Introduction
Biological invasions pose a major challenge to ecosystems with consequences for community structure and biodiversity (Dunn et al. 2012). The global spread of introduced species is altering native communities and is rapidly accelerating due to human activities (Mooney and Cleland 2001). In recent years, there has been growing interest in the role of parasites in invasion success (Tompkins et al. 2010). Much of the research involving the role of parasites in invasions has focused on direct interactions, including competition and predation studies, and on their subsequent effects on community dynamics (Wilson et al. 1998, Corbin and D’ Antonio 2004). However, indirect effects are believed to play a key role in the struc- turing of ecological communities (Holt 1977, Miller 1994, Bonsall and Hassell 1998, Peacor and Werner 2001). The extent to which indirect effects influence the invasion process and their consequences on native communities remains unexplored (White et al. 2006). Parasites can indirectly structure ecological com- munities within the same trophic level through parasite mediated competition (Bowers and Turner 1997). During biological invasions, the absence of parasites can indirectly enhance the competitive ability of an exotic species (Torchin et al. 2003) by reallocating resources against defense to competitive traits (Blossey and Notzold 1995).Parasites can affect the outcome of competitive interactions between exotic and native species through both density-mediated indirect effects (DMII) and trait-mediated indirect effects (TMII). For instance, competitive interactions between the native ant Solenopsis geminata and invasive ants Solenop- sis. invicta were altered by the phorid parasitoid Pseudacteon browni (Morrison 1999). In the presence of phorid flies, S. geminata had to defend themselves against P. browni resulting in 50 % decline in resource retrieval thereby giving the invasive ant S. invicta a competitive advantage. As such, Pseudacteon phorid fly parasitoids (Diptera: Phoridae) have been used as a biological control agent against S. invicta populations (Mehdiabadi et al. 2004). Interspecific competitive trade-offs are believed to play an important role in the structuring of ant communities (Lebrun and Feener 2007). The discovery-dominance trade-off describes the ability of a species to discover a food resource versus the ability of a species to dominate a food resource (Fellers 1987, Savolainen et al. 1988). The discovery-dominance trade-off allows for species coexistence and has been documented in studies involving only a few ant species (Lynch et al. 1980, Perfecto 1994, Morrison 1996, Feener et al. 2008). However, exotic ants are believed to break down this trade-off in their introduced range by excelling at both traits (Holway 1999). Another trade-off involves the ability of species to defend itself against natural enemies in contrast to maximizing their competitive abilities (Lebrun and Feener 2007). For instance, specialized phorid fly parasitoids have been found to attack host ants and limit their foraging activities resulting in the frequent loss of resources to their competitors (Feener and Brown 1992) (Orr et al. 1995, Morrison et al. 2000, Philpott 2005). Together these trade-offs interact with one another to influence community structure in such a way that a species ability to maximize it’s competitive potential is balanced by its vulnerability to parasitism (Adler 1999). An optimal fitness strategy is to min- imize competitive ability to the level of the entire ant assemblage in order to avoid parasitism (Adler et al. 2007). While the presence of phorids has been found to reduces foraging rates to baits, phorids may not always determine the outcome of competitive interactions between native and exotic ant species (Morrison 1999). In addition, phorids were not influential in competitive outcomes between Solenopsis invicta and Solenopsis geminata in a laboratory setting (Morrison 2000). Furthermore, the spatial distribution of hosts can affect the searching efficiency and attack rates by phorids (Philpott et al. 2009). Empirical studies are needed to address the relative importance of phorid parasitoids in influencing invasion dynamics and community structure. Our study focuses on the invasion dynamics of W. auropunctata and competi- tive interactions with the arboreal ant Linipethema iniquum in Puerto Rico. W. auropunctata is native to Central and South America and has in recent decades expanded to island groups in the Carribean and Pacific Oceans (Foucaud et al. 2010). It has also spread to parts of Western Africa, including Gabon and Cameroon, and most recently to the Middle East (Walker 2006) (Ndoutoume-Ndong and Mikissa 2007, Vonshak et al. 2009, Mikkisa et al. 2013, Wetterer 2013). Within its native range, W. auropunctata is regarded as a common, but sub-dominant species as it faces intense competition by dominant ant species (unpublished data). W. auropunctata is widely distributed in Puerto Rico and can reach high populations densities on coffee farms (Wetterer 2013). At our site, phorid parasitoids were found to be attacking L. iniquum workers (Pseudacteon sp.). This study addresses whether trait-mediated indirect effects by phorids mediate competition and facilitate the invasion of W. auropunctata in Puerto Rico (fig 4.1). We exam- ined the effects of phorid parasitoids on resource competition between host L. iniquum and non-host W. auropunctata and the effects of the spatial distribution of ants on phorid searching efficiency.
Materials and Methods
Study sites and species
The study was conducted in the months of February, June, and July, in 2015 and 2016, on a coffee farm in Puerto Rico. The field site consisted of a 5-hectare plot within a high-shade organic coffee farm located in the central mountainous region in the municipality of Orocovos (18.175850, -66.4155700). The plot consisted primarily of coffee and banana trees where W. auropunctata and L. iniquum were found to be nesting. W. auropunctata invasion into the Carribean region is the result of multiple introductions due to human- induced dispersal and likely originates from it’s native habitat along the northern coast of South America (Foucaud et al. 2010). The Caribbean region represents an important zone from where secondary dispersal that led to the global expansion of W. auropunctata. The native range of L. inipethema extends from South to Central America into the eastern parts of the Caribbean (Wild 2008). L. iniquum is an arboreal ant species and has been found nesting in hollow twigs and leaf petioles of plantain in Puerto Rico (Wheeler 1908). We surveyed the 5-ha plot to determine the abundance and spatial distribution of W. auropunctata and L. iniquum populations (fig 4.2). Tuna baits were placed along transects of coffee and banana trees spaced out every 2 meters. Baits were subsequently checked every 30 minutes to determine the presence or absence of W. auropunctata and L. iniquum. In order to examine competitive interactions a small plot (30 m x 14 m) was established along the edges of W. auropunctata and L. iniquum territorial boundaries. A total of 324 baits were placed on trees in the large plot (within the high-shade organic farm). Surveys were completed in June and July of 2015.