Non-trophic networks
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Historically, species interactions have been considered primarily in terms of trophic interactions. More recently, mutualisms such as pollination and seed dispersal have been scrutinized within the framework of ecological networks. Currently, networks that integrate both types of interactions are being constructed and analyzed.
Species Interactions
Biological interactions result from the fact that organisms in an ecosystem interact with each other, and that no organism is isolated from its surroundings. There are six possible combinations: mutualism, commensalism, neutralism, amensalism, predation, and competition . These range from mutually beneficial through neutral to mutually harmful interactions. Observing and estimating the fitness costs and benefits of species interactions is very problematic. The way interactions are interpreted can profoundly affect the ensuing conclusions.
Interactions Characteristics
Direct vs. indirect
- Indirect: mediated through other species
Physically involved vs. not
- Eating species X versus releasing harmful chemicals
Prevalence
- Proportion of the population affected
Negative/ Positive
- Cost/benefit balance
Strength
- Death vs. slight reduction in fitness
Relationship in space and time
- Geographical proximity
- Length and duration of interaction
In the same way that trophic cascades can occur, it is expected that ‘interaction cascades’ take place. Thus, it should be possible to construct ‘effect’ networks which parallel in many ways the energy or matter networks common in the literature. By assessing the network topology and constructing models, we might better understand how interacting species affect each other and how these effects spread through the network. In certain instances, it has been shown that indirect trophic effects tend to dominate direct ones (Patten, 1995)—perhaps this pattern will also emerge in non-trophic interactions.
By analyzing network structures, one can determine keystone species that are of particular importance. A different class of keystone species is what are termed ‘ecosystem engineers’. Certain organisms alter the environment so drastically that it affects many interactions that take place within a habitat. This term is used for organisms that “directly or indirectly modulate availability of resources (other than themselves) to other species, by causing physical state changes in biotic or abiotic materials”. Beavers are an example of such engineers. Other examples include earthworms, trees, coral reefs, and planktonic organisms. Such 'network engineers' can be seen as “interaction modifiers”, meaning that a change in their population density affects the interactions between two or more other species.
Interesting examples
- Wolbachia/arthropods
- Beneficial endosymbionts
- competitors
- Vectors
- Viruses
- Multi-faceted interactions (eg. Manduca sexta/ Datura wrightii )
Questions
- Can the complexities of biology ever be captured in schematics?
- How do we accurately detect and evaluate non-visible interactions?
- How much predictive power do these networks have for population dynamics?
- What same patterns might emerge within different communities?
Sources
- C.G. Jones, J.H. Lawton and M. Shachak, Positive and negative effects of organisms as physical ecosystem engineers, Ecology 78 (1997), 1946–1957.
- V. Vasasa, F. Jordan. Topological keystone species in ecological interaction networks: Considering link quality and non-trophic effects. Ecological Modelling 196 ( 2006 ) 365–378.
- Fath B. Network mutualism: Positive community-level relations in ecosystems. Ecological Modelling. 208, 1 (2007), 56-67.
- Patten, B.C., 1995. Network integration of ecological extremal principles: exergy, emergy, power, ascendency, and indirect effects. Ecol. Model. 79, 75–84.