TITLE: Initial PhD ideas
DATE: 2017-09-25
AUTHOR: John L. Godlee
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I'm now just over 1 month into my 48 month PhD program. I've spent
a lot of time reading and trying to wrap my head around my project.
Now I feel like I'm ready to at least have a stab at saying what my
PhD is going to be about. Though this all might change again over
the next few months as I start to write my PhD plan and
confirmation report.
The original title of the PhD I applied for was "Biodiversity –
Ecosystem Function Relationships in Southern African Woodlands".
The basic idea that has been reported by lots of independent
studies is that as biodiversity increases, so do the levels of
various ecosystem functions. Biodiversity is most commonly measured
as species richness (i.e. the number of species in a given area).
Ecosystem functions are just any rate process that the ecosystem
performs, one of the most commonly used is the rate at which CO2 is
fixed into biomass via photosynthesis, taking into account respired
CO2. This idea has been used a lot to justify biodiversity
conservation work, under the assumption that if biodiversity
declines, ecosystem functions and their associated benefits to
humankind will also decline.
There is however, still lots of controversy about how strong the
effects of the BEFR are in natural systems and what the drivers of
the BEFR are. A lot of studies of the BEFR have been in mesocosms,
or small grassland patches. These studies are often criticised for
not mirroring natural systems enough to be used to inform policy.
Often they study one ecosystem function within a limited timeframe
and with a small species richness. Often species addition/removal
is random, which doesn't mirror what happens in natural systems
under stress.
In the past, studies have identified the following as potential
drivers of the BEFR:
- Niche complementarity - As more species are added, they
necessarily differ in the niche space they occupy so they can fill
more of the total niche space and optimise resource usage.
- Selection effects - As more species are added, there is a
higher probability that one of those species will maximise
productivity. e.g. Grimes 1998
The above potential drivers of the BEFR differ in their reasoning.
Niche complementarity doesn't take into account community
composition, while Selection effects realy on community
composition. Of course, it's likely that in a natural system it's
probably a combination of the two.
Of course there are other reasons why ecosystem functioning, e.g.
productivity might vary over space:
- Resource availability - As resource availability increases,
productivity responds to that by increasing productivity
- Environmental stress - An increase in environmental stress such
as fire intensity might lead to an increase or a decrease in
productivity, the exact mechanisms behind this are complex.
Southern African Woodlands are understudied and unique
The "Southern African Woodlands" bit is part of why this project is
interesting. The map below from (Clarke et al. 2017) shows that
African study sites (and the tropics in general) are severely under
represented in the Biodiversity - Ecosystem Function (BEFR)
literature.
Also, a lot of these theories about the BEFR were drawn from
studies in temperate environments, so it's not a given that the
same processes will occur in the dry tropics. Indeed, in the wet
tropics, there isn't such a strong BEFR, and there are various
reasons for that. One of the trains of thought that is getting a
lot of attention at the moment is the role of the environment in
mediating the strength (i.e. the steepness of the slope), of the
BEFR. In the temperate studies, it's often found that as you
decrease resource availability, facilitation effects become
stronger than competition effects, so that if you add another
species to the ecosystem, the ecosystem functioning is likely to go
up more than if the community was under no stress and species were
competing more. I'm not sure I agree with that idea, surely in a
woodland, even if there is drought stress, the trees are still
going to compete for that little water availability because they
are close together. The competition effect might even increase.
There isn't much data on the traits of different species in African
woodlands, but it makes sense that species with different traits
would react differently to environmental change, so the structure
of the woodland is going to change as different populations change
in abundance depending on their response to environmental change.
It also makes sense that as the relative abundance of each species
changes under environmental change, the level of various ecosystem
functions will also change at the plot level, because different
species provide different ecosystem functions. These two sources of
variation between woodlands muddy the waters in the search for a
general BEFR, but possibly if you can control for those sources of
variation, or at least account for them properly in statistical
analysis you could go some way to teasing out a relationship.
Possible studies
Obviously one of the main jobs of this PhD is to use data from many
woodland plots across Southern Africa to try to quantify the BEFR,
and figure out what the drivers of the BEFR are. For example, does
the shape of the BEFR vary across space according to water
availability. How does it vary according to woodland type.
I'd like to do a study that looks at how different woodland types
and their species composition (i.e. their biodiversity) affect how
they will respond to increasing atmospheric CO2 concentrations. For
example, some woodlands might be made up predominantly by a single
species that is very fast growing and can take advantage of the
extra CO2, while others might not. I suppose leaf traits would also
have a lot ot do with this. I've heard of a general distinction
between species that invest in woody growth, and those that invest
in leafy growth, maybe that has something to do with it.
I think that the understorey of savanna-woodland mosaics is
understudied. You might expect that the structural aspects and
diversity of a woodland canopy might affect the diversity and
composition of the understorey. Understorey species might affect
the provision of rare ecosystem services and functions, they will
also have a big role in the spread of fire through those
understories.
Lastly, it looks like I might be going to Angola for 2 weeks in
January, to get involved in overseeing the collection of some data
that will get put into the SEOSAW database. The trip should be a
good chance for me to get some more fieldwork experience (go to a
cool place) and make some connections with the people who are
collecting the data. Making these measurements will be one of the
first cataloging efforts in the woodlands in this part of the
African continent and could be a neat paper in itself if I can make
some connection between the initial biomass and its partitioning
amongst species of different traits. That study would also be a
nice way to kick off a second PhD chapter on the role of community
composition on the BEFR. Then I'd have a chapter on the role of
environmental variation on the BEFR.
The lack of a BEFR in the wet tropics, possibly worldwide
In the wet tropics, species richness is very high to begin with.
Many studies of the BEFR in experimental landscapes have shown that
the BEFR saturates at high biodiversity, so maybe if you take away
a species it won't actually make any difference. This equates to
there being high functional redundancy in the wet tropics. The most
plausible theory in my mind as to why this is the case is that
because environmental conditions are quite amenable, it is likely
that competition interactions are powerful enough that species
co-exist, even within the same niche space, at least at the plot
scale and higher, this leads to a lot of functional redundancy. If
you remove a species from the niche space, a suitably similar
species will have the ability to fill that niche space quickly, so
there is no loss of function.
In dry ecosystems however, there aren't that many tree species to
begin with, so you would expect there to be a big effect on
productivity if you removed a species, as a big area of niche space
has been removed. However, this line of reasoning relies on the
assumption that the tree species occupy different niches, while
occupying adjacent space, but adult trees in savannahs might not
compete with each other due to them being so spaced apart. So,
instead of there being a species richness - function relationship,
maybe there is a composition - function relationship, and the
function merely relies on the presence of certain species over
others.
Early predictions
y thoughts at the moment are that because African woodlands are
quite species poor to begin with, the addition of a tree species
could greatly increase the productivity of the woodland. But I also
expect that this effect will depend on a minimum threshold of tree
density to begin with. At really low tree densities the effect of
adding a species would depend more on the productivity of that
species, because the niche space isn't filled enough to promote
niche partitioning.
