Biological Invasions of Fresh Water: Empirical Rules and Assembly Theory
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Biological Conservation 78 (1996) 149-161 Copyright © 1996 Elsevier Science Limited Printed in Great Britain. All rights l~served PII: S0006-3207(96)00024-9 0006-3207/96/$15.00 +.00 ELSEVIER BIOLOGICAL INVASIONS OF FRESH WATER: EMPIRICAL RULES AND ASSEMBLY THEORY Peter B. Moyle & Theo Light Department of Wildlife and Fisheries Biology, University of California, Davis, CA 95616, USA Abstract biotic invasions into aquatic systems is still limited Because the integrity of aquatic ecosystems is being chal- (Li & Moyle, 1981; Lodge, 1993a,b). The general result lenged worldwide by invading species, there is a growing of deliberate introductions has been termed the need to understand the invasion process and to predict 'Frankenstein Effect' because so many well-intentioned the success and effects of invading species. Case histories introductions have had unexpected consequences, usually offish invasions in streams, lakes, and estuaries indicate negative (Moyle et al., 1986). The need to increase our that invading species and systems being invaded interact ability to predict the consequences of invasions and to in idiosyncratic ways that are often hard to predict, understand invasion processes has been emphasized by largely because of the role of environmental variability in the enormous economic and social costs of some recent determining the outcomes of invasions. We nevertheless invasions, the growing interest in halting biodiversity loss, present a conceptual model of aquatic invasions and a and the increased difficulty of managing ecosystems dozen empirically-derived rules that seem to govern most subjected to repeated invasions. aquatic invasions. While these rules are limited in their Recently, generalizations about aquatic invasions usefulness, they do seem to have more predictive value than have been coming from two main approaches, analyses rules derived from community assembly theory. Copyright of case histories of individual invasions (e.g. Nichols © 1996 Elsevier Science Limited et al., 1990; Moyle & Light, 1996) and applications of recent developments in ecological theory, especially Keywords: invasions, introduced fish, assembly rules, community assembly theory (Pimm, 1989; Case, 1991; stream ecology, lake ecology, aquatic communities, Townsend, 1991; Lodge, 1993a,b). In this paper, we estuaries. consider both approaches. After providing a brief introduction to community assembly theory, we present a conceptual model of invasions. We then review a INTRODUCTION series of case histories of biological invasions for Freshwater and estuarine biotas are changing rapidly which we have some idea of community-level effects of worldwide (Moyle & Leidy, 1992; Allan & Flecker, the invasions. Based in part on these examples, 1993). Human-mediated invasions of aquatic organisms we propose a series of empirical rules for use in pre- are associated with these changes, which commonly dicting the effects of fish invasions into freshwater include extirpation of native organisms. Among the environments. Finally, we ask how useful the predic- more spectacular cases are the elimination of 200+ tions of assembly theory are to the study of aquatic species of haplochromine cichlids following the intro- invasions. duction of Nile perch Lates nilotica into Lake Victoria, Africa, the alteration of the Laurentian Great Lakes COMMUNITY ASSEMBLY THEORY ecosystems by invading sea lamprey Petromyzon marinus, zebra mussel Dreissena polymorpha and other species, Community assembly theory stresses process and his- and the complete domination of lowland rivers in the tory and seeks explanation for community patterns in Western United States by non-native fish and invertebrates the context of dynamic rather than static (equilibrium) (Moyle, 1986). On the other hand, there are numerous community structure. It holds that all present commu- documented instances where invading species have nities were built up by a process of sequential invasions become established without extinctions of native organ- and extinctions, a process which is continuous. This isms and seem to have become integrated into the local places the study of present-day invasions, human medi- biota. It is clear that our ability to predict the effects of ated or otherwise, into the larger context of community development, i.e. it focuses on the interaction between Correspondence to: P. B. Moyle. (Fax) (916) 752 4154; e-mail: the invader and the target community (Lodge, 1993b). [email protected] The discovery of assembly rules could therefore serve 149 150 P. B. Moyle, T. Light to put community ecology on a more predictive basis A CONCEPTUAL MODEL OF BIOLOGICAL (Nee, 1990; Keddy, 1992). Community assembly has INVASIONS been explored through examination of natural patterns (Diamond, 1975), laboratory and field experimentation Most invasion biology is based on a common concep- (Alford & Wilbur, 1985; Wilbur & Alford, 1985; Gilpin tual model in which the invasion process is depicted as et al., 1986; Robinson & Dickerson, 1987; Robinson & having three basic phases: arrival, establishment, and Edgemon, 1988; Drake, 1991), and computer simula- integration (Vermeij, this issue; Fig. 1). Most invasion tion (Post & Pimm, 1983; Drake, 1990b; Strange, studies focus on one or at most two of these phases 1995). A number of generalizations relevant to invasion and ours is no exception, focusing on the establishment biology have emerged from this work (Case, 1991). and integration of invading species in aquatic commu- These include: nities. Our conceptual model borrows from assembly theory the idea of a community constructed over time (1) invasion success decreases with species richness, from a pool of potential invaders. This species pool has supporting Elton's (1958) hypothesis, and the been greatly expanded for most present communities amount of time the community has been accu- by human activities, both through deliberate introduc- mulating species; tions or the establishment of new dispersal routes (e.g. (2) there is a priority effect for residents: invasion, Carlton, 1985). Each newly arriving species meets with even by a superior competitor, is difficult at low what Elton (1958) termed 'ecological resistance' to its numbers of invaders; establishment, which is here considered to consist of (3) invasion resistance increases with the strength of three interactive elements: environmental, biotic, and community interaction coefficients; demographic resistance. Assem- bly theory has empha- (4) the final community state depends on invasion sized the role of biotic resistance (mainly predation and order, thus multiple stable states are possible competition) in the success and failure of invasions from a single species pool; (Case, 1991). Demographic factors (e.g. reproductive (5) communities can shift among alternative, predictable traits) influence the invasion success of parasites (A. P. states as invasions and extinctions proceed. Dobson, pers. comm.), social insects (Moiler, this issue), Assembly theory, like much of the literature on biotic invasions (Li & Moyle, 1981; Lodge, 1993a,b), is concerned primarily with the biological characteristics kx~\\\~mthropogenically exp~ded~Xx~ of the invading species and of the ecosystems being ~,x'x~'----'---------------',~ invaded, on the assumption that competition and pre- dation are the major processes determining the out- come of an invasion. Most simulations assume that all members of the species pool are physiologically capable of persisting in the environment (Post & Pimm, 1983; dlsper~J routes, human mmapon Drake, 1990b), while laboratory studies hold the physi- cal environment constant in order to focus on biotic I environmentalresistance interactions (Robinson & Dickerson, 1987; Drake, temperature, AOW, ehe.~Msa~y, 8b'ucture~ Iblo~lc resou~ 1991). Keddy (1992) depicts the invasion-assembly process as proceeding by a series of deletion rules, with the first filter determined by the environment and sub- /. \ sequent ones by competition, an approach which ignores the interaction between biotic and abiotic factors. preyavai~bilRy, compeUUon, ~ I numbeJ~h2moduced, preston I ,'usemse w parasRes I abWw to lacre~e when rare Although Drake (1990b) suggests that environmental % %% disturbance can reset assembly trajectories, this effect is %% not explored in either modeling or experimental studies (but see Strange, 1995). We will argue later in this paper that this is the area where assembly theory needs I lnte~"atl°n I modifying to reflect the reality of biological invasions. Our experience with invading fishes in California abundm~ce ~fts streams and estuaries suggests that if abiotic conditions functtonaJ shll'ts are appropriate for an invader, it is likely to become established, regardless of the biota already present; conversely, invasion failures are best attributed to failure to adapt to abiotic conditions rather than to biotic resistance on the part of the recipient community Fig. 1. A conceptual model of invasions, which assumes three (Baltz & Moyle, 1993; Moyle & Light, 1996). basic stages: arrival, establishment, and integration. Freshwater invasions 151 and pines (Rejmfinek & Richardson, this issue), and are mento sucker Catostomus occidentalis. Although the often listed among traits of successful invaders (e.g. r flow regimes of the river and its tributaries are largely vs K strategists, Lodge, 1993a,b). However, we view unmodified, the drainage has been altered by human environmental factors as the most critical in determin- activity. There