Allelochemically-based interspecific interactions

Math/ Compu~. Mode&g, Vol. 13, No. 6, pp. 31-34, Printed in Great Britain. All rights reserved

1990

08957177/90$3.00+ 0.00 Copyright 6 1990Pergamon Press plc

ALLELOCHEMICALLY-BASED INTERSPECIFIC INTERACTIONS D. S. MULYK Department of Entomology, University of Alberta, Edmonton, Alberta T6G 2G1, Canada Abstract-Allelochemicals are chemicals that mediate interactions between different species. Subclasses of allelochemicals are defined and examples of each are given. A simple mathematical model of tannin production in oak leaves is reviewed.

1. INTRODUCTION

During almost every moment of an organism’s existence, there is some form of interaction with an organism of another species. Most readily observed and modelled are the “classical” predator-prey and herbivore-plant relationships [l, 21; however, more subtle (at least to our eyes) interactions are constantly going on. The spacing patterns observed in sage (Salvia spp.) communities [3,4], polyphagy (consumption of more than one plant species) or monophagy (single host plant) in herbivores [5,6], orientation to habitats by migrating shore insects [7] and filtering rates of Daphnia,

[ 151: allomones, kairomones and synomones. Allomones are allelochemicals pertinent to the biology of an organism (organism 1) but which, upon contacting an individual of another species (organism 2), evoke a behavioral and/or physiological response adaptatively favorable to organism 1 but not organism 2 [15]. Plant feeding deterrents (e.g. alkaloids, terpenes, flavonoids) [6, 16, 171 and toxins, which are often higher concentrations of deterrent compounds [18], provide protection against most herbivores. Defensive secretions, such as the musk odor of skunks and venoms are allomones. Defensive secretions do not have to be repellent; some may allow for “chemically-based crypsis” which make the herbivores that release them indistinguishable from their host plants to hide themselves from olfactory-guided predators [ 191. Predators release allomones that beguile (inquilines sharing ant nests [20]), confuse (slave-making ants [21]) or attract and incapacitate (chigger nymphs feeding 31

D. S. MULYK

32

on Collembola [22]) their prey. Allelopathic agents (chemicals which have an inhibitory or regulatory effects on other organisms) are also allomones [23-251. Kairomones are allelochemicals pertinent to the biology of an organism (organism 1) which, upon contacting an individual of another species (organism 2) evoke a behavioral and/or physiological response adaptatively favorable to organism 2 but not organism 1 [15]. Cucurbitacins (a group of terpenes) are allelopathic chemicals responsible for the bitter taste of zucchini skins and other related vegetables, thus deterring most potential herbivores; however Diubrotica spp. (Chrysomelidae; leaf beetles) use these chemicals as feeding stimulants [26, 271 and in this sense cucurbitacins are kairomones. Bark beetles emit aggregation pheromones during the initial stages of an attack on a host tree, which attract conspecifics to aid in overwhelming the tree’s natural defenses. Clerid beetles, predators of bark beetles, also respond to the aggregation pheromone, leading them to their prey [28, 291; hence bark beetle pheromones are kairomones for some clerids. Parasitoids (an insect that feeds within the body of another organism eventually causing the death of the host) respond positively to the body odor [30] and metabolic products [3 1, 321 of their hosts. Synomones are allelochemicals pertinent to the biology of an organism (organism 1) which, upon contacting an individual of another species (organism 2), evoke a behavioral and/or physiological response adaptatively favorable to both organisms [15]. This group of allelochemicals includes floral scents and nectars used by insect pollinators to locate flowers [33, 341. The insect obtains food (nectar and sometimes pollen) while the plant receives and/or forwards pollen (male gametes) to conspecifics. Plants may afford protection for their pollinators by providing them with food and secondary substances [35]. Chemicals released by a plant upon feeding by an insect herbivore that attract that herbivore’s parasitoids [36-381 are synomones to the plant and parasitoid, but are allomones to the herbivore. Confusion in the literature about the proper classification of an allelochemic cue abound [ 14, 151. Some of the confusion arises because of the producer-receiver allelochemic concepts adopted in many of the older papers [14], whereas allelochemical interactions should be considered as cost-benefit relationships [15]. An allelochemical does not have a single role but has different roles depending upon the specific interaction examined. For examples, tomatine, an alkaloid produced by tomatoes, is a feeding deterrent (allomone) for the cabbage butterfly (Pieris brassicae) [39], while it is a feeding stimulant (kairomone) for the Colorado potato beetle (Leptinotarsa decemlineata) [40]. At high concentrations tomatine becomes an allomone for the Colorado potato beetle [41].

3.

A MATHEMATICAL

MODEL

OF

TANNIN

PRODUCTION

IN

OAK

LEAVES

Antonelli and coworkers [4245], in a recent series of papers, have mathematically modelled the effects of allelochemicals in species interactions. Feeny [46] and Rhoades [47] propose that the proportion of resources allocated by plants to chemical defense is dependent on the relative abundance of the plant in the community. The predominant members of the community utilize dosage-dependent chemical defenses against herbivores. A mathematical model [42], in which tannin production in oak leaves is allometrically related to leaf biomass for a system with a single insect species, is briefly reviewed below. Huxley’s [4X] allometric law holds, via linear regression, for many species of conifers and oaks [49]. By letting Q denote the average leaf biomass (dry weight) for a stand of oaks, x is the allometric constant for oak in relation to breast height diameter, C is an experimentally derived constant based on leaf weight and growth, and BHD denotes the average breast height diameter. Then, In Q = x ln(BHD)

+ C

holds with specific values of x and C for each species. Rhoades’ plant response where T denotes the average total tannin content of leaves, is expressed as T=d.Qg,

for some portionally

constant

mechanism

[42],

O
d. Note that if g = 1, then T/Q = d (no response),

but, if g = 0, then

Allelochemicaliy-based interspecificinteractions

33

T = d; thus, for a small amount of defoliation we obtain a maximal response. Continued derivation (see Ref. [42]) leads to the tannin production equations:

d(ln T)

dt

=xg

.[,l,,,,]

and dN dt = nN( 1 - aN), where x is as above, CIis the inverse of the carrying capacity, N is the average number of leaves per tree, ;1 is the intrinsic rate of increase of leaves during the growing season and t is the time interval during the growing season. This model gives a very basic mathematical description of the tannin production observed, by Feeny [46], during the periodic defoliation of oaks by the winter moth (Operophtera brumata). More field experiments still need to be done to determine if this allelochemic model holds for other species interactions. 4. CONCLUSION The importance of allelochemicals in ecological systems is vastly underrated. It is becoming increasingly clear that the allelochemical web is more complex than first thought. Allelochemical data fused with traditional ecological ideas (energy pyramids, food webs and community organization) is a powerful tool for investigating ecosystems. Acknowledgemenfs-Thanks are due to E. R. Fuller, M. Eymann, critical reviews of the manuscript.

D. A. Pollock,

G. R. Pohl and T. G. Spanton

for their

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