understanding biodiversity, its benefits and how it is threatened

By Dr J Floor Anthoni (2001)

 extinction Extinction is forever. What are its major causes? Classification of endangered species. How many species? Extinction is frightening. The mathematics of scarcity. 
 measuring diversity Scientists have made enormous progress in cataloguing the planet's species but they are not even half way there. In the meantime, they have also become better at estimating how many species remain unknown, and how many species must have disappeared recently. Diversity and area size. Factors affecting island reserves. 


related pages
on this web site
Conservation principles: understanding conservation of the environment and how it can fail (30p)
Resource management: important knowledge for understanding biodiversity and conservation (30p)
Global threats to humans, atmosphere, land and sea. (20 p)
Marine habitats: an introduction to the life-determining factors in the sea. (16 p)
The intertidal rocky shore: principles of the rocky shore zoning, and an identification of shore species. (80p)
Myths and fallacies in marine conservation, marine reserves, MPAs and marine science. (large)
Biorealms of the planet: the major biospheres and their differences. (4 p)
Red Data Book of NZ: a summary of the list of endangered species in New Zealand. (9 p)
Sitemap: discover the gems in the Seafriends web site from our complete site map (11p)
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Death is an important part of evolution. Unsuccessful species must die early, preferably before being able to reproduce. In the end, every individual dies, in order to make room for its offspring. Likewise, extinction, the death of an entire species, is part of the natural process. The planet has known several major extinctions, often caused by an external event, like the impact of an extra-planetary body (meteor). Such mass extinctions have led to the emergence of entirely new species and communities.

extinction is forever.
nature is a hard taskmaster: it punishes failure with the death of the species.

biodiversity and extinctionsThis graph, derived from all the known fossil finds, shows how biodiversity evolved over time. It took a very long time before life had developed a reproducing cell. Then in the Cambrian epoch, the number of species and families soared, only to level off some 450 million years ago. Three extinctions followed, the largest one at the end of the Permian, heralding the epoch of the dinosaurs. Then biodiversity took off again, suffering yet another great extinction at the end of the Cretaceous, which saw the end of the dinosaurs and the rise of mammals.

The extinctions caused by humans are profound and estimated at 100-1000 times the natural rate, indeed comparable to that caused by meteorite impact (the believed cause of the extinction of the dinosaurs). But their causes are different, and entirely related to the increase and spread of the human population and its wastefulness (see also threats for more details):

Species extinction is a gradual process, although some extinctions have happened fast and suddenly, like that of the Mauritian dodo and the American passenger pigeon. Most extinctions go unnoticed because we either are unfamiliar with the species, or do not like it (insects). Because extinctions are irreversible changes in global biodiversity, scientists are very concerned about measuring them accurately.
The best guess of biologists is that species are disappearing between 100 and 1000 times as fast as they were before Homo sapiens arrived. But our impact is different from the mass extinctions of the past. They wiped out whole groups of animals, notably the dinosaurs, whereas humans are picking off individual species. In the past, biodiversity recovered as species spread into new ecological niches, but humans are wiping out niches as well as organisms. Wildlife will have a tough time regenerating. 

The winners after the mass extinction that finished off the dinosaurs are about to become the losers. One in four mammal species and one in eight bird species face a high risk of extinction in the near future: the population of each species is expected to fall by at least a fifth in the next 10 years. Almost all are endangered by human activity. The invertebrates are tipped to dominate the new world order. Only around 0.1 per cent of the 1.6 million known species are thought to be threatened, though many undiscovered species are likely to be dying out before we even know of their existence. 

As global climate change shifts temperatures across the planet, species may not be able to follow fast enough. According to UNEP, they will have to migrate 10 times as fast as they did after the last ice age. Many won't make it. 
If all these species became extinct, we would lose 85 genera of mammals and 38 genera of birds. For mammals alone, this is equivalent to the destruction of 850 million years of evolutionary history. Extinction strikes in groups because related species tend to be vulnerable to the same hazards, such as introduced species or habitat loss.

The Earth's biodiversity is a capital resource that provides vital ecosystem services, goods such as food, fuel, fiber and medicines, and the aesthetic, recreational and cultural riches associated with nature. It remains a poorly understood scientific frontier, with an estimated 90% of species yet to be discovered and described. Alarmingly, this biological diversity is being lost across the planet, at all levels of organization from genes, through species, to landscapes. Some experts predict that up to  two thirds of all plant and animal species may be lost by the end of this century. Biologists generally believe that these losses represent a threat to Earth system functioning and the well-being of human societies. All sources of evidence for a particular extinction must be thoroughly documented, and these sources should be made available to anyone interested. Researchers should be able to check source information for themselves so as to judge the merits of inclusion of a species on an extinction survey.

(Source not recorded. Sorry)

As dead as a dodo
The extinct dodo from Mauritius IslandThe dodo (Raphus cucullatus) is an extinct flightless bird related to the pigeon, and as large as a turkey.  It had short legs, an enormous beak, stubby wings, and a tuftlike tail with curly feathers. It laid a single egg on the ground. 

European sailors, having rounded the Cape of Good Hope, were in desperate need of food, which they found in plentiful supplies on the island of Mauritius, between Madagascar and India. Dodos moved clumsily and slowly and were easy to catch. They also had a nice taste, while their copious amounts of fat made them easy to preserve. Pigs and monkeys brought to the island by Portuguese sailors during the 1500's destroyed the eggs and ate the young.  The dodo died out about 1680. The heads and feet of a few dodos are preserved in museums, but their relatives (solitaires) on Reunion and Rodrigues Islands, are known only from pictures, from accounts written by travellers, and from bones that were found on Reunion and Rodrigues islands. 

The dodo and solitaires belong to the pigeon and dove order, Columbiformes.  They are in the dodo and solitaire family, Raphidae.  The Reunion solitaire is classified as Raphus solitarius, and the Rodrigues solitaire as Pezophaps solitaria. The Reunion solitaire became extinct in about 1750, and the Rodrigues solitaire about 1800.

But the story of the dodo does not end here. A tree known as Calvaria major, also confined to the island of Mauritius, was a puzzle to botanists for centuries. Once quite common on the island, it had gone into decline until only 13 ancient trees, all over 300 years old, were left. Although they produced plenty and healthy-looking seeds, none ever germinated. In the mid-1970s, an American ecologist, Stanley Temple, came up with an explanation. He suggested that the tree's large fruits had formerly been eaten by the dodo, whose stone-filled gizzard exerted a powerful crushing pressure. Birds often swallow stones to make their gizzards (stomachs) work even better.
To withstand this onslaught, the tree's seeds adapted a thick-walled seed coat for protection. The dodo's gizzard pounded away at this coat, making it much thinner, and perhaps cracking it a bit. When deposited by the bird, the seed could germinate. As the dodos became extinct, the seeds could no longer escape from their thick-walled armaments, and the trees slowly died out, as no replacement seedlings sprouted. Only by human intervention can this tree species now survive.

(Adapted from: Linda Gamlin and Gail Vines: The evolution of life. 1986.)

Classification of endangered species
In order to interbreed, the males and females of a species must be able to find each other. It often requires a minimum density or a place where many can congregate safely. For a species to survive natural disasters like disease outbreaks, there have to be many individuals with a high variability in genes, living in separate groups, but being able to mingle occasionally.

The International Union for the Conservation of Nature (IUCN) has defined the following grades of endangeredness (for more detail see the NZ Red Data Book and for more authoritative detail, visit www.redlist.org:

status population comments
. The species is suspected of being endangered or even critical, but not enough data is available, either because it lives cryptically (hidden), or in places difficult to get to, or because no investigations have been made so far. Data Deficient is therefore not a category of threat or Lower Risk.
. The species is no longer able to fulfil its ecological role in its ecosystem. If it is a keystone species on which many others depend, it will bring major changes. In that case, many dependent species will also be endangered. Economically extinct is when numbers are so low that exploitation becomes uneconomic.
rare . Rare species may not necessarily be endangered. They have evolved in ways to exist in small numbers spread far apart and over large areas, by for instance being able to reproduce asexually as well. Species are rare and endangered when their populations are small and in decline.
lower risk
? When it does not satisfy the criteria for any of thecategories CR, EN or VU.
CD. Conservation Dependent when subject to a conservation programme targeted towards the taxon in question, the cessation of which would result in the taxon qualifying for one of the threatened categories above within a period of five years. 
NT. Near Threatened. Taxa which do not qualify for Conservation Dependent, but which are close to qualifying for Vulnerable. 
LC. Least Concern. Taxa which do not qualify for CD or NT.
<10,000 The species is vulnerable because its natural gene pool is becoming rather small. Lack of genetic diversity is the threat. They are believed to become endangered if the cause of their decline is not removed.
<2,500 The species is in such small numbers that lack of individuals has become a threat. A breeding collapse due to lack of genetic diversity becomes a possibility.
<250 The species cannot survive unnatural deaths or sudden adverse conditions and needs special care. At this level, the species can most likely no longer be saved. Extinction is only a matter of time. Only a major conservation effort like captive breeding and re-introduction can postpone extinction.
in the wild 
When it is known only to survive in cultivation, in captivity or as a naturalised population (or populations) well outside the past range. A taxon is presumed extinct in the wild when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual.
0 No reliable sightings for 50 years. It is hard to say when a species has become extinct. Some species such as the New Zealand kakapo, the flightless parrot, have been rediscovered. However, their numbers usually remain critical. Species living in the 20th century (from 1900 on) and now extinct, qualify. Species in zoos don't qualify. 

People are obsessed with fire-fighting, courageously and with much effort saving a species from the brink of extinction, but to the grand scheme of nature, this is not important. We have already lost a huge numbers of species, and will lose a great deal more. It is therefore important to focus on those that are not in a critical state, and to provide them with the habitat and space they need. This is already costly and difficult enough.

Scientists are rushing to know precisely how many species exist on Earth, and to even register the extinct ones, but would all this effort change our actions? Would it really be beneficial to those who survive? Would it even be beneficial to us?
We know that the only acceptable and effective course of conservation consists of limiting the spread of people, limiting the population and stopping habitat destruction and fragmentation. We know what to do. Now we need to do it.

f026502: Hector's dolphin in New Zealand
f026502: Hector's dolphins (Cephalorhynchus hectori) are endemic to New Zealand, and their numbers are estimated at around 3000, classifying this small species as endangered. In various places, fishing methods have been restricted to prevent them from drowning in set nets but the dolphin's main enemy is degradation of the sea by runoff from the land. These dolphins live strictly coastally, which brings them into conflict with people and also in highly degraded waters.
f210411: yellow-eyed penguin
f210411: New Zealand's yellow-eyed penguin visits the land for nesting, where it is threatened by habitat destruction and predation. In this sheltered bay of the Flea Bay (Pohatu) marine reserve near Christchurch, it is protected by the vigilance of its caretakers.

fish species of New ZealandHow many species?
Scientists differ enormously over the estimated number of species on the planet. They are furthermore hampered by the fact that no integrated library of species exists, although a beginning has just been made. They also suffer from the low regard for taxonomy, the description and classification of organisms, due to it being considered a low level of science. However, they may be wrong on this, because biodiversity is the total stock of genetic information of the planet, a huge library of life.

Another problem is that we simply haven't looked in all places. In the diagram on right, the number of known fish species is shown for New Zealand. As soon as the Extended Economic Zone was ratified, and scientists started to systematically sample the area, a large number of new species, never before seen by human eyes, was discovered. They now discover two new fish species every month, and scores of other unknown organisms, as long as they keep looking.

Here follow two recent developments in the effort to put meaning to the concept of biodiversity and extinction.

Sound taxonomic information is crucial. An understanding of the classification, distribution, and evolution of taxa underpins the baseline knowledge of species-level diversity. Thus, any attempt to conserve this biodiversity requires the same level of understanding (McNaughton, 1994; Nielson and West, 1994; Kottelat, 1995; Brookes, 1998). In other words, "The name is the key to knowledge. No name, no information. Wrong name, wrong information" (BioNET-International, cited by Powledge, 1998). Recognizing this, the 1996 Convention on Biological Diversity adopted a Global Taxonomy Initiative to formally establish that "a sound taxonomic...knowledge base is a prerequisite for environmental assessment, ecological research, and the conservation of biological diversity...."
Eldredge (1992) stated, "the mechanism of extinction may lie squarely within the province of ecology, but we measure extinction taxonomically, squarely within the realm of systematics."
GBIF project (March 2001): ALL creatures great and small, from the mightiest whale to the humblest bacterium, will soon have an entry in the first complete Who's Who of the natural world. When finished, the Global Biodiversity Information Facility, or GBIF, will be available free on the Internet to anyone investigating the biology and ecology of the planet.  The initiative to set up the GBIF came from the Organisation for Economic Co-operation and Development in Paris. The deal was that once at least 10 nations had agreed to stump up $2 million between them, the facility would be a going concern.
 This week in Brussels, 12 founder nations announced GBIF's official launch. The dozen include rich and poor nations, from the US and Japan to Slovenia and Ecuador. Britain and France are notable absentees, though both are interested. The OECD is now accepting bids to host the GBIF secretariat. Spain, Denmark and the Netherlands have expressed an interest.
 The idea is to link all the world's public databases on biodiversity. "It always surprises people to know there's no catalogue of all the 1.8 million named species on Earth," says Jim Edwards of the US National Science Foundation, who was the initiative's prime mover. Not only will the compendium list all species, it will also assemble a directory of the 3 billion natural specimens collected and stored in museums and seed banks around the world. One major objective of the organisers is to persuade some of the world's greatest museums to computerise records which are still on paper or card. "There are millions of specimens in museums such as the Smithsonian in Washington DC and the Natural History Museum in London, but only a fraction are digitised. (Source: Environment News Service)

Estimated number of species on Earth
species described estimated numbers still to be described
viruses 5,000 500,000
bacteria 4,000 400,000 - 3 million
fungi 70,000 1.0 - 1.5 million
protozoa 40,000 100,000 - 200,000
algae 40,000 200,000 - 10 million
plants 250,000 300,000 - 500,000
vertebrates 45,000 50,000
roundworms 15,000 500,000 - 1.0 million
molluscs 70,000 200,000
crustaceans 40,000 150,000
spiders, mites 75,000 750,000 - 1.0 million
insects 950,000 8 - 100 million
Data from Groombridge (1992), showing that the task of describing biological species is in its infancy.
Some marine phyla not mentioned.

Extinction is frightening
Extinction is irreversible and forever. Suppose humans became extinct. It would mean that nowhere on the entire planet, an organism could be found, which looked like us. Not a white man, or negro or eskimo. Nobody, and there's no way back!

Extinction is frightening for a number of reasons:

Prehistoric colonisation of islands in the Pacific and Indian Oceans by humans and their commensals (rats, dogs and pigs) may have led to the extinction of as many as a quarter of the world's bird species. Since 1600, as many as 484 animal and 654 plant species are recorded as having gone extinct. This is almost certainly an underestimate, especially for tropical regions. 
Because of the world-wide loss of natural habitats that has already taken place, tens of thousands of species are already committed to extinction. It is not possible to take preventive action to save all of them.
The graph shows how an almost constant extinction rate suddenly escalated for island communities, as populations there reached their limits. A wave of extinctions for continent communities followed later, but abated due to the green revolution, which requires a smaller area for cultivation.
Extinct species over time
World's threatened species
endangered vulnerable rare indeterminate total
mammals 177 199 89 68 533
birds 188 241 257 176 862
reptiles 47 88 79 43 257
amphibians 32 32 55 14 133
fishes 158 226 246 304 934
invertebrates 582 702 422 941 2647
plants 3632 5687 11485 5302 26106
Source: World Conservation Monitoring Centre (WCMC), 1995 in UNEP GBA 1995.

Mathematics of scarcity

Mathematics of scarcityAs the human population enters the era of scarcity, nothing will ever be the same again. In the era of plenty the human population could grow without immediate penalty, although delayed effects such as desertification, salinisation and global warming will still haunt us much later, penalising us for the growth spurt of the past 40 years, from 3 to 6 billion people. The coming 40 years, however, will look quite different again because of reaching environmental limits before growing at least twice as much as the past 50 years.
In the diagram I have attempted to use mathematics in order to show the quite counter-intuitive effects of doubling the human population once again, from 6 billion to 11 billion. This is an unusual diagram, because it does not show a time line, but only a symbolic whole in scales from 0 to 1 (which is equal to 0 - 100%). Horizontally, the part used, and vertically the partial effect.

Human use of the biosphere is represented by the thick black line. It starts at the bottom left with zero human population, and it ends at the top right with all space occupied by humans. How fast we climb up this black line, depends on how much habitat we change and how many wastes we produce, thus our dependence on both our cropland and ecoservices. It is affected by population size and economic activity.

In order to survive, humans need ecoservices, to recycle their wastes (see chapter above). Consider the area under the black curve, our use and the area above it, that available to nature. Then it is clear that the unused part will be declining as people use more. In the era of plenty (before now), the services exceeded our needs, giving us 100% service (red line). We observe the blue skies, the clean air, enough drinking water, and our wastes not causing major problems.

However, at the halfway point, this is going to change rapidly according to the formula (1-x) / x, describing the amount of nature remaining, divided by the population, which amounts to personal ecosystems services (red curve). As one can see, it will suddenly start plummeting down, causing serious problems perhaps as early as the year 2020, problems which are not yet apparent today.

As the area used by people increases, it correspondingly leaves less for nature. From what we know about the relationship between area size and number of species, extinctions can be expected to follow the green curve. It starts at the top left corner, where zero humans are leaving 100% of the planet for other species, and it ends at the bottom right where 100% of human use leaves no room for other species. At the 50% point, mid-way, 0.25 x 50%= 12.5% of species are threatened with extinction by the amount of habitat destroyed by humans. They are on the way out. Up to 25% loss of all species can be expected by 2030. After that, more still. This in turn reduces the quality of nature's services. In all, the near future may herald some sudden and 'unexpected' nasty changes.

The World Watch Institute (www.worldwatch.org) reports that in the year 2000, some 1.2 billion people worldwide struggle to survive on $1 day or less. 1.2 billion people lack access to safe drinking water and 2.9 billion have inadequate access to sanitation. About 150 million children are malnourished, and more than 10 million children under 5 will die in 2001 alone.
The question arises, how many will be in famine in 2020 or in 2040? An intial reaction would be that the proportion would be the same, so for a doubling in population, one would expect twice as many in dire need (2.4 billion). One may expect that biotechnology comes to the rescue, just in time to save all. But the fact is that GE does not have a usable technology today. It took the green revolution 50 years to where it is now. Most of the increase in food came from bringing new land under culture. But both the water supply and land have come to their limits, and are now even reducing in quantity. Knowing that famine is never spread equitably, but only to those who can not afford the minimum, it is more likely that famine will more than double, it is even possible that it could swell the underfed to 6 billion (five-fold increase). This is also part of the logic of scarcity.

It appears that Man is administering to himself the same prescription for extinction as he did to other species: habitat destruction and introduced species (himself). Note in this respect, that not a word was said about our presently perceived threats, of the end of fossil fuel, global warming, sea levels rising and so on. It is likely that our main problems will arrive from somewhere else, like habitat destruction (nature + human habitat), and that they have not shown up yet. However, once they do show, the problems will arrive thick and fast, according to the logic presented here. But a word of caution is in place.

First of all, humans are not spread evenly over the planet, so some places will notice the effect of human habitat destruction much sooner and more severe than others. Secondly, where humans have destroyed the habitat for other species, they have often planted a habitat capable of providing some human ecosystems services, such as timber forests, rubber plantations and so on. Since most of our ecosystem services are provided by bacteria, which are more resilient than higher organisms, our waste treatment may follow a different logic. Finally, the oceans are very large and mainly unchanged. They will continue providing ecosystems services, even though the land, rivers and coastal seas may have suffered severely. Also remember that some countries are further up the slope than others.

On an optimistic note, it follows that whatever we can do now to reduce the harmful side effects of our actions, will help to slow down our climb up the black ramp, with its unavoidable consequences.

The recycling of carbon dioxide, as an example of an ecosystem service
Every organism on Earth depends on the carbon cycle, part of which consists of returning carbon dioxide (CO2) to plants. The amount of carbon dioxide produced by humans, has increased considerably during the Industrial Revolution, and is increasing at an accelerating and alarming rate. As a result, more of this hothouse gas will be retained in the atmosphere, possibly leading to global warming. Perhaps surprisingly, most of what we produce, disappears from the atmosphere, but scientists are not sure how and where to. The oceans can dissolve large quantities of this gas, but plants are probably the main cause. To put it in perspective: humans produce about 8 Pg/year, the atmosphere contains 500 Pg, increasing with 0.2 Pg/year; all plants and trees contain 600 Pg, cycling 60 Pg each year; and all soil organisms, mulch and peat contain 1300 Pg. By comparison the oceans contain 37,000 Pg in CO2. (One Petagram equals a billion tonnes, Gt). 
For plants to absorb all human made CO2, they need to grow about 1% faster each year, which amounts to a couple of extra leaves for a shrub. Of course, this can hardly be noticed. But for plants to do so, they need water, nutrients and a suitable temperature. Where water is scarce or temperatures low, plants cannot be expected to contribute much to the recycling of CO2. But nutrients are not available in unlimited supplies either. Many 'eclipse' forests (such as tropical rain forests) have used up all available nutrients in their soils, and they too cannot be expected to be great contributors. Thus there exists a real limit as to how much nature can recycle carbon dioxide, and this limit may be reached in the foreseeable future, resulting in an 'unexpectedly' rapid rise in the concentration of CO2 in the atmosphere. This particular ecosystem service will then behave like the red line in above diagram. Note that the concentration of CO2 in the atmosphere needs to rise a little in order to enable plants to absorb this gas faster. This is what we are seeing now.

Note in this respect that it is widely believed that burning coal is a bad thing because it produces more CO2 per unit of energy than gas or even oil. But this myopic view may well be wrong because coal contains the complete and balanced fertiliser to enable plants to sequester the emitted CO2 entirely, whereas gas is almost devoid of nutrients, and oil contains unbalanced quantities of it. Burning coal from this perspective could well amount to zero-carbon emission. (Truth is often stranger than fiction!)

A separate chapter will be devoted to Global Warming.

Reader please note that the mathematics of scarcity is not mainstream science, but consists of my own ideas. Discussion welcome.



Species diversity by areaMeasuring diversity

When measuring the number of species in a square area (like a quadrat), one very quickly arrives at a high number of species. Each time the experiment is repeated, one finds a few more species, but their numbers increase slowly. When the observations are plotted in a graph, the top graph results. In it we see that the key 20 species are always found together. To find the next ten, takes an area very much larger. When plotting the results on a double-logarithmic scale, a straight line can be drawn through the observations, and it can be described in a mathematical formula. From this line it is now possible to predict how many different species will be found in an area of 1000m2 and 10,000m2, assuming that the habitat stays the same.
This finding can be used in reverse, to calculate how many species will be lost from a habitat if it is reduced in size. As a rule of thumb, it boils down to one quarter of the size reduction. Thus a 12% reduction in area leads to a loss of 3% of the species. It may not happen immediately, but slowly over time.
Species diversity as function of area: S = c x A^z or dS= z x dA   Diversity increase as area increases
S=diversity within a taxonomic group; c= depends on group; z=0.2-0.3; A=area

If 2% of habitat is destroyed, extinction is 0.25 x 2% =0.5%
World habitats already changed by 50% thus already 12% of all species is lost or threatened to extinction. But this increases very rapidly since the remaining area decreases very rapidly.

The above equation is often expressed as "A tenfold decrease in area results in a two-fold loss in species", thus "By conserving 10% of the area, 50% of species is lost". Take an area of 100 units 100^0.3= 3.981= 4 species. Now reduce it tenfold 10^0.3= 1.995= 2, a loss of 50% of species.

The way the number of species depends on area, has important consequences for the size of protected areas. The smaller these are, the fewer species will be protected. In the example above, 35 species were found in 300m2, but 20 were found in every 10m2. A reserve for the most common 20 at 10,000 individuals each, would require 10,000x10m2 or 10ha. To protect the next 15 species, would need a reserve of 10,000x300= 300ha.

It is often argued that 10 reserves of 30ha would achieve the same, but there are a number of reasons why this is not so:

The relationship between evapotranspiration and plant productivity above (eco21.gif) also has important consequences for the size of protected areas, since all life ultimately depends on the amount of green food available. A mountain puma in a cold climate would need much more territory than a lion in a savannah grassland, which needs more than a jaguar in a tropical rainforest. Cold climate reserves may need to be 10x larger than warm climate reserves. Likewise dry climate reserves must be larger than wet climate reserves. Protected areas must attempt to protect both the total functionality and resilience (sustainability) of the ecosystem, which is severely compromised by a small size.

May R M, 1975: Patterns of species abundance and diversity, in Ecology of Species and Communities pp81-20. (eds M Cody, J M Diamond) Harvard Univ Press.

How large an area is needed to protect a breeding stock of 250 tigers?
If those 250 tigers were all there was left, it would amount to a critically endangered species, which is less than the bare minimum for survival. Bengal tiger: 3750 left; Siberian tiger: <200. South China tiger: 50 in zoos. Indochinese tiger: <1500. Sumatran tiger: 650. Bali tiger, Caspian tiger, and Javan tiger are now extinct.
A male tiger (180kg) patrols and defends a territory of about 52km2 against other males, and he shares this space with 2-4 females (140kg). Thus about 4 tigers require 52km2. Thus 250 tigers require an area of 3250km2 plus a boundary zone of at least 25%, gives about 4000km2 (40 x 100 km). By comparison, Kruger National Park (1884) in Africa is 60 x 350 km, or precisely 19,455 km2, room for 1500 lions, 900 leopards, and 250 cheetahs.

By the 1990's, there were about 1,500 national parks in the world. These parks protect about 3.9 million square kilometres in over 120 countries. Average size: 2600km2.

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