Terrestrial food web of the Malpelo Fauna and Flora Sanctuary, Colombia

Knowledge regarding the structure of food webs and its factors is one of the significant objectives of ecology, particularly in areas with a great range of species-rich communities. However, food webs are more than a set of species interacting with each other (Bascompte and Jordano, 2007; Montoya et al., 2006). These interactions produce ecosystemic attributes and patterns that can generate diverse mechanisms that model and modify the structure and functioning of the food web, playing an essential role in ecosystem stability maintenance (de Ruiter et al., 2005; Tylianakis et al., 2010).

Focusing on a single component of the food web (e.g., a specific species or group) neglects that its existence and survival depend on interactions with other components (Bascompte and Jordano, 2007; Fontaine et al., 2006). Thus, focusing on species conservation does not necessarily maintain the structural integrity of the food web, whereas preserving its structure helps maintain biodiversity (Bascompte and Jordano, 2007; Bastolla et al., 2009). Another important factor is the preservation of emergent food web characteristics (e.g., stability –maintenance of its main structure and dynamics–) and its resilience (e.g. the ability to cope with disturbances by being resistant to impacts), which require monitoring of attributes such as connectivity (the degree of connection, linkage diversity, average path length), compartmentalization, and nesting, particularly when species or habitats are threatened by pollutants or other disturbances (Tylianakis et al., 2010). The approach to resource and habitat management and conservation must regard the ecosystem holistically (Raffaelli, 2006; Pranovi and Link, 2009), considering its structure and functioning through energy flows, trophic relationships (Feng et al., 2017; Gamito et al., 2020), and species interaction strength (Preisser et al., 2005; Werner and Peacor, 2003).

Understanding the trophic interactions between food web components is important when explaining community dynamics and species’ impacts on food web compartments (Navia et al., 2010; Bornatowski et al., 2014). This improves our understanding of how direct and indirect effects on these components can propagate throughout the system, altering abundance and connectivity with other components (Werner and Peacor, 2003) and possibly resulting in local extinctions leading to secondary extinctions and influencing populations of coexisting species (Pimm and Lawton, 1980). This implies that conservation priorities must agree with the maintenance of food web stability. Therefore, drawing inferences on the propagation of direct and indirect effects within food webs facilitates a better understanding of the energy flow (Stevens et al., 2000; Navia et al., 2010), the position (e.g., centrality), and the role of species. Moreover, monitoring key food web attributes allows inferences about each species’ propagation and the consequences of their interactions with other species and other secondary effects (Dambacher et al., 2010; Navia et al., 2010), generating valuable information to achieve conservation goals.

Most studies have described food webs (Bascompte and Jordano, 2007) and the effects of different factors (e.g., environmental and anthropogenic) on the dynamics, productivity, and stability of ecosystems (Rezende et al., 2009; Zetina-Rejón et al., 2015) as supports for conservation. However, there are few efforts to understand the community dynamics of a complex oceanic system such as Malpelo Island, where ecological processes are highly influenced by sea-land interactions (Caut et al., 2012; Polis et al., 1997).

Food web studies generate information on the competition, nutrient dynamics, cascade effects (Winemiller and Polis, 1996), and community structure at an individual, intermediate, and group levels (Pimm, 1980) of complex relationships between components and their properties (Balasundaram et al., 2005) and structural patterns (Milo et al., 2002). This enables the design of ecosystem-based management and conservation strategies (Borgatti, 2002; Whipple et al., 2000) using methods that create simplified models of food webs, i.e., topology. This facilitates the evaluation and prediction of the model’s qualitative dynamics based on the concept of community structure, which is visualized with nodes and links in the food web, where nodes correspond to species (i.e., predator or prey) and links represent their trophic interactions. Moreover, the food web’s topological properties provide essential measures for common limitations in graph theory. In ecology, identifying highly influential food web nodes (Borgatti, 2005) can be represented as keystone species (Jordán et al., 2006; Mills et al., 1993).

Malpelo Island is an ideal study area for analyzing food webs due to its geographic isolation and the convergence of currents, which results in an ideal site for the aggregation of endemic and migratory species. The high ecological values of the area have allowed Malpelo Island to become the current largest marine protected area (MPA) in the Colombian Pacific, known as the Malpelo Fauna and Flora Sanctuary (FFS) (Ministerio del Ambiente y Desarrollo Sostenible, 2017). It has also been included as part of the World Heritage for Humanity by UNESCO, among other nominations, in recognition of its high conservation value (Management Plan, 2015). However, the MPA’s management and conservation strategies are carried out from an individual approach (i.e., species or groups) and not from an ecosystem approach due to the lack of information on its ecological dynamics.

Currently, some studies have focused on food habits for terrestrial and marine species of the Malpelo FFS (Table 1); however, studies related to their food webs are scarce. There are only two studies focused on the analysis of food webs. The first one modeled the Malpelo FFS terrestrial food web from direct observation and trophic studies of a few terrestrial species, thus representing some energy flows within the system. Nevertheless, this study left many questions regarding energy flow unanswered due to the need for more data (Wolda, 1975). The second study focused on the terrestrial invertebrate community, representing energy fluxes and biomass production (Calero et al., 2011). Both studies highlight the importance of the Nazca booby Sula granti in maintaining the terrestrial ecosystem. However, these studies do not describe the attributes and structural patterns of the food web, nor do they identify those key species that keep the system stable. Thus, the trophic dynamics of the terrestrial ecosystem still poorly understood, and management and conservation measures focus on species rather than adopting an ecosystemic perspective. Therefore, the identification of structural patterns and the role of species can contribute to an integrated approach to conservation efforts.

Based on these premises, the objectives of this study were: 1) to describe the structure of the terrestrial food web of Malpelo FFS from a topological approach, 2) to identify key species, 3) to identify the formation of terrestrial trophic substructures (i.e., trophic sub-webs and motifs) as indicators of stability and resilience to disturbances, and 4) to evaluate the resilience of the food web from each trophic component (i.e., trophogroups).