Species Importance in a Heterospecific Foraging Association Network
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Oikos 122: 1325–1334, 2013 doi: 10.1111/j.1600-0706.2013.00101.x © 2013 The Authors. Oikos © 2013 Nordic Society Oikos Subject Editor: Paulo Guimares Jr. Accepted 10 January 2013 Species importance in a heterospecific foraging association network Hari Sridhar, Ferenc Jordán and Kartik Shanker H. Sridhar ([email protected]) and K. Shanker, Centre for Ecological Sciences, Indian Inst. of Science, Bangalore-560012, India. – F. Jordán, The Microsoft Research – Univ. of Trento Centre for Computational and Systems Biology, Piazza Manifattura 1, IT-38068 Rovereto, TN, Italy. There is a growing recognition of the need to integrate non-trophic interactions into ecological networks for a better understanding of whole-community organization. To achieve this, the first step is to build networks of individual non- trophic interactions. In this study, we analyzed a network of interdependencies among bird species that participated in heterospecific foraging associations (flocks) in an evergreen forest site in the Western Ghats, India. We found the flock network to contain a small core of highly important species that other species are strongly dependent on, a pattern seen in many other biological networks. Further, we found that structural importance of species in the network was strongly correlated to functional importance of species at the individual flock level. Finally, comparisons with flock networks from other Asian forests showed that the same taxonomic groups were important in general, suggesting that species importance was an intrinsic trait and not dependent on local ecological conditions. Hence, given a list of species in an area, it may be possible to predict which ones are likely to be important. Our study provides a framework for the investigation of other heterospecific foraging associations and associations among species in other non-trophic contexts. Species in communities are enmeshed in a web of multiple i.e. temporary social groups formed by individuals of differ- interactions. For a clear picture of community organization, ent species (Morse 1977). Individuals joining such groups it is not enough just to focus on species identities, but also are known to benefit either through direct foraging facili- to understand the interaction web (Paine 1966, Ings et al. tation, better protection from predators, or improved forag- 2008). Incorporation of network analysis in ecology has ing as a consequence of reduced vigilance (Sridhar et al. helped this endeavor by offering a suite of metrics at the 2009). Across multiple such groups in an area, populations species, interaction and whole-community level (Estrada of different species can be viewed as linked in an ecological 2007). While the network approach has largely been network of interdependencies. restricted to trophic interactions such as food webs and Heterospecific foraging associations have been docu- plant-animal mutualisms (Ings et al. 2008; although mutu- mented from numerous taxa including invertebrates, fishes, alistic networks also include indirect non-trophic inter- birds and mammals (reviewed by Morse 1977). Here we actions, e.g. pollinators linked through a shared host-plant) examined the most prominent and speciose of these and host–parasite interactions (Blick et al. 2012), emphasis associations, mixed-species bird flocks (flocks hereafter) in has been placed on incorporating direct non-trophic inter- terrestrial habitats. Flocks are known to form important actions (Kéfi et al. 2012, Pocock et al. 2012) in order to subunits of terrestrial bird communities the world over gain a more complete picture of community organization. A (Goodale et al. 2010) with substantial proportions of total first step towards this is building and understanding direct bird species, particularly insectivores, in each community non-trophic interaction networks. participating in flocks (King and Rappole 2001). Flocks can Direct non-trophic interactions are usually thought of range in size from just two heterospecific individuals to over only in terms of competition; recent work has however a hundred individuals representing tens of species (Terborgh highlighted the prevalence of positive interactions in com- 1990). Different lines of evidence, such as obligate flock munities and the need to incorporate them into ecological participation of species (Munn and Terborgh 1979, Jullien theory (Bruno et al. 2003). Till date, only a few positive and Thiollay 1998) and links between flock participation interactions, such as cleaner fish mutualisms (Sazima et al. and life-history traits (Jullien and Clobert 2000), suggest 2010) and nurse plant-seedling facilitations (Verdú and that flocks play a fairly important role in species’ ecologies. Valiente-Banuet 2008), have been visualized and examined Given this, it is likely that interdependencies among flock using the network approach. In this study, we extend it to a participant species could scale up to influence patterns class of positive interactions that are fairly common in a for- at higher levels (Goodale et al. 2010) including other eco- aging context, namely heterospecific foraging associations, logical networks of which these species form a part. 1325 In this study we focused on species importance in the with other Asian wet forest sites showed that importance flock network. Quantifying species importance allows us values were strongly congruent at species, genus and family to identify hubs or ‘keystone species’ i.e. those that are likely levels. to have disproportionately large effects on flock ecology and evolution and whose removal could have cascading effects on communities in question (Jordán and Scheuring Material and methods 2002, Estrada 2007, Pocock et al. 2012). This approach also allows us to examine whether processes at the level of Empirical data set individuals scale up to influence properties at the commu- nity level (Bascompte and Stouffer 2009, Stouffer 2010). We conducted fieldwork in an evergreen forest site in Anshi Numerous studies, across flock systems from different National park (15°00′97.8″N, 74°38′72.2″E) situated in geographic regions, have clearly shown two kinds of species the Western Ghats mountain range along the west coast to be functionally important in flocks: those which are of India (Fig. 1). We collected data between January and found in conspecific groups (intraspecifically gregarious March in 2010 and 2011, which is the non-breeding species) and those which feed by catching insects in air with season for most evergreen forest bird species. Data collec- short flights from stationary positions (sallying species; tion was carried out along twelve trails in a 26 km2 Hutto 1994, Goodale and Kotagama 2005, Sridhar et al. focal study area around the Anshi nature camp and Anshi 2009, Srinivasan et al. 2010). Individuals of such species village (Fig. 1). The minimum distance between flock are often the ‘nucleus’ or leaders of flocks, presumably locations on different trails was 500 m. Each trail was sur- because of the direct benefits that other species gain by veyed four times in 2010 and three times in 2011. On associating with them (Hutto 1994, Sridhar et al. 2009). each trail walk, carried out between 08:30–15:00 h, two Quantifying species importance in the flock network allows observers searched for mixed-species bird flocks. Flocks us to examine the link between functional importance in were defined as roving associations of two or more species, individual flocks and emergent community importance. staying together for at least five min. The five minute This is particularly important since studies have demon- cutoff was used to minimize the possibility of including strated that the local extinction of functionally important chance groupings of independently moving heterospecifics. species can result in the cessation of mixed-species flock However, to obtain reasonable ‘snapshots’ of flock composi- activity even when other flock participants are present. tion and minimize the likelihood of species turnover, we (Stouffer and Bierregaard 1995). Going beyond the specifics restricted observation time to 15 min per flock. We desig- of particular study sites, it is also useful to examine the nated a species as belonging to the flock if it was within generality of species importance in communities i.e. whether 10 meters of at least one other heterospecific individual. species importance is linked to a specific ecological setting While it is possible that heterospecifics gain benefits of asso- or is an intrinsic characteristic of a species (Stouffer et al. ciation even at distances greater than 10 m, this cutoff is 2012)? The latter, if true, will mean that a species’ impor- often used in flock studies in dense forest habitats (Hutto tance in networks can be predicted by its taxonomic or 1994) because of poor detectability beyond 10 m. We phylogenetic classification. included all species seen at least once during flock observa- In this study we examined an ecological network tion as being part of the flock. Though species differed in formed as a result of flock participation in an evergreen for- their detectabilities, given the lengths of our observations est site in the central Western Ghats, India. Nodes in this (5–15 min) it is likely that a species, if present in the network represent species and edges represent level of co- flock, would have been detected. Nevertheless, even if we occurrence in flocks. Edges in ecological networks typically missed recording species of poor detectability in some flocks,