Habitat-dependent advertisement call variation in the monkey frog Phyllomedusa nordestina

The acoustic adaptation hypothesis (AAH) predicts that acoustic signals are selected to propagate more efficiently in the habitat where they are normally transmitted. Several studies corroborated the AAH for primates and birds, but evidence for frogs is contentious: While most studies failed to support the AAH, recent studies have shown that within-species variation conforms to the predictions of the AAH. Herein, we test the AAH by comparing advertisement calls of Phyllomedusa nordestina from two contrasting habitats (Atlantic Rainforest and Caatinga) and by testing the influence of the amount of vegetation around calling sites on acoustic parameters of frog calls. The interval between pulses was significantly different between individuals from the Atlantic Rainforest and from the Caatinga, and the number of pulses was correlated with the amount of vegetation around calling sites. Hence, our results indicate that multiple evolutionary forces may act simultaneously on the advertisement calls of frogs.     

Testing drivers of acoustic divergence in cicadas (Cicadidae: Tettigettalna)

Divergence in acoustic signals may have a crucial role in the speciation process of animals that rely on sound for intra-specific recognition and mate attraction. The acoustic adaptation hypothesis (AAH) postulates that signals should diverge according to the physical properties of the signalling environment. To be efficient, signals should maximize transmission and decrease degradation. To test which drivers of divergence exert the most influence in a speciose group of insects, we used a phylogenetic approach to the evolution of acoustic signals in the cicada genus Tettigettalna, investigating the relationship between acoustic traits (and their mode of evolution) and body size, climate and micro-/macro-habitat usage. Different traits showed different evolutionary paths. While acoustic divergence was generally independent of phylogenetic history, some temporal variables’ divergence was associated with genetic drift. We found support for ecological adaptation at the temporal but not the spectral level. Temporal patterns are correlated with micro- and macro-habitat usage and temperature stochasticity in ways that run against the AAH predictions, degrading signals more easily. These traits are likely to have evolved as an anti-predator strategy in conspicuous environments and low-density populations. Our results support a role of ecological selection, not excluding a likely role of sexual selection in the evolution of Tettigettalna calling songs, which should be further investigated in an integrative approach.     

Call degradation in diploid and tetraploid green toads

According to the 'environmental selection' hypothesis, the physical characteristics of environments influence the evolution of long-range acoustic signals by favouring those properties that decrease sound attenuation and distortion with distance. Different environments could favour different acoustic properties and therefore contribute to the intra- and interspecific variation of calls. In the present paper, we investigate whether this hypothesis could explain the differences between the advertisement calls of three taxa of central-Asian green toads: lowland diploids, highland tetraploids and lowland tetraploids. The pattern of propagation of 12 natural calls (four for each taxon) was analysed in nine localities of Kyrgyzstan and Kazakstan. We broadcast the calls and recorded them along a trasect at distances of 2, 4, 8, 16, and 32 m from the speaker, to estimate sound attenuation and propagation. Attenuation was quantified from the oscillogram (by directly calculating the SPL of calls) and from the power spectrum (by measuring the relative amplitude of the fundamental frequency) , whereas degradation was estimated by cross-correlating spectrograms of the same call at different distances. Results show that: (1) the pattern of sound propagation significantly differs among localities in relation to the different vegetation and environmental noise; (2) in most localities, call attenuation and degradation differ significantly among the three taxa ; (3) such differences are not consistent to those expected under the hypothesis of environmental selection: independent of altitude, lowland tetraploid calls fare worse than both diploid and highland tetraploid calls, whereas diploid and highland tetraploid calls show different patterns of propagation in a few localities only.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 11–26.     

Low connectivity and declining genetic variability along a depth gradient in Corallium rubrum populations

This study examines the possible effect of depth on the connectivity and genetic variability in red coral (Corallium rubrum; Octocorallia: Alcyonacea) populations. Patterns of genetic structuring along a depth gradient (from 20 to 70 m) were investigated in two locations of the western Mediterranean coast (northern Catalan and eastern Ligurian Seas) using 10 microsatellite loci. Strong patterns of genetic structuring among the samples were found both within and between the two study sites. In both locations, consistent patterns of reduction in genetic variability along the depth gradient were also observed, suggesting that depth has an important role in determining the patterns of genetic structure in Corallium rubrum. Moreover, a threshold in connectivity was observed among the samples collected across 40–50 m depth, supporting the hypothesis that discrete shallow- and deep-water red coral populations occur. This finding has major implications for management strategies and the conservation of commercially exploited deep red coral populations.     

Elevation as a barrier: genetic structure for an Atlantic rain forest tree (Bathysa australis) in the Serra do Mar mountain range,SE Brazil

Distance and discrete geographic barriers play a role in isolating populations, as seed and pollen dispersal become limited. Nearby populations without any geographic barrier between them may also suffer from ecological isolation driven by habitat heterogeneity, which may promote divergence by local adaptation and drift. Likewise, elevation gradients may influence the genetic structure and diversity of populations, particularly those marginally distributed. Bathysa australis (Rubiaceae) is a widespread tree along the elevation gradient of the Serra do Mar, SE Brazil. This self‐compatible species is pollinated by bees and wasps and has autochoric seeds, suggesting restricted gene dispersal. We investigated the distribution of genetic diversity in six B. australis populations at two extreme sites along an elevation gradient: a lowland site (80–216 m) and an upland site (1010–1100 m.a.s.l.). Nine microsatellite loci were used to test for genetic structure and to verify differences in genetic diversity between sites. We found a marked genetic structure on a scale as small as 6 km (F_ST = 0.21), and two distinct clusters were identified, each corresponding to a site. Although B. australis is continuously distributed along the elevation gradient, we have not observed a gene flow between the extreme populations. This might be related to B. australis biological features and creates a potential scenario for adaptation to the different conditions imposed by the elevation gradient. We failed to find an isolation‐by‐distance pattern; although on the fine scale, all populations showed spatial autocorrelation until ~10‐20 m. Elevation difference was a relevant factor though, but we need further sampling effort to check its correlation with genetic distance. The lowland populations had a higher allelic richness and showed higher rare allele counts than the upland ones. The upland site may be more selective, eliminating rare alleles, as we did not find any evidence for bottleneck.     

Genetic diversity of Saccharina latissima (Phaeophyceae) along a salinity gradient in the North Sea–Baltic Sea transition zone

The North Sea–Baltic Sea transition zone constitutes a boundary area for the kelp species Saccharina latissima due to a strong salinity gradient operating in the area. Furthermore, the existence of S. latissima there, along Danish waters, is fairly patchy as hard bottom is scarce. In this study, patterns of genetic diversity of S. latissima populations were evaluated along the salinity gradient area of Danish waters (here designated brackish) and were compared to reference sites (here designated marine) outside the gradient area, using microsatellite markers. The results showed that the S. latissima populations were structured into two clusters corresponding to brackish versus marine sites, and that gene flow was reduced both between clusters and between populations within clusters. In addition, results provided empirical evidence that marginal populations of S. latissima in the salinity gradient area exhibited a distinct genetic structure when compared to marine ones. Brackish populations were less diverse, more related, and showed increased differentiation over distance compared to marine populations. The isolation of the brackish S. latissima populations within the salinity gradient area of Danish waters in conjunction with their general low genetic diversity makes these populations vulnerable to ongoing environmental and climate change, predicted to result in declining salinity in the Baltic Sea area that may alter the future distribution and performance of S. latissima in the area.     

Genetic Consequences of Habitat Fragmentation in Black-and-Gold Howler (<Emphasis Type="BoldItalic">Alouatta caraya</Emphasis>) Populations from Northern Argentina

Human-induced habitat fragmentation might seriously affect behavioural patterns and the survival of species whose ecological requirements strongly depend on specific environmental conditions. We compared the genetic structure and dispersal patterns of 2 populations of Alouatta caraya (Plathyrrhini, Atelidae) to understand how habitat reduction and fragmentation affect gene flow in this species. We sampled individuals from 7 groups living in continuous forest (CF, n = 46, 22 males and 24 females), and 11 groups that inhabit a fragmented forest (FF, n = 50, 24 males and 26 females). FST values based on 11 microsatellite loci showed a recent genetic differentiation among groups in the FF. In contrast, the CF showed no differentiation among groups. Further, FST values between sexes, as well as kinship relationships, also exhibited differences between habitats. In the CF, both males and females disperse, leading to nondifferentiated groups composed of adults that are not close relatives. Conversely, in the FF, some groups are differentiated, males disperse more than females, and groups are composed of closely related adult females. Our results suggest that habitat fragmentation modifies the dispersal patterns of black-and-gold howlers. These differences between habitats may reflect a reduced gene flow, providing genetic evidence that suggests that habitat fragmentation severely limits the howler’s ability to disperse. An increasing level of isolation due to uncontrolled deforestation may cause similar loss of genetic diversity on other arboreal primates, and nonprimates that depend on forest continuity to disperse, reducing their abilities to cope with environmental changes.     

Differentiation of Mating Vocalizations in Birds: Acoustic Features in Mainland and Island Populations and Evidence of Habitat-Dependent Selection on Songs

Local environments can act as selective agents on some characteristics of birds’ songs, whereas other song traits may not reflect local genetic adaptation. Geographic variation in songs of two Australian bird species (red‐capped robins Petroica goodenovii, western gerygones Gerygone fusca) was studied to examine one component of the ‘habitat‐dependent selection’ hypothesis. This hypothesis suggests that: (1) the detailed spectral characteristics of male songs are an evolved response to local habitat conditions affecting signal propagation and detection and (2) parallel evolution of other fitness traits sets up the potential for assortative mating by female choice. To examine the first part of the hypothesis, I made comparisons among widespread mainland populations and an island population using two levels of analysis: a typological analysis of song morphology (phonology: notes, syllables, syntax, temporal pattern, repertoires) and a spectral analysis of acoustic characteristics of songs (mean frequency, Wiener entropy, frequency modulation) using an automated procedure of feature extraction (Sound Analysis Pro). Spectral analysis was also used to extract values of the same acoustic features from the background sound environment of each recorded population. The typological analysis revealed no differences among mainland populations of either species, but large differences between mainland songs and those on the island. In contrast, the spectral analysis revealed acoustic divergence among populations, both mainland and island. For both species, Wiener entropy of songs correlated negatively with that of the ambient sound environment, consistent with predictions of the habitat‐dependent selection hypothesis of environmental selection on signal design.     

The mitochondrial control region of the black howler monkey,Alouatta caraya (Primates,Platyrrhini), and the development of new primers

The analysis of polymorphic genetic markers is necessary to assess the genetic variability of natural populations in order to develop effective conservation strategies. However, the molecular genetic variability of most neotropical primates is not well known. Therefore, we describe 13 primers for the amplification and sequencing of the mitochondrial control region that were developed for Alouatta caraya. Six of these primers gave unique bands on four other platyrrhine species (Alouatta palliata, Ateles paniscus, Ateles belzebuth and Saimiri boliviensis) showing their utility for control region amplification and sequencing in these platyrrhine species.     

Predicting the success of an invader: Niche shift versus niche conservatism

Invasive species can encounter environments different from their source populations, which may trigger rapid adaptive changes after introduction (niche shift hypothesis). To test this hypothesis, we investigated whether postintroduction evolution is correlated with contrasting environmental conditions between the European invasive and source ranges in the Asian tiger mosquito Aedes albopictus. The comparison of environmental niches occupied in European and source population ranges revealed more than 96% overlap between invasive and source niches, supporting niche conservatism. However, we found evidence for postintroduction genetic evolution by reanalyzing a published ddRADseq genomic dataset from 90 European invasive populations using genotype–environment association (GEA) methods and generalized dissimilarity modeling (GDM). Three loci, among which a putative heat‐shock protein, exhibited significant allelic turnover along the gradient of winter precipitation that could be associated with ongoing range expansion. Wing morphometric traits weakly correlated with environmental gradients within Europe, but wing size differed between invasive and source populations located in different climatic areas. Niche similarities between source and invasive ranges might have facilitated the establishment of populations. Nonetheless, we found evidence for environmental‐induced adaptive changes after introduction. The ability to rapidly evolve observed in invasive populations (genetic shift) together with a large proportion of unfilled potential suitable areas (80%) pave the way to further spread of Ae. albopictus in Europe.     

Genomic scanning using AFLP to detect loci under selection in the moss Funaria hygrometrica along a climate gradient in the Sierra Nevada Mountains,Spain

The common cord moss Funaria hygrometrica has a worldwide distribution and thrives in a wide variety of environments. Here, we studied the genetic diversity in F. hygrometrica along an abiotic gradient in the Mediterranean high mountain of Sierra Nevada (Spain) using a genome scan method. Eighty‐four samples from 17 locations from 24 to 2700 m were fingerprinted based on their amplified fragment length polymorphism (AFLP) banding pattern. Using PCA and Bayesian inference we found that the genetic diversity was structured in three or four clusters, respectively. Using a genome scan method we identified 13 outlier loci, which showed a signature of positive selection. Partial Mantel tests were performed between the Euclidean distance matrices of geographic and climatic variables, versus the pair‐wise genetic distance of the AFLP dataset and AFLP‐positive outliers dataset. AFLP‐positive outlier data were significantly correlated with the gradient of the climatic variables, suggesting adaptive variation among populations of F. hygrometrica along the Sierra Nevada Mountains. We highlight the additional analyses necessary to identify the nature of these loci, and their biological role in the adaptation process.     

Use of intraspecific variation in thermal responses for estimating an elevational cline in the timing of cold hardening in a sub‐boreal conifer

To avoid winter frost damage, evergreen coniferous species develop cold hardiness with suitable phenology for the local climate regime. Along the elevational gradient, a genetic cline in autumn phenology is often recognised among coniferous populations, but further quantification of evolutionary adaptation related to the local environment and its responsible signals generating the phenological variation are poorly understood. We evaluated the timing of cold hardening among populations of Abies sachalinensis, based on time series freezing tests using trees derived from four seed source populations × three planting sites. Furthermore, we constructed a model to estimate the development of hardening from field temperatures and the intraspecific variations occurring during this process. An elevational cline was detected such that high‐elevation populations developed cold hardiness earlier than low‐elevation populations, representing significant genetic control. Because development occurred earlier at high‐elevation planting sites, the genetic trend across elevation overlapped with the environmental trend. Based on the trade‐off between later hardening to lengthen the active growth period and earlier hardening to avoid frost damage, this genetic cline would be adaptive to the local climate. Our modelling approach estimated intraspecific variation in two model components: the threshold temperature, which was the criterion for determining whether the trees accumulated the thermal value, and the chilling requirement for trees to achieve adequate cold hardiness. A higher threshold temperature and a lower chilling requirement could be responsible for the earlier phenology of the high‐elevation population. These thermal responses may be one of the important factors driving the elevation‐dependent adaptation of A. sachalinensis.     

Spatial Patterns of Haplotype Variation in the Epiphytic Bromeliad Catopsis nutans

Identifying factors governing the origin, distribution, and maintenance of Neotropical plant diversity is an enduring challenge. To explore the complex and dynamic historical processes that shaped contemporary genetic patterns for a Central American plant species, we investigated the spatial distribution of chloroplast haplotypes of a geographically and environmentally widespread epiphytic bromeliad with wind‐dispersed seeds, Catopsis nutans, in Costa Rica. We hypothesized that genetic discontinuities occur between northwestern and southwestern Pacific slope populations, resembling patterns reported for other plant taxa in the region. Using non‐coding chloroplast DNA from 469 individuals and 23 populations, we assessed the influences of geographic and environmental distance as well as historical climatic variation on the genetic structure of populations spanning >1200 m in elevation. Catopsis nutans revealed seven haplotypes with low within‐population diversity (mean haplotype richness = 1.2) and moderate genetic structure (F_ST = 0.699). Pairwise F_ST was significantly correlated with both geographic and environmental distance. The frequency of dominant haplotypes was significantly correlated with elevation. A cluster of nine Pacific lowland populations exhibited a distinct haplotype profile and contained five of the seven haplotypes, suggesting historical isolation and limited seed‐mediated gene flow with other populations. Paleodistribution models indicated lowland and upland habitats in this region were contiguous through past climatic oscillations. Based on our paleodistribution analysis and comparable prior phylogeographic studies, the genetic signature of recent climatic oscillations are likely superimposed upon the distribution of anciently divergent lineages. Our study highlights the unique phylogeographic history of a Neotropical plant species spanning an elevation gradient.     

Genomewide scan for adaptive differentiation along altitudinal gradient in the Andrew's toad Bufo andrewsi

Recent studies of humans, dogs and rodents have started to discover the genetic underpinnings of high altitude adaptations, yet amphibians have received little attention in this respect. To identify possible signatures of adaptation to altitude, we performed a genome scan of 15 557 single nucleotide polymorphisms (SNPs) obtained with restriction site‐associated DNA sequencing of pooled samples from 11 populations of Andrew's toad (Bufo andrewsi) from the edge of the Tibetan Plateau, spanning an altitudinal gradient from 1690 to 2768 m.a.s.l. We discovered significant geographic differentiation among all sites, with an average F_ST = 0.023 across all SNPs. Apart from clear patterns of isolation by distance, we discovered numerous outlier SNPs showing strong associations with variation in altitude (1394 SNPs), average annual temperature (1859 SNPs) or both (1051 SNPs). Levels and patterns of genetic differentiation in these SNPs were consistent with the hypothesis that they have been subject to directional selection and reflect adaptation to altitudinal variation among the study sites. Genes with footprints of selection were significantly enriched in binding and metabolic processes. Several genes potentially related to high altitude adaptation were identified, although the identity and functional significance of most genomic targets of selection remain unknown. In general, the results provide genomic support for results of earlier common garden and low coverage genetic studies that have uncovered substantial adaptive differentiation along altitudinal and latitudinal gradients in amphibians.     

Population Structure and Growth Patterns of Opuntia echios var. gigantea along an Elevational Gradient in the Galípagos Islands1

Cacti growing in forests potentially experience growth limitation due to reduced light availability. To test this hypothesis, we studied the population structure of Opuntia echios var. gigantea at 15 sites on the south side of Isla Santa Cruz, Galípagos Islands, Ecuador. Populations were located in communities ranging from arid scrub at low elevations to closed‐canopy tropical dry forest at higher elevations. Ordination confirmed the existence of a strong elevation‐vegetation gradient. Opuntia abundance peaked at lower elevations (ca 30 m), with lower densities in closed‐canopy sites. For populations in scrub vegetation, density declined fairly regularly with plant height. Populations in forested sites had few plants of intermediate height, suggesting periodic recruitment. Scrub populations had random dispersion, while those in forests were aggregated. The change in spatial pattern may be related to a change in primary reproductive mode from asexual propagation via fallen fruits to propagation via fallen cladodes. Height was significantly correlated with stem diameter. Intercepts of these relationships increased toward higher elevations, probably in response to the increasing height of the surrounding canopy.     

ACOUSTIC COMMUNICATION DISTANCES OF A GOMPHOCERINE GRASSHOPPER

ABSTRACT

I studied in the laboratory and in the field the communication distance of the gomphocerine grasshopper Chorthippus biguttulus, which lives close to the ground in stony, short grass habitats. Using sound measurements and behavioural tests, I examined the frequency dependent attenuation and temporal degradation of the song pattern. Sound measurements (2–40 kHz) in the field revealed that excess attenuation increases with increasing frequency. Close to the ground, excess attenuation reaches 18 dB/m. On the basis of (i) excess attenuations, (ii) tympanic receptor threshold curves and (iii) song spectra, perception distances of female tympanic receptors for male songs and vice versa were calculated. Behavioural tests revealed that, especially in short and stony grass vegetation, the typical habitat of Ch. biguttulus, the temporal song pattern is strongly degraded. Thus, in the laboratory and in tall grass vegetation, sound attenuation was found to limit the communication distance. In contrast, in short grass vegetation, degradation of the temporal pattern was found to limit the communication distance of males listening to female songs. I argue that the exploitation of fast amplitude modulations for song recognition restricts the acoustic communication of grasshoppers to short distances (1–2.2 m). In this respect it seems adaptive to stridulate at low intensities.     

Bioclimatic regions influence genetic structure of four Jordanian Stipa species

Strong environmental gradients can affect the genetic structure of plant populations, but little is known as to whether closely related species respond similarly or idiosyncratically to ecogeographic variation. We analysed the extent to which gradients in temperature and rainfall shape the genetic structure of four Stipa species in four bioclimatic regions in Jordan. Genetic diversity, differentiation and structure of Stipa species were investigated using amplified fragment length polymorphism (AFLP) molecular markers. For each of the four study species, we sampled 120 individuals from ten populations situated in distinct bioclimatic regions and assessed the degree of genetic diversity and genetic differentiation within and among populations. The widespread ruderals Stipa capensis and S. parviflora had higher genetic diversity than the geographically restricted semi‐desert species S. arabica and S. lagascae. In three of the four species, genetic diversity strongly decreased with precipitation, while genetic diversity increased with temperature in S. capensis. Most genetic diversity resided among populations in the semi‐desert species (Φ_ST = 0.572/0.595 in S. arabica/lagascae) but within populations in the ruderal species (Φ_ST = 0.355/0.387 S. capensis/parviflora). Principal coordinate analysis ( PCoA) and STRUCTURE analysis showed that Stipa populations of all species clustered ecogeographically. A genome scan revealed that divergent selection at particular AFLP loci contributed to genetic differentiation. Irrespective of their different life histories, Stipa species responded similarly to the bioclimatic gradient in Jordan. We conclude that, in addition to predominant random processes, steep climatic gradients might shape the genetic structure of plant populations.     

Urban population genetics of slum‐dwelling rats (Rattus norvegicus) in Salvador,Brazil

Throughout the developing world, urban centres with sprawling slum settlements are rapidly expanding and invading previously forested ecosystems. Slum communities are characterized by untended refuse, open sewers and overgrown vegetation, which promote rodent infestation. Norway rats (Rattus norvegicus) are reservoirs for epidemic transmission of many zoonotic pathogens of public health importance. Understanding the population ecology of R. norvegicus is essential to formulate effective rodent control strategies, as this knowledge aids estimation of the temporal stability and spatial connectivity of populations. We screened for genetic variation, characterized the population genetic structure and evaluated the extent and patterns of gene flow in the urban landscape using 17 microsatellite loci in 146 rats from nine sites in the city of Salvador, Brazil. These sites were divided between three neighbourhoods within the city spaced an average of 2.7 km apart. Surprisingly, we detected very little relatedness among animals trapped at the same site and found high levels of genetic diversity, as well as structuring across small geographical distances. Most F_ST comparisons among sites were statistically significant, including sites <400 m apart. Bayesian analyses grouped the samples in three genetic clusters, each associated with distinct sampling sites from different neighbourhoods or valleys within neighbourhoods. These data indicate the existence of complex genetic structure in R. norvegicus in Salvador, linked to the heterogeneous urban landscape. Future rodent control measures need to take into account the spatial and temporal linkage of rat populations in Salvador, as revealed by genetic data, to develop informed eradication strategies.     

Speciation in a keystone plant genus is driven by elevation: a case study in New Guinean Ficus

Much of the world's insect and plant biodiversity is found in tropical and subtropical ‘hotspots’, which often include long elevational gradients. These gradients may function as ‘diversity pumps’ and contribute to both regional and local species richness. Climactic conditions on such gradients often change rapidly along short vertical distances and may result in local adaptation and high levels of population genetic structure in plants and insects. We investigated the population genetic structure of two species of Ficus (Moraceae) along a continuously forested elevational gradient in Papua New Guinea. This speciose plant genus is pollinated by tiny, species‐specific and highly coevolved chalcid wasps (Agaonidae) and represented by at least 73 species at our study gradient. We present results from two species of Ficus sampled from six elevations between 200 m and 2700 m a.s.l. (almost the entire elevational range of the genus) and 10 polymorphic microsatellite loci. These results show that strong barriers to gene flow exist between 1200 m and 1700 m a.s.l. Whereas lowland populations are panmictic across distances over 70 km, montane populations can be disjunct over 4 km, despite continuous forest cover. We suggest that the limited gene flow between populations of these two species of montane Ficus may be driven by environmental limitations on pollinator or seed dispersal in combination with local adaptation of Ficus populations. Such a mechanism may have wider implications for plant and pollinator speciation across long and continuously forested elevational gradients if generalist insect pollinators and vertebrate seed dispersers also form populations based on elevation.     

Competition at the population level along a standing crop gradient: a field experiment in successional grassland

I measured competitive responses of experimentally-established populations of the perennial grass, Andropogon gerardi, across a complex gradient of standing crop and species composition in the successional grasslands of southwest Michigan. The goal was to assess whether long-term (three year) population-level responses of Andropogon to competition matched the inferences made from a previous phytometer study that examined transplant responses to competition across this same gradient over a single growing season.Replicate experimental populations of Andropogon were established at seven grassland sites by sowing seed into 0.5×0.5 m plots that had been denuded of all vegetation. During the first year of the study, all Andropogon populations were maintained as monocultures by hand weeding. At the end of the first growing season, half of the monocultures were selected for continued weeding and half were left open to invasion by competitors for three years. Invasion of the unweeded populations by neighboring plants varied strongly among sites and was positively correlated with standing crop. Increased susceptibility to invasion and competition resulted in the extinction of the unweeded Andropogon populations at the two most productive sites, supporting the hypothesis that Andropogon is restricted by competition to low productivity sites in these grasslands. The finding that the intensity of competition was positively correlated with standing crop is consistent with the previous transplant study, suggesting that short-term experimental assays of competition on the growth of individual transplants may have predictive value for longer-term outcomes of competition at the population level.