Download Influences of habitat structure, climate, disturbances and predation

SYLVIA 39 / 2003 SUPPLEMENT
Influences of habitat structure, climate,
disturbances and predation on population
dynamics of Black Grouse in the northern
Ore Mountains
Sven Herzog & Torsten Krüger
Dozentur für Wildökologie und Jagdkunde, Technische Universität Dresden, Pienner Straße 8,
D-01737 Tharandt, Germany; e-mail: [email protected]
Herzog S. & Krüger T. 2003: Influences of habitat structure, climate, disturbances and predation on population dynamics of Black Grouse in the northern Ore Mountains. Sylvia
39(suppl.): 9–15.
In the Ore Mountains (Erzgebirge), Black Grouse (Tetrao tetrix) populations increased during
the late 1980s and early 1990s, whereas at the same time a decrease of nearly all Central
European populations of this species was observed. Since the late 1990s, we have observed
a population decrease again. The scope of the present paper is to give a hypothesis explaining
this phenomenon on the basis of several factors being of potential influence, such as: (1)
change of habitat structure, (2) effects of weather and/or climate, (3) disturbance by man,
especially tourism, and (4) predation. Habitat structure was found to be a major factor influencing population dynamics. Additionally, a considerable influence might result from the
impact of weather, although a statistical assessment points to a merely minor relationship. For
single parts of the study area, we cannot exclude that also increased tourism does impose
stress on the Black Grouse population due to permanent disturbances.
Keywords: Black Grouse, Tetrao tetrix, population dynamics, habitat structure, forest decline,
climate, tourism, predation
INTRODUCTION
Right from the beginning, the 20th
century has experienced a decline of
Black Grouse (Tetrao tetrix) populations in almost all Central European
countries. Vast parts of the former distribution range (Glutz von Blotzheim et al.
1973) have been abandoned. According
to Popp & Müller (1966) an estimated
stock of Black Grouse in West Germany
of 13,000 individuals were distributed
over all regions of Germany still in 1966.
After almost 20 years Black Grouse had
become extinct in most parts of
Germany. In the remaining regions, as
for instance in Niedersachsen (Lower
Saxony) the population size had decrea-
sed from an appreciable stock of 8,900
to as few as 157 individuals (Klaus 1996).
This trend has even continued until now
despite intensive protective measures as
well as the reintroduction to the wild
and conservation programmes. Because
of this development Black Grouse is
threatened with extinction in almost all
countries of Central Europe.
An increase of Black Grouse population has been evident in the northern
part of the Ore Mountains (Erzgebirge,
State of Saxony) since the late
1980s (Krüger & Herzog 2000a,b). This
tendency was obvious until 1993 with
a subsequent decrease in Black Grouse.
Due to lack of data, however, a complete
9
Herzog S. & Krüger T. / Black Grouse in the Ore Mts.
presentation of the development of
Black Grouse populations in the Ore
Mountains since the beginning of the
20th century is not possible.
For more profound studies of the
causes of decline of the populations
living in the Ore Mountains precise
knowledge of the development of the
populations is needed. Therefore this
paper will give a first attempt for
a causal analysis of the development of
Black Grouse populations in the northern part of the Ore Mountains over the
period from 1980 until today.
METHODS
was performed by annual counts of
lekking cocks. Prior to this, lek counts
were carried out only in the area of
Satzung. The number of the lekking
cocks was doubled and regarded as the
available population (Klaus et al. 1990).
Lekking Black Grouse were counted in
all investigation areas at the same time
(1st May of each year) to exclude repeated counts of the same individuals.
Furthermore, lek observations were
conducted prior to and after the day of
counting, to get a more general picture.
All available data of reproduction as well
as observations outside the mating and
rearing period were examined and
evaluated, if possible.
Time and area of investigation
In order to prevent the results from
being influenced by short-term fluctuations of populations the period since
1980 was chosen for the investigations.
Occurrences of Black Grouse in the
Ore Mountains are largely confined to
the mountainous crests. Based on observations, four areas of Black Grouse
occurrence can be differentiated for the
northern part of Ore Mountains, located
on German territory:
(1) Fürstenau (3,600 ha): Fürstenwalde
up to Mt. Kahleberg, along the
border
(2) Deutscheinsiedel (900 ha): along the
border east of Deutscheinsiedel
(3) Satzung (700 ha): bounds and adjacent state forest along the border
(4) Cunnersdorf (c. 30 ha): village of
Rosenthal along the border
Detailed descriptions as well as maps of
the study areas are shown by Krüger
(2004).
Assessment of Black Grouse
population size
Since 1987 the assessment of the Black
Grouse stock in the Ore Mountains
pertained to all investigation areas and
10
Assessment of influencing factors
The medium-term change of habitat
structure was evaluated by a series of
aerial photographs as well as data from
forest management planning (Forsteinrichtung). Long-term habitat changes
which may substantiate the decline of
the species prior to the investigation
period have been studied with respect
to the development of the bog areas as
the most significant type of open landscape in the region.
As parameters evaluating the influence of weather and climate, we chose
diurnal data of minimum and maximum
temperature, precipitation and the
height of snow cover.
The effects of disturbance by man
have been studied using especially
tourism characteristics as numbers of
visitors, overnight-stays and hotel capacity around the study areas.
For assessing the potential influence
of predation, data referring to hunting
harvests of relevant species as well as
forest damages by mice as an indicator
of alternative prey (Angelstam et al.
1984, Wegge & Storaas 1990) have been
analysed.
SYLVIA 39 / 2003 SUPPLEMENT
To evaluate the hypothetic correlations between the single factors and the
development of Black Grouse populations (measured by the number of
lekking cocks, see above), bivariate
correlation analyses have been carried
out using SPSS software.
RESULTS
Development of the Black Grouse
population in the northern Ore
Mountains since 1980
In the northern Ore Mountains region
most of Black Grouse disappeared in
the period between 1965 and 1980
(Saemann 1987, Klaus 1991, 1993). At the
beginning of the investigations in 1980
it was merely one area (Satzung) that
was significantly colonised. In the
subsequent years only few individuals
could be occasionally recorded
(Table 1). In some years any signs of the
occurrence of this species were missing
at all. Starting from the mid-1980s,
a slight increase of the population was
observable, occasional sightings increased, and first courtship counts in the
investigation area yielded good results.
Already in 1987 fifteen courting
cocks could be confirmed again in the
Ore Mountains. Unfortunately, at that
time an intensive abundance assessment was not yet possible. Almost
complete data for the four northern
(German) subpopulations exist since
1988 or 1989, with the exception of 1991,
when no courtship counting was
possible in Fürstenau investigation area.
The development of the subpopulations
is illustrated in Table 1; the Black Grouse
stock of the German part of the Ore
Mountains reached its maximum in
1993 with approximately 120 specimens
(57 courting cocks). However, yet in
subsequent years, starting from 1995,
Table 1. Results of courtship counts in the Ore Mountains.
year
Altenberg
Fürstenau
Satzung
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1980
14
6
10
16
16
22
24
18
–
16
–
–
8
7
5
7
11
10
17
10
10
11
10
8
6
3
2
2
2
3
4
2
2
Deutscheinsiedel
4
3
2
6
6
11
20
20
18
15
15
8
4
2
0
Cunnersdorf
0
2
4
3
2
0
total
28
14
19
33
34
54
57
50
29
39
23
14
15
4
2
2
3
4
2
2
11
Herzog S. & Krüger T. / Black Grouse in the Ore Mts.
a rapid decrease was apparent. The individuals were no more observed so
frequently except for the courtship, and
also detection by finds of, e.g., prey residues and droppings declined in the
subsequent years. The actual lowest
number was recorded in 1998 with 14
courting cocks. The development in the
four northern areas, however, took
a somewhat different course.
According to the oral communications of local forest executives and also
according to our observations, which
were not based on exact numbers, the
highest stock of the Black Grouse was
reached at the end of 1980s and at the
beginning of 1990s. Later on the quantity of this species decreased dramatically.
Concerning the investigation areas in
the Ore Mountains, a total of 43 proofs
of reproduction could be obtained; 33
of which refer to the time period from
1980 to 1998. These proofs include
8 finds of nests with eggs and 35 observations of broods.
Based on the available data, reproduction in Black Grouse habitats on the
German part of the mountains can be
proven for a long period. The average
clutch size was 8.1 and the number of
chicks per brood 5.5, which corresponds to values published in literature
(Klaus 1990).
The development of vegetation
shows a typical development during the
last decades. Vegetation heights as well
as crown densities increased nearly
continuously since the reforestation of
the clearcuts after forest decline from
the mid-1970s to the mid-1980s.
Especially the areas with vegetation
heights below 1 m, which are most rele-
Table 2. Results of correlation analysis (effect: + positive, – negative, significance: * significant,
** highly significant).
potentially influencing factor
vegetation height <0.3 m (class 1)
vegetation height 0.3–1 m (class 3)
vegetation height 1–4 m (class 3)
vegetation height >4 m (class 4)
change from class 1 to class 2
change from class 2 to class 3
change from class 3 to class 4
days with snow height >30 cm
rainfall (breeding period)
number of rainy days (breeding period)
end of frost period
average temperature (breeding period)
average temperature (July)
rainfall (July)
number of rainy days (July)
tourist overnight stays in the study areas
hotel capacity
hunting bags (Red Fox, Saxony)
hunting bags (Red Fox, study areas)
hunting bags (Wild Boar, Saxony)
hunting bags (Wild Boar, study areas)
mice damage
12
proportion of variation
explained by the factor
effect
significance
71.8%
64.2%
80.1%
52.4%
70.8%
–
–
–
–
–
–
48.9%
–
–
–
80.7%
94.9%
81.9%
–
48.7%
–
–
–
–
+
–
–
0
0
0
0
0
0
+
0
0
0
–
–
+
0
+
0
0
**
**
**
*
**
n.s.
n.s.
n.s.
n.s.
n.s.
n.s.
*
n.s.
n.s.
n.s.
*
**
**
n.s.
*
n.s.
n.s.
SYLVIA 39 / 2003 SUPPLEMENT
vant especially for reproductive behaviour, decreased to less than 10% of the
total area.
The results of correlation analysis are
summarised in Table 2.
According to this, habitat structure is
assumed to be the most important
factor with respect to the population
development.
Concerning the factor weather/climate, a slight coincidence between
mean temperature during the breeding
period and (perhaps) between snow
heights over several years and population density may exist.
Disturbance by tourism is correlated
to population development also for
single parameters.
Assessment of the influence of
predation is also not possible from the
present database. We found no clear
hypothesis for the results from hunting
statistics as an indicator for population
development of the most relevant
species.
DISCUSSION
As we can see from the results, Black
Grouse in the northern Ore Mountains
shows an increase during about one
decade, whereas before the species
came near to extinction and, actually,
a decrease is observable again.
Although the species forms a metapopulation colonising the southern part
of the Ore Mountains as well, with
a more or less intensive exchange of
individuals and although the assessment of population development
started later for the southern part of the
area, these findings call for an analysis
of the relevant factors.
The medium-term changes of habitat
were evaluated using aerial photographs and data of forest management
planning. Long-term habitat changes,
which best substantiate the decline of
the species prior to the investigation
period, are actually studied with respect
to the development of the bog areas.
Both the medium term change of
habitat structure due to the landscape
development after the large-scale forest
die-backs and the long term situation
(especially in the northern part of the
Ore Mountains) of drained and cultivated peat bogs seem to be major
factors influencing the population
dynamics of the species. Similar results
for the southern (Czech Republic) part
of the Ore Mountains have been obtained by Bejček et al. (1998).
For Norwegian forests, there is also
the experience, that forest management
influences the brood habitats of Black
Grouse especially by clear cuttings in
a positive way (Børset & Krafft 1973).
At the present time of evaluations it
can be shown that 90% of the former
bogs have been drained, afforested or
subjected to peat utilisation since 1830.
Consequently, the habitat of Black
Grouse has diminished markedly due to
anthropogenic impacts. This development was reverted from 1979 onwards
when extreme forest damage began to
occur on an area larger than 10,000 ha
on the German and 50,000 ha on the
Czech side. Due to forest decline over
vast stretches of territory and the subsequent clearance of the areas, large-scale
Black Grouse habitats turned up again.
The available observations and population assessments indicate that these
areas were colonised in a relatively short
period of time. Depending on the size
of the damaged areas and the forest
management performed there, the
cleared areas were grown with trees and
the gaps filled over the past decade
again, which caused Black Grouse to
vanish from these sites. This is clearly
apparent from the habitats at Deutscheinsiedel. Still today other areas comprise
13
Herzog S. & Krüger T. / Black Grouse in the Ore Mts.
big potential habitats (Altenberg) for
Black Grouse, but also here the populations diminished. In this region there
are a considerably increased number of
visitors, especially the 1-day tourists, as
well as continual checks by the Federal
Border Guard (Bundesgrenzschutz) that
could impose stress on the habitats due
to disturbances. Significant explanations of the influence of predators are not
possible on the basis of the hunting
statistics.
Based on the available results,
a considerable influence on the development of Black Grouse populations
must actually be ascribed to the impact
of weather, although a statistical assessment, using daily data of minimum
temperature, precipitation and height of
snow cover during the winter season
points to only minor relationships.
Regarding the effect of disturbances,
changes in road density and the signposting along hiking trails as well
tourism characteristics (visitors and
overnight stays) in the investigation area
are used for analysis. Here, the overnight stays are assumed to be a factor of
potential influence.
For assessing the possible influence
of predators, data referring to hunting
harvests of the relevant wildlife species
are available. Based on this data (and
keeping in mind that this database may
be weak), we found no clear and unequivocal influence of predators.
However, although the major part of
the population fluctuations of Black
Grouse are to be explained by influences of the changing habitat, the other
factors have to be taken into consideration for future studies.
Especially the predator situation
remains quite unclear and has to be
investigated by fine-scale telemetric
studies including also the potential
predators.
14
SUMMARY
The Ore Mountains were seriously influenced by forest decline during the
1970s and 1980s. This phenomenon, as
a consequence, resulted in the occurrence of huge clearcut areas along the
top of the mountains, being reforested
with immission-tolerant tree species.
The present paper is founded on the
hypothesis that population dynamics of
Black Grouse was critically influenced
by this landscape development. The
investigations are based on the results of
Black Grouse population monitoring
programs in the Ore Mountains. This
monitoring
programs
summarise
a period of 19 years in the German part
of the study area. In the Czech Republic,
the data have been recorded over
a period of 6 years. The data are discussed with special respect to the changes
in the habitat structure due to forest
decline processes.
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15