Keep track of individual organisms’ birth rate & mortality rate
Dynamics of species interactions
Keep track of overall types of interactions, e.g. competitive, consumer–resource
Species diversity over space and time
Patterns over large and small scales
Now
Ecology of ecosystems
How are abiotic properties distributed on Earth, what shapes their distributions, and how they affect life on Earth.
Core themes from the Syllabus
Ecological systems are dynamic, meaning that their properties can change over time.
Ecological systems feature feedbacks, meaning that the dynamics of one component of a system often affects another.
The dynamics and wellbeing of ecological systems are tightly intertwined with the dynamics and wellbeing of human societies
Understanding ecological systems requires us to confront uncertainty, which can arise because of limited knowledge of the system, or due to inherently stochastic processes.
Scope of ecosystem ecology
Patterns and drivers of major energy/substance fluxes
Over this week, we will consider the global drivers of nitrogen and water
We will frequently talk in terms of “pools” and “fluxes”
Today’s case study: how precipitation shapes ecological patterns, and vice-versa.
NPP = Carbon stored by photosynthesis - carbon lost during respiration
Some questions that an ecosystem ecologist might study
What determines the productivity of a system, and why is it so different across the globe?
How does productivity relate to biodiversity?
How might ecosystem productivity be affected by human activities?
What determines the productivity of a system, and why is it so different across the globe?
First-order explanation: productivity is shaped by precipitation and temperature
The question then becomes, what shapes precipitation and temperature?
Structural explanation
Dynamic explanation
Structural factors behind temperature
Spherical shape -> Latitudinal gradient in temperature
Tilted axis -> Seasonal variation in temperature
Structural factors behind temperature
A first-order explanation
Angle of incidence varies – perpendicular at equator, not so at the poles
To reach the Earth’s surface, light and energy from the sun passes through Earth’s atmosphere
Particles in the atmosphere reflect back some energy – so the more atmosphere that the light/energy passes through, the less of it reaches Earth’s surface
The path to the tropics is shorter than the path to the poles.
Productive tropical forests dominate in lowland areas of the tropics.
But…
Patterns of precipitation are not driven by “structural” factors alone
Consider the impacts of trees on precipitation
How much water does an “average” oak tree transpire annually?
Patterns of precipitation are not driven by “structural” factors alone
Landscapes with vegetation that transpire more water will have larger evapotranspirative flux, which moves water from the groundwater pool to the atmospheric pool
Depending on what happens to the atmospheric water, this can cause more rainfall
Human modified landscapes often have reduced rates of evapotranspiration
(fig. 2 from Spracklen et al. 2018, Annual Reviews of Environment and Resources)
Large reductions in rainfall expected due to land-use change
(fig. 2 from Spracklen et al. 2018, Annual Reviews of Environment and Resources)
Changes in rainfall can lead to “regime shifts”
A more subtle story of the water cycle in the Amazon basin
This analysis provides compelling observational evidence that rainforest transpiration during the late dry season plays a central role in initiating the dry-to-wet season transition over the southern Amazon
The fate of the southern Amazon rainforest depends on the length of the dry season.
The length of the dry season also depends on the rainforest.
Feedbacks at the ecosystem scale
Cloud seeding as a way to “trigger” more rain?
Why does this matter?
Large-scale ecosystem processes are not just ‘set in stone’
They are always in relationship with the biological processes that unfold
This means that disruptions to the biology can have important consequences to the ecosystem as a whole
