ABRAHAM MABELIS
Introduction
During the last decades the built-up
area of Warsaw expanded very fast. As a result several green areas
disappeared, while the remaining ones are getting more and more under urban
pressure. Consequently, more green areas will disappear (Szulczewska and
Kaliszuk 2003). This will affect the liveability of the city. Green areas
are important for improving the urban climate: trees and bushes increase
air humidity, give some cooling on hot days, produce oxygen and filter
the air. Moreover, urban green retains rain-water and its soil has important
cleaning properties: some noxious substances can be broken down, at least
if the soil is not too much polluted. Urban green has not only an ecological
function, but is also important for recreation. For both functions it is
important to keep biodiversity at a high level.
The Convention of Biological Diversity
(Rio de Janeiro 1992), which was signed by many countries, aims at the
maintenance of ecosystem, species and genetic diversity. This Treaty also
has consequences for local communities, according to Local Agenda 21. The
city council can meet its obligations by establishing a sustainable green
infrastructure. The maintenance of such a robust green infrastructure is
not only important for the survival of native plants and animal species,
but also for maintaining a healthy environment and for giving citizens
the opportunity to relax and experience nature. In order to prevent alienation
of citizens from nature the presence of native species should be favoured
above introduced exotic species, although the aim to maintain a high level
of biodiversity in a city emanates mainly from the wish to keep it liveable
and healthy (Kelcey 1978, Trojan 1981a, Goode 1989).
Ecological qualities of urban nature
should be guaranteed despite urban processes. Therefore, a strategy should
be followed in which water, energy and waste flows are managed properly
(Tjallingii 1993, 1995, 1996). Moreover, fragmentation of urban green should
be prevented as much as possible. Generally, urban green is less fragmented
in the outskirts of the city than in the centre, where green areas are
generally smaller and more isolated by roads and buildings.
For evaluating environmental quality
the presence and absence of species which can disperse very well, but are
sensitive to pollution, may be useful. For example the distribution of
some Lichen species, gives an indication of differences in air quality.
Similarly, water quality can be measured by changes in the presence of
fish species and dragon flies, which are sensitive to water pollution.
For evaluating the spatial quality
of urban nature, the distribution of common species, which possess a poor
dispersal capacity, gives an indication of the isolation of suitable habitat
patches of these species. Fragmentation of urban green by roads, houses,
parking places, etc. will affect the exchange of individuals between their
local populations and hence their survival probability.
Before discussing the relation between
the green infrastructure of a city and its biodiversity, some characteristics
of the urban environment will be discussed.
Urban environment
In flat land a city like Warsaw rises
up as a rock island. Buildings with holes, crevices and hiding places can
be suitable for rock-dwelling birds and bats to hide, reproduce or hibernate,
while old walls (with calcium- rich cement between the stones) can be suitable
as habitat for wall plants. Moreover, cellars, which are constant cool
and moist, add a kind of cave environment to the city. Several species
use cellars to hibernate, like some beetles and night butterflies.
A city can also be considered as
a heat island: the average temperature is higher in a city than in its
surroundings, despite the fact that the sun will shine here less often
and less fierce than in the countryside, due to air pollution. However,
a great deal of the city surface is covered by buildings, asphalt and flat
stones, which can preserve heat for a long time. The relative high temperature
of the city is suitable for thermophilic species. Consequently, the northern
limit of the distribution area of several thermophilic species is situated
in cities. Examples are known of plants (Sukopp and Wurzel 1995), digger
wasps (Whiteley 1994), longhorned beetles (Burakowski and Nowakowski 1981b),
rove beetles and glow worms (Klausnitzer 1989) and springtails (Sterzy?ska
1982), among others. Thanks to the warmer city climate birds can produce
more clutches of eggs per year (Klausnitzer 1989). Moreover, winters are
less severe in cities. Consequently water will freeze less easily, which
may be profitable for water birds. On the other hand, the warmer climate
of a city is disadvantageous for some insect species: e.g. a parasite can
miss the connection with its host species (Garbarczyk 1982).
A city can also be considered as
a dry island. Its stony environment causes that the air of a city is dryer
than in its surroundings (Blume 1993). Generally the soil is also dryer
in a city, despite the fact that it rains more often (Kuttler 1993). It
appears that the proportion of hygrophilic species of several groups of
insects will be lower inside a city than in its surroundings (Ba?kowska
1981a, Pisarski and Kulesza 1982, Chudzicka and Skibi?ska 1994). A shift
from hygrophilic species to dry-tolerant species from the outskirts of
a city to the centre is ascertained in Warsaw for several groups of animals:
worms (Kasprzak 1986), spiders (Klausnitzer 1987), harvest-spiders (Czechowski
et al. 1981b), springtails (Sterzy?ska 1982, 1987), carabid beetles (Czechowski
1981a, 1982), lady beetles (Czechowska and Bielawski 1981), flies (Górska
1981), scorpion flies (Czechowska 1982), ants (Pisarski and Czechowski
1978, Pisarski 1982b) and parasitic wasps (Garbarczyk 1982). However, it
appears that dry- tolerant species with a narrow ecological amplitude occur
less often in the city of Warsaw (Pisarski and Kulesza 1982).
Soil and water in a city is enriched
with nutrients from garbage and dung. Garbage from vegetable or animal
origin is easy attainable in a city and attracts many animal species (Spirn
1984). It explains the great proportion of animals here which live
on organic garbage (Trojan et al. 1982, Ba?kowska et al. 1985). The
combination of a relative warm microclimate and the availability of an
abundance of food makes a city attractive for many insect species (Frankie
and Ehler 1978). A lot of them are most common in urban environments and
are therefore known as synanthropic species. The number of species which
live from organic substances (detritus) decreases to the centre of the
city, e.g. mites (Niedba?a et al. 1982). Part of these detritus-eating
species, which can live up in green areas in the centre occur here in great
densities. These species are synanthropic and have a wide distributional
range (J?dryczkowski 1981). Several carrion-beetles and carrion-eating
flesh-flies (Calliphoridae) belong to this category (Klausnitzer 1989,
Trojan et al. 1982, respectively). Species which larvae live from dung,
like dung-flies, profit from the presence of dogs. Especially the dung-fly
Scataphaga stercoraria can be very common locally (Draber-Mo?ko 1981a).
The proportion of synanthropic species of many animal groups increases
from the outskirts of Warsaw to the centre, as found in dung-flies, midges,
sow-bugs, millipedes and spiders (Górska 1981,Wegner 1982, J?dryczkowski
1981, J?dryczkowski 1982, Krzy?anowska et al. 1981, respectively).
The high density of some mammal and
bird species in the city is also explained by the availability of plenty
of food (Luniak 1980, 1981, 1983; Sukopp 1990). Especially garbage-, seed-
and fruit-eaters can find more than sufficient food. A city gives shelter
to relatively more omnivorous birds, like the magpie, jackdaw, rook and
starling. Some species, like the pigeon, have adapted their menu to city
life and changed from seed-eaters to omnivorous birds (Klausnitzer 1989).
A few birds of prey, like the peregrone falcon and kestrel, take
advantage of this situation and catch more birds in the city than in its
surroundings (Klausnitzer 1989).
Organic garbage and dung increases
the nutrient content of the soil. Consequently, certain plant species of
nutrient-rich soils will dominate at the cost of species of nutrient-poor
soils. The result of this process is that insects which are dependent of
plants from nutrient-poor vegetations will decrease in number and may eventually
disappear.
Enrichment of waters with nitrogen
and phosphorous may also have an impoverishing effect on flora and fauna.
It stimulates the growth of blue-green algae (Cyanophyta), which by decomposition
may cause oxygen deficiency with devastating effects for water life (Douglas
1983).
In general air pollution increases
towards the centre of a city (e.g. for Warsaw: see Wyrwick? 1995). This
becomes evident by considering the distribution of Lichen species which
are sensitive to SO2 (van Dam et al. 1986, Sukopp and Werner 1983). Air
pollution has also a negative effect on the vitality of trees. Decreasing
vitality of trees may lead to an increase of plant sucking insects, like
cicadas (Chudzicka et al.1979, Chudzicka 1986ab) and aphids (Pisarski and
Czechowski 1978). In contrast, leaf-eating insects, like caterpillars,
beetles and sawflies, occur generally in low densities in the centre of
a city (Pisarski 1976, Chudzicka and Skibi?ska 1994). The effects of acid
deposition on the soil fauna will be strongest in sandy areas with a low
content of organic material. Increasing acidification of the soil may result
in a strong decrease of earthworms and certain soil-arthropods, like springtails
and click-beetles (Sterzy?ska 1987, Nowakowski 1986). The quality of the
soil can also be influenced by trampling. Intensive trampling causes soil
condensation, which affect adversely the number of micro-arthropod species
(Garay and Nataf 1982).
In general disturbance of green areas
increases also nearer to the centre of a city. Consequently, species which
are sensitive to this factor will decrease in numbers and may in the end
disappear from disturbed areas e.g. soil- and bush breeding birds may disappear
from green areas which are visited intensively and birds which can sing
only softly will disappear from green areas near noisy roads (Luniak 1981,
1983). However, it seems that traffic noise is not a nuisance for most
bird species: the composition of the breeding population did not change
significantly over a distance of 1-400 m from a busy road in Warsaw (Luniak
1981).
A city is also a light-island. Illumination
can disturb animals which are active at night: it can hamper their foraging
(e.g. owls) and their orientation (e.g. night butterflies and migrating
amphibians). Disorientation may increase the risk to become a road victim.
Consequently, local populations of sensitive species may eventually
disappear (de Moolenaar et al. 1997).
The urban environmental factors,
mentioned above, can explain the decrease in number of species of animals
and plants from the outskirts of a city to the centre, as is apparent from
inventories which are carried out in several cities, like in Warsaw (Pisarski
1982a, among others). However, the decrease of number of species
to the centre can also be explained by the smaller size and more isolated
position of habitat patches of many species, due to the increasing density
of buildings and roads. The question arises in what respect urban biodiversity
will be affected by these factors.
Urban biodiversity
Urban biodiversity can be kept on
a high level by maintaining characteristic ecotope types (ecosystems) and
species. The maintenance of biodiversity should not be aimed at maximizing
the number of species, because many exotic species are introduced by man,
intentionally or accidentally. In general green urban areas, like parks
and gardens, differ from their potential natural vegetation. Amply half
of higher plant species which occur in the centre of a city are introduced
by man (Jackowiak 1994). Part of these species which established recently
can only live up to a couple of years in this new environment. No efforts
are required to keep them. The same can be said of weedy invaders.
Urban fauna is also strongly influenced
by man, not only by transporting parasites, fleas, lice and mites with
help of house animals, but also by importing products and accompanied invertebrate
species. Many of these species occurred originally in (sub) tropical areas,
but are nowadays distributed over the whole world. In a moderate climate
zone they can only survive the winter by using heated residences, store
houses and glass houses, like the eastern cockroach (Blatta orientalis),
the pharao ant (Monomorium pharaonis) and the house cricket (Acheta domesticus).
The aspiration to maintain a high level of biodiversity in the city does
not refer to such introduced species, but to the maintenance of characteristic
urban species which established without the help of man.
The number of characteristic species
which occur in a city will be greater the more ecosystems are present.
The same applies to a more restricted area, like a park. The larger the
park the easier it is to maintain different ecotypes: e.g. woodland, shrub,
grassland, marshland and water. Not only stenotopic species, which are
bound to a specific ecotype, can profit from ecotope variability, but also
species which need a combination of biotopes for their living and reproduction,
e.g. amphibians hibernate in woodland and reproduce in water. Variation
of composition and structure of the vegetation within a specific ecotype
contributes also to species diversity, e.g. some species, which can be
found in deciduous forests cannot be found in coniferous forests, and vice
versa and some species which can be found in dense woodland cannot be found
in open woodland, and vice versa. In general only part of a biotope type
is suitable as habitat for a species. For example the nuthatch needs old
deciduous trees (with holes) for building its nest, while the willow-warbler,
as a ground breeder, prefers young open parts of a deciduous forest with
some undergrowth.
For maintaining biodiversity at a
high level it will be necessary that areas with a specific biotope type
(ecosystem) should not become isolated to the extent that many indigenous
species can no longer exchange individuals. Therefore, several ecosystems
should be maintained in a proper setting of a network which makes the exchange
of individuals of characteristic species possible.
Green infrastructure
Water ways, like the river Vistula,
as well as roads and railways to other towns, are important determining
factors for the way a city expands. As a result the built- up area of Warsaw
has a lobate outline: the areas in between the built-up offshoots are relatively
green. These so-called green wedges can function as a corridor for the
dispersal of species from the outskirts to the centre of the city. However,
there are many barriers to pass, like roads and railways. This is especially
difficult (or impossible) for species which cannot fly. On the other hand
many of these species can disperse lengthwise along verges of roads and
railways. In this respect broad verges, if properly managed, can function
as habitat and corridor for characteristic species. Such verges are part
of the green infrastructure of the city, together with nature reserves,
parks, allotment gardens and cemeteries.
A proper urban green infrastructure
contributes to the biodiversity of the city and makes it more attractive
as a residence. Its contribution to the species richness depends upon the
size, quality and configuration of green areas. The presence of some
relief contribute to biotope differences, e.g. the west side of the old
river bed is marked by an escarpment, along which some parks are situated
on the transition of the low river terrace (with wet areas) to the 10 –
25 m high terrace of the old river bed: an ideal situation for the establishment
of a green corridor (Wolski 1999, Szulczewska and Kaliszuk 2003).
Part of the green infrastructure
of Warsaw is connected with a blue infrastructure. The Vistula river with
its adjacent riparian forest, the streams which flow into the river and
the oxbow lakes, which are remnants of the old river bed, are all parts
of its blue infrastructure. Species which are living in (or near) water
have a higher survival probability if these waters are part of a habitat
network, that is to say if they are not only suitable as habitat, but if
they can also function as dispersal corridor or stepping stone.
For estimating the survival probability
of species a distribution map of their habitat is needed. For good dispersing
species the total habitat area will be most important for their survival,
while for poor dispersing species number, size and configuration of habitat
patches will be crucial. In the next paragraphs area, connectivity and
quality of habitat patches will be discussed, successively.
Habitat area
In general the size of a local population
will decrease if the area of its habitat patch will become smaller. This
is most obvious in the case of territorial species which are evenly distributed
over the habitat patch. The smaller the population the vulnerable it will
be for temporary disadvantageous environmental circumstances (environmental
stochasticity) and for random changes in numbers, due to natality, mortality
and migration (demographic stochasticity). Moreover, loss of genetic information
may occur if the size of a population will become very small. Species
which need a large foraging area are most sensitive to habitat reduction.
For example a breeding pair of the tawny owl needs a territory of 20 -
40 ha, dependent of the amount of food which is available. Consequently,
the probability of the tawny owl to be present in a forest patch in Warsaw
appeared to be greater the larger its area (Wilcoxon: P<0.01; data from
Jab?onski 1991).
On theoretical grounds it is assumed
that the minimum viable bird population will be at least 20 breeding pairs
(Verboom et al. 1997). This implies that a viable population of tawny owls
should have the disposal of a foraging area of 400 – 800 ha. This cannot
be realized in one large habitat area, but only in a network of habitat
patches (woodlots), between which exchange of individuals is possible.
Local populations should become extinct one after the other if there should
be no immigration of individuals from other patches. In the case of the
tawny owl immigration is still possible if the distance between habitat
patches is less than 15 km (Mooij 1982). The regional persistence of the
species depends upon the proportion between the extinction of local populations
and the (re)colonization of empty habitat patches. The turnover of local
populations depends upon the size and connectivity of the habitat patches,
the dispersal capacity of the species and the permeability of the city
landscape for the species. It is argued that in a situation where the habitat
of a species is fragmented the total habitat area needed for the persistence
of such a metapopulation should be much greater than in a situation where
its habitat is not fragmented (Verboom et al. 1997). This will be especially
the case for species with a poor dispersal capacity.
Stenotopic species, which are fastidious
about habitat quality, are also sensitive to habitat reduction, because
a decreasing proportion between area and perimeter may affect its habitat
quality more severely, due to increasing deteriorating effects from its
surroundings. On the basis of distribution data of stenotopic forest carabids
it is concluded that a patch of woodland with a diameter of less than 80
m is unsuitable for them (Mader and Mühlenberg 1980). This may mainly
be due to negative influences from the surroundings on the microclimate
of the patch, although other factors may play a role as well, like pollution,
disturbance and dunging (by dogs). In Warsaw the butterfly Maculinea teleius
is threatened by habitat reduction and even by disappearance of its habitat.
The caterpillars of this rare butterfly are dependent on Sanguisorba officinalis
as food-plant and on nests of Myrmica-ants for their hibernation. The butterfly
is protected by law, but will disappear from Warsaw as the plan to build
houses on that location will be realized.
A reduction of habitat area will
generally result in a decrease of stenotopic species, while eurytopic species,
with a wide ecological amplitude, will become more dominant. In Warsaw
it is found that the species composition of different invertebrate groups
will shift to more common species, the smaller the area of the habitat,
as found in carabid beetles (Czechowski 1981a), cockchafers (Kubicka 1981),
leafbeetles (W?sowska 1981, 1986), noctuids (Wegner 1982, Winiarska 1986),
bees (Barnaszak 1982), wasps (Skibi?ska 1978, 1982a, 1986b), digger wasps
(Skibi?ska 1982b, 1986ac), parasite wasps (Sawoniewicz 1982, 1986), ants
(Pisarski 1982b), flies (Ba?kowska 1981abcd, Draber-Mo?sko 1981acd, Durska
1981, Trojan 1981b), midges (Wegner 1982) and harvest-spiders (Czechowski
1981b).
The conclusion is that habitat loss,
as a result of the reduction or disappearance of green areas, will lead
to the extinction of local populations of species and may even lead to
their disappearance from the city.
Habitat connectivity
Local populations of species fluctuate
in numbers. They will eventually become extinct, unless they can be rescued
by immigrating individuals from other habitat patches (Brown and Kodric-Brown
1977, among others). The immigration rate of a habitat patch depends upon
the reproduction and dispersal capacity of the species, the distance to
the nearest occupied habitat patches and the resistance of the area in
between (habitat connectivity). A species can persist in a city as long
as the extinction of its local populations can be compensated by the colonisation
of unoccupied habitat patches. This may be possible if the patches are
connected. In other words if they are part of a habitat network.
Species with a good dispersal capacity
can reach all parts of a city. The environment will select where they will
occur. Part of these species, which are fastidious for habitat quality,
may be useful as indicators for deterioration of the environment.
Among the good dispersers are not
only species which are able to fly, but also species which can be transported
by the wind over long distances (e.g. spiders and seeds of anemochorous
plants), species which can be easily transported by mammals (e.g. parasites
and seeds of zoochorous plants) and species which can be transported by
water (e.g. fishes and seeds of hydrochorous plants).
Most bird species can fly very well
and can reach all parts of a city. Even relatively poor dispersers, like
pheasant and partridge, are able to colonise isolated habitat patches in
a city: the pheasant was observed on fallow land in the centre of Warsaw
rather soon after the species was introduced in a park at 2.5 km distance
(Luniak 1983). Early in the morning, when there is less traffic, it will
be able for the species to cross roads. Most bird species, which are adapted
to the urban environment, like blackbird, house-sparrow, starling and jackdaw,
cross busy roads regularly in daytime. In Warsaw it is even observed that
a golden oriole crossed a busy road (Luniak 1981, Mabelis, unpubl.). This
species breeds rarely in cities and is rather shy. It is stated that the
distance of a park to the city centre has no significant effect on the
composition of its breeding population (Luniak 1983). The composition will
be mainly determined by the diversity of vegetation types (ecosystems).
However, the probability that a particular species will breed in a park
depends upon the total habitat area within.
Many flying invertebrates can disperse
very well through a city, especially small insects like aphids, flies,
bees, wasps, bumblebees, ant-queens, butterflies, dragon flies and macropterous
beetles. However, even these flying insects need green elements as stepping
stones and corridors for their dispersal. For example the cinnabar moth
(Tyria jacobaea) could disperse far into the city of London via road- and
railway verges, on which food-plants of the caterpillar grow (Plant 1994).
Also long-winged grasshoppers are dependent on linear habitat elements
for their dispersion in a built-up environment: the oak bush-cricket (Meconema
thalassinum) disperse along lanes with oak trees and the great green bush
cricket (Tettigonia viridissima) along road and railway verges with
a ruderal vegetation. An interruption of such a corridor by a road, canal
or bridge, is not an insurmountable barrier for most flying insects, although
it may reduce their dispersion.
Most species which live in dynamic
environments, like fallow land, have a good dispersal capacity. It means
that extinction of local populations of these species, due to succession
of the vegetation or to cultivation, can be compensated by the colonisation
of habitat patches, which become available elsewhere in the city. However,
Crowe (1979) found less flowering plants in more isolated fallow lands.
It appears that even the colonisation probability of good dispersing species
is affected by the distance between habitat patches. the park and the connectivity
of the park with breeding areas elsewhere.
For animals which can only walk or
creep, roads, parking places and water ways are important dispersal barriers.
Nevertheless, small mammals, like the house-mouse, the longtailed fieldmouse
and the brown rat can reach all parts of a city (Klausnitzer 1989, Sukopp
1990, Szacki et al. 1994). Even larger mammals, like the fox and the beech
marten, penetrate a city sometimes very deeply at night (Harris 1986, Klausnitzer
1989, Sukopp 1990). These species disperse best in city quarters with large,
joining gardens and in green strips with bushes. Also mice and voles, which
avoid paved roads (Mader and Pauritsch 1981), use such green structures
during dispersal (Liro and Szacki 1987, 1994; Szacki et al. 1994). A subterranean
species , like the mole, can disperse via road verges and joining gardens,
although structure and food supply of the soil is decisive if he will use
them (Haeck 1969). Moles avoid built-up and paved areas. Young ones may
cross an asphalt road now and then, but often they cannot reach isolated
gardens and parks (Sukopp 1990). Hedgehogs can disperse in a city via road
verges, parks and gardens. They cross traffic roads at night, which causes
many traffic victims. Consequently, they cannot reach isolated areas (Klausnitzer
1989). That holds also for the squirrel, which is active in daytime and
will cross a traffic road rarely (Klausnitzer 1989, Sukopp 1990). The result
is that less mammal species occur in the centre than in the outskirts of
a city.
Roads are also important barriers
for amphibians and reptiles. Consequently, reptiles occur generally only
in the outskirts of a city. However, amphibians can penetrate a city by
means of water courses and ponds (Mazgajska 1996, among others). This applies
especially to the common toad and the small water-salamander (Sukopp and
Werner 1987).
Invertebrate species, which cannot
fly, like brachypterous carabid beetles, generally will not cross a paved
road (Mader 1988). Consequently, most of these species don’t occur in the
city centre (Czechowski 1982). An exception is Carabus nemoralis, a forest
species, which occurs on several places in the centre of Warsaw. These
local populations may be relict populations, but it is also possible that
the species is able to penetrate the city via allotment gardens, road and
railway verges with a ruderal vegetation. The species was not found in
green strips, which are mowed or raked regularly (Czechowski 1982).
Not only nature reserves and parks
can be used by species as dispersal corridors and/or as habitat, where
they can reproduce, but also allotment gardens, cemeteries, road and railway
verges. For most invertebrate species which disperse over short distances,
the availability of habitat will be conditional for their occurrence in
such anthropogenic ecotopes. Therefore it is important to pay also attention
to the ecological quality of these green areas.
Habitat quality
Many ecotypes (biotopes) can be distinguished
in a city, ranging from artificial to natural. The quality of natural biotopes
generally decreases to the centre of a city, but this does not hold for
artificial biotopes: some species occur mainly in the city centre, where
old buildings are present, e.g. swift and kestrel (Luniak et al. 2001).
Biotope quality can be maintained (or improved) by proper planning and
management. A few biotope types will be discussed: woodland, grassland,
fallow land and water.
Woodlands are relatively stable:
they have a rather constant microclimate. In such an environment stenotopic
forest species can be expected. However, stability of woodlots may be less
in the centre of a city than in the outskirts, where it will be less intensively
used and managed. Less forest species were found in parks of Warsaw than
in its surroundings in case of carabid beetles (Czechowski 1979, 1982),
weevils (Cholewicka 1981), rove beetles (Klausnitzer 1989), julides (J?dryczkowski
1982), spiders (Krzy?anowska et al. 1981), springtails (Sterzy?ska 1982)
and several other groups. This is not only due to the more isolated position
of woodlots for species with a poor dispersal capacity, but also to disturbance
(e.g. by dogs) and intensive management of urban woodland. For example,
raking leaf litter intensively has a serious reducing effect on the species
richness of snails (Klausnitzer 1987, 1989), worms (Kasprzak 1986), julides
(J?dryczkowski 1982), spiders (Krzy?anowska et al. 1981), cicada (Chudzicka
1982, 1986), carabid beetles (Czechowski 1982) and parasite wasps (Garbarczyk
1982). Locally the species spectrum of carabid beetles, click beetles and
springtails shifted towards underground living species, due to intensive
measures of management (Czechowski 1982, Nowakowski 1982, Sterzy?ska 1982,
1987), while the spectrum of ant species shifted towards species which
live in trees (Czechowski et al. 1990). The poverty of invertebrates on
such localities is of great disadvantageous for many mammal species and
birds (Dickman 1987, Mulsow 1982, respectively; see Fig. 1). For example
the robin breeds only in parks of Warsaw where leaf litter was not raked
and birds which breed on the ground or in bushes prefer localities where
management is lacking or very extensive (Luniak 1981).
The removal of old trees and dead
wood, reduce also the forest fauna. Consequently, less species of longhorn
beetles were found in woodland within Warsaw than in its surroundings (Burakowski
and Nowakowski 1981). The forest fauna can also be impoverished by removing
bushes and herbs, as ascertained for spiders, carabid beetles, snails and
birds (Krzy?anowska et al. 1981, Czechowski 1982, Klausnitzer 1987, Luniak
1981, respectively). Situations with a gradual transition of woodland via
bushes to an open herb vegetation are richest in species.
Grasslands are often intensively
managed, especially in parks for recreation activities. Consequently, these
grasslands are poor in species. This holds also for grasslands between
buildings. Often these grasslands are mowed frequently, while raking leaf
litter will impoverish the fauna even more.
In parks with large grassland areas
it is possible to enlarge the number of species by applying different mowing
regimes on different localities: parts which are mowed frequently for intensive
recreation and playing and parts which are mowed once (or twice) a year
for enjoying flowers and insects. By keeping these differences in management
constant over years the number of species will increase, while the extinction
probability of their local populations will decrease, due to spreading
their extinction risk in space (Den Boer 1968, 1981). In short: local biodiversity
can be enhanced by striving for diversity in space, while keeping the management
constant in time. Flower-rich grasslands with a diversified structure are
richest in species. They should be managed (mowed or grazed) extensively.
Wastelands are dynamic: those areas
are generally only temporary available for species. New wasteland will
be colonised first by species with a good reproduction and dispersal capacity.
Many of these pioneers are eurytopic and live rather short. A great deal
of the pioneer-plants are annuals and most insect species are polyphagous.
In the coarse of time the proportion of perennial plants will increase
and consequently the number of food specialists among the plant- eating
insects. Such vegetations possess a rich entomofauna: at least 90 insect
species are more or less dependent on the creeping thistle (Cirsium arvense)
and related species (Zw?lfer 1965, Redfern 1983) and up to 100 insect species
feed on such common species as Artimisia vulgare and Tanacetum vulgare
(Klausnitzer 1968). Wastelands are good foraging areas for seed- and insect-eating
birds. They are also good habitat for the house mouse, longtailed fieldmouse
and common vole and this makes wastelands popular as foraging area for
birds of prey and owls (Klausnitzer 1987).
However, habitat quality of fallow
lands decreases if they are used as dumping place for rubbish. Especially
chemical waste can impoverish the fauna, e.g. heavy metals are harmful
to springtails and other saprophagous species (Sterzy?ska 1987). Also synthetic
substances are detrimental for the soil fauna (Kohsiek et al. 1994).
Railway emplacements contribute
also to the biodiversity of a city. The flower-rich vegetation on such
nutrient-poor soils offers suitable habitat for many insect species, like
digger bees and digger-wasps, under the condition that no pesticides are
used.
Water courses and ponds make the
urban environment more attractive. Their contribution to biodiversity depends
for a great deal on their water quality. Enrichment of the water with nutrients
should be opposed as much as possible. The presence of a border vegetation
will have a purification effect on water quality (Tourbier and Pierson
1976). Such a vegetation can be developed by making the slope of the border
locally flat. The nature quality of a border vegetation can generally be
enlarged by increasing border variation, in cross direction, as well as
in length direction.
Planning and management
Biodiversity in a city can be kept
at a high level by implementing a sustainable green infrastructure. In
Warsaw the concept of an Urban Natural System was presented in the Warsaw
Master Plan of 1992. The aim was to protect the most valuable green areas.
However, a comparison with the adjusted Plan of 2001 it reveals that some
green areas vanished (Szulczewska and Kaliszuk 2003). Protection of green
areas on the level of the city should be effective, but as remarked
in Warsaw Master Plans (1990, p.32): The problem of uncontrolled development
will be particularly difficult to solve correctly in Warsaw, where the
districts have an important voice in the matter.
Biotope patches which will become
smaller and more isolated will loose species, which are sensitive for habitat
fragmentation. In practice it will be possible to increase habitat area
of some target species by adjusting management. For example if the marshland
of Zakole Wawerskie would be managed properly, then the habitat area of
many characteristic and rare species will increase, and hence their survival
probability. In many cases it will also be possible to increase the survival
probability of species by improving the connectivity with other habitat
patches by planning and implementing corridors. The river Vistula and its
banks, which are partly covered by riparian forest and flower-rich meadows,
is a good example of such a blue-green corridor. If properly managed it
will have an important habitat and dispersal function for water and marshland
species. Another example is the planned system of linear parks along the
escarpment of the old riverbed in southern Warsaw (Scarpa Ursynówska).
It is also desirable to plan green corridors along streams, although such
ideas are not always implemented during the execution of a development
plan (for Bia?o??ka Dworska, see: Ba?kowska et al. 1985). In practice the
concept of an Urban Natural System will be part of “a coherent package
of objectives, principles and priorities for the desired quality of green
areas in the public domain“ (Meeus 1989). The question is how the implementation
of an ecological network of habitats, corridors and stepping stones for
the survival of species can be integrated in the planning of urban projects.
In many cases it will be possible to link the ecological network with a
recreational network, a traffic network and a network for proper water
management (Tjallingii 2003). For example broad verges along roads and
railways can be used as corridors by cyclists and/or pedestrians, as well
as by animal species (Mabelis 2004, Figs. 2, 3). Another example: wasteland
can be used by plants and animals as habitat and/or stepping stone, but
also by children to play and experience nature. The value of wasteland
for these functions is generally underestimated.
In order to keep biodiversity at
a high level monitoring of nature quality is necessary. A monitoring programme
can be part of an Ecological Policy Plan of the city. Monitoring has two
functions: to determine if policy targets concerning biodiversity are attained
(evaluating or controlling function) and to signalize unexpected changes
in environmental quality (signalizing function). For both functions it
will be necessary to relate measured data to possible causes of changes
in environmental quality (diagnostic capacity). So it is not enough to
collect data, but they should be analysed and interpreted (Fig 4). For
the interpretation it is desirable to measure also possible causal factors,
like pollution and recreation pressure. The following steps should be described
in a monitoring programme:
1. targets for measurements: data
for comparing them with standard values or to establish trends.
2. objects (e.g. species) and variables
(e.g. size of local populations), which should be measured.
3. sample strategies: localities,
method and frequency of sampling.
4. organisation structure: description
of responsible executors who will collect, analyse and interpret the data.
For the registration of trends in
environmental quality a map on which the distribution of different ecosystems
(biotope types) is given will be helpful for measuring the total area per
biotope type, as well as the size and configuration of biotope patches.
The quality of biotope patches can
be measured by describing the composition and structure of their vegetation
and in the case of water its clearness and the presence of a submersed
and emerged vegetation. The nature quality of biotopes can be kept at a
high level by maximizing the number of species which are characteristic
for that biotope type (Mabelis 2000). For the maintenance of characteristic
and rare species in the city it is desirable to collect yearly data about
the population size of a few representatives.
Changes in the size and configuration
of green areas will affect the liveability of a city for citizens and its
viability for plants and animals. Many green areas in Warsaw have no destination
at the moment. For city planners it will be useful to make predictions
about the consequences of the expansion of built-up areas for the
survival of species, because it will make it easier to make a choice between
alternatives. For this aim species can be selected which are easily detectable,
which differ in survival strategies and which belong to different functional
groups (e.g. reducers, herbivores, carnivores).
For people who are responsible for
the management of green areas it will be good if they realise the consequences
of methods used for the species richness of those areas: it may lead to
more environmental friendly decisions. A problem is that many green areas
in Warsaw are used by some people as dumping places for their rubbish.
It concerns not only verges along roads and railways, but also nature reserves.
In order to prevent pollution a good communication between responsible
authorities and citizens is necessary. Citizens could be stimulated by
authorities and non-governmental organisations (NGO’s) to care for nature
in the environment where they are living. However, this is only possible
if the City Council, local authorities and managers of urban green will
feel responsible for the maintenance of a high level of biodiversity in
the city.
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