Seed dispersal networks in tropical forest fragments: Area effects, remnant species, and interaction diversity

Abstract

Seed dispersal interactions involve key ecological processes in tropical forests that help to maintain ecosystem functioning. Yet this functionality may be threatened by increasing habitat loss, defaunation, and fragmentation. However, generalist species, and their interactions, can benefit from the habitat change caused by human disturbance while more specialized interactions mostly disappear. Therefore, changes in the structure of the local, within fragment, networks can be expected. Here we investigated how the structure of seed dispersal networks changes along a gradient of increasing habitat fragmentation. We analyzed 16 bird seed dispersal assemblages from forest fragments of a biodiversity‐rich ecosystem. We found significant species–, interaction–, and network–area relationships, yet the later was determined by the number of species remaining in each community. The number of frugivorous bird and plant species, their interactions, and the number of links per species decreases as area is lost in the fragmented landscape. In contrast, network nestedness has a negative relationship with fragment area, suggesting an increasing generalization of the network structure in the gradient of fragmentation. Network specialization was not significantly affected by area, indicating that some network properties may be invariant to disturbance. Still, the local extinction of partner species, paralleled by a loss of interactions and specialist–specialist bird–plant seed dispersal associations, suggests the functional homogenization of the system as area is lost. Our study provides empirical evidence for network–area relationships driven by the presence/absence of remnant species and the interactions they perform. Abstract in Spanish is available with online material.

Large wild herbivores slow down the rapid decline of plant diversity in a tropical forest biodiversity hotspot

“…Our results contribute to this body of work, indicating that LH conservation and rewilding could slow down compositional change by filtering out the arrival of new species into communities. In the context of tropical forests, it is also noteworthy that the combined effects of climate change and land use are contributing to tree loss, forest functional changes and the emergence of fire, leading to a process of increasing faunal and floristic savannization (Alroy, 2017;Emer et al, 2020;Sales et al, 2020;Staal et al, 2020). LH might contribute to decelerate such process by decelerating compositional change and filtering against invasive species and life-forms typical of savanna-like forests.…”

“…Tropical rainforests are subjected to unprecedented anthropogenic pressures that compromise their integrity and erode their diversity. Tree loss, widespread defaunation, changes in the functional structure of interactions and the emergence of fire threaten to drive many of such hyper-diverse ecosystems into a nonlinear process of floristic and faunal savannization (Alroy, 2017;Emer et al, 2020;Galetti et al, 2021;Sales et al, 2020;Staal et al, 2020). To halt the ecological meltdown of these hyper-diverse ecosystems, effective conservation and restoration measures are required to preserve the diversity in both old-growth and secondary tropical forests (Rozendaal et al, 2019).…”

Deforestation Simplifies Understory Bird Seed-Dispersal Networks in Human-Modified Landscapes

“…Indeed, forest loss can act as an important selective force, filtering frugivorous birds with specific phylogenetic and functional features, and thus interfering in the seed dispersal networks (Peña et al, 2020). In particular, recent studies reported significant changes in network structure (e.g., higher nestedness and lower modularity) after forest loss in Atlantic forest sites (Emer et al, 2020;Marjakangas et al, 2020) or changes in bird functions in deforested landscapes (Coster et al, 2015). Here, we found highly simplified seed-dispersal networks with only a few pairs of interacting species, whose structural properties have been depleted due to the loss of forest cover in these human-modified landscapes.…”

“…In contrast, species homogenization in more disturbed landscapes can concentrate interactions within some dominant species in the system, reducing interaction evenness (Rodewald et al, 2014). Similarly, the loss of species or decreasing species abundance in smaller forest fragments can affect the number of links per species by reducing the number of partners available for the interactions to occur (Muñoz et al, 2017;Emer et al, 2020).…”

Importance of species traits on individual-based seed dispersal networks and dispersal distance for endangered trees in a fragmented forest

“…Habitat fragmentation is one of the most important drivers of the decline in species diversity ( Dirzo et al., 2014 ; Haddad et al., 2015 ; Cao and Zhang, 2022 ), which may trigger cascading effects on ecological networks ( Taubert et al., 2018 ; Peters et al., 2019 ). In most remnant habitat patches, mutualistic networks for animal-mediated pollination and seed dispersal are disrupted by species loss ( Schupp et al., 2017 ; Grass et al., 2018 ) and affected by the size and quality of habitat patches ( Spiesman and Inouye, 2013 ; de Bomfim et al., 2018 ; Emer et al., 2019 ). Compared with large habitat patches, small habitat patches are unable to support the survival of most large-bodied species owing to their lower food quality and greater interspecific competition ( Hart et al., 2017 ; Ferretti and Fattorini, 2021 ).…”