The Ecological Processes that Underpin Ecological Restoration  

Tawanda Tarakini1,2 , Xingxing Liang1
1 Faculty of Biological Sciences, University of Leeds, LS2 9JT, Leeds, UK
2 Department of Wildlife and Safari Management, Chinhoyi University of Technology, Bag 7724 Chinhoyi, Zimbabwe
Author    Correspondence author
International Journal of Molecular Ecology and Conservation, 2012, Vol. 2, No. 3   doi: 10.5376/ijmec.2012.02.003
Received: 04 Jun., 2012    Accepted: 10 Jul., 2012    Published: 20 Jul., 2012
© 2012 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Tarakini and Liang, 2012, The Ecological Processes that Underpin Ecological Restoration, International Journal of Molecular Ecology and Conservation, Vol.2, No.3 (doi: 10.5376/ijmec.2012.02.0003)

Abstract

The science of restoration ecology is closely linked to ecological knowledge and practical restoration principles are often heavily guided by ecology theory. Knowing that there are sections in community theory that are currently poorly understood, it might be dangerous if restoration principles are based on what science offers for these sections. The need to focus on dispersal, disturbance, colonization and succession dynamics is particularly important since many of our project sites are highly degraded and disjointed from a healthy regional pool of colonists. In this article, the importance of these major ecological theories and how relevant they are in restorations today are evaluated. Restoration work is set to bring not only corrections in functionality and community structure of degraded systems but also benefiting basic research in community ecology.

Keywords
Colonisation; Dispersal; Disturbance; Ecological restoration; Succession

 Probably the most conclusive definition of restoration is captured in Jordan et al (1987) and SER (2004) as an intentional activity that initiates or accelerates the recovery of an ecosystem with respect to its health, integrity and sustainability returning to a system which is a close approximation of its condition prior to disturbance. In most cases, disturbance is caused by humans, although impacts to ecosystems may be caused or aggravated by natural agencies (e.g. wildfire, floods, storms), such that an ecosystem cannot recover its pre-disturbance state or its historic developmental trajectory (SER, 2004). Restoration involves removing/ modifying a specific disturbance, and allowing ecological processes to bring about an independent recovery in the process (SER, 2004). Planning for ecological restoration and conservation are fundamentally based on conceptual or theoretical models of nature (Naveh, 1994) and restorations are aimed at assisting or initiating recovery, in contrast to ecosystem management where people work to guarantee the continued well-being of an ecosystem (Young, 2000).

From the early 1980s, restoration ecology has evolved searching ways to improve conceptual basis, for example from succession theory, community assemblage rules, threshold dynamics, state transitions and differentiation of niches (Suding and Hobbs, 2009). There are still a lot of knowledge gaps in ecology that should raise concerns about which principles should be used in restoration, and the current work are my opinions to this debate. There is need for pluralism in approaches to studies of communities, and restoration ecology may provide an ideal way of demonstrating how pluralistic approaches can bridge the knowledge gaps in natural and managed communities (Jordan et al., 1987). A reference ecosystem might be needed to have a model for planning an ecological restoration project, and later serve in the evaluation of that project (Palmer et al., 1997). There is considerable evidence that a feedback exists between species composition and ecosystem processes and that many ecosystem processes will develop over different time scales (Palmer et al., 1997). This means that restoration in practice may involve the setting of sequential, multi-step goals like: restore desired community structure, monitor the development and verify that linkages between community structure and function have been established. There are current debates as to whether restoration should focus on restoring community structure or ecosystem function, since restoration is unique in that work is done to achieve a process at the end, not a product. Restoration ecology has at its core the assumption that many degradative forces are temporary, and that some proportion of habitat loss and population decline is recoverable (Young, 2000).

1 Succession, species interactions, and their natural cycles
Attempts have been made to elucidate succession regularity of populations and communities in man-manipulated or natural restoration processes (Jiang and Shen, 2007). There are generalised expectations during secondary plant succession and these including a high degree of predictability in the sequence of different life forms (Bazzaz, 1996), a decrease of the rate of succession as the system approaches its mature state (Myster and Pickett, 1994). As Clement’s theory suggest, convergence in community structure will be towards a single climax, or divergence be expected along multiple alternative states of ecosystem development (Bazzaz, 1996). Many succession authors indeed concur to the idea that as succession proceeds, total nitrogen and organic matter increase, chemical and physical characteristics improve (e.g. Du et al., 2007). Three mechanisms were proposed by Connel and Slatyer (1977) to determine succession: facilitation (early colonisers create suitable conditions for others), tolerance (ability to stand competitive environment) and inhibition (restrict/prevents existence of other species). The changes in nutrition during succession are different according to the types, extent and primary conditions of disturbance. Care must therefore be taken as successions in restorations do not always take the theoretical stages (Jordan et al., 1987), in most cases we do not know the stage that disturbance will have changed the developmental trajectory of the system. Due to our lack of knowledge on the amount of time each state can take during restoration, we might fall for the trap to quickly conclude that a restoration project is succeeding or failing. Succession theory should therefore be used as guidelines in planning and monitoring restorations. However if community development is highly predictable, it is likely to be feasible to manipulate natural succession processes to accelerate restoration (Palmer et al., 1997).

Competition has been used to explain the decline of early species, especially those that have long persistence tails. The alternative hypotheses explaining the decline in early species suggest that it is due to changes in environmental conditions (e.g. Du et al., 2007). At this time it we should leave the debates and realize that these two theories can be applied in different conditions (we cannot use the general theories for terrestrial/marine, woodland/grasslands and rainforest/deserts ecosystems). Applying competition knowledge in restoration allow managers to actively control the populations of competitors to have a desired community assemblage at a given time, however care need to be taken as species niches are not defined by abrupt boundaries. For example competition can be used as a tool in limiting the establishment of seedlings in restorations (Holl, 2002). Plants complete for light, space and nutrients but the most evident competition is aboveground from pasture grasses, where exotic species can be very aggressive. If these are not controlled they will tend to dominate site.

On a pair-wise basis, knowledge in ecology suggests that species also interact in other methods like predation, commensalism and mutualism. For example in lake Orantino, lake trout (Salvelinus namaycush) restoration efforts resulted in a lot of spawning fish of hatchery-origin with no detectable natural recruitment, and predation by the non-native alewise (Alosa pseudoharengus) has been used to explain to explain the trend on lake trout fry (Krueger et al., 1995). This means although we can use science knowledge to control invasive and other common species, we might get surprises in restoration programmers.

Scientists are finding more information each year on aboveground and belowground components of ecosystems as these are strongly linked through a range of both direct and indirect interactions that operate across levels of ecological organization (e.g. Kardol and Wardle, 2010). Alteration of trophic interactions in aboveground food webs can influence soil food webs and vice versa (Hines, 2006), causing reciprocal shifts in community composition between the plant and soil communities (Van der Heijden and Horton, 2009). Networks such as those between plants and mycorrhizal fungi interactions are increasingly recognized as critical in restoration plant growth especially in associations between plants and consumers, and the ultimate composition of aboveground and belowground communities. Processes such as nutrient and carbon influxes have been proved to be driven by aboveground and belowground linkages at the community level (Fortunel, 2009). Some aboveground-belowground processes have already been utilised (for example interactions of plants with aboveground herbivores and the abiotic soil environment) while others are yet to be exploited in restoration ecology (Kardol and Wardle, 2010).   We might therefore try to fully utilise these aboveground and belowground links as there is a lot potential to gain in restoration output using cheap materials (herbivores, mycorrhizal fungi etc.).

Hydrology has been ranked as the most important factor that will influence the success of a wetland restoration (Mitsch and Gosselink, 1993). Factors such as depth, duration, and periodicity of inundation in a wetland are often related to plant establishment, litter decomposition, soil properties, plant distribution and growth (Kolka et al., 2000). The importance of identifying patterns in the hydrological characteristics in evaluating the impact of restoration activities, and the potential for long-term restoration success should not be underestimated. Drainage patterns influence the oxidation state of the substrate, partly determining the chemical form of soil and pore water constituents, microbial and vegetative communities being supported, and the rates of degradation, mineralization, and subsidence in a wetland ecosystem (Mitsch and Gosselink, 1993). The water table, which may be influential in supporting first colonizers, can also be lowered by woodlands. By regulating the water table (and the associated hydrological cycles) restorationist can actually determine what kind of plants to establish in a wetland.

2 Dispersal, colonization and migrations
Although recreating some appropriate abiotic site conditions is necessary, it is often not sufficient in restoring species-rich communities such as grassland communities (Bischoff, 2002). Shortage of propagules can be a serious bottleneck in plant restorations, and seed banks of some plants especially grassland species are transient or of short-term (McDonald et al., 1996). Input of seeds from outside is often necessary to re-establish populations. The probability of such inputs depends on the distance to source populations and dispersal abilities of the species. Currently there is relatively limited knowledge on dispersal abilities of plant species so that it can fully applied in restoration ecology (Bishoff, 2002) and the relationship between dispersal/colonization dynamics and habitat arrangement, particularly in the face of natural and human-induced disturbances, is also poorly understood for most systems (Palmer et al., 1997). Some studies suggest that mowing and grazing (e.g. Strykstra et al., 1997) can enhance dispersal processes in grasslands. In many tropical regions seed dispersal by animals dominates as the form of dissemination of propagules and has the potential to facilitate restoration of native vegetation on degraded sites (Wunderle, 1997). However efficiency of dispersal by animals can be limited by isolation of a restoration site, availability of dispersers and size of seeds (Strykstra et al., 1997). Because of such challenges, initial restoration work normally involve active planting of vegetation, before complementation by dispersal sources and large-seeded species might have to planted as well as due to their relative immobility, to return to primary forest. An understanding of the general movements of species in areas (source sink relationships) gives restoration managers insights; not all species needs to be reintroduced but if movements are facilitated then species can migrate to new sites. Reserves are therefore important not only to secure vital ecosystem interactions, water quality and ecological memory but also as potential “species banks” that can be allowed to disperse to degraded sites. Reserves will help in re-establishing ecological memory for ecosystem resilience, natural successional processes and capacity for renewal.

It is now generally being now accepted that communities are dynamic assemblages of species, in which all species should be regarded as potential colonist, because fluctuations and disturbances allow particular species to establish at certain times (Jordan et al., 1987). Early colonist are typically short lived, fast growing species and reproduce early while older succession fields are dominated by longer lived perennial species. Although the persistence and abundance of species later in succession depends on competitive ability, their ability to inhibit growth of other species, or simply their longevity (Connell and Slatyer, 1977), each species need to have the ability to establish/colonize an area. In the past few years, the concept of colonization credit (number of species yet to colonize a patch) was proposed as a spatially explicit framework to assess recovery in species richness following restoration (Cristofoli and Mahy, 2010). Having such studies for target species might be expensive yet very crucial in restorations especially if this information is augmented with habitat quality information. Conservation and restoration work must urgently target strict specialists, showing colonization problems in degraded patches.

3 Species disturbance theory and multiple species assemblages
Natural disturbances shape community structures (spatially and temporally) to create their range of variability that will control ecological processes and habitat variability (Jõgiste et al., 2007). These disturbances regulate processes such as nutrient and carbon cycles, temporal-spatial distribution of input litter ultimately the net primary production of ecosystems. Since it is known that disturbances operate at multiple spatial scales; often fine/coarse- scale disturbances interact in complex ways to produce multi-scale habitat diversity from microhabitats to landscape patterns (Sudding and Hobbs, 2009). The scale is a therefore an important question when applying natural disturbance concepts to restoration exercises. The same level of disturbance can cause catastrophic effects on a small area while having mild effects on a relatively larger area. An example is a localized mudflow that may destroy lots of species but if it occurs in a forested mountain range, it creates other habitats thereby increasing diversity of the forest. If disturbance passes the intermediate stage then degradation is triggered. Basing on intermediate disturbance theory, a dominant species will "rule" in areas with little disturbance while in areas with high disturbance only species that are highly adapted to the disturbance will survive (Hobbs and Norton, 1996). In either way, there will be low biodiversity, but a perpetual low level disturbance (e.g. floods and fire) allows more diversity to flourish in the area. Restorationist can intentionally bring in disturbance in the form of grazing, bush control and forest product harvesting.

Equilibrium states concepts are no longer broadly accepted by ecologists as adequately explaining community structure. The amount of information learnt from stead states is limited compared to responses by ecosystem to external disturbances that expose the dynamic connections of the systems. Often, responses to disturbances are not necessarily linear, meaning perturbations must be extreme (Jordan et al., 1989). Many communities exist in perpetual states of non-equilibrium or dynamic equilibria where natural disturbance prevents most populations from reaching maximum densities (Pickett et al., 1992). It should therefore be noted during restorations that due to the non-equilibrium state of ecosystems, application of any method will not guarantee a single result; the ecosystem trajectory might be slightly be different from theory. Each attempted plan should therefore be given enough time for managers to be sure of its success/failure, and evaluation and monitoring should be continual.

Since restoration efforts often involve a focus on multi-species assemblages (Palmer, 1997); community ecology theory becomes central and extremely relevant to restorationists (Godefroid et al., 2010). The composition and richness of plant assemblages are both important to ecosystem functions such as productivity, carbon and nitrogen dynamics and canopy architecture. Restorationists should concentrate on restoring functional groups if the initial goal is to restore a functional community, but if goal is to restore community structure then efforts should be to understand spatial ecology, patch dynamics, and metapopulation theory, and these are topics that we unfortunately do not have sufficient knowledge particularly in a restoration context (Palmer et al., 1997).

4 Discussions
It is crucial that restorationists be able to understand how ecosystems became degraded, an idea of how to reverse the degradation, and how to efficiently initiate recovery processes (Hobbs and Norton, 1996). It should also be known that flexibility in setting restoration objectives is not deviations from basing restoration on ecological science, but rather, it is a clear recognition that incomplete knowledge of past ecosystem states, changes in the global environments, costs, and the scale of degradation calls for a pragmatic approach (Suding and Hobbs, 2009). Ecological knowledge should therefore be taken as guidelines in restorations but being cautious because of complexity of nature and our lack of full understanding dynamics of it, habitats and ecosystems may not always respond according to the theories of science. 

5 Implications for practice
     If endpoints are not known, then it is better to consider ecosystem function rather than species composition.      Turn from hard engineering solutions to ecologically based restorations
     Use community theory as guidelines in seeding processes in restoration or creating opportunities for dispersal
     Adopt pluralism and be flexible in approaches to restore communities
     Continual monitoring and evaluation of projects is critical
     Formulate well-accepted criteria for each ecosystem type to assess and report restoration outcomes

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