Effects Of Environmental Heterogeneity Research Articles

The effects of environmental heterogeneity on root growth and root/shoot partitioning.

The purpose of this Botanical Briefing is to stimulate reappraisal of root growth, root/shoot partitioning, and analysis of other aspects of plant growth under heterogeneous conditions. Until recently, most knowledge of plant growth was based upon experimental studies carried out under homogeneous conditions. Natural environments are heterogeneous at scales relevant to plants and in forms to which they can respond. Responses to environmental heterogeneity are often localized rather than plant-wide, and not always predictable from traditional optimization arguments or from knowledge of the ontogenetic trends of plants growing under homogeneous conditions. These responses can have substantial impacts, both locally and plant-wide, on patterns of resource allocation, and significant effects on whole-plant growth. Results from recent studies are presented to illustrate responses of plants, plant populations and plant communities to nutritionally heterogeneous conditions. Environmental heterogeneity is a constant presence in the natural world that significantly influences plant behaviour at a variety of levels of complexity. Failure to understand its effects on plants prevents us from fully exploiting aspects of plant behaviour that are only revealed under patchy conditions. More effort should be invested into analysis of the behaviour of plants under heterogeneous conditions.

GROUPING LOCATIONS FOR EFFICIENT CASSAVA EVALUATION IN MALAWI

Cassava, a crop widely adapted in the tropics, has the important attribute of withstanding adverse environmental conditions better than do many other staple crops. The performance of an individual genotype, however, is influenced by the environment in which it grows. In Malawi, the heterogeneity of agro-ecologies requires the cumbersome and costly assessment of new cassava genotypes at many sites. This study was conducted, therefore, to test the feasibility of selecting only a few locations for cassava evaluation that would be representative of all the agro-ecologies in which cassava is grown in Malawi. Enormous environmental effects, largely contributed by the interaction between season and location, were manifested. Genotype×environment interaction, due largely to a third level interaction (genotype×season×location), was highly significant for all the traits studied. A principal component analysis scatter plot showed no particular grouping of environments, but a pair-wise comparison showed that some of the locations had limited genotype×environment interaction, indicating that it would be sufficient to use one of these sites for evaluating these traits. The value of the residual was often large, probably as an effect of environmental heterogeneity in the test sites. The authors conclude that cassava genetic improvement will continue to be slow if Malawi is used as a single breeding zone. They recommend a much finer grouping of the locations and the use of smaller plot sizes to allow more clones to be tested at more sites for the same cost. Locations may be selected for intensive cassava breeding work from those that give the best discrimination between genotypes while having insignificant genotype×environment interactions in a relatively large number of environments.

The effect of environmental heterogeneity on clonal behaviour of Prunella vulgaris L.

We studied the response of a clonal herb, Prunella vulgaris, to its position in a gap, the identity of neighbouring graminoid species, and simulation of competition for light. The growth characteristics, such as stolon length, internode length and number of stolons were recorded in competitive environments with graminoid bunches of Molinia caerulea, Juncus effusus and Nardus stricta and in different types of gaps in a manipulative field experiment. Two pot experiments studied the effect of reduced light and of low red/far red ratio (R/FR) on the plant growth characteristics. In both field and pot experiments we monitored stolon orientation in a treatment divided into two parts (competition/shaded or gap/unshaded). Methods of circular statistics were used to evaluate stolon orientation. The graminoid bunches had equivalent effects on plant vegetative growth – plants had longer stolons and internodes than plants growing in gaps. The effects of treatments on other characteristics (amounts of stolons and leaves) were not significant. In divided treatment, plants responded by accumulation of stolons and leaves in vegetation removed part, and avoided the vegetated part. In the pot experiments the plant biomass and stolon number decreased in low R/FR and in low irradiance treatments, but the internodes in low R/FR were 26% longer compared to the control. Centroids were uniformly distributed in divided low R/FR treatment contrary to low irradiance treatment where they showed preference for unshaded part. No dependency of stolon length on its orientation was found in divided treatments of both experiments. Prunella vulgaris in heterogeneous environment distinguished favourable and less favourable conditions and produced more stolons with shorter internodes under favourable conditions. This resulted in concentration of resource acquiring structures in favourable patches.

The use of spatial analysis to measure the effect of environmental heterogeneity on genetic variation in Trifolium species from Sardinia

Trifolium species are native to the Mediterranean Basin where they are an important component of permanent pastures. A number of species have become naturalized in Australia and are agriculturally important in annual pastures. To understand the importance of genetic and ecotypic variation in the adaptation of Trifolium species to a new environment, seeds of three predominantly inbreeding and two predominantly outcrossing species of Trifolium were collected from 12 sites in Sardinia in 1998. Detailed ecogeographic information was collected at each site. The progeny were grown at the University of Western Australia Field Station at Shenton Park, Western Australia in 1999, and were scored for 10 morphological characters. Spatial analysis was applied to the data to improve the estimation of accession means, and to investigate the relationship with environmental variables that characterized the sites of collection. The spatially adjusted means were used as the basis for a principal components analysis. Ecogeographical factors at the sites of collection, particularly soil pH, are suggested to be more important than breeding system in determining the extent of genetic variation within the species. The species that showed the greatest genetic variation between accessions were the predominantly inbreeding species T. glomeratum and T. subterraneum, and the predominantly outcrossing species T. nigrescens. It is suggested that the wide genetic variation of these three species is largely due to their being in an ecogeographic environment close to their optimum, and to the possession of a mating system that is neither completely outcrossing nor completely self-fertilizing. The remaining two species, T. tomentosum and T. resupinatum, both occur more frequently on alkaline soils, and it is suggested that for the collected accessions of these two species the acid soils of Sardinia are a stressful environment that does not promote high levels of genetic variation.

The effects of environmental heterogeneity on the performance of Glechoma hederacea: the interactions between patch contrast and patch scale

Summary 1 We report on an experimental investigation into the effects of contrast in patch quality and patch size upon the performance of the clonal herb Glechoma hederacea in nutritionally heterogeneous environments. 2 There were six treatments providing different degrees of patch contrast. These ranged from maximum contrast, in which patches consisted entirely of compost or sand, to homogeneity, in which all patches contained 50% of each. These were combined factorially with two patch size treatments. Overall, all treatments provided the same quantity of compost and sand, and the same area and volume of higher and lower quality patches. 3 Clones in the large‐patch environments produced significantly more root biomass in both rich‐ and poor‐ quality patches than the clones in the small‐patch environments. At both scales there was a decline in root biomass in the rich patches, accompanied by an increase in root biomass in the poor patches, as patch quality converged. 4 The proportion of the root biomass of clones that was located in nutrient‐rich patches was greatest at high contrast and declined gradually to become equal in all patches, as contrast between patches diminished to homogeneity. This effect was independent of patch scale. There was a close match between root placement and nutrient availability in different quality patches in all treatments except that with the highest contrast. 5 At both patch scales, root : shoot ratios of clone parts in rich patches rose (and those in poor patches declined) as patch contrast increased. This species is therefore capable of morphological specialization at a local level when clones grow in heterogeneous environments. These effects were also greater at the larger patch scale. 6 The effect of patch contrast on total clone yield was significantly modified by patch scale. At lower contrast, yield was similar at both scales, whereas at higher contrast, yield was significantly greater in the large‐patch treatment. 7 Although G. hederacea can match its morphology to environmental heterogeneity in some habitats, it is less able to do so when patches are small‐scale and highly contrasting. These limitations cause considerable differences in the yield that can be achieved from a given quantity of resource provided in different configurations.

Effects of environmental heterogeneity on Lepidoptera and Hymenoptera populations in Eucalyptus plantations in Brazil

For 5 months, insects from the Lepidoptera and Hymenoptera orders were collected at five sites: in the interior of a Brazilian native forest, in the zone of contact between the native forest and a Eucalyptus plantation, and in three sites within the Eucalyptus plantation, at 200, 400 and 600 m from the edge of the native forest. Lepidoptera were collected with light traps and divided into three groups in relation to the damage they cause to Eucalyptus: primary pests, secondary pests, and other abundant but non-pest species. Hymenoptera were collected with Malaise traps and categorized as predators, egg parasitoids or larval parasitoids. In general, the heterogeneity hypothesis was supported: total lepidopteran abundance was higher and total natural enemy abundance was lower in the Eucalyptus plantation than within the native forest or at the edge of the native forest and the Eucalyptus plantation. Negative correlations were found between the abundance of Lepidoptera and that of egg parasitoids and larval parasitoids. No significant differences in the abundance of lepidopteran pests and hymenopteran natural enemies were found among the three sites within the Eucalyptus plantation. Plantations of Eucalyptus intercalated with native vegetation could reduce outbreaks of lepidopteran pests by increasing numbers of Hymenoptera natural enemies of these pests.

Environmental heterogeneity as a strategy for pest management in Eucalyptus plantations

The effect of environmental heterogeneity on the distribution and abundance of Lepidoptera was tested in Eucalyptus plantations interwoven with natural vegetation. Collections were carried out in Aracruz, ES, Brazil, in five sites along a 1000 m transect starting in a native woodland and penetrating stands of Eucalyptus grandis and Eucalyptus saligna. Collections were undertaken with light traps, three times a month, from April to August, 1993, recording the number of individuals per morphospecies in each site. A positive correlation was found between the number of sites where each species was recorded and its mean local abundance ( r 2 = 0.45; P < 0.01; n = 790), indicating that the more ubiquitous Lepidoptera species are also those that were locally more abundant. No pest species was recorded among those very abundant. Pest species generally presented moderate abundances and only three out of ten were found at all sites. It seems, therefore, that pest species were constrained by the heterogeneity conferred by the coexistence of Eucalyptus plantations and remnants of native vegetation.

Soft selection and quantitative genetic variation: a laboratory experiment

The effect of environmental heterogeneity on the genetic variation of different quantitative characters was studied in two laboratory and two recently captured populations of Drosophila melanogaster. Two different culture media (habitats R and G) were used. Coarse-grained heterogeneity with independent density control in each habitat (R + G), and fine-grained (R/G) heterogeneity were simulated in population cages. Control populations in both R and G habitats were also maintained. Genetic differences for oviposition-site preference, larval preference and/or within-habitat viability were found between subpopulations sampled from different media. This happened in all four populations maintained on R + G, two populations maintained on R/G, and one control population. Thus, environmental heterogeneity seems to protect genetic variability responsible for between-habitat genetic differentiation, particularly when such heterogeneity corresponds to the 'soft selection' model (R + G). However, for the quasi-neutral trait sternopleural bristle number, no genetic between-habitat differentiation, nor increased heritability were observed in populations maintained under any kind of environmental heterogeneity. Hence, although soft selection seems to be a real force in determining adaptation to heterogeneous environments, the genetic variability maintained may be small in relation to the whole genome.

Inferences on the inheritance of congenital anomalies from temporal and spatial patterns of occurrence.

Most congenital anomalies are believed to result from interactions between genetic and environmental determinants, whose relative importance is not generally established. Temporal and spatial patterns allow inferences on the underlying transmission processes; in particular, it is possible to discriminate between sporadic and nonsporadic genetic factors, and to find evidence for the effects of environmental heterogeneity in time and space. We studied the occurrence of 14 anomalies in 14 registries of Western Europe. Four basic patterns have been identified: (1) Chromosomal abnormalities have uniform incidences and do not show significant geographical variation, in agreement with the expected consequences of randomly scattered nondisjunction events. (2) The homogeneous spatial distributions of three severe malformations (renal agenesis, oesophageal atresia, ano-rectal atresia) are consistent with both the effects of fresh mutation and segregation of detrimental alleles. (3) A decrease of similarity of incidences with distance has been observed for neural tube defects, and this is the expected consequence of isolation by distance on genetically determined traits. (4) For facial clefts, polydactyly, and hypospadias, all postulated processes poorly account for the observed temporal and spatial patterns.

Factors Influencing Movements of Pteronarcys Nymphs in Pacific Creek, Grand Teton National Park, Wyoming

Controversy continues over the relative importance of resource limitation and intraspecific competition in stream ecosystems (Grossman et al 1982, Hart 1983). The chief trouble is separating density independent effects (e.g., Knight and Gaufin 1963) from effects that are density-dependent (i.e., biological) (Peckarsky 1979). Movements of marked individuals can reveal fine detail about intraspecific coupetition or effects of environmental heterogeneity. My research uses individually marked stoneflies (Pteronarcys californica) to study movement patterns. Pteronarcys was chosen because it is large enough to tag and can be caught by electroshocking. The work has 2 parts: a descriptive study of stonefly novements, and a series of manipulations to probe factors affecting them. The descriptive study was conducted in 1987: (1) to determine feasibility of catching and marking large numbers of stoneflies; (2) to obtain life history data (sex, size, phenology); (3) to find out the size of Pteronarcys home ranges; and (4) to discover maximum distance and maximum rate the stoneflies can move. Purposes (3) and (4) were intended to guide formulation of the manipulation study by providing an estimate for sample and quadrat sizes. The manipulation study, (July-November 1988), was a randomized block ANOVA design. Food and stonefly density were manipulated in 4m2 quadrats in Pacific Creek. Purposes of the manipulation were: (1) to see if stoneflies use available habitat differentially; (2) to discover the relative importance of food availability in explaining stonefly distribution; (3) to test for intraspecific interactions as determinants of movement; (4) to look for interactions between food and intraspecific factors. I present here some results from 1987 and summarize data from 1988 (only now beginning to be analyzed).

Landmarks or Obstacles: The Effects of Spatial Heterogeneity on Bumble Bee Foraging Behavior

Nectar-collecting bumble bees were followed on their foraging routes between plants of Hieracium aurantiacum and the effects of environmental heterogeneity on interplant flight distance and directionality were examined. In both natural environments and those in which landmarks (environmental features protruding above the herb layer) were removed, there was a significant inverse relationship between residence time per flowering head and distance of the next inter-plant flight made. However, bumble bees moved differently in areas containing landmarks versus uniform flower stands in two respects: 1) Inter-plant flight distances were significantly longer in uniform areas. 2) Significantly fewer backwards-oriented flights were made in uniform areas. Thus, bumble bees are likely to move more rapidly through uniform habitats than those with boundary or landmark features. Studies of optimal foraging behavior in pollinators, and gene flow in the plants that they service, should consider more comprehensively the range of habitats in which the organisms are found.

Prey-predator models in spatially heterogeneous environments

The effects of environmental heterogeneity on models of prey-predator systems are investigated. Refuge behaviour is found in a continuous gradually varying environment. In this situation we do not necessarily get oscillating population cycles. The stabilizing effect observed depends on environmental variation and is not produced by diffusion alone. Our conclusions are fairly independent of the details of the model.