The taxonomic distribution of rare and common species among families in the vascular plant flora of Fennoscandia

Many plant traits are not randomly distributed among families. The question considered here is ‘are rarity and commonness of vascular plants in Fennoscandia randomly distributed among families?’ If more rare or more common species are found within a family, this may give some initial indications about which traits may predict rarity and commonness of species. A species was defined as rare or common based on its abundance and on the number of grid squares it occupies. 1521 naturally occurring species in 229 75×75 km grid squares were used. Permutation tests were performed to assess statistically if rarity and commonness are randomly distributed among families. Several families can be identified as having more rare or more common species than would be expected under a random allocation model. However, there are little deviations from what would be expected if rarity and commonness were randomly distributed among families in the whole Fennoscandian flora. It is proposed that the arbitrary geographical limits of the study area may account for the lack of any clear patterns of rarity and commonness among and between families.     

How plant life‐history and ecological traits relate to species rarity and commonness at varying spatial scales

Comparative studies investigating relationships between plant traits and species rarity and commonness were surveyed to establish whether global patterns have emerged that would be of practical use in management strategies aimed at the long‐term conservation of species. Across 54 studies, 94 traits have been examined in relation to abundance, distribution and threatened status at local, regional and geographical spatial scales. Most traits (63) have yet to be the focus of more than one study. Half of the studies involved less than 10 species, and one‐quarter did not replicate rare–common contrasts. Although these features of the literature make it difficult to demonstrate robust generalizations regarding trait relationships with species rarity, some important findings surfaced in relation to traits that have been examined in two or more studies. Species with narrow geographical distributions were found to produce significantly fewer seeds (per unit measurement) than common species (in four of six studies), but did not differ with respect to breeding system (five of five studies). The majority of traits (including seed size, competitive ability, growth form and dispersal mode) were related to rarity in different ways from one study to the next. The highly context‐dependent nature of most trait relationships with rarity implies that application of knowledge concerning rare–common differences and similarities to management plans will vary substantially for different vegetation types and on different continents. A comparative analysis of distribution patterns in relation to several life‐history and ecological traits among 700 Australian eucalypt species was then performed. A significantly dispro­portionate number of tall species and species with long flowering durations had wide geographical ranges. Trait relationships with distribution were explored further through the development of a methodology incorporating multiple spatial scales. Eight theoretical categories were described illustrating variation in distribution patterns (and hence rarity and commonness) across small, intermediate and large spatial scales, based on the spatial structure of species occurrence across the Australian landscape. Each eucalypt species was placed into a category, and trait variation was explored across all species in relation to distribution patterns across multiple spatial scales. This approach yielded important information about trait relationships with distribution among the eucalypts, linking the spatial structure of points‐of‐occurrence with patterns of rarity and commonness. With the pressing need to protect increasing numbers of threatened species and slow rates of extinction, the development and refinement of a broadly usable methodology for rarity studies that encompasses multiple spatial scales, which can be used for any geographical location, will be useful in both conservation and management.     

Growth and reproduction of New Zealand

In New Zealand, as elsewhere, research on rare species has been dominated by autecological studies of individual threatened species. Limitations of this approach are that it involves no comparison with related common species which may have similar traits, and that the minimal sample size prevents generalisation about causes and consequences of rarity. We report on experimentally determined growth and reproductive traits of 10 rare and common Acaena(Rosaceae) species from two taxonomic sections (sect. Ancistrum and sect. Microphyllae). We examined the relationship between rarity or commonness and relative growth rate, mode of vegetative expansion, morphology/presentation of reproductive structures and reproductive allocation. Rarity and commonness were defined according to geographic range size, measured as the number of 10-km grid squares containing at least one record of the species. There were tendencies across both taxonomic sections for species with large range size to have higher relative growth rates and in section Microphyllae, faster lateral expansion. Among section Ancistrum species, common species tended to produce inflorescences for a shorter period and held their capitula higher above the canopy, but other reproductive attributes showed little association with range size. In section Microphyllae all reproductive traits tended to be positively associated with range size. This was mainly due to the single very common species having high fecundity. The lack of strong patterns among our results may reflect insufficient sample size or that the rare species represent different types of rarity.     

A theoretical model linking interspecific variation in density dependence to species abundances

Understanding the factors that govern the commonness and rarity of individual species is a central challenge in community ecology. Empirical studies have often found that abundance is related to traits associated with competitive ability and suitability to the local environment and, more recently, also to negative conspecific density dependence. Here, we construct a theoretical framework to show how a species’ abundance is, in general, expected to be dependent on its per-capita growth rate when rare and the rate at which its growth rate declines with increasing abundance (strength of stabilization). We argue that per-capita growth rate when rare can be interpreted as competitive ability and that strength of stabilization largely reflects negative conspecific inhibition. We then analyze a simple spatially implicit model in which each species is defined by three parameters that affect its juvenile survival: its generalized competitive effect on others, its generalized response to competition, and an additional negative effect on conspecifics. This model facilitates the stable coexistence of an arbitrarily large number of species and qualitatively reproduces empirical relationships between abundance, competitive ability, and negative conspecific density dependence. Our results provide theoretical support for the combined roles of competitive ability and negative density dependence in the determination of species abundances in real ecosystems, and suggest new avenues of research for understanding abundance in models and in real communities.     

The contribution of geographically common and rare species to the spatial distribution of biodiversity

Motivation and aim

Mapping the spatial distribution of biodiversity is critical for understanding its fundamental drivers (e.g. speciation, environmental filtering) as well as for conservation assessment. An important dimension of this topic is how the distributions of subsets of species contribute to the overall distribution of biodiversity. Although studies have previously investigated the role of geographically common and rare species in determining these patterns, their respective contributions appear to vary between studies. Knowing which species contribute disproportionately to the spatial distribution of biodiversity enables the identification of key indicator species for biodiversity assessments across large areas and is important for prioritising areas for conservation actions. An extensive review of the literature was carried out to synthesise research on how geographic rarity contributes to spatial patterns of biodiversity. We identify potential explanations for the discrepancies in findings between studies and identify opportunities for further research.

Results

Many studies on the contribution of geographic commonness and rarity to the spatial distribution of biodiversity focus on species richness. A prevalent view is that common (widespread) species contribute disproportionately, although this is not ubiquitous across studies due to factors such as the geographic extent from which relative rarity is quantified. We identify research pathways that will further improve our knowledge of how geographically common and rare species shape the spatial distribution of biodiversity including the impact of spatial scale on species contributions and the incorporation of biodiversity components beyond taxonomic alpha diversity, that is functional and phylogenetic diversity.

Main conclusions

Future research should incorporate multiple biodiversity components and model scale dependency. This will further our knowledge on the underlying processes that shape the spatial variation of biodiversity across the planet and help inform biological surveys and conservation activities.     

Common and rare species respond to similar niche processes in macroinvertebrate metacommunities

Ecologists have long investigated why communities are composed of a few common species and many rare species. Most studies relate rarity to either niche differentiation among species or spatial processes. There is a parallel between these processes and the processes proposed to explain the structure of metacommunities. Based on a metacommunity perspective and on data on stream macroinvertebrates from different regions of Brazil, we answer two questions. 1) Are sets of common and rare species affected by similar niche and spatial processes? 2) How does the community composition of common and of rare species differ? The main hypothesis we test is that common species are mainly affected by environmental factors, whereas rare species are mostly influenced by dispersal limitation. We used variation partitioning to determine the proportion of variation explained by the environment and space in common and rare species matrices. Contrary to our expectations, evidence supported the idea that both common and rare species are affected mainly by environmental factors, even after controlling for the differing information content between common and rare species matrices. Moreover, the abundance of some common species is also a good predictor of variation in rare species matrices. Niche differences are unlikely to be the sole cause of patterns of rarity in these metacommunities. We suggest that sets of common and rare species react to similar major environmental gradients and that rare species also respond to processes that operate at a more fine‐grained spatial scale, particularly biotic interactions. We extend the view that species sorting is the dominant process structuring metacommunities and argue that future studies focusing on rarity would benefit from a metacommunity perspective.     

Both rare and common species make unique contributions to functional diversity in an ecosystem unaffected by human activities

Aim

Rare species typically contribute more to functional diversity than common species. However, humans have altered the occupancy and abundance patterns of many species—the basis upon which we define “rarity.” Here, we use a globally unique dataset from hydrothermal vents—an untouched ecosystem—to test whether rare species over‐contribute to functional diversity.

Location

Juan de Fuca Ridge hydrothermal vent fields, Northeast Pacific Ocean.

Methods

We first conduct a comprehensive review to set up expectations for the relative contributions of rare and common species to functional diversity. We then quantify the rarity and commonness of 37 vent species with relevant trait information to assess the relationship between rarity and functional distinctiveness—a measure of the uniqueness of the traits of a species relative to traits of coexisting species. Next, we randomly assemble communities to test whether rare species over‐contribute to functional diversity in artificial assemblages ranging in species richness. Then, we test whether biotic interactions influence functional diversity contributions by comparing the observed contribution of each species to a null expectation. Finally, we identify traits driving functional distinctiveness using a distance‐based redundancy analysis.

Results

Across functional diversity metrics and species richness levels, we find that both rare and common species can contribute functional uniqueness. Some species always offer unique trait combinations, and these species host bacterial symbionts and provide habitat complexity. Moreover, we find that contributions of species to functional diversity may be influenced by biotic interactions.

Main conclusions

Our findings show that many common species make persistent, unique contributions to functional diversity. Thus, it is key to consider whether the abundance and occupancy of species have been reduced, relative to historical baselines, when interpreting the contributions of rare species to functional diversity. Our work highlights the importance of testing ecological theory in ecosystems unaffected by human activities for the conservation of biodiversity.     

Seven forms of rarity in mammals

Conservation biologists have identified threats to the survival of about a quarter of the mammalian species; to identify patterns of rarity and commonness of mammals, we studied a global sample of 1212 species (about 28% of the mammals) using the ‘7 forms of rarity’ model (in which species are roughly divided into above and below the median for local population density, species’ range area, and number of habitat types). From a niche‐based hypothesis of abundance and distribution, we predicted that mammals would exhibit a bimodal pattern of rarity and commonness, with an overabundance of species in the relatively rarest and most common categories; and just such a significant bimodal pattern emerged, with over a quarter of the species classified as exceedingly rare and a further quarter very common, supporting the niche‐based hypothesis. Orders that include large mammals, including perissodactyls, primates, diprotodonts, and carnivores, exhibited significantly high proportions of relatively rare species; and tropical zoogeographic regions, especially Indomalaya, had relatively high proportions of species in the rarest category. Significant biases in the available data on mammals included under‐sampling of small species like rodents and bats, and a relative paucity of data on zoogeographic regions outside of North America and Australia. Mammalian species listed as of conservation concern by the IUCN occurred in all cells of the model, indicating that even relatively common species can be listed as threatened under some conditions; but we also found that sixty‐three species were relatively rare in all three criteria of the 7‐forms model but were not listed as threatened, indicating potential candidates for further study. Mammals may be a group of animals where rarity or commonness is a natural aspect of species biology, both confirming and perhaps partly explaining the large proportion of mammals assigned threatened status.     

Rarity facets of biodiversity: Integrating Zeta diversity and Dark diversity to understand the nature of commonness and rarity

Measuring commonness and rarity is pivotal to ecology and conservation. Zeta diversity, the average number of species shared by multiple sets of assemblages, and Dark diversity, the number of species that could occur in an assemblage but are missing, have been recently proposed to capture two aspects of the commonness‐rarity spectrum. Despite a shared focus on commonness and rarity, thus far, Zeta and Dark diversities have been assessed separately. Here, we review these two frameworks and suggest their integration into a unified paradigm of the “rarity facets of biodiversity.” This can be achieved by partitioning Alpha and Beta diversities into five components (the Zeta, Eta, Theta, Iota, and Kappa rarity facets) defined based on the commonness and rarity of species. Each facet is assessed in traditional and multiassemblage fashions to bridge conceptual differences between Dark diversity and Zeta diversity. We discuss applications of the rarity facets including comparing the taxonomic, functional, and phylogenetic diversity of rare and common species, or measuring species'' prevalence in different facets as a metric of species rarity. The rarity facets integrate two emergent paradigms in biodiversity science to better understand the ecology of commonness and rarity, an important endeavor in a time of widespread changes in biodiversity across the Earth.     

Can We Distinguish Plant Species that are Rare and Endangered from Other Plants Using Their Biological Traits?

Understanding the factors responsible for species rarity is crucial for effective species conservation. One possible approach to obtaining information about causes of species rarity is to compare rare and common species. We analyzed the biological and ecological traits of critically endangered (CR) plant species of the Czech Republic. We compared the vegetative, generative and ecological traits of CR species with: i) common closely related species (a form of phylogenetic correction), ii) common closely related species sharing the same habitat (i.e., excluding pairs not sharing the same habitat, because many differences in species traits can be caused by adaptation to a specific habitat type) and iii) all plants of the Czech Republic. Information about species traits was mainly obtained from literature and databases. Comparison with common closely related species showed that CR species are smaller, flower for shorter periods, and have higher proportions of self-compatibility and higher terminal velocities. CR species also differ in their mode of dispersion, and their ecology and distribution. Comparison with species from the same habitat gave similar results. Comparison with the whole flora produced slightly different results, with additional differences in pollination mode and seed mass. The results of all three types of comparison suggest that critically endangered species of the Czech Republic are small, competitively inferior species, with some differences in the generative part of their life cycle, and occur mainly in open, unproductive habitats.     

Book Review

Abstract

Most studies on population ecology of bryophytes have involved common species. However, some studies have compared life history strategies in rare and common species. We review the life history strategies (life expectancy, sexual and asexual reproduction, spore production, spore size and dispersal) for species that are rare in relation to pattern and persistency of suitable habitat patches. In particular, we discuss the dynamics on two levels, within and among localities, for different categories of rare species. We predict that most rare species will be found to have restricted dispersal capacities but higher than average life expectancies of local subpopulations. Natural rarity is distinguished from human-induced rarity and species rare for the latter reason are distinguished as 'threatened'.     

Visualizing connectivity of ecological and evolutionary concepts—An exploration of research on plant species rarity

Understanding the ecological and evolutionary factors that influence species rarity has important theoretical and applied implications, yet the reasons why some species are rare while others are common remain unresolved. As a novel exploration of scientific knowledge, we used network analysis conceptually to visualize the foci of a comprehensive base of >800 studies on plant species rarity within the context of ecology and evolution. In doing so, we highlight existing research strengths that could substantiate novel syntheses and gaps that could inspire new research. Our results reveal strong integrated foci on population dynamics with other ecological concepts. In contrast, despite the potential for ecological and evolutionary processes to interact, few studies explored the interplay of environmental factors and microevolutionary patterns. The cellular and molecular biology, physiology, and plasticity of rare plant species within both ecological and evolutionary contexts similarly provide avenues for impactful future investigations.     

Effect of population size on the prospect of species survival

Many recent studies have demonstrated a negative effect of small population size on single plant traits. However, not much is known about the actual consequences of reduced plant performance on the long-term prospect of species survival. I studied the effect of population size on population growth rate and survival probability in the rare perennial herbScorzonera hispanica occurring in fragmented grasslands. Its performance was measured using several traits related to reproduction in 21 populations ranging in size from 3 to 2475 plants. These data were then connected with data on full demography of the species from three of the studied populations. Two different matrix models differing in the number of transitions based on measurements in the populations differing in size were used to explore the relationship between population size and population growth rate. Both matrix models showed that despite the decline in seed production in small populations, population growth rate is never significantly different from one, and the populations could thus be expected to survive in the long run. Calculations of extinction probabilities that take into account demographic and environmental stochasticity, however, showed that populations below 100 flowering individuals have a high probability to become extinct. This demonstrates that demographic and environmental stochasticity is an important driver of the fate of small populations in this system. This study demonstrates that estimation of population growth rate can provide new insights into the effect of population size on population growth and survival. It also shows how matrix models enable the combination various pieces of information about the single populations into one overall measure, and may provide a useful tool for the standardization of studies on the effects of population size on population performance.     

The predation cost of being a male: implications for sex-specific rates of ageing

Evolutionary theory predicts that the rate of extrinsic (i.e. age- and condition-independent) mortality should affect important life history traits such as the rate of ageing and maximum lifespan. Sex-specific differences in mortality rates due to predation may therefore result in the evolution of important differences in life history traits between males and females. However, quantifying the role of predators as a factor of extrinsic mortality is notoriously difficult in natural populations. We took advantage of the unusual prey caching behaviour of the barn owl Tyto alba and the tawny owl Strix aluco to estimate the sex ratio of their five most common preys. For all prey species, there was a significant bias in the sex ratio of remains found in nests of both these owls. A survey of literature revealed that sex-biased predation is a common phenomenon. These results demonstrate that predation, a chief source of extrinsic mortality, was strongly sex-biased. This may select for alternate life history strategies between males and females, and account for a male life span being frequently lower than female lifespan in many animal species.     

Rarity and persistence

Rarity is a population characteristic that is usually associated with a high risk of extinction. We argue here, however, that chronically rare species (those with low population densities over many generations across their entire ranges) may have individual‐level traits that make populations more resistant to extinction. The major obstacle to persistence at low density is successful fertilisation (union between egg and sperm), and chronically rare species are more likely to survive when (1) fertilisation occurs inside or close to an adult, (2) mate choice involves long‐distance signals, (3) adults or their surrogate gamete dispersers are highly mobile, or (4) the two sexes are combined in a single individual. In contrast, external fertilisation and wind‐ or water‐driven passive dispersal of gametes, or sluggish or sedentary adult life habits in the absence of gamete vectors, appear to be incompatible with sustained rarity. We suggest that the documented increase in frequency of these traits among marine genera over geological time could explain observed secular decreases in rates of background extinction. Unanswered questions remain about how common chronic rarity actually is, which traits are consistently associated with chronic rarity, and how chronically rare species are distributed among taxa, and among the world's ecosystems and regions.     

Genetic conservation and management of the California endemic,Torrey pine (Pinus torreyana Parry): Implications of genetic rescue in a genetically depauperate species

Rare species present a challenge under changing environmental conditions as the genetic consequences of rarity may limit species ability to adapt to environmental change. To evaluate the evolutionary potential of a rare species, we assessed variation in traits important to plant fitness using multigenerational common garden experiments. Torrey pine, Pinus torreyana Parry, is one of the rarest pines in the world, restricted to one mainland and one island population. Morphological differentiation between island and mainland populations suggests adaptation to local environments may have contributed to trait variation. The distribution of phenotypic variances within the common garden suggests distinct population‐specific growth trajectories underlay genetic differences, with the island population exhibiting substantially reduced genetic variance for growth relative to the mainland population. Furthermore, F1 hybrids, representing a cross between mainland and island trees, exhibit increased height accumulation and fecundity relative to mainland and island parents. This may indicate genetic rescue via intraspecific hybridization could provide the necessary genetic variation to persist in environments modified as a result of climate change. Long‐term common garden experiments, such as these, provide invaluable resources to assess the distribution of genetic variance that may inform conservation strategies to preserve evolutionary potential of rare species, including genetic rescue.     

How species traits and affinity to urban land use control large-scale species frequency

Aim Although urban areas only occupy c. 2.8% of the earth's land surface, urbanization threatens biodiversity as areas of high human population density often coincide with high biodiversity. Therefore, nature conservation should concentrate on both remote areas and densely populated regions. Protecting rare plant species in rural and urban areas can contribute to the protection of biodiversity. We therefore need to understand why species are rare. Studies on causes of rarity often concentrate on either plant traits or extrinsic threats (such as habitat fragmentation or nitrogen enrichment). However, there are only a few studies that combine traits and extrinsic threats, although such analyses might clarify causes of rarity. We assessed how the affinity of vascular plant species to urban land use (‘urbanity’) interacts with plant traits in determining species frequency. Location Germany, resolution c. 12 km × 11 km. Methods Species with a low frequency may be rare because they occur in rare habitats or because of other reasons, although their habitat is frequent. Therefore, we calculated the frequency of species corrected for habitat frequency, i.e. relative species frequency. We explained relative species frequency by the interactions of species traits and species affinity to urban land use using generalized linear models. Simultaneous autoregressive error models controlled for phylogenetic relationships of species. Results Relative species frequency depends on species affinity to urban land use, independent of the different interactions between traits and urbanity used as predictors. The higher the urbanity the higher is species frequency. Urbanity interacts with species preferences towards temperature and soil acidity. Moreover, dispersal, nitrogen preferences and origin explain relative species frequency, amongst others. Main conclusions Many rare species, especially those preferring cool or acidic habitats might already have disappeared from urban areas. Analyses that combine species traits and environmental effects can explain the causes of rarity and help to derive better conservation strategies.     

Life history tactics of darters (Percidae: Etheostomatiini) and their relationship with body size, reproductive behaviour, latitude and rarity

Patterns of covariation of life history traits of darters in the genus Etheostoma are reviewed. The primary pattern is associated with body size. Large darters grow faster, mature at a larger size, produce bigger clutches, and have longer reproductive and life spans, and shorter spawning seasons, than do small darters. When the effects of size are removed statistically, the dominant secondary pattern matched the r-K continuum from fast-growing, short-lived, primarily semelparous species with many small ova and a high reproductive effort (r-species) to slow-growing, long-lived, iteroparous species with few large ova and a low reproductive effort (K-species). Variation in life history traits is also influenced by reproductive behaviour, latitude, and rarity (as measured by geographic range). There are significant differences in the primary and secondary life history patterns among reproductive guilds. Latitude and rarity are not correlated with these primary and secondary patterns. Instead, they account for variation of tertiary patterns. Rare species may not match the reproductive performance of more common and widely distributed species. Future studies of life history traits in darters should focus on species whose reproductive behaviour differs from that of the species reviewed in this study, and on the demographic characteristics of rare or declining populations.     

Timescales of population rarity and commonness in random environments

This is a mathematical study of the interactions between non-linear feedback (density dependence) and uncorrelated random noise in the dynamics of unstructured populations. The stochastic non-linear dynamics are generally complex, even when the deterministic skeleton possesses a stable equilibrium. There are three critical factors of the stochastic non-linear dynamics; whether the intrinsic population growth rate (lambda) is smaller than, equal to, or greater than 1; the pattern of density dependence at very low and very high densities; and whether the noise distribution has exponential moments or not. If lambda < 1, the population process is generally transient with escape towards extinction. When lambda > or = 1, our quantitative analysis of stochastic non-linear dynamics focuses on characterizing the time spent by the population at very low density (rarity), or at high abundance (commonness), or in extreme states (rarity or commonness). When lambda >1 and density dependence is strong at high density, the population process is recurrent: any range of density is reached (almost surely) in finite time. The law of time to escape from extremes has a heavy, polynomial tail that we compute precisely, which contrasts with the thin tail of the laws of rarity and commonness. Thus, even when lambda is close to one, the population will persistently experience wide fluctuations between states of rarity and commonness. When lambda = 1 and density dependence is weak at low density, rarity follows a universal power law with exponent -3/2. We provide some mathematical support for the numerical conjecture [Ferriere, R., Cazelles, B., 1999. Universal power laws govern intermittent rarity in communities of interacting species. Ecology 80, 1505-1521.] that the -3/2 power law generally approximates the law of rarity of 'weakly invading' species with lambda values close to one. Some preliminary results for the dynamics of multispecific systems are presented.     

Eutrophication and fragmentation are related to species’ rate of decline but not to species rarity: results from a functional approach

Due to ubiquitous eutrophication and fragmentation, many plant species are actually threatened in Europe. Most ecosystems face an overall nutrient input leading to changes in species composition. Fragmentation is effectively influencing species survival. We investigate if two different measures of species performance of 91 calcareous grassland species–rate of decline and rarity—are related to comparable traits and hence processes. On the one hand we expected that species rate of decline is mainly determined by the processes of eutrophication and fragmentation. On the other hand we hypothesized that the importance of site characteristics may overwhelm the effect of eutrophication and fragmentation for species rarity. Hence, we compared persistence traits responding to eutrophication, dispersal traits being related to fragmentation and ecological site factors for decreasing and increasing species and for rare and common species. The results suggest that increasing species had better means of long-distance dispersal and were more competitive than decreasing species. In contrast, there were hardly any differences in traits between rare and common species, but site characteristics were related to species rarity. Rare species were in the main those with ecological preferences for warm, dry, light and nutrient poor conditions. This study may represent a basis for the assessment of plant species threat. Applying the deduced knowledge about the life history of decreasing versus increasing species to habitat-scale approaches it is possible to predict which species may become threatened in the future simply from the combination of their trait values. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.