Predicting the consequences of disperser extinction: richness matters the most when abundance is low

Effects of habitat disturbance on the reproductive ecology of Arum pictum ssp. sagittifolium: from pollination to seedling recruitment

BACKGROUND AND AIMS

The loss of natural habitats may strongly affect the fitness of plants that depend on animals for reproduction. However, very little is known regarding the differential effects of habitat disturbance on the distinct phases of the reproductive cycle of plants, especially in non-rewarding species.

METHODS

We assessed the effects of habitat disturbance on the entire reproductive cycle of Arum pictum ssp. sagittifolium, a species with deceptive pollination that is endemic to the western Mediterranean Basin. For this, we performed hand-pollination and bagging experiments, evaluated the role of pollinators and dispersers on reproduction, and estimated seedling recruitment in three natural and three disturbed populations (according to their surrounding natural habitat) in Mallorca Island.

KEY RESULTS

Pollinators were sphaerocerid flies (mainly Coproica, with ~50 % of visits) and staphylinid beetles, and were required for sexual reproduction. Habitat disturbance differently affected the reproductive phases of A. pictum ssp. sagittifolium. Habitat disturbance had a positive effect on Shannon pollinator diversity (but not on pollinator richness), and total pollinator and Coproica abundance were three times higher in disturbed habitats, where overall seed production was also ~30 % higher in natural habitats. Seed production increased with Coproica abundance, but only in natural habitats. Seed dispersers of A. pictum ssp. sagittifolium were birds, mainly Sylvia atricapilla. Although habitat disturbance did not influence disperser diversity or abundance, the majority of seedlings appeared under adult plants and in natural habitats.

CONCLUSIONS

Overall recruitment was higher in natural habitats, but this effect could have been masked by only assessing pollinator and disperser numbers, as processes related to the quality of these interactions might be influencing fitness. Our study highlights the need to study different reproductive phases and their multiple components and processes to properly understand the effects of habitat disturbance on the regeneration of plant populations.

Plant-frugivore network simplification under habitat fragmentation leaves a small core of interacting generalists

Habitat fragmentation impacts seed dispersal processes that are important in maintaining biodiversity and ecosystem functioning. However, it is still unclear how habitat fragmentation affects frugivorous interactions due to the lack of high-quality data on plant-frugivore networks. Here we recorded 10,117 plant-frugivore interactions from 22 reservoir islands and six nearby mainland sites using the technology of arboreal camera trapping to assess the effects of island area and isolation on the diversity, structure, and stability of plant-frugivore networks. We found that network simplification under habitat fragmentation reduces the number of interactions involving specialized species and large-bodied frugivores. Small islands had more connected, less modular, and more nested networks that consisted mainly of small-bodied birds and abundant plants, as well as showed evidence of interaction release (i.e., dietary expansion of frugivores). Our results reveal the importance of preserving large forest remnants to support plant-frugivore interaction diversity and forest functionality.

Smaller communities, such as those on islands, under ecological network simplification reduce interactions between specialist organisms.

Mechanisms underlying interaction frequencies and robustness in a novel seed dispersal network: lessons for restoration

As human-caused extinctions and invasions accumulate across the planet, understanding the processes governing ecological functions mediated by species interactions, and anticipating the effect of species loss on such functions become increasingly urgent. In seed dispersal networks, the mechanisms that influence interaction frequencies may also influence the capacity of a species to switch to alternative partners (rewiring), influencing network robustness. Studying seed dispersal interactions in novel ecosystems on O'ahu island, Hawai'i, we test whether the same mechanisms defining interaction frequencies can regulate rewiring and increase network robustness to simulated species extinctions. We found that spatial and temporal overlaps were the primary mechanisms underlying interaction frequencies, and the loss of the more connected species affected networks to a greater extent. Further, rewiring increased network robustness, and morphological matching and spatial and temporal overlaps between partners were more influential on network robustness than species abundances. We argue that to achieve self-sustaining ecosystems, restoration initiatives can consider optimal morphological matching and spatial and temporal overlaps between consumers and resources to maximize chances of native plant dispersal. Specifically, restoration initiatives may benefit from replacing invasive species with native species possessing characteristics that promote frequent interactions and increase the probability of rewiring (such as long fruiting periods, small seeds and broad distributions).

The impact of habitat loss on pollination services for a threatened dune endemic plant

Habitat loss is currently a major threat to biodiversity, affecting species interactions, such as plant-pollinator interactions. This is particularly important in self-incompatible plants relying on pollinators to reproduce and sustain their populations. Here, we evaluated how habitat loss affects the pollination system, plant individual-pollinator species interaction network, and plant reproductive fitness of the self-incompatible Jasione maritima var. sabularia, a threatened taxon from dune systems. This plant is a pollinator generalist, visited by 108 species from distinct taxonomic groups. Results suggest that increasing habitat loss led to a significant decline in pollinator richness, increased pollen limitation, and a decrease in reproductive fitness of J. maritima var. sabularia. Visitation rate per individual did not significantly change with available area, indicating that the quality of pollen differed across populations. The topology of the network between J. maritima var. sabularia individuals and its pollinator species did not change, which may be attributed to the stability in the core of pollinator species. This suggests that the lower fitness of plants with increasing habitat degradation may be explained not only by the lower richness of peripheral pollinators but also by the genetic structure of the plant populations, as there is a possible higher transference of less quality pollen by pollinators, ultimately compromising the persistence of plant populations. Our study highlights the need of future studies to integrate the fine details provided by individual-level networks, which will increase our understanding of the pattern of species interactions and its consequences for the fitness of threatened plant populations.

Cascading Impacts of Seed Disperser Loss on Plant Communities and Ecosystems

Seed dispersal is key to the persistence and spread of plant populations. Because the majority of plant species rely on animals to disperse their seeds, global change drivers that directly affect animals can cause cascading impacts on plant communities. In this review, we synthesize studies assessing how disperser loss alters plant populations, community patterns, multitrophic interactions, and ecosystem functioning. We argue that the magnitude of risk to plants from disperser loss is shaped by the combination of a plant species’ inherent dependence on seed dispersal and the severity of the hazards faced by their dispersers. Because the factors determining a plant species’ risk of decline due to disperser loss can be related to traits of the plants and dispersers, our framework enables a trait-based understanding of change in plant community composition and ecosystem functioning. We discuss how interactions among plants, among dispersers, and across other trophic levels also mediate plant community responses, and we identify areas for future research to understand and mitigate the consequences of disperser loss on plants globally.

The frugivory network properties of a simplified ecosystem: Birds and plants in a Neotropical periurban park

Frugivory networks exhibit a set of properties characterized by a number of network theory-derived metrics. Their structures often form deterministic patterns that can be explained by the functional roles of interacting species. Although we know lots about how these networks are organized when ecosystems are in a complete, functional condition, we know much less about how incomplete and simplified networks (such as those found in urban and periurban parks) are organized, which features are maintained, which ones are not, and why. In this paper, we examine the properties of a network between frugivorous birds and plants in a small Neotropical periurban park. We found a frugivory network composed of 29 species of birds and 23 of plants. The main roles in this network are played by four species of generalist birds (three resident, one migratory: Myiozetetes similis, Turdus grayi, Chlorospingus flavopectus, and Dumetella carolinensis) and three species of plants (one exotic, two early successional: Phoenix canariensis, Phoradendron sp., and Witheringia stramoniifolia). When compared to reference data from other locations in the Neotropics, species richness is low, one important network-level metric is maintained (modularity) whereas another one is not (nestedness). Nestedness, a metric associated with network specialists, is a feature this network lacks. Species-level metrics such as degree, species strength, and module roles, are not maintained. Our work supports modularity as the most pervasive network-level metric of altered habitats. From a successional point of view, our results suggest that properties revealed by species-level indices may be developed at a later time, lagging the acquisition of structural elements.

Life history traits predict colonization and extinction lags of desert plant species since the Last Glacial Maximum

Variation in life-history strategies can affect metapopulation dynamics and consequently the composition and diversity of communities. However, data sets that allow for the full range of species turnover from colonization to extinction over relevant time periods are limited. The late Quaternary record provides unique opportunities to explore the traits that may have influenced interspecific variation in responses to past climate warming, in particular the rate at which species colonized newly suitable habitat or went locally extinct from degrading habitat. We controlled for differences in species climate niches in order to predict expected colonization and extinction sequences recorded in packrat middens from 15 localities in the Mohave, Sonoran, and Chihuahuan deserts of North America. After accounting for temperature niche differences, we tested the hypotheses that dispersal syndrome (none, wind, vertebrate), growth form (herb, shrub, tree) and seed mass mediated variation in postglacial colonization lags among species, whereas clonality (clonal, non-clonal), growth form, and seed mass affected extinction lags. Growth form and dispersal syndrome interactively affected colonization lags, where herbaceous species lacking long-distance dispersal mechanisms exhibited lags that exceeded those of woody, wind or vertebrate-dispersed species by an average of 2,000-5,000 yr. Growth form and seed mass interactively affected extinction lags, with very small-seeded shrubs persisting for 4,000-8,000 yr longer than other functional groups. Taller, vertebrate-dispersed plants have been shown in other studies to disperse farther than shorter plants without specialized dispersal mechanisms. We found that variation along this axis of dispersal syndromes resulted in dramatic differences in colonization rates in response to past climate change. Very small seeded shrubs may have a unique combination of long vegetative and seed bank lifetimes that may allow them to persist for long periods despite declines in habitat condition. This study indicates that readily measurable traits may help predict which species will be more or less sensitive to future climate change, and inform interventions that can stabilize and promote at-risk populations.

Direct and cascading effects of landscape structure on tropical forest and non-forest frugivorous birds

Global biodiversity is increasingly threatened by land-use change, but the direct and indirect drivers of species diversity in human-modified tropical landscapes are poorly known. Forest-dependent species are expected to be particularly sensitive to changes in landscape composition (e.g., forest loss) and configuration (e.g., increase of forest edges), both directly and indirectly through cascading landscape effects on local patterns of forest structure and resource availability. In contrast, non-forest-dependent species are probably more strongly related to landscape changes than to local forest patterns, as these species are able to use resources not only from the forest, but also from other landscape elements over larger spatial scales. We tested these hypotheses using structural equation modeling. In particular, we sampled 20 landscapes (115 ha each) from the Brazilian Atlantic rainforest to assess the effect of landscape-scale forest cover and amount of forest edges on the diversity of frugivorous birds, both directly and indirectly through the effect that these landscape variables may have on vegetation complexity and fruit biomass. We separately assessed the response of forest-dependent and non-forest-dependent frugivores to infer potential mechanisms underlying bird assemblages in fragmented landscapes. The diversity of forest-dependent birds mainly decreased with the simplification of vegetation complexity in more deforested landscapes, but increased with increasing fruit biomass in more forested landscapes (indirect effects). Both patterns were significant, thus supporting a strong bottom-up control, i.e., local habitat simplification and resource scarcity in highly deforested landscapes limits the maintenance of forest-dependent birds. Conversely, but as expected, non-forest-dependent birds were more strongly and directly related to landscape-scale patterns. In particular, landscapes with higher forest edge amount showed higher bird species diversity, probably because the increasing length of ecotones and interspersion/juxtaposition of different habitat types in landscapes with more forest edges can increase resource availability and foraging efficiency of non-forest-dependent birds. As the seed dispersal services offered by forest-dependent species cannot be ecologically compensated for by the proliferation of non-forest-dependent species, preventing forest loss is imperative to maintain forest-dependent birds and forest regeneration in this vanishing biodiversity hotspot.

Pollination networks from natural and anthropogenic-novel communities show high structural similarity

The Anthropocene is marked by an unprecedented homogenisation of the world's biota, confronting species that never co-occurred during their evolutionary histories. Interactions established in these novel communities may affect ecosystem functioning; however, most research has focused on the impacts of a minority of aggressive invasive species, while changes inflicted by a less conspicuous majority of non-invasive alien species on community structure are still poorly understood. This information is critical to guide conservation strategies, and instrumental to advance ecological theory, particularly to understand how non-native species integrate in recipient communities and affect the interactions of native species. We evaluated how the structure of 50 published pollination networks changes with the proportion of alien plant species and found that network structure is largely unaffected. Although some communities were heavily invaded, the proportion of alien plant species was relatively low (mean = 10%; max. = 38%). We further characterized the pollination network in a botanic garden with a plant community dominated by non-invasive alien species (85%). We show that the structure of this novel community is also not markedly different from native-dominated communities. Plant-pollinator interactions revealed no obvious differences regarding plant origin (native vs. alien) or the native bioregion of the introduced plants. This overall similarity between native and alien plants is likely driven by the contrasting patterns of invasive plants (promoting generalism), and non-invasive aliens, suggested here to promote specialization.

Moving from frugivory to seed dispersal: Incorporating the functional outcomes of interactions in plant-frugivore networks

There is growing interest in understanding the functional outcomes of species interactions in ecological networks. For many mutualistic networks, including pollination and seed dispersal networks, interactions are generally sampled by recording animal foraging visits to plants. However, these visits may not reflect actual pollination or seed dispersal events, despite these typically being the ecological processes of interest.

Frugivorous animals can act as seed dispersers, by swallowing entire fruits and dispersing their seeds, or as pulp peckers or seed predators, by pecking fruits to consume pieces of pulp or seeds. These processes have opposing consequences for plant reproductive success. Therefore, equating visitation with seed dispersal could lead to biased inferences about the ecology, evolution and conservation of seed dispersal mutualisms.

Here, we use natural history information on the functional outcomes of pairwise bird–plant interactions to examine changes in the structure of seven European plant–frugivore visitation networks after non‐mutualistic interactions (pulp pecking and seed predation) have been removed. Following existing knowledge of the contrasting structures of mutualistic and antagonistic networks, we hypothesized a number of changes following interaction removal, such as increased nestedness and lower specialization.

Non‐mutualistic interactions with pulp peckers and seed predators occurred in all seven networks, accounting for 21%–48% of all interactions and 6%–24% of total interaction frequency. When non‐mutualistic interactions were removed, there were significant increases in network‐level metrics such as connectance and nestedness, while robustness decreased. These changes were generally small, homogenous and driven by decreases in network size. Conversely, changes in species‐level metrics were more variable and sometimes large, with significant decreases in plant degree, interaction frequency, specialization and resilience to animal extinctions and significant increases in frugivore species strength.

Visitation data can overestimate the actual frequency of seed dispersal services in plant–frugivore networks. We show here that incorporating natural history information on the functions of species interactions can bring us closer to understanding the processes and functions operating in ecological communities. Our categorical approach lays the foundation for future work quantifying functional interaction outcomes along a mutualism–antagonism continuum, as documented in other frugivore faunas.