Disentangling Vine-Invaded Tropical Landscapes: From Individual Vine Patches to Vine Networks
Delgado Rivera, Diana L.
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Vines, like their woody counterparts, lianas, are increasing in abundance in several parts of the World (Warshauer et al. 1983, Mackey et al. 1996) including both tropical (Shrestha and Dangol 2014) and temperate regions (Figure 1.1; Miller 1997). Vines can smother vegetation (McGaughey 2011) and human-made structures alike (Miller 1997) by forming extensive patches that resemble dense green blankets that cover vast areas. Vine invasions are often attributed to individual species of alien origin (Warshauer et al. 1983, Mackey et al. 1996, Miller 1997). In Puerto Rico it is very common to find extensive vine patches and based on work conducted elsewhere, I was expecting that few introduced vine species were involved in these vine invasions. For sure I knew that vine proliferation had become a recurrent and costly problem for the Puerto Rico’s Electric Power Authority -PREPA- and that it even captured the attention of the Puerto Rican Senate (e.g., Resolución del Senado # 1696, 2010). Soon it became clear that vine patches were composed of multiple vine species, both of native and alien in origin, and that they were altering the configuration of the landscape. This fueled my interest to characterize the structure and composition of vines patches, investigate the factors influencing their composition and examine possible ways in which vine patches organize in complex networks connecting vine communities in a landscape. Also, I wanted to examine how the different types of biotic interactions taking place within vine communities could regulate their spread and overall invasive success. The invasive success of vines most likely reflects 1) their intrinsic characteristics, 2) the prevailing conditions in the places where they have established (Lake and Leishman 2004), and 3) the type of biotic interactions that they develop with other species in the invaded area (Belote and Weltzin 2006). Here I examine these three aspects to gain a better understanding of the dynamics behind the observed vine invasions. In order to understand the relationship between intrinsic and extrinsic vine traits and their proliferation status, in chapter two I compiled an extensive database to characterize Puerto Rico’s contemporary vine assemblage and establish relationships between vine proliferation status and plant traits. I examined the association between vine origin and proliferation status with taxonomy, distribution, intrinsic, and extrinsic traits in order to understand the aggressive proliferation of vines, indistinct of their origin. Then, I used a suite of traits, both intrinsic and extrinsic that can help predict proliferation status in vines. Results showed that vine origin was associated only to one of seven of the traits examined, which allow the grouping of alien and native proliferating species together when analyzing which proliferating status was associated with the various traits. In addition, we were able to successfully predict vine proliferation status based on five of traits, namely fruit type, use, abundance, distribution, and seed dispersal mode. In chapter three I focused on determining which abiotic and biotic variables influence both vine diversity and species composition in vine communities. For this I went to the field and measured the abundance of each vine species present in a subset of vine patches found along a complex mosaic of environmental and land cover variables. Results showed that both vine diversity and species composition within vine communities can be explained by a combination of composite climatic, edaphic, topographic and land cover variables. In particular, climatic variables such as those related to extreme temperatures and precipitation, appeared to be the most important influencing vine diversity within vine patches. In terms of species interactions vines represents a particular example of host - parasite interactions because they, like all climbing plants, are structural parasites that need of other structures for physical support (Forseth and Innis 2004). In consequence, host availability and diversity most likely influences the structure of vine communities (Nabe-Nielsen 2001). Nevertheless, most studies that focus on climbing plant communities -principally working with lianas- (with the exception of Molina-Freaner et al. 2004) examine separately the influence of abiotic variables (Molina-Freaner et al. 2004, DeWalt et al. 2006, Swaine and Grace 2007, DeWalt et al. 2010) and that of climbing plants hosts’ preference (DeWalt et al. 2000, Nabe-Nielsen 2001, Carrasco-Urra and Gianoli 2009, Leicht-Young et al. 2010). Highlighting the need to incorporate together the influence of abiotic and biotic interaction in the study of vine communities in order to get a better understanding of what may be influencing their current expansion. I propose the use of graph theory as a way to incorporate both abiotic and biotic interactions in the modeling of vine invasions. Graphs or networks are simple representations of complex systems whose structure, connectivity, and function can be described through a series of metrics (Urban and Keitt 2001, Minor and Urban 2008). Applying this approach to vine spread can help untangle the dynamic and complex processes behind these invasions and design management plans for vine-invaded landscapes (Ferrari et al. 2014, Stewart-Koster et al. 2015). Among other things, this type of approach can help predict and prevent further vine invasions (Sutrave et al. 2012, Stewart-Koster et al. 2015). In consequence, in chapter four, I used a graph theory approach to model the connectivity of structural hosts of the most common and prolific vine in my study area, Mikania micrantha. I also modeled the effect of different biotic interactions among vine species in the size, structure and connectivity of these host-parasite networks. Through the creation of a series of networks that varied in terms of scale and biotic interactions between Mikania micrantha, its hosts and other vine species present in the vine patches (e.g., Pueraria phaseoloides), I showed that competition between M. micrantha and P. phaseoloides had little effect constraining the size, connectivity and structure of the network of M. micrantha’s. Finally, in chapter five I revisit the most important findings of my dissertation and how they have help gain a better understanding of vine invasions. I also highlight the applicability of the approaches used in my research, such as the use of networks to understand the dynamics of vine spread. This approach can easily be applied to other vine species as well as other host-parasite systems and help inform management and conservation plans.