Big plants—do they limit species coexistence?

Aims According to conventional theory, larger plant species are likely to inflict more intense competition on other (smaller) species. We tested a deducible prediction from this: that a larger species should generally be expected to impose greater limits on the number of species that can coexist with it.Methods Species richness was sampled under plant canopies for a selection of woody species ('host' species) that display a wide range of adult sizes (from small shrubs to large trees), growing within natural vegetation of the Interior Douglas-fir zone of southern British Columbia, Canada. These data were compared with species richness levels sampled within randomly placed plots within the host species habitat.Important findings A prominent host species size effect on species richness was detected but only narrowly at the small end of the species size range. Across most (90%) of the increasing size range of host species, the number of species residing under the host canopy showed no significant decrease relative to the number expected by random assembly, based on species richness within randomly defined equivalent areas within the habitat of the host species. This apparent 'null effect', we suggest, is explained not because these larger species have no effect on community assembly. We postulate that larger species are indeed likely to be more effective in causing competitive exclusion of some smaller species (as expected from conventional theory), but that any potential limitation effect of this on resident species richness is offset for two reasons: (i) larger species also generate niche spaces that they cannot exploit under their own canopies and so have minimal impact (as competitors) on smaller species that can occupy these niches and (ii) certain other small species—despite small size—have effective competitive abilities under the severe competition that occurs within host neighbourhoods of larger species. These and other recent studies call for re-evaluation of traditional views on the role of plant size in affecting competitive ability and community assembly.     

When does environmental variation most influence species coexistence?

The ability of environmental variation to affect species coexistence is much studied, yet environmental variation is not always important. I present an approximate calculation for the long-run growth rate of a species in the presence of spatially and temporally correlated environmental variation. I then perform a factorial numerical experiment, varying the mean seed dispersal distances, competition radii, and overwinter seed survival probabilities for two competing species for an array of variational regimes, noting the effects on their long-run growth rates. I find, first, that purely spatial variation has a greater capacity for influence than variation with a temporal component. Second, spatiotemporal variation can promote coexistence as strongly as purely temporal variation or more so, given the right species traits. Third, if the environmental variation has a spatial component, traits which enable species to become spatially segregated promote coexistence most strongly. That is, it is the possibility of spatial segregation which gives spatial variation its large potential to promote coexistence.

The relative of species pools in determining plant species richness: an alternative explanation of species coexistence?

Explanations of the pattern of species have traditionally relied on small-scale, local processes occurring in ecological time. Differences in species richness have associated with different mechanisms avoiding competition, such as spatiotemporal heterogeneity (weaker competitors may find a more favourable place or time) or environmental stress (competition is assumed to be less intensive under difficult conditions). More recently, large-scale process have been taken into account, raising such questions as: which plant species may potentially grow in a certain community? Are evolutionary processes and species dispersal responsible for the differences between communities? The species-pool theory attempts to answer these general questions, and information about species pools is needed for the design of experiments where the number of species in a community is manipulated.     

Do allopatric male Calopteryx virgo damselflies learn species recognition?

There is a growing amount of empirical evidence that premating reproductive isolation of two closely related species can be reinforced by natural selection arising from avoidance of maladaptive hybridization. However, as an alternative for this popular reinforcement theory, it has been suggested that learning to prefer conspecifics or to discriminate heterospecifics could cause a similar pattern of reinforced premating isolation, but this possibility is much less studied. Here, we report results of a field experiment in which we examined (i) whether allopatric Calopteryx virgo damselfly males that have not encountered heterospecific females of the congener C. splendens initially show discrimination, and (ii) whether C. virgo males learn to discriminate heterospecifics or learn to associate with conspecifics during repeated experimental presentation of females. Our experiment revealed that there was a statistically nonsignificant tendency for C. virgo males to show initial discrimination against heterospecific females but because we did not use sexually naïve individuals in our experiment, we were not able to separate the effect of innate or associative learning. More importantly, however, our study revealed that species discrimination might be further strengthened by learning, especially so that C. virgo males increase their association with conspecific females during repeated presentation trials. The role of learning to discriminate C. splendens females was less clear. We conclude that learning might play a role in species recognition also when individuals are not naïve but have already encountered potential conspecific mates.     

Morphology and coexistence of congeneric ectoparasite species: reinforcement of reproductive isolation?

Assuming that differences or similarities in morphology among congeneric parasite species living in the same habitat are not a random pattern, several hypotheses explaining morphological differences were tested: (i) reproductive isolation, (ii) niche restriction resulting from competition, and (iii) niche specialization. Congeneric monogenean (platyhelminth) ectoparasites parasitizing the gills of one host species were used as an ecological model. Morphometric distances of the attachment organ and morphometric distances of the copulatory organ between species pairs were calculated, Levin's niche size and Renkonen niche overlap indices were applied. Our results support the prediction that the function of niche segregation is to achieve reproductive isolation of related species in order to prevent hybridization (reinforcement of reproductive barriers). Parasite species living in the same niche differ greatly in the size of copulatory organ. Moreover, species coexistence is facilitated by an increase in morphometric distances of copulatory organ and niche centre distances. Our results also show that species living in overlapping niches have similar attachment organs, which supports the prediction that morphologically similar species have the same ecological requirements within one host and suggests small effects of interspecific competition for the evolution of morphological diversity of attachment organs. Specialist adaptations also seem to facilitate species coexistence and affect the niche distribution within host species. Parasite species that can colonize more than one host species, i.e. generalists, occupy more distant niches within host species than strictly host-specific parasites. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76, 125–135.     

How does spatio-temporal disturbance influence species diversity in a hierarchical competitive system? Prospective order of species coexistence and extinction

To address how habitat destruction and hierarchical competition among species affect the spatio-temporal dynamics of a multi-species community, we present a compartment model in which multiple species undergo dispersal and competitive interactions in a patchy habitat arranged in a two-dimensional lattice. We assume that disturbances are periodically imposed on some parts of the lattice in a block, followed by a period free of disturbance. For convenience, species are ranked in order of competitive ability. We further assume that the intrinsic growth rate of species i, _i , and the dispersal ability, D _i , increase in decreasing order of rank. Our model can analytically determine the exact number of surviving species when disturbance is absent. In the presence of disturbance, we numerically examine how spatio-temporal changes in environmental heterogeneity affect species coexistence and extinction, for the case in which the value of _i /D _i monotonically increases or decreases with rank. The results demonstrate that (1) when the interspecific competition is smaller than the intraspecific competition, we can provide predictions on the prospective order of species to be driven extinct and the order of potential species to revive with increasing extents of disturbance; (2) when the interspecific competition is stronger than intraspecific competition, a small difference in the disturbance level can lead to drastic changes in the species composition, their densities and the order of species extinction. In addition, comparison with other similar models reveals that differences in species interaction in local population dynamics critically affect the disturbance-mediated species diversity.     

Extreme cost of rivalry in a monandrous species: male–male interactions result in failure to acquire mates and reduced longevity

Mating system variation is profound in animals. In insects, female willingness to remate varies from mating with hundreds of males (extreme polyandry) to never remating (monandry). This variation in female behaviour is predicted to affect the pattern of selection on males, with intense pre-copulatory sexual selection under monandry compared to a mix of pre- and post-copulatory forces affecting fitness under polyandry. We tested the hypothesis that differences in female mating biology would be reflected in different costs of pre-copulatory competition between males. We observed that exposure to rival males early in life was highly costly for males of a monandrous species, but had lower costs in the polyandrous species. Males from the monandrous species housed with competitors showed reduced ability to obtain a mate and decreased longevity. These effects were specific to exposure to rivals compared with other types of social interactions (heterospecific male and mated female) and were either absent or weaker in males of the polyandrous species. We conclude that males in monandrous species suffer severe physiological costs from interactions with rivals and note the significance of male–male interactions as a source of stress in laboratory culture.     

Body size and species coexistence in consumer–resource interactions: A comparison of two alternative theoretical frameworks

Species coexistence involving trophic interactions has been investigated under two theoretical frameworks—partitioning shared resources and accessing exclusive resources. The influence of body size on coexistence is well studied under the exclusive resources framework, but has received less attention under the shared-resources framework. We investigate body-size-dependent allometric extensions of a classical MacArthur-type model where two consumers compete for two shared resources. The equilibrium coexistence criteria are compared against the general predictions of the alternative framework over exclusive resources. From the asymmetry in body size allometry of resource encounter versus demand our model shows, counterintuitively, and contrary to the exclusive resource framework, that a smaller consumer should be competitively superior across a wide range of supplies of the two resource types. Experimental studies are reviewed to resolve this difference among the two frameworks that arise from their respective assumptions over resource distribution. Another prediction is that the smaller consumer may have relatively stronger control over equilibrium resource abundance, and the loss of smaller consumers from a community may induce relatively stronger trophic cascades. Finally, from satiating consumers’ functional response, our model predicts that greater difference among resource sizes can allow a broader range of consumer body sizes to coexist, and this is consistent with the predictions of the alternative framework over exclusive resources. Overall, this analysis provides an objective comparison of the two alternative approaches to understand species coexistence that have heretofore developed in relative isolation. It advances classical consumer–resource theory to show how body size can be an important factor in resource competition and coexistence.     

Does a trade‐off between current reproductive success and survival affect the honesty of male signalling in species with male parental care?

Recent theory predicted that male advertisement will reliably signal investment in paternal care in species where offspring survival requires paternal care and males allocate resources between advertisement and care. However, the predicted relationship between care and advertisement depended on the marginal gains from investment in current reproductive traits. Life history theory suggests that these fitness gains are also subject to a trade‐off between current and future reproduction. Here, we investigate whether male signalling remains a reliable indicator of parental care when males allocate resources between current advertisement, paternal care and survival to future reproduction. We find that advertisement is predicted to remain a reliable signal of male care but that advertisement may cease to reliably indicate male quality because low‐quality males are predicted to invest in current reproduction, whereas higher‐quality males are able to invest in both current reproduction and survival to future reproduction.     

Lethal male–male interactions in Eurasian lynx

Eurasian lynx (Lynx lynx) is considered a solitary, territorial felid with a low degree of direct intraspecific interactions. Between 2002 and 2011 we observed four aggressive interactions between five different male Eurasian lynx, where two were lethal. All interactions occurred during the mating season and three of the interactions resulted in takeover of the home range by the intruder. Thus, in this study we demonstrate that aggressive interactions, sometimes with severe consequences, occur in male Eurasian lynx and suggest that they are driven primarily by access to female lynx in the mating season.     

Heterogeneity in soil water and light environments and dispersal limitation: what facilitates tree species coexistence in a temperate forest?

In the present study, we analysed the habitat association of tree species in an old‐growth temperate forest across all life stages to test theories on the coexistence of tree species in forest communities. An inventory for trees was implemented at a 6‐ha plot in Ogawa Forest Reserve for adults, juveniles, saplings and seedlings. Volumetric soil water content (SMC) and light levels were measured in 10‐m grids. Relationships between the actual number of stems and environmental variables were determined for 35 major tree species, and the spatial correlations within and among species were analysed. The light level had no statistically significant effect on distribution of saplings and seedlings of any species. In contrast, most species had specific optimal values along the SMC gradient. The optimal values were almost identical in earlier life stages, but were more variable in later life stages among species. However, no effective niche partitioning among the species was apparent even at the adult stage. Furthermore, results of spatial analyses suggest that dispersal limitation was not sufficient to mitigate competition between species. This might result from well‐scattered seed distribution via wind and bird dispersal, as well as conspecific density‐dependent mortality of seeds and seedlings. Thus, both niche partitioning and dispersal limitation appeared less important for facilitating coexistence of species within this forest than expected in tropical forests. The tree species assembly in this temperate forest might be controlled through a neutral process at the spatial scale tested in this study.     

Competition and species coexistence in a metapopulation model: Can fast asymmetric migration reverse the outcome of competition in a homogeneous environment?

We investigate whether asymmetric fast migration can modify the predictions of classical competition theory and, in particular revert species dominance. We consider a model of two species competing for an implicit resource on a habitat divided into two patches. Both patches are connected through constant migration rates and in each patch local dynamics are driven by a Lotka-Volterra competition system.Local competition is asymmetric with the same superior competitor in both patches. Migration is asymmetric, species dependent and fast in comparison to local competitive interactions. The species and patches are taken to be otherwise similar: in both patches we assume the same carrying capacities for both species, and the same growth rates and pair-wise competition coefficients for each species.We show that global dynamics can be described by a classical Lotka-Volterra competition model. We found that by modifying the ratio of intraspecific migration rates for both species all possible combinations of global species relative dominance can be achieved. We find specific conditions for which the local superior competitor is globally excluded. This is to our knowledge the first study showing that fast asymmetric migration can lead to inferior competitor dominance in a homogeneous environment. We conclude that disparity of temporal scales between migration and local dynamics may have important consequences for the maintenance of biodiversity in spatially structured populations.     

When is a parasite species a species?

Regrettably, 140 years after the publication of Darwin's Origin of Species, we face the grotesque situation that we still do not know what is a species whose origin Darwin wanted to explain. A generally applicable species definition is not available. Is there a basic unit of biodiversity above the level of individuals? Do we try to define something that does not exist in reality? The strong potential for the evolution of genetic variability in parasites together with the importance of species diagnosis for applied fields of parasite research make biodiversity research a key role in parasitology. Frequent occurrence of sympatric speciation, clonal reproduction, selfing, sib mating or parthenogenesis imply exceptional conditions for the evolution of gene pool diversities in parasites.     

Nuptial feeding by male bushcrickets: an indicator of male quality?

Male bushcrickets transfer a spermatophore at mating that consistsof a sperm-containing ampulla and a product of the accessoryglands, the spermatophylax, that is consumed by the female duringinsemination. Male Requena verticalis produce functionally differentspermatophores depending on the availability of sexually receptivefemales. They will maintain high mating frequency by providinga gift sufficient to ensure sperm transfer, or will invest parentallyin females when their mating frequency is low. We examined therelationship between male quality and nuptial feeding underconditions where males invest in ejaculate protection or inparental investment. When investing in ejaculate protection,males reduced the quality of the spermatophylax meal by reducingboth the concentration of protein and the absolute amount ofprotein it contained. There was no relationship between malephenotype and gift size or quality. Moreover, we could findno evidence for the recently advanced hypothesis that femalescan exercise mate choice by interfering with insemination. However,when males were investing parentally, we found a positive associationbetween spermatophylax size and male size, but no relationshipbetween protein content and male size. Males with high levels offluctuating asymmetry invested more heavily in the nutritionalcontent of their spermatophylaxes than did symmetrical males.Thus, male quality does influence nuptial feeding, but in amanner predicted by a model of indirect fitness benefits frommate choice.     

How do species interactions affect species distribution models?

One of the most promising recent advances in biogeography has been the increased interest and understanding of species distribution models – estimates of the probability that a species is present given environmental data. Unfortunately, such analyses ignore many aspects of ecology, and so are difficult to interpret. In particular, we know that species interactions have a profound influence on distributions, but it is not usually possible to incorporate this knowledge into species distribution models. What is needed is a rigorous understanding of how unmeasured biotic interactions affect the inferences generated by species distribution models. To fill this gap, we develop a general mathematical approach that uses probability theory to determine how unmeasured biotic interactions affect inferences from species distribution models. Using this approach, we reanalyze one of the most important classes of mechanistic models of competition: models of consumer resource dynamics. We determine how measurements of one aspect of the environment – a single environmental variable – can be used to estimate the probability that an environment is suitable with species distribution models. We show that species distribution models, which ignore numerous facets of consumer resource dynamics such as the presence of a competitor or the dynamics of depletable resources, can furnish useful predictions for the probability that an environment is suitable in some circumstances. These results provide a rigorous link between complex mechanistic models of species interactions and species distribution models. In so doing they demonstrate that unmeasured biotic interactions can have strong and counterintuitive consequences on species distribution models.     

Are hybrid species more fit than ancestral parent species in the current hybrid species habitats?

Hybrid speciation is thought to be facilitated by escape of early generation hybrids into new habitats, subsequent environmental selection and adaptation. Here, we ask whether two homoploid hybrid plant species (Helianthus anomalus, H. deserticola) diverged sufficiently from their ancestral parent species (H. annuus, H. petiolaris) during hybrid speciation so that they are more fit than the parent species in hybrid species habitats. Hybrid and parental species were reciprocally transplanted into hybrid and parental habitats. Helianthus anomalus was more fit than parental species in the H. anomalus actively moving desert dune habitat. The abilities to tolerate burial and excavation and to obtain nutrients appear to be important for success in the H. anomalus habitat. In contrast, H. deserticola failed to outperform the parental species in the H. deserticola stabilized desert dune habitat, and several possible explanations are discussed. The home site advantage of H. anomalus is consistent with environmental selection having been a mechanism for adaptive divergence and hybrid speciation and supports the use of H. anomalus as a valuable system for further assessment of environmental selection and adaptive traits.