UNIVERSITY OF NOVA GORICA SCHOOL OF ENVIRONMENTAL SCIENCES Irena KRANJC Indirect Effects in Ecology Menthor: Marko Debeljak Ljubljana, 2015.

UNIVERSITY OF NOVA GORICASCHOOL OF ENVIRONMENTAL SCIENCES

Irena KRANJC

Indirect Effects in Ecology

Menthor: Marko Debeljak

Ljubljana, 2015

Interrelations among ecosystem components and processes can be subdivided into direct and indirect effects:

Direct effect - restricted to the direct effect of one component (or process) on another, and are attributable to an explicit direct transaction of energy and/or matter between the components in question)

Indirect effect - those which are not comply with the above restriction

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In nineteenth century the significance of indirect interactions was well realized and was accounted for in the classic studies of Darwin, Dokuchaiev, Gumboldt, Engels, and many other scientists.

In the twentieth century, appreciation of indirect effects in nature received considerable acceleration, due to:

accumulating interdisciplinary knowledge of natural ecosystems, development of appropriate mathematical techniques, urgent necessity to resolve the growing problems of environmental

damage, resulting from the uncurbed expansion of the human population backed by the advances of the technological progress.

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Popularization of Vernadsky’s fundamental theories about the ‘biosphere’, the ‘noo¨sphere’, and interrelations between biota and geochemical cycling stimulated investigations of indirect effects even more.

Among a lot of possible indirect effects, there are five that have been most commonly studied:

Interspecific competition Apparent competition Trophic cascades Indirect mutualism and commensalism Interaction modification

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Interspecific competition – two species (or several) compete for the same resource. An increase in Component 1 will lead to the increased consumption of the shared resource (Component 2), and consequently to the decrease in a competitor (Component 3)

two predators sharing the same prey two microbial species whose growth is limited by the availability of

the same nutrient

indirect effect

Figure 1:Interspecific competition

1 3

2

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Apperent competition – two species have a common predator

an abundant population of species 1 sustains a high density population of predator 2, who, in turn, may limit the population of another prey species 3.

Indirect effect indirect effect

Figure 2: Apperent competition

1 3

2

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Trophic cascades – Trophic cascades involve propagation of the effect along a vertical trophic chain consisting of three or more components connected by grazing or predation.

an increase/decrease in Component 4 will lead to the decrease/increase in Component 3, increase/decrease in Component 2, and decrease/increase in Component 1.

These effects are particularly well studied in aquatic food chains, terrestrial systems

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Indirect mutualism and commensalism – Indirect mutualism and commensalism involve a consumer – resource interaction coupled with either exploitative or interference competition.

Starfish and snails reduce the abundance of mussels, a dominant space occupier and increase the abundance of inferior sessile species.

The presence of grazers on oyster farms in Australia increases oyster recruitment by removing algae, who otherwise preempt the available spaces. An increase in species 1 should lead to a decrease in species 2 and an increase in species 3.

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Interaction modification – the relationship between a species pair is modified by a third species

Interaction modification’is often, and quite rightly, considered as a principally different type of indirect effect. By coupling interaction modifications with other types of relationships (e.g.,trophic), one may arrive at possibilities of numerous relationships. One of the more simple of such combinations may be exemplified with an indirect effect of grazers and certain agricultural practices on the population density of foxes and the rodent in Eastern Europe (V. Takarsky, personal communication)

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Lower grazing rates lead to a denser and taller grass cover, enabling more successful hunting of predators.

Conversely, higher grazing rates lead to a lower grass cover, thus enhancing the detection of predators by the rodents. As a result, increase in grazing may have an indirect positive effect on the rodent population and an indirect negative effect on the population of foxes

Figure 4: Predators and Rodents 10

Classifications of Indirect Effects

Indirect effects can be characterized in a number of ways and may conveniently contribute to the toolbox for comparative ecosystem analysis:

related to the characteristics of exerting, receiving, and transmitting compartments

presence/absence of a lag phase before the manifestation of a response,

strength of the interaction (particularly in relation to the direct interactions) and its directionality (e.g., whether it is isotropic or anisotropic),

dependence on a specific ecosystem context, importance for the functioning of the compartments involved, importance for structural (e.g., successional or evolutionary) changes

in the populations involved and the whole biological community, and significance for overall ecosystem functioning.

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Indirect Effects

All the relations not restricted to the effects of a direct transaction of matter and energy between the adjacent ecosystem components are treated as indirect.

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Examples of Occurrence and Importance of Indirect Effects

Indirect Effects in Terrestrial Environment Indirect Effects in Aquatic Systems Role of Abiotic Components Indirect Effects of Global Relevance Indirect Effects and Industrial Ecology Evolutionary Role of Indirect Effects

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Indirect Effects in Terrestrial Environment

In the end of nineteenth century the school of thought founded by Dokuchaiev had developed a theory that soil was a product of complex interactions between climate and geological and biological components of the terrestrial landscape.

Indirect effects in terrestrial ecosystems relate to the dependence of plant nutrient supply on mineralization of nutrients by soil biota, and to the propagation of these effects through the food chain.

Soil fauna may help to disperse microorganisms crucial for plant functioning and biogeochemical cycling, and physically modify the habitat, thus changing environmental conditions for all the biological community. Plants, in turn, modify the habitat for other organisms, for example, by producing litter, providing shade, shelter, etc.

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Indirect Effects in Aquatic Systems

Recent studies of indirect effects in aquatic environment variously involved a combination of the empirical approach and an application of statistical techniques, methods of network analysis, simulation modeling using ‘What if’ scenarios, and sensitivity analysis.

One of the perhaps most frequently addressed examples of indirect effects in aquatic environment relate to trophic cascades,

which involve propagation of the effect along a vertical trophic chain consisting of three or more components connected by grazing or predation. Daskalov was recently investigated a decrease in the top predator’s population in the Black Sea due to overfishing resulted in a ‘trophic casade’, leading to an increase in the abundance of planktivorous fish, a decline in zooplankton biomass, and an increase in phytoplankton crop.

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Role of Abiotic Components

In ecology, it is widely recognized that species interaction can be mediated by a nonliving resource, and that a species can potentially exert a selective force on another species through nontrophic interactions. It should also be noted that in nature many species are very well adapted to modify their community and habitat.

beavers by changing the habitat’s hydrological regime, humans by initiating dramatic changes in global climate and geochemical fluxes, earthworms by increasing aeration and redistributing organic matter in soil

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Indirect Effects of Global Relevance Indirect relationships important on the global or sub global

scale are often separated from their cause spatially and/or temporally.

For example, the dramatic increase in volcanic activity (possibly caused by the impact of an asteroid) at the end of the Mesozoic era is thought to have led to the extinction of dinosaurs, which arguably stimulated the eventual evolution of mammals (including humans).

Figure 5 : Volcanic activity17

The increased production and use of fertilizers in the 1950s led to the increased phosphate inputs, eutrophication, and decrease in water quality in many lakes, ponds, and reservoirs during the subsequent decades.

Figure 6: Water pollution

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The increased consumption of fossil fuels in the twentieth century led to the increased emissions of carbon dioxide, which were eventually followed by global warming and an apparent increase in the frequency of natural disasters.

Global climate change (principally related to the increased concentrations of greenhouse gases) is still one of the most discussed topics in ecology and environmental sciences in general.

Figure 7: Effects of Global Warming 19

Indirect Effects and Industrial Ecology

Industrial ecology is based on the analogy between natural and industrial ecosystems, and aims to facilitate the development of industrial recycling and cascading cooperative systems by minimizing the energy consumption, generation of wastes, emissions, and input of raw materials.

One of the commonly used methods of industrial ecology is ‘life cycle assessment’ (LCA). It studies the environmental aspects and potential impacts throughout a product’s life ("cradle to grave"), from raw material acquisition through production, use and disposal.

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Similar to LCA, but usually with considerably narrower system boundaries, are methods of energy analysis, including energy footprinting (which, effectively, constitutes calculations of how much energy is spent and saved /recovered in all the processes included within the chosen system boundary) and net energy analysis (which in addition to the detailed energy budgeting involves calculation of indicators such as incremental energy ratio and absolute energy ratio).

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Another method popular in ‘industrial ecology’ is ‘ecological footprinting’. The method estimates the area necessary to support (i.e., in terms of, for example, production of food, energy, processing of wastes) current, past, or probable future functioning of particular geographical units.

Only the ecofootprints of Australia and Canada appear to fit inside their borders. The rest of the ‘developed’ countries appear to live on the expense of other territories.

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Evolutionary Role of Indirect Effects Indirect effects often promote coexistence and the role of

indirect effects should, in general, increase in the course of evolution.

In grassland communities containing Rumex spp., insect herbivory (by Gastrophysa viridula) appears to be a cost inherent in the development of plants resistance to pathogenic fungi (Uromyces rumicus). infection of plants with endophytic fungi often enhances plants competitive abilities via deterring grazers by production of toxic compounds (as a result, some plants might have coevolved together with their endophytes)

It should be noted, that indirect effects are important for the evolution of not only natural, but also industrial ecosystems.

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Approaches and Techniques Used to Detect and Measure Indirect Effects

Detection and measurements of indirect effects are often far from straightforward, and are mostly based on the intuition, common sense, and prior knowledge of any particular system.

Abrams and co authors described two major approaches adopted in ecological studies:

Theoretical Experimental

Some ecologist think that the methodological continuum to study indirect interactions is best represented by a triangle with observational, experimental, and theoretical nodes.

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Theoretical approach - observations (and/or carefully considered experimental data) are used together with theoretical considerations to construct a model capable of investigating interactions among the components incorporated in the model structure.

Experimental approach - densities of individual species are manipulated (e.g., by total removal) in microcosms or experimental plots, and statistical analysis (e.g., ANOVA, ANCOVA) are subsequently applied to estimate the magnitude of indirect effects of manipulations on densities of other species.

Mathematical methods which have been used in studies of indirect effects in natural ecosystems are statistical methods (PCA, factor analysis, CCA, ANCOVA, ANOVA), simulation modeling (e.g., using ‘what if scenarios’, sensitivity and elasticity analysis), and methods of network analysis.

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Problems and Implications for Environmental Management

There are many problems associated with studies of indirect effects.

Complexity and Uncertainty Separation in Time Separation in Space Defining System Boundaries

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Implications for Environmental Management

Qualitative and quantitative account of indirect effects is becoming a common part of environmental management, and is indispensable for successful application of landscape engineering, biomanipulation, biogeochemical manipulation, strategic environmental assessment (SEA), and environmental impact assessment (EIA), etc. In particular, mathematical methods can be helpful in this respect.

A combination of empirical and theoretical work should precede any practical steps, and any desk study should be backed up by a thorough monitoring plus experimental program.

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Current and Further Directions

Further investigations of indirect effects are important both for enhancing our understanding and therefore improving management of specific ecosystems, and for general development of ecology.

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Questions1.How can we subdivided interrelations among ecosystem components and processes?

2. Which are five most commonly studied indirect effects?

3. List three examples of Importance of Indirect Effects

4. What kind of Approaches and Techniques we Use to Detect and Measure Indirect Effects?

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