Abiotic Heterogeneity Research Articles

Direct and indirect effects of understorey bamboo shape tree regeneration niches in a mixed temperate forest

Plant cover plays a major role in shaping the nature of recruitment microsites through direct (resource mediated) and indirect (consumer mediated) interactions. Understorey plants may differentially affect seedling establishment, thus contributing to regeneration-niche separation among canopy tree species. We examined patterns of early tree seedling survival resulting from interactive effects of understorey bamboo (Chusquea culeou) and resident consumers in a mixed temperate Patagonian forest, Argentina. Newly germinated seedlings of Nothofagus dombeyi and Austrocedrus chilensis were planted in bamboo thickets and non-bamboo patches, with or without small-vertebrate exclosures. We found species-specific patterns of seedling survival in relation to bamboo cover. Nothofagus survival was generally low but increased under bamboo, irrespective of cage treatment. Desiccation stress accounted for most Nothofagus mortality in open, non-bamboo areas. In contrast, Austrocedrus survival was highest in non-bamboo microsites, as most seedlings beneath bamboo were killed by small vertebrates through direct consumption or non-trophic physical damage. There was little evidence for a negative impact of bamboo on tree seedling survival attributable to resource competition. The balance of simultaneous positive and negative interactions implied that bamboo presence facilitated Nothofagus early establishment but inhibited Austrocedrus recruitment via apparent competition. These results illustrate the potential for dominant understorey plants to promote microsite segregation during early stages of recruitment between tree seedlings having different susceptibilities to water stress and herbivory. We recognise, however, that patterns of bamboo-seedling interactions may be conditional on moisture levels and consumer activity during establishment. Hence, both biotic and abiotic heterogeneity in understorey environments should be incorporated into conceptual models of regeneration dynamics and tree coexistence in forest communities.

La heterogeneidad ambiental en un matorral xerófilo

Se caracterizó la heterogeneidad biótica y abiótica tanto espacial como temporal en dos zonas del matorral xerófilo de la Reserva Ecológica del Pedregal de San Ángel(REPSA). Se identificaron nueve grupos florísticos, los cuales presentaron una distribución diferencial de las especies vegetales presentes en la zona. Cada uno de los grupos fue nombrado respecto a las especies indicadoras, las cuáles fueron consideradas como determinantes de la fisonomía de la comunidad. Utilizando los análisis multivariados(TWINSPAN y CANOCO) se observó que la composición de especies de los grupos se modifica espacial y temporalmente. Este estudio sugiere que la variación de algunos factores abióticos como la profundidad del suelo y la pendiente influye de diferente manera en la heterogeneidad del sistema. La heterogeneidad temporal fue evidente al existir una diferente composición florística en la zona a lo largo de la temporada de lluvias y la temporada de secas.

Changes in the Herbaceous Communities on the Landslide of the Casita Volcano, Nicaragua, during Early Succession

This study examined the main changes in the herbaceous communities during the first four years of succession in a large landslide on Casita Volcano, Nicaragua, located in a densely populated area that has a tropical dry climate. Our main objective was to determine the major pathways of change in community features, such as richness, biovolume, species composition, and abundances of plant traits and to verify if they varied between the different landslide areas based on abiotic heterogeneity and landscape context. Number, percent cover, mean height and traits of herbaceous species, and several abiotic factors related to fertility and geomorphological stability of substrates were sampled in 28 permanent plots. Environmental heterogeneity strongly influenced early successional changes in the herbaceous communities during the four years of the study. Biovolume increased in the unstable and infertile areas and decreased in the stable and fertile landslide areas. In most zones, species richness decreased significantly because of the expansion of a few dominant species that developed a large cover and excluded other species. Those dominant species were mainly responsible for changes in species composition and included annual forbs, e.g., Calopogonium mucunoides and Stizolobium pruriens, graminoids that have rhizomes or stolons, e.g., Hyparrhenia rufa, and perennial nitrogen-fixing forbs, e.g., Clitoria ternatea. They might be important in determining future successional patterns on the Casita Volcano landslide.

Landslide early succession in a neotropical dry forest

Early succession on large landslides in highly humanized areas that have a tropical dry climate is not well studied. This study documented vegetation recovery during the first 4 years after disturbance at a landslide on Casita Volcano, Nicaragua. We aimed to determine the main pathways and causes of change in community features, such as richness, biovolume, and species composition and verify the role played by environmental heterogeneity. Data consisting on number, covers and mean height of woody species and several abiotic factors related to fertility and stability of substrates were obtained from permanent plots in previously defined zones. Pathways of early succession were highly contingent on abiotic heterogeneity and landscape context and were mainly controlled by abiotic factors associated with fertility of substrates, and incidence of human disturbances. Those results might form the basis of a model of early succession on landslides located in densely populated areas within tropical dry ecosystems. Our results suggest that, rather than focusing research on large-scale disturbances, the study of succession in landslides of the type that occurred on Casita Volcano must point towards the response of ecosystems to a much more complex disturbance regime, in which human-induced disturbances play a major role.

The Relationship Between Geodiversity and Habitat Richness in Šumava National Park and Křivoklátsko PLA (Czech Republic): A Quantitative Analysis Approach

The Relationship Between Geodiversity and Habitat Richness in Šumava National Park and Křivoklátsko PLA (Czech Republic): A Quantitative Analysis ApproachThis paper focuses on the development of a quantitative method for evaluating the relationship between abiotic heterogeneity and habitat richness at the landscape level. The study took place in the Křivoklátsko protected landscape area and Šumava national park (Czech Republic). Our initial hypothesis was that habitat richness should be high in areas with high abiotic heterogeneity, and vice versa. GIS vector layers of habitat were used for the formulation of habitat richness. A geological layer, a digital terrain model and hydrographic layers were used to determine abiotic heterogeneity. The study areas were overlain by a grid square and habitat richness and abiotic heterogeneity were assessed in each study cell. The data obtained were used in a statistical model (multiple spatial linear regression, with maximum credibility). The results of the statistical model indicated a significant influence of abiotic heterogeneity on habitat richness.

AQUATIC PLANT COMMUNITY INVASIBILITY AND SCALE-DEPENDENT PATTERNS IN NATIVE AND INVASIVE SPECIES RICHNESS

Invasive species richness often is negatively correlated with native species richness at the small spatial scale of sampling plots, but positively correlated in larger areas. The pattern at small scales has been interpreted as evidence that native plants can competitively exclude invasive species. Large-scale patterns have been understood to result from environmental heterogeneity, among other causes. We investigated species richness patterns among submerged and floating-leaved aquatic plants (87 native species and eight invasives) in 103 temperate lakes in Connecticut (northeastern USA) and found neither a consistently negative relationship at small (3-m2) scales, nor a positive relationship at large scales. Native species richness at sampling locations was uncorrelated with invasive species richness in 37 of the 60 lakes where invasive plants occurred; richness was negatively correlated in 16 lakes and positively correlated in seven. No correlation between native and invasive species richness was found at larger spatial scales (whole lakes and counties). Increases in richness with area were uncorrelated with abiotic heterogeneity. Logistic regression showed that the probability of occurrence of five invasive species increased in sampling locations (3 m2, n = 2980 samples) where native plants occurred, indicating that native plant species richness provided no resistance against invasion. However, the probability of three invasive species' occurrence declined as native plant density increased, indicating that density, if not species richness, provided some resistance with these species. Density had no effect on occurrence of three other invasive species. Based on these results, native species may resist invasion at small spatial scales only in communities where density is high (i.e., in communities where competition among individuals contributes to community structure). Most hydrophyte communities, however, appear to be maintained in a nonequilibrial condition by stress and/or disturbance. Therefore, most aquatic plant communities in temperate lakes are likely to be vulnerable to invasion.

Environmental Control of Early Succession on a Large Landslide in a Tropical Dry Ecosystem (Casita Volcano, Nicaragua)

ABSTRACTWe described the plant communities on a large landslide in a human‐dominated area of tropical dry forest landscape (Casita Volcano, Nicaragua) and in the adjacent forest, 3 yr after landslide occurrence. At both sites, we determined the relationships between spatial changes in environmental factors and the spatial distribution of species and plant traits. Subsequently, we tested the hypothesis that the compositional similarity between the landslide and the forest increased with a decrease in the distance from the forest edge and the width of the landslide. In the forest, the spatial distribution of species and plant traits was determined mainly by an elevational gradient that was associated with the amount of bare soil, whereas, on the landslide, there was no such gradient but species distributions were influenced mostly by the presence of residual agricultural and forest soils and human disturbance. We did not find an increase in compositional similarity between the landslide and the forest at the edge or in the narrow zones of the landslide. Compared to other landslides, the recovery process was strongly influenced by the extreme abiotic heterogeneity, climate seasonality, and human use in the area. The study of succession in tropical dry landslides located in densely populated zones should focus on understanding the response of regional ecosystems to a complex disturbance regime in which human‐induced disturbances play a major role.

Patterns and ecological consequences of abiotic heterogeneity in managed cork oak forests of Southern Spain

AbstractSpatial heterogeneity of abiotic factors influences the structure and function of forests and must be taken into account for their conservation and sustainable management. In this study, we evaluate the heterogeneity of abiotic environmental variables in managed cork oak (Quercus suber L.) forests in southern Spain at patch, site and regional scales. The extent of spatial heterogeneity depended on the environmental variable examined and the scale considered. For example, soil Mn and P and light availability in the understorey were very heterogeneous at the regional scale, while soil N had low regional heterogeneity, but high spatial variability, at patch scale, attributed to open overstorey and grazing disturbance. There was a general trend of increasing heterogeneity with spatial scale. We also study the effects of a silvicultural practice—shrub clearing on the forest environment and its consequence for spatial heterogeneity. Shrub clearing increased understorey light and decreased its spatial heterogeneity with idiosyncratic effects on soil properties and their spatial heterogeneity at each site. Finally, we compare the heterogeneity (estimated by the coefficient of variation) obtained in these cork oak forests with a database compiled from published studies on other forest environments. The comparison revealed a remarkable extent of abiotic heterogeneity in the cork oak forests studied, suggesting that a sustainable management of these forests should combine intrinsic and human induced abiotic heterogeneity to preserve crucial ecological processes and to maintain high levels of biodiversity.

Response of tree seedlings to the abiotic heterogeneity generated by nurse shrubs: an experimental approach at different scales

Spatial heterogeneity of abiotic factors influences patterns of seedling establishment at different scales. In stress‐prone ecosystems such as Mediterranean ones, heterogeneity generated by shrubs has been shown to facilitate the establishment of tree species. However, how this facilitation is affected by spatial scale remains poorly understood. We have experimentally analysed the consequences of the abiotic heterogeneity generated by pioneer shrubs on survival, growth and physiology of seedlings of three important tree species from Mediterranean mountains (Acer opalus ssp. granatense, Quercus pyrenaica and Quercus ilex). Patterns of abiotic heterogeneity and seedling performance were studied at two scales differing in grain: 1) the microhabitat scale, by using open interspaces as controls of the effect of different shrub species, and 2) the microsite scale, analysing the effects of fine‐grain heterogeneity (within‐microhabitat heterogeneity). Results showed that, at the microhabitat scale, seedling establishment of the three tree species significantly benefited from the modification of the abiotic environment by nurse shrubs. However, we found shrub‐seedling interactions to be species‐specific, due to differential modification of both aboveground (light availability) and belowground (soil compaction, water content, and fertility) abiotic factors by nurse shrub species. Heterogeneity at the within‐microhabitat scale was rather high, although it did not significantly affect seedling performance of any of the tree species. The study demonstrates that the effects of the abiotic heterogeneity generated by shrubs are not consistent across the range of spatial scales considered. The regeneration niche of tree species becomes very complex at fine spatial scales, and thereby estimators of abiotic heterogeneity are valuable descriptors of spatial patterns of seedling establishment only when microsite “noise” is averaged out at greater scales.

POPULATION DIFFERENTIATION DECREASES WITH DEPTH IN DEEP-SEA BIVALVES

The deep sea is the largest ecosystem on Earth. Recent exploration has revealed that it supports a highly diverse and endemic benthic invertebrate fauna, yet the evolutionary processes that generate this remarkable species richness are virtually unknown. Environmental heterogeneity, topographic complexity, and morphological divergence all tend to decrease with depth, suggesting that the potential for population differentiation may decrease with depth. To test this hypothesis, we use mitochondrial DNA (16S rRNA gene) to examine patterns of population differentiation in four species of protobranch bivalves (Nuculoma similis, Deminucula atacellana, Malletia abyssorum, and Ledella ultima) distributed along a depth gradient in the western North Atlantic. We sequenced 268 individuals from formalin-fixed samples and found 45 haplotypes. The level of sequence divergence among haplotypes within species was similar, but shifted from between populations at bathyal depths to within populations at abyssal depths. Levels of population structure as measured by phiST were considerably greater in the upper bathyal species (N. similis = 0.755 and D. atacellana = 0.931; 530-3834 m) than in the lower bathyal/abyssal species (M. abyssorum = 0.071 and L. ultima = 0.045; 2864-4970 m). Pairwise genetic distances among the samples within each species also decreased with depth. Population trees (UPGMA) based on modified coancestry coefficients and nested clade analysis both indicated strong population-level divergence in the two upper bathyal species but little for the deeper species. The population genetic structure in these protobranch bivalves parallels depth-related morphological divergence observed in deep-sea gastropods. The higher level of genetic and morphological divergence, coupled with the strong biotic and abiotic heterogeneity at bathyal depths, suggests this region may be an active area of species formation. We suggest that the steep, topographically complex, and dynamic bathyal zone, which stretches as a narrow band along continental margins, plays a more important role in the evolutionary radiation of the deep-sea fauna than the much more extensive abyss.

POPULATION DIFFERENTIATION DECREASES WITH DEPTH IN DEEP-SEA BIVALVES

The deep sea is the largest ecosystem on Earth. Recent exploration has revealed that it supports a highly diverse and endemic benthic invertebrate fauna, yet the evolutionary processes that generate this remarkable species richness are virtually unknown. Environmental heterogeneity, topographic complexity, and morphological divergence all tend to decrease with depth, suggesting that the potential for population differentiation may decrease with depth. To test this hypothesis, we use mitochondrial DNA (16S rRNA gene) to examine patterns of population differentiation in four species of protobranch bivalves (Nuculoma similis, Deminucula atacellana, Malletia abyssorum, and Ledella ultima) distributed along a depth gradient in the western North Atlantic. We sequenced 268 individuals from formalin-fixed samples and found 45 haplotypes. The level of sequence divergence among haplotypes within species was similar, but shifted from between populations at bathyal depths to within populations at abyssal depths. Levels of population structure as measured by ΦST were considerably greater in the upper bathyal species (N. similis = 0.755 and D. atacellana = 0.931; 530–3834 m) than in the lower bathyal/abyssal species (M. abyssorum = 0.071 and L. ultima = 0.045; 2864–4970 m). Pairwise genetic distances among the samples within each species also decreased with depth. Population trees (UPGMA) based on modified coancestry coefficients and nested clade analysis both indicated strong population-level divergence in the two upper bathyal species but little for the deeper species. The population genetic structure in these protobranch bivalves parallels depth-related morphological divergence observed in deep-sea gastropods. The higher level of genetic and morphological divergence, coupled with the strong biotic and abiotic heterogeneity at bathyal depths, suggests this region may be an active area of species formation. We suggest that the steep, topographically complex, and dynamic bathyal zone, which stretches as a narrow band along continental margins, plays a more important role in the evolutionary radiation of the deep-sea fauna than the much more extensive abyss.

Interannual changes in folivory and bird insectivory along a natural productivity gradient in northern Patagonian forests

Trophic regulation models suggest that the magnitude of herbivory and predation (top‐down forces) should vary predictably with habitat productivity. Theory also indicates that temporal abiotic variation and within‐trophic level heterogeneity both affect trophic dynamics, but few studies addressed how these factors interact over broad‐scale environmental gradients. Here we document herbivory from leaf‐feeding insects along a natural rainfall/productivity gradient in Nothofagus pumilio forests of northern Patagonia, Argentina, and evaluate the impact of insectivorous birds on foliar damage experienced by tree saplings at each end of the gradient. The study ran over three years (1997–2000) comprising a severe drought (1998–1999), which allowed us to test how climatic events alter top‐down forces. Foliar damage tended to increase towards the xeric, least productive forests. However, we found a predictable change of insect guild prevalence across the forest gradient. Leaf miners accounted for the greater damage recorded in xeric sites, whereas leaf chewers dominated in the more humid and productive forests. Interannual folivory patterns depended strongly on the feeding guild and forest site. Whereas leaf‐miner damage decreased during the drought in xeric sites, chewer damage increased after the drought in the wettest site. Excluding birds did not affect leaf damage from miners, but generally increased chewer herbivory on hydric and xeric forest saplings. Indirect effects elicited by bird exclusion became most significant after the drought, when total folivory levels were higher. Thus, interannual abiotic heterogeneity markedly influenced the amount of folivory and strength of top‐down control observed across the forest gradient. Moreover, our results suggest that spatial turnovers between major feeding guilds may need be considered to predict the dynamics of insect herbivory along environmental gradients.

Alternative stable states and regional community structure

Many models of local species interactions predict the occurrence of priority effects due to alternative stable equilibria (ASE). However, few empirical examples of ASE have been shown. One possible explanation for the disparity is that local ASE are difficult to maintain regionally in patch dynamic models. Here we examine two possible mechanisms for regional coexistence of species engaged in local ASE. Biotically generated heterogeneity (e.g., habitat modification that favors further invasion by conspecifics) results in regional exclusion of one species at equilibrium. In contrast, abiotic heterogeneity due to spatial variation in resource supply ratios generates local-scale ASE and ensures regional coexistence with sufficiently broad environmental gradients. Abiotic heterogeneity can result in a species that is the dominant competitor over some of its range being excluded if the area where it is dominant is too small. Biotic heterogeneity can lead to alternative stable landscapes or regional priority effects, while abiotic heterogeneity results in regional determinism. Broad environmental gradients in resource supply favor regional coexistence of species that exhibit local ASE.

THE EFFECTS OF SALINITY AND NUTRIENTS ON A TRITROPHIC SALT-MARSH SYSTEM

In recent years, ecologists have begun to develop a more fine-grained and integrative approach to examining the importance of top-down and bottom-up effects on herbivore populations by investigating how changes in abiotic heterogeneity affect the relative roles of these forces. We performed two factorial field experiments to determine how increasing salinity and nutrient supply affected the relative strengths of top-down and bottom-up forces among the gall-making midge, Asphondylia borrichiae, its host plant Borrichia frutescens, and a suite of parasitoids. Salinity was increased by the addition of salt pellets, and nutrient supply was increased by the addition of fertilizer. In both experiments, parasitism pressure was decreased by trapping hymenopteran parasitoids with yellow sticky traps. In both experiments, bottom-up manipulations had significant effects on gall density. Elevated salinity levels decreased the number of galls per 200 Borrichia stems, and fertilization increased the number of galls. In...

Environmental stress and reproductive mode in Mesaphorura macrochaeta (Tullbergiinae, Collembola)

Summary Sexual populations of the normally parthenogenetic collembolan Mesaphorura macrochaeta have been reported from localities associated with abiotic stress and low environmental heterogeneity. Here, we present the results from an investigation of two localities where both sexual and parthenogenetic forms of the species occur, a sandy beach at the seaside, and a copper polluted field, in Jutland, Denmark. The beach was studied over a transect of 50 meters, stretching from the edge of the water straight inland. The contaminated field was studied over a 140 meter long transect across a copper gradient spanning 20 to 3000 mg Cu/kg dry soil. At both transects we found a fundamental change in species composition of the collembolan community. In the unstressed areas M. macrochaeta constituted a minor part of the community but it was totally dominant in the stressed habitats. Also, the sex ratio changed radically at both sites. We found about 40% males in populations from the areas characterized by severe stress and low environmental heterogeneity, while populations from the unstressed areas showed an all-female composition. The observations conflict with current hypotheses for geographic parthenogenesis and the relationship between reproductive mode and environmental heterogeneity. Additionally, the genetic diversity, measured by RAPDs, was higher among the parthenogens than in the sexual population at the beach site. These findings denote that environmental heterogeneity is not a crucial factor for reproductive strategy in this case, indicating either a species-specific phenomenon, or illustrating a special form of environmental stress promoting sexual reproduction.

The community dynamics of epiphytic midge larvae across the pelagic–littoral interface: do animals respond to changes in the abiotic environment?

In some littoral systems, abiotic heterogeneity results from the edge of vegetation towards the shore as macrophytes gradually impede mixing between pelagic and littoral water. During the summer of 1994, we investigated whether such abiotic heterogeneity influenced epiphytic midge larvae (Diptera:Chironomidae) inhabiting a stand of Scirpus americanus in Saginaw Bay, Lake Huron. After macrophytes became well established, gradients in six parameters of water quality (turbidity, alkalinity, pH, dissolved oxygen, conductivity, and chloride) formed horizontally from the macrophyte edge towards the shore. In outer portions of the stand where physico-chemical properties were influenced by the influx of pelagic surface waves, chironomid abundance increased to 1076 larvae · stem-1. Diversity (H prime ) in these areas remained constant over the summer with grazing and filter-feeding taxa equally represented. Further into the macrophyte bed where water quality indicated highly reduced mixing, midge abundance never exceeded 27 individuals · stem-1 and larval biomass was reduced 2-4 fold. Loss of filter-feeding taxa resulted in a dramatic decline in diversity in these areas after formation of the abiotic gradients. This study suggests that pelagic-littoral water exchange may result in environmental heterogeneity that directly or indirectly influences epiphytic invertebrate community structure and function.