Notes towards Biodiversity Chapter 4
Introductory/Title Slide (1)
Hello. My name is Gwen Raitt. I will be presenting this chapter on global biodiversity
and its decline.
Oh Dear!No one knows how much biodiversity there is or how much will be lost. The
multiple levels of biodiversity mean that no single measurement for biodiversity is
possible (Wikipedia Contributors 2006). This chapter briefly considers measures of
ecosystem and genetic diversity before concentrating on the species inventory and
estimates of global species numbers and species extinction rates. The present species
inventory contains the 1.4—1.8 million species already described but does not contain
much information about most of these species (Dobson 1996, Lovejoy 1997, Stork 1997).
Biodiversity at the Ecosystem Level
Inventorying ecosystems is complicated by the fact that it is difficult to set boundaries for
an ecosystem (Hawksworth & Kalin-Arroyo 1995), so there is no standard classification
system for ecosystems (Hawksworth & Kalin-Arroyo 1995). Ecosystems are usually
classified at two levels: globally (with climatic determinants) and locally/regionally
(with vegetation and species diversity determinants) (Hawksworth & Kalin-Arroyo
1995). The existing global classifications are inadequate (Bisby 1995). Global
ecosystem classification is of little value at the scales that are important for conservation
(Hawksworth & Kalin-Arroyo 1995). Remote sensing provides ways to assess and
monitor vegetation structure and phenology – aspects of ecosystems (Hawksworth &
Kalin-Arroyo 1995). The picture shows the Normalised Difference Vegetation Index
(NDVI) for Africa for the months (starting at the top left) January, April, July and
October.
Biodiversity at the Genetic Level
Genetic diversity may be considered/compared at three levels (Hawksworth & KalinArroyo 1995): variability between individuals within a population, variability between
populations within a species and diversity between species (Hawksworth & Kalin-Arroyo
1995).
Heterozygosity (the proportion of loci that carry two or more alleles) is used to quantify
the variability between individuals within a population and variability between
populations within a species (Hawksworth & Kalin-Arroyo 1995). Comparisons of
heterozygosity between species do not quantify how different the species are, merely how
different their internal variability is. The picture shows heterozygous parents (both
colours) and their homozygous (one colour) and heterozygous offspring. The picture is
simplified to show only one gene expressing a single trait. The proportion of
heterozygosity depends on: the evolutionary rates of the proteins or DNA used to
measure the variability, the breeding system of the organism and the degree of
connectivity between the populations (Hawksworth & Kalin-Arroyo 1995).
Problems with the Existing Species Inventory (1)
The precise number of recognized species is not known (Groombridge 1992,
Hawksworth & Kalin-Arroyo 1995, Stork 1997). There are several reasons for this.
One reason is the lack of a single definition for a species (Groombridge 1992, Bisby
1995, Hawksworth & Kalin-Arroyo 1995, Gaston 1996). Not all species definitions are
comparable (Groombridge 1992, Gaston 1996). Using different species definitions to
determine the number of species within a given taxon results in different total numbers of
species for that taxon (Gaston 1996, Williamson 1997), for example using a species
definition similar to that used by botanists on birds more than doubles the number of
species identified (Williamson 1997). Another reason is that the quality of the taxonomy
is not consistent. The inventory contains poor taxonomy as well as sound taxonomy
(Groombridge 1992, Hawksworth & Kalin-Arroyo 1995). The present inventory
incorporates unknown amounts of synonymy. Synonymy refers to a single species being
described and named more than once (Groombridge 1992, Hawksworth & Kalin-Arroyo
1995, Stork 1997). Several factors contribute to the occurrence of synonymy. Firstly,
there is no recognized central register of species though there are registers for some
groups (Stork 1997). Secondly, holotype (“type”) specimens may be difficult to access
as they may occur in collections geographical far from the site at which they were
discovered and traveling to view relevant collections is expensive – may be prohibitively
so (Stork 1997). Finally, the natural variation of a new species is unknown – different
forms may be given different names (Stork 1997). Determining the number of
recognized species at any time is not given priority, possibly because there is not much
biological significance in the data (Groombridge 1992).
Problems with the Existing Species Inventory (2)
The existing partial inventory is biased towards (Groombridge 1992): species that appeal
to humans such as the giant panda (Ailuropoda melanoleuca), pests such as the cat flea
(Ctenocephalides felis), organisms not requiring complex procedures or expensive
equipment to study, larger size (the smaller the organism the less likely it is to be
studied), easily distinguishable species that are readily sorted and species that are easily
accessed (Groombridge 1992).
Gaps in the Species Inventory
Gaps in the inventory may be considered in terms of physical location and in terms of the
categories of organisms. Usually the physical localities are expected to yield diversity in
the categories of organisms that are not well inventoried.
Fitter et al. (2005) identified about 500 species of soil organisms (bacteria, protozoa and
nematodes) at a single site in Scotland (Fitter et al. 2005). This high alpha diversity will
only lead to a large contribution to the global total if the species turnover (beta and
gamma diversity) is also high.
The ‘hype’ about tropical forest canopies is not backed by clear evidence (Groombridge
1992, Hawksworth & Kalin-Arroyo 1995). There is a possibility that the tropical forest
floor has greater diversity than the canopy (Groombridge 1992, Stork 1997).
Marine benthic organisms show high alpha diversity but this would not lead to high
global species totals unless the turnover of species (beta and gamma diversity) is also
high. Range sizes that are known are larger than those found on land and do not appear
to support the idea of a high species turnover (Groombridge 1992, Hawksworth & KalinArroyo 1995).
The total richness of parasites and other symbionts will depend on levels of host
specificity (Groombridge 1992, Hawksworth & Kalin-Arroyo 1995). Levels of
parasitism depend on host size, host defenses and the population structure of the host – if
the potential host is difficult to find it will not have many obligate parasites
(Groombridge 1992)
Nematodes may potentially be found as parasites, in marine sediments and in soils
(Groombridge 1992, Hawksworth & Kalin-Arroyo 1995) but little is known about species
turnover (Groombridge 1992).
Fungi have high alpha diversity but there is insufficient information on latitudinal
gradients and factors concerning turnover rates such as range size (Groombridge 1992,
Hawksworth & Kalin-Arroyo 1995). Species concepts are difficult to apply to
microorganisms and there is little information on range sizes (Groombridge 1992).
Perceptions of potential bacterial diversity have grown from studies suggesting that the
numbers of unculturable bacteria are much greater than those of culturable bacteria
(Hawksworth & Kalin-Arroyo 1995).
Plenty of evidence exists of high terrestrial arthropod diversity at most scales (the
smallest scale is excluded). Insects formed the largest portion of all described species
(Groombridge 1992, Hawksworth & Kalin-Arroyo 1995, Erwin 1997).
Clockwise from the top left, the pictures show a soil profile, the canopy of a rainforest,
marine sediments, an insect larva covered in parasitic wasp pupae and a nematode worm.
Numbers (in Thousands) of Described Species
The numbers quoted here are from Groombridge (1992) and Hawksworth & KalinArroyo (1995). Hawksworth & Kalin-Arroyo (1995) base their figures on Groombridge
(1992) with updates. Please note that Groombridge (1992) lists vertebrates and
Hawksworth & Kalin-Arroyo (1995) list chordates. Since vertebrates are a subset of
chordates, the latter term was preferred. Differences in the numbers reflect a
combination of new species, discovered synonymies and the year to which the authors
counted (Groombridge 1992, Hawksworth & Kalin-Arroyo 1995).
Erwin’s Estimate of 30 million Arthropod Species
Erwin (1982) estimated that the global total of arthropod species was as large as thirty
million. He based his estimate on a single study of the beetles collected by insecticide
fogging the canopy of Luehea seemannii (a tropical evergreen tree) in Panama with some
information from a study in Brazil (weevil numbers) (Erwin 1982, Groombridge 1992,
Stork 1997). This estimate is used to demonstrate some of the problems with estimates
based on single datasets.
The following data were used to make the estimate. Three seasons of sampling nineteen
Luehea seemannii trees yield 955+ species of beetles excluding weevils (Stork 1997).
From Brazil, he obtained information that weevil numbers ≈ leaf-beetles numbers so
suggest ~206 weevils per tree species. Enquiry supplied the estimate of about 50 000
tropical tree species (Stork 1997).
Erwin made three assumptions. The first was that about 13.5 % of the total number of
beetle species per tree canopy were host specific. The second was that beetles make up
40 % of tropical canopy arthropods. The third was that the forest canopy to forest floor
ratio is two to one. Erwin actually added a third of the number of canopy species to the
total number of canopy species (Stork 1997). Erwin’s actual assumption was a tropical
canopy to tropical floor arthropod species ratio of three to one though his stated
assumption was at least two to one.
Calculating Erwin’s Estimate
955 (beetle species minus weevil species) plus 206 (weevil species) equals 1 161 beetle
species per tree species canopy. Round up to 1 200 beetle species per tree species
canopy. If 13.5 % of the beetle species per tree species canopy are host specific then
there are 162 host specific beetle species per tree species canopy. The remaining 1 038
beetle species are transient (Stork 1997). Multiplying the host specific beetle species by
the estimated number of tropical tree species yields 8 100 000 host specific beetle species
in the tropical tree canopy. Add the 1 038 transient species and there are 8 101 038
beetle species in the tropical tree canopy. If beetles make up 40 % of all arthropods then
there are 20 252 595 arthropod species in the tropical tree canopy. Adding one third of
the total tropical tree canopy species to account for the tropical forest floor arthropods
(Stork 1997) (note that this actually results in a three to one canopy to floor ratio) gives
26 935 951 tropical forest arthropod species. If the number of non-tropical arthropod
species was estimated at about 3 100 000 then the global total is approximately thirty
million arthropod species.
More Recent Data and the Impacts on Erwin’s Estimate
For Erwin’s assumption of 13.5 % host specificity, temperate and provisional tropical
findings suggest that host specificity is less than 5 % (Stork 1997). Though latitudinal
variation in the proportions of species from different guilds of insects is probable, the
figures from widely spread studies suggest that beetles make up 20—25 % of the total
number of arthropods, not the 40 % assumed by Erwin (Stork 1997). Raw data from two
studies suggest that the canopy to forest floor arthropod species ratio should at least be
reversed (1:2 not 2:1 as Erwin suggested). There is also evidence to suggest that a large
portion of the fauna will be found in both ecotones (Groombridge 1992, Stork 1997).
Using these figures and following the calculations shown before, the estimate of the
global total for arthropod species becomes 39 113 680 arthropod species in the world.
Changing the ratios used by Erwin makes a big impact on the total estimate (39 113 680
vs. 30 000 000).
Problems with Single Sample Extrapolations
All estimates are affected by the accuracy of the figures used. The
accuracy/completeness of counts from a single sample is open to question (Stork 1997).
The calibration of ratios for single sample extrapolations is generally poor or nonexistent. Erwin’s estimate is a case in point (Groombridge 1992). There is an often
unstated underlying assumption that the relationships used in the extrapolation scale
evenly (Groombridge 1992).
Useful Ratios for Estimation
All extrapolations from existing data involve one or more assumptions. The main one
being that a ratio occurring in a known situation is also true in an unknown situation
(Groombridge 1992). All ratios need to be calibrated to support the assumption that the
ratio holds under different conditions. Samples used to generate ratios are calibrated
against inventories. Six categories of known to unknown species richness ratios are
useful for extrapolation (Hawksworth & Kalin-Arroyo 1995): “group A to group B (e.g.
butterflies to beetles), subgroup to group” (e.g. beetles to insects), “sample to inventory
(e.g. of a site), area A to area B (e.g. site A to site B), smaller scale to larger scale (e.g.
site to country)” and “habitat/stratum to inventory (e.g. of a site)” (p 121 Hawksworth &
Kalin-Arroyo 1995).
Other Forms of Estimation
Time-series of species descriptions may be used to estimate future growth within groups
of organisms. Application to groups that form major parts of the total biodiversity has
not been successful and even for well known groups, the results may be misleading
(Groombridge 1992, Hawksworth & Kalin-Arroyo 1995).
The inverse relationship between body size and number of species may be used to
extrapolate for larger organisms but appears to break down at lengths of less than one
millimetre (Groombridge 1992, Stork 1997). For the smallest organisms, the question of
species definition becomes important. Dispersal by air or water is easier for these
organisms so allopatric speciation is less likely (Hawksworth & Kalin-Arroyo 1995,
Stork 1997).
The proportions of species in the different trophic levels of the food web allow fairly
reliable generalization without reference to host specificity. The data on host specificity
is insufficient for reliable estimation. Vegetation structure may yield better predictions of
the numbers of parasites than the number of species does (Groombridge 1992).
Specialist opinion has usually not been systematically collected. A specialist’s estimates
depend on their exposure to data on especially the richness of poorly studied regions
(Groombridge 1992). Specialists’ estimates tend to be conservative (Groombridge 1992,
Hawksworth & Kalin-Arroyo 1995).
Global Species Estimates
All the estimates are in thousands of species (i.e. 4 = 4 000 species). The figures quoted
here are from Groombridge (1992) and Hawksworth & Kalin-Arroyo (1995). Please note
that Groombridge (1992) lists vertebrates and Hawksworth & Kalin-Arroyo (1995) list
chordates. Since vertebrates are a subset of chordates, the latter term was preferred.
The probable accuracy of the estimates as listed on the far right uses the following set of
definitions: ‘Good’ covers estimates that are “almost certainly accurate within a factor of
two”, ‘Moderate’ refers to estimates that are “almost certainly accurate within a factor of
five”, estimates regarded as having ‘poor’ accuracy are “almost certainly accurate within
a factor of ten” and estimates that are “not certainly within an order of magnitude” are
‘very poor’ (p 118 Hawksworth & Kalin-Arroyo 1995).
Present versus Background Extinction RatesSpecies life spans and background
extinction rates are estimated from the fossil record. Estimations from the fossil record
are uncertain, as the time resolution is rarely better than 103—104 years (Barbault &
Sastrapradja 1995). For mammals, a species life span is about one million years and the
background extinction rate is about one species every two hundred years.
For organisms with adequate data from the fossil record, the average life span is between
five and ten million years which means a background extinction rate of about one species
every thousand years (Barbault & Sastrapradja 1995). The present overall extinction rate
is estimated to be at least 500 times the background extinction rate (Gaston & Spicer
1998). For birds, the present rate is about a thousand times the background rate (Barbault
& Sastrapradja 1995).
Estimating Future Extinction RatesTwo principle forms of estimation of extinction
rates exist. The first estimates extinction rates based on rates of habitat loss and the
species-area relationships from island biogeography (Groombridge 1992, Barbault &
Sastrapradja 1995, Smith et al. 1995, Dobson 1996). The second group of estimates
relies on Red Data lists. Rates at which species change categories on Red Data lists may
be used to determine extinction rates or extinction rates may be estimated using speciesby-species assessments (Barbault & Sastrapradja 1995, Stork 1997). In a well
documented location such as Britain, if the accuracy of the Red Data list status is of
groups such as birds and invertebrates is comparable then relative extinction rates can be
estimated. In Britain the mean relative extinction rate between birds and insects is 7.1. If
this ratio holds then since about 1 %of birds have become extinct since 1600, the ratio
suggests that about 0.14 % of insects (about 11 200 species if the total number of insect
species globally is eight million) have become extinct. Testing this in Britain, 99 insect
species have not been seen since 1900. Using the ratio suggests that eleven bird species
should have vanished in the same period. This was found to be the case though two bird
species had recolonised the island between 1800 and 1949 (Stork 1997).
Problems with Estimates Based on Habitat Loss and Species-Area RelationshipsThe
accuracy of estimates based on habitat loss and species-area relationships is low because:
detailed knowledge of species distribution and endemism is lacking though they are
known to be unevenly distributed, habitat loss is not evenly distributed, the rate of habitat
loss is uncertain, changes at a global level (such as climate) are not considered, genetic
impoverishment (loss of genetic diversity through loss of local populations and/or
decreasing population size) is not considered, the likely survival of species in modified
habitats is uncertain, the pattern of habitat loss is uncertain – loss resulting in
fragmentation increases species loss and protection of key areas reduces species loss, the
species-area relationship varies between taxonomic groups, past disturbance and habitat
loss are unknown, changes in future human behaviour are uncertain (Groombridge 1992,
Barbault & Sastrapradja 1995).
Species Most Threatened with Extinction (Barbault & Sastrapradja 1995)Based on
ecological theory, species with the following traits are more prone to extinction: large
organisms (e.g. blue whales Balaenoptera musculus and elephants (Loxodonta africana)),
those feeding highest in the food chain, those with chronically small population sizes
(Barbault & Sastrapradja 1995, Dobson 1996), “those with small ranges or distributions,
those linked by biology to threatened habitats or ecosystems” (p. 236 Barbault &
Sastrapradja 1995) (e.g. tropical forest species), “those that evolved in isolation (e.g.
island” organisms), “those with poor dispersal and colonization abilities (p. 236 Barbault
& Sastrapradja 1995, Dobson 1996), “those with colonial nesting habits” (e.g. cape
gannets (Sula capensis)), migratory species (p. 236 Barbault & Sastrapradja 1995,
Dobson 1996) (e.g. greater striped swallows (Hirundo cucullata)), “those dependent on
unreliable resources and those with little evolutionary experience of disturbances” (p. 236
Barbault & Sastrapradja 1995).
Some CautionsNeither global biodiversity estimates nor estimated extinction rates
contribute to either conservation practice or tackling the root causes of biodiversity loss
(Groombridge 1992).
Data on extinction rates are optimistic in that genetic impoverishment is not taken into
account (Groombridge 1992). Each species is the potential ancestor of future species so
abnormal extinction rates affect future speciation (Barbault & Sastrapradja 1995).
Last slide
I hope that you found chapter 3 informative and that you will enjoy chapter 4.