Probing the role of propagule pressure, stochasticity, and Allee effects on invasion success using experimental introductions of a biological control agent

Abstract

1. Although most populations of non-native species arriving in new environments fail to establish, mechanisms behind failed biological invasions are still poorly understood. 2. Propagule pressure has been found to be a dominant driver of establishment success, underpinned by processes such as stochasticity and Allee effects. While studies have revealed the presence of a component Allee effect in field populations, empirical support for demographic Allee effects has been limited. 3. We used the leaf-feeding beetle Neolema ogloblini, a biological control agent against the plant Tradescantiafluminensis, as a proxy invasive species to experimentally study the process of establishment. We investigated how the initial size of the population released affects the probability of establishment and population growth in the first season after introduction at isolated sites in New Zealand. 4. The probability of establishment was found to increase with numbers of individuals released. A significant quadratic relationship was found between population size released and per capita population growth rate; that is per capita population growth rate initially increased as population size released increased but decreased at higher population levels. 5. These results support the presence of a demographic Allee effect. However, as the influence of stochasticity (environmental and demographic) could not be completely separated from that of a demographic Allee effect, we conclude that extinction probability in the experimental populations of N. ogloblini was influenced by both a demographic Allee effect and stochasticity. 6. Our study demonstrates and supports the concept that experimental biocontrol agent releases can be used for testing hypotheses regarding invasion biology.

The role of propagule pressure in experimental bark beetle invasions

“…Aside from experimental releases of biological control agents (e.g. Grevstad, 1999; Williams et al, 2021) and historical intentional introductions by new settlers (e.g. bird introductions by naturalization societies [Veltman et al, 1996]), such experimental approaches have rarely been conducted.…”

“…Though ecological theory predicts that elevated propagule pressure during the colonization process increases the probability of long‐term establishment, attributing failed invasion events to insufficient propagule pressure is difficult because most failed colonization events go undetected. Some empirical studies relating propagule size and colonization have been conducted, either in mesocosms (Drake et al, 2005; Kaul et al, 2016; Ketola et al, 2017; Kramer & Drake, 2010) or in field conditions (Davis et al, 2004; Grevstad, 1999; Kramer et al, 2009; Williams et al, 2021), but such empirical quantification of invasion thresholds are generally rare.…”

“…Experimental invasions (releases) can be powerful tools for probing the effects of propagule pressure on colonization as well as quantifying Allee effects (Grevstad, 1999; Williams et al, 2021). In this study, we used bark beetles (Scolytinae), a globally important pest group (Lantschner et al, 2020), as model systems to experimentally test how propagule pressure and Allee effects influence population growth and colonization success.…”

Resistance from a resident heterospecific affects establishment success of a globally invasive freshwater fish

“…With one or more resident killifish, guppy population size was low, even at the highest introduction propagule size. A better understanding of how the combined effects of biotic resistance and Allee effects interact with environmental stochasticity, such as seasonal stream flow, should be important in evaluating management strategies for invasive species in general (e.g., Williams et al, 2021).…”

Synergies between Heat Disturbance and Inoculum Size Promote the Invasion Potential of a Bacterial Pathogen in Soil

“…In addition, increasing empirical evidence has suggested that the size of the introduced inocula is vital in determining the invasion success of alien species [ 3 , 4 ]. Alien pathogens with larger inoculum sizes are more likely to invade successfully than species with smaller inoculum sizes, as the population growth of many introduced species with small inoculum sizes tends to be negative, making these species vulnerable to extirpation; this is summarized by the Allee effect [ 5 ].…”