Population Ecology of Insect Invasions and Their Management∗

ANRV330-EN53-20 ARI 2 November 2007 19:36 Population Ecology of Insect Invasions and Their Management∗ Andrew M. Liebhold and Patrick C. Tobin Forest Service, U.S. Department of Agriculture, Northern Research Station, Morgantown, West Virginia 26505; email: [email protected], [email protected] Annu. Rev. Entomol. 2008. 53:387–408 Key Words First published online as a Review in Advance on Allee effect, establishment, nonindigenous species, spread, September 17, 2007 stratified dispersal The Annual Review of Entomology is online at ento.annualreviews.org Abstract This article’s doi: During the establishment phase of a biological invasion, popula- 10.1146/annurev.ento.52.110405.091401 tion dynamics are strongly influenced by Allee effects and stochastic Copyright c 2008 by Annual Reviews. dynamics, both of which may lead to extinction of low-density pop- All rights reserved ulations. Allee effects refer to a decline in population growth rate 0066-4170/08/0107-0387$20.00 with a decline in abundance and can arise from various mechanisms. ∗ The U.S. Government has the right to retain a Strategies to eradicate newly established populations should focus nonexclusive, royalty-free license in and to any on either enhancing Allee effects or suppressing populations below copyright covering this paper. Allee thresholds, such that extinction proceeds without further inter- vention. The spread phase of invasions results from the coupling of population growth with dispersal. Reaction-diffusion is the simplest form of spread, resulting in continuous expansion and asymptotically constant radial rates of spread. However, spread of most nonindigenous insects is characterized by occasional long-distance dispersal, which results in the formation of isolated colonies that grow, co- alesce, and greatly increase spread. Allee effects also affect spread, generally in a negative fashion. Efforts to slow, stop, or reverse spread should incorporate the spread dynamics unique to the target species. 387 ANRV330-EN53-20 ARI 2 November 2007 19:36 INTRODUCTION Table 1 The three successive invasion phases with corresponding management activities Geographical barriers such as oceans, moun- Arrival: transport of tain ranges, and glaciers have compartmen- Invasion phase Management activities a nonindigenous talized the world’s biota into disjunct commu- Arrival International quarantines species to new areas nities during the ∼400 million years of insect Inspection outside of its native evolution. As continents separated or collided, Establishment Detection range climates changed, and oceans receded, species Eradication Establishment: ranges have constantly shifted, sometimes Spread Domestic quarantines growth of a Barrier zones population to with the result that species were introduced sufficient levels such into communities where they had previously that natural not evolved. But these changes have been Biological invasions can be broken down extinction is highly relatively slow. Increases in the world’s hu- into three distinct population processes: ar- unlikely man population over the last 1000 years have rival (the process by which individuals are Spread: expansion brought about changes in our behavior that transported to new areas outside of their of the nonindigenous have greatly accelerated the breakdown of native range), establishment (the process by species’ range into new areas barriers to species movement. For example, which populations grow to sufficient lev- the conquest of far reaches of the world by els such that extinction is highly unlikely), Europeans was accompanied by movements and spread (the expansion of an invading of various species both intentionally (e.g., species’ range into new areas) (25, 74, 92, introduction of domestic species such as the 119) (Table 1). Here we concentrate on pop- honey bee) and accidentally (e.g., inadvertent ulation processes operating during the estab- movement of species, such as cockroaches, lishment and spread phases. We then dis- hitchhiking on vessels) (21). More recent cuss management activities associated with sophistication of human societies has re- the establishment and spread phases and clar- sulted in remarkable increases in worldwide ify how knowledge of population processes movement of humans and their goods. This can be used in the selection of effective strate- mobility has unfortunately resulted in a rapid gies. The arrival phase is an equally important acceleration of introductions of insect species phase of the invasion process but mostly does beyond their native ranges (31, 62, 69, 108, not involve population processes; readers are 121). encouraged to consult other sources (34, 52, Many intentionally introduced insects are 81, 149, 127) that have investigated invasion generally considered beneficial, and most ac- pathways and arrival processes. cidentally introduced insect species rarely reach high population levels and are seldom noticed. However, a highly conspicuous mi- ESTABLISHMENT nority of nonindigenous species sometimes While rising levels of world trade and travel become particularly abundant and cause con- have resulted in an ever-increasing arrival of siderable ecological, economic, and evolu- alien species, most of them have failed to es- tionary impacts (85, 100, 129). The reasons tablish (124, 146). The establishment phase why populations of alien species sometimes thus represents a critical period during which explode and the characteristics of their im- populations grow and expand their distribu- pacts are not covered in this review. Instead, tion such that extinction is highly unlikely. we focus here on the basic population pro- Founder populations typically are small and cesses that operate during the invasion pro- consequently are at great risk of extinction. cess and how this information can be applied Generally, the smaller the founder popula- to develop effective strategies for mitigating tion, the less likely is establishment (79, 84). invasions. This is conceptually illustrated by historical 388 Liebhold · Tobin ANRV330-EN53-20 ARI 2 November 2007 19:36 records of introductions of natural enemies a as part of biological control programs, in which establishment frequencies are consis- tently higher from releases of large numbers of individuals (6, 35, 50). Much of what we know about the popu- N /N 1 t +1 t lation biology of low-density invading populations is extracted from a rich literature on the population ecology of rare species (i.e., conservation biology). All populations are affected by stochastic abiotic influences (e.g., weather), but low-density populations are par- CK N ticularly influenced by such effects. We can t mathematically represent the generational change in population density as b = + ε , Nt+1 f (Nt ) t 1. where N is population density in year t or + t 1, f (Nt) is a function that encompasses ε birth and death processes, and t is varia- N /N 1 tion due to stochasticity. In addition to en- t +1 t vironmental stochasticity, all populations are affected by demographic stochasticity, which refers to random variation in birth and death processes (32). The important result of demographic and environmental stochasticity is that low-density populations (e.g., newly N founded invading populations) can be driven t to extinction purely due to inimical random Figure 1 variation. However, there is another factor Schematic representation of the Allee effect. Change in population density, contributing to the extinction of low-density Nt+1/Nt is plotted as a function of density at the beginning of the populations that must also be considered: generation, Nt.(a) Illustration of equilibria. When density is less than C, the Allee threshold, it will decrease toward extinction. When density exceeds Allee effects. C, it will increase toward K, the carrying capacity. When populations exceed the carrying capacity, they will decrease. (b) Illustration of eradication strategies. The first strategy is to reduce the population density (solid purple Allee Effects dot) to a density that is below the Allee threshold. In the second strategy, the Warder Allee (1) studied animal ecology and Allee threshold (solid red dot) is increased to a level that exceeds the population density. Both strategies result in population extinction. is generally thought to be among the first to recognize the concept that animal populations must be composed of some minimum num- of extinction from the perspective of conser- ber of individuals to remain viable. Certain vation biology (128). Of late, there has been processes may lead to decreasing net popula- growing recognition of its importance dur- tion growth with decreasing density, and thus ing the establishment phase of biological in- there may exist a threshold below which low- vasions (26, 68, 133). Allee effect: density populations are driven toward extinc- Causes of Allee effects include failure to decreased population tion (Figure 1a). This phenomenon is known locate mates (8, 50), inbreeding depression growth correlated with decreasing as the Allee effect (19, 24), and it has been (66), the failure to satiate predators (39), and abundance identified as critical to understanding patterns the lack of cooperative feeding (17). In many www.annualreviews.org • Invasion Population Ecology 389 ANRV330-EN53-20 ARI 2 November 2007 19:36 cases, Allee effects arise from several causes, The ability to reproduce asexually and although multiple sources of Allee effects do other forms of reproductive behavior can also not necessarily act additively and

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