Disturbance can involve any factor which disrupts ecosystem function by altering environmental conditions (wiki). The Intermediate Disturbance Hypothesis (IDH) states that in order to optimise local species diversity, ecological disturbance must occur at a moderate frequency (wiki). Disturbance can vary in severity, occur at several temporal and spatial scales and be a result of natural or anthropogenic events (www.nature).
Natural disturbance (e.g. fires, floods, volcanic eruptions) often prevents populations from reaching their carrying capacity (Wiens 1984; DeAngelis & Waterhouse 1987; Pickett et al. 1992 – Ecol theory). Anthropogenic disturbance ,however, has a larger detrimental impact on habitat. In the past, the influence of humans on ecosystems has been limited; yet modern impacts have more recently accelerated in gravity and range (wiki). Anthropogenic disturbance can change and damage habitat, alter ecological function and increase the probability of invasive species (invasive – Holzner et al. 1983; Mills et al. 1994). Invasive species can cause local species extinction as well as preventing native species from being re-established (Simberloff 1990; Vitousek 1990). Once settled, the removal of these species is often challenging, as they are less affected by competition or predation than native species (ecol theory). Anthropogenic activities can also alter natural disturbance events (e.g. – flood prevention methods, control of wildfires; www.nature).
The restoration of disturbance regimes may be crucial. Restoration projects should aim to instigate or accelerate recovery after a disturbance, or to re-establish natural disturbance cycles (www.nature). It is essential to determine the level of variability which is tolerated in restored systems (Duarte 1991; Li & Reynolds 1994; Horne & Schnieder 1995; Palmer & Poff 1997; White & Walker 1997) and to recognise and minimise the difference between natural and anthropogenic disturbance (wiki). Restored sites must be continually monitored and manipulated accordingly (ecol theory) so once a species has been established, the dimensions of natural disturbance regimes (size/intensity/ duration/seasonality etc) can be established (ecol theory). If this is not possible, restorationists should attempt to initiate deliberate disturbances which best replicate historical disturbances (wiki). It is also important to note that species cannot merely be reintroduced; the manner by which disturbances affect the sustainability of a restored community must also be taken into account (ecol theory).
Ecological succession is the progressive change in the composition and interactions of a community over time (Putnam 1994 – ecol theory). After a disturbance occurs, an ecosystem will have a fairly simple level of organisation (few dominant species)(wiki). Given time, the community will develop to be more complex (numerous interdependent species)(wiki).
Restoration can occur passively or actively. Passive restoration allows succession to occur naturally, once the origin of disturbance has been eliminated (nature). Active restoration involves more human input. In mildly or moderately disturbed habitats, restorationists may try to increase the rate of natural succession. In other words, allow the community to develop in the same way as it would naturally, but assisting it to reach the final stage more quickly (ecol theory). However, in critically disturbed habitats, more concentrated efforts may be necessary and environmental conditions may need to be recreated to facilitate the processes of succession (wiki). Under these circumstances, restorationists may overlook early stages of succession in order to establish late successional species sooner (ecol theory). In some cases, underlying damage, such as soil damage, may also require restoration before the later stages of succession can begin (Whisenant 1999 – ecol of rest).
It has been found that succession theory is not always appropriate (Connell & Slatyer 1977 – ecol theory) Community patterns are often considerably unpredictable due to both disturbances and stochastic events (Fisher 1983; Levin 1989; Roughgarden 1989 – ecol theory). As a result, successional theory has been deemed irrelevant in several systems (Roughgarden et al. 1987; Olafsson et al. 1994 – ecol theory).
Assuming it allows the prediction of community patterns, the knowledge of succession theory may be essential to restoration ecology (ecol theory). In systems with strong interactions between species and rare or highly predictable disturbances this has been found to be particularly true as community progression is principally ruled by local interactions (ecol theory).
Species Interactions – predation/herbivory/mutualism/competition/
Community Assembly Theory
The study of processes that occur across distinct landscapes and the abiotic and biotic responses to these processes is known as landscape ecology (Turner 1989 – linking rest). The need for the study of landscape ecology has arisen due to anthropogenic alterations to habitats (Linking rest). Patch isolation, contiguity, size and shape (in terms of complexity) are all features which can be used to describe a landscape and these have all been found to have a large impact on both animal and plant populations (Turner & Gardner 1990; Robbins & Bell 1994 – linking rest).
According to the theory of island biogeography, populations are most likely to become extinct if a habitat is small and/or isolated from other populations. One of the assumptions of this theory states that the area between patches of habitat is both hostile and uniform. This theory assumes that the area between habitat patches (matrix) is both uniform and inhospitable. (Previous studies have suggested that the organisation of habitat types in a matrix can influence the movement of organisms and/or the role of landscapes (Gustafson & Gardner 1996 – linking rest).)
Small and isolated habitats are also more susceptible to the detrimental effects of habitat fragmentation (nature). Habitat fragmentation occurs when a habitat becomes disconnected as a result of anthropogenic or natural events (nature). This causes the formation of small isolated patches of suitable habitat (nature). These habitats have a lower carrying capacity and have decreased species diversity and are therefore more vulnerable to inbreeding and local extinction (nature). As well as this, fragmentation increases the proportion of edge and decreases the gap between edges (nature). Edge effects may alter environmental factors, in turn affecting the flux of organisms and other substances across landscapes (Holland et al. 1991 and references within; Robinson et al. 1992). Systems are sometimes dependent on the flow of individuals between habitats. This sustains the community structure, implying that fragmentation can influence recruitment and therefore affect the resilience of communities (Palmer et al. 1997 – linking rest). The role of restoration usually involves improving connectivity between patches by constructing or re-establishing links. This may be done with the use of corridors or stepping stones.
Ecol of rest
nutrient and energy fluxes are essential components of ecosystem function and stability at a range of spatial and temporal scales (Ehrenfeld & Toth 1997; Aronson et al. 1998; Bedford 1999; Peterson & Lipcius 2003).
Ecosystem function describes the foundational processes of natural systems, including nutrient cycles and energy fluxes. These processes are the most basic and essential components of ecosystems. An understanding of the full complexity and intricacies of these cycles is necessary to address any ecological processes that may be degraded. A functional ecosystem, that is completely self-perpetuating (i.e. no management required), is the ultimate goal of restorative efforts. Because these ecosystem functions are emergent properties of the system as a whole, monitoring and management are crucial for the long-term stability of an ecosystem.
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