Download CHAPTER 22 Biodiversity in Ecosystems

CHAPTER 22
Biodiversity in Ecosystems
Introduction
Modern interest in ecosystems amidst the general public stems largely from two key
properties which have attracted headlines – namely, biodiversity and stability. This
chapter explains the scientific bases of the two concepts, and the nature of the debates
which surround them, including whether the two are themselves interrelated. As with
many terms used in biogeography, both concepts are capable of several, not just one,
definition. This complicates their study and the study of links between them. There are
always the dangers of simplification and generalization which, although attractive to the
student and to the teaching of concepts, are fundamentally flawed. Interest in the two
concepts is essentially based on the potentially serious impact which human actions can
have upon them. Thus the adverse effects of ‘development’ upon ‘the environment’
nowadays usually lead us to concentrate upon ‘biodiversity’ and ‘ecosystem stability’.
Chapter summary
Definitions of diversity
•
In talking about diversity in a comparative way, it is necessary to define precisely
the spatial and temporal limits of the community being studied.
•
The species richness, s, the number of species in a community, is the simplest
measure of diversity.
•
The Shannon index of diversity is based on information theory and takes into
account both species richness and equitability, or relative abundance of species.
•
Connectance measures the number of interactions between species in a
community.
•
Whittaker introduced alpha diversity to measure number of species and beta
diversity to measure diversity caused by environmental variability.
Factors influencing diversity: time and energy
•
The number of species in taxonomic classes declines as one moves from the
Equator to the poles.
•
The time hypothesis suggests that the latitudinal gradient is based on the length of
time available for speciation, i.e. since the upheavals of the Pleistocene era.
•
However, from the fossil record it is clear that the trend has existed through all
geological ages and was certainly evident in the Tertiary period, before the
Pleistocene.
•
The species–energy hypothesis relates diversity to the energy available for
ecological processes, based on the larger amounts of solar energy in low latitudes.
•
However, many high-energy ecosystems of high productivity (estuaries, salt
marshes) have very few species.
Factors influencing diversity: stability and speciation
•
The stability hypothesis of Sanders suggests that areas of environmental risk and
stress are places of low diversity.
•
 onversely, regions of stable environmental conditions, where climate changes
C
little, are areas of high diversity.
•
Other ecologists (MacArthur and May) have shown that groups of competing
species can tolerate a greater degree of overlap between their respective niches
when environmental conditions are more constant.
•
Another suggestion is that environmental variability is higher than was previously
thought in the tropics, and that soil variations and micro-climate variations are
more important than was previously thought.
•
It is also suggested that barriers in the tropics allow more species to develop in
isolation, whereas those species would be eliminated in temperate and polar
climates.
Factors influencing diversity: area
•
The theory of island biogeography states that there is a direct relationship between
the number of species on an island and its size.
•
The second element in the theory of island biogeography is the distance effect,
whereby the farther away from continents the islands are, the lower is the species
richness.
•
The relationship between species number and area may be a useful one in the
design of nature reserves, but it is complicated by the different sizes of ranges of
different species.
•
The present area of tropical rain forests is about 8 million km2, or about half the
area in the immediate post-glacial some 10,000 years ago.
•
Many tropical countries are attempting to preserve rain forest by requiring
landowners to leave at least 50 per cent of their land under forest.
Definitions of stability
•
Stability can be defined in at least five ways: stable, resilience, persistence,
resistance, variability.
•
A stability domain defines the range of conditions under which a system is stable;
the mid-point of this domain is the point of equilibrium.
•
A range of natural and human-induced perturbations can affect the stability of
ecosystems.
•
MacArthur defined stability using information theory in a manner similar to the
Shannon index of diversity.
•
The variability over time of a biological population is an important index of
stability, and can be quantified by the coefficient of variation.
British birds: a stability example
•
A major obstacle to the study of the stability of biological populations is the lack
of long runs of data.
•
The British Trust for Ornithology has recorded bird populations on sample plots
since 1962, and the data can be used to study population stabilities.
•
Sometimes changes in numbers can be ascribed to natural, usually climatic,
causes; at other times, changes appear to reflect both natural and human-action
impacts.
•
The study of various species of British birds points to the difficulty of using a
single species as an indicator of environmental change.
•
For many years it was thought that species in land ecosystems had constant
variability over time (‘white noise’); however, recent work by Pimm and
Redfearn suggests that variability increases with time (‘red noise’).
Relationship between diversity and stability
•
Elton was a strong supporter of the hypothesis that there is a causal connection
between complexity and stability in ecosystems.
•
Elton’s evidence was based on six points: simple mathematical models, simple
laboratory experiments, the theory of island biogeography, human ecosystems,
tropical rain forests, and the tundra.
•
Evidence against Elton’s hypothesis has been building up since the 1970s and
includes the detailed work of May on a whole range of mathematical models.
•
Field evidence is also available to illustrate how some species-poor communities
can be very resilient and some species-rich communities very unresilient.
•
Different diversity–stability questions give conflicting answers, which explains
why the debate was so controversial in the past and why there is so much more
work to be done.
Threats to global biodiversity
•
About 1000 plant species are known to have become extinct in the past 2000
years, and about 25,000 are currently threatened.
•
Red Data Books produced by the IUCN given details of all threatened species.
•
The greatest threats to diversity are habitat loss, commercial exploitation,
competitive introductions, eradication of pests, and disease.
•
The Biodiversity Convention adopted by many countries in 1992 requires each
nation to produce a national conservation strategy.
•
First World countries depend upon twenty major crop plants, whereas the world’s
indigenous peoples use more than 50,000 plants for food, fibre and medicines.
CASE STUDY Changes in Biodiversity in British Ecosystems due to Land-Use
Change 1939-2050
We have seen what enormous changes have taken place in the ecosystems and vegetation
cover of the British Isles over the past 10 millennia (Chapter 20). They will not be the
last. In the present century, factors such as global warming, changes in the economics of
farming, public attitudes towards blood sports, and the increasing use of the countryside
for tourism, recreation and access will inevitably bring further change.
Starting with wartime demands for food in World War 2, British farming has witnessed a
massive intensification and drive for productivity and profitability. ‘Improvements’
included land drainage, mechanisation, the use of agro-chemicals (fertilisers, herbicides,
insecticides, molluscicides), and structural changes to the rural landscape such as field
consolidation and hedgerow removal. Transformation of the countryside has been
achieved at a great cost to wildlife. Wildflowers, birds, insects and mammals have been
eliminated at an alarming rate.
‘Weeds’ had been an inevitable hindrance to good crop production previously, and in the
1930s cornfields were frequently yellow with charlock, red with poppies and white with
wild radish. Improved techniques for cleaning seeds, and the use of herbicides to combat
weeds in arable crops, has meant that weed-free crops of cereals and ryegrass pastures are
now the norm. According to English Nature in 2000, “of all the wild flowers in the
English landscape, those of our traditional arable fields are probably declining the
fastest.” Nan Sykes (2001) has made a study of cornfield flowers in North Yorkshire. She
estimates that 24 of the 45 species still found in and around arable fields are ‘at risk’. A
further 8 species have become locally extinct, and 13 species are ‘rare’ in North
Yorkshire (Table 1).
Table 1 Status of Cornfield Flowers in North Yorkshire (after Sykes 2001).
STATUS
Extinct locally,
rare in UK
Rare locally
COMMON NAME
corn buttercup
corncockle
cornflower
corn gromwell
fluellen
pheasant eye
red hemp nettle
weasel snout
Bugloss
cornfield spurge
corn marigold
corn mint
field woundwort
flixweed
LATIN NAME
Ranunculus arvensis
Agrostemma githago
Centaurea cyanus
Lithospermum arvense
Kickxia elatine
Adonis annua
Galeopsis angustifolia
Misopates orontium
Anchusa arvensis
Euphorbia exigua
Chrysanthemum segetum
Mentha arvensis
Stachys arvensis
Descurainia sophia
lesser toadflax
night-flowering catchfly
shepherd’s needle
small white fumitory
treacle mustard
Venus’ looking glass
Chaenorrhinum minus
Silene noctiflora
Scandix pectin-veneris
Fumaria parviflora
Erysimum cheiranthoides
Legousia hybrid
The dilemma is, what conservation measures can be taken to preserve cornfield flowers
and their associated insect, small mammal, butterfly and bird populations, alongside
modern farming? Although their nuisance value is well documented, little is known about
their potential value and wider ecological role. The Royal Botanic Gardens, Kew,
maintain a Millennium Seed Bank where seeds are freeze dried to ensure survival of
nationally endangered species, and cornfield plants form part of this collection. Also,
there are projects such as the Cornfield Flower Project, Ryedale Folk Museum, Huttonle-Hole, North Yorkshire, where a reserve for cornfield flowers has been created. To
some extent the balance has been restored by recent changes in EU farm support policies
with the Single Farm Payment system, and the introduction of positive management
instruments such as Environmentally Sensitive Areas (ESAs) and Countryside
Stewardship. A further conservation measure is that some recent agricultural support
schemes include ‘conservation headlands’, whereby a 10 metre wide strip on the field
edge is left untreated by herbicides. This encourages germination of long-lived seed,
although it cannot help species already locally extinct.
A further threat to the ecological integrity and biodiversity of British ecosystems in the
21st century will come from increasing tourism, recreation and access to the countryside.
The question of public access to uncultivated countryside, i.e. the mountain, moor, heath,
down and common land of England and Wales, was a contentious issue during the 20th
century. On the whole, farmers and landowners were implacably opposed, although
public bodies such as local planning authorities and National Park Authorities improved
the rights-of-way network. Only at the beginning of the 21st century has campaigning for
open access borne fruit and brought legislation to secure the right-to-roam. The
Countryside Rights of Way (CROW) Act (2000) created a new legal right of access on
foot, horse and cycle to about 1.6 million hectares in England and Wales (Countryside
Agency 2001, 2002).
Apart from studies of footpath erosion in areas like the Peak District and Lake District
National Parks, where heavy use of the most popular footpaths and beauty spots has
caused serious damage, there is not much research on the disturbance to ecosystems by
public access. Trampling causes a change in species composition of vegetation, breaks
the stems of plants, and leads to reductions in vigour, regeneration and ground cover.
Trampling also affects soils by losses of organic matter, porosity, and air- and waterinfiltration. Other negative impacts of access come from the increased risk of fire, wearand-tear on roadside verges, litter and the picking of wildflowers.
The question arises, can occasional, low intensity use of open countryside lower
biodiversity and ecological integrity? Birds provide an area where there has been some
research by NGOs such as the Royal Society for the Protection of Birds (RSPB) and the
Ramblers Association, as well as public bodies such as Natural England and National
Park Authorities. Table 2 shows possible consequences of the expansion of public rightsof-way on bird populations.
Table 2 Possible consequences of recreation on bird populations.
DISTURBANCE
Temporary disruption
to feeding or resting:
EFFECTS
Less critical in late summer,
post breeding season
More critical when food supply
restricted and daylight/feeding
time reduced
Effects on breeding
adults:
Disturbed when looking for
nest site
Disturbed when incubating
eggs
Taking cover after alarm calls
of parent bird
Effects on habitat from
accidental fire
POSSIBLE RESULTS
• bird takes cover or
flies to undisturbed
area
• exhaustion due to
energy loss
• vulnerable to
predators
• delay in breeding
• increased
competition for sites
• heat loss to eggs
• non-incubation
• more predation
• feeding and brooding
interrupted
• predators attracted by
alarm calls
• loss of breeding
cover and food
supply
The balance between successful breeding and recreational disturbance is a fine one.
Natural England has estimated that whereas in upland Britain a habitat such as blanket
bog would take 600 – 800 passages to be damaged, breeding birds such as curlew, black
grouse, hen harrier and merlin can be negatively affected by as little as one passage by a
member of the public. Marion Shoard (1999 p. 345) describes the problem as follows:
“Walkers are unlikely to trample over nests as they are so widely scattered, but if
they get close enough to scare a sitting bird off the nest, they may expose eggs
and young to an increased risk of predation, particularly if their numbers make
disturbance frequent.”
References
Countryside Agency (2001) New Rights, New Responsibilities: what the new countryside
access arrangements will mean to you.
Cheltenham.Countryside Agency (2002) Drawing the boundaries: mapping and
consultation of the new countryside access rights. Cheltenham.
Shoard, M. (1999) A Right to Roam. Oxford: Oxford University Press.
Sykes, N. (2001) Cornfield flowers: have they gone for ever? York: PLACE Research
Centre, York St John College.
Discussion topics
1. ‘Biodiversity is much more than species diversity.’ Discuss different definitions
of biodiversity.
2. How do species arise? Give examples of the different mechanisms involved.
3. Why do you think that it is important to preserve biodiversity?
4. ‘Ecological communities are dauntingly complex.’ Discuss what is known about
the relations between biodiversity and stability in ecosystems. What questions
remain to be answered?
Further reading
Colinvaux, P. (1993) Ecology 2, New York: Wiley. The second edition of the
Introduction to Ecology (1973). Clear and well illustrated discussion of the debate on
biodiversity and its causes.
Elton, C.S. (1958) The Ecology of Invasions by Plants and Animals, London: Methuen. A
classic text. Plenty of illustration and a detailed treatment of the theory of island
biogeography.
Gaston, K.J. and Spicer, J. (1998) Biodiversity: an introduction, Blackwell. A readable
and approachable introduction to the many strands of the biodiversity debate.
Groombridge, B., ed. (1992) Global Biodiversity: Status of the Earth’s living resources,
London: Chapman and Hall. A detailed study of the current situation regarding
threats to species and to ecosystems of high diversity.
Tudge, C. (2005) The secret life of trees, London: Allen Lane. A readable and
comprehensive account of tree species, their distribution, evolution and uses. Devotes
much space to the debate on tropical tree biodiversity, and the many hypotheses
proposed in explaining global gradients of tree diversity.
United Nations Environment Programme (UNEP) (1995) Global Biodiversity
Assessment, Cambridge: Cambridge University Press. The official and detailed study
coming out of the Rio conference on global biodiversity. Packed with useful
discussion and example.
Wilson, E.O. (1992) The Diversity of Life, London: Penguin. This is perhaps the most
scholarly and up-to-date account of the nature and causes of diversity. Very readable
and full of examples.
UN Environment Programme (1995) Global Biodiversity Assessment, Cambridge:
Cambridge University Press. The official and detailed study coming out of the Rio
conference on global biodiversity. Packed with useful discussion and examples.
Wilson, E. O. (1992) The Diversity of Life, London: Penguin. A scholarly account of the
nature and causes of diversity. Very readable and full of examples.
Web resources
http://www.unep-wcmc.org
The World Conservation Monitoring Centre (WCMC) at Cambridge, is managed by the
United Nations Environment Programme (UNEP). It produces policy documents on
biodiversity, and is a source of many data on biodiversity and conservation across the
globe.
http://www.iucnredlist.org
The International Union for the Conservation of Nature (IUCN) (also known as the
World Conservation Union) maintains lists of threatened species giving rise to concern. It
publishes Red Data Books for the different classes of organisms whose future is
threatened.
There are many web sites covering the topics of evolution and the origins of biodiversity.
These below are especially useful:
http://tolweb.org/tree/phylogeny.html
Tree of Life project
http://aw.com/ide/Media/JavaTools/popcmpex.html
Principle of competitive exclusion
http://www.evotutor.org/Speciation/SpeciationA.html
Overview of evolutionary processes
http://www.evotutor.org/Speciation/Sp1A.html