CONSERVATION BIOLOGY BSC 3052
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400 TRENDS in Ecologyamp Evolution Vol16 No7 July 2001 Scale and species numbers H Charles J Godtray and John H Lawton One between body size and species diversity7 the number of described species in logarithmic size classes first increases as one moves from larger to smaller organisms but then begins to decline again when a threshold of 1 mm is reached This pattern is still not fully understood but clearly involves changes in the speciation and extinction rates as a function of body size modulated by resource availability as perceived by species of different sizesx 7 um n pauiw 39 L Ufapvhiva quot quot a39wp u mm m Iv 39 39 39 39 39 39 39 39 39 39 W 39 39 39I39I hnlumuu r 39 39 39 mnm Inrallu 39 u I speciation anu m 39L 39 ne quot between r Ham H Charles I Godiray Population Biology Dept oi Biology Imperial College at Silwood Park Ascot Berkshire UK email c9odlrayicacuk these The number of organisms in a particular area is determined by sp eciation and extinction and by immigration and local loss The relative importance of these processes depends upon the scale of the investigation We discuss a series of recent studies that have made important contributions to disentangling these is sues at the global regional and local scales Ecological processes are of course crucial to many theories of SPECIATION see Glossary itself a topic discussed by Schluter in this issuel At a global scale Consider first the largest scale what determines the number of species on Earth At this scale the 1 L L andthe Men in Black would disagree local loss of applv and thetotal r is Illebi tury of we iation d t events It is possible that species numbers are at a dynamic equilibrium at which the rates of extinction and speciation precisely balance each other and that many suitable niches are unoccupied or it could be that the number of species increases until all niches are filled up For some taxa the fossil record is good enough to investigate these is sues directly see Benton and Pearsonz this issue We know that the history of life on Earth is punctuated by episodes of mass extinction after which diversity recovers However what is less certain and what depends crucially on issues such as the quality of the fos sil record and the TAXONOMIC level at which the analysis is conducted is whether the recovery is best explained by an exponential or a logistic model The significance ofa logistic relation ship is that the asymptote might re ect a ceiling or equilibrium for global biodiversity at any particular level of complexity3 At global scales speciation and extinction have generated several obvious patterns in species richness One of the most striking is the relation ship 53471315 1 of the body size diversity relation ship is made harder by the fact that we tend to know les s about the biology and systematics of organisms as their d Although new bird and mammal species continue to be discovered and the application of molecular techniques will throw up surprises such as the recent demonstration that there are at least eight species of mouse lemurs Microceb us spp in Madagascar9 we essentially know how many large animal and plant species there are on Earth For medium sized terrestrial animals such as insects there is far greater uncertainty although alarming estimates that there might be 100 million species of arthropod10 are now generally thought to be too hi gh with most entomologists arguing for figures in the range 2 8 million Uncertainty increases as the organism gets smaller or the habitat is less well explored such as the marine or subterranean envirorm39ients In a forest in south central Cameroon the proportion of animal species that expert taxonomists could assign to known named species was inversely related to the log of the geometric mean body length of each taxon from 0 for birds and 1 for butterflies through to 30 80 for beetles ants and termites in various forest strata to over 90 for soil nematodes Free living nematodes with body sizes spanning the crucial 1 mm length class could be one of the most speciose animal taxa on Earth but also one of the most poorly known Cros sing the 1 mm threshold we encounter true microbes and the number of described species begins to fall It is possible that this dip re ects a lack of study 1 to invest39 microbial diversity have only been developed in recent decades Indeed the frontiers of life are continually being pushed back as new communities of archaea and bacteria using non solar sources of energy are discovered underground or on the sea floor see for example the recent report13 of symbiotic consortia of archaea and bacteria anaerobically oxidising methane in submarine sediments Moreover tallying the described species of prokaryotes using a eukaryote species concept might not make sense for organisms that often share DNA an M n mnmnnmm A THENDSin Ecologyamp Evolution Vol16 No7 July 2001 401 Fig 1 Thellagellated prntninnn having a speciesspeci c pattern The agellum beats to create Windermere on Imesy at Bland Finlay t 1 and 114 r I r 39 Liicui tree like patterns of evolution Organisms lt1 mm in size differ from larger creatures in more than just their genetics As has previously been emphasized1516 they are small enough to be dispersed passively in the atmosphere and sufficiently abundant that populations of at least free living species could be relatively homogeneous on a global scale Such pervasive gene flow might lower speciation rates leading to a true drop in the global diversity of the smallest organisms Recent work not with bacteria but with protists supports the 39bu gs are everywhere39 hypothesis Ciliated protozoa have a rich micro structure that allows traditional MORPHOLOGICAL SPECIES CONCEPTS to be applied successfully Using various enrichment techniques 85 different species of ciliates were isolated from a sample from a volcanic crater lake in Victoria Au stralia17 The number in itselfis not significant because larger samples and different enrichment cultures would doubtles s have increased the figure B ut what was amazing is that none of the species from this little explored site was new and the type localities of all but one of the species were in Northern Europe the exception being known only from tropical Africa A similar exercise with breeding birds ignoring human introductions might at most yield three to four species in common between Northern Europe and Victoria Neither is there something odd about ciliates Chrysomonad agellate species in the genus Paraphysomonas Fig 1 can be identified by their structure they have siliceous scales that are preserved after death and morphological and genetic species concepts agree Globally there are 50 described species and 32 of them were found in Priest Pot a 1 ha pond in the Lake District UK More significantly the relative abundance of species in Priest Pot was highly correlated with their world wide 2 waved from a I of surveys of the genus In these very small organisms local and global distributions appear tightly linked httplreelrendscom At a regional scale One of the major discoveries of the heroic age of explorer naturalists was that biodiversity was not constant throughout the Earth but tended to peak at or near the equator and to decline towards the poles This pattern occurs in most but not all terrestrial taxa19 and is also found in some marine groups although the overall picture there is far less clear Tremendous effort has gone into explaining this pattern and the problem if anything is now a surfeit of hypotheses with perhaps 30 competing ideas although not all of these are independent In an attempt to reduce this mass of ideas several authors have explored models of species diversity that incorporate a minimum number of biological processes For example global biota was constructed by randomly choosing the latitudinal midpoints and ranges ofa set ofsimulated distributions subject to the constraint that there is a hard latitudinal limit to the placement of ranges2122 Species with midpoints near the equator can have greater latitudinal range than those towards the poles and this is sufficient to generate higher species diversity at the equator However variants of the null model where midpoints are distributed uniformly acros s latitude tend to predict smaller ranges towards the poles at variance with the common but not universal empirical pattern known as the Rapoport effect real range sizes increase towards the poles It would be remarkable if such simple null models reproduced all the details of global patterns in species richness and that is not their point Rather they serve to identify features of the patterns that require biological rather than statistical explanations One ofthe few universal rules in ecology is that more species are found in larger areas and the idea has been championed2324 that area might explain latitudinal patterns in species diversity Tropical areas are larger than Polar Regions because the Earth is a sphere because the north and south tropics abut and because climate changes more slowly with latitude at the equator The greater area of the Tropics generates the tropical polar decrease in species richness2324 by the same mechanisms that generate other continental scale species area relationships A recent review of the area hypothesis concluded that although area almost certainly has a role in determining the latitudinal gradient in species numbers it is unlikely to be whole story In particular some of the assumptions about how climate affects species ranges and how the probability of ALLOPATRIC SPECIATION varies with range size might affect the predictions of the hypothesis Another idea with a long pedigree is that biodiversity is correlated with energy flow into the environment for example with net primary productivity or some related variable2527 At more local levels diversity tends to peak at intermediate levels of productivity although at the regional level 1 usually linear recent Box 1 Species diversity and scale TRENDS in Ecologyamp Evolution Vol16 No7 July 2001 diversity in the same way as ambient solar energy in uences plant diversity or do the complex trophic interactions that constitute natural food webs magnify or diminish these processes To answer these questions we need to move from simply comparing species diversities in different regions to looking at how the structure of food webs is in uenced by factors such as diversity productivity and other variables3 31 To what extent are local communities simply haphazard sub samples of the regional species pool uonsiue 39 39 39 39 39 Traditionally J L diversityy 39 39 39 the extent quotquotquot r varies over space 3 139 39 t the quot 1 H J r J g l l ya yhas l t quot 39 quot39 which diversity 39 aand mgv At the local level geographical scales 1 l uiwnity39 39 do 39 quot sculvquotAlquot scale that of individuals a Type II saturating response can occur asa habitat fills up becauseir 1 i n r ground Hnwever quot 39 39 39 quot 39aType response must occur It is very difficult to predict the type of pattern to be expected in between these two scales In the absence of species 39 39 quot quot quot dienumber 39 quot 39 39 n a 3 r u 1 a u nspecies eadi of whim does best in one of ndifferent patdi types where it excludes all other 39 Ifthe scale at 39 quot 39 39 39 39 of the patch then a 1 3 n 1 and y nand in comparisons across regions that vary in A r quot3 Tquot 39 quot However increasing dispersal can lead 39 39 39 quot 39 39 39 quot an 0and Y quotyand 139quot 39 39 39 quot is thatmost i species pool rather than absolute levels of saturation References a Whittaker R H 1960Vegeta on of the Siskiyou Mountains Oregon and California Eco Monogr 30 2797338 b Loreau M quot and gamma diversity Eco Left 3 73776 C Landg R multiple communi es Oikos76 5713 d Caley M J and Schluter D 1997 Therela onship between local and regional divery39ty Ecology78 70780 e Sn39vastava7D S Jamil 39 1 1 39 39 pitfalls and poten als J Anim Ecol 68 1716 mm analyses have found that measures are better correlated with diversity J u st two explanatory variables energy and rainfall were able to explain approximately 80 of woody plant diversity in 25 000 km2 grid squares in southern Africa28 although the t is exaggerated by spatially correlated error terms The mechanism postulated to underlie these relation ships is that high energy input allows a greater biomass to be supported and this in turn allows more species to coexist in a given area primarily through a reduction in the risk of extinction B ut the chain of causation is probably complex especially as one moves away from plants to secondary and tertiary consumers of energy Do quot 1 in plants hi on hilplllreelrendscom as opposed to non random assemblages of species determined by competition predation and other ecological processes As several authors have recently ed the cur Wu depends on scale that is on exactly what is meant by local A simple way to explore this issue is to plot local versus regional species richness for sites of similar size and structure3334 If local richness is determined by statistical sampling processes a linear Type I relationship is predicted whereas non linear Type II saturating relation ships might indicate other processes at work The exact nature of these other processes is elusive but several workers have argued that ecological mechanisms in particular resource competition should generate Type I local regional relationships However the majority of workers have found Type I relation ships linear or with little evidence of saturation Does this imply that competition predation and other ecological proces ses are of minor importance in structuring communities The dif culties of deducing process from pattern using this type of plot have recently been reviewed Using a hierarchical decomposition of species diversity Box 1 it has been argued that the relation ship between local and regional species diversity is crucially dependent on scale and that both statistical and biological processes can generate the two types of local regional diversity plots Thus although this approach has been important in demonstrating the role of regional diversity in in uencing local species patterns local regional diversity plots are probably arelatively coarse tool for in e Ligatiu g p underlying community structure 35 37 So how might one understand the processes determining species diversity at more local levels Much of current community and population ecology is devoted to this issue and there have been several interesting recent developments for example in exploring diversity and species area effects using orest inventories38 40 However we concentrate here on one particular problem the relation ship between local productivity and diversity Recall that at regional scales diversity tends to increase monotonically with productivity At local levels there is a marked tendency for diversity to peak at intermediate levels of productivity although a recent THENDSin Ecologyamp Evolution Vol16 No7 July 2001 403 Fig 2 Thediversily 0 population were similar rnduced with pemissinnlrnm Rel 47 unstirred cultures diversity peaked at intermediate 10 nutrient levels whereas in stirred cultures diversity 09 was low and independent of productivity Fig 2 Thus 08 a unimodal diversity productivity relation ship is 507 Y 12quot inL C ll 06 higher trophic levels 05 Of course pseudomonad phenotypes are not 2 04 species and their generation and extinction might D 03 differ from equivalent processes among true species 02 Yet in this system without sex the parallels between 01 bacterial variants and orthodox species are close So 0 what ecological process underlies this pattern This 0 1 2 3 4 5 6 7 system has been modelled47 using a variant of a model Nutr em concentrat on Ln of the maintenance of diversity in heterogeneous K 49 that 39 l 41 34bit At low only the variants meta analysis of diversity productivity patterns suggests a much more varied range of relation ship s41 However the pattern is certainly widespread and hypotheses involving both competition and predation have been put forward to explain unimodal patterns It has been suggested that low and high productivity environments are characterized by intense competition for different resources and the plant species that best competes for the appropriate limiting resource becomes dominant At levels is universally limiting and this allows a greater diversity of plants to persist Alternative explanations involve the roles of higher trophic levels in maintaining diversity W 5 Since the classic experiments on removing a key stone predator we have known that higher trophic levels can promote ecological diversity Pos sibly in low productivity environments insufficient energy is available to higher trophic levels to maintain diversity whereas in very high productivity environments predators are so abundant that they reduce diversity by driving extinct all but the prey species most able to survive their onslaughts Of course these and other processes could combine possibly idio syncratically to determine local diversityproductivity patterns While these issues continue to be explored in field experiments two recent studies using bacterial systems offer novel approaches to this problem The first47 uses the common aerobic bacterium Pseudomouas uorescens A previous experiment using the same system48 had s own that a single genotype of bacterium could give rise through mutation to a remarkable rangcn 39 quot 39 quot in an unstirred culture bottle occupied separate ecological niches the culture medium body the vessel surfaces and the cultureair interface Ifthe culture bottle is shaken this diversity fails to appear It thus seems that diversity is maintained through negative frequency dependent selection in a spatially structured environment In the more recent experiments the same bacterial strain was used but nutrient concentrations were also manipulated by three orders of magnitude In innu hllplllreelrendscom with the highest absolute fitnes s are able to maintain themselves as productivity increases more variants are viable and the negative frequency dependence identified in the first experiment allows coexistence But in nutrient rich culture the absolute fitness of the best variant in its own niche is so high that it can displace all other variants even though it does not perform as well in foreign niches 1 ion that or higher trophic levels are not required for a unimodal diversity productivity relationship does not mean they are unimportant in the field Recent experiments with a different microbial system have shown how the relative importance of competition and predation changes with habitat productivity Although these experiments do not directly show that predation leads to a unimodal diversity productivity relationship the results are consistent with models that make this prediction45 see also recent experiments with ciliate communities Using two strains of Escherichia coli one of which was relatively resistant to the bacteriophage T2 and the other relatively vulnerable a tradeoff between resistance to the phage and efficiency of resource glucose utilization has been demonstrated51 54 When the two bacterial strains and phage were cultured at low productivities the competitively les s successful but more resistant strain declined whereas the reverse occurred at high productivity Although the qualitative predictions of the theory were confirmed the precise rates of change of strain densities were not exactly as predicted by a model of the system One possibility is that spatial heterogeneity in the chemostats in uenced the outcome another is that there were unanticipated nonlinearities in the interaction A further complicating factor is the ap earance of completely resistant E coli39 strains in high productivity replicates This analogue of speciation is perhaps peculiar to microbial systems re ollrce Conclusions Understanding the processes determining species numbers requires the marshalling of all the tools and approaches available to ecologists At global and Review questions BSC3052 FST and PVA 1 Genetic data gives a long term View on structure of populations7 often we assume that the local population are in Hardy Weinberg equlibrium and the are able to calculate allele frequencies and using those we can calculate expected heterozygosities Calculate Recipe below the two statistics from the data of 7 cticious7 deer populations below and discuss what the results mean Locus ADH Florida Keys Everglades Central Florida FF 92 36 25 SF 4 48 50 SS 2 16 25 Homozygosity F FST between Keys and Everglades FST between Keys and Central Florida FST between Everglades and Central Florida Recipe calculate F as mentioned in the slides on inbreeding7 calculate FST using the expected heterozygosities7 in the slides you will see H 7 H H U this is identical to your notes a FST 1 7 i F ST HT HT HW is the heterozygosity within a subpopulation7 since we have two subpopulation for each of the pairwise analyses take the average of both7 HT is the expected heterozygosity assuming the pair of population is panmictic you need to calculate the allele frequency of F or S for a pair7 for example the Keys and Everglades will result in apF 9242 36482198 Answer Homozygosity F for each population is 94987 36 161007 50100 allele frequency pF allele 2055045 0495 2073027 039 FST Florida Keys Everglades Central Florida Florida Keys 1 020 034 Everglades 001 Page 1 Review questions BSC3052 FST and PVA 2 The cticious sightings of Elaphe guttata in Southwood was used to calculate transition rates from Juveniles to adults 0255 Staying with the adult class 045 having 2 eggs per adult per year 9 Sketch a stage graph label all parts and ll in the transition rates Assume that the transition rates are correct and that the snakes mature within a year 2 0255 0 2 P 0255 045 calculate the number of juveniles and adults in year 2001 when we use the values of 2000 where we found 30 juveniles and 14 adults Year Juveniles Adults Total 2000 30 14 44 2001 28 1395 42 What is the growth rate between year 2000 and 2001 compare this with the count based method above Show the projection matrix Growth rate A same calculaation as in count based methods once one has summed up all the stages in a year 0 Show a graph with time on the X axis and the growth rate A on the y axis Calculate A for the years 2001 up to 2006 using the start condition 30 juveniles 14 adults Page 2