Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.     

Why are there so few fish in the sea?

The most dramatic gradient in global biodiversity is between marine and terrestrial environments. Terrestrial environments contain approximately 75-85% of all estimated species, but occupy only 30 per cent of the Earth's surface (and only approx. 1-10% by volume), whereas marine environments occupy a larger area and volume, but have a smaller fraction of Earth's estimated diversity. Many hypotheses have been proposed to explain this disparity, but there have been few large-scale quantitative tests. Here, we analyse patterns of diversity in actinopterygian (ray-finned) fishes, the most species-rich clade of marine vertebrates, containing 96 per cent of fish species. Despite the much greater area and productivity of marine environments, actinopterygian richness is similar in freshwater and marine habitats (15 150 versus 14 740 species). Net diversification rates (speciation-extinction) are similar in predominantly freshwater and saltwater clades. Both habitats are dominated by two hyperdiverse but relatively recent clades (Ostariophysi and Percomorpha). Remarkably, trait reconstructions (for both living and fossil taxa) suggest that all extant marine actinopterygians were derived from a freshwater ancestor, indicating a role for ancient extinction in explaining low marine richness. Finally, by analysing an entirely aquatic group, we are able to better sort among potential hypotheses for explaining the paradoxically low diversity of marine environments.     

Why are there so many species of bumble bees at Dungeness?

WILLIAMS, P. H., 1989. Why are there so many species of bumble bees at Dungeness? Dungeness is unique in the British Isles in that it has more species of bumble bees than any other locality. Three ideas about what governs the number of species at a locality are examined by locking at patterns of flower visits at Dungeness in comparison with those at Shoreham, a species-poor locality also in Kent. The species of bumble bees that are present at Dungeness but absent from Shoreham show no association in their distributions among 2 km grid-squares in Kent with the species of food-plants that they prefer at Dungeness, nor is there any correlation between the diversity of bees and diversity of food-plants at Dungeness and Shoreham. From the information available, Dungeness is most likely to have more species of bumble bees because it has a particularly high density of the more nectar-rich flowers that bumble bees can use. Bumble bees feed most profitably from deep flowers because these contain more nectar than shallow flowers, although direct access to deeper flowers is ultimately limited by the length of each bee's proboscis. The distribution of worker proboscis lengths among species in the species-pool in Kent is clumped about a median of 7.9 mm. The best foraging conditions for the maximum number of species should be provided when flowers of similar depths are present in sufficiently large numbers for all foragers to make near-optimal flower choices. Although there is no difference in median between the distributions of the bees' proboscis lengths and the depths of the flowers they use at Dungeness, at Shoreham the flower depths used are shorter than the proboscis lengths. Among the food-plants at Dungeness, high densities of Teucrium scorodonia and Echium vulgare are likely to be especially important.     

Why are there so few resistance-associated mutations in insecticide target genes?

The genes encoding the three major targets of conventional insecticides are: Rdl, which encodes a gamma-aminobutyric acid receptor subunit (RDL); para, which encodes a voltage-gated sodium channel (PARA); and Ace, which encodes insect acetylcholinesterase (AChE). Interestingly, despite the complexity of the encoded receptors or enzymes, very few amino acid residues are replaced in different resistant insects: one within RDL, two within PARA and three or more within AChE. Here we examine the possible reasons underlying this extreme conservation by looking at the aspects of receptor and/or enzyme function that may constrain replacements to such a limited number of residues.     

Why so few?

Nobel Prize Women in Science: Their struggles and momentous discoveries (1998). S. Bertsch McGrayne. Carol Publishing Group, 448 pp. $19.95 paper ISBN 0806520256.     

Santa Rosalia revisited: Why are there so many species of bacteria?

The diversity of bacteria in the world is very poorly known. Usually less than one percent of the bacteria from natural communities can be grown in the laboratory. This has caused us to underestimate bacterial diversity and biased our view of bacterial communities. The tools are now available to estimate the number of bacterial species in a community and to estimate the difference between communities. Using what data are available, I have estimated that thirty grams of forest soil contains over half a million species. The species difference between related communities suggests that the number of species of bacteria may be more than a thousand million. I suppose that the explanation for such a large number of bacterial species is simply that speciation in bacteria is easy and extinction difficult, giving a rate of speciation higher than the rate of extinction, leading to an ever increasing number of species over time. The idea that speciation is easy is justified from the results of recent experimental work in bacterial evolution.     

Why are there so many coexisting species of lizards in Australian deserts?

Because Australian skinks of the genus Ctenotus display very high local species richness in arid-zone spinifex grasslands but not in mesic habitats, these lizards have been used as ”model organisms” to ask why ecologically similar taxa coexist under some circumstances but not others. Previous work has involved detailed studies within small areas, and has looked for differences in ecological processes between arid versus mesic habitats. We suggest a radically different explanation for the high species-richness of arid-zone Ctenotus, by shifting attention to a larger spatial scale: the regional species pool. Analyses of the geographic distributions of Ctenotus species confirm that more species coexist at sites in the arid-zone (mean =9.3 species per site) than in other climatic zones (means 2.4–7.6). However, the total number of species occurring within the arid-zone is actually lower, per km² of habitat, than is the case in some other climatic zones. That is, arid-zone Ctenotus show a higher local (alpha) species diversity, but a lower regional (gamma) diversity, than their mesic-habitat congeners. This apparent paradox occurs because most arid-zone species occur over vast areas (mean =1,035,000 km²), whereas congeners from other climatic zones have smaller geographic ranges (200–373,000 km²). The broad distributions of arid-zone taxa reflect the great spatial homogeneity in climatic conditions in this zone. That is, the ”climate spaces” occupied are similar for Ctenotus species from all bioclimatic regions. Thus, a given amount of climatic space translates into a larger geographic distribution (and hence, more sympatry) in the arid-zone than in other areas. In summary, the high number of coexisting Ctenotus species in arid-zone habitats may simply reflect the facts that the arid zone is large (so that many species have evolved therein) and climatically homogeneous (so that any species evolving in that habitat type can disperse very widely, and thus overlap with many other species). Our approach explains much of the variance in local-assemblage species richness from regional to site scales; but explanations invoking biological attributes of the species concerned, the nature of their interactions with other species or with particular resources (such as prey or shelter) may still be significant at microhabitat scales. For lizard communities in Australia, species richness at a site may be determined more by continental biogeography rather than by ecological interactions. Received: 28 June 1999 / Accepted: 14 April 2000     

Why are there so few examples of entomopathogenic fungi that manipulate host sexual behaviors?

For entomopathogenic fungi (EPF), it may be particularly advantageous to manipulate host behavior in order to increase conspecific encounters that facilitate transmission of conidia. To achieve this, some EPF apparently alter the chemical communication and signaling of insect sexual behavior, but there are very few documented examples. Here, we identify and discuss the few known examples and propose two categories of EPF-induced alterations in insect sexual behavior. First, EPF may induce changes in how healthy conspecifics of the opposite sex perceive and respond to the infected individual via chemical or visual cues, which promote the display of sexual behaviors towards the infected individual. Second, EPF may directly change sexual behavior of the infected individual to promote conspecific encounters. We conclude that the scarcity of observed examples is likely caused by the difficulty of detecting subtle changes in insect sexual behavior, but also because manipulation of host sexual behavior is a trait primarily present in understudied host-specific EPF.     

Why are there so few evolutionary transitions between aquatic and terrestrial ecosystems?

Insects and flowering plants have rarely invaded the sea. Explanations for this have traditionally centered on the unique shortcomings of these groups in the marine environment. We show, however, that transitions among terrestrial, freshwater, and marine environments are infrequent in all major plant and animal clades except tetrapod vertebrates. In general, well-adapted incumbents are at a competitive advantage over would-be invaders from a physically different habitat. Data on the times and places of transition are consistent with our contention that evolutionary transitions among physically different environments are most likely when incumbents in the recipient environment exist in a regime of low-intensity competition and prcdation, as in terrestrial communities of the middle Paleozoic or the land biotas of oceanic islands. Freshwater environments, in which inferred intensities of predation are lower than in most marine and terrestrial environments, offer less biotic resistance to invaders than do communities in the sea or on land. Most invaders respond to novel physical circumstances by shutting down their metabolic machinery, and therefore add to their subordinate status as competitors with active incumbents. Only active tetrapods, particularly those with high and endothermically driven rates of metabolism, have successfully overcome this limitation.     

Why are there so few smart mammals (but so many smart birds)?

The expensive brain hypothesis predicts an interspecific link between relative brain size and life-history pace. Indeed, animals with relatively large brains have reduced rates of growth and reproduction. However, they also have increased total lifespan. Here we show that the reduction in production with increasing brain size is not fully compensated by the increase in lifespan. Consequently, the maximum rate of population increase (rmax) is negatively correlated with brain mass. This result is not due to a confounding effect of body size, indicating that the well-known correlation between rmax and body size is driven by brain size, at least among homeothermic vertebrates. Thus, each lineage faces a 'grey ceiling', i.e. a maximum viable brain size, beyond which rmax is so low that the risk of local or species extinction is very high. We found that the steep decline in rmax with brain size is absent in taxa with allomaternal offspring provisioning, such as cooperatively breeding mammals and most altricial birds. These taxa thus do not face a lineage-specific grey ceiling, which explains the far greater number of independent origins of large brain size in birds than mammals. We also predict that (absolute and relative) brain size is an important predictor of macroevolutionary extinction patterns.     

Why are there so many insect species? Perspectives from fossils and phylogenies

Over half of all described species are insects, but until recently our understanding of the reasons for this diversity was based on very little macroevolutionary evidence. Here I summarize the hypotheses that have been posed, tests of these hypotheses and their results, and hence identify gaps in knowledge for future researchers to pursue. I focus on inferences from the following sources: (i) the fossil record, normally at family level, and (ii) insect phylogenies, sometimes combined with: (iii) the species richness of insect higher taxa, and (iv) current extinction risks. There is evidence that the species richness of insects has been enhanced by: (i) their relative age, giving time for diversification to take place; (ii) low extinction rates. There is little evidence that rates of origination have generally been high or that there are limits on numbers of species. However, the evidence on macroevolutionary rates is not yet so extensive or coherent as to present unequivocal messages. As regards morphological, ecological, or behavioural hypotheses, there is evidence that diversity has been enhanced by (iii) flight or properties resulting from it like enhanced dispersal, (iv) wing folding, and (v) complete metamorphosis. However, in all these cases the evidence is somewhat equivocal, either because of statistical issues or because evidence from different sources is conflicting. There is extensive evidence that diversity is affected by (vi) the ecological niche. Comparative studies indicate that phytophagy generally increases net diversification rates, and reduces extinction risk. However, niche specialization is also associated with an increase in extinction risk. Small body size (vii) is often associated with low extinction risk in comparative studies, but as yet there is no solid evidence that it consistently enhances net rates of diversification. Mouthpart diversity (viii) has generally increased over time in the insects, but cannot explain the apparent great increase in diversity seen in the Cretaceous and Tertiary. Sexual selection and sexual conflict (ix) are two processes that are widespread in insects, and there is comparative evidence linking both to increased diversification. Although some comparative evidence links tropical distributions (x) to increased rates of diversification, the extent to which latitudinal richness gradients are unusual in insects is equivocal. There is little to no direct evidence from fossils and phylogenies that insect diversity has generally been affected by (i) sensory- or neuro-sophistication, (ii) population size or density, (iii) generation time or fecundity, (iv) the presence of an exoskeleton or cuticle, (v) segmentation or appendage diversity, (vi) adaptability or genetic versatility, though all of these remain plausible hypotheses awaiting further tests. The data suggest that the insect body ground plan itself had no direct effect on insect diversity. Thus, whilst studies to date have given substantial understanding, substantial gaps still remain. Future challenges include: (i) interpreting conflicting messages from different sources of data; (ii) rating the importance of different hypotheses that are statistically supported; (iii) linking specific proximate to specific ultimate explanations and vice versa; and (iv) understanding how different ultimate hypotheses might be dependent on each other.     

Darwin's second 'abominable mystery': Why are there so many angiosperm species?

The rapid diversification and ecological dominance of the flowering plants beg the question "Why are there so many angiosperm species and why are they so successful?" A number of equally plausible hypotheses have been advanced in response to this question, among which the most widely accepted highlights the mutually beneficial animal-plant relationships that are nowhere better developed nor more widespread than among angiosperm species and their biotic vectors for pollination and dispersal. Nevertheless, consensus acknowledges that there are many other attributes unique to or characteristic of the flowering plants. In addition, the remarkable coevolution of the angiosperms and pollination/dispersal animal agents could be an effect of the intrinsic adaptability of the flowering plants rather than a primary cause of their success, suggesting that the search for underlying causes should focus on an exploration of the genetic and epigenetic mechanisms that might facilitate adaptive evolution and speciation. Here, we explore angiosperm diversity promoting attributes in their general form and draw particular attention to those that, either individually or collectively, have been shown empirically to favor high speciation rates, low extinction rates, or broad ecological tolerances. Among these are the annual growth form, homeotic gene effects, asexual/sexual reproduction, a propensity for hybrid polyploidy, and apparent "resistance" to extinction. Our survey of the literature suggests that no single vegetative, reproductive, or ecological feature taken in isolation can account for the evolutionary success of the angiosperms. Rather, we believe that the answer to Darwin's second "abominable mystery" lies in a confluence of features that collectively make the angiosperms unique among the land plants.