Disturbances

From an ecosystem point of view disturbances can be natural or human caused. We are mostly talking about herbivores in this lecture

Natural

• Climate and weather conditions
• herbivores
• Fires

Human caused

etc

Natural disturbances can be directly/indirectly modified by human actions

Response to disturbance

• Can cause a change in biomass or species composition
• A strong disturbance can reach a tipping point and cause a regime shift, such as forest to tundra. This could even be a permanent change
  • Birch forest transitioning to a conifer forest due to increased herbivory - might be not possible to return

Disturbance dynamics

• Most commonly researched in North America
• Each species is adapted to different levels to live in an environment shaped by disturbances
  • Related to the environment that prevailed during the species' evolutionary history and its typical changes can help us understand how it will interact with climate change
  • Reindeer lichen is an important example

Carbon stock

!ECGS-039_Vegetation_TV, p.5

Disturbances and diversity

• Peak species diversity is in ecosystems with medium frequency of disturbances

Recent changes in northern vegetation

• In northern Norway, vegetation communities have changed in last 30y
• Lichen dominanted heaths have reduced, which could be due to increased reindeer herds

Tømmervik, H. et al. 2004. Vegetation changes in the Nordic Mountain birch forest: the influence of grazing and climate change. Arctic, Antarctic, and Alpine Research 36: 323-332

• Remote sensing based analysis can measure this

Reindeer (caribou)

• Tundra forms are migratory, they move higher for the summer and in the winter lower or southern or towards forest
• Domesticated tundra caribou have been moved further south than their natural range by caribou herders, resulting in ecosystem changes
  • This is because of fencing. They don't migrate the same as the historical movement
  • On the Russian side of the border they are more similar to their natural migration with herders (not fenced), similar to how it was in Finland 100 years ago
• You can see reindeer tracks during migration are visible from the air. This is a migration path in an isthmus in between two lakes:
  !ECGS-039_Vegetation_TV, p.9

Reconstructing decline of lichen cover on Finnish-Norwegian border

• Finland-Norway border fence built in 1950s, preventing reindeer migration
• Different reindeer herding practices show a marked difference in lichen cover between the countries

Wallenius T., Bjerke J., Erlandsson, R., Kolari T., KumpulaT., RäsänenA., Tahvanainen, T.; Tømmervik, H., Villoslada, M., Virtanen T. 2025. Reconstructing the historical decline of lichen cover across the reindeer fence of the Finnish-Norwegian border. Ambio 54(10): 1683-170

• In this image, the browner side is Finnish and whiter side Norwegian:
  !ECGS-039_Vegetation_TV, p.11
• In this place where the image was taken, they only come to this area in the winter, wheras in Finland they are fenced in a smaller area and are there year round so they eat more reindeer lichen

Methods

• Using old air photos, Landsat, Sentinel, UAV images,
• convert to black and white rasters
  • Reindeer lichen is easy to identify on black and white imagery since its white
• lichen cover and biomass estimates from 400 plots to calibrate regression model
• Used to make biomass maps

Results

• Lichen cover decreased in both countries, but especially in Finland
• This can not be accounted for only from herbivory, probably trampling in the summer is an important factor
• A MSc thesis is also looking at vegetation changes - mosses have increased. But we don't have the full results yet, still waitingn on publication
  !ECGS-039_Vegetation_TV, p.14

Insects and Climate

Important bug facts

• They are ectotherms so they need very specific temperature optima
• They have very high fecundity and short life spans (Life History Adaptations)
• Good dispersal capacity
  This means they have potential for quick population responses to climate variations. They can grow massively if there is no factor limiting their growth.

How much does climate determine and regulate insect population size?

E.g. is insect growth density-dependent or density independent?

• Density dependent and other climatic factors both influence abundances by varying intensities at different times or sites
• Climate also modifies the relative importance of biotic effects
• Weather may synchronize insect abundances and outbreaks over large geographic areas
  • This can be good weather that results in outbreaks or bad weather that wipes them out
• Since each species has very specific conditions and reproductive patterns (20,000 species in Finland) it is diffcult to give general predictions
  • Every year the Finnish lepidoptera conference adds hundreds of new species for Finland - whether they are immigrating here or newly described
  • So we can describe general patterns of categories of species, or focus on certain species as a mdoel
• An obvious pattern in biogeography is a decrease in species number towards the poles. Exceptions:
  • Aphids and ichneumonids are most abundant in temperate regions
  • sawflies are most abundant in cool regions

Insect life cycle patterns (Life History Evolution)

1. Overwintering stage (egg, larvae, pupae or adult) - is it above or below the snow?
   • If they are under the snow, if there is enough snow, winter temperatures are not so important
   • Maritime arctic climates are difficult for insects (Iceland or Svalbard) because there isn't enough snow to overwinter underneath
2. Voltinism: Several generations per growing season
   • larva can take two or three years to get through the cycle in some species
3. Solitary vs. gregarious
   • Some group organisms can use Cooperation defensive techniques for predators

Insect diets

1. Hervibores
   • Can be mono-, oligo- or polyphagous - specific diets can limit them but also allow them to be very successful
2. Parasites/parasitoid and predators (Exploitation)
   • Parasitoids kill the host, parasites cause problems for the host but don't kill them
3. Detritus eaters

Feeding habits

1. Sucking of liquids, like in aphids, mosquitoes, true bugs.
2. Chewing (moth and butterfly larva, most beetles, hymenoptera)

Simplified model of insect response to environmental warming:

An interesting chart from Bale et al:^[1]
!ECGS-039_Vegetation_TV, p.19

Main conclusions

• Species with large ranges are less affected by climate change
• In temperate reasons, temperature mostly affects winter survival
• At northern latitudes higher temperatures extend the summer, including thermal budget for growth and reproduction
• Herbivores show a number of distinct life-history strategies to exploit plants with different growth forms and strategies, affected differently by climate warming.

Butterfly and moth distributions and climate

• Usually these follow isotherms
• Range is typically limited by temperature in the colder regions
• When climate warms, species distribute polewards and to higher altitudes
• Range shifts are more pronounced in northern europe than southern europe

Finnish Macrolepidoptera

• Tarmo's 1999 study mapped lepidoptera diversity in different bio zones of Finland - diversity decline about 50 species per 100km to north
• In 2019 "Red Book"(?) 18% of lepidoptera species were threatened, open tundra habitat species were 33% threatened.

Bark beetle damage in Alaska

• In Alaska and Canada there has been lots of forest insect damage, especially spruce bark beetle which are easy to map with remote sensing or aerial survey
• Might be coming soon to Finland, there is some damage in Russia close to the border
• Kenai peninsula has really high spruce beetle mortality in th 1990s
• In Finland we have fewer spruce species, but not so much damage, we will see what happens

Moth outbreaks in mountain birch

Background

• Mountain birch Betula pubescens ssp. czerepanovii is the forest-line forest in Fennoscandia
• The auntumnal moth causes large birch deaths
• Observations over 100 years in Finland, also studied in Sweden
• After trees are killed, reindeer are really good at eating the new sprouts and preventing the birch from regrowing
  • Some experiments with fencing or restricting reindeer from accessing birch forests killed by beetle

This interesting image shows different bands of moth attack, related to wind and temperature. The dead birches at the very bottom had overwintered eggs protected by snow, but in the lee of the hill there is not enough snow so the birches can live. A GIS based model was developed for this^[2]
!ECGS-039_Vegetation_TV, p.31

Life cycle

• Eggs over-winter under the snow
• Eggs are laid individuallt to birch twigs, especially in betweenlichens
• Larvae emerge during bud burst, and they can disperse by balooning on a string, spread by wind
• Larvae need young leaves to grow well for 2-8 weeks depending on temperature and food quality
• They are polyphagous and can eat other pants as well
• They pupate in the ground in the moss or humus layer in a loose cocoon until autumn
• Adults flyin autumn, most active at night, do not feed
• The females are weak flyers and just lay their eggs wherever (limited dispersal capacity)
  • They are almost wingless, this image shows a male and a female:
    !ECGS-039_Vegetation_TV, p.83
• Egg number depends on the size of the female. 1mg weight adds 2.6 eggs on average, can vary from a couple to 300

Temporal variation in outbreaks

• Outbreaks in 9-10 year cycles
• High phase lasts typically 2-3 years where leaf feeding can be 100%
• Cycle amplitude changes regionally and between cycles.

Outbreaks and age of birch

• The effect is strongest on very old (100y) birch stands, since they prefer to lay eggs on old lichen-rich twigs
• The 100 year birch cycle thus affects the amplitude of the 10 year moth cycle

Why are the bad outbreaks only in the north?

• These moths are widely distributed but the bad outbreaks are only in Northern scandanavia
• It's thought that because of lower species richness here. Southern regions have more generalist predators to prevent the outbreaks

Causes of population cycles

• This can occur if delayed density-dependent factors effect the population
• Potential delayed density dependent factors

Delayed inducible resistance

• It seems that birch leaves decline in quality a few hours after being eaten by larvae: Delayed Inducible Resistance
  • Is this a biological response? Or just the larvae eating all the nutrients right away
  • In experiments this can effect to following generations, but this was not a significant factor affecting population collapse at the end of the cycle
• It seems that population collapse might just be because they run out of food

Predators

• Not likely because they have rapid responses, so they cannot cause the cycle

Parasitism

• Parasitoid wasps can only cause the dynamic if they are specialists

Conclusion

• It seems like parasitism is the cause (parasitoid wasps): the rate increases following moth population increase and is near 100% near the collapse

Birch recovery

• Recovery has been poor and a lot of the affected areas have turned to treeless tundra
• Liekly due to intensive reindeer grazing
• There have been some plans to arrange reindeer grazing to aid recovery, but some say its better to keep it as tundra
  • It might be that climate change is going to increase the birch population anyways, so reindeer are kind of keeping it in check

GIS model to predict this based on field study

^[2:1]
Linear regression based on temperature, altitude and flow accumulation, and predict how this will occur in future climate scenarios
!ECGS-039_Vegetation_TV, p.58

Ecosystem effects

• After they eat the birches, they are generalists so they eat all the ground vegetation too
• Sometimes this can result in graminoids growing due to increased fertilization from all the dead leaves
  • Incerased mosses and Festuca sedge was observed in Kilpisjärvi region

Conclusions

• outbreaks have spread to new areas due to watmer winters
• Both winter moth and autumnal moth are causing more outbreaks
• Severity of damage might decrease due to better recovery of birches in warmer summers and more active parasitoids and predators