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The ecological processes that underpin ecological restoration
Tawanda Tarakini1,2 and Liang Cindy1...
1 Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT
2 Department of Wildlife and Safari Management, Chinhoyi University of Technology, Bag 7724 Chinhoyi,
Zimbabwe.
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.
Key words: colonisation, dispersal, disturbance, ecological restoration, succession.
Introduction
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
predisturbance 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 & 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
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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).
Succession, species interactions, and natural cycles
Attempts have been made to elucidate succession regularity of populations and communities in manmanipulated or natural restoration processes (Jiang & 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 & 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. Feng 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. Feng et al. 2007). At this time it we should leave the debates and realise 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
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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 programmes.
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 & 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 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 & 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 & 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 & Gosselink
1993). The water table, which may be influential in supporting first colonisers, 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.
Dispersal, colonisation 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 reestablish 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 humaninduced 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
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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 & Slatyer 1977), each species
need to have the ability to establish/colonise an area. In the past few years, the concept of colonisation credit
(number of species yet to colonise a patch) was proposed as a spatially explicit framework to assess
recovery in species richness following restoration (Cristofoli & 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 colonisation problems in degraded patches.
Disturbance theory and multi 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 & 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 localised 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 & 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
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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).
Discussion
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 & 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 & 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.
Implications for practice
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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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