Introduction
Anticipating change in the planetary system and especially the biosphere has become a modern preoccupation. The need to know, and to know instantaneously, what is happening to different ecosystems across the globe has brought ecologists out of corners of research departments into the limelight.
Mass media and the culture of open access to information has exposed environmental issues to far higher levels of scrutiny than even the most progressively minded activist would ever have dreamed of (Krebs, 1999 p. 12). Saving endangered species and protecting, the environment has now become a national pastime in many countries and of significant global institutional concern.
Nevertheless, simple ‘knee-jerk’ reactions to change will not be enough to mitigate against any long-term ecological damage. There has to be a resolute effort to comprehend how ecosystems function. This can only be achieved through a solid theoretical foundation.
This essay examines how ecological theory can be applied in the degradation of tropical forest ecosystems due to agriculture, and their restoration from exhausted agriculture lands.
The theories that are going to be discovered in this essay will be applicable on high diversity tropical forests while the ecological system under study is terrestrial. The levels of the biological hierarchy addressed include ecosystem, community, population and individual while the ecological mechanisms affected are behavioral, physiological, and chemical.
High diversity tropical forests are often deforested for conversion to agriculture. This is evident in the large areas of tropical dry forests in Central America that have been converted to cotton plantations (Aide et al., 2000 p. 32).
Although it has been argued that tropical forest have low levels of nutrients and agricultural practices on a site are limited to several years, some agricultural practices have continued for years, and are eventually abandoned.
This can be due to a number of reasons such as reduced productivity, economic changes or socio-political changes (Ehrlich& Ehrlich, 1992 p. 20). The mammoth question that arises from this abandonment is what the fate of these pastures becomes.
The prospect of secondary forests recovering to a similar state to the original forests is almost unfathomed. In deed, research in the past has shown that numerous factors affect the restoration of forests and species constituents after abandonment of agriculture. The rate of recovery lies between two extremes. The first scenario consists of forests that have been subjected to slash and burn agriculture covering a small area.
In this case, the rate of recovery is usually high. The other extreme is that of sites that have suffered extreme soil degradation and frequent fires. In this latter case, the conditions inhibit the recovery of the original forest and can lead to a different ecosystem (Brown & Lugo, 1990 p. 100).
Ecological restoration is, thus, a challenging task. Nevertheless, owing to the high diversity of these forests, and their influence on global water and carbon cycles, it is important that people understand how to manage and restore.
Agricultural Degradation of Tropical Forest Ecosystems
Tropical forest ecosystems are degraded through various ways. The causes can be classified into three categories. The first category consists of natural causes. The second category comprises of human activities while the third one is through underlying causes. Under natural category, degradation of highly diversified tropical ecosystems is caused by hurricanes, natural flames pests or floods.
All these causes deteriorate the nutrient content of the soil hence, killing some of the organisms in the soil and hindering growth of plant cover (Laurance, 1999 p.115). The second category of causes of degradation of ecosystems consists of human activities. The include logging, mining and oil extraction, construction of dams and roads, cattle ranching, and agriculture expansion.
These human activities lead to clearing of tropical forests, which causes habitat shift, or even in extreme cases, extinction of endangered species. The underlying causes of degradation of tropical forest ecosystems include mistaken policy interventions, governance weaknesses, as well as broader socioeconomic and political causes.
The effects of degradation of tropical forest ecosystems are many and varied. In deed, deforestation and degradation of forests have become a global concern in the recent past. One of the major effects of deforestation is global warming. Tropical deforestation not only produces substantial carbon emissions, but it also reduces the size of forest carbon sink (Lugo, Parrotta, & Brown, 1993 p. 117).
This can lead to an amplifying in the global atmospheric carbon dioxide stock hence resulting to global warming. Apart from affecting the carbon cycle, changes in forest cover can also affect the terrestrial energy balance especially through albedo, evaporation rates and aerosol emissions (Schwartz et al., 2000 p. 300).
Albedo is the proportion of solar radiation reflected back into space. A low albedo implies that very little solar radiation is reflected while more is absorbed by the surface, thus warming the land. A high albedo means that a high proportion of solar radiation is reflected back into space while less is absorbed. Nature is usually presumed to respond to gradual change in a smooth manner.
However, studies on forests and other ecosystems have shown that sudden, drastic switches to a contrasting state can perturb smooth change. Although diverse events can lead to these shifts, it has been proved that loss of resilience usually prepares the ground for a switch to an alternative state.
All ecosystems are exposed to these gradual changes in climate, nutrient loading, habitat fragmentation or biotic exploitation. As such, strategies for sustainable management of these ecosystems should emphasize on maintaining resilience (Folke et al., 2004 p. 167).
Deforestation of tropical forest ecosystems leads to a disturbance of local species communities’ distribution, disturbance of trophic levels and biodiversity loss. It also alters the existing ecological conditions and resource availability. For instance, in case where human activities are responsible for the deforestation.
This means that the animals that previously occupied the area intended for the activity Such as construction of roads will have to be displaced. This disturbs their harmonious coexistence and alters the tropic level as some of the animals that may previously have depended on them for food will be forced to find alternative sources of food (Lewis, 2009 p. 100).
The plant cover, which is home to different species, will be cleared and this, in turn, alters the establishing ecological conditions, which results to reduction in resource availability, and eventual biodiversity loss probably due to starvation. Forests are very important in the maintenance of biological diversity. They are estimated to be home to half of the world’s total biological diversity.
Tropical forests are especially richly endowed. Forest fragmentation exacerbates the impacts on biodiversity of overall deforestation and forest degradation by blocking migration routes and making access easier for further exploitation by humans and entry by invasive species.
As mentioned earlier, human activities are among the notorious causes of degradation of tropical forest ecosystems. However, not all human activities pose the same measure of threat to these forests. One of the human activities that are proving to be a dangerous threat to tropical forests is agriculture. As human population increases day by day, food consumption also increases.
This results to a quest for new lands for subsistence and large scale commercial plantations. Given that tropical forests are considered productive due to their fertility, they are bound to be cleared for conversion to farming land (Wilcove & Pin Koh, 2010 p. 1003). This phenomenon has adverse effects on the ecosystem.
As noted earlier, this may lead to biodiversity loss since some organisms or species are forced to migrate or die in the processed due to starvation. This can also be attributed to a change in the existing ecological conditions, hence altering the livelihood of the species living in that ecosystem. The original tropical systems are not spared either. Organisms are forced to feed on alternative sources of food (Lal, 2008 p. 20).
As if agriculture is not enough to pose a serious threat to forest ecosystems, human beings have gone ahead to engage in unsustainable agricultural activities. These are characterized by overgrazing, excessive cultivation, rotations that lack pasture legumes, bare soil and fallowing practices, as well as insufficient soil testing for nutrient levels and fertilizer use (Balvanera et al., 2006 p 150).
Other characteristics of unsustainable farming methods include excessive clearing of deep-rooted perennial native species causing a rise in groundwater levels (Matson et al., 1997 p. 507). These practices affect tropical forest in various scales and ways.
For instance, overgrazing leads to soil erosion occasioned by insufficient plant cover while rotations that lack pasture legumes result to decline in soil nutrients and biological activity. If land is subjected to these poor farming methods for a long time, it becomes degraded. Its fertility and ability to support biodiversity deteriorates (Parrotta, 1992 p. 127).
The ecological mechanisms of the land such as the soil pH change due to some of unsustainable farming methods like cultivating in waterlogged areas. This increases the acidity of the soil, hence forcing some species of organisms to shift to other areas. Human beings are quick to abandon such lands because they are no longer productive (Carney & Matson, 2006 p. 230).
This is in spite of the fact that they are the ones who have caused the degradation of these lands. After land has been degraded, it can be restored to its approximate initial state. One way of land restoration is through primary succession. This process deploys a number of restoration ecology theories such as succession single equilibrium succession theory and multi-equilibrium succession theory.
Restoration Ecology
Restoration ecology is the process of aiding the recuperation of an ecosystem that has been despoiled, damaged or ruined. This definition is very broad, but it shows explicitly that restoration is not something theoretical without any practical obligations, but has also to do with engagement and intervention in current social and environmental affairs (Van Andel & Aronson, 2005 p. 12).
In deed, restoration ecology is sometimes portrayed as degradation in reverse. With whatever assistance may be needed from a restoration practitioner, a degraded ecosystem undergoes natural succession that returns it to its original state.
In so doing, it regains its community structure and composition and its ecological functions and processes return to normal. This implies that technically, a degraded land is one that has lost its community structure, ecological function or process (Kobayashi, 2004 p.15).
The restoration process involves two phases. These include reclamation and restoration. These phases are based on the type of barrier that needs to be crossed before the restoration is complete. Under reclamation, two barriers need to be overcome. These are abiotic and biotic factors.
The abiotic barrier barriers require physical modification in order to bring the systems to anew level of stability associated with a new ‘higher’ level of function. The biotic barrier may be occasioned by a lack of suitable species or interaction between them and abiotic components.
They can be addressed by active modification. The second phase is the restoration of a programme meant to restore ecosystem function and structure. Improved management is also required in order to attain a fully functional ecosystem (Van Andel & Aronson, 2005 p. 13).
Successional Model
Traditionally, restoration efforts have emphasized on means to re-establish historical abiotic conditions to enhance the natural return of the vegetation. This succession-based approach posits that once the historical, physical environment is re-established natural successional processes will return the biotic systems to its original state.
Natural succession, also called biological succession or ecological succession has varied definitions (Walker, Walker &Hobbs, 2007 p. 23). They range form simple ones like ‘process of vegetation change’ to ‘a hypothetically orderly sequence of change in plant communities leading to a stable climax community’. Going by the latter definition, natural, succession restores an impaired ecosystem to a complex and stable state.
The term succession in this commonly used sense infers an endpoint that will be attained, and that will mark the termination of the successional process. Traditionally, ecologists have designated this hypothetical endpoint as the climax, at which point the biotic communities on site reach stability and equilibrium with their environment (Clewell & Aronson, 2007 p. 46).
The whole sequence of successional stages that result to a climax or endpoint is called a sere, and intermediate stage within a sere can be termed as a seral community. However, this approach is unpredictable in other cases.
The successional model makes several assumptions. To begin with, it assumes that environmental conditions remain stable through out the development of a sere. This could take centuries to complete due to the long generation of many types of trees.
Critiques of the concept of a single equilibrium climax argue that environmental conditions commonly change more rapidly than it takes equilibrium to develop. Another assumption of the succession theory is the existence of strong internal regulation and feedback of ecosystems, in which one process reinforces another in a synchronous fashion (Chazdon, 2008 p. 159).
Regulation can be imposed by harsh climatic conditions, such as cold weather. An ecosystem is viable for restoration using the succession model if it falls under two situations. The first scenario is whereby the degree of impairment is modest and probably requires little or no repair of abiotic conditions on site.
In this case, restoration can be achieved through manipulations of the biotic community, such as replenishing biotic diversity that was reduced due to degradation (Brown, & Lugo, 1994 p. 100).
The other situation is in extreme cases such as harsh environments where plant diversity is already low, and ecosystem processes are restricted by the environment. In such cases, the potential for flux and patch dynamics is reduced, and the attainment of the former state is probable.
The last scenario indicates that the succession model can be used to restore degraded land due to agriculture to tropical forest ecosystems. This is because such abandoned lands have less plant diversity and require the introduction of the initial abiotic and biotic factors to attain the original state (McGlade, 1999 p. 56).
Alternative States Model
Recent experiments show that degraded systems are resilient to conventional reinstatement efforts. This is due to constraints such as alterations in landscape connectivity and organization, loss of indigenous species pools, changes in species domination, trophic relations and invasion by exotics and concomitant effects on biogeochemical processes.
As such, there is an increasing demand for alternative ecosystem state models that entail system thresholds and feedbacks in ecology restoration. These models hold that the system can shift abruptly between two or more states (Suding, Gross & Houseman, 2004 p. 47).
Models of alternative states incorporate positive feedbacks and alternative internally reinforced states. They are increasingly deployed in the prediction of whether or not a system is likely to collapse due to gradual changes in climatic factors, human exploitation of biotic resources, habitat loss and fragmentation.
These models are also useful in providing a constructive framework for developing management tools to restore systems that have already collapsed to a degraded state. The advantage of this approach is that it recognizes that the dynamics of the degraded state are unique from those in the new or target state and that the trajectory to recover will probably be different from that of degradation.
Alternative state model can be used to convert degraded agricultural land into tropical forest through a number of ways (Suding, Gross & Houseman, 2004 p. 49).
For instance, abandoned agricultural fields characterized by sealed surfaces and slow infiltration rates can be restored into tropical forest through appropriate interventions. In this scenario, any rainfall that occurs is lost to runoff, hence, creating a positive feedback for drier conditions support less vegetation and less undergrowth holds less water. Without intervention, the ecosystem cannot recover.
This positive feedback can be by restoration treatments tha6t roughen the soil. After micro-catchments are dug and water is held, vegetation regenerates spontaneously, creating a structure that catches more water (Holl, Loik & Lin, 2000 P. 345).
The incorporation of alternative state model ideas into practice involves consideration of a number of factors. To begin, the specific restoration goal should be determined. Goals may concern particular species composition, system structure or function.
Although restoration goals generally concern the return of a system to a state approximating condition before degradation, goals pertaining to the restoration or creation of particular services or functions might be more practicable in some areas. After determining the specific restoration goal, the next step is identifying constraints in the degraded ecosystem (Suding, Gross & Houseman, 2004 p. 51).
Apart from disturbance and physical constraints, there are four types of biotic constraints that hinder restoration. These include biogeochemical feedbacks, trophic-level interactions, propagule limitation, and regional environmental change. These constraints can be identified through experimentation.
After identifying the constraints, the next stage is to prioritize them. This happens especially when multiple constraints exist. High priority should be a reserve of those constraints that can be tackled simultaneously, require the least cost to cross recovery thresholds, and those that have strong dependencies or feedbacks to the rest of the ecosystem. The prioritized constraints should then be addressed.
The actions taking on them are determined by the specific constraints. Where possible, efficacy should be tested before implementation. The next stage of incorporating alternative state model ideas involves characterizing the changed system (Suding, Gross & Houseman, 2004 p. 53).
A monitoring program is adopted to determine whether the goals of restoration are met and which constraints persist. At this stage, goals may have to be re-evaluated, and the process is re-initiated. However, if goals have been met, focus can now shift to maintenance of the system instead of restoration.
Conclusion
In conclusion, the restoration of high diversity tropical forests following the abandonment of agricultural lands offers a challenge for restoration ecologists. The appropriate restoration strategy relies on the magnitude of degradation, the desired rates of recovery and the desired similarity to species composition in the initial forest.
This is because if the goal of a restoration strategy is not only to recover forest structure but also species composition similar to the initial forest, an extra intervention will be required. This essay holds that natural regenerations can be an effective way for restoring tropical secondary forests, and so is the alternative state model.
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