Decay of interspecific avian flock networks along a disturbance gradient in Amazonia

Abstract

Our understanding of how anthropogenic habitat change shapes species interactions is in its infancy. This is in large part because analytical approaches such as network theory have only recently been applied to characterize complex community dynamics. Network models are a powerful tool for quantifying how ecological interactions are affected by habitat modification because they provide metrics that quantify community structure and function. Here, we examine how large-scale habitat alteration has affected ecological interactions among mixed-species flocking birds in Amazonian rainforest. These flocks provide a model system for investigating how habitat heterogeneity influences non-trophic interactions and the subsequent social structure of forest-dependent mixed-species bird flocks. We analyse 21 flock interaction networks throughout a mosaic of primary forest, fragments of varying sizes and secondary forest (SF) at the Biological Dynamics of Forest Fragments Project in central Amazonian Brazil. Habitat type had a strong effect on network structure at the levels of both species and flock. Frequency of associations among species, as summarized by weighted degree, declined with increasing levels of forest fragmentation and SF. At the flock level, clustering coefficients and overall attendance positively correlated with mean vegetation height, indicating a strong effect of habitat structure on flock cohesion and stability. Prior research has shown that trophic interactions are often resilient to large-scale changes in habitat structure because species are ecologically redundant. By contrast, our results suggest that behavioural interactions and the structure of non-trophic networks are highly sensitive to environmental change. Thus, a more nuanced, system-by-system approach may be needed when thinking about the resiliency of ecological networks.

Keywords: Amazon; forest fragmentation; heterogeneous landscapes; insectivorous birds; mixed-species flocks; network theory.

Figure 1.

Mixed-species flocks showed substantial variation in both (a) species richness and (b) encounter rate across a habitat gradient in the Brazilian Amazon. Bars represent mean±s.e. (Online version in colour.)

Figure 2.

Box plots show that species in mixed-species flocks in more preserved habitats (PF and 100 ha) (a) had a greater number of interspecific interactions (normalized degree) and (b) a higher frequency of interactions (weighted degree) than in degraded forest habitats (SF, PSF and 10 ha). (Online version in colour.)

Figure 3.

Example of networks and habitat configurations for three flocks found in PF, 10 ha fragment and SF habitat types in the Brazilian Amazon. Differences in network structure reflect the decay of interspecific interactions in mixed-species flocks across a disturbance gradient. Edge thickness and transparency in each network are proportional to numbers of interactions. Interaction values at the lowest 10% are set to transparent. Nodes sizes are proportional to flock attendance. Legends for species with participation above 6% are given. Ai, Automolus infuscatus; Ao, Automolus ochrolaemus; Cc, Cariothraustes canadensis; Ch, Cacicus haemorrhous; Dl, Deconychura longicauda; Eg, Epinecrophylla gutturalis; Gy, Glyphorhynchus spirurus; ; Hd, Herpsilochmus dorsimaculatus; Hm, Hylophilus muscicapinus; Ho, Hylophilus ochraceiceps; Lf, Lanio fulvus; Mg, Myiopagis gaimardii; Mi, Mionectes sp.; Ml, Myrmotherula longipennis; Mm, Myrmotherula menetriesii; Mu, Myrmotherula brachyura; Mx, Myrmotherula axillaris; Pc, Piprites chloris; Pf, Piculus flavigula; Pr, Philydor erythrocercum; Rm, Ramphocaenus melanurus; Ro, Rhynchocyclus olivaceus; Ta, Thamnomanes ardesiacus; Tc, Thamnomanes caesius; Ts, Tolmomyias assimilis; Tu, Tachyphonus surinamus; Tv, Trogon viridis; Vl, Vireolanus leucotis; Xm, Xenops minutus; Xy, Xiphorhynchus pardalotus. (Online version in colour.)

Figure 4.

Cumulative degree distributions highlight differences in mixed-species flock network structure across a habitat gradient in the Brazilian Amazon. Pairs of histograms for observed and randomized networks show that degraded habitats are characterized by low medians and high skew while more intact habitats have higher medians and lower skew. Dashed lines represent the median normalized degree. (Online version in colour.)

Figure 5.

Mixed-species flock cohesiveness (clustering) and attendance show strong positive correlation with vegetation structure. (a) Relationship between flock global clustering coefficient and mean vegetation height. (b) Relationship between species attendance and mean vegetation height. The y-axis represents the cumulative amount of time all species in a given habitat participated in a flock. Flocks (points in the graph) are shaded for habitat type (medium grey, PF; light grey, 100 ha; white, 10 ha; dark grey, PSF; black, SF). Dashed line represents the β coefficient from the model and the grey shaded area is the 95% confidence interval. (Online version in colour.)