Insect Influences in the Regulation of Plant Populations and Communities

Abstract

This cha pter presents an overview and some specilic concepts and examples concerning the diverse roles of insects in regulating or inl1uencing platH populations. Also considered are some conseque nces of these populationcelllered roles and of o the r, broader roles of inseCls that affect plant community composition, functioning, and maintenance. These roles are seen as e ffects on density, age and quality, pauernsof seasonal express ion, spatial distribution, and stability of specific populations and, to an extent, the ... communities of which they are a pari (cL Chapters 2, 12). Both short-term functioning and more distan t evolutio nary processes are considered. T he spcctrum of these influences embrace ( I) local movemelll, d yna mics, and natural control and (2) larger-sca le move ment , d ynamics, and evolutionary change, all commonly affected by various physical and biotic inll uences, including their heteroge neity in time and space (cl". Cha pters 6-18, 23). Insects can se rve such roles as exploiters, allies, or competitors of other species. T heir roles as pollinators are presellled in Chapter 20, and omilted here. We here emphasize examples where insects are sig nificant in a regulatory sense (C hapter 12). Some populations of plants (as of a nimals) may be regulated thro ugh the reciprocal predator-prey interaction in volving insects, and the structure, fu nctio nin g, and slability of some biotic COIll Illunities may thus be strongly inl1uenced by insects. Most terrestrial plants, and also animals and microorganisms, are in some way closely associated with insects. Coevolution of insects and va riotls associated organisms has produced some clearly mutual benelits; and for others roles mainly as food , one for the o ther. For some, a stable predatorprey (host- parasitoid) relationship at the population interaction level can be seen, despite the violence of individual to individual interaction. Some insects mediate interspeci fic competition between ot her o rga nisms, including plants, affecting their dynamics, resource sharing, and succession (Chapters 2, 15) . T hrough their exploitation of epidemic po pulatio ns and mediating roles, some insects contribute to reestablishing a more natural (primeval) community composition (e.g., through reducing the density of o ne spl."C ies of plant where <In increased density has been ca used by human or other distu rbance of balance, or in mediating the species richness of the « biotic community). Over long periods of lime, it is presumed that individuals' adaptations associated wilh such activities have undergone a fine tunin g, cont ributing to ex ist ing community integrity and relative stability. Epidemics of herbivores resulting from disturbance may cause destruction of much of certain plant po pulations. Whether this occllrs in patches or more generally wilt affect the rate of resto ration of the plant S(;l nd. Rate of resto ration wi ll depend Llpon the type and age distribution of the stand and the characteristics of the individual plant species. For smail patches of destruction, compensations (e.g., growth of ncig hboring surviv ing plants or branc hes) may achieve a rapid adjustment: fo r la rger patches, restoration could only be achieved by establishment and maturing of recruit plants. T his would be a 10llg lime for a redwood forest but much less for vegetation of annllal plants.