Climax+communities

In [|ecology], a **climax community**, or **climatic climax community**, is a biological [|community] of [|plants] and [|animals] which, through the process of [|ecological succession] — the development of vegetation in an area over time — has reached a steady state. This equilibrium occurs because the climax community is composed of species best adapted to average conditions in that area. The term is sometimes also applied in [|soil] development. Wikipedia []
 * 1) Succession
 * a. Succession is a sequential change in species within a community.
 * b. //Primary Succession//
 * i. Begins with bare rock; takes very long time.
 * ii. Weathering of wind and rain plus //pioneer species// such as lichens and mosses begin to build up soil.
 * iii. //Herbaceous// or //grass stage// grows on deeper soil and shades out shorter pioneer species.
 * iv. Pine trees or deciduous trees eventually take root; form climax community.
 * c. //Secondary Succession//
 * i. Begins in abandoned field with soil layers already in place.
 * ii. Moves more rapidly to climax community.
 * d. Early researchers assumed climax communities were determined for each environment; today recognized to be outcome of competition among whatever species are present.
 * 1) Climax Communities Are More Stable
 * a. Early stages of succession show most growth and are most productive.
 * b. Pioneer communities lack diversity, make poor use of inputs and lose heat and nutrients.
 * c. As succession proceeds, species variety increases and nutrients recycle more.
 * d. Climax communities make fuller use of inputs and maintain themselves-they are more //stable//.
 * e. Humans replace climax communities with simpler communities. (Table 32.2)

Climax Communities
[|Climax communities] are those whose net production and utilization are in equilibrium. and whose cycling of nutrients occurs directly between the organisms and decomposing material via exchange pools (rather than from hard to get at environmental reservoirs). Biodiversity is greater in [|climax communities] and food webs become more elaborate. Finally [|climax communities] are much more stable than the transition stages leading up to them.

Problem with Climax Concept: Ultimately, even if succession tends towards a steady state, the time required to achieve this state is unrealistically long; in most cases, external disturbances and environmental change occur so frequently that the realization of a climax community is unlikely, and therefore it has come to be regarded as a less useful concept. Long-term vegetation dynamics are now more often characterized as resulting from the action of stochastic factors. (A stochastic process is one whose behavior is non-deterministic in that a state does not fully determine its next state)

The climax community continues to propagate itself and tends to remain relatively unchanged overtime. Disruptive events like fires, hurricanes, blights, or human influence may temporarily cause new and different communities to form (ie. fields, pine forests, swamps), but over time these eventually succeed back to the climax community.

** Community stability may be due to lack of disturbance or community resistance or resilience in the face of disturbance. ** ** Some Definitions ** The simplest definition of **stability** is the absence of change. A community or ecosystem may be stable for a variety of reasons. One reason may be that there has been no disturbance. For instance, the benthic communities of the deep sea may remain stable over long periods of time because of constant physical conditions. The type of stability resulting from an absence of disturbance, if it exists, is not particularly interesting to ecologists. Ecologists are more interested in how communities and ecosystems may remain stable even when exposed to potential disturbance. Consequently ecologists generally define stability as the persistence of a community or ecosystem in the face of disturbance. Stability may result from two very different characteristics. **Resistance** is the ability of a community or ecosystem to maintain structure and/or function in the face of potential disturbance. However, stability may also result from the ability of a community to return to its original structure after a disturbance. The ability to bounce back after disturbance is called resilience. A resilient community or ecosystem may be completely disrupted by disturbance but quickly return to its former state. What causes communities and ecosystems to be resilient? The phenomenon of resilience takes us back to succession. Remember that we defined succession as the gradual change in plant and animal communities in an area following disturbance or the creation of new substrate. It is succession that restores a community disrupted by disturbance. Succession is the basis for resilience. Ecologists ask many questions about stability. Are some communities and ecosystems more resistant than others? What factors determine differences in resistance among communities and ecosystems? Are some ecosystems and communities more**resilient** than others? What factors determine the rate of recovery of community structure and ecosystem processes following disturbance? However, few studies have been conducted at scales appropriate to address these questions. One of the main problems faced by ecologists interested in community and ecosystem stability is the need to conduct detailed studies over a long period of time. There are a few studies that meet this requirement; one of them is the Park Grass Experiment. ** Lessons from the Park Grass Experiment ** The Park Grass Experiment is the prototype of all long-term experimental studies in ecology. It was started at the Rothamsted Experimental Station in Hertfordshire, England, between 1856 and 1872. The purpose of the experiment was to study the effects of several fertilizer treatments on the yield and structure of a hay meadow community. Because the Park Grass Experiment has continued without interruption for nearly one and a half centuries, it provides one of the most valuable records of long-term community dynamics. That record provides some unique insights into the nature of community stability. Jonathan Silvertown (1987) used data from the Park Grass Experiment to respond to the suggestion that existing studies do not conclusively demonstrate that any ecological community is stable. Silvertown pointed out that the Park Grass Experiment is one of the few studies of terrestrial communities that have been carried out in sufficient detail and over sufficient time to provide a test of stability that meets the rigorous requirements suggested by Connell and Sousa. The composition of the plant community at the Park Grass Experiment has been monitored since 1862. This record reveals at least one level of stability. Over this period, virtually no new species have colonized the meadow. Changes in the community have occurred as a consequence of increases or decreases in species already present in the meadow at the beginning of the experiment. Silvertown used variation in community composition as a measure of stability. He represented composition as the proportion of the community consisting of grasses, legumes, or other species. The analysis of composition was restricted to the period from 1910 to 1948 to avoid the early period of the experiment when the meadow community was adjusting to the various fertilizer treatments. Figure 20.23 shows the relative proportions of grasses, legumes, and other plants on plots receiving three different treatments: plot 3, no fertilizer; plot 7, P, K, Na, and Mg; and plot 14, N, P, K, Na, and Mg. The differences in vegetation on the three plots were mostly produced by the different fertilizer treatments and developed early in the Park Grass Experiment. **// FIGURE 20.23 //**// Proportions of grasses, legumes, and other plant species under three experimental conditions (data from Silvertown 1987). //   The proportion of grasses, legumes, and other plants in the study plots varied from year to year, mainly in response to variation in precipitation. Despite this annual variation, figure 20.23 indicates that the proportions of three plant groups remained remarkably similar over the interval of the study. A quantitative analysis of trends in biomass revealed no signifi - cant changes in the biomass of the three plant groups in plots 3 and 7 and only a minor, but statistically significant, decrease in the biomass of grasses on plot 14. In other words, the data presented in figure 20.23 show remarkable stability in the proportion of grasses, legumes, and other species. Does the stability of Silvertown's three major groups of plants in the Park Grass Experiment hold up if we examine community structure at the species level? It tams out that while the proportions of grasses, legumes, and other species remained fairly constant, populations of individual species changed substantially. Mike Dodd and his colleagues (1995) used census data from 1920 to 1979 to examine plant population trends. The result of their analysis showed that some species increased in abundance, some decreased, some showed no trend, while others increased and then decreased (fig. 20.24). **// FIGURE 20.24 //**// Patterns of species abundance during 60 years of the Park Grass Experiment (data from Dodd et al. 1995). //   The contrasting results obtained by Silvertown and by Dodd's project suggest that whether a community or ecosystem appears stable may depend upon how we view it. At a very coarse level of resolution, the Park Grass community has remained absolutely stable. It was a meadow community when the Park Grass Experiment began in 1856 and it remains so today. When Silvertown increased the resolution m distinguish between grasses, legumes, and other species, the community again appeared stable. However, when Dodd and his colleagues increased the resolution still further and examined trends in the abundances of individual species, the Park Grass community no longer appeared stable. Are there stable natural communities? The answer to this question may depend upon how you make your measurements. The ecologist interested in addressing any question concerning community stability is faced with several practical problems. Generally, an adequate study requires a great deal of time, which limits the possibility of replication. One solution to this problem is to study communities and ecosystems, such as Sycamore Creek, Arizona, that undergo more frequent disturbance and show relatively rapid recovery. These systems offer the opportunity to compare recoveries from multiple disturbances. ** Replicate Disturbances and Desert Stream Stability ** Numerous studies of disturbance and recovery in Sycamore Creek, Arizona, have produced a highly detailed picture of community, ecosystem, and population responses. This detailed picture suggests that ecologists have just begun to probe the subtleties of ecological stability. For instance, one study shows that resistance in the spatial structure of the Sycamore Creek ecosystem underlies spatial variation in ecosystem resilience. Maury Valett and his colleagues (1994) studied the interactions between surface and subsurface waters in Sycamore Creek in order to study the influence of these linkages on ecosystem resilience. They tested the hypothesis that ecosystem resilience is higher where hydrologic linkages between the surface and subsurface water increase the supply of nitrogen. They proposed a controlling role for nitrogen because it is the nutrient that limits primary production in Sycamore Creek. Valett and his colleagues intensely studied two stream sections at middle elevations in the 500 km2 Sycamore Creek catchment. They measured the flow of water between the surface and subsurface along these reaches with devices called piezometers. Piezometers can be used to measure the vertical hydraulic gradient, which indicates the direction of flow between surf'ace water and water flowing through the sediments of a streambed. Positive vertical hydraulic gradients indicate flow from the streambed to the surface in areas called upwening zones. Negative vertical hydraulic gradients indicate flow from the surface to the streambed, which occurs in downwelling zones. Zero vertical hydraulic gradients indicate no net exchange between surface waters and water flowing within the sediments. Areas with zero vertical hydraulic gradients are called stationary zones. Valett and his colleagues measured vertical hydraulic gradient along the lengths of both study sections, producing hydrologic maps for both. The upper end of each study reach was an upwelling zone. The middle reaches were stationary zones and the lower reaches were downwelling zones. Figure 20.25 shows the distributions of these zones across one of the study reaches. **// FIGURE 20.25 //**// Patterns of upwelling and downwelling in a reach of Sycamore Creek, Arizona (data from Valett et al 1994). //   The concentration of nitrate in surface water in the two study reaches varies directly with vertical hydraulic gradient (fig. 20.26). Upwelling zones, which are fed by nitrate-rich waters upwelling from the sediments, have the highest concentrations of nitrate. Nitrate concentrations gradually decline with distance downstream through the stationary and downwelling zones. **// FIGURE 20.26 //**// Relationship of nitrate to vertical hydraulic gradient in Sycamore Creek, Arizona (data from Valett et al. 1994). //   The higher concentrations of nitrate in the upper reaches of each study section are associated with higher algal production. Figure 20.27 indicates that algal biomass accumulates at a higher rate in upwelling zones compared to downwelling zones. Valett and his colleagues used rate of algal biomass accumulation as a measure of rate of recovery from disturbance. Because the rate at which algal biomass accumulates in upwelling zones is so much higher than in downwelling zones, they concluded that the rate of ecosystem recovery is higher in upwelling zones. This pattern supports their hypothesis that algal communities in upwelling zones are more resilient. **// FIGURE 20.27 //**// Changes in algal biomass, measured as chlorophyll a. following flooding at upwelling and downwelling zones (data from Valett et al. 1994). //  The team also found that while flash floods devastated the biotic community, the spatial arrangement of upwelling, stationary, and downwelling zones remained stable. In other words, this aspect of the spatial structure of the Sycamore Creek ecosystem is highly resistant to flash flooding. The location of upwelling, stationary, and downwelling zones remained stable in the face of numerous intense floods. The spatial 'stability of the Sycamore Creek ecosystem in the face of potential disturbance is an example of ecosystem resistance. However, what is the source of this stable spatial structure? This spatial stability can be explained by considering geomorphology, especially the distribution of bedrock. Subsurface water is forced to the surface in areas where bedrock lies close to the surface. Upwelling zones in Sycamore Creek are located in such areas, and since flooding does not move bedrock, the locations of upwelling zones are stable. Therefore, this aspect of ecosystem stability is controlled by landscape structure. Consequently, the ecologist trying to understand the organization and dynamics of the Sycamore Creek ecosystem must consider the structure of the surrounding landscape. []