Biodiversity (short for biological diversity) refers to ther variety of life-forms that exist in a particular place (see Box 9.1). For the first time in 65 million years, the world's biodiversity is declining (Wilson, 1992; Pimm, 1995; Heywood and Watson, 1995). Pressures from human society are the main cause and, in New Zealand, these pressures have been intense and widespread. The decline comes at a time when many people are becoming aware of the economic value of wild species (e.g. in nature tourism, new crop and livestock production, and new gene and drug discoveries) and of the 'ecological services' they provide (e.g. in pest and weed control, carbon dioxide absorption, waste and nutrient processing and fish stock nurseries). On a more ethical plane, increasing numbers of people are coming to value Earth's diverse organisms simply for what they are - as fellow beings with a right to exist.
This chapter outlines the nature and state of our biodiversity, as far as we can assess it, the historical and current pressures threatening it, and our responses, as a society, to the challenge of protecting and sustaining it. Before looking at the specific situation in New Zealand, it is important to place New Zealand's biodiversity problems in a global and historical context. In terms of Earth's 4.6 billion year history, biodiversity is a recent development, even though life itself is very old. The earliest known rocks, from Greenland, are almost 3.9 billion years old. Some scientists have interpreted their unusually high ratio of carbon-12 to carbon-13 as evidence that primitive life emerged not long after the first oceans had formed on the cooling Earth (Day, 1994; Balter, 1996; Hayes, 1996; Mojzsis et al., 1996; Nisbet and Fowler, 1996).
The earliest undisputed fossil evidence of actual life forms - bacteria - is from 3.46 billion-year-old rocks in Australia (Schopf, 1993; Knoll, 1994). More ambiguous evidence for early life consists of fossilised mineral deposits called stromatolites which can be formed by either bacteria or geological processes. The oldest fossil stromatolites are 3.5 billion years old, but their biological origin cannot be stated with certainty (Grotzinger and Rothman, 1996; Hecht, 1996; Walter, 1996).
For perhaps two thirds of life's history, biodiversity was limited to single-celled organisms. Various species of bacteria ruled the oceans for 1 to 2 billion years before eventually giving rise to more advanced one-celled organisms called protists which, in their turn, eventually gave rise to everything else. Protists are commonly divided into 'algae' (micro-organisms that absorb their energy from sunlight, through a process called photosynthesis) and 'protozoa' (micro-organisms that absorb their energy from living or dead organic matter and are non-photosynthetic).
It was not until 1.7 billion years ago that the earliest multi-celled organisms left clear traces in the fossil record - tiny leaf-like seaweeds which were probably the first brown algae (Shixing and Huineng, 1995). Some species of green and red algae may also have became multi-celled about this time, though, so far, their earliest known fossils are only 1 billion years old (Kerr, 1995a). About 1 to 1.2 billion years ago, a line of protozoans began to follow the algal example and adopt multi-cellular forms (Vermeij, 1996). Although these tiny, soft-bodied, first animals left no fossil traces, the time of their origin can be dated from the genes of their modern descendants (Wrayet al.,1996).
It was hundreds of millions of years before atmospheric oxygen levels became high enough to sustain larger, more metabolically complex, organisms (Grahamet al., 1995). That is when animals began to show up in the fossil record. Numerous species of 2-10 cm circular blobs called Ediacarans appeared in the oceans between 540 and 570 million years ago (Grotzingeret al., 1995; Kerr, 1995c). Soon after, biodiversity really began to take off. In a great burst of evolutionary innovation, known as the Cambrian Explosion, two separate kingdoms of multi-celled organisms - the fungi and the animals - proliferated into the most diverse collections of organisms on Earth (Kerr, 1993). The species within these kingdoms have continued to evolve and diversify, but their main body plans were laid down between 500 and 530 million years ago when the five main divisions (or phyla) of fungi and the 35 or so phyla of animals evolved (Benton, 1995; Bowring et al., 1993; Sepkoski, 1993).
The explosion of animal phyla produced sponges (phylum Porifera), jellyfish, sea anemones and corals (phylum Cnidaria), sea stars and sea urchins (phylum Echnodermata), shellfish, snails and squid (phylum Mollusca), millipedes, armour-plated trilobites and the ancestors of crabs, insects and spiders (phylum Arthropoda) and microscopic eel-like creatures called conodonts which introduced teeth and backbones to the world and ultimately gave rise to the vertebrates - fish, amphibians, reptiles, birds and mammals (phylum Chordata).
These are just some of the better known animal phyla. Many others consist of tiny, soft-bodied animals known only to a few thousand dedicated scientists. At least a dozen phyla are made up of worm-shaped animals, some of which are familiar and some obscure. These include: flatworms (phylum Platyhelminthes), ribbon worms (phylum Nemertea), jaw worms (phylum Gnathostomulida), horsehair worms (phylum Nematomorpha), spiny headed worms (phylum Priapulida), segmented worms, which include leeches and the common earthworms (phylum Annelida), beard worms (phylum Pogonophora), arrow worms (phylum Chaetognatha) and, the most diverse phylum of all, the roundworms or nematodes (phylum Nemata).
The nematodes are found living in soil, plants and other animals from the polar regions to the tropics. However, they have proliferated most dramatically in the deep ocean bathyl regions where muddy slopes fall away from the continental shelves. Mud-dwelling marine nematodes are believed by some to have more species than all other life-forms combined, accounting for perhaps three-quarters of the globe's biodiversity (Grassle and Maciolek, 1992; Lambshead, 1993; Pearce, 1995b). Even the more conservative estimate of 400,000 nematode species is a daunting prospect for the world's 20 or so nematode taxonomists, whose list of identified roundworms so far runs to only 25,000 species (Hawksworth et al., 1995).
While biodiversity was dramatically expanding in the sea, multi-celled green algae and fungi were colonising the ancient shorelines. About 460 million years ago the green algae gave rise to the first plants - liverworts which soon gave rise to mosses and hornworts (Niklas, 1994; Palmer, 1995a). Many of these early plants formed symbiotic relationships with fungi which persist to the present day. About 90 percent of plant species rely on symbiotic fungi for one or more key nutrients (Kendrick, 1992). These plant-fungi communities laid down a carpet of soil and vegetation which lured the first land animals ashore about 450 million years ago (Gordon and Olson, 1995).
The pioneer land animals were members of the Arthropod phylum, whose modern descendants include: myriapods (e.g. millipedes, centipedes), arachnids (e.g. scorpions, spiders, mites), crustaceans (e.g. slaters, crabs, barnacles, shrimps) and insects (e.g. beetles, flies, butterflies). The ancestral arthropods were probably armoured myriapods whose hard yet flexible exoskeletons and jointed legs made them ideal 'Moon buggies' to explore and survive in their new environment (Palmer, 1995b). They were closely followed by the first land vertebrates, the amphibians (e.g. salamanders and frogs). Flatworms, earthworms, leeches and nematodes also wriggled ashore, alongside slugs and snails. With 90 percent of Earth history gone, the stage was now set for biodiversity to flourish on land.
Biodiversity is the variety of life. It includes "diversity within species, between species and of ecosystems and the processes that maintain them." (United Nations, 1992; Department of Conservation, 1994a). Biodiversity is most often measured asspecies diversity (the number of different species in a given area), but can also be measured as genetic diversity(the variety of genes within a population), or ecological diversity (the number of different ecosystems or ecological processes in an area). Most biodiversity research focuses on species, either as the fundamental unit of study, or as the unit which enables other aspects of biodiversity to be studied (Wilson, 1992). As closed gene pools, species are the key taxonomic units in studies of genetic diversity, and as sets of specially adapted organisms, they are the key functional units in ecology.
A species is commonly defined as a group of organisms belonging to the same evolutionary lineage and capable of interbreeding under natural conditions. However, this definition does not cover all species (Gibbons, 1996b; Wilson, 1992). Some are just at the point in their evolutionary divergence where they still have the physical capacity to interbreed but never do in nature because of behavioural differences or geographical barriers (e.g. lions and tigers; donkeys and horses, sheep and goats). Others reproduce asexually and so never interbreed (e.g. most bacteria, and some protozoans, plants, animals, and fungi). Asexual species clone themselves with each new generation so their only source of genetic variation is through accumulated mutations over long spans of time. Without the interbreeding criterion, it is difficult to know when asexual lineages can be considered separate species. Despite these difficulties, the species concept embraces most living things and remains the key unit of study in biology (Wilson, 1992).
Genetic diversity refers to differences in the nucleic acid molecules (DNA and RNA) within a population. Every organism contains a species-specific set of DNA molecules (or chromosomes). Each chromosome is made up of tens of thousands of smaller units, called genes, which, together, determine the organism's structure and function by controlling the chemistry and growth of every body part. Most genes come in several different versions (called alleles). Allele differences among individuals can cause minor variations in certain traits (e.g. height, colouring, temperament etc.). New alleles occasionally arise by random mutation. Some cause inherited diseases or handicaps, most are harmless, and some are beneficial.
The greater the variety of alleles in a population, the more genetically diverse it is said to be, and the greater its survival prospects. A high level of genetic diversity maximises the species'ability to adapt to new environmental pressures. Recent research suggests that genetic diversity declines once populations fall below 5,000 individuals and harmful alleles become more widespread as a result of inbreeding (Lande, 1995).
An ecosystem is an assemblage of species that interact with each other and their physical environment in a particular location. These interactions are called ecological processes. Most ecosystems can be characterised in broad terms by their dominant species or their main environmental features (e.g. beech forests, tussock grasslands, streams, lakes, wetlands, estuaries, etc.). Whether they are true systems, in the sense of being orderly and predictable with mutually inter-dependent parts and discrete boundaries, is still a debating point among scientists and philosophers, many of whom argue that most ecosystems are too disorderly and diffuse to be so neatly described (Botkin, 1990; Davis, 1984; Degan et al., 1987; Golley, 1994; Goodman, 1975; Jackson, 1994; Kay and Schneider, 1994; Moffat, 1996; Sagoff, 1985; Shrader-Frechette and McCoy, 1993; Simberloff, 1980; Worster, 1990).
However, whether seen as systems or as mere clusters of species, there is little doubt that ecosystems contain diverse ecological processes, many of which are vital for the survival of at least some species. In some cases, these vital ecological processes even include periodic disturbance (Pickett and White, 1985). For example, recent research in the United States has shown that some species in stream and grassland ecosystems, namely those that are specially adapted to colonise disturbed sites (e.g. short grasses, mayflies) and those that prey on the colonisers (e.g. grazing animals, fish), would become extinct without episodic floods and fires (Leach and Givnish, 1996; Tilman, 1996; Wootton et al., 1996). From such research it is clear that measures to conserve biodiversity must encompass not only species and genes, but also ecological processes and landscape management. Although this chapter focuses primarily on the most tangible and measurable level of biodiversitythe speciesthe importance of maintaining biodiversity at the ecological level is stressed throughout. For more information on the major water and land ecosystems, see Chapters 7 and 8.
Today Earth's extended family stretches thinly over its surface. Between 1.4 and 1.9 million living species have been scientifically identified (Wilson, 1992; May, 1992; Hawksworth et al., 1995). Anywhere between 3 and 112 million species may exist in all, most of them tiny animals, fungi and micro-organisms which have yet to be described. A rough working figure is about 14 million species (Hawksworth et al., 1995; Benton, 1995). Furthermore, many species contain genetically distinct subspecies and varieties.
- The standard approach to classifying this diversity is to group all species into five broad 'kingdoms'. However, recent genetic studies have shown that Earth's family tree is more complex than this (see Box 9.2). Furthermore, the number of known species in each of the five 'kingdoms' is constantly being revised as new ones are named and described. According to the Global Biodiversity Assessment commissioned by the United Nations Environment Program (UNEP), the latest estimate of the number of species scientifically described is 1.75 million, divided up as follows (Hawksworth et al.,1995):
- 4,000 known Bacteria (single-celled organisms with no nucleus; broadly divided into two groups: the archaebacteria which live in extremely hot or chemically hostile environments; and the more widely distributed eubacteria, such as Escherichia coli, Rhizobia and cyanobacteria);
- 80,000 known Protists (mostly single-celled organisms with a cell nucleus; broadly divided into: photosynthesisers, oralgae, some of which are multicellular; and non-photosynthesisers, or protozoa, such as ciliates, flagellates and amoeba);
- 270,000 known Plants (multicellular organisms which are immobile and obtain their food from the sun by photosynthesis and from the soil by absorption; broadly divided into: thenon-vascular bryophytes, mainly mosses and liverworts; and the vascular tracheaphytes, such as ferns, trees, flowering shrubs and grasses);
- 72,000 known Fungi (multicellular organisms which are immobile and obtain their food by absorption from the soil or from other organisms; broadly divided into: eumycota, the true fungi; and myxomycota, or slime moulds - which are now recognised as protists); and
- 1,320,000 known Animals (multicellular organisms which are usually mobile and obtain their food by eating other organisms; broadly divided into: invertebrates, animals that have no backbone, such as sponges, jellyfish, worms, shellfish, arthropods, etc., which account for 34 of the 35 animal phyla; andvertebrates, which belong to the Chordate phylum, comprising fish, amphibians, reptiles, birds and mammals).
To these, some people add a sixth 'kingdom' consisting of about 4,000 viruses. Viruses are sometimes described as 'sub-organisms' because they are not living cells. However, these tiny protein capsules contain genetic material (DNA or RNA) and, on entering living cells, they hijack the cell's chemical machinery to reproduce themselves. Viruses appear to have originated as rogue DNA and RNA from various unrelated life-forms and may actually belong to several kingdoms.
Within each of these kingdoms, organisms are classified hierarchically into taxonomic groups (or taxa). The classification of our own species can show how this system works. At the top level, each kingdom (in our case, the Animal kingdom) is divided into broad groups called phyla (except, for some reason, the Plant kingdom whose phyla are more plainly calleddivisions). Within the Animal kingdom we belong to the Chordate phylum. Each phylum, in turn, is divided intosub-phyla (in our case, the Vertebrates), thenclassesand sub-classes (among the vertebrates, we belong to the Mammal class), thenorders and sub-orders (we belong to the Primate order of mammals), and then families and sub-families (in our case, a Primate family called Homininae, which also includes the great apes).
Textual description of figure 9.1 (a & b)
The five kingdoms are animalia, plantae, mycota (funghi) prostista (including all bacteria) and protista (including protozoans and algae). Animalia, plantae and mycota are all off one branch with prostista and protista being separate branches.
The three domains are Eucarya, Archaebacteria and Eubacteria. From the first living cell, life branches in Eubacteria and Archaebacteria, with Eucarya branching off Archaebacteria.
The groups in each domain, in the order they branch off, are:
First living cell
- Aquifex and relatives
- Thermatoga and relatives
- Green non-sulphur bacteria
- Green sulphur-reducing bacteria
- Chlamydia; Spirochetes (e.g. Leptospirosis); Flavobacteria and Bacteriodes
- Gram positive bacteria; Purple bacteria (e.g. E. coli) - some members of this branch have been incorporated into Eucarya as organelles
- Cyanobacteria and relatives - many members of this branch have been incorporated into Eucarya as organelles
- Pyrodictium; Thermofilium and Thermoproteus; Sulfolobus and Desulfurococcus
- Methanomicrobioles and Exterme halophiles
- Diplomonads (e.g. Giardia); Trichomonads; Microsporindians
- Loboid amoeba
- Sarconoid amoeba
- Amoebiod slime mould
- Red algae
- Golden algae; Brown algae; Yellow algae and Diatoms
- Sporozoans; Dinoflagellates and Cilliates
- Acanthamoeba, Plants and Green algae
- Fungi, Choanoflagellates and Animals
This is a composite view based on several published sources which differ on some details.
The final stage of classification is the most important because it is at this level that species are assigned their scientific names (in Latin)first a 'surname' which indicates the genus they belong to (ours is Homo, meaning human) and then a species name (sapiens, meaning wise). Closely related species are generally given the same genus name and referred to as sibling species. Human classification is anomalous in this regard, dating from the founder of scientific taxonomy, Carl Linnaeus. In 1747, he wrote to a friend that he feared the clergy's reaction if he were to classify humans and chimpanzees together (Sagan, 1977). It is now known that, besides their physical and psychological similarities, humans and chimpanzees are 98.4 percent identical genetically. Some scientists and philosophers now argue that a reclassification is long overdue (Cavalieri and Singer, 1993; Diamond, 1991).
Sometimes, where a species contains populations that are genetically different, but not different enough to warrant a separate species name, a third sub-species name may be added - as for the extinct Neanderthal people of Europe and the Middle East (Homo sapiens neandertalensis). Sub-species present a special conservation problem. Apart from the usual threats, their existence is also threatened by interbreeding which preserves their genes but not their physical distinctiveness.
Despite the growing awareness of life's diversity, the important task of identifying and classifying the world's species (taxonomy) has been difficult in recent years as an increasing proportion of public and private research funding is channelled into applied rather than pure research. Increasingly, priority is being given to the urgent task of learning more about ecosystem management and the management of biodiversity for commercial uses, such as fisheries, bioprospecting and biotechnology.
In New Zealand it is now down to a few dozen taxonomists in museums, universities and research institutes to describe and name our tens of thousands of unknown species. An increasing proportion are retired scientists working part-time because there is no one else to do the work. In such circumstances, firm statistics on the number of species are hard to establish. However, based on work to date, some rough estimates can be made.
Bacterial biodiversity is poorly understood, yet may be greater than all the other kingdoms combined. About 3-4,000 species have been described worldwide, but this could be just the tip of a vast iceberg. The recentGlobal Biodiversity Assessment gives a working figure of 1 million bacteria, with a possible high of 3 million (Hawksworth et al., 1995). Yet, even this may be an under-estimate. It is often assumed that each species of plant, animal and fungus harbours at least one specifically adapted bacterial species. If so, the total number of bacteria must be at least equal to all other species combined - about 13 million, to quote the working figure, but possibly as high as 112 million (Hawksworth et al., 1995).
Of the remaining kingdoms, the Animals are undoubtedly the most diverse, with most of the honours going to the tiny, less glamourous species. Nematodes (roundworms) are the most diverse sea animals in the world, with maybe millions of species, and the arthropods (specifically, the insects) are the most diverse land animals. About 950,000 insect species have been formally identified, but the vast majority are still undescribed and the total number could be 8 million or more (Hawksworth et al., 1995).
Our phylum, the Chordates, contains about 45,000 known species, the vast majority of which are vertebrates (animals with spinal columns). These are spread over five classes: about 20,000 fishes; 4,000 amphibians; 7,000 reptiles; 9,000 birds; and 4,000 mammals. Mammalian biodiversity is dominated by two groups of small animals - about 1,700 rodent species (rats, mice, hamsters, squirrels, etc.) and nearly 1,000 species of bats.
Fungi belong to the next most diverse kingdom after the animals - the Mycota. They are believed to number at least 1.5 million species globally, though only about 70,000 have been described (Hawksworth et al., 1995). The most visible kingdom of all, that of the Plants, is well behind the animals and fungi in diversity, but is vital to the success of these kingdoms by providing the raw material for their food.
Most plant species are angiosperms (flowering plants). The total number of identified angiosperm species is almost 240,000, with many more still awaiting discovery in the tropics and in remote areas. The non-flowering plants, such as the bryophytes (e.g. mosses and liverworts), the ferns, and the gymnosperms (e.g. cone-bearing trees) evolved much earlier than the angiosperms but have fewer surviving species. The known non-flowering plants total less than 30,000 species.
The five-kingdom view of biodiversity groups Earth's millions of species into five great 'kingdoms' (Whittaker, 1959). Three of these kingdoms contain multi-celled organismsthe energy eaters (Animals), the energy absorbers (Fungi) and the energy photosynthesisers (Plants). The Animals have more species than the other kingdoms combined, followed by the Fungi, then the Plants and, finally, the two 'lower' kingdomsthe Protists (which are mostly single-celled but contain some multi-celled species) and the Bacteria (which are entirely single-celled). The main difference between the two 'lower' kingdoms is that Bacteria have simple cells in which the genetic material floats freely (prokaryote cells), while Protists have more complex cells in which the genes are housed in a protective bag or nucleus (eukaryote cells). The Protists are divided into photosynthesisers (Algae) and non-photosynthesisers (Protozoans).
Recently, genetic research has shown that the five-kingdom classification gives a somewhat distorted view of biodiversity. It minimises the diversity of the single-celled organisms while exaggerating that of the animals, fungi and plants. Although animals, fungi and plants have far more known species than the smaller life-forms, they are all relatively recent twigs on Earth's family tree (see Figure 9.1). The genetic studies show that, despite having fewer known species, the humble bacteria are the most widespread and genetically variable organisms on the planet, with the protists not far behind.
The new family tree revealed by this research has three main branches, or 'domains', each made up of many 'kingdoms' (Embleyet al., 1994; Woese et al., 1990; Morell, 1996). The two oldest domains consist entirely of bacteria - the Archaea or Archaebacteria (primitive bacteria) and the 'true' Bacteria or Eubacteria. To date, only 4,000 bacteria species have been scientifically described, but 1 million or more species may exist (Hawksworth et al., 1995). The third domain, called the Eucarya or Eukaryota, contains all the organisms which have a cell nucleus. This includes all the protists as well as the animals, plants and fungi. The Eucarya are genetically closer to the Archaebacteria than the Eubacteria, though their cells also incorporate organelles (mitochondria and chloroplasts) of Eubacteria origin (see below).
The Archaebacteria live in extreme environments similar to those found on primeval Earth. Instead of oxygen they 'breathe' sulphur, methane and halogens (chlorine and fluorine) and many are 'thermaphiles' (heat-lovers), thriving in places such as volcanic vents and thermal pools. Their unusual chemistry is of interest to drug and chemical companies and may shed light on the ultimate origins of life (Day, 1994). Many scientists believe the archaebacteria are a relic group from Earth's early days, though dissenters argue that they are a specialist group which arose more recently. The latest molecular research suggests that they and the Eubacteria split into separate domains about 2 billion years ago, and that the archaebacteria subsequently gave rise to our domain, the Eucarya, about 1.5 billion years ago (Doolittleet al., 1996).
The Eubacteria generally prefer less extreme environments than the archaebacteria, though some of the most primitive ones are also heat-lovers. Among the eubacteria are the well-known disease-causing groups (e.g. Campylobacter, Staphylococci, Chlamydia, Rubella, Salmonella, Shigella etc.), as well as many harmless species, such as the common, intestine-dwelling, faecal coliforms and enterococci (e.g. Escherichia coli). Many species are also beneficial, such as the various Rhizobiabacteria which enable plant roots to obtain nitrogen and the many soil-dwelling decomposers.
One of the most important and diverse groups Eubacteria encompasses is the cyanobacteria and their close relatives (e.g. Prochloron). These photosynthesising bacteria were once lumped in with the algae and called 'blue-green algae'. Although they are single-celled, some cyanobacteria form colonies while others form microscopic filaments of linked cells which occasionally show division of labour with some cells carrying on photosynthesis while others produce spores or attach to a surface. Cyanobacteria are found in almost every moist environment, in the sea, in fresh water and on land. They provide the beginning of the food chain in the ocean, being eaten by protozoans, which in turn are eaten by animals from corals to whales. They also provide the manure for Asia's rice paddies, the root fertiliser for tropical cycad plants, and they can form choking blooms in eutrophic lakes, producing toxins which mammals find unpleasant. Despite their name, many marine strains are red rather than blue-green, and one such strain (Trichodesmium) gives the Red Sea its name.
Fossil imprints in Australian rocks dating back 3.5 billion years have been interpreted as colonies of cyanobacteria or similar photosynthesising bacteria. This suggests that the first steps toward an oxygenated atmosphere were taken by lowly bacteria early in Earth's history. The cyanobacteria were also co-creators of the higher photosynthesising organisms - the algae and the plants. This ancestral connection occurred when various species of cyanobacteria were swallowed, but not killed, by hungry protozoans. Instead of being digested, the bacteria managed to survive inside the protozoans, continuing to reproduce and, most importantly, photosynthesise.
This formed the start of a symbiotic relationship in which the cyanobacteria provided a continuous supply of high energy carbohydrates to their protozoan hosts and, in return, enjoyed a ready supply of nutrients which had been captured by the larger host as well as a safe indoor life. The specialised descendants of these symbiotic cyanobacteria are still present in the cells of today's plants and algae as chloroplasts, the small 'organelles' which actually carry out the process of photosynthesis. Genetic studies have shown that this fusion of protozoan and cyanobacteria cells occurred separately in at least five different protistan 'kingdoms' forming at least five unrelated 'kingdoms' of algae.
Algae is the name collectively given to all the photosynthetic protistans. Although about 40,000 species have been described, some 200,000 may exist (Hawksworth et al., 1995). Algae differ from cyanobacteria in having a nucleus and chloroplasts. Most are single-celled 'micro-algae' and some are multi-celled 'macro-algae'. Classifications vary, but six groups exist. The Chlorophytes, or Green Algae, are mostly single-celled but contain some multi-celled species (which eventually gave rise to the mosses and other plants). TheChrysophytes are all single-celled. They include the numerous yellow-brown Diatoms and the Yellow and Golden Algae. ThePhaeophyta, or Brown Algae, are genetically part of the Chrysophyte 'kingdom' but have been traditionally put in a separate group because they are multi-celled seaweeds whereas the others are single-celled floating phytoplankton. TheRhodophytes, or Red Algae, include single-celled and multi-celled species and make up the red seaweeds. The Pyrrophytes, or Dinoflagellates, are single-celled and abundant. They are best known for the handful of species that form toxic algal blooms (see Chapter 7, Box 7.12). The Euglenophytes are green flagellates - single cells that swim by the action of one or more whip-like tails.
Protozoans are the non-photosynthesising protists. About 40,000 species are known, but some 200,000 may exist (Hawksworthet al., 1995). They are all single-celled with the closest attempt at multi-cellularity being the colonies or 'fruiting bodies' formed by slime moulds. The slime moulds are sometimes classed as fungi, on the strength of this and their relatively advanced cell structure, but genetically are not closely related to the fungi at all. The protozoans range from primitive cyst forming species, which have no means of propelling themselves about, such as the parasite, Giardia lamblia (see Chapter 7, box 7.11), through to the complexchoanoflagellates, whose cell structure is almost identical to that of the first animals, the sponges. In between are amoeba, which move by undulating their bodies, flagellates, which swim with whip-like tails, and ciliates, whose bodies are lined with small hair-like cilia which enable them to move about.
A feature which distinguishes the lower protozoans (such as Giardia) from all other Eucarya is their lack of an important 'organelle' called the mitochondrion. Just as chloroplasts convert the Sun's energy and carbon dioxide into high-energy food molecules, the mitochondria convert those food molecules into tiny fuel packages called ATP which can be used efficiently to power activities anywhere in the cell. The lower protozoans and the bacteria have no mitochondria, and genetic studies have revealed why. Like chloroplasts, mitochondria have evolved from an ancestral bacterial invader - in this case a non-photosynthesising purple bacteria. This seems to have happened only once, well before the cyanobacteria got in on the act, when a purple bacteria took up residence inside a Giardia -like protozoan. The symbiotic relationship between the mitochondrial bacteria and their protozoan hosts enabled the higher protozoans and algae to use energy more efficiently and diversify into the millions of larger species that now exist. These tangled relationships show that life's diverse organisms are indeed a single family, with more interconnections than Darwin ever imagined.
Evolution is a slow creator and environmental crises are swift destroyers. Five times in the past 500 million years Earth's biodiversity was devastated by catastrophes that together wiped out perhaps 90 percent of the species that ever existed. We, and all living things, are descendants of the handful that got through. The catastrophes appear to have had various causes, ranging from the impacts of asteroids to tectonic plate movements, volcanic eruptions, oceanic gas eruptions and rapid climate changes (Benton, 1993, 1995; Erwin, 1993; Jablonski, 1986; Knoll et al.,1996; Raup and Sepkoski, 1982; Renne et al., 1995; Stanley and Yang, 1994; Ward, 1994; Wilson, 1992).
The greatest catastrophe known, the PermianTriassic (P-T) crisis of 250 million years ago, occurred when active volcanoes covered 2.5 million square kilometres of shallow sea in what is now Siberia and when the world's deep oceans were belching up large volumes of carbon dioxide. Lava from the volcanoes formed the vast Siberian Traps. Over a million years, episodic eruptions produced huge quantities of steam and sulphurous clouds whose prolonged 'volcanic winter' may have lowered temperatures and brought acid rain to much of the planet (Renne et al., 1995; Kerr, 1995b).
Evidence has also been found of a great build-up of carbon dioxide about this time in stagnant ocean depths, where sluggish currents had allowed vast amounts of decaying algae and bacteria to sink to the bottom. It is argued that this carbon dioxide-rich water eventually surged to the surface, killing sea life and emitting a toxic cloud which killed land animals and triggered rapid climate change (Knoll et al., 1996; Kerr, 1995d). A small-scale version of this happened in 1986 when Lake Nyos, in Africa's Cameroons, released a toxic cloud that killed people and animals.
The last great biodiversity crisis, 65 million years ago, extinguished about half the world's species but is best known for ending the 160-million-year reign of the dinosaurs. Scientists attribute the extinctions to drastic climate changes, but have debated for a decade what might have triggered such changes (Glen, 1995; Kerr, 1994). Some argue that the extinctions occurred gradually over several hundred thousand years and might have been caused by repeated episodes of volcanic activity (Officer, 1993). Volcanic eruptions about this time did, in fact, produce one of the largest outpourings of lava ever known, forming the Deccan Traps of India.
However, most scientists now think that, big as the Deccan eruptions were, their impact was dwarfed by that of the asteroid that punched a 180-kilometre wide indent into what is now Mexico's Yucatan Peninsula (Swinbourne, 1993; Ward, 1994). The buried crater was discovered by oil company geologists. From its dimensions, the meteorite is estimated to have been 10 km across and to have hurtled into what was then a shallow sea at nearly 90,000 km per hour. The impact is believed to have produced huge tidal waves, winds of up to 1,000 km per hour, and vast clouds of dust and vapour as two great fireballs of molten rock erupted, one after the other (Alvarez et al., 1995; Hecht, 1995). The fallout from this impact is still visible even in New Zealand where traces of the meteor's rare metal, iridium, are found in 65 million year old sedimentary rocks. A far-flung fragment of the asteroid itself was found in sea-floor sediments near Hawaii (Kerr, 1996).
It took 10 to 20 million years of evolution for the number of species to increase again. Among the successful evolutionary lines were the mammals and, among them, the primates, whose ape branch eventually sprouted our ancestor and close relativethe chimpanzee. The recency of our chimpanzee origin is reflected in the extraordinarily high genetic similarity between our two species (King and Wilson, 1975a, 1975b; Diamond, 1991) and in a rapidly growing sequence of fossils connecting us to chimpanzee-like ancestors who lived 3.5 to 4.5 million years ago (ago (Brunet et al., 1995; Clarke and Tobias, 1995; Culotta, 1995b; Leakeyet al., 1995; Shreeve, 1996; White et al., 1993, 1994; WoldeGabriel et al., 1994; Wood, 1994).
For several million years, various species or sub-species of hominids came and went until biologically modern humans (Homo sapiens sapiens) emerged in Africa less than 200,000 years ago (some 5,000-7,000 generations) (Ayala, 1995; Bowcock et al., 1994; Gibbons, 1994, 1995; Goldstein et al., 1995; Nei, 1995; Penny et al., 1995; Waddle, 1994; Wilson and Cann, 1992).
Our species began expanding out of Africa about 70,000-120,000 years ago (3,000-5,000 generations), dogging the 2 million-year-old footsteps of an earlier hominid (Homo erectus) which had already faded into the fossil records of China, Africa and Europe (Gibbons, 1995; Lewin, 1994; Mellars, 1996; Stringer and Gamble, 1993; Stringer and McKie, 1996) and survived only in South-east Asia (Lewin, 1996b; Swisher et al., 1996).
The first emigration waves spread eastward along the equator toward India and South East Asia about 100,000 years ago (Stringer and McKie, 1996). Some groups fanned down into Melanesia and Australia (maybe 50-60,000 years ago), while others headed north-east into Asia and the Arctic circle (perhaps 60,000 years ago) and north-west into Europe (about 40,000 years ago). The Asian branches radiated in all directions, eventually sending offshoots into the Americas (about 15,000 years ago) and the islands of Micronesia and Polynesia (beginning about 2,000 years ago). The process of expansion, migration, mixing and warfare has repeated itself since, both within and between these regions, producing a diversity of peoples, languages and cultures but a disturbing uniformity in their environmental effects (Cavalli-Sforza, 1991, Cavalli-Sforza et al., 1993, 1994).
Right from the start, human invasions of new ecosystems caused extinctions and ecological changes. Among the first victims were the Neanderthal people of Europe, descendants of Homo erectus and cousins of our own species (Stringer and Gamble, 1993; Stringer and McKie, 1996; Gibbons, 1996a) and probably the remnant Homo erectus people of South-east Asia who were still in the region when the first Homo sapiens arrived (Lewin, 1996b; Swisher et al., 1996). Thousands of birds and mammals also became extinct, not just in Europe, but in Australia, Madagascar, the Americas and the Pacific Islands (Culotta, 1995a; Diamond, 1991; Flannery, 1994; Martin and Klein, 1984; Pimm et al., 1995). The Pacific extinctions were the most recent and their scale is only beginning to be understood (Diamond, 1995a; Steadman, 1995; Wragg and Weisler, 1994). As many as 2,000 of the original 3,000 Pacific birds may have disappeared following human contact (Pimm, 1995).
It was the invention of agriculture, however, between 7,000 and 12,000 years ago (300-600 generations) that enabled humans to replace whole ecosystems with just a few species of crop plants and stock animals. The revolution occurred independently in the grasslands and river valleys of Iraq, Egypt, India, China and Mesoamerica. It was made possible by the closing of the last Ice Age whose chill had dominated the global climate for the previous 100,000 years. It is only within the last two centuries (8 generations) that the invention of combustion engines and electricity generation has extended our technological power, enabling us to invade, degrade and remove virtually any ecosystem in the world.
Our towns and farms now cover 37 percent of the globe's land surface, and our activities have significantly disturbed natural habitat over 52 percent of the total land area (World Resources Institute, 1994). Extinctions have soared well beyond natural rates as we replace ecosystems with pasture, crops and settlements and convert their resources into human flesh, artefacts and waste. For the first time since mammals became the dominant land animals Earth's species are disappearing faster than evolution can replace them (Wilson, 1992; Pimm, 1995). Current extinction rates are estimated to be 20-200 times higher than the natural 'background' rates, and, on current population and economic trends, are predicted to reach anywhere between 200 and 1,500 times the natural rate within the next century, wiping out all currently threatened species (Pimm et al., 1995).
Most of the known victims are creatures like us, large land animals, whose size, habitat and small natural populations, make them particularly vulnerable to predation and habitat loss. Globally, about 11 percent of mammals and birds are considered threatened and about 3 percent of reptiles and fish (World Resources Institute, 1994). The order of mammals that we belong to, the Primates, is one of the worst affected, with 66 percent of its species now threatened - including our closest living relatives, the chimpanzees, gorillas and orangutans (Ceballos and Brown, 1995). As the pressures increase, the global family appears to be heading toward another great catastrophe. This time, however, the cause is not a mindless volcano or asteroid, but the most clever species on the planet - which means that, for the first time, the scale of the catastrophe can be a matter of choice (Ward, 1994).
In 1992, world leaders made their choice by signing the Convention on Biological Diversity (or Biodiversity Convention) at the United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro. In signing the Convention they agreed to take measures to prevent the biodiversity crisis from turning into another global catastrophe. The Convention's preamble recognises that biodiversity is important for evolution and for maintaining life-sustaining systems, as well as for its intrinsic value and its ecological, genetic, social, economic, scientific, educational, cultural, recreational and aesthetic values (United Nations, 1992). It also notes that the "fundamental requirement for the conservation of biological diversity is the in-situconservation of ecosystems and natural habitats and the maintenance and recovery of viable populations of species in their natural surroundings".
Article 6 of the Convention requires each member country to develop national strategies, plans or programmes for conservation and sustainable use of biodiversity and to integrate these as much as possible into relevant sectoral or cross-sectoral plans, programmes and policies. The Convention states that scientific uncertainty should not be used as a reason for postponing conservation measures. New Zealand ratified the Convention in 1993 and, in doing so, embraced the challenge of protecting one of the world's most ravaged but distinctive outposts of biodiversity.