Two-species Treatments Research Articles

The existence and strength of higher order interactions is sensitive to environmental context.

One strategy for understanding the dynamics of any complex system, such as a community of competing species, is to study the dynamics of parts of the system in isolation. Ecological communities can be decomposed into single species, and pairs of interacting species. This reductionist strategy assumes that whole-community dynamics are predictable and explainable from knowledge of the dynamics of single species and pairs of species. This assumption will be violated if higher order interactions (HOIs) are strong. Theory predicts that HOIs should be common. But it is difficult to detect HOIs, and to infer their long-term consequences for species coexistence, solely from short-term data. I conducted a protist microcosm experiment to test for HOIs among competing bacterivorous ciliates, and test the sensitivity of HOIs to environmental context. I grew three competing ciliate species in all possible combinations at each of two resource enrichment levels, and used the population dynamic data from the one- and two-species treatments to parameterize a competition model at each enrichment level. I then compared the predictions of the parameterized model to the dynamics of the whole community (three-species treatment). I found that the existence, and thus strength, of HOIs was environment dependent. I found a strong HOI at low enrichment, which enabled the persistence of a species that would otherwise have been competitively excluded. At high enrichment, three-species dynamics could be predicted from a parameterized model of one- and two-species dynamics, provided that the model accounted for nonlinear intraspecific density dependence. The results provide one of the first rigorous demonstrations of the long-term consequences of HOIs for species coexistence, and demonstrate the context dependence of HOIs. HOIs create difficult challenges for predicting and explaining species coexistence in nature.

The slippery nature of ectomycorrhizal host specificity: Suillus fungi associated with novel pinoid (Picea) and abietoid (Abies) hosts

ABSTRACT Suillus is among the best-known examples of an ectomycorrhizal (ECM) fungal genus that demonstrates a high degree of host specificity. Currently recognized host genera of Suillus include Larix, Pinus, and Pseudotsuga, which all belong to the pinoid clade of the family Pinaceae. Intriguingly, Suillus sporocarps have been sporadically collected in forests in which known hosts from these genera are locally absent. To determine the capacity of Suillus to associate with alternative hosts in both the pinoid and abietoid clades of Pinaceae, we examined the host associations of two Suillus species (S. punctatipes and S. glandulosus) through field-based root tip sampling and seedling bioassays. Root tip collections underneath Suillus sporocarps were molecularly identified (fungi: nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 [ITS barcode]; plant: trnL) to assess the association with multiple hosts. The bioassays contained both single- and two-species treatments, including a primary (Larix or Pseudotsuga) and a secondary (Picea, Pinus, or Abies) host. For the S. punctatipes bioassay, an additional treatment in which the primary host was removed after 8 mo was included to assess the effect of primary host presence on longer-term ECM colonization. The field-based results confirmed that Suillus fungi were able to associate with Abies and Tsuga hosts, representing novel host genera for this genus. In the bioassays, colonization on the primary hosts was detected in both single- and two-species treatments, but no colonization was present when Picea and Abies hosts were grown alone. Removal of a primary host had no effect on percent ECM colonization, suggesting that primary hosts are not necessary for sustaining Suillus colonization once they are successfully established on secondary hosts. Collectively, our results indicate that host specificity is more flexible in this genus than previously acknowledged and help to explain the presence of Suillus in forests where recognized hosts are not present.

Top-Down Effects of Salamanders on Macroinvertebrates in Fishless Headwater Streams

Salamanders are the top predators in many fishless headwater streams, and intraguild interactions among stream salamanders are well documented. However, little is known about the top-down effects of salamanders on stream food webs or how intraguild interactions mediate these effects. To investigate the effects of salamanders on macroinvertebrate communities of headwater streams, we conducted an experiment in stream mesocosms to test for effects of two stream salamander species, namely, Eurycea bislineata and Gyrinophilus porphyriticus, alone or in combination, on benthic and emerging macroinvertebrate density, biomass, and community composition. We also assessed intraguild interactions between these salamander species by comparing Eurycea bislineata survival and G. porphyriticus growth in single-species versus two-species treatments. Gyrinophilus porphyriticus reduced benthic macroinvertebrate densities when alone but not when co-occurring with E. bislineata. There were no effects of salamanders on benthic macroinvertebrate biomass or community composition and no effects on emerging macroinvertebrate density, biomass, or community composition. Eurycea bislineata survival decreased and G. porphyriticus weight increased in two-species treatments, suggesting that intraguild predation was occurring. Overall, although some of our findings are equivocal, these results suggest that salamanders can exert top-down control on macroinvertebrate communities in fishless headwater streams, decreasing benthic macroinvertebrate density. But this effect is dependent on the salamander species present, and can be removed by intraguild interactions between salamander species.

Does fungal competitive ability explain host specificity or rarity in ectomycorrhizal symbioses?

Two common ecological assumptions are that host generalist and rare species are poorer competitors relative to host specialist and more abundant counterparts. While these assumptions have received considerable study in both plant and animals, how they apply to ectomycorrhizal fungi remains largely unknown. To investigate how interspecific competition may influence the anomalous host associations of the rare ectomycorrhizal generalist fungus, Suillus subaureus, we conducted a seedling bioassay. Pinus strobus seedlings were inoculated in single- or two-species treatments of three Suillus species: S. subaureus, S. americanus, and S. spraguei. After 4 and 8 months of growth, seedlings were harvested and scored for mycorrhizal colonization as well as dry biomass. At both time points, we found a clear competitive hierarchy among the three ectomycorrhizal fungal species: S. americanus > S. subaureus > S. spraguei, with the competitive inferior, S. spraguei, having significantly delayed colonization relative to S. americanus and S. subaureus. In the single-species treatments, we found no significant differences in the dry biomasses of P. strobus seedlings colonized by each Suillus species, suggesting none was a more effective plant symbiont. Taken together, these results indicate that the rarity and anomalous host associations exhibited by S. subaureus in natural settings are not driven by inherently poor competitive ability or host growth promotion, but that the timing of colonization is a key factor determining the outcome of ectomycorrhizal fungal competitive interactions.

Eco-Evolutionary Interaction in Competing Phytoplankton: Nutrient Driven Genotype Sorting Likely Explains Dominance Shift and Species Responses to CO2

How ecological and evolutionary processes interact and together determine species and community responses to climate change is poorly understood. We studied long-term dynamics (over approximate-ly 200 asexual generations) in two phytoplankton species, a coccolithophore (Emiliania huxleyi) and a diatom (Chaetoceros affinis), to increased CO2 growing alone or competing with one another in co-occurrence. To allow for rapid evolutionary responses, the experiment started with a standing genetic variation of nine genotypes in each of the species. Under co-occurrence of both species, we observed a dominance shift from C. affinis to E. huxleyi after about 120 generations in both CO2 treatments, but more pronounced under high CO2. Associated with this shift, we only found weak adaptation to high CO2 in the diatom and none in the coccolithophore in terms of species´ growth rates. In addition, no adaptation to interspecific competition could be observed by comparing the single to the two-species treatments in reciprocal assays, regardless of the CO2 treatment. Nevertheless, highly reproducible genotype sorting left only one genotype remaining for each of the species among all treatments. This strong evolutionary selection coincided with the dominance shift from C. affinis to E. huxleyi. Since all other conditions were kept constant over time, the most parsimonious explanation for the dominance shift is that the strong evolutionary selection was driven by the experimental nutrient conditions, and in turn potentially altered competitive ability of the two species. Thus, here observed changes in the simplest possible two-species phytoplankton "community" demonstrated that eco-evolutionary interactions can be critical for predicting community responses to climate change in rapidly dividing organisms such as phytoplankton.

Phylogenetic diversity is a better predictor of wetland community resistance to Alternanthera philoxeroides invasion than species richness.

Highly biodiversity communities have been shown to better resist plant invasions through complementarity effects. Species richness (SR) is a widely used biodiversity metric but lacks explanatory power when there are only a few species. Communities with low SR can have a wide variety of phylogenetic diversities (PD), which might allow for a better prediction of invasibility. We assessed the effect of diversity reduction of a wetland community assemblage typical of the Beijing area on biotic resistance to invasion of the exotic weed Alternanthera philoxeroides and compared the reduction in SR and PD in predicting community invasibility. The eight studied resident species performed similarly when grown alone and when grown in eight-species communities together with the invasive A.philoxeroides. Variation partitioning showed that PD contributed more to variation in both A.philoxeroides traits and community indicators than SR. All A.philoxeroides traits and community indicators, except for evenness index, showed a linear relationship with PD. However, only stem length of A.philoxeroides differed between the one- and two-species treatments, and the diversity index of the communities differed between the one- and two-species treatments and between the one- and four-species treatments. Our results showed that in natural or semi-natural wetlands with relatively low SR, PD may be a better predictor of invasibility than SR. When designing management strategies for mitigating A.philoxeroides invasion, deliberately raising PD is expected to be more efficient than simply increasing species number.

Heterospecific shoaling in an invasive poeciliid: shared history does not affect shoal cohesion

Social behaviour potentially plays an important role in invasion success. New colonists, for example, may glean useful information about predators and food by interacting with native heterospecifics. The extent to which invaders benefit from such social interactions could hinge on their prior exposure to other species. Here we asked how the shoaling decisions of the Trinidadian guppy, Poecilia reticulata, a successful invasive species, are mediated by their shared history with a heterospecific, the phenotypically similar Micropoecilia picta. To do this, we monitored shoal cohesion in single-species treatments and in treatments where M. picta was present. We predicted that shoal cohesion would be greater in single- than in mixed-species shoals. We also hypothesized that mixed-species shoals consisting of fish with a shared history would be more cohesive than those where the two species had hitherto occurred allopatrically. We found that shoal cohesion did not differ between single- and two-species treatments, or in relation to shared history with M. picta. However, while guppies were more often found in mixed-species than single-species shoals, they were more likely to have a conspecific individual as their nearest neighbour within mixed-species shoals. These results show that guppies willingly shoal with heterospecifics, even in the absence of a shared history, but also that the resulting shoals are not randomly assembled. This flexibility in shoaling may confer a crucial advantage in the initial stages of invasion.

Daily Temperature Fluctuations Alter Interactions between Closely Related Species of Marine Nematodes.

In addition to an increase in mean temperature, climate change models predict decreasing amplitudes of daily temperature fluctuations. In temperate regions, where daily and seasonal fluctuations are prominent, such decreases in daily temperature fluctuations can have a pronounced effect on the fitness of species and on the outcome of species interactions. In this study, the effect of a temperature regime with daily fluctuations versus a constant temperature on the fitness and interspecific interactions of three cryptic species of the marine nematode species complex of Litoditis marina (Pm I, Pm III and Pm IV) were investigated. In a lab experiment, different combinations of species (monospecific treatment: Pm I and Pm IV and Pm III alone; two-species treatment: Pm I + Pm IV; three-species treatment: Pm I + Pm IV + Pm III) were subjected to two different temperature regimes: one constant and one fluctuating temperature. Our results showed that fluctuating temperature had minor or no effects on the population fitness of the three species in monocultures. In contrast, interspecific interactions clearly influenced the fitness of all three species, both positively and negatively. Temperature regime did have a substantial effect on the interactions between the species. In the two-species treatment, temperature regime altered the interaction from a sort of mutualism to commensalism. In addition, the strength of the interspecific interactions changed depending on the temperature regime in the three-species treatment. This experiment confirms that interactions between the species can change depending on the abiotic environment; these results show that it is important to incorporate the effect of fluctuations on interspecific interactions to predict the effect of climate change on biodiversity.

Oviposition by Aedes aegypti and Aedes albopictus: influence of congeners and of oviposition site characteristics.

We investigated the oviposition behavior of Ae. aegypti and Ae. albopictus. In particular we examined whether small-scale site characteristics and the presence of conspecifics or congeners altered oviposition by these mosquitoes. Various combinations of females of the two species were allowed to oviposit inside cages among either vegetation (potted plants) or structural components (wood and concrete blocks). Numbers of eggs deposited per female were compared between species, sides, and treatments. Most significant differences between treatments and species involved differences between single species and mixed species treatments. Ae. aegypti deposited more eggs/female in the vegetation side than in the structure side whereas the opposite pattern was evident for Ae. albopictus. Ae. aegypti females had higher frequency of skip oviposition than Ae. albopictus. An average of 63% of the containers in the two-species treatments contained eggs of both species, with more frequent joint occurrences observed in the treatment with three females of each species than in the treatments with one of each. Our results point to the existence of various interactions between gravid Ae. aegypti and Ae. albopictus females at or near the oviposition sites but further experimental work is necessary to fully characterize the interactions and their specific mechanisms.

Species-specific effect of macrobenthic assemblages on meiobenthos and nematode community structure in shallow sandy sediments

Three functionally different macrofaunal species (the filter- and/or surface deposit-feeding polychaete Hediste diversicolor, and the suspension-feeding bivalves Mya arenaria and Cerastoderma glaucum) were introduced as single- and two-species treatments into microcosms containing sandy sediment with a natural meiofaunal community. H. diversicolor is a burrowing species building a system of galleries, C. glaucum lives actively near the sediment surface acting as a biodiffuser and M. arenaria buries deeply and leads a sessile lifestyle. It is shown that H. diversicolor extended the vertical distribution of meiofauna into deeper sediment layers compared to the control and non-Hediste treatments. The response of the nematode community varied significantly among treatments and was dependant on the macrobenthic species composition but not on the species number. Nematode assemblages in all treatments with the polychaete, both in monoculture and with either bivalve, differed significantly from those recorded in other treatments and were more similar than replicates within any other single treatment. H. diversicolor also appeared to have stimulated nematode species diversity. The present study demonstrated that the impact of macrobenthic assemblages on meiofauna is not a simple summation of individual species effects but is species specific.

Structure of earthworm burrows related to organic matter of a constructed Technosol

Literature considering earthworms as a biological agent of Technosol functioning, especially those constructed in order to reclaim degraded areas (e.g. industrial wasteland), is scarce. The main objectives of the following work is: (i) to describe the structure of burrows produced by two different eco-morphological groups of earthworms in a constructed Technosol and (ii) to link their burrowing activity to Technosol organic matter transfers and carbon content in one-species and two-species combinations. For these purposes, the contributions of Lumbricus terrestris as an epi-anecic earthworm and Aporrectodea caliginosa as an endogeic one to such functioning aspects of a constructed Technosol, resulting from soil engineering processes (e.g. choice of proportions, parent materials) were assessed using laboratory microcosm experiments. The Technosol studied was composed of green waste compost, treated industrial soil and paper mill sludge. Earthworms were inoculated separately and together in the constructed Technosol over a period of 75days. Ultra-structural analysis of randomly selected burrows of the one-species treatments and the soil of the control treatment were sampled to describe their structure. Functional consequences on the organic matter in the Technosol were assessed by studying remaining surface litter mass, transfer of surface organic matter to depth and by measuring soil carbon content.At the ultrastructural scale, the burrowing activity of the two eco-morphological groups of earthworms locally modified the organo-mineral associations of the Technosol. Burrows presented a similar structure for both species, with a looser internal cutan including some organic elements and microbial activity tracks (closed to the lumen) and a compacted external mineral cutan (distant from the lumen). However, structural differences were observed between species. L. terrestris burrows contained a visible organo-mineral interface, resulting from the interface between the internal cutan, which was much more organic than for the A. caliginosa one, and the external mineral cutan. For L. terrestris, mucus was present as a large stratum visible between the interface and the external mineral cutan, whereas the mucus was scattered for A. caliginosa. Aggregation also differed between species. Even though both burrows presented some organo-mineral aggregates, plant organic matter particularly contributed to the aggregates for L. terrestris, whereas aggregation was essentially bacterial for A. caliginosa. As is the case for “natural soils”, these results confirmed the notions of i) “ecosystem engineers” in constructed Technosols, by demonstrating that earthworms create organo-mineral structures with a similar structure and ii) eco-morphological groups by distinguishing differences between these structures. At a larger scale (microcosm scale), L. terrestris buried significantly more surface organic matter into depth than A. caliginosa. However, almost no effect of either of the two earthworms (one-species and two-species treatments) on soil carbon content was noticeable.It is suggested that not enough soil carbon measures were made given the temporal and spatial scales of this experiment or earthworm effects may have been masked in the man-made soil built with materials with high initial carbon content. Although sometimes slight at the microcosm scale, differences suggested that earthworm combinations of dissimilar eco-morphological groups led to different effects on organic transfers and carbon content of the constructed Technosol. Developing knowledge about effects of soil fauna diversity in constructed Technosols using reliable tools (e.g. trait-based approaches) is required. It would better predict the effects of biological agents such as earthworms in soil engineering and in turn improve the ecological restoration of such Technosols.

Soil organisms shape the competition between grassland plant species

Decomposers and arbuscular mycorrhizal fungi (AMF) both determine plant nutrition; however, little is known about their interactive effects on plant communities. We set up a greenhouse experiment to study effects of plant competition (one- and two-species treatments), Collembola (Heteromurus nitidus and Protaphorura armata), and AMF (Glomus intraradices) on the performance (above- and belowground productivity and nutrient uptake) of three grassland plant species (Lolium perenne, Trifolium pratense, and Plantago lanceolata) belonging to three dominant plant functional groups (grasses, legumes, and herbs). Generally, L. perenne benefited from being released from intraspecific competition in the presence of T. pratense and P. lanceolata. However, the presence of AMF increased the competitive strength of P. lanceolata and T. pratense against L. perenne and also modified the effects of Collembola on plant productivity. The colonization of roots by AMF was reduced in treatments with two plant species suggesting that plant infection by AMF was modified by interspecific plant interactions. Collembola did not affect total colonization of roots by AMF, but increased the number of mycorrhizal vesicles in P. lanceolata. AMF and Collembola both enhanced the amount of N and P in plant shoot tissue, but impacts of Collembola were less pronounced in the presence of AMF. Overall, the results suggest that, by differentially affecting the nutrient acquisition and performance of plant species, AMF and Collembola interactively modify plant competition and shape the composition of grassland plant communities. The results suggest that mechanisms shaping plant community composition can only be understood when complex belowground interactions are considered.

Influence of a dominant consumer species reverses at increased diversity

Theory and experiments indicate that changes in consumer diversity affect benthic community structure and ecosystem functioning. Although the effects of consumer diversity have been tested in the laboratory and the field, little is known about effects of consumer diversity in the subtidal zone, one of the largest marine habitats. We investigated the grazing effects of sea urchins on algal abundance and benthic community structure in a natural subtidal habitat of the Galápagos Islands. Three species of urchins (Eucidaris, Lytechinus, and Tripneustes) were manipulated in inclusion cages following a replacement design with three levels of species richness (one, two, and three species) with all possible two-species urchin combinations. Identity was the main factor accounting for changes in the percentage of substrate grazed and benthic community structure. Two out of the three two-species assemblages grazed more than expected, suggesting a richness effect, but analyses revealed that this increased grazing was due to a sampling effect of the largest and commercially valued urchin species, Tripneustes. Benthic community structure in treatments with Eucidaris, Lytechinus, and Tripneustes alone was significantly different at the end of the experiment, suggesting that resource use differentiation occurred. Communities in Tripneustes enclosures were characterized by abundant crustose coralline algae and grazed substrate, while those without it contained abundant green foliose algae (Ulva sp.). An unexpected emergent property of the system was that the most species-rich urchin assemblage underyielded, grazing less than any other assemblage with Tripneustes, effectively reversing its dominant influence observed in the two-species treatments. While further experiments are needed to discern the mechanisms of underyielding, it may be related to changing interspecific interactions as richness increases from two to three species or to density-dependent Tripneustes grazing. This study highlights the general importance of evaluating consumer richness effects across the entire range of species richness considered, as the performance of the most species-rich consumer assemblage could not be predicted by manipulations of intermediate levels of consumer species richness.

Root tip competition among ectomycorrhizal fungi: Are priority effects a rule or an exception?

Competition for root colonization among ectomycorrhizal fungi is well documented, but the mechanisms determining competitive outcomes are not clearly understood. In a previous study, we observed that timing of colonization (i.e., a priority effect) had a significant effect on the outcome of competition between two ectomycorrhizal (EM) fungi in the genus Rhizopogon. In this study, we explicitly tested the role of priority effects in competition among EM fungi by experimentally manipulating the timing of colonization of four Rhizopogon species on Pinus muricata seedlings. In a first experiment, we set up 12 two-species combinations, in which seedlings were first inoculated from spores with one species, grown for three months, and then inoculated with an equal density of spores of a second species and grown for an additional three months. Root tip occupation in the two-species treatments was determined by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis of internal transcribed spacer region (ITS) of rDNA. In a second experiment, we further examined competitive interactions between two Rhizopogon species using split-root P. muricata seedlings. One side of the root system was pre-colonized by one species, and spores of the second species were added to the other side of the root system in all same and different species pair-wise combinations. We found that for three of the four species (R. occidentalis, R. salebrosus, R. vulgaris), the outcome of competition in the first experiment depended strongly on the timing of colonization, with the first colonizing species always being the competitive dominant. For R. evadens, however, initial colonization did not prevent significant subsequent colonization by R. occidentalis and R. vulgaris. This appeared to be caused by the lower colonization of R. evadens compared to the three other species. In the second experiment, we observed that the portion of the split root system that was initially uncolonized remained receptive to colonization when spores were added. The amount of colonization of R. occidentalis and R. salebrosus on the side of the root system to which they were added was not significantly influenced by species identity on the other side of the seedling. In combination, these results confirm that priority effects do play a major role in dynamics of EM root tip colonization, at least in the early colonization of seedlings, and that the proportion of the root system occupied by a species appears to be a key factor determining competitive success.

Competitive interactions among three ectomycorrhizal fungi and their relation to host plant performance

1. Competition strongly influences many species assemblages, but its role in mycorrhizal fungal interactions is not well understood. We examined interactions among three ectomycorrhizal (ECM) species to determine if the structure of competition could be characterized by either competitive networks (where no clear hierarchy exists in the outcome of competition between various species pairs) or competitive hierarchies (where one species out competes all other species). 2. Using a bioassay experiment, we inoculated Pinus muricata seedlings with three Rhizopogon species (R. occidentalis, R. salebrosus, and R. vulgaris) in single-, two-, and three-species treatments. After 7 months, we assessed the relative abundance of each species in each treatment using real-time PCR of internal transcribed spacer rDNA. 3. We found that R. occidentalis was strongly inhibited by R. vulgaris and R. salebrosus in all competition treatments. In contrast, R. vulgaris and R. salebrosus had similar ECM biomasses in the two-species treatment, but R. vulgaris had significantly higher biomass than R. salebrosus in the three-species treatment. 4. In the single-species treatments, seedlings colonized by the competitive dominants had higher shoot biomass and total leaf nitrogen, but also higher percentage ECM biomass. In the multi-species treatments, seedlings had either equivalent or somewhat lower shoot biomass and total leaf nitrogen than their respective single-species treatments. 5. Synthesis. Our results indicate that ECM competition does not appear to be characterized by strict networks or hierarchies. Instead, the outcome is dependent on the conditions of the local environment in which it occurs. There also does not seem to be a clear relationship between ECM competitive ability and plant performance, but competition does appear to negatively affect the ability of ECM fungi to provide benefits to their hosts.

Determining the outcome of field‐based competition between two Rhizopogon species using real‐time PCR

Interest in the ecology of ectomycorrhizal (ECM) fungi has increased considerably, but little is known about interspecific interactions among ECM species. We examined competitive interactions between Rhizopogon occidentalis and R. salebrosus at Point Reyes National Seashore, California, USA. At three field sites, species abundances were compared in single- and two-species treatments on Pinus muricata seedlings inoculated with spores. Competition for root tips was assessed using real-time polymerase chain reaction (PCR) of internal transcribed spacer rDNA. In general, we found strong competitive exclusion of R. salebrosus by R. occidentalis, with >or= 75% of the seedlings in the two-species treatment colonized exclusively by R. occidentalis after 5 and 10 months. However, on the seedlings that were co-colonized, we observed no significant difference in the abundances of R. salebrosus and R. occidentalis, suggesting that once R. salebrosus was established, it was no longer competitively inferior. There were no significant differences in survival, growth, or percentage leaf nitrogen of seedlings colonized with either Rhizopogon species, but both growth and percentage leaf nitrogen were significantly higher for ECM than non-ECM seedlings. We also observed strong positive correlations between actual ECM root tip weight and that inferred from real-time PCR for both species, indicating that this method provided an accurate assessment of root tip occupation and hence ECM competitive dynamics. In conjunction with a previous experiment, our results indicate that competition between these two Rhizopogon species occurs similarly in both field and laboratory settings and that when colonizing from spore, timing largely determines the outcome of initial competitive interactions.

Priority effects determine the outcome of ectomycorrhizal competition between two Rhizopogon species colonizing Pinus muricata seedlings

Competition is often considered a fundamental process influencing assemblage structure, yet little is known about competition among ectomycorrhizal (EM) fungi. Here, we examine competitive interactions between Rhizopogon occidentalis and Rhizopogon salebrosus in a 6-month microcosm study. Pinus muricata seedlings were grown in three EM treatments: R. occidentalis, R. salebrosus, and R. occidentalis and R. salebrosus. At 2, 4, and 6 months, five seedlings per treatment were harvested and the EM root tip biomass of each species was determined. Root tips in the two-species treatment were identified using molecular techniques. R. occidentalis had similar EM root tip biomass when grown alone or in the presence of R. salebrosus. By contrast, R. salebrosus had significantly lower EM root tip biomass when grown with R. occidentalis than when grown alone, indicating it was a competitive inferior under the conditions tested. Competition was driven by differences in timing of colonization resulting in a strong priority effect for R. occidentalis. Our results, together with two earlier studies, indicate competition may play a more important role in EM interactions than previously recognized.

Temperature-Dependent Interactions between Juvenile Steelhead and Sacramento Pikeminnow in Laboratory Streams

We examined the temperature dependence of interactions between juvenile steelhead Oncorhynchus mykiss and juvenile Sacramento pikeminnow Ptychocheilus grandis in laboratory streams. Growth of dominant steelhead in water 20-23°C was reduced by more than 50% in trials with Sacramento pikeminnow compared with trials with steelhead alone. Comparison of the two-species treatment with one with an equal density of fish comprised of steelhead alone indicated that the per capita effect of Sacramento pikeminnow was similar to the intraspecific effect of steelhead in water 20-23°C. In contrast, the growth of dominant steelhead in water 15-18°C was unaffected by Sacramento pikeminnow. The temperature-dependence of the effect of Sacramento pikeminnow on dominant steelhead was also reflected in the size of the area defended by steelhead and in the behavior of Sacramento pikeminnow under the two thermal regimes. Compared with all other treatment combinations, dominant steelhead defended the smallest areas when accompanied by Sacramento pikeminnow in warm water. Behavioral interactions with steelhead initiated by Sacramento pikeminnow were about 50 times more frequent in warm water. For the growth of subdominant steelhead, the effect of intraspecific competition exceeded the interspecific effect of Sacramento pikeminnow regardless of water temperature. While thermal regime affects the distributions of these two species, this experiment suggests that it also affects the outcome of competition between them.

Differences in growth and behaviour influence asymmetric predation among early-instar dragonfly larvae

Libellulid dragonflies lay large egg clutches. When eggs of two or more species are deposited at the same time and patch, one can expect a high density of early-instar larvae. Thus, interspecific interactions should be evident. We studied the interaction of two species, Sympetrum fonscolombii and Orthetrum cancellatum, which typically co-occur in French rice fields. We experimentally simulated the laying of egg clutches of both species at the same time and site. Survival of O. cancellatum was reduced compared with that of controls without S. fonscolombii and also with that of S. fonscolombii in the two-species treatment. At the end of the experiment, mean head width of S. fonscolombii was greater than that of O. cancellatum, which may be one reason for the differential survival. In a second experiment we observed the behaviour of pairs of early-instar larvae of both species in different size combinations. Small larvae of S. fonscolombii reduced locomotory activity in the presence of larger O. cancellatum. In contrast, small O. cancellatum did not do so in the presence of larger S. fonscolombii. This behavioural difference may also cause asymmetric interspecific predation.