Developing an ecological context for allelopathy

Scott J. Meiners Chui-Hua Kong

Laura M. Ladwig Nikki L. Pisula

Kimberly A. Lang

Received: 28 March 2012 / Accepted: 7 June 2012

Springer Science+Business Media B.V. 2012

Abstract There has been a renewed interest in

allelopathy as a plantplant interaction as more plant

ecologists have become involved in studying biolog-

ical invasions. This resurgence highlights a major

deficiency in our understanding of allelopathythe

lack of a well-developed ecological context for the

interaction. In contrast to allelopathy, the plantplant

interaction of competition has a strong theoretical

foundation as well as a large body of supporting

empirical studies. We suggest that the plant-herbivore

defense literature provides a mature and well-devel-

oped framework from which a broader ecological

context for allelopathy can be developed. Here, we

discuss three broad classes of questions, drawn from the

herbivore defense literature, which may help to develop

an appropriate ecological context for allelopathy. These

questions focus on (1) variation in allelopathic expres-

sion within species, (2) community level variation in

allelopathy across species, and (3) variation in the

impacts of allelopathy on associated species. Address-

ing such broad population and community level themes

in a variety of systems will be necessary to fully develop

an ecological context for allelopathy and provide a

theoretical basis for understanding its role in plant

communities.

Keywords Allelopathy Allocation Competition Cost of defense Impacts Variation

Introduction

Allelopathy is broadly defined as any chemical-

mediated interaction among plants, though it is

typically thought of as a mechanism of inhibition

(Rice 1974). Within ecological research, allelopathy

has rapidly become a favored mechanism of non-

native species impacts and success (Lawrence et al.

1991; Heisey 1996; McCarthy and Hanson 1998;

Roberts and Anderson 2005; Abhilasha et al. 2008;

S. J. Meiners (&)Department of Biological Sciences, Eastern Illinois

University, Charleston, IL 61920, USA

e-mail: sjmeiners@eiu.edu

C.-H. Kong

Department of Ecology, College of Resources and

Environmental Sciences, China Agricultural University,

Beijing 100193, China

L. M. Ladwig

Department of Biology, University of New Mexico,

Albuquerque, NM 87131, USA

N. L. Pisula

Gewalt Hamilton Associates, Inc., 850 Forest Edge Drive,

Vernon Hills, IL 60061, USA

K. A. Lang

Department of Biology, Bradley University, Peoria,

IL 61625, USA

123

Plant Ecol

DOI 10.1007/s11258-012-0078-5

Gomez-Aparicio and Canham 2008; Inderjit et al.

2008; Lankau et al. 2009). While there has been some

conceptual development in the context of invasion

(e.g., Callaway and Aschehoug 2000; Thorpe et al.

2009), allelopathy as an ecological phenomenon has

evolved relatively slowly since its inception therefore

its importance to communities and populations

remains unclear (Wardle et al. 1998).

In contrast to allelopathy, there is a very strong

conceptual basis for the understanding of herbivore

defenses (e.g. Atsaat and ODowd 1976; Coley et al.

1985; Rosenthal and Kotanen 1994; de Jong 1995).

This rich body of theory allows us to predict the types of

species and tissues that may be defended, the flexibility

of defense (i.e., inducible or constituent), the evolu-

tionary constraints of allocation to defenses and shared

evolutionary history with herbivores. As allelopathy

may be thought of as a chemical defense against

competitive suppression, we may expect to see similar

ecological patterns to the interactions between plants

and their herbivores. Herbivore defenses and allelo-

chemicals are similar in that they require allocation of

energy and nutrients and should incur costs to the

organism that must be balanced with the benefits of

protection. However, defenses against herbivores differ

from allelopathy in the directness of the interactions

involved. While herbivore activity is a direct impact on

the affected plant, allelopathic interactions rely on

chemicals released into the environment. The allelo-

pathic interaction is therefore mediated by soil chem-

ical properties and soil microbial communities, and

may directly influence plant growth or indirectly

through altered microbial associations. Despite the

more direct nature of plantherbivore interactions, they

represent a reasonable starting point to guide our

understanding of allelopathy.

Drawing heavily from the conceptual foundation of

plant antiherbivore defenses, we briefly outline three

broad research questions that should be addressed to

build an appropriate ecological context for allelopathy

(Fig. 1). Our goal is to simply point out parallels

between the two areas of research and illustrate

productive avenues for allelopathic research. These

questions are not the only challenges to the study of

allelopathy as there are many methodological issues

that must still be resolved. However, we believe that

by addressing these broad questions plant ecologists

will be more able to understand the importance of

allelopathy in natural systems.

What are the sources of variation in allelopathy

within species?

The vast majority of allelopathic research focuses on

whether a species is allelopathic and what the

causative agent may be. The conclusion of many of

these allelopathic studies is that the test species is

determined to be either allelopathic, or not. While

such studies are beneficial, they neglect to address the

potential for intraspecific variation in allelopathy.

From the herbivore defense literature, we know that

within a species there may be considerable variation in

defensive chemistry based on genetic variation in

chemical production, the environmental conditions

that an individual is growing in, the exposure of the

individual to herbivores, or the developmental stage of

the individual (Coley et al. 1985; Siemens et al. 2002;

Agrawal 2004). Allelopathic studies need to include

sources of intraspecific variation to fully understand

the ecological context of allelopathy at the population

level. Central to the population ecology of allelopathy

is the idea of energetic costs. Plants that allocate

resources to allelochemicals are expected to incur

costs to growth and reproduction that natural selec-

tion should balance with the benefits of reduced

competition.

Genetic variation

As ecologists, we know remarkably little about the

variation in allelopathy that would serve as the base

variance upon which selection would function. In

contrast, genetic controls on herbivore defenses are

commonly examined (e.g. Gols et al. 2008). As with

any trait, we would expect genetic variation among

individuals even when allelopathy is strongly advan-

tageous. Genetic variation may allow local adaptation

which would favor increased allocation to allelopathy

in competitive environments and a decrease when the

costs exceed the benefits. Some evidence for genetic

variation in allelopathy comes from the agricultural

literature that evaluates crop varieties based on

allelopathic control of weed species (e.g. Weston

1996). Selection imposed from crop breeding has

generated variation in allelopathy among varieties,

though this was not the original selection goal

(Bertholdsson 2004). We may expect similar variation

in plant populations, where allelopathy may be

beneficial and therefore selected for. For example,

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123

Lankau (2008) found genetic variation in the produc-

tion of allelopathic chemicals in Brassica. In this

study, allocation towards chemical defense resulted in

greater interspecific competitive ability, but reduced

intraspecific competition from allocation costs. These

types of studies suggest that understanding genetic

variation and selective pressures may yield important

insights into allelopathy as an ecological process

(Lankau et al. 2009). Identifying genotypes of varying

allocation to allelopathy will allow for the quantifica-

tion of the cost of allelopathy by comparing growth

rates in the presence and absence of competition and

will provide opportunities to understand the underlying

tradeoffs that constrain allelopathy. Furthermore, geno-

types with varying degrees of allelopathy can provide

experimental opportunities to test for the direct benefits

of allelopathy as well as its impact on experimental

plant communities.

Environmental variation

The ability of plants to produce defensive chemicals is

often constrained by the environment in which the

plant is growing (Coley et al. 1985). Environments

that are limiting in resources needed to construct

particular allelochemicals or are physiologically

stressful may alter the production of allelochemicals

(Kong et al. 2002; Kong et al. 2004; Rivoal et al.

2011). Allelochemicals, just as herbivore defense

chemicals, may be primarily carbon molecules or may

contain nitrogen (Duke and Dayan 2006) and may be

favored by different growing conditions. The respon-

siveness of plants to the environment would result in

phenotypic plasticity across landscapes that vary in

resource availability and competitive interactions.

Understanding the basis for environmentally-driven

variation will allow the prediction of conditions under

which we would expect allocation to allelochemicals

and potentially strong effects of allelopathy. An

environment-allelopathy linkage would also suggest

a specific triggering mechanism for allelopathy.

Though allelopathy is commonly considered a com-

petitive mechanism, it is unknown whether it functions

to alleviate competition specific to water, light or

mineral nutrients or competition in general. Compe-

tition has been found to both increase and decrease the

abundance of allelochemicals (Kong et al. 2002;

Rivoal et al. 2011), leaving its role as a constraint on or

trigger for allelopathy unclear.

Inducible versus constituent allelopathy

Many species maintain a base level of antiherbivore

chemicals, increasing their production only if the plant

is damaged by an herbivore (e.g. Harvell and Tollrian

1999; Siemens et al. 2002). Flexibility in defense

effectively minimizes the cost of producing and

maintaining defenses in the absence of herbivores,

Species 1

GeneticEnvironmentalInducible

Species 3

GeneticEnvironmentalInducible

Species 2

GeneticEnvironmentalInducible

Variation in impactsSpecificityEvolutionary responses

Variation among speciesPrevalence of allelopathyPlant strategiesInvasion

Species n

GeneticEnvironmentalInducible

Variation within species

Fig. 1 Conceptual diagram that highlights the primary sourcesof variation in allelopathy within plant communities. Though

dealt with separately in this article, there is clearly potential for

interactions among these sources of variation. The study of

allelopathy must incorporate a broader view that includes the

diverse range of interactions and contingencies into its

conceptual framework. This approach will allow the develop-

ment of a more complete theoretical basis for allelopathy and its

role in plant communities

Plant Ecol

123

though an energetic cost is incurred from the initial

damage. As there are likely costs to allelopathy

(Lankau 2008), it may be selectively advantageous

for plants to similarly vary allelochemical production

based on the presence/absence of interspecific com-

petitors (Kong et al. 2002). For example, allocation

of carbon and mineral nutrients towards growth and

reproduction instead of allelochemicals when com-

petition is minimal would maximize fitness in that

environment. Interestingly, allelochemicals may

function as both an inhibitor of competing plant

species and defense against herbivores (Wardle et al.

1998; Cipollini 2004; Thelen et al. 2005; Cipollini

et al. 2008), leading to more a complex suite of

potential regulators. Establishing a causal link-

age between competitive environment and allelopa-

thy would strengthen its role as a competitive

mechanism.

Tolerance versus resistance to competition

Another major concept used to explain variation in

defense in the plant-herbivore literature is the trade-off

between resistance (defensive) and tolerance traits.

Plant species may allocate resources to defenses

against herbivore damage or may develop physiolog-

ical responses such as compensatory growth, resource

use efficiency or utilization of stored carbohydrates to

minimize the impact of herbivore damage on plant

fitness (Strauss and Agrawal 1999; Stamp 2003).

While the presence of a trade-off is not always found

between defensive and tolerance traits (Mauricio et al.

1997; Leimu and Koricheva 2006), similar processes

may also occur with competition. Allelopathy in this

context would be the resistance end to the strategy

gradient where the release of chemicals would inhibit

the competitive ability of neighboring plants. Toler-

ance characteristics may be similar to those proposed

in the herbivory literature. Tolerant plants may

respond to competitive pressures by rapid root prolif-

eration (a form of scramble competition), compensa-

tory allocation to root or shoot tissues to increase

resource uptake, increased efficiency of resource

utilization or phenological shifts in resource uptake

with storage. While the potential for such alternate

strategies has not been explored in the context of

allelopathy, it would help to explain variation in

allelopathic strength among populations and species.

What are the sources of variation in allelopathy

among species?

Plant species are expected to exhibit a range in levels

of antiherbivore defenses with some species very

poorly defended, others heavily defended and many

species intermediate between the two extremes. Var-

iation in defense is set by the constraints imposed by

differences in allocation tradeoffs, life history and

evolutionary context across species. Allelopathy stud-

ies typically focus on a single target species that is

a priori thought to be allelopathic. This scenario

generates challenges for understanding variation in

allelopathy among species as the pool of studies is

likely biased towards more allelopathic species. As

case studies often determine whether a species is

allelopathic or not (a binary response), it makes

comparisons among species more difficult. To place

allelopathy in an accurate ecological context, we need

to address the continuous range of allelopathic inter-

actions across communities (Pisula and Meiners

2010).

Prevalence of allelopathy in communities

Allelopathy is typically treated as a somewhat rare

plant characteristic. When experiments are conducted,

one species is designated as allelopathic and the

other(s)the target speciesare assumed to be non-

allelopathic. In contrast to allelopathy, the herbivore

defense literature embraces the diversity of defensive

modes and levels within plant communities. As

competition is as ubiquitous, if not more so than

herbivory, it is possible that allelopathy is similarly

widespread in plant communities. To address the

community level aspects of allelopathy, we will need

surveys of large numbers of species (e.g. Moral and

Cates 1971) or meta-analyses of individual studies.

However, surveys, like those of any plant trait, must

include a range of species to avoid biases based on the

identity of the species selected for study. As larger and

more detailed surveys are developed, phylogenetic

patterns will need to be addressed, particularly with

regard to the types of chemicals produced. Ultimately,

knowing the prevalence of allelopathy may help us to

understand its importance in plant communities. This

information may also help us to understand why

documenting the community impacts of allelopathy

are so difficult (Wardle et al. 1998).

Plant Ecol

123

Ecological correlates of allelopathy

Within the herbivore defense literature is the concept

of whether species should grow or defend (Herms and

Mattson 1992). Selection may favor different strate-

gies based on an individuals potential life span and

other life history characteristics. If we were to place

allelopathy into Grimes C-S-R view of plant strate-

gies (Grime 1977), we would expect allelopathy, as a

competitive mechanism, to be predominately found

with the C-strategists. In this set of tradeoffs, ruderals

would typically not be exposed to competitive inter-

actions, and therefore would be less likely to be

allelopathic. Stress tolerant species may also be

allelopathic to capitalize on limited resources, but this

may be more context-specific. For example, produc-

tion of allelochemicals by light limited understory

species would be unlikely to increase light availability

to the forest floor and may detrimentally reduce

allocation to growth. However, if species in the

understory were limited by soil resources, production

of allelochemicals may be beneficial. While Grimes

suite of plant strategies may be a good starting place,

allelopathy may not cleanly fit into the scheme as

annuals (ruderals) are often found to be allelopathic

(e.g. Bertholdsson 2004; Wang et al. 2005; Lankau

2008). Broad scale analyses may also be able to find

linkages between environmental variables and alle-

lopathy. The benefit of looking for either life history or

environmental correlates of allelopathy is that it will

help us to predict the type of species and communities

where allelopathy may be important. Building this

context will also suggest a clear ecological function

for allelopathy within plant communities.

Linkage with invasion

Allelopathy is an often-cited mechanism of the

success and impacts of non-native species in intro-

duced habitats (Callaway and Aschehoug 2000;

Abhilasha et al. 2008; Inderjit et al. 2008) and is at

least partially responsible for the resurgence of interest

in allelopathy. Though part of this effect may be the

sensitivity of the native species to the invaders

chemicals (see below), allelopathy as a plant trait

may be important in determining which non-native

species become successful. To effectively determine

whether invasion success is linked with allelopathy we

would need to appropriately compare invaders with

native species (van Kleunen et al. 2010). A strong

argument for allelopathy as a key trait in invasion

would be if dominant non-native species have a

greater incidence of allelopathy than dominant native

species. If non-native and native species have similar

rates of allelopathy, then allelopathy may be a general

mechanism of dominance rather than one specific to

the invasion process. Alternatively, it may be that

non-native invaders are not more likely to be allelo-

pathic, just more likely to be tested than native species.

This scenario would not suggest that allelopathy is

unimportant in any individual invasion, just that it is

not a mechanism of dominance unique to non-native

species. Allelochemical production may still be

important to an invaders success if the environmental

constraints of the new habitat result in increased

production or if the new community is more suscep-

tible to those chemicals (Inderjit et al. 2011).

What are the sources of variation in the impacts

of allelopathy?

Ultimately we are interested in understanding what

allelopathy does in plant communities, just as we are

interested in how herbivores structure plant commu-

nities and populations. Allelopathy impacts need to be

examined at two temporal scales, ecological and

evolutionary, to fully understand their importance.

Again, the focus here is to generate a broader view of

allelopathy by looking at larger ecological patterns

rather than individual species as case studies.

Specificity of effect

Studies often identify differences among target spe-

cies in their response to an allelopathic plant species

(e.g. Abhilasha et al. 2008). Similar to antiherbivore

defenses, we may expect to see selection on the

efficacy of allelochemicals analogous to the selective

pressures imposed by generalist and specialist herbi-

vores. Rare or unpredictable herbivores are unlikely to

impose large selective pressures on plant populations,

whereas herbivores that consistently damage a species

should shape the chemical defenses towards their

activity. Similarly, strong competitors that are consis-

tently present in a community may generate stronger

selective pressures for allelopathy and select for

chemicals that have a strong specificity. For example,

Plant Ecol

123

herbaceous dicots in grasslands would more consis-

tently compete with grasses than with other herba-

ceous species. An effective allelochemical in this

situation may be much more effective on grasses than

other life forms. Therefore, we can postulate that

plants with a more unpredictable pool of competitors

may evolve allelochemicals with broader effective-

ness the equivalent of generalist herbivore defenses.

Identifying the rules that govern species responses to

allelopathy would address some of the difficulty in

showing community level impacts of allelopathy

(Wardle et al. 1998). As competitive interactions are

not equivalent across a community, likewise we

should not expect allelopathy to have uniform effects.

Selection in response to allelopathy

Not only should we expect selection to shape the

specificity of allelopathy, but we should also expect

selection to shape species response to allelopathy. If

the production of allelochemicals generates a consis-

tent selective pressure on resident species, then

genotypes that are less affected by the allelochemicals

should be favored in the community (Callaway et al.

2005). Over time, this trend may lead to suites of

allelopathic species and their primary competitors that

are adapted to the chemicals produced. Populations

without exposure to allelochemicals may therefore

remain more susceptible to their effects (Callaway

et al. 2005). In the context of invasion, this is called the

novel weapons hypothesis, where species nave to an

allelochemical may be more affected than those which

have coevolved with the allelopathic competitor

(Thorpe et al. 2009; Kim and Lee 2011). If the

selective pressure maintaining allelochemical produc-

tion is alleviated, this change may lead to shifts in

allocation from allelochemicals towards growth and

reproduction as has been seen in herbivore defenses

(Blossey and Notzold 1995; Inderjit et al. 2011).

Conclusions

The general themes proposed here are not the only

challenges to understanding the complexity and

importance of the ecology of allelopathy, nor is this

an exhaustive list of potential avenues for research.

There are still methodological issues associated with

examining large suites of species as well as in

separating out competitive effects from allelopathic

ones (e.g., Lau et al. 2008). Other important classes

of questions not dealt with here involve the role of

allelopathy in ecosystems and the interaction of

allelochemicals with soil processes (Wardle et al.

1998; Inderjit et al. 2011). From a developmental

perspective, the study of allelopathy is still in its

ecological infancy and needs to mature conceptually

before its importance to populations and communities

can be fully assessed. While the plant defensive

literature provides a reasonable starting point to

develop a broader context to allelopathy, we must

also remember the long history and controversies that

characterized its development (Stamp 2003). It is

important that we not only learn from the outcomes of

those debates, but also from the process so that we may

avoid similar pitfalls. By critically examining the

conceptual foundation of plant defenses, we may

wisely select ideas that may be appropriate to the field

of allelopathy and move forward.

The time is ripe for ecologists to develop a broader

conceptual view of allelopathy. Many studies focus on

determining whether a species is allelopathicthe

equivalent of asking whether a species is defended from

herbivores. While this approach is important to those

interested in the autecology of that species, it does not

directly move the broader field forward. The case study

approach is an important first step to determine whether

allelopathy can be important. As the answer to this query

is clearly yes, it is now time to move forward, build upon

the rich foundation of case studies, and develop the

ecological context of allelopathy.

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Plant Ecol

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Developing an ecological context for allelopathyAbstractIntroductionWhat are the sources of variation in allelopathy within species?Genetic variationEnvironmental variationInducible versus constituent allelopathyTolerance versus resistance to competition

What are the sources of variation in allelopathy among species?Prevalence of allelopathy in communitiesEcological correlates of allelopathyLinkage with invasion

What are the sources of variation in the impacts of allelopathy?Specificity of effectSelection in response to allelopathy

ConclusionsReferences