Explaining co-existance in plant communities: Fertilisation and CC

Question 1

The effects of fertilistion on Diversity

Answer 1

1. Removes nutrient limitation + removes small-scale heterogeneity that allows coexistence
   -> Based on resource ratio hypothesis
2. Adding nutrients shifts competition to aboveground for light = more asymmetric competition (tall grasses often dominate fertilised grasslands)

Study: Competition for Light Causes Plant Biodiversity Loss After Eutrophication
-> If add light back to understorey, then increased biomass and increased richness (interactive effect of fertiliser and light)
-> Negative effect of fertiliser removed
-> Doesn’t work in nature tho (can’t just add light bulbs)
->So best way to protect richness is NOT fertilise and add light

Counter hypothesis: Density hypothesis
- More above ground competition leads to mortality of small individuals from all species -> random loss of species and little effect on biodiversity

Question 2

Park Grass (Rothamstead, started in 1856)

Answer 2

• oldest ecological experiment: control, fertilisers (P, K, Mg, Na, with N ), CaCO3 lime, manure plots
• tested hay yield + specie composition / richness
• Unmanipulated plots had the highest species richness and those with ammonium sulphate had the worst as increased soil pH and richness.
• As biomass increased species richness decreased linearly and as pH decreased species richness decreased non-linearly.

Question 3

Natural expeiment: the effect of fertiliser on Calcareous grassland

Answer 3

Calcareous grassland:
- Previously covered a lot of Southern England
- V species rich w/ low biomass (40 species per m2 quadrate)
- V nutrient poor soils (particularly P as chalk binds up P)
- Traditionally used for grazing (sheep) which also controlled dominant species

land use change
1900s -> shift to arable and land used for horticulture -> land was improved using chemical fertiliser

-> 47% priority habitat lost over ~50 yrs from 1960
-> 97% unimproved grassland lost over ~50 yrs form 1932
-> reminaing grassland also poor due to lack of grazing and nitrogen deposition from the air acting as natural fertiliser

Question 4

Restoring diversity in Calcareous grassland

Answer 4

The verges of the Ridgeway National Trail are the only regions of calcareous grassland that remain -> sheep grazing resotration not a possibility

Restoring diversity by:

a) Grazing sheep – stop tall grass excluding shorter forbs
b) Annual cutting at end of summer (replaces sheep) - remove cut material
c) Reduce soil fertility
-> cut and remove cut material (eg. mow) or remove topsoil
d) Resow seeds (as short lived seed bank)

results:
-> Resowing only effective if do other management strategies (effect increases as the intensity of management interventions increases- best is to remove topsoil)

Question 5

Factors leading to diversity

Answer 5

**Reource heterogeniety and limitation **
- R* theory and ratio theory

Example: Tropical Rainforests on Oxisols (Amazon basin)
* highly weathered soils of tropical and subtropical regions
* Nutrients poor but exceptional plant diversity

Example: the negative effect of fertilisation

**Competition vs co-existance trade off **

**Other trophic interactions **
- Parasites
- herbivores

Question 6

Species co-existance, competition trade off

Answer 6

Trade off in competitive ability (R of species) and its colonising ability allows co-existance [Tilman, 1994]
- When study plants at Cedar Creek he noticed that there was a competitive hierarchy (R* based on N) and also an inverse colonisation hierarchy

For multiple species to co-exist the poorer competitor must have the greater colonisation rate.

Equation for 1 species:

dp/dt= cp(1-p) - mp

c= colonisaiton rate
P= proportion of sites occupied
m= mortality rate

Equilibrium patches: 1-m/c (so c > m)

1 species cannot fully occupy a habitat at equilibirum as c therefore cannot be greater than m

Equation for 2 species:
- Assumes competitive assymetry
- species 1 completely unaffected by presence of species 2 = unlikely in real life
- Species 2 only lives in space species 1 has left and additional loss due to competitive displacement

dp1/dt= cp1(1-p1) - m1p1

dp2/dt= cp2(1-p1-p2) - m2p2 - c1p1p2

Stable co-existance is possible if the species with the higher competitive ability also has a greater colonisation rate.

Species can be continually added to the model, colonising the space left behind.

Relaxing assumption of competitive asymmetry -> diversity collapse (can get 2 species coexisting, but rarely more)

Question 7

Example: Seed mass vs number

Answer 7

• More seeds: better coloniser
• Larger seeds: better competitor

Tilman’s model predicts that the large-seeded species will be able to exclude the smaller-seeded ones if their colonisation limitation is overcome.

Study used seed addition to remove the trade-off and ensure that each species produced the same number of seeds.

Resulted in large seed dominance and little co-existance.

Another example may be asexual vs sexual reproduction

Question 8

Janzen-Connell effect: diversity maintained by species-specific herbivores or parasites

Answer 8

When seeds or seedlings occur at high density or close to adult conspecifics they are more vulnerable to attack from specialized natural enemies such as pathogens and insect herbivores

Herbivores:
- Predation on seeds = v high around parent / conspecifics (as many seeds produced and fall attract herbivores)
- seed does better if travels far from parent
- limits pop’n growth as can’t grow near conspecifics and recruitment rates decrease as pop’n increase
- If each species has own specialist specific herbivore you can get many species coexisting

Example: Black cherry tree
- predated by mice and chip munks

Parasites:

• As plants grow it get species-specific community in soil around roots (nematodes, bacteria etc)
• a species may not grow well in soil it has previously occupied (other species don’t mind as pathogens not specific to them)
• limits pop’n growth as recruitment falls

Study:
- Petermann et al (2008) used soil from 24 common European grassland species that had been growing as monocultures for three years
- Checked whether grassland species grow better on own or another species’ soil – does better on another’s
- effect removed if sterilise soil showing importace of parasite.

Study:
- A 2010 study of 8 tree species in Panama found evidence for strong Janzen–Connell effects. Seedlings tended to do worse when grown with soil microorganisms from under their own species or when grown in the field under their own species, compared to when grown under other species.

Question 9

Overview

Answer 9

What maintained diversity?
- Limiting resources and heterogeniety (proved through addition of fertiliser -> removes limitation/ heterogeniety and makes light limiting)
- Competition vs co-existance trafe-off (collapses when remove assymetry)
- Other trophic scales (herbivores/ parasites)

The effects of fertilistion on Diversity
1. Removes nutrient limitation + small-scale heterogeneity
2. shifts competition to aboveground for light

Restoring diversity in calcareous grasslands -> many mechanisms involving reduing nutrients and returning seeds

Question 10

the Nutrient Network

Answer 10

International organisation looking at the effect of fertilisation on grass land

• Standard experimental process that can be conducted anywhere in the world-> e.g the savannas of South Africa and Tanzania to Swiss mountain meadows, and from North American prairies to the Australian Outback
• Each year, scientists apply nitrogen, phosphorous or potassium fertilizers, either singly or in combination, to square-meter-sized plots. At the end of the seasonal growing cycle, they cut the grasses to measure biodiversity and biomass—the amount of vegetation within a plot—and reapplied fertilizer to begin the next year’s cycle
• Biodiversity declined at all these varied sites

Question 11

N fertiliser

Answer 11

It is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide and Amino acids

NH3 fertiliser = largest industrially produced chemical on planet – Haber-Bosch (2% of world’s energy used to reduce N2 to ammonia as need high temp + pressures)

V high nitrate runoff leading to eutrophication

50 fold increase in N production by humans since 1910 (not enough N in just manure)

Malnourishment correlated w/ poor soils w/ low N levels

Soybean can fix up to 450kg N per hectare = best N2 fixing legume due to 1000s yrs of selective breeding soybeans = important N source + animal forage + crop rotation

Question 12

P fertiliser

Answer 12

Phosphorus in plants is key in capturing, storing, and converting the sun’s energy into biomolecules

Globally 10kg P added per ha agricultural land = 3kg in Africa, up to 25kg in parts of Europe

Only 15-30% P taken up by crops – need to increase efficiency P uptake not just add more