Effects of livestock grazing on plant species richness

1
Effects of livestock grazing on plant species
richness

• Juha Pykälä
• Finnish Environment Institute
• 30.8.2002
• Management of meadows - Nordic and Baltic
  solutions
• Hotel Männikäbi, Saaremaa

2
Effects of grazing I

• The effects of grazing on plant species depend on
  the environmental conditions, species and breed
  of the animal, the grazing intensity and timing
  of grazing
• - decrease of plant biomass
• - decrease of vegetation height, decrease of size
  of plant shoots, increase of the number of plant
  shoots
• - in dry and mesic sites species of dry habitats
  benefit
• - warmer microclimate in summer

3
Effects of grazing II

• - decrease of nutrients in soil
• - soil pH may increase
• - compaction of soil due to treading
• - nutrient redistribution due to dung and urine
• - decrease of seed set
• - increase of dispersal of plant propagules
  (endo- and epizoochory)
• - animals select certain plants and avoid others
• - increase of structural heterogeneity of
  vegetation, because different vegetation types
  are utilized with different efficiency

4
Livestock grazing usually increases vascular
plant species richness

• Usually very prominent increase on small spatial
  scales (increase in species density)
• - mesic grasslands number of species / m2
• grazed 25-30, abandoned 10-15
• Less obvious increase on large spatial scales
• However, positive effects of grazing on species
  richness seem to dominate

5
Number of plant species in mesic grasslands in SW
Finland
6
More species benefit of grazing than suffer from
it

• Grazing usually increases plant species richness
  in all kinds of habitats (e.g. grasslands,
  forests, rich fens, shores).
• Earlier, when most of the northern European land
  area was influenced by livestock grazing, species
  sensitive to grazing may have strongly declined.
• However, this is poorly documented.

7
Effects of cattle grazing on Finnish vascular
plants restricted to shores of the Baltic Sea
(Pykälä 2000)
8
Response of plants of rich fens (n44) to
combined effect of mowing and cattle grazing
based on literature data. i increase, d
decrease, ? no data (Pykälä 2001)
9
Number of native and archaeophyte vascular plants
in Finland assigned into 10 different soil
nitrogen level (1-9 from very low to very high,
x indifferent) groups. Assignment following
Ellenberg et al. (1991).
10
Number of native and archaeophyte vascular plants
in Finland assigned into 10 different soil pH
level (1-9 from very acid to alkaline, x
indifferent) groups. Assignment following
Ellenberg et al. (1991).
11
Number of native and archaeophyte vascular plants
in Finland assigned into 10 different light level
(1-9 from very shady to full sunlight, x
indifferent) groups. Assignment following
Ellenberg et al. (1991)
12
Why grazing increases plant species richness I

• - decrease of the amount of nitrogen and
  phosphorus in soils
• - slow down the increase of trees and shrubs
• - increase of light intensity and temperature
• - decrease of dominant species
• - decrease of litter
• - decrease of plant size

13
Livestock partially compensates for the role of
extinct large herbivores

• - megaherbivores and several other large
  herbivores extinct before and after the latest
  Ice Age
• - it has been argued that savanna-like vegetation
  was common in (temperate) Europe (see Gerken
  Görner 1999, Vera 2000)
• - lack of evidence for that, closed forests
  predominated, but naturally open and half-open
  vegetation more abundant and longer-lasting than
  today, probably due to grazing and browsing of
  large herbivores and fires (Swenning 2002)

14

• Partial compensation of human suppressed natural
  disturbances such as fires, flooding and
  uprooting of old trees i.e. grazing has been
  very important for maintaining populations of
  many early successional native species (Pykälä
  2000).

15
Similarities between natural disturbances (fires,
flooding etc.) and livestock grazing and mowing

• - defoliation, decrease of biomass of living and
  dead plants and litter
• - decrease of trees and bushes
• - increased light availability
• - warmer microclimate
• - increased seed-dispersal distances
• increased landscape-level heterogeneity

16

• Livestock grazing do not replace natural
  disturbances (different ecosystem responses), but
  have been crucial for the survival of many plant
  species otherwise dependent on natural
  disturbances

17
Mitigation of the negative effects of
eutrophication and acidification on many plant
species (Pykälä 2000, 2001)

• - grazing decreases amount of N and P in soils
• - control of invasive species of nutrient-rich
  habitats, e.g. reed (Phragmites australis)
• - e.g. vegetation in a grazed seashore may be
  closer to natural than vegetation in a
  non-managed shore invaded by reed
• - grazing may increase soil pH
• - some plants persist on more acid soils, when
  areas are grazed

18
Grazing intensity

• Generally, intermediate grazing intensity is
  considered most beneficial to plant species
  richness.
• However, there is not enough data on the effects
  of different grazing intensities to evaluate, if
  this is correct.
• Furthermore, many plant species seem to benefit
  of high grazing intensities.
• For maintaining regional species richness areas
  with low, intermediate and high grazing
  intensities are needed.

19
Differences between different grazing animals

• Species numbers are increased by grazing of
  cattle, sheep and horses.
• There is a lack of studies comparing effects of
  different grazing animals on plant species
  richness.
• Probable order
• cattle gt horses gt sheep lt goats

20
Why cattle usually most suitable

• - less selective with regard to food than other
  grazing animals
• - grazes not so close to the ground than sheep
  and horses (vegetation height probably closer to
  optimal for high plant species richness)
• - cattle suitable for all habitats, sheep and
  horses avoid wet, soft bottom ground

21

• Number of species sensitive to sheep grazing is
  probably higher than that sensitive to cattle
  grazing.
• Sheep most suitable for dry grasslands and tree
  and shrub control

22
Nutrient enrichment decreases species richness in
grazed areas

• Main reasons for nutrient enrichment
  (eutrophication)
• - fertilization
• - supplementary forages
• - grazing of semi-natural areas together with
  cultivated pastures
• - air pollution

23
At present, in most grazed areas nutrient
enrichment has occurred

• Major problem, because most plant species occur
  in nutrient poor (low amount of nitrogen and
  phosphorus) soils
• In most of the presently grazed area grazing
  management is not positive to plant species
  richness due to nutrient enrichment
• It is crucial that nutrient enrichment is
  prevented when areas are grazed for maintaining
  biodiversity.

24
In Finland

• The present area of grazed semi-natural
  vegetation probably ca. 1 compared to that a
  century ago.
• The present area of mowed semi-natural vegetation
  less than 0.1 compared to that a century ago.
• This area appears to be below the threshold value
  for many species to survive.
• Thus, overgrown former grasslands should be
  restored on a large scale.