Lecture 6: Habitat loss, fragmentation and edge effects Flashcards
Lecture aims
1.Define land conversion vs. land degradation
2.Discuss the impacts of habitat loss and fragmentation
3.Discuss biophysical impacts of land-use change in terms of 3D structure and microclimate
Land conversion/ land degradation
see globalforestwatch.org,map
*Globally, tree cover remains widespread (pale green)
*But intact forest landscapes without human impact (dark green) are much smaller in area
*What is going on in these degraded forest landscapes?
Hansen et al. (2013) Science
Potapovet al. (2017) Science Advances
Human Footprint Index (HFI)
*HFI is an aggregate index combining many different forms of human impact, from agricultural land to built environment to roads and railways
*widespread intense human impact seen as orange & red
*However, there is also a lot of blue and bluey-green, where there is often still human impact but it is relatively less
See Venter et al 2016 Nature Communications
What makes a forest?
There are many different ways we might conceive of forests
There is no exact definition because it depends on how forest is used and how it is detected
Definitions changed as emphasis moved from timber to conservation to carbon sequestration
We also changed how we measure forest cover – used to be measured with ground survey plots, now we use satellite imagery
See figures from Chazdon et al 2016 Ambio
Approaches to forest management
See figure from Chazdon et al 2016 Ambio:
1700-Now
Timber management: Forest lands should be managed primarily to sustain timber production.
Forest definition: land bearing vegetative associations dominated by trees of any size (FAO 1953). Temporarily
unstocked areas and plantations are considered forest. In 1990 this definition changed to land with tree crown
cover (or equivalent stocking level) >10 % and area of >0.5 ha with trees > 5 m at maturity (FAO FRA 2000).
1960’s - Now
Conservation: Intact forests should be protected to conserve biological diversity. Forest management should
minimize ecological impact and maximize ecosystem functions and species interactions.
Forest definition: a dynamic complex of plant, animal and micro-organism communities and their abiotic
environment interacting as a functional unit, where trees are a key component of the system (CBD).
1988-Now
Climate change mitigation: Forest conservation, reforestation, and afforestation can reduce global warming
through reducing carbon emissions sources and increasing carbon sinks.
Forest definition: a minimum area of land of 0.05 - 1.0 hectares with tree crown cover (or equivalent stocking
level) of more than 10 - 30 per cent with trees with the potential to reach a minimum height of 2 - 5 meters at
maturity in situ (UNFCCC 2001).
2010-Now
Earth stewardship: Forests are complex adaptive systems whose resilience is intimately linked with society.
Ecosystem services of forests are important for poverty alleviation and sustainable development.
Forest definition: a complex system composed of heterogeneous assemblages of individual agents (e.g., trees,
animals, humans), closely interacting through flows involving markets, goods and various other ecosystem
services (Chapin et al. 2010)
Forest conversion = going from forest to non forest state
examples:
forest to pasture
forest to cropland
forest to goldmine
Clearly this is an intense and immediate form of land-use change as we have completely lost the habitat that was there. Agriculture is the main driver of wholesale land conversion
Forest degradation = still a forest but in a degraded state
The 38% of forests that remain globally are often in a degraded state
There are many ways in which a forest can become degraded – any loss of ecosystem function or resilience could be seen as degradation.
These are common and widespread forms of forest degradation:
Selective logging
Fragmentation
Defaunation
Defaunation
See figure from - Gardner et al. (2019) Nature Communications
Removing animals for meat or the pet trade results in reduced seed dispersal
Natural forest:
High fauna (natural) vertebrate community
Primates, caviomorph rodents and large-gaped birds disperse large-seeded trees
Defaunated forest:
Low fauna vertebrate community
Regeneration of large-seeded tree species is reduced, being replaced by small-seeded or abiotically dispersed tree species
Deforestation
^Permanent tree removal as the goal
Deforestation:
Complete removal of trees for the conversion of forest to another land use such as agriculture, mining, or towns and cities. It results in a permanent conversion of forest into an alternative land use.
The trees are not expected to regrow.
Commodity-driven deforestation
Conversion of forests to other land uses such as agriculture (including Oil palm and cattle ranching), mining, or energy infrastructure.
Urbanization
Conversion of forests to towns, cities and
urban infrastructure such as roads.
Thinning of the canopy - a reduction in the density of trees in the area - but without a change in land use.
See OurWorldinData figures in notes
Forest degradation
^Allowing trees to regrow
Forest Degradation
The changes to the forest are often temporary and it’s expected that they will regrow.
Shifting agriculture
Small-scale clearing of forest for subsistence, slash-and-burn farming
Forestry products
Logging and management of forests for
products such as timber, paper and pulp.
Wildfires
Natural fires, or management
through deliberate burning
See OurWorldinData figures in notes
impacts of habitat loss and fragmentation
Habitat loss and fragmentation do not exist in isolation. While habitat loss in one area has a more dramatic biodiversity impact on that specific area, its effects radiate into neighbouring habitat
Fragmentation = the division of habitat into smaller and more isolated fragments separated by a matrix of human-transformed land cover
*Unless habitat loss is complete, it causes fragmentation of remaining habitat
Habitat is not only smaller in area, but it becomes disconnected and has a different geometry with relatively more edge habitat
More edge than interior in fragmented habitat which means more of the habitat is degraded and not suitable to support as much biodiversity
See Haddad et al 2015 Science Advances:
*Because habitat loss and fragmentation effects are difficult to disentangle, studies have been designed to try to tease apart the different components of fragmentation
*Reducing habitat area causes declines
*As does keeping area the same but increasing patch isolation
*As does keeping area the same but increasing the amount of edge
Why does isolation of habitat patches matter? Metapopulation dynamics
Metapopulation dynamics
Disconnected patches can no longer exchange individuals and genetic material
See figure from: Ancrenaz et al. (2020)
https://doi.org/10.1101/2020.05.17.100842
*In this example from a preprint looking at orangutan metapopulation dynamics, there are three populations with males making lost distance movement between them
*If populations were too far apart or the matrix was too inhospitable, the populations would become completely isolated – resulting in genetic isolation leading to inbreeding
Why does isolation of habitat patches matter? Landscape connectivity
Species need to be able to move across a landscape for a variety of reasons
*The more isolated patches are, the more difficult it is for species to move across landscapes
*Relevant for many species, at different timescales
in species with:
large home ranges
seasonal migrations
range shifts under climate change
See: Ramalho et al. (2023) Biological Conservation
See also: the nature conservancy map:
www.maps.tnc.org/migrations-in-motion/#3/19/-78
^Different colours refer to different taxa.
These are the pathways we expect species in tropical south America to follow to track their preferred climate. Usually headed uphill and poleward. Areas of resistance such as cropland and urban areas are included in this mapping to predict future movement.
Whether they can actually do this depends on how connected the start point is to the end point – they may not make it all the way
Climate connectivity = landscape connectivity along a climate gradient
Where should a species go to avoid warming under climate change?
^ It depends on how species move & what the barriers to movement are
see figures in notes
Due to lack of foresight most species will be pushed towards the easiest route e.g. the 20 degree species moves to the nearest small mountains and then moves up the mountain so would be fine for a while. The 16 degree species would be likely to move up the mountain and end up in an escalator to extinction situation as the peak becomes inhospitable over time.
Over 62% of tropical forest area (~10 million km2) is already incapable of facilitating range shifts to analogous future climates
^ Senior et al. (2019) Nature Climate Change
Figure in notes: The analysis used to map climate connectivity across tropical forests
*Red means that there is no pathway to somewhere cooler under climate change, blue means there is a pathway to somewhere cooler
*Across the tropics there is a mixture of both, but large areas with no climate connectivity
Why does the amount of edge habitat matter?
*Because habitat loss and fragmentation effects are difficult to disentangle, studies have been designed to try to tease apart the different components of fragmentation
*Reducing habitat area causes declines
*As does keeping area the same but increasing patch isolation
*As does keeping area the same but increasing the amount of edge
Edge effect = physical & ecological changes associated with habitat edge
As a result of forest loss, 70% of remaining forest is within 1 km of forest edge
What happens in edge habitat?
*Climatic differences at the edge – less vegetation means more solar radiation, more wind and less evapotranspiration (lower humidity)
*Leads to vegetation changes – higher tree mortality, more pioneer species, more likely to be colonised by invasives
See Nunes et al 2023 Nature communications: study using terrestrial laser scanning to identify tree shape
^Vegetation structure changes at habitat edge and changes in the actual shape of trees occurs. Resulting in a loss of carbon storage in edge habitat.
*Changes in climate and soil leads to changes in both overall vegetation structure and tree allometry
*Both of these contribute to a loss of above-ground biomass, which means loss of above-ground carbon
20% of the world’s remaining forest now found within 100 m of an edge, 50% within 500 m and 70% within 1 km Haddad et al. (2015) Science Advances
The distance of edge effects varies - it can penetrate inwards for over 500 m in some cases.
Parra-Sanchez & Banks-Leite (2020) Scientific Reports
Threatened forest interior species reach peak abundances only at sites farther than 200-400 m from forest edges Pfeifer et al. (2017) Nature
Biophysical impacts of land-use change in terms of 3D structure and microclimate
Edge effects are one way in which land-use change impacts vegetation structure and microclimate
see figure in notes:
Edge effect impacts on tree structure :
-thinner branches
-thinner trunk
-Narrower crown
-Reduced wood storage
-lower vertical crown size
-lower horizontal growth
Climate term definitions
(De Frenne et al. (2024) Methods in Ecology & Evolution)
Macroclimate = the average atmospheric conditions of a large geographic region, independent of local topography, soil and vegetation
Microclimate = the thermal and hydric conditions in the immediate vicinity of organisms or ecosystem processes of interest, as driven by atmospheric conditions interacting with the earth’s surface
Mesoclimate = conditions in which climatic variations are caused by the wholesale movement of air masses, where variation is typically most evident at scales ranging from hundreds or metres to kilometres e.g. cold air drainage
See figure from Bramer et al 2018
Advances in Ecological Research
*If you zoom in from macro to micro scales, there are many physical processes governing the climate
*This causes microclimate at ecologically relevant scales to typically deviate from the macroclimate recorded by weather stations
Edge effect in Boreal, Temperate and Tropical forest
De Frenne et al 2020 Gllobal change biology
This review illustrated edge effect well (even though this was not the paper focus)
see figure in notes
Looking at boreal forest first as an example – in an open field the wind profile shows decreasing wind speed closer to the ground
Some buffering provided by a simple plantation, but almost complete buffering by a primary forest – complex, dense veg structure can completely remove effect of wind
LUC to plantation increasing wind speed
Next, looking at short-wave radiation in a temperate forest – short-wave radiation is high and homogenous in an open field
Also high where there are gaps in forest canopy, but lower in dense primary forest – this changes seasonally
Finally to temperature in a tropical forest – high and homogenous in an open field
Buffered in a secondary forest, but even more buffered in the denser vegetation of a primary forest
Forests function as a thermal insulator
See De Frenne et al. 2019
Nature, Ecology and Evolution
*On average air temperature is 4.1 degrees cooler inside forests than outside, and this difference can be even bigger in the tropics
*When you remove or degrade forest, you often reduce this buffering effect
How does land use impact local climate change?
Senior et al 2017 Ecology and Evolution
see figure showing increase in above ground temp. In different land use conditions
*This we explored by collating temperature data from the tropics, spanning a gradient of vegetation cover
*Up to 14 degrees hotter in cropland, compared to primary forest (black dashed lined)
*Even before adding on climate change
*Interestingly, however, temperature below-ground (blue) doesn’t change much
*Potential for climate at a finer scale to buffer coarser scale changes above-ground – need to zoom in further
see Santos et al 2024 Agricultural and Forest Meteorology
unlogged forest has higher canopy reducing temperature variations
*In a more recent paper, we used terrestrial laser scanning to quantify the change in vegetation structure caused by logging – you can see here that the canopy is higher in unlogged forest
*This translates to a difference in microclimate temperature – unlogged forest (teal) is less variable, being warmer at night and cooler during the day than in logged forest
Summary
*Both land conversion and land degradation are pervasive, & the former contributes to the latter
*Habitat fragmentation impacts biodiversity through more than loss of habitat area – also isolation of habitat and increasing prevalence of edge effects
*Both land conversion and land degradation change vegetation structure and microclimate
*-> How might this interact with climate change?
