The role of propagule pressure in explaining species invasions

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Human-mediated species invasions are a significant component of current global environmental change. There is every indication that the rate at which locations are accumulating non-native species is accelerating as free trade and globalization advance. Thus, the need to incorporate predictive models in the assessment of invasion risk has become acute. However, finding elements of the invasion process that provide consistent explanatory power has proved elusive. Here, we propose propagule pressure as a key element to understanding why some introduced populations fail to establish whereas others succeed. In the process, we illustrate how the study of propagule pressure can provide an opportunity to tie together disparate research agendas within invasion ecology.

Introduction

Propagule pressure (also termed ‘introduction effort’ [1]) is a composite measure of the number of individuals released into a region to which they are not native [2]. It incorporates estimates of the absolute number of individuals involved in any one release event (propagule size) and the number of discrete release events (propagule number). As the number of releases and/or the number of individuals released increases, propagule pressure also increases. Here, we concentrate on the role of propagule pressure in determining the establishment success or failure of non-native populations (Figure 1). However, increasing propagule pressure might be equally important in understanding the rate of geographical spread of non-native populations (e.g. [3]) (Box 1). Determining the role of propagule pressure within these different invasion contexts should lead to broader ecological and management insights into invasive species biology.

Section snippets

Variation in invasion success

Propagule pressure is an ‘event-level’ characteristic 1, 4: that is, it can differ for each introduced population. This is in contrast to location- and species-level traits, which are constant across repeated introductions (e.g. [5]). For example, rainbow trout Oncorhynchus mykiss have been purposefully introduced into Africa, Australia, New Zealand, Europe and many other locations worldwide [6]. We can explain the success of rainbow trout using its life history or ecological traits (e.g.

The idiosyncrasy of the invasion process

The importance of propagule pressure to establishment success can also explain why the invasion process often appears so idiosyncratic. In a recent exploration of the role of propagule pressure in the establishment of non-native bird populations, Cassey and colleagues [16] identify two reasons for this importance. First, this research showed that most variation in propagule size was clustered among release events [16]. Thus, a single species that has been introduced to several islands or

Understanding the role of propagule pressure

The strength of the relationship between propagule pressure and establishment success does not, however, negate roles for location and species 20, 21, although it is the interaction of these factors with propagule pressure that might be the more important element to furthering our understanding of invasions. In that regard, we highlight four research agendas that can promote our understanding of the role of propagule pressure within invasions beyond recognizing the importance of large numbers.

Conclusions

Elucidating the factors that determine introduction success involves disentangling the influence of characteristics of the species and environment, and the idiosyncrasies of specific introduction events [4]. From the array of possibilities available, propagule pressure is emerging as a single consistent correlate of establishment success. Propagule pressure can explain not only why some introductions succeed when others fail, but also why introduction success is so idiosyncratic. However,

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