Editing Theoretical ecology (section)

==Ecosystem ecology==
{{See also|Ecosystem models}}

Introducing new elements, whether [[Biotic component|biotic]] or [[abiotic]], into [[ecosystem]]s can be disruptive. In some cases, it leads to [[ecological collapse]], [[trophic cascade]]s and the death of many species within the ecosystem. The abstract notion of [[ecological health]] attempts to measure the robustness and recovery capacity for an ecosystem; i.e. how far the ecosystem is away from its steady state. Often, however, ecosystems rebound from a disruptive agent. The difference between collapse or rebound depends on the [[toxicity]] of the introduced element and the [[resilience (ecology)|resiliency]] of the original ecosystem.

If ecosystems are governed primarily by [[stochastic]] processes, through which its subsequent state would be determined by both predictable and random actions, they may be more resilient to sudden change than each species individually. In the absence of a [[balance of nature]], the [[species composition]] of ecosystems would undergo shifts that would depend on the nature of the change, but entire ecological collapse would probably be infrequent events. In 1997, [[Robert Ulanowicz]] used [[information theory]] tools to describe the structure of ecosystems, emphasizing [[mutual information]] (correlations) in studied systems. Drawing on this methodology and prior observations of complex ecosystems, Ulanowicz depicts approaches to determining the stress levels on ecosystems and predicting system reactions to defined types of alteration in their settings (such as increased or reduced energy flow), and [[eutrophication]].<ref>Robert Ulanowicz (). ''Ecology, the Ascendant Perspective''. Columbia Univ. Press. {{ISBN|0-231-10828-1}}.</ref>

[[Ecopath]] is a free ecosystem modelling software suite, initially developed by [[NOAA]], and widely used in fisheries management as a tool for modelling and visualising the complex relationships that exist in real world marine ecosystems.

===Food webs===
[[Food web]]s provide a framework within which a complex network of predator–prey interactions can be organised. A food web model is a network of [[food chain]]s. Each food chain starts with a [[primary producer]] or [[autotroph]], an organism, such as a plant, which is able to manufacture its own food. Next in the chain is an organism that feeds on the primary producer, and the chain continues in this way as a string of successive predators. The organisms in each chain are grouped into [[trophic level]]s, based on how many links they are removed from the primary producers. The length of the chain, or trophic level, is a measure of the number of species encountered as energy or nutrients move from plants to top predators.<ref name="Post93">{{cite journal|last=Post|first=D. M.|title= The long and short of food-chain length|year=1993|journal = Trends in Ecology and Evolution|volume=17|issue=6| pages=269–277|doi=10.1016/S0169-5347(02)02455-2}}</ref> [[Food energy]] flows from one organism to the next and to the next and so on, with some energy being lost at each level.  At a given trophic level there may be one species or a group of species with the same predators and prey.<ref>{{cite book | author= Jerry Bobrow, Ph.D.|author2=Stephen Fisher | title= CliffsNotes CSET: Multiple Subjects| year= 2009| edition= 2nd| publisher= John Wiley and Sons| page= 283| url= https://books.google.com/books?id=BAaYNjlrJDcC&q=%22presumed+to+share+both+predators+and+prey%22&pg=PR1| isbn= 978-0-470-45546-3}}</ref>

In 1927, [[Charles Sutherland Elton|Charles Elton]] published an influential synthesis on the use of food webs, which resulted in them becoming a central concept in ecology.<ref>Elton CS (1927) Animal Ecology. Republished 2001. University of Chicago Press.</ref> In 1966, interest in food webs increased after [[Robert T. Paine (zoologist)|Robert Paine's]] experimental and descriptive study of intertidal shores, suggesting that food web complexity was key to maintaining species diversity and ecological stability.<ref>{{cite journal | author = Paine RT | year = 1966 | title = Food web complexity and species diversity | journal = The American Naturalist | volume = 100 | issue = 910| pages = 65–75 | doi = 10.1086/282400 | s2cid = 85265656 }}</ref> Many theoretical ecologists, including [[Robert May, Baron May of Oxford|Sir Robert May]] and [[Stuart Pimm]], were prompted by this discovery and others to examine the mathematical properties of food webs. According to their analyses, complex food webs should be less stable than simple food webs.<ref name=May />{{rp|75–77}}<ref name=Pimm />{{rp|64}} The apparent paradox between the complexity of food webs observed in nature and the mathematical fragility of food web models is currently an area of intensive study and debate. The paradox may be due partially to conceptual differences between persistence of a food web and equilibrial [[Ecological stability|stability]] of a food web.<ref name=May>May RM (2001) [https://books.google.com/books?id=BDA5-ipCLt4C&dq=%22Stability+and+Complexity+in+Model+Ecosystems%22&pg=PR7 ''Stability and Complexity in Model Ecosystems''] Princeton University Press, reprint of 1973 edition with new foreword. {{ISBN|978-0-691-08861-7}}.</ref><ref name=Pimm>Pimm SL (2002) [https://books.google.com/books?id=tjHOtK4amfQC&q=Pimm+%22Food+Webs%22 ''Food Webs''] University of Chicago Press, reprint of 1982 edition with new foreword. {{ISBN|978-0-226-66832-1}}.</ref>

===Systems ecology===
[[Systems ecology]] can be seen as an application of [[general systems theory]] to ecology. It takes a [[holistic]] and interdisciplinary approach to the study of ecological systems, and particularly ecosystems. Systems ecology is especially concerned with the way the functioning of ecosystems can be influenced by human interventions. Like other fields in theoretical ecology, it uses and extends concepts from [[thermodynamics]] and develops other macroscopic descriptions of complex systems. It also takes account of the [[Ecological energetics|energy flows]] through the different [[trophic level]]s in the ecological networks. Systems ecology also considers the external influence of [[ecological economics]], which usually is not otherwise considered in ecosystem ecology.<ref>R.L. Kitching, ''Systems ecology'', University of Queensland Press, 1983, p.9.</ref> For the most part, systems ecology is a subfield of ecosystem ecology.