Spatial Niche Differentiation Research Articles

Diversity of larger consumers enhances interference competition effects on smaller competitors

Competition between large and small species for the same food is common in a number of ecosystems including aquatic ones. How diversity of larger consumers affects the access of smaller competitors to a limiting resource is not well understood. We tested experimentally how species richness (0–3 spp.) of benthic deposit-feeding macrofauna changes meiofaunal ostracods’ incorporation of fresh organic matter from a stable-isotope-labeled cyanobacterial bloom, using fauna from the species-poor Baltic Sea. Presence of macrofauna mostly decreased meiofaunal incorporation of bloom material, depending on the macrofauna species present. As expected, the species identity of macrofauna influenced the incorporation of organic matter by meiofauna. Interestingly, our results show that, in addition, species richness of the macrofauna significantly reduced meiofauna incorporation of freshly settled nitrogen and carbon. With more than one macrofauna species, the reduction was always greater than expected from the single-species treatments. Field data from the Baltic Sea showed a negative correlation between macrofauna diversity and meiofaunal ostracod abundance, as expected from the experimental results. We argue that this is caused by interference competition, due to spatial niche differentiation between macrofauna species reducing the sediment volume in which ostracods can feed undisturbed by larger competitors. Interference from macrofauna significantly reduces organic matter incorporation by meiofauna, indicating that diversity of larger consumers is an important factor controlling the access of smaller competitors to a limiting food resource.

Effects of algal diversity on the production of biomass in homogeneous and heterogeneous nutrient environments: a microcosm experiment.

BackgroundOne of the most common questions addressed by ecologists over the past decade has been-how does species richness impact the production of community biomass? Recent summaries of experiments have shown that species richness tends to enhance the production of biomass across a wide range of trophic groups and ecosystems; however, the biomass of diverse polycultures only rarely exceeds that of the single most productive species in a community (a phenomenon called ‘transgressive overyielding’). Some have hypothesized that the lack of transgressive overyielding is because experiments have generally been performed in overly-simplified, homogeneous environments where species have little opportunity to express the niche differences that lead to ‘complementary’ use of resources that can enhance biomass production. We tested this hypothesis in a laboratory experiment where we manipulated the richness of freshwater algae in homogeneous and heterogeneous nutrient environments.Methodology/Principal FindingsExperimental units were comprised of patches containing either homogeneous nutrient ratios (16∶1 nitrogen to phosphorus (N∶P) in all patches) or heterogeneous nutrient ratios (ranging from 4∶1 to 64∶1 N∶P across patches). After allowing 6–10 generations of algal growth, we found that algal species richness had similar impacts on biomass production in both homo- and heterogeneous environments. Although four of the five algal species showed a strong response to nutrient heterogeneity, a single species dominated algal communities in both types of environments. As a result, a ‘selection effect’–where diversity maximizes the chance that a competitively superior species will be included in, and dominate the biomass of a community–was the primary mechanism by which richness influenced biomass in both homo- and heterogeneous environments.Conclusions/SignificanceOur study suggests that spatial heterogeneity, by itself, is not sufficient to generate strong effects of biodiversity on productivity. Rather, heterogeneity must be coupled with variation in the relative fitness of species across patches in order for spatial niche differentiation to generate complementary resource use.

Coexistence of two sympatric and morphologically similar bat species Rhinolophus affinis and Rhinolophus pearsoni

Ecologists have long focused on the coexistence of sympatric species. Here, we investigated two horseshoe bat species, Rhinolophus affinis and Rhinolophus pearsoni inhabited in the same cave, for their foraging strategies, niche differentiation, prey selection, and their coexistence status. These two species of horseshoe bats were different in the dominant frequency of their echolocation calls, but similar in their morphology. We found evidence for prey selectivity although there was a high degree of overlap in prey categories and sizes. R. affinis and R. pearsoni foraged on 16 and 7 categories insects, respectively, with Pyralidae, Geometridae, Melolonthidae dominating their diets. The degree of trophic niche overlap was 0.69. Pairwise comparisons suggested that there was no obvious differentiation in prey categories and size. However, high prey availability in the environment (Simpson diversity index = 0.79 and Margalef richness index = 4.12) contributed to their coexistence by dampening the interspecific competition. Since there are one or more mechanisms facilitating species coexistence in a community, our results suggest that the spatial niche differentiation in foraging microhabitats and in foraging habitats at landscape scale may promote the coexistence of the two bat species. However, additional field data are needed to confirm this speculation.

Tree species composition and environmental relationships in a Neotropical swamp forest in Southeastern Brazil

We examined the relationships between topography, soil properties and tree species compo- sition in a Neotropical swamp forest in southeastern Brazil. Plots were sampled in the forest, encompass- ing three different soil ground water regimes along the topographical declivity. All non-climbing plant individuals with trunk height (1.3 m were sampled. A canonical correspondence analysis—CCA—of the species-environmental relationships grouped tree species according to drainage and chemical soil conditions. A total of 86 species were found, being 77 species in the inferior, 40 species in the intermediate and 35 species in the superior topographic section. Some species were among the 10 most abundant ones, both in the overall sampled area and in each topographical section, with alternation events occur- ring only with their abundance position. However, substantial differences in floristic composition between sections were detected in a fine spatial scale, due to higher number of species, diversity index (H 0 ) and species unique (exclusives) in the inferior topographic section. These higher values can be attributed to its higher spatial heterogeneity that included better drained and seasonally waterlogged soils, higher soil fertility and lower acidity. The increase of the soil water saturation and the uniform conditions derived from the superficial water layer has led to a lower number of species and an increase on the palm trees abundance in the intermediate and superior sections. Our results showed that at a small spatial scale niche differentiation must be an impor- tant factor related to the increase of the local diversity. The wide distribution of the most abundant species in the studied area and the increase of local diversity corroborate the pattern of distribution of species in larger scales of swamp forests, in which the most abundant species repeat themselves in high densities in different remnants. However, the floristic composition of each remnant is strongly variable, contributing to the increase of regional diversity.

Vertical distribution of an ectomycorrhizal community in upper soil horizons of a young Norway spruce (Picea abies [L.] Karst.) stand of the Bavarian Limestone Alps

The vertical niche differentiation of genera of ectomycorrhiza (ECM) was assessed in a 17-year-old Norway spruce (Picea abies [L.] Karst.) plantation on a mountainous dolomitic site (1,050 m above sea level) of the Bavarian Limestone Alps. We determined ECM anatomotypes, recorded the abundance of corresponding ECM root tips and classified them into groups of ECM exploration types, which refer to the organisation and the extent of their extramatrical mycelia. The abundance of ECM was highest in the organic soil layers, compared to the mineral soil horizon. The ordination of the ECM communities and of the exploration types revealed segregation related to soil horizon properties. While Cenococcum geophilum preferred the organic soil layers, Lactarius spp., Tomentella spp. and Craterellus tubaeformis were generally most abundant in the mineral soil horizons. Cenococcum geophilum was the predominant species, possibly based on enhanced competitiveness under the prevailing site conditions. The short-distance exploration types (e.g. C. geophilum) preferentially colonised the organic soil layer, whereas the contact types (e.g. most of the Tomentella spp., C. tubaeformis) together with medium-distance types (e.g. Amphinema byssoides) were primarily associated with the underlying A-horizons. Therefore, the soil horizons had an important effect on the distribution of ECM and on their community structure. The spatial niche differentiation of ECM genera and exploration types is discussed in regard to specific physico-chemical properties of soil horizon and the assumed ecophysiological strategies of ECM.

Niche differences in phenology and rooting depth promote coexistence with a dominant C4 bunchgrass

Spatial and temporal niche differentiation are potential mechanisms of plant species' coexistence in many communities, including many grasslands. In a 6-year field experiment, a dominant prairie bunchgrass, Schizachyrium scoparium, excluded species sharing similar phenology and rooting depth, but coexisted with species differing in phenology and rooting depth. We used a series of experimental plots that differed in S. scoparium abundance to measure the effect of S. scoparium on abundances of other species and on soil nitrate concentrations across time and depth. Because we found that S. scoparium most strongly reduced soil nitrate levels at mid-season and at shallow depths, we hypothesized that at higher S. scoparium abundance, species with mid-season phenology and shallow roots would be excluded and that deep-rooted and early season species would be more likely to persist. As predicted, the proportional number and cover of species exploiting different niches than S. scoparium (early season and deep-rooted species) increased across the S. scoparium abundance gradient. This work provides novel empirical evidence for the factors that contribute to species coexistence in the field. Our study suggests that spatial and temporal niche differentiation promote species coexistence in these grasslands.

Mutualism or cooperation among competitors promotes coexistence and competitive ability

In the Lotka-Volterra competition model and similar competition models, species can only coexist if between-species competition is weaker than within species. Prior modeling and field studies have shown that coexistence can be promoted by reducing the competitive ability between species through spatial or temporal niche differentiation. Colonization, disturbance, aggregations in patchy habitats, resource transport and supply rates, predation, sex ratio mediation have been suggested to promote species coexistence. The role of mutualism in promoting species coexistence has never been quantitatively studied. In this study, by starting from the traditional Lotka-Volterra competition model, a new general model with introduction of mutualism between two competitive species is proposed. We hypothesized that the interaction of one species to the other is flexible instead of always negative; the zero growth isoclines of the competing species are parabolic. Three conclusions are drawn from this modeling study. First, mutualism is a new way of promoting coexistences of two species. Second, mutualism often increases the carrying capacities of both species, and then promotes their competitive abilities. Third, the mutualism-competition model is also appropriate to describe the dynamics of cooperation and competition between individuals or groups within species, and mutualism between plants and animals. The behind mechanism lies in mutualism or cooperation reduced the severity of competition at low density. Inferior competitor, if cooperative to superior competitor, is also possible to survive. Since mutualism or cooperation is commonly seen among competitors or between prey and predators, its role in shaping social or community structure is worth to explore.

Mortality and Parasitism in West African Tortoise Beetles (Coleoptera: Cbrysomelidae)

Two West African tortoise beetle species, Acrocassis roseomarginata Boheman 1854 and Aspidimorpha quinquefasciata Fabricius 1801, feed on the same host plant species but show a small scale spatial niche differentiation, preferring sun or shade microhabitats respectively. Various mortality factors of the two species were quantified in the field, and the hypothesis was tested whether larval parasitism and other mortality factors might be responsible for this spatial niche separation. Total larval mortality of A roseomarginata was significantly higher than of A quinquefasciata in both years (97.3% vs 76.4% in 1995 and 95.7% vs 57% in 1997). A roseomarginata suffered high rates of parasitism by Brachymeria straeleni Schulz (Hymenoptera: Chalcididae) with 73.0% and 71.4% of pupae parasitized in 1995 and 1997 respectively; A quinquefasciata-larvae were almost not parasitized. The survival rate of A roseomarginata from oviposition until pupation was very low (1.1 individuals surviving of 100 eggs laid). Neither total mortality nor parasitism in the different microhabitats could explain spatial niche differentiation in the two beetle species. However, if parasitism was excluded, residual mortality (caused by predators, abiotic factors or pathogens) was consistent with the abundance data in sun- and shade microhabitats in one of the two years of the study. Thus, the initial hypothesis whether mortality was responsible for the spatial abundance pattern of the two beetle species, was contradicted. However, a factor of residual mortality might contribute to selection for spatial niche differentiation in some years.

Spatial and trophic niche differentiation in two sympatric lizards (Tropidurus torquatus and Cnemidophorus ocellifer) with different foraging tactics

Abstract In a ‘restinga’ habitat of southeastern Brazil, we studied the food habits and the microhabitat use of two lizards with distinct foraging modes: the tropidurid Tropidurus torquatus, a sit‐and‐wait predator, and the teiid Cnemidophorus ocellifer, a wide forager. The diet of the two species differed strongly, indicating a low level of similarity in their trophic niche. The sit‐and‐wait predator fed mainly on mobile prey, whereas the wide forager fed mainly on sedentary prey (larvae). The spatial niche breadth of T. torquatus was larger than that of C. ocellifer. Despite interspecific differences, the two species overlapped greatly in micro‐habitat use. The data indicate that at Linhares the two lizard species differed more in food resources than in microhabitat, and that most of the food differences reflect the foraging patterns of the species.

Spatial heterogeneity and niche differentiation in oceanic zooplankton

The annual mean abundance (1958 to 1986) and geographical distributions of four of the major biomass species of copepod in the northern North Atlantic Ocean are shown together with their seasonal vertical distributions (1971 to 1974) from Ocean Weather Station I. The arcticboreal species Euchaeta norvegica and Calanus finmarchicus have sympatric distributions with their maximum numerical abundance in the cold water current system between Labrador and Greenland. The distribution of the temperate species Pleuromamma robusta and Metridia lucens are allopatric with respect to the two previous species but have sympatric distributions centering around the north-eastern oceanic region. The four species dominate the copepod biomass of the oceanic epiplankton and minimise interspecific competition by a) seasonal displacement of their main reproductive periods, b) occupying different trophic levels, c) having diverse reproductive strategies, d) residing in different positions in the water column and e) having different migratory behaviour throughout the year. The vertical distributions and migratory behaviour of these four copepods suggest a highly structured community with individual species occupying distinct niches with minimum overlap between competitors; even though the vertical distributions of the species are constantly changing through diel, ontogenetic and seasonal migrations.

Spatial heterogeneity and niche differentiation in zooplankton of Lake Huron

Spatial heterogeneity and niche differentiation in zooplankton of Lake Huron