosa de manejo de A. cupreata sugerimos
[Show abstract][Hide abstract] ABSTRACT: It has been suggested that tropical defaunation may unleash communitywide
cascading effects, leading to reductions in plant diversity. However,
experimental evidence establishing cause–effect relationships thereof is
poor. Through a 5 year exclosure experiment, we tested the hypothesis that
mammalian defaunation affects tree seedling/sapling community dynamics
leading to reductions in understorey plant diversity. We established plot triplets
(n ¼ 25) representing three defaunation contexts: terrestrial-mammal
exclosure (TE), medium/large mammal exclosure (PE) and open access controls
(C). Seedlings/saplings 30–100 cm tall were marked and identified
within each of these plots and re-censused three times to record survival
and recruitment. In the periods 2010–2011 and 2011–2013, survival was
greater in PE than in C plots and recruitment was higher in TE plots than in
C plots. Overall, seedling density increased by 61% in TE plots and 23% in
PE plots, whereas it decreased by 5% in C plots. Common species highly consumed
by mammals (e.g. Brosimum alicastrum and Ampelocera hottlei) increased
in their abundance in TE plots. Rarefaction curves showed that species diversity
decreased in TE plots from 2008 to 2013, whereas it remained similar for
C plots. Given the prevalence of tropical defaunation, we posit this is an
anthropogenic effect threatening the maintenance of tropical forest diversity.
Proceedings of the Royal Society B: Biological Sciences 12/2014; 282(1800). DOI:10.1098/rspb.2014.2580 · 5.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chronosequences, commonly used to assess succession, have been questioned because of their failure to project successional trajectories. Here, we develop a simple analytical approach combining both chronosequence and dynamic data to test the power of age of abandonment and site factors to explain and predict succession. The approach proceeds by first fitting statistical models relating age to attribute values (the chronosequence model) and their observed changes (the dynamic model) to test explanatory power. Predictive power is then tested by bootstrapping the chronosequence model to derive confidence intervals for expected changes and comparing them with the dynamic model. Finally, residuals from both models are tested against site factors. The procedure was applied to six attributes (basal area, plant density, mean plant height, species richness, evenness, and composition) of the woody community (plants >1 cm dbh within 0.1-ha plots) in nine abandoned cattle pastures (0–12 yr) and three old growth tropical dry forests monitored over 6 yr. Age explained 60–97 percent of the variance in community attributes and only 32–57 percent in observed changes. It significantly overestimated basal area and mean height, while species richness and composition were highly predicted. Besides age, management history also explained successional dynamics. Our results suggest age is not necessarily a reliable predictor of short-term successional dynamics, and explanatory power is not indicative of predictive power. Because of this low reliability, caution is needed when applying chronosequences to evaluate ecosystem services' recovery. The analytical approach developed here contributes to a better exploration of those possible limitations.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
An increasing number of studies have documented chronosequence and dynamic patterns of forest regeneration and secondary succession in old fields in the tropics. However, there is a lack of knowledge about the driving ecological mechanisms, especially regarding plant-vertebrate interactions. Here, we explored the role played by vertebrate herbivores in determining structural changes of regenerative tree communities along secondary succession in abandoned agricultural fields in the La Selva Lacandona, southeastern Mexico. In February of 2005, seven abandoned cornfields, with fallow ages between 0 and 22 years were selected to establish a vertebrate exclusion experiment. In each field a 10 x 50 m plot was delimited where 20 subplots (1 x 2.5 m each) were randomly established. Ten subplots were used as controls and ten were surrounded with metallic mesh (1 m height, 0.5 inch aperture) to exclude vertebrates. All seedlings and saplings 10-200 cm height were tagged, measured, and identified. New censuses were conducted at approximately three-four months intervals during five following years, recording new recruits and the height of survivors.
In total, more than 3,000 individual plants were monitored. Relative to controls, exclosures significantly changed plant density, species density, species replacement rate, and reduced height growth. On average, in absence of vertebrates, plant density increased 82%, while plant height showed a decrease of 211%. These results show that, by consuming seeds and or seedlings, vertebrate herbivores can reduce possible effects of thinning of the community that could happen for competition or interference when plant density is high. The increase in plant abundance in the exclosures was followed by an increase of 216% en species density and by a faster change in species composition, indicating that vertebrate herbivores shape species diversity, filtering the species that can participate along succession. The exclosure effects changed with fallow age, generally becoming stronger in older secondary forests. Our results show that vertebrate herbivores play a critical role in determining the structure and species composition over the forest secondary succession in abandoned tropical fields.
[Show abstract][Hide abstract] ABSTRACT: This work analyzed the structural variation of tree and shrub communities present in different vegetation types (subtro- pical scrub, oak forest, mixed forest, pine and fir forest) across altitudinal gradients and soil depth at the Cuitzeo Basin, Michoacán. The basin covers 4,000 km2 and altitudinal range of 1,700 to 3,420 m a.s.l. In the entire basin we sampled 50 sites 1,000 m2 each, recording a total of 4,941 plants with a diameter at breast height ≥ 1 cm, representing 164 species, 88 genera and 47 families. Fa- milies with more species were Asteraceae (30, 18.4% of total species) and Fagaceae (16, 9.8%). Above 2,500 m a.s.l. were found pine, fir and mixed (pine-oak) forests; below 2,300 m a.s.l. were found the oak forest and subtropical scrub. Plant density was similar among vegetation types but different indicators of community biomass increased with altitude and soil depth. The subtro- pical scrub and mixed forest were the most diverse in species and coniferous forest the less diverse. There was a high turnover of species within and among vegetation types (index of Bray-Curtis similarity, range: 5.4-25.4%). The vegetation composition varied significantly associated with altitude and soil depth. We offer some guidelines for the conservation of the important vegetation complex present in the Cuitzeo Basin.
[Show abstract][Hide abstract] ABSTRACT: Rainforests are undergoing severe deforestation and fragmentation worldwide. A huge amount of small forest patches are being created, but their value in conserving biodiversity and forest structure is still controversial. Here, we demonstrate that in a species-rich and moderately-managed Mexican tropical landscape small rainforest patches (<100 ha) can be highly valuable for the conservation of tree diversity and forest structure. These patches showed diverse communities of native plants, including endangered species, and a new record for the country. Although the number of logged trees increased in smaller patches, patch size was a poor indicator of basal area, stem density, number of species, genera and families, and community evenness. Cumulative species-area curves indicated that all patches had a similar contribution to the regional species diversity. This idea also was supported by the fact that patches strongly differed in floristic composition (high β-diversity), independently of patch size. Thus, in agreement with the land-sharing approach, our findings support that small forest patches in moderately-managed landscapes should be included in conservation initiatives to maintain landscape heterogeneity, species diversity, and ecosystem services.
PLoS ONE 06/2014; 9(6):e98931. DOI:10.1371/journal.pone.0098931 · 3.23 Impact Factor