Chapter 19: Ecosystem Stability, Competition, and Nutrient Cycling
Danielle Quist and Emily Zelenka
19.1 Introduction:
A. Effect of biological diversity on stability of ecosystems
1. 1960-70’s it was believed that higher diversity leads to more stability
2. Three different origins of stability
i. External environments is constant
ii. Community has high resilience to disturbances
iii. Community has high resistance to disturbances
3. Very few studies on population fluctuations and species diversity, but the
few that were done have been inconclusive or shown that the more diverse
communities are less stable
Question: Why do you think some cases show less stability when there
is more diversity?
19.2 Stability of Model Ecosystems
A. May’s definition of stability: a system is stable when it is recovered to a state of
equilibrium after a perturbation
1. Unstable systems rarely seen and do not last long, either becomes stable or
extinct
2. His studies show the complete opposite of what we have been learning:
likelihood of stability increases with decreased species
B. Predator-prey and host-parasite relationships (+/-) stabilize
C. Mutualistic or symbiotic interactions (+/+) may cause positive feedbacks,
resulting in outbursts and/or extinctions of both species. Why? Examples?
D. In food webs, systems with higher average numbers of feeding linkages are less
stable
E. In systems with increasing species numbers, connectance should decrease if the
community is to be stable. Why?
19.3 Competition and the Loss of Diversity
A. Removal of higher trophic level leads to extinction in lower levels (ex. Paine)
1. Releases pressure from predations or consumption
2. Increased competition between the prey species
3. Competitive exclusion= success of a competitor leads to extinction of
others
Question: During the past century, do you think the loss of upper trophic levels
is the key cause of increased numbers of extinction? Why would that be?
B. Human pollution can also affect extinction through competition
1. W. Europe, increased nitrogen improves productivity of plants
2. Competitive displacement occurs, fast-growing displace slow
C. Evidence for competitive exclusion
1. Study comparing fertilized and unfertilized meadow plots showed 75%
decline in species in the fertilized plots. Why?
2. Berendse studied effect of fertilizers on slow-growers in a normal system
and system of only slow-growers (see Figure 19.2 p415)
Question: Does competition occur before outside/human influences on resources
or does it only develop as a result of the population growth? Or do stabilizing
types of competition break down into destabilizing ones when conditions are
favorable?
19.4 Stabilizing Consequences of Competitive Interactions
A. A quantitative competition theory is needed in order to examine the
stabilizing/destabilizing consequences of the competitive interactions.
1. De Wit and Ennik (1958) introduced model for competition in higher plants
2. They developed an equation (p416) as a model between animal crop
species,
3. It allows the assessment of individual limiting resources and the variables
affecting the use of them by each plant
4. Isocline graphs can be made with this equation to further understand the
competition between species (Figure 19.3, 19.4)
B. Categories of competition interactions can have stabilizing effects on community
1. “or model”: competition between 2 species whose populations are limited
by 2 different resources
2. “+ model”: competition between 2 species over 1 resource that is has 2
different forms or is in 2 different parts of the environment
Question: What would happen if the species were no longer limited by these
resources?
19.5 Effects of Organisms on Their Physical Environment
A. Stability is not only linked with interactions between populations, but also to
interactions between species and their interactions with their environment
B. Lovelock uses the Daisyworld model to show how natural selection favoring
certain colored plants could regulate the temperature of a planet
C. Back to reality… organisms affect soil fertility!
Question: What other ways do organisms affect their physical environment?
19.6 Features Affecting Plant Fitness Under Different Nutrient Supply Conditions
A. Plants in nutrient-poor environments adapt by maximizing assimilation of
nutrients and minimizing loss of nutrients
B. In nutrient-rich environments, they succeed in competition by maximizing growth
rates and adapting to maximize intake of other resources
C. Plants with high nutrient retention and have slow growth rates (Erica) do better in
nutrient-poor environments
D. Species with high loss rates and have fast growth rates (Molinia) do well in
nutrient-rich systems
19.7 Consequences of the Different Effects of Plant Species on the Nutrient Cycle
A. Plants like Molinia can increase the nutrient level and get caught in a positive
feedback cycle
B. There are 2 different strategies to restrict nutrient losses
1. Increase the life span of organs, such as leaves
2. Withdrawing an important part of the nutrients in leaves and other plant
parts before abscissions
C. Leaf litter decomposability is affected by the plant species that produce it
1. Plants with fast growth rates have highly decomposable litter
2. The litter affects the nutrient level in the soil, thus affecting the physical
environment
Question: The Arctic tundra plant diversity is very limited by the lack of
available nitrogen; how will global warming affect the ecosystem of the tundra?
19.8 Conclusions
A. Based on the previously stated definition of stability, diversity does not contribute
to stability
Question: Under a different definition of stability, would diversity have a more
positive effect?
B. Human interference/interactions have caused large losses of biodiversity where
they may have intended to help, thanks to competitive exclusion due to changes
such as predator removal or increased nutrient supplies.
Question: Which do you think caused the dramatic losses: the changes
themselves, or the rapid rate at which the changes took place?