Transcript Gleason
Successful Ecologists Biography Schedule Organismal Ecology, William Murdoch - Tim Assal group - Oct 7 Population Ecology, ??? - ??? - Oct 14 Community Ecology, ??? – Jeremy Sueltenfuss group - Oct 21 Ecosystems Ecology, Dave Schimel – Kevin Wilcox group - Oct 28 Nov 9, 11 Dan Simberloff – Jared Stabach group Unknown Ecosystem Ecologist – Ed Gage group Unknown ecologist – Nell Campbell group Gene Likens – Ben Gannon group 1. Approach How Ecology is “done” What is the relationship between diversity and invasion? Large scale, Observational Small scale, Experimental 3. Perspective Vegetation Patterns & Succession 2. Process Kaibab population crash Succession & Chronosequences Gleason Clements Equilibrium Nonequilibrium Inference Synthesis Understanding Re-examination Alternative Ecology & paradigm shifts – an “old” example (Clements/Gleason) and a new challenge? (succession) ----------------------------“Balance of Nature” vs. “Non-equilibrium view” In Chemistry – what is the difference between a compound and a mixture? • The properties of compounds are different from those of their constituent elements. This is one of the main criteria for distinguishing a compound from a mixture. • Another criterion for distinguishing a compound from a mixture is that the constituents of a mixture can usually be separated by simple means, but the components of a compound can only be separated by a chemical reaction. • Conversely, mixtures can be created by mechanical means alone, but a compound can only be created by a chemical reaction. Q. Are natural ecosystems/communities more like compounds or mixtures? The “Balance of Nature” as the classical ecological paradigm • Both western and eastern roots Eastern philosophy – nature seen in terms of unity, interplay, harmony… Western society – divine order, earth is a fellow creature, nature is a finely tuned machine… Early ecologists – concepts of stability and equilibrium embedded in the view that populations, communities and ecosystems are self-regulating systems, kept in stable equilibrium if left alone… Many early ecologists seemed to believe that nature could be understood in terms of a balance of destructive and conservative forces, but that if left undisturbed, nature would maintain a permanence of structure and function… (Thus, “no management” is good management? Is this the view today?) Concepts derived from this view: Steady-state, Stability, Homeostasis View that at an equilibrium state, all factors or processes leading to change are resisted or balanced by the system… Diversity – stability debate has its roots in this perspective… Concepts in literature: • Resistance – the capacity of an ecological system to resist external perturbation • Resilience – the rate at which an ecological system returns to its equilibrium state after a perturbation • Persistence – the capacity of an ecological system to stay within some predefined bounds despite perturbations • Constancy – the degree of variation in system properties over time Support for the “Balance of Nature” or Equilibrium paradigm • Density-dependence and negative feedback mechanisms • Particularly important in population ecology • Virtually all of the mechanisms proposed to explain long-term coexistence of species can be related to trade-offs in some components of their biology at equilibrium (Chesson 1991)…Tilman’s Resource Ratio hypothesis In sum: Equilibrium paradigm focused on closed, selfregulating systems and the factors that allow populations to persist, and species to coexist in stable communities and downplayed the importance of spatial and temporal variation and history. Non-equilibrium perspective Emphasis on: Openness Transient dynamics Stochastic processes Fluctuations in population densities are an essential feature of non-equilibrium communities with chance and historical contingency important Non-equilibrium systems are also characterized by non-linear interactions and thresholds such that they can be quite sensitive to small perturbations Non-equilibrium systems may exhibit abrupt, discontinuous changes in system behavior as a result of certain parameters crossing an apparent boundary (Wu and Loucks 1995) A key point: Non-equilibrium systems are less predictable, but not necessarily less persistent… F. Clements F.E. Clements (1916) conceived of the community as a sort of superorganism • Species were tightly bound together both now and in their common evolutionary history • Mutualism and coevolution play an important role in the evolution of species making up the association • Individuals, populations, and communities bore a relationship to each other which resembled that between cells, tissues and organisms •Species found in a community have similar distributional limits • Thus community boundaries are clearly delineated • Communities, not species, are the essential unit On the landscape, associations of species would appear along environmental gradients as groups of species with high fidelity to specific associations, with little overlap between associations, and narrow ecotones (boundaries between adjacent communities). Figure 22.1a is from (Smith and Smith 2001). Clements' theories on communities were also linked to succession. Nature of succession-The community can reproduce itself, “repeating with essential fidelity the stages of its development” (Clements 1916) • Each stage of succession represented a step in the development of the superorganism • Seral stages modify the environment so that species of that stage can no longer exist there and they prepare the site for replacement by species of the next stage • The process continues until the vegetation arrives at the self-reproducing climax According to Clements: • Succession was a predictable, directional, inevitable process • The climax represented a community at some equilibrium or steady state with the physical and biotic environment • Only one climax would exist for a region whose characteristics were determined by climate and edaphic conditions. Clements presented a deterministic unidirectional view of succession The philosophical structure of Clements's holistic approach was similar to advances in other sciences of that time. In geology and geomorphology theorists presented views that contained various stages of maturity, and compared landscape evolution to a developing organism. H. Gleason Gleason viewed the community as consisting of individual species that respond independently to environmental conditions. • Nature of the community-“the vegetation of an area is merely the resultant of two factors, the fluctuation and fortuitous immigration of plants and an equally fluctuating and variable environment” (Gleason 1926). • Species, not communities, are the essential unit. • Species change in abundance along environmental gradients so gradually that it is not practical to divide vegetation into associations. • The relationship between coexisting species within a community is a result of similarities in their requirements and tolerances, not as a result of strong interactions or common evolutionary history • What is referred to as a community is merely the group of species found to coexist under any particular set of environmental conditions On the landscape, such communities of plants would appear to not be clearly associated with each other and there would be no distinct boundaries between one community and an adjacent one. Gleason – Nature of succession-succession results from the individual responses of different species to the prevailing environmental conditions. Plants involved in succession are those that arrive first on the site and are able to establish themselves under prevailing environmental conditions. As time passes, plants modify the environment and competition and other interactions among species determine the final outcome Gleason’s views, even though they included less determinism (more chance) and holism than Clements, still had many equilibrium characteristics… Competition is an equilibrium concept… A nonequilibrium view: Populations, communities and ecosystems may be far from equilibrium because of variable frequencies of disturbance and climatic fluctuations Main forces determining community membership are colonization and extinction rates of species and the time since disturbance, not species interactions The metapopulation viewpoint (systems are open, not closed) and spatial structure of the environment (both abiotic and biotic) play important roles in determining community membership There is a strong historical component to community membership…Assembly order and priority effects are important For many years, Clementsian views dominated Ecology and Gleason was ignored… Today? Recognition that both perspectives are important Non-equilibrium focus Equilibrium focus The future? Will ecological dynamics be driven primarily by disturbance and random weather fluctuations? Are contemporary ecosystem dynamics being driven more by chronic resource alterations (CO2, N, H2O), instead of discrete disturbances? Simple model of directional change Resource alteration Response Community change Community re-ordering Physiological response Time A. Disturbance B. Global Change Biota Biota Resources Resources Discrete disturbance Chronic resource alteration Smith et al. 2009 (Ecology 90:3279-3289)