Ecosystem stability: Concept (resistance and resilence)

Ecosystem stability: Concept (resistance and resilience), 

INTRODUCTION

Man has always inhabited two worlds. One is the ‘natural-world’ of plants, animals, air, water, and soil of which man himself is a part; while other is the ‘built-world’ of social and cultural institutions and artifacts which he created for himself by using science and technology, and political organization. Both the natural and socio-cultural worlds constitute an important part of the environment.

Thus, Environment is a quite comprehensive term that includes not only the areas of air, water, plants, and animals, but also other natural and man-modified features like transportation systems, land use characteristics, community structures, as well as economic stability. In other words, the environment is made up of both biophysical and socio-economic elements. As a result of environmental degradation due to unrestrained industrial and technological progresses and over-exploitation of natural resources; there has been increasing awareness in environmental issues in sustainability and the better management of development in harmony with the environment, in recent decades. Associated with this environmental awareness has been the introduction of new legislation, emanating from national and international agencies, that seeks to influence the relationship between development and the environment.

ECOSYSTEM STABILITY

We are at that point in the course where we are switching gears. In last four units, we have focusing on the biological diversity, environmental pollution and their control measures, and conservation strategies for biological wealth. In this unit, we are going to be reading about ecosystem stability and function, perturbation and restoration, invasive plant species and their ecology, and ecological management. We will be reading about environmental impact assessment and sustainable development.

Every ecosystem is subject to perturbations such as climate, nutrient fluctuation, loss of biodiversity, and introduction of exotic species, that can alter ecosystem structure and function. The degree to which ecosystems respond to perturbations depends their ability to withstand perturbation and maintain normal function (resistance) and/ or recover from disturbance (resilience).

It is commonly believed that the more diverse an ecosystem is, the more stable it will be. These two components of stability, in essence, address how a system responds to disturbances and knowledge about them is key to developing ecosystem recovery and restoration efforts. Before we can read about this relationship, however, we need to decide just what diversity really is.

Whittaker [1972] distinguished three types of diversity.

1.    Alpha diversity—diversity within a particular area or ecosystem

2.    Beta diversity—the change in diversity between ecosystems

3.    Gamma diversity—the overall diversity in a landscape comprised of several ecosystems

Rosenberg et al. [2002] studied microsatellite variation in human beings at several hundred loci. They were able to distinguish five major geographical groupings of populations: Africa, Eurasia, East Asia, America, and Oceania. Of the total diversity in human populations, roughly 10% of the diversity is a result of differences among the different geographical groupings. Diversity within each geographical grouping corresponds to alpha diversity, the total diversity within human beings corresponds to gamma diversity, and the proportion of diversity due to differences among geographical populations (10%) corresponds to beta diversity.

Most analyses for conservation purposes have focused only on species diversity attempting to identify regions with a large number of species. You all probably realize, however, that there are several aspects to diversity :

·         Number of different species

·         Relative abundance of different species

·         Ecological distinctiveness of different species, e.g., functional differentiation

·         Evolutionary distinctiveness of different species

We also won’t discuss formal definitions of ecological diversity, which are primarily definitions of alpha diversity although they can be generalized to allow partitioning of gamma diversity into its alpha and beta components. These definitions treat all species as equivalent, ignoring aspects of ecological and evolutionary distinctiveness. Until relatively recently, many experimental evaluations of the diversity-stability hypothesis did the same.

DIVERSITY AND STABILITY

Over the past few decades, it has been commonplace for conservationists to appeal to the diversity-stability hypothesis as a component of their arguments for the importance of conserving biological diversity.

Consider, for example, the following passage from Barry Commoner’s book, The Closing Circle : The amount of stress which an ecosystem can absorb before it is driven to collapse is also a result of its various interconnections and their relative speeds of response. The more complex the ecosystem, the more successfully it can resist a stress. Like a net, in which each knot is connected to others by several strands, such a fabric can resist collapse better than a simple, unbranched circle of threads – which if cut anywhere breaks down as a whole. Environmental pollution is often a sign that ecological links have been cut and that the ecosystem has been artificially simplified

Principles

Robert MacArthur [2002] proposed measuring the stability of an ecosystem by measuring the number of alternative pathways it contains through which energy can flow. He justified this measure by arguing that an ecosystem with many pathways, representing an abundance of species organized in a complex food web, tends to equilibrate fluctuations in population as predators will switch from less abundant to more abundant prey species, lowering population densities of the more common and allowing the density of the less common to increase.

There are six reasons for thinking the hypothesis to be true:

Evidence from mathematical models suggests that those with few species are inherently unstable.

Laboratory experiments are consistent with the mathematical models.

Habitats on small islands are more susceptible to invasion than are those on continents.

Less diverse habitats of cultivated or planted land are more susceptible to invasion than undisturbed habitat.

Highly diverse tropical forest ecosystems are relatively resistant to pest invasion.

Orchard spraying, which simplifies ecological relationships, tends to increase the likelihood of severe oscillations in pest populations.

In 1975, Daniel Goodman summarized the mounting evidence against the diversity stability hypothesis by responding to each of Elton’s arguments for it.

2 Models of more complex communities showed just the opposite of what Elton asserted. The more species that interact the less likely the system is to be stable.

The data suggesting vulnerability of islands to invasions by pest species may result from accidents of distribution or other special characteristics of islands.

Crops and orchard tree planted in pure stands do not represent equilibrium low diversity systems. It is difficult to find evidence that naturally low diversity communities are more susceptible to invasion than naturally high diversity communities.

The tropical biota is so diverse and complex that large fluctuations might go unnoticed. Furthermore, there is evidence that even highly diverse systems can be dramatically altered by invaders, e.g., the impact of the crown-of-thorns starfish on coral reefs.

Empirical results

Tilman and Downing [14] suggest that primary productivity in more diverse plant communities is more resistant to, and recovers more fully from, a major drought.

207 plots of prairie grasslands differing in species richness from 1 to 26.

Measured resistance as relative rate of community biomass change from 1986, the year before a drought, to 1988, the peak of the drought.

Drought resistance is an increasing function of community diversity.

Saturates at about 10–15 species.

More diverse communities are more resistant than less diverse communities, but they don’t have to be very diverse.

Recent results on the relationship between bacterial species diversity and community respiration (a measure of total microbial activity) show that there are diminishing returns as the number of species in the bacterial community increases [1]. The strong diminishing returns associated with increases in species diversity are likely to be a general feature of relationships between ecosystem processes and species richness.

In a similar experiment Tilman et al. [1996] found that plant cover is an increasing function of species richness and lower concentrations of inorganic soil nitrogen, presumably because of greater nitrogen uptake in more diverse communities.

Experiments of van der Heijden et al., (1998) on mycorrhizal diversity suggest that plant species composition and community structure are more sensitive to the present or absence of particular mycorrhizal associates when the diversity of mycorrhizal fungi is low. Similarly, plant species diversity, nutrient capture, and productivity are increasing functions of mycorrhizal diversity.

But there are two possible explanations for patterns like these:

More diverse communities could increase the chances that at least one species within them is highly productive.

More diverse communities may be able to tap resources more effectively because the differ in strategies for resource acquisition.

Cardinale et al. [2006] perform a meta-analysis of 111 field, greenhouse, and laboratory studies that manipulated species diversity to determine its effect on abundance and biomass. They found that

Decreasing diversity is, on average, associated with decreased abundance, decreased biomass, or both.

The standing biomass of the richest polyculture tends to be no different from that of the most productive monoculture.

“Collectively [their] analyses suggest that the average species loss does indeed affect the functioning of a wide variety of organisms and ecosystems, but the magnitude of these effects is ultimately determined by the identity of species that are going extinct”. Using a somewhat different approach Grace et al. reach a similar conclusion: “[T]he influence of small-scale diversity on productivity in natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity”.

FUNCTIONAL DIVERSITY

Diaz and Cabido [2001] point out that experiment like those just described focus only on the number of species present, not on the functions, they play in an ecosystem. They summarize evidence from a variety of studies suggesting that ecosystem processes depend on functional diversity far more strongly than on species diversity per se. They suggest two plausible explanations:

Functional redundancy

Two or more species in a particular ecosystem may play essentially the same role in ecosystem processes. It may for example, make relatively little difference to the nitrogen dynamics which particular species of legumes are present, only that there are some nitrogen-fixing plants present. The loss of species with similar functional effects should have relatively little effect on ecosystem processes.

Functional insurance

The more divergent species in an ecosystem are with respect to their influence on ecosystem processes, the smaller the number required to buffer an ecosystem against change. Species with similar functional effects that differ in functional response may buffer ecosystems against externally imposed change because the species that influence each ecosystem response may respond differently.

CONCEPTS OF STABILITY

Part of the problem here is that it’s not entirely clear what we mean by stability, nor what aspect of diversity we are considering.

Are we concerned only with the number of species in the community and its relation to stability, are we concerned with how evenness relates to stability, or are we concerned with some combination of both? Work I am aware of that considers the problem focuses only on species diversity, i.e., the number of species present, and only recently has begun to consider the degree of functional diversity represented.

In one sense this may be legitimate. After all, part of the reason conservationists have invoked the diversity-stability hypothesis is to justify concern about the loss of individual species.

We may also be missing something important. If other aspects of diversity play an important role in the structure and function of ecosystems, a focus on the number of species alone may blind us to the role that evenness plays in the ability of ecosystems to respond to changes in energy and nutrient inputs.

Various meanings depending on context. There are at least three ways in which stability might be defined.

1.    Lack of change

2.    Ability to return quickly to a previous state

3.    Not going extinct

Some ecologists use more specific terms :

Constancy

“Tendency of an ecosystem to maintain homeostasis (i.e. remain constant over time)” Homeostasis = equilibrium. This is the definition most people use when talking about ecological “stability” i.e. remains at equilibrium.

What remains at equilibrium? Examples: No long-term loss of:

§  Biomass production

§  Species composition

§  Population structure

§  Resources: organic matter, moisture, nutrients, soil characteristics

The ability of a community to resist changes in composition and abundance in response to disturbance. Not a particularly useful concept of stability for conservationists because.

Few, if any, ecosystems could be described as constant.

Even those that have powerful mechanisms for reacting to environmental fluctuations do so through internal changes that return the system as quickly as possible to a stable state. But these involve responses and changes. It seems better to regard them as examples of resiliency than of constancy.

Resilience

The ability of a community to return to a prior state (equilibrium) after disturbance.

Elasticity : how quickly community returns to equilibrium after disturbance

Amplitude : how much disturbance community can tolerate and still return to equilibrium

Resistance : the force needed to change the community

The ability of a community to return to its pre-disturbance characteristics after changes induced by a disturbance. Resiliency corresponds to stability the way it is studied in mathematical models. Are deviations from equilibrium reduced with time (stable) or amplified with time (unstable)? Still, it has little applicability to actual ecosystems. It measures a system’s tendency to return to a single stable point, but

Many ecological systems appear to have multiple stable points. If disturbance remains below a particular threshold, it will return to its predisturbance configuration. If it exceeds that threshold, it may move to a new configuration.

Furthermore, most ecological systems change not only in response to disturbance but also in response to natural, successional change.

There is little evidence that ecological communities ever represent an equilibrium configuration from which it would make sense to study perturbations.

Constancy and resiliency have this in common: both focus on species persistence and abundance as measures of stability.

Persistence

Not going extinct. Usually refers to populations of endangered species, as in “the population is stable”.

Dynamic stability

A system is dynamically stable if its future states are determined largely by its own current state, with little or no reference to outside influences. In many ways this seems to correspond with our intuitive notions of stability and to make sense of the relationship between diversity and stability.

Recall the quote from Commoner: “The more complex the ecosystem, the more successfully it can resist a stress.”

A system that is dynamically stable is one that is relatively immune to disturbance. A rapidly spinning gyroscope is dynamically stable because the gyroscopic forces that it generates resist external forces that would alter is plane of rotation.

It reflects our hope that stable systems should be able to maintain themselves without human intervention.

A diverse biological system is more likely to be dynamically stable than one that is not diverse because in diverse communities biotic interactions may often play a larger role in a species’ success than its interactions with the physical environment. To the extent that changes in the system are driven by biotic interactions, it is dynamically stable, since characteristics of the system itself are determining its future state.

Ives and Carpenter (2007) suggest a different approach to understanding community stability.

–        Alternative stable states

–        Non-point attractors

–        Pulse perturbations

–        Press perturbations

–        Extinctions

–        Invasions

Their approach strikes as quite useful, first because it emphasizes that systems move to a region different from the one from which they were perturbed and second because it reminds us that things other than diversity, like the frequency and character of perturbation, may affect the stability of ecosystems.

Biological integrity

Biological integrity refers to a system’s wholeness, including presence of all appropriate elements and occurrence of all processes at appropriate rates.

1.    What are “appropriate elements”?

2.    What are “appropriate rates” of processes?

By definition, naturally evolved assemblages possess integrity but random assemblages do not. Therefore, provides justification for management focusing on native species rather than introduced ones. This seems like the logical fallacy of affirming the consequent, but

Species composition of lakes exposed to nutrient enrichment or acidification responds more quickly and recovers more slowly than processes like primary production, respiration, and nutrient cycling.

Shifts in biotic composition do not necessarily lead to changes in process rates.

Angermeier and Karr suggest that these observations means a focus on integrity rather than diversity makes sense. To me it makes more sense to conclude that species changes are a more sensitive indicator of what is going on than process changes.

Loss of native species from a system is truly “a canary in the mine,” a warning of process changes that may have consequences much larger than we suspect.