Relative Importance Of Environmental Factors Research Articles

Hydroperiod and Cattle Use Associated with Lower Recruitment in an r-Selected Amphibian with a Declining Population Trend in the Klamath Mountains, California

Abstract Recent population declines in amphibians associated with mortality in early life stages highlight the need for a better understanding of the environmental factors related to successful survival to metamorphosis. In our study, we closely examine the relative importance of environmental factors to three stages of recruitment for Cascade frogs (Rana cascadae), a declining amphibian, in high-elevation wet meadows. Our results show that local dynamics are strongly associated with breeding site selection, the number of egg masses, and the number of individuals that survive to metamorphosis per egg mass. Rana cascadae does not tend to breed in wet meadows where nonnative trout are present. Survival to metamorphosis per egg mass is lower when hydroperiod of pools is shorter, cattle use is higher, and native insect predators are present. Our results suggest that future management efforts to conserve R. cascadae should strive to protect and restore wet meadows free from nonnative trout, containing many poo...

Does land use change alter water and nutrient dynamics of phreatophytic trees in the Central Monte desert?

AbstractIn the woodlands of the Central Monte desert (Argentina), phreatophyte vegetation is exposed to different levels of disturbance. Livestock and settlement activity produce mainly nutrient inputs and partial vegetation removal, with a consequent reduction on water use by vegetation. We hypothesize that because of the increased soil water and nutrient resources associated with livestock stations, water stress will be relieved for the remaining Prosopis flexuosa trees, favouring water and nutrient status, and plant growth at the plant scale. The goal of this research was to analyse the physiological responses of P. flexuosa trees that grow in interdune valleys under different land uses, and the changes in relative importance of environmental factors controlling these responses. We compared the ecophysiological responses of adult P. flexuosa trees in two contrasting types of disturbance: disturbed (livestock stations) and relatively undisturbed (control woodlands) stands, in the 2011–2013 growing seasons. Pre‐dawn and midday leaf water potential and stomatal conductance were higher in livestock stations than in control woodlands, suggesting a better water status of P. flexuosa in livestock stations. Isotopic composition (lower δ13C, and higher δ15N) and foliar nitrogen concentration (lower C:N ratios) indicated lower water use efficiency and higher nitrogen absorption from the soil in livestock stations. Both growth of new leaves (foliar area) and the length of young branches were higher in the first season for both land uses, but branch length was higher in livestock stations. This is consistent with the idea that vegetation removal and nutrient contribution of settlement activities improved water and nutrient status of remaining vegetation, and modified the relative importance of factors controlling ecophysiological processes. We concluded that physiological responses and vegetative growth of P. flexuosa were not only affected by meteorological conditions (rainfalls, vapour pressure deficit), but also by the ecohydrological changes caused by changes in land use. Copyright © 2015 John Wiley & Sons, Ltd.

Physical, chemical, and management-related drivers of submerged macrophyte occurrence in Mediterranean farm ponds

The construction of ponds for irrigation has proliferated during last decades in Mediterranean regions. If properly constructed and managed, ponds may simultaneously meet conservation and agricultural objectives: the preservation of certain macrophytes might improve water quality for irrigation purposes. However,theeffectsofmanagementareoftenneglected when analyzing the main drivers of organisms commu- nity structure. Here we investigated the effect of environmental variables, construction type and man- agement practices on the occurrence of submerged macrophytes (vascular plants, charophytes,filamentous algae). Total submerged macrophyte coverage and richness did not vary with pond type, but the presence of submerged macrophytes was significantly higher in embankmentponds.Artificialpondsshowedthehighest charophyte richness but lowest vascular plant occur- rence. Total submerged macrophyte occurrence mainly correlated with conductivity and littoral vegetation, whilst the relative importance of environmental factors was higher than variables related to pond type and management.However,embankmentpondsshowedthe highest richness of vascular plants and filamentous algae, and artificial ponds contributed to the regional pool of charophyte species. Thus, construction pond type and management levels might be relevant to understandsubmergedmacrophytesoccurrenceinman- made ponds, since the three pond types contributed to the local and regional species pool.

The threatened epiphytic lichen Lobaria pulmonaria in the Iberian Peninsula: Genetic diversity and structure across a latitudinal gradient

The current genetic diversity and structure of a species plays a marked role in the species' future response to environmental changes. Identification of the factors that might ensure the long-term viability of populations along its distribution area is therefore important for conserving biodiversity. In this work, infraspecific genetic diversity and structure of the threatened lichen Lobaria pulmonaria was investigated along a latitudinal gradient, spanning the Spanish latitudinal range of L. pulmonaria. Eighteen populations in Northern, Central, and Southern Spain were analysed using six specific fungal microsatellites of L. pulmonaria. Genetic diversity indices were calculated and compared among populations. Genetic differentiation was assessed using AMOVA and Bayesian methods. Additionally, a redundancy analysis was used to estimate the relative importance of environmental factors on the genetic variation among populations. Annual precipitation was the only factor affecting the genetic diversity probably through its influence on population and thallus size of L. pulmonaria, and significantly higher levels of genetic diversity were detected in southern populations. Isolation by distance was not significant, being environmental variables most important factors controlling genetic variation in L. pulmonaria populations.

Drivers of Intensity and Prevalence of Flea Parasitism on Small Mammals in East African Savanna Ecosystems.

The relative importance of environmental factors and host factors in explaining variation in prevalence and intensity of flea parasitism in small mammal communities is poorly established. We examined these relationships in an East African savanna landscape, considering multiple host levels: across individuals within a local population, across populations within species, and across species within a landscape. We sampled fleas from 2,672 small mammals of 27 species. This included a total of 8,283 fleas, with 5 genera and 12 species identified. Across individual hosts within a site, both rodent body mass and season affected total intensity of flea infestation, although the explanatory power of these factors was generally modest (<10%). Across host populations in the landscape, we found consistently positive effects of host density and negative effects of vegetation cover on the intensity of flea infestation. Other factors explored (host diversity, annual rainfall, anthropogenic disturbance, and soil properties) tended to have lower and less consistent explanatory power. Across host species in the landscape, we found that host body mass was strongly positively correlated with both prevalence and intensity of flea parasitism, while average robustness of a host species to disturbance was not correlated with flea parasitism. Cumulatively, these results provide insight into the intricate roles of both host and environmental factors in explaining complex patterns of flea parasitism across landscape mosaics.

Vegetation, distance to the coast, and aeolian geomorphic processes and landforms in a transgressive arid coastal dune system

The main objective of this study was to determine the nature and relative importance of environmental factors that shape the distribution of dune vegetation in the Maspalomas dune field in the Canary Islands, an arid transgressive dune system. Environmental factors were defined according to their expression related to the linear distance from the coast and to the local aeolian geomorphic process-landform context. Plant communities were characterized through an integration of field mapping, digital orthophoto interpretation and GIS analysis. The vegetation of the study area consists of 19 plant communities. In areas with active sedimentary processes, vegetation tends to be localized in slacks. In areas with more stabilized landforms, vegetation appears at both dunes and slacks. In conclusion, the main factor controlling the distribution of vegetation in the transgressive dune system of Maspalomas is the local aeolian sedimentary process-landform context. Distance to the coast is not as significant as an influence. The gradient structuring of environmental factors associated with retentive coastal dune systems and expressed as distance to the coast does not fully capture the biogeomorphic dynamism of transgressive dune fields.

Environmental determinants of tropical forest and savanna distribution: A quantitative model evaluation and its implication

AbstractThe distributions of tropical ecosystems are rapidly being altered by climate change and anthropogenic activities. One possible trend—the loss of tropical forests and replacement by savannas—could result in significant shifts in ecosystem services and biodiversity loss. However, the influence and the relative importance of environmental factors in regulating the distribution of tropical forest and savanna biomes are still poorly understood, which makes it difficult to predict future tropical forest and savanna distributions in the context of climate change. Here we use boosted regression trees to quantitatively evaluate the importance of environmental predictors—mainly climatic, edaphic, and fire factors—for the tropical forest‐savanna distribution at a mesoscale across the tropics (between 15°N and 35°S). Our results demonstrate that climate alone can explain most of the distribution of tropical forest and savanna at the scale considered; dry season average precipitation is the single most important determinant across tropical Asia‐Australia, Africa, and South America. Given the strong tendency of increased seasonality and decreased dry season precipitation predicted by global climate models, we estimate that about 28% of what is now tropical forest would likely be lost to savanna by the late 21st century under the future scenario considered. This study highlights the importance of climate seasonality and interannual variability in predicting the distribution of tropical forest and savanna, supporting the climate as the primary driver in the savanna biogeography.

Risk map for cutaneous leishmaniasis in Ethiopia based on environmental factors as revealed by geographical information systems and statistics.

Cutaneous leishmaniasis (CL) is a neglected tropical disease strongly associated with poverty. Treatment is problematic and no vaccine is available. Ethiopia has seen new outbreaks in areas previously not known to be endemic, often with co-infection by the human immunodeficiency virus (HIV) with rates reaching 5.6% of the cases. The present study concerns the development of a risk model based on environmental factors using geographical information systems (GIS), statistical analysis and modelling. Odds ratio (OR) of bivariate and multivariate logistic regression was used to evaluate the relative importance of environmental factors, accepting P ≤ 0.056 as the inclusion level for the model's environmental variables. When estimating risk from the viewpoint of geographical surface, slope, elevation and annual rainfall were found to be good predictors of CL presence based on both probabilistic and weighted overlay approaches. However, when considering Ethiopia as whole, a minor difference was observed between the two methods with the probabilistic technique giving a 22.5% estimate, while that of weighted overlay approach was 19.5%. Calculating the population according to the land surface estimated by the latter method, the total Ethiopian population at risk for CL was estimated at 28,955,035, mainly including people in the highlands of the regional states of Amhara, Oromia, Tigray and the Southern Nations, Nationalities and Peoples' Region, one of the nine ethnic divisions in Ethiopia. Our environmental risk model provided an overall prediction accuracy of 90.4%. The approach proposed here can be replicated for other diseases to facilitate implementation of evidence-based, integrated disease control activities.

Environmental change drives long-term recruitment and growth variation in an estuarine fish

How individuals respond to environmental change determines the strength and direction of biological processes like recruitment and growth that underpin population productivity. Ascertaining the relative importance of environmental factors can, however, be difficult given the numerous mechanisms through which they affect individuals. This is especially true in dynamic and complex estuarine environments. Here, we develop long-term otolith-based indices of recruitment and growth for estuary perch Percalates colonorum (Bemm River, Australia), to explore the importance of intrinsic (individual, demographic) and extrinsic (hydrologic, climatic, density-dependent) factors in driving estuarine fish productivity. Analyses involved a novel zero-inflated specification of catch curve regression and mixed effects modelling. The 39years of recruitment and 46years of growth data, spanning a period of environmental change including severe drought, displayed considerable inter-annual variation. Recruitment success was strongly related to high freshwater inflows during the spawning season, suggesting that these conditions act as spawning cues for adults and potentially provide favourable conditions for larvae. Individuals displayed age-dependent growth, with highest rates observed at younger ages in years characterized by warm temperatures, and to a lesser degree, greater magnitude base inflow conditions. We detected systematic among-year-class growth differences, but these were not attributable to year class strength, suggesting that environmental conditions experienced by individuals as juveniles can have long-lasting effects of greater importance to population productivity than density-dependent growth responses. The primacy of temperature in driving growth variation highlights that under-appreciated climatic variation can affect estuarine fish productivity through direct physiological and indirect food web mechanisms. We predict that climatic warming will promote individual growth in southerly populations of P. colonorum but concurrently limit recruitment due to forecast reductions in spawning season river discharge. Disparate trait responses are likely in other fishes as they respond to multiple and changing environmental drivers, making predictions of future population productivity challenging.

The effects of environmental change on the spatial and environmental determinants of community-level traits

An advantage of trait-based approaches to ecology is the ability to predict the response of a species assemblage to environmental change through trait–environment relationships. Because species assemblages are also known to be affected by spatial processes, variation in community-level traits may be similarly affected by spatial structure. Furthermore, the importance of spatial structure may vary with changes to the environment. Using a dataset describing a local stream fish assemblage and environmental variables, we examine the relative contribution of environmental and spatial factors in explaining variation in community-level traits across seasons. We also test for any spatial structuring of community-level traits. For most traits, seasonal environmental change did not seem to alter the relative importance of environmental factors. Traits that did not vary consistently with environmental variables across seasons exhibited significant spatial structure. Overall, relationships between traits and environmental variables seemed to operate on a continuum with ‘environmental traits’ (those that were strongly correlated with environmental variables in response to environmental change) at one end to ‘spatial traits’ (those that did not correlate with environments, but exhibited spatial structure) at the other. We suggest that the distinction between these types of traits is important, as different modeling approaches would be appropriate in using community-level traits to predict the response of species assemblages to environmental change.

Environmental Variables Explain Genetic Structure in a Beetle-Associated Nematode

The distribution of a species is a complex expression of its ecological and evolutionary history and integrating population genetic, environmental, and ecological data can provide new insights into the effects of the environment on the population structure of species. Previous work demonstrated strong patterns of genetic differentiation in natural populations of the hermaphroditic nematode Pristionchus pacificus in its La Réunion Island habitat, but gave no clear understanding of the role of the environment in structuring this variation. Here, we present what is to our knowledge the first study to statistically evaluate the role of the environment in shaping the structure and distribution of nematode populations. We test the hypothesis that genetic structure in P. pacificus is influenced by environmental variables, by combining population genetic analyses of microsatellite data from 18 populations and 370 strains, with multivariate statistics on environmental data, and species distribution modelling. We assess and quantify the relative importance of environmental factors (geographic distance, altitude, temperature, precipitation, and beetle host) on genetic variation among populations. Despite the fact that geographic populations of P. pacificus comprise vast genetic diversity sourced from multiple ancestral lineages, we find strong evidence for local associations between environment and genetic variation. Further, we show that significantly more genetic variation in P. pacificus populations is explained by environmental variation than by geographic distances. This supports a strong role for environmental heterogeneity vs. genetic drift in the divergence of populations, which we suggest may be influenced by adaptive forces.

Spatiotemporal variation in the origin of C4 grasses: δ13C analysis of grass pollen from the southeastern United States

The C4-plant functional type is a major evolutionary and ecological success. However, the relative importance of environmental factors, such as climatic conditions and atmospheric CO2 concentrations, in driving the origin of C4 grasses in different parts of the world remains poorly understood. We determined the carbon-isotope composition of 612 individual grains of grass pollen in seven sediment samples from the southeastern United States from the middle/late Eocene. By applying a Bayesian model to these data, we determined the probability of rejecting (at p<0.05) the null model that each sample contained pollen from only C3 grasses. We could not reject the null hypothesis for any sample when using a range of potential values of δ13C of atmospheric CO2. These results suggest that C4 grasses were not present on the landscape during the middle/late Eocene, a conclusion consistent with a limited number of studies from more northern and western regions of North America. In contrast, prior studies indicate that C4 grasses existed during this time in parts of Europe and South America. Together these results imply variations in the location, time, and environmental conditions associated with the origin of C4 grasses.

Epiphytic Diatoms along Environmental Gradients in Western European Shallow Lakes

Diatom‐based assays have been successfully associated worldwide with the tropic status of lakes. Several studies have demonstrated a correlation between epiphytic diatoms and nutrient load in shallow lakes and wetlands. We examine the relative importance of environmental factors in explaining the structure of epiphytic diatom communities in a set of Western European shallow lakes. The effects of lake physical/chemical, morphometric, and geographical variables on diatom assemblages were tested using distance‐based, canonical correspondence and regression analyses. Our results show that epiphytic diatom communities respond, mainly to physical/chemical variables, overriding the effects of lake depth, size, and location. The clustering of studied systems based along a total phosphorus concentration gradient concurs with previous classification of diatom taxa and nutrient state. Assessment of epiphytic diatoms provided a potential method for shallow lake classification.

Spatial Scale or Amplitude of Predictors as Determinants of the Relative Importance of Environmental Factors to Plant Community Structure

AbstractThe literature on tropical rain forest plant‐community relationships with environmental factors usually does not recognize that the relative importance of environmental factors recorded in each study might be due to their amplitude of variation within sites. Geographic scale, however, is recognized as an important modulator of this relative importance. To disentangle the effects of scale and environmental amplitude, ferns and trees in two landscapes of the same size (each 25 km2) with different soil‐fertility amplitudes but similar soil‐texture range were sampled in central Amazonia. We found that major determinants of community structure were the same for ferns and trees. Texture was the main predictor of community structure in the site with homogeneous soil fertility, while availability of exchangeable cations was the main predictor in the site with a wider fertility range. When both sites were analyzed together, soil fertility was the main predictor of community structure and soil texture segregated floristic subgroups within certain ranges of the soil‐fertility gradient. We conclude that: (1) floristic patterns for trees and ferns are congruent; (2) floristic variation depends on the amplitude of the studied gradients, more than on geographical scale; (3) limiting factors are not necessarily the most important predictors of compositional patterns; and (4) communities are structured hierarchically. Therefore, landscape structure (meaning which combinations of environmental factors, their amplitude of variation and which part of the gradient is found within the landscape) affect our perception of the relative importance that environmental factors will have as predictors of species composition.

Scale‐dependent influence of environmental variables on species distribution: a case study on five coastal benthic species in the Baltic Sea

Statistical modelling can be used to relate biological survey data to environmental factors, thereby providing a basis for predictive mapping of species or communities. Although environmental variables vary and influence biota at different scales, models are often fitted without discussion of how scale dependency influences the results. In this study, we analysed the relative importance of environmental factors for the distribution of aquatic species as a function of extent, using data on the cover of five common benthic species (four macrophytes and one animal), from 1731 sites along the Swedish Baltic Sea coast. We modelled the cover and distribution of the five species in relation to salinity, depth, slope, wave exposure and substrate in scale steps from 25 to 1500 km, and analysed the relative contribution of the environmental variables to each species model. The average total deviance explained by the models was generally quite high, and decreased with increasing scale for all macrophyte species, while it increased for the animal, the Baltic Sea blue mussel Mytilus edulis. The relative contribution of different environmental variables changed with scale, and responses also differed between species. The average contribution of salinity increased for all species when moving from local to Baltic Sea scale, and for the Baltic Sea blue mussel it was the single most important factor at the Baltic Sea scale. The average contribution of depth decreased with increasing scale for all species. However, regardless of scale, depth was the most important environmental factor to explain the distribution of all but one of the investigated macrophyte species. This shows that fine scale predictor variables can be of major importance also for species distribution models covering large areas.

An Investigation of Factors Affecting the Decline of Delta Smelt (Hypomesus transpacificus) in the Sacramento-San Joaquin Estuary

The delta smelt is an annual fish that is endemic to the Sacramento-San Joaquin Estuary and is protected under federal and California Endangered Species Acts. Record low abundances have occurred since 2004. Three questions are addressed here: What is the relative importance of environmental factors with direct effects on abundance? Do factors that may have indirect effects provide an explanation of abundance changes? Are effects of environmental factors better accounted for individually or as criteria defining the volume of water with suitable abiotic attributes? Strong evidence was found of density-dependent population regulation. The density of prey was the most important environmental factor explaining variations in delta smelt abundance from 1972 to 2006 and over the recent period of decline in the abundance of the fish. Predation and water temperature showed possible effects. Entrainment of delta smelt at south Delta pumping plants showed statistically significant effects on adult-to-juvenile survival but not over the fish's life cycle. Neither the volume of water with suitable abiotic attributes nor other factors with indirect effects, including the location of the 2 ppt isohaline in the Delta in the previous fall (“fall X2”), explained delta smelt population trends beyond those accounted for by prey density. [Supplementary materials are available for this article. Go to the publisher's online edition of Reviews in Fisheries Science for the following free supplemental resources: information on factor selection and specification; and estimating the volume of abiotic habitat.]

Experimental assessment of the survival and performance of forest herbs transplanted beyond their range limit

Recent anthropogenic climate change is already affecting the range dynamics of many organisms worldwide. Understanding the relative importance of environmental factors and historical processes in determining species’ geographic ranges is crucial for predicting the impacts of climate change. Here, we experimentally assessed the survival and performance of four understorey forest plants (Ornithogalum pyrenaicum, Scilla bifolia, Iris foetidissima and Symphytum tuberosum) transplanted beyond their natural range limit over a 7-year period. Since the survival probability of plants transplanted beyond their natural geographic range is expected to decrease with the home-away distance, the species represent a gradient in the distance between the transplant site and their natural geographic range limit (from 50 to 500km). Three of the four species still survived seven years after introduction. Appreciable survival outside their natural geographic range partly suggests that the ranges of these plant species may be controlled by dispersal limitation. Importantly, an interspecific trend towards decreasing survival with increasing distance from the range limit was apparent. Initial competition from other understorey plants generally limited the survival and performance of the transplanted species. In conclusion, the good survival and performance of the most ‘nearby’ species emphasizes the importance of dispersal limitation in determining range limits, while the poor survival of the ‘farther’ species suggests that their occurrence is (co-)controlled by environmental factors such as the local climate. Finally, we briefly discuss our results within the context of human-mediated assisted migration in the face of climate change.

Evolution of eggshell structure during rapid range expansion in a passerine bird

Summary1. Environmental factors such as temperature, humidity and partial oxygen pressure can affect avian eggshell structure because gas exchange across the shell must allow sufficient water loss while preventing dehydration of the embryo. Studies of species with known chronology of colonization of novel environments provide a powerful insight into the relative importance of ecological factors shaping the evolution of eggshell structure.2. Here, we examined changes in eggshell structure that accompanied rapid range expansion of house finches (Carpodacus mexicanus) across North America. We analysed thickness and pore density in eggshells from three ecologically distinct populations: the native desert population in southwestern Arizona and two 30‐year‐old populations in the northwestern (north‐west Montana) and southeastern (south‐east Alabama) parts of the species’ range. We also conducted cross‐foster exchanges of freshly laid eggs within and between the northwestern and southeastern populations to examine consequences of population differences in eggshell structure on embryo development.3. Eggshell structure was most distinct in a recently established population inhabiting higher humidity environment (southeastern Alabama), where eggs were the largest, eggshells the thickest and pore density the lowest. Populations that experienced highly distinct ambient temperatures (southwestern Arizona and northwestern Montana) nevertheless had similar eggshell structure. These results were corroborated by experiments where humidity differences between cross‐fostered nests had twice the effect on embryo survival compared to the effect of change in ambient temperature. Correspondingly, experimental egg exchanges between southeastern. Alabama and northwestern Montana populations were associated with fourfold increase in embryo mortality compared to within‐population egg exchanges.4. We document rapid evolution of eggshell structure in response to colonization of novel environments and establish the relative importance of environmental factors on avian eggshells. We discuss these results in relation to population variation in incubation behaviour and its ability to shield eggshell structure from the selection exerted by novel environments.

Children's Environmental Health Experience and Interest Among Pediatric Care Providers in Vietnam

Background. In rapidly developing regions of the world such as Southeast Asia, marked industrialization with insufficient regulation places children at increased risk for significant environmental exposures. Health care providers have a key role in identifying, treating and preventing environmentally-related illnesses. Objectives. The authors undertook a survey of pediatric health care providers in Vietnam in order to gain preliminary insight into environmental hazard knowledge, and attitudes and beliefs regarding the relative importance of environmental factors in child health that could guide future initiatives towards building up capacity for children's environmental health in the region. Methods. A formal written and self-administered survey instrument was adapted from the World Health Organization's Children's Environmental Health Survey and translated into Vietnamese. The survey was administered via convenience sampling after formal introduction to children's environmental health (CEH) was made through lectures or meetings with pediatric care providers affiliated with the major children's hospitals and pediatric departments in Vietnam. Results. One hundred forty-one pediatric care providers completed the survey. Most indicated environmental factors are considered to be very important in child health (84%); 98% felt the magnitude of the problem is increasing; and air pollution is seen as the top environmental health issue facing the country. The most commonly identified problems in their clinical experience included: food poisoning due to microbiological agents (85%); pesticide poisoning (77%); tobacco smoke exposure (75%); and inadequate sanitation (60%). Although most (80%) endorsed asking about children's environmental conditions in clinical practice, a little more than a third (39%) were confident taking an environmental exposure history. For most key topics, less than half had received specific training. A majority (63%) of survey respondents were very interested in more environmental health training. Conclusions. Pediatric health care providers in Vietnam believe that environmental hazards in child health is an important topic that is routinely encountered in their care of patients, but training, experience and self-efficacy in these topics are limited.

Seasonal dynamics of zooplankton community in four Mediterranean reservoirs in humid area (Beni Mtir: north of Tunisia) and semi arid area (Lakhmes, Nabhana and Sidi Saâd: center of Tunisia)

The zooplankton community was studied in four Mediterranean reservoirs to assess the relative importance of environmental factors as determinants of zooplankton dynamic in the four seasons. The water temperature and hydrology variations affect the distribution of zooplankton. A positive correlation was established between the total zooplankton and the water temperature ( r=0.9, n=9, p<0.05). Fourteen zooplankton species were identified. Seasonal changes in the density (ANOVA, F=3.7, d.f=36, p<0.01) and the biomass of total zooplankton (ANOVA, F=4.4, d.f=36, p<0.001) were observed. Our results suggest that planktivorous fish may not modify the zooplankton dynamics in Beni Mtir reservoir (oligotrophic). On the contrary, in Sidi Saâd reservoir (mesotrophic), fish predation has major effects on seasonal zooplankton dynamics.