Congeneric Competition Research Articles

Elevational distribution of birds in an Eastern African montane environment as governed by temperature, precipitation, and habitat availability

AbstractTropical species, many of which have limited thermal tolerances and narrow ecological niches, are likely to experience elevational shifts due to climate change. However, our ability to predict these shifts is limited by a scarcity of baseline data on current distributions and a lack of understanding of drivers of species ranges. To characterize the elevational distributions of a suite of Afromontane birds, we conducted annual point counts between 2013 and 2018 at 297 plots across habitats and elevations (2416–4303 m) in Volcanoes National Park, Rwanda. We assessed elevational and habitat preferences via indicator analyses, and modeled abundance distributions as a function of temperature, precipitation, habitat availability, and congeneric competition. Although most of the 35 focal species were distributed across habitats and elevational ranges, 20 species particularly associated with narrow elevational range (<300 m) and 24 species were strongly associated with one to three habitat types. Abiotic factors significantly correlated with abundance distributions of 33 species (temperature 33, precipitation 17), and biotic factors with distributions of 31 species (habitat 30, competition 7). Temperature and habitat availability were particularly associated with upper elevational limits (31 and 26 species, respectively, vs. 10 and 6 at lower limits), whereas precipitation affected both limits similarly (17 lower, 16 upper), and competition had a limited role at either limit (4 lower, 3 upper). That the elevational distribution of Afromontane birds results from a species‐specific combination of biotic and abiotic factors is crucial information in our effort to predict climate change effects in this region.

Metabolic potential of microbial community and distribution mechanism of Staphylococcus species during broad bean paste fermentation

Although the microbial diversity and structure in bean-based fermented foods have been widely studied, systematic studies on functional microbiota and mechanism of community forms in multi-microbial fermentation systems were still lacking. In this work, the metabolic pathway and functional potential of microbial community in broad bean paste (BBP) were investigated by metagenomics approach, and Staphylococcus, Bacillus, Weissella, Aspergillus and Zygosaccharomyces were found to be the potential predominant populations responsible for substrate alteration and flavor biosynthesis. Among them, Staphylococcus was the most abundant and widespread functional microbe, and closely related Staphylococcus species were diverse and ubiquitously distributed, with the opportunistic pathogen S. gallinarum being the most abundant Staphylococcus specie isolated from BBP. To explain the dominance status of S. gallinarum and species distributions of Staphylococcus genus, we tested the effects of abiotic and biotic factors on three Staphylococcus species using a tractable BBP model, demonstrating that adaptation to environmental conditions (environmental parameters and other functional microbes) led to the dominant position and species coexistence of Staphylococcus, and congeneric competition among Staphylococcus species further shaped ecological distributions of closely related Staphylococcus species. In general, this work revealed the metabolic potential of microbial community and distribution mechanism of Staphylococcus species during BBP fermentation, which could help traditional factories to more precisely control the safety and quality of bean-based fermented foods.

Competitive drivers of interspecific deviations of crown morphology from theoretical predictions measured with Terrestrial Laser Scanning

AbstractTree crown morphology is a key driver of forest dynamics, determining not only the competitiveness of an individual but also the competitive effect exerted on neighbouring trees. Multiple ecological theories, including metabolic scaling theory (MST), predict crown morphology from first principles, but typically lack consideration of competition. The accurate quantification of crown morphology to test theoretical predictions, and the canopy interactions that could alter them, has historically been limited by the simplicity and associated error of traditional crown measurements.In this study, we calculate high‐resolution two‐ and three‐dimensional crown metrics from Terrestrial Laser Scanning data for 1,441Pinus sylvestris,P.nigra,Quercus fagineaandQ. ilextrees from a water‐limited forest community in central Spain and test height‐crown metric scaling relationships. We demonstrate new TLS methods to define symmetric and asymmetric neighbourhood metrics based on tree height, crown size and neighbour projected crown area, and test the importance of neighbourhood genus diversity on crown morphology by separating competition from congeneric and heterogeneric neighbours.Competition negatively impacted all crown metrics except crown depth where onlyP.nigrashowed sensitivity. Asymmetric competition was the strongest driver of pine crown morphology, but oaks were more sensitive to symmetric competition, in line with shade tolerance expectations. Congeneric competition reducedQ.fagineacrown size and changed its shape, but we found no significant effects of heterogeneric neighbours. Most species and crown dimensions had height‐crown scaling exponents below those predicted by MST, which may be due to water limitation effects. Pines and oaks showed large differences in crown depth to height scaling, with the former shallower and the latter deeper, in contrast to theoretical predictions.Synthesis. Our study is the first to demonstrate the ability of TLS to characterise crown morphology from leaf‐wood separated clouds and competitive neighbour effects in a water‐limited forest community, and to use TLS metrics to test ecological crown scaling theory. Most crown metrics scaled below theoretical predictions. Pines were more sensitive to competition by larger neighbours and oaks to crowding from all neighbours, with competition from neighbours of the same genus having a consistent negative effect.

Species‐specific variation in germination rates contributes to spatial coexistence more than adult plant water use in four closely related annual flowering plants

Abstract Spatial partitioning is a classic hypothesis to explain plant species coexistence, but evidence linking local environmental variation to spatial sorting, demography and species' traits is sparse. If co‐occurring species' performance is optimized differently along environmental gradients because of trait variation, then spatial variation might facilitate coexistence. We used a system of four naturally co‐occurring species of Clarkia (Onagraceae) to ask whether distribution patchiness corresponds to variation in two environmental variables that contribute to hydrological variation. We then reciprocally sowed Clarkia into each patch type and measured demographic rates in the absence of congeneric competition. Species sorted in patches along one or both gradients, and in three of the four species, germination rate in the ‘home’ patch was higher than all other patches. Spatially variable germination resulted in the same three species exhibiting the highest population growth rates in their home patches. Species' trait values related to plant water use, as well as indicators of water stress in home patches, differed among species and corresponded to home patch attributes. However, post‐germination survival did not vary among species or between patch types, and fecundity did not vary spatially. Synthesis. Our research demonstrates the likelihood that within‐community spatial heterogeneity affects plant species coexistence, and presents novel evidence that differential performance in space is explained by what happens in the germination stage. Despite the seemingly obvious link between adult plant water‐use and variation in the environment, our results distinguish the germination stage as important for spatially variable population performance.

Habitat use, interspecific competition and phylogenetic history shape the evolution of claw and toepad morphology in Lesser Antillean anoles

AbstractEcologically functional traits are the product of several, at times opposing, selective forces. Thus, ecomorphological patterns can be disrupted locally by biotic interactions, such as competition, and may not be consistent across lineages. Here, we studied the evolution of claws and toepads in relationship to macrohabitat (vegetation), use of structural microhabitat (perch height) and congeneric competition for two distantly related Lesser Antillean anole clades: the Anolis bimaculatus and Anolis roquet series. We collected univariate and geometric morphometric data from 254 individuals across 22 species to test the hypotheses that functional morphology should covary with both vegetation and perch height and that the presence of a competitor may disrupt such covariation. Our data showed predictable associations between morphology and macrohabitat on single-species islands but not when a congeneric competitor was present. The outcomes of competition differed between series, however. In the A. bimaculatus series, species with a sympatric congener diverged in claw and toepad traits consistent with functional predictions, whereas A. roquet series anoles showed either no association between habitat and morphology or the opposite pattern. Our results demonstrated that ecomorphological patterns across macrohabitats can be disrupted by competition-driven microhabitat partitioning and that specific morphological responses to similar ecological pressures can vary between lineages.

Soil microbial communities alter conspecific and congeneric competition consistent with patterns of field coexistence in three Trifolium congeners

Abstract Coexistence and diversity in plant communities depend upon outcomes of plant competition. Competition and coexistence can be mediated by abiotic soil nutrient differences as well as by soil microbial communities. The latter effects occur through various mechanisms including negative plant–soil feedbacks, when plants foster the build‐up of specialized pathogenic microbes, which ultimately reduce conspecific, but not heterospecific, densities. Microbial mutualists can have generalized associations with host plants, and by associating with multiple species might affect coexistence by conferring different levels of benefit to hosts. We examined the effects of abiotic differences and soil microbial communities, including mutualistic nitrogen‐fixing rhizobial bacteria, on coexistence processes and asked whether these interactions inform patterns of co‐occurrence in natural communities. We measured plant–soil feedbacks in the greenhouse for three native Trifolium species that either highly co‐occurred or were spatially repulsed at our field site. Using size‐fractioned soil microbial inocula prepared from field‐collected soils, we explored the effect of soil microbes on nodulation and the outcome of competition (relative interaction intensity). We also examined the effects of soil origin (home vs. away soil) on the outcome of competition between species. Soil microbes had strong positive effects on plant growth and nodulation. Microbes in general reduced the strength of plant competition relative to competition in sterilized soil and altered the relative strength of interactions with conspecific vs. congeneric neighbours, which often occurred in ways predicted to enhance coexistence. In one pair of highly co‐occurring Trifolium, competition was strong in sterilized soils, but these species facilitated one other in the presence of the full microbial community. The net effect of these microbes in general reduced competition while also increasing performance over that in sterilized soil, suggesting a role for soil microbial mutualists in host coexistence. Synthesis. Our results demonstrate that interactions between plants and diverse soil communities can alter plant–plant interactions and plant–soil feedbacks. They may increase niche differences and equalize fitness differences in ways consistent with observed co‐occurrence in the field.

The amplification of the mitochondrial genome of the endangered buffy-tufted-ear marmoset Callithrix aurita (Primates: Cebidae) for massive parallel sequencing using the HiSeq 2500 platform

Callithrix aurita is a native small primate species from the Brazilian Southeastern Atlantic mountains. Considered as vulnerable and figuring the IUCN Red List, the species currently struggles with a reduced population size and faces several obstacles such as congeneric competition and the obvious anthropic pressure. Here we describe our amplification strategy to obtain high quality DNA of the endangered marmoset species Callithrix aurita from the South American Atlantic Rainforest, for use on massive parallel sequencing with the HiSeq 2500 platform. Two fragments of about 9Kb were successfully amplified comprising the fragments F1 165-9229 and F2 8531-639 of the mitochondrial genome of the Callithrix aurita. The quantification of the DNA was made using the Qubit fluorimeter and the fragments are now under sample preparation and sequencing using the Nextera DNA Kit within the HiSeq 2500 platform.

Transcriptomic responses to conspecific and congeneric competition in co‐occurring Trifolium

Summary Species coexistence requires differential response to inter‐ and intraspecific competition, typically conceptualized as niche differentiation. Coexistence of close relatives therefore poses an interesting scenario with regards to niche differentiation since these species generally have many traits in common due to shared ancestry. Native Californian Trifolium assemblages are locally diverse and represent a unique system for understanding competitive interactions among close relatives. We conducted two similar greenhouse studies in which Trifolium fucatum was grown alone, with a conspecific competitor, and with a congeneric competitor (Trifolium macraei). In the first study, we assessed biomass production in T. fucatum, and in the second study we conducted an RNAseq analysis of T. fucatum roots to test for differentially expressed genes that may mediate competitive interactions and potentially coexistence. Compared to plants grown alone, competition (i.e. growth in the same pot) with a conspecific resulted in a greater reduction in biomass than competition with a congener, as predicted by theory. However, competition with a congener resulted in twice as many differentially expressed genes as competition with a conspecific. Annotations of identified genes differentiating congeneric from conspecific competition suggest several functions attributed to interactions with third‐party organisms, including nodulation with rhizobial mutualists, and defence responses against pathogens and herbivores. In addition, salt‐responsive genes and an iron transporter were differentially expressed in congeneric competition, and comparisons of sodium and iron concentrations in field soils where these species are found showed that T. fucatum occurs in higher sodium and iron microsites than T. macraei. Thus, the transcriptome highlighted abiotic niche axes worth pursuing in future ecological experiments as potential mediators of coexistence. We also found a subset of genes that responded similarly to both congeneric and conspecific competition in both direction and magnitude, indicating some conserved responses to competition, regardless of neighbour identity. Synthesis. Transcriptomic analyses represent novel tools for identifying the molecular mechanisms underlying interactions among species. Working iteratively with ecological experimentation and observation, transcriptomes may allow us to identify novel dimensions of the n‐dimensional niche that determine species’ distributions and their ability to coexist.

The role of competition, ecotones, and temperature in the elevational distribution of Himalayan birds.

There is clear evidence that species' ranges along environmental gradients are constrained by both biotic and abiotic factors, yet their relative importance in structuring realized distributions remains uncertain. We surveyed breeding bird communities while collecting in situ temperature and vegetation data along five elevational transects in the Himalayas differing in temperature variability, habitat zonation, and bird richness in order to disentangle temperature, habitat, and congeneric competition as mechanisms structuring elevational ranges. Our results from species' abundance models representing these three mechanisms differed markedly from previous, foundational research in the tropics. Contrary to general expectations, we found little evidence for competition as a major determinant of range boundaries, with congeneric species limiting only 12% of ranges. Instead, temperature and habitat were found to structure the majority of species' distributions, limiting 48 and 40% of ranges, respectively. Our results suggest that different mechanisms may structure species ranges in the temperate Himalayas compared to tropical systems. Despite recent evidence suggesting temperate species have broader thermal tolerances than tropical species, our findings reinforce the notion that the abiotic environment has significant control over the distributions of temperate species.

Niche partitioning between close relatives suggests trade-offs between adaptation to local environments and competition.

Niche partitioning among close relatives may reflect trade-offs underlying species divergence and coexistence (e.g., between stress tolerance and competitive ability). We quantified the effects of habitat and congeneric species interactions on fitness for two closely related herbaceous plant species, Mimulus guttatus and Mimulus laciniatus, in three common habitat types within their sympatric range. Drought stress strongly reduced survival of M. guttatus in fast-drying seeps occupied by M. laciniatus, suggesting that divergent habitat adaptation maintains this niche boundary. However, neither seedling performance nor congeneric competition explained the absence of M. laciniatus from shady streams where M. guttatus thrives. M. laciniatus may be excluded from this habitat by competition with other species in the community or mature M. guttatus. Species performance and competitive ability were similar in sympatric meadows where plant community stature and the growing season length are intermediate between seeps and streams. Stochastic effects (e.g., dispersal among habitats or temporal variation) may contribute to coexistence in this habitat. Habitat adaptation, species interactions, and stochastic mechanisms influence sympatric distributions for these recently diverged species.

Distribution and Morphological Variation of Microcebus spp. Along an Environmental Gradient in Southeastern Madagascar

The lemurs of Madagascar are known for their extraordinary levels of speciation. However, the mechanisms and environmental conditions that led to this diversity remain obscure. We used 3 species of Microcebus (M. griseorufus, M. murinus, M. rufus) occurring along an environmental gradient as a model to investigate 1) how the different species are distributed in relation to variation in environmental conditions and ecotones; 2) whether or not the morphology of a given species varies in relation to environmental conditions; and 3) whether or not there is evidence for morphological character displacement to reduce congeneric competition in sympatry vs. allopatry. The 3 species of Microcebus show clear associations with specific habitat types. Distributions overlap at ecotones. Nevertheless, the ecotone between dry spiny and gallery forest represents a species boundary between Microcebus griseorufus and M. murinus while the ecotone between dry spiny forest and evergreen humid forest represents the species boundary between M. murinus and M. rufus. Different ambient conditions are not reflected in changes in body measurements of Microcebus murinus living in different vegetation formations. There is no indication for character displacement in sympatry vs. allopatry. Thus, differences in body mass or other morphological characteristics do not contribute to species separation between Microcebus griseorufus and M. murinus. The results confirm the importance of ecotones as species boundaries as a condition postulated for the radiation of lemur and other species on Madagascar. They also demonstrate different habitat affinities of seemingly very similar lemur species and thus illustrate our very limited understanding of the actual selection pressures, adaptations of lemurs to their environments, and their possible response to interspecific competition.

The competitive ability of three mymarid egg parasitoids (Gonatocerus spp.) for glassy-winged sharpshooter (Homalodisca coagulata) eggs

Abstract Parasitoid longevity, utilization of Homalodisca coagulata (Say) (Hemiptera: Cicadellidae) eggs of different ages, and progeny survival rates were determined in the laboratory for Gonatocerus ashmeadi Girault, Gonatocerus triguttatus Girault, and Gonatocerus fasciatus Girault (Hymenoptera: Mymaridae). Ovipositional behavior and aggression between females were investigated when all three species were simultaneously presented H. coagulata egg masses in the laboratory. Results from the longevity study demonstrated that when females were provisioned with honey–water solution, female G. ashmeadi survived up to 171.9% longer than G. triguttatus and G. fasciatus , whereas, survival was equivalent between G. triguttatus and G. fasciatus . Results from the egg age utilization study showed that G. ashmeadi , G. triguttatus , and G. fasciatus most efficiently utilized eggs 1–6, 3–6, and 1–2 days of age, respectively, and that exploited egg age ranges overlapped between species. Gonatocerus ashmeadi parasitized a significantly higher (up to 45.0 and 62.6%) proportion of H. coagulata eggs aged 1–6 days of age compared with G. triguttatus and G. fasciatus , respectively. Additionally, in competition studies, overall parasitism by G. ashmeadi was significantly higher (up to 76.0%) compared with G. triguttatus and G. fasciatus . Results from behavioral observations of females concurrently searching for H. coagulata egg masses showed that G. ashmeadi allocated the greatest proportion of time to resting/grooming (26.5%) and oviposition (25.8%), while G. triguttatus allocated significantly more (up to 61.6%) time to resting/grooming compared with all other activities. Female G. fasciatus spent the greatest proportion of time resting/grooming (40.0%) and off leaves with H. coagulata egg masses (39.6%). G. ashmeadi and G. triguttatus allocated 2.1 and 1.3% of time to aggressively interacting and defending egg masses from congenerics, whereas, this was not observed for G. fasciatus . Results suggest that G. ashmeadi may show the most potential as a biological control agent of H. coagulata , and that successful widespread establishment and impact by G. fasciatus on H. coagulata in California is unlikely unless this species can efficiently exploit low density populations of H. coagulata in early spring when congeneric competition for egg masses is low.

Competition among small colonies of Agaricia: the importance of size asymmetry in determining competitive outcome

Scleractinian corals may experience competition at early life history stages, particularly where aggregated settlement decreases the space available for growth. The outcome of such inter- actions can affect post recruitment success and the relative abundance of coral species. The purpose of this study was to determine the importance of competition among small colonies (≤ 4 cm diameter) of Agaricia on shallow (≤10 m depth) reefs in St. John (US Virgin Islands) and Jamaica. The associa- tion between the density of small Agaricia and the frequency of congeneric encounters was exam- ined, together with the relationship between density of Agaricia and reef rugosity (topographic com- plexity). Rugosity was analyzed because it indirectly affects competitive interactions by providing cryptic habitats that promote aggregated settlement and coral-coral interactions. To test one possible mechanism determining the outcome of such interactions, the relationship between size asymmetry of competing colonies and competitive outcome was assessed. The results demonstrate that competi- tion among small Agaricia is common, particularly where aggregated settlement is favored by high topographic complexity. In other words, competitive interactions among small Agaricia are affected by both physical (e.g. topographic complexity) and biological (e.g., settlement patterns) factors. The outcome of such interactions is influenced by the difference in size of competing colonies, with dom- inant colonies generally being larger than subordinate colonies. Further research is required to iden- tify the mechanistic basis of dominance, but currently the most likely hypotheses are that large colonies are dominant because they have greater energy resources than small colonies, or intrinsi- cally faster growth rates. The high frequency of competition among Agaricia colonies in St. John and Jamaica suggests that further studies of congeneric competition within the Scleractinia may prove valuable in understanding the population biology of this taxon.

Oak canopy effects on the distribution patterns of two annual grasses: the role of competition and soil nutrients

Within the oak woodlands of California there is often a distinct shift in the botanical composition between the open grassland and the herbaceous understory beneath oak canopy. Botanical sampling at two woodland sites indicated that the annual grass Bromus diandrus was dominant under deciduous blue oak canopy, while a congener, Bromus hordeaceus, was dominant in open grassland. We examined the relative importance of congeneric competition and edaphic factors in creating these differences in species distribution in two separate field experiments that manipulated both congeneric and intraspecific competition, as well as soil type. We used the demographic measure of relative reproductive rate as an index of population growth. In general, demographic performance correctly predicted the distribution of the two annual grasses in the field. Our results indicate that reduced abundance of B. hordeaceus under canopy reflects the negative effects of competition with B. diandrus. In contrast, B. diandrus is little affected by competition from B. hordeaceus. The reduced abundance of B. diandrus in open grassland may result, in part, from its inability to adapt as well as B. hordeaceus to lower nutrient availability in soils of the open grassland.

Reproduction, Population Structure, and Effects of Congeneric Competition on the Crevice Spiny Lizard, Sceloporus poinsetti (Iguanidae), in Southwestern New Mexico

Reproduction, Population Structure, and Effects of Congeneric Competition on the Crevice Spiny Lizard, Sceloporus poinsetti (Iguanidae), in Southwestern New Mexico

Biochemical Evidence for Congeneric Competition as a Factor Restricting Gene Flow Between Populations of a Darter (Percidae: Etheostoma)

Echelle, A. A., A. F. Echelle, and B. A. Taber (Biology Department, Baylor University, Waco, Tx. 76703 and Biology Department, Southwest Missouri State University, Springfield, Mo.) 1976. Biochemical evidence for congeneric competition as a factor restricting gene flow betweerw populations of a darter (Percidae: Etheostoma). Syst. Zool. 25:228-235. Electrophoretic analysis of proteins encoded by polymorphic loci demonstrates that Etheostoma spectabile populations show greater differentiation in allelic frequencies in an area of sympatry with E. radiosum than in an area of allopatry. The data are more compatible with the assumption that degree of gene-flow rather than natural selection is the primary mechanism maintaining local differentiation. Increased heterogeneity in sympatry is attributed to exclusion of spectabile from downstream areas through competition with radiosum, thereby isolating spectabile in headwater areas and restricting gene flow between populations. In sympatry, spectabile showed greater differentiation than radiosum, presumably because of greater gene flow in the latter. [Pisces; Percidae; Etheostoma; darters; proteins; electrophoresis; differentiation: gene-flow: natural selection: geograDhical isolation.] As a group, members of the darter genus Etheostoma are primarily adapted for life in upland streams with hard substrates and clear, relatively fast-flowing waters. Downstream habitats become increasingly unsuitable for most Etheostoma. As a result, gene flow between headwater populations is restricted and marked differentiation may occur even within small geographical areas (Echelle et al., 1975). This is reflected by the high degree of speciation that has occurred in the genus. Our purpose is to demonstrate that, as a result of competition, geographical isolation and differentiation may occur even within areas of continuously suitable physical habitat. This was omitted from Mayr's (1963) survey of barriers to gene flow, but Hubbs and Springer (1957) alluded to it with their suggestion that competitive exclusion by the mosquitofish, Gambusia affinis, has contributed to speciation in the G. nobilis species group. In this paper we provide biochemical evidence for this effect. The orangethroat darter, E. spectabile, is ubiquitous in riffle habitats of streams of the Red River drainage in southcentral Oklahoma which are outside the range of the -orangebellv darter, E. radiosum. However, within the range of E. radiosum, i.e., from Blue River eastward, E. spectabile tends to be restricted to springs and headwater areas of spring-fed streams. In downstream areas E. radiosum is ubiquitous in riffle habitat whereas E. spectabile is generally rare or absent. This seems to be a clear example of competitive exclusion. In Arkansas, E. spectabile is abundant in Ozarkian areas where E. radiosum is absent, but it is much more sparsely distributed in southwest Arkansas where E. radiosum is abundant (Buchanan, 1973), further implying a competitive relationship. Our objective was to assess the degree of homogeneity in allelic frequencies among E. spectabile populations in two tributaries of the Red River, Clear Boggy River and Washita River. These are, respectively, within and outside the range of E. radiosum in southern Oklahoma. Also, Clear Boggy populations of E. radiosum were examined. We hypothesized that E. spectabile should show greater heterogeneity in the area of sympatry than in the area of allopatry, and that E. spectabile in sympatry should show greater heterogeneity than associated populations of E. radiosum.

Shift in the Structural Habitat of Anolis Opalinus Due to Congeneric Competition

The Jamaican Anolis opalinus has different perch height characteristics depending on locality. Toward the center of the island, around Mandeville, A. opalinus characteristically perches low, being classified as a trunk—ground species. In Kingston, however, A. opalinus perches high, classified as a trunk—crown species, Evidence indicates the Kingston A. opalinus would also perch low except that interaction with Anolis I. Lineatophus forces A. opalinus into perch sites higher than appear to be the species' preference.

SOME CALANOID COPEPODS FROM ARIZONA WITH NOTES ON CONGENERIC OCCURRENCES OF DIAPTOMUS SPECIES

Seven species of Diaptomus are reported from Arizona, including 4 new to the state. D. shoshone, D. nudus, and D. leptopus occur on the Colorado Plateau; D. clavipes and D. siciloides are abundant in the mountainous area below the Mogollon Rim; D. albuquerquensis occurs rarely in turbid ponds on the Plateau and in the Sonoran Desert; D. dorsalis lives in permanent, oligohaline ponds near Phoenix.Thirty‐four examples of congeneric coexistence are presented, including 3 from Arizona. In 24 instances, coexistence involved species of different subgenera. Differences in length of co‐occurring species are common and are particularly marked in temporary ponds and saline lakes. Genetic changes in the nature of character displacement may modify dimensions of coexisting species, although environmental factors other than congeneric competition also play a part. Adult diaptomids from Arizona tanks may be larger when early instars are passed during summer rather than winter. Extrinsic factors thus tend to blur morphological character displacement. Modified life cycles and ecological displacement are further intrinsic mechanisms whereby habitats are partitioned, although not revealed by ordinary sampling techniques.