R-3 Repository :: Browsing by Author "Dunham, Amy E."

Browsing by Author "Dunham, Amy E."

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Above and below ground impacts of terrestrial mammals and birds in a tropical forest
(Blackwell Publishing, 2008) Dunham, Amy E.
Understanding the impact of losing trophic diversity has global significance for managing ecosystems as well as important theoretical implications for community and ecosystem ecology. In several tropical forest ecosystems, habitat fragmentation has resulted in declines and local extinctions of mammalian and avian terrestrial insectivores. To assess the ability of a tropical rainforest community in Ivory Coast to resist perturbation from such loss of trophic diversity, I traced feedbacks in above and below ground communities and measured changes in nutrient levels and herbivory rates in response to an experimental exclosure of avian and mammalian terrestrial insectivores. I present evidence that loss of this functional group may result in increased tree seedling herbivory and altered nutrient regimes through changes in the abundance and guild structure of invertebrates. Exclusion of top predators of the forest floor resulted in increased seedling herbivory rates and macro-invertebrate ( 5 mm) densities with strongest effects on herbivorous taxa, spiders and earthworms. Densities of microbivores including Collembola, Acarina and Sciaridae showed the opposite trend as did levels of inorganic phosphorus in the soil. Results were evaluated using path analysis which supported the presence of a top down trophic cascade in the detrital web which ultimately affected turnover of phosphorus, a limiting nutrient in tropical soils. Results illustrate the potential importance of vertebrate predators in both above and belowground food webs despite the biotic diversity and structural heterogeneity of the rainforest floor.
Assessing the impacts of nonrandom seed dispersal by multiple frugivore partners on plant recruitment
(Ecological Society of America, 2015) Razafindratsima, Onja H.; Dunham, Amy E.
Directed dispersal is defined as enhanced dispersal of seeds into suitable microhabitats, resulting in higher recruitment than if seeds were dispersed randomly. While this constitutes one of the main explanations for the adaptive value of frugivore-mediated seed dispersal, the generality of this advantage has received little study, particularly when multiple dispersers are involved. We used probability recruitment models of a long-lived rainforest tree in Madagascar to compare recruitment success under dispersal by multiple frugivores, no dispersal, and random dispersal. Models were parameterized using a three-year recruitment experiment and observational data of dispersal events by three frugivorous lemur species that commonly disperse its seeds. Frugivore-mediated seed dispersal was nonrandom with respect to canopy cover and increased modeled per-seed sapling recruitment fourfold compared to no dispersal. Seeds dispersed by one frugivore, Eulemur rubriventer, had higher modeled recruitment probability than seeds dispersed randomly. However, as a group, our models suggest that seeds dispersed by lemurs would have lower recruitment than if dispersal were random. Results demonstrate the importance of evaluating the contribution of multiple frugivores to plant recruitment for understanding plant population dynamics and the ecological and evolutionary significance of seed dispersal.
Battle of the sexes: Cost asymmetry explains female dominance in lemurs
(Elsevier, 2008) Dunham, Amy E.
Biodiversity maintenance in food webs with regulatory environmental feedbacks
(Elsevier, 2007) Rauscher, Daniel; Brown, Christopher G.; Dunham, Amy E.; Bagdassarian, Carey K.
Although the food web is one of the most fundamental and oldest concepts in ecology, elucidating the strategies and structures by which natural communities of species persist remains a challenge to empirical and theoretical ecologists. We show that simple regulatory feedbacks between autotrophs and their environment when embedded within complex and realistic food-web models enhance biodiversity. The food webs are generated through the niche-model algorithm and coupled with predator–prey dynamics, with and without environmental feedbacks at the autotroph level. With high probability and especially at lower, more realistic connectance levels, regulatory environmental feedbacks result in fewer species extinctions, that is, in increased species persistence. These same feedback couplings, however, also sensitize food webs to environmental stresses leading to abrupt collapses in biodiversity with increased forcing. Feedback interactions between species and their material environments anchor food-web persistence, adding another dimension to biodiversity conservation. We suggest that the regulatory features of two natural systems, deep-sea tubeworms with their microbial consortia and a soil ecosystem manifesting adaptive homeostatic changes, can be embedded within niche-model food-web dynamics. r 2007 Elsevier Ltd. All rights reserved.
Carry-over effects in space: Beyond single species studies and towards metacommunity dynamics
(2014-04-25) Van Allen, Benjamin G; Rudolf, Volker H. W.; Miller, Thomas E.; Dunham, Amy E.; Lane, David M.
The behavioral and physical traits of adults can be strongly influenced by conditions experienced during development. Consequently, variation in natal habitat quality across a landscape and through time can also lead to differences in the traits of adults. When individuals move across the landscape, this could create carry-over effects where differences in the natal habitat quality of colonizers influence population dynamics and species interactions in new habitats. I studied how these carry-over effects, which are known to alter individual traits and population dynamics, scale up to larger effects on community and metacommunity (multiple communities across a landscape connected by dispersing individuals) dynamics. I tested these questions on carry-over effects in a spatial context using a Tribolium spp. flour beetle system with habitat patches of flour. I generated carry-over effects by using flour types which predictably alter the traits of flour beetles who develop in them. The first chapter identified that carry-over effects which alter population dynamics occur in this system. It also discovered novel and powerful mechanisms for carry-over effects to influence population dynamics. The second chapter shows how carry-over effects and population density interact to affect dispersal decisions, which is important for understanding how carry-over effects will propagate across landscapes. The third chapter shows that carry-over effects can decisively alter competitive dynamics and outcomes, but that how may not be immediately predictable from their influence on single population dynamics. Finally, my fourth chapter manipulated whether carry-over effects occurred across multi-patch landscapes to test whether they influence species dynamics across a metacommunity. This final chapter shows that at the landscape scale, carry-over effects can have even more strong emergent effects. For example, carry-over effects increased population sizes across landscapes by ~10% while dramatically promoting different species under different conditions of dispersal and habitat arrangement. Thus, the influences of carry-over effects on population and community dynamics at the landscape level may be strong, but context dependent on other spatial processes. Further work to understand how landscape habitat patterns and dispersal shape the influence of carry-over effects on metacommunity dynamics could improve understanding of natural population and community dynamics.
Changing species interactions and processes in tropical forests in the Anthropocene
(2020-01-31) Lamperty, Therese; Dunham, Amy E.
Human activities have caused widespread species declines and habitat degradation, particularly in tropical forests. These changes are not just consequences of global change, but they are also themselves drivers of change. This is because as vertebrate assemblages are altered, ecological and evolutionary processes can be influenced by changes in species interactions. The consequences of this can be profound, but they are not fully understood. Here, we address three critical but understudied aspects of ecological and evolutionary cascades triggered by current global change patterns. In Chapter 1, we show that defaunation in an Afrotropical system can indirectly increase understory vegetation. We also report a sharp decrease in termite abundances and a 25% lower contribution of invertebrates to decomposition in defaunated compared to faunally-intact forest. Overall, this chapter indicates defaunation may indirectly affect understory vegetation and invertebrate communities with consequences for foodweb dynamics and processes on the forest floor that ultimately influence nutrient cycling. In Chapter 2, we take a finer-scale approach to look at how defaunation in a neotropical forest affects seed dispersal and the resulting spatial genetic structure of a dominant, animal dispersed palm. Using a genetic approach we find evidence for the first time that even in a generalist tree species (i.e. one able to gain dispersal from a broad suite of frugivores), defaunation can affect seed-dispersal, as indicated by the higher spatial genetic structure we find associated with defaunation. Ultimately, this chapter has implications for how defaunation influences the maintenance and spatial distribution of genetic variation for tropical plant communities. In Chapter 3, we move from investigating how species interactions and ecological processes are affected by local extinctions to how they are influenced by local habitat disturbance, an equally ubiquitous threat. In studying frugivore visitation to host plants, we find that indicators of past forest disturbance are associated with lower fruit removal rates and altered community composition of frugivore mutualists feeding on fruits of a neotropical palm. This chapter demonstrates that localized differences in forest structure resulting from past disturbance can influence species interactions for decades, potentially influencing seed dispersal services, plant demography and forest regeneration patterns.
Chapter 3: Status, trends and future dynamics of biodiversity and ecosystems underpinning nature's contributions to people.
(IPBES, 2018) Cormier-Salem, Marie-Christine; Dunham, Amy E.; Gordon, Christopher; Belhabib, Dyhia; Bennas, Nard; Duminil, Jérôme; Egoh, Benis N.; Elahamer, Aisha Elfaki Mohamed; Moise, Bakwo Fils Eric; Gillson, Lindsey; Haddane, Brahim; Mensah, Adelina; Mourad, Ahmim; Randrianasolo, Harison; Razafindratsima, Onja H.; Taleb, Mohammed Sghir; Shemdoe, Riziki; Dowo, Gregory; Amekugbe, Millicent; Burgess, Neil; Foden, Wendy; Niskanen, Leo; Mentzel, Christine; Njabo, Kevin Y.; Maoela, Anicia Malebajoa; Marchant, Robert; Walters, Michele; Yao, Adou Constant; Archer, E.; Dziba, L.; Mulongoy, K.J.; Maoela, M.A.; Walters, M.
Drivers and consequences of animal foraging behavior on seed dispersal and plant community composition
(2021-10-11) Tonos Luciano, Jadelys M.; Dunham, Amy E.
Dispersal is essential to coexistence and community assembly. Animal dispersal is particularly important in hyper-diverse tropical communities. However, dispersal is generally represented simplistically, ignoring its behavioral basis and the resulting variation among species and individuals. Though the importance of behavior is increasingly recognized, further work is needed to link the details of disperser behavior to their repercussions for plant communities. Here we examine the influence of disperser behavior on spatial patterns of plant-animal interactions, seed deposition and plant community diversity. Chapter 1 showed how frugivore consideration of the intrinsic and extrinsic traits of the available fruiting resources can result in drastic inequalities in plant-animal interactions. A few highly visited plants in central locations, with rich neighborhoods and large fruit crops, increased visitation to near neighbors and contributed to a spatially modular interaction structure. This chapter suggests the need to examine individual-level interactions in plant-animal mutualisms, as frugivore foraging behavior may structure interactions in a way that cannot be predicted from the species-level. Chapter 2 examined how skewed foraging patterns may structure patterns in seed dispersal and considered the need to incorporate such foraging decisions into models of seed movement. We show that incorporating frugivore foraging decisions, in respect to highly preferred plants, influenced dispersal patterns in ways that cannot be accurately modeled with traditional dispersal models. This chapter suggests the need to incorporate individual-level variation in plant frugivore interactions, and the resulting directionality of movement, into future models of seed dispersal. Chapter 3 studied how frugivorous disperser foraging behavior may shape the spatial structure of plant community diversity. We found that biotic dispersal can be associated with higher or lower diversity near biotically dispersed plants, while abiotically dispersed plants showed little deviation. Additionally, different dispersal modes can vary in their influence on spatial diversity patterns, though the strength of such effects varied across sites relying on different disperser communities. This chapter indicates how even broadly categorizing disperser behavior can illuminate the mechanisms generating plant community structure.
Ecological drivers of intraspecific variation in seed dispersal services of a common neotropical palm
(Wiley, 2021) Lamperty, Therese; Karubian, Jordan; Dunham, Amy E.
Through frugivory and seed dispersal, vertebrates influence plant demography and forest regeneration. Variation in local habitat surrounding fruiting plants can influence frugivore foraging decisions, thereby creating intraspecific variation in seed dispersal services. However, we have little knowledge of drivers of local variation in frugivory. Here, we investigate factors that may influence frugivore diversity and fruit removal at the level of individual plants. We focus on a common understory palm within a continuous Chocó forest with mixed land-use histories in Ecuador. The density of pioneer tree species in the genus Cecropia around focal palms was negatively related to fruit removal and the diversity of frugivores visiting palms. This may relate to the fact that the presence and abundance of Cecropia species often indicate the existence and severity of past disturbances. Local Cecropia density was also related to an overall shift in the frugivore community that corresponded with an increase in fruit removal by lower-quality seed dispersers (rodents). We also found that the local density of fruiting conspecifics was positively related to frugivore diversity, but not fruit removal. Our results provide information on drivers of intraspecific inequalities in plant populations across tropical forest landscapes. The reduction in fruit removal and frugivore diversity associated with local Cecropia abundance suggests that seed dispersal services can be sensitive to fine-scale variation in habitat structure. Furthermore, because Cecropia are often indicative of past disturbances, this indicates that even small-scale habitat degradation by humans can have lasting effects by creating localized pockets of forest unfavored by frugivores.
Evaluating effects of habitat loss, hunting, and El Niño on a threatened lemur
(Elsevier Science, 2008) Erhart, Elizabeth M.; Dunham, Amy E.; Wright, Patricia C.; Overdorff, Deborah J.
Madagascar ranks as one of the world’s top extinction hotspots because of its high endemism and high rate of habitat degradation. Global climate phenomena such as El Nin˜ o Southern Oscillations may have confounding impacts on the island’s threatened biota but these effects are less well known. We performed a demographic study of Propithecus edwardsi, a lemur inhabiting the eastern rainforest of Madagascar, to evaluate the impact of deforestation, hunting, and El Nin˜ o on its population and to re-evaluate present endangerment categorization under the IUCN. Over 18 years of demographic data, including survival and fecundity rates were used to parameterize a stochastic population model structured with three stage classes (yearlings, juveniles, and adults). Results demonstrate that hunting and deforestation are the most significant threats to the population. Analysis of several plausible scenarios and combinations of threat revealed that a 50% population decline within three generations was very likely, supporting current IUCN classification. However, the analysis also suggested that changing global cycles may pose further threat. The average fecundity of lemurs was over 65% lower during El Nin˜ o years. While not as severe as deforestation or hunting, if El Nin˜ o events remain at the current high frequency there may be negative consequences for the population. We suggest that it is most critical for this species continued survival to create more protected areas, not only to thwart hunting and deforestation, but also to give this endangered lemur a better chance to recover from and adapt to altered climate cycles in the future.
Frugivores bias seed-adult tree associations through nonrandom seed dispersal: a phylogenetic approach
(Wiley, 2016) Razafindratsima, Onja H.; Dunham, Amy E.
Frugivores are the main seed dispersers in many ecosystems, such that behaviorally driven, nonrandom patterns of seed dispersal are a common process; but patterns are poorly understood. Characterizing these patterns may be essential for understanding spatial organization of fruiting trees and drivers of seed-dispersal limitation in biodiverse forests. To address this, we studied resulting spatial associations between dispersed seeds and adult tree neighbors in a diverse rainforest in Madagascar, using a temporal and phylogenetic approach. Data show that by using fruiting trees as seed-dispersal foci, frugivores bias seed dispersal under conspecific adults and under heterospecific trees that share dispersers and fruiting time with the dispersed species. Frugivore-mediated seed dispersal also resulted in nonrandom phylogenetic associations of dispersed seeds with their nearest adult neighbors, in nine out of the 16 months of our study. However, these nonrandom phylogenetic associations fluctuated unpredictably over time, ranging from clustered to overdispersed. The spatial and phylogenetic template of seed dispersal did not translate to similar patterns of association in adult tree neighborhoods, suggesting the importance of post-dispersal processes in structuring plant communities. Results suggest that frugivore-mediated seed dispersal is important for structuring early stages of plant-plant associations, setting the template for post-dispersal processes that influence ultimate patterns of plant recruitment. Importantly, if biased patterns of dispersal are common in other systems, frugivores may promote tree coexistence in biodiverse forests by limiting the frequency and diversity of heterospecific interactions of seeds they disperse.
Increasing women's participation in community-based conservation: key to success?
(Indian Ocean e-Ink, 2015) Razafindratsima, Onja H.; Dunham, Amy E.
Nonrandom seed dispersal by lemur frugivores: mechanism, patterns and impacts
(2015-04-23) Razafindratsima, Onja Harinala Franckline Eva; Dunham, Amy E.; Rudolf, Volker H. W.; Siemann, Evan; Masiello, Caroline A.; Erhart, Elizabeth M.
Frugivores act as seed-dispersal agents in many ecosystems. Thus, understanding the roles and impacts of seed dispersal by frugivores is important to understand the structure and diversity maintenance of plant communities. Frugivore-mediated seed dispersal is behaviorally driven, generating nonrandom patterns of seed dispersion; but, we know relatively little about how this might affect plant populations or communities. I examined how frugivores affected plants from the individual level to the population and community levels. To do this, I used modeling, trait-based and phylogenetic approaches combined with field observations and experiments, focusing on seed dispersal by three frugivorous lemur species in the biodiverse rainforest of Ranomafana National Park, Madagascar. An analysis of traits suggested that 84% of trees in Ranomafana are adapted for animal dispersal, of which more than 70% are dispersed by these three lemur species indicating their role as generalist dispersers. The distribution of fruit and seed size of bird-dispersed species was nested within the wide spectrum of size distribution associated with lemur dispersal. Nonrandom seed dispersal by these three frugivores increased per capita recruitment of the seeds of a long-lived canopy tree, Cryptocarya crassifolia, by four-fold compared with no dispersal, even though it was not an overall advantage compared to random dispersal. The three frugivores not only dispersed seeds away from parent and conspecific adult-trees, but also biased seed dispersal toward certain microhabitats. By using fruiting trees as seed dispersal foci, lemur frugivores structure the spatial associations between dispersed seeds and adult-trees nonrandomly, in terms of fruiting time, dispersal mode and phylogenetic relatedness. However, lemur-dispersed tree species were not more likely to be each other’s neighbors as adults. Interestingly, co-fruiting neighboring trees sharing lemur dispersers were more phylogenetically distant than expected by chance despite a phylogenetic signal in lemur dispersal mode, although there was no phylogenetic signal in the timing of fruiting among lemur-dispersed tree species. Results suggest an important link between frugivore foraging behavior and the spatial, temporal and phylogenetic patterning of seed dispersal. The critical role of frugivores in structuring seed dispersion and seed-adult plant associations has critical implications for plant-plant interactions, biodiversity patterns and community structure.
Restoration ecology of ecosystems invaded by Triadica sebifera (Chinese tallow tree): theory and practice
(2013-07-24) Gabler, Christopher; Siemann, Evan; Dunham, Amy E.; Bedient, Philip B.; Rudolf, Volker H. W.
Invasive exotic species threaten biodiversity and ecosystem functions globally, creating need for and encumbering ecological restoration. When restoring exotic plant-dominated ecosystems, reinvasion pressure is the rate of new exotic recruitment following mature exotic removal. It can vary broadly among similarly invaded habitats and is crucial to restoration outcomes and costs, but is difficult to predict and poorly understood. Initial results from the experimental restoration of a wetland dominated by Triadica sebifera led us to develop the ‘outgrow the stress’ hypothesis. It holds: (1) Variation in reinvasion pressure is driven by differences in propagule abundance and spatiotemporal availability of realized recruitment windows, which are defined by abiotic conditions and biotic interactions. (2) Differences in reinvasion pressure become masked by exotic dominance when increases in niche breadth during development enable exotic persistence across sites where recruitment windows range from frequent to episodic. We validated this hypothesis. First, we used greenhouse and field experiments to quantify Triadica’s moisture niche early in development. By two months post-germination, seedling tolerances broadened to include conditions unsuitable for germination. This clearly demonstrated a rapid ontogenetic niche expansion, which could decouple mature Triadica density and average reinvasion pressure. Second, we used a greenhouse mesocosm experiment to quantify how recruitment window duration, competition and fertility impacted population-level Triadica establishment in stressful environments. As ‘outgrow the stress’ predicts, longer windows increased Triadica success and multi-factor interactions were common, with competition and fertility effects varying among environmental contexts. Third, we substantiated predictions of ‘outgrow the stress’ regarding propagule availability and soil moisture by manipulating these in a multi-site field experiment spanning eleven experimental restorations of Triadica-dominated habitats along a moisture gradient. Triadica reinvasion pressure varied broadly among sites but correlated with moisture and fertility. Propagule availability drove reinvasion in favorable environments, but availability of suitable conditions trumped propagules in extreme environments. Competition reduced Triadica performance and sometimes survival. Triadica prevalence reduced native plant prevalence. Six restorations require minimal Triadica management for success. This work advances our understanding and enables better predictions of reinvasion pressure and invasions in general. Accurate predictions enhance restoration efficiency by informing site selection and optimal management strategies.
Simplified Communities of Seed-Dispersers Limit the Composition and Flow of Seeds in Edge Habitats
(Frontiers Media S.A., 2021) Razafindratsima, Onja H.; Raoelinjanakolona, Nasandratra Nancia; Heriniaina, Rio R.; Nantenaina, Rindra H.; Ratolojanahary, Tianasoa H.; Dunham, Amy E.
Edge effects, driven by human modification of landscapes, can have critical impacts on ecological processes such as species interactions, with cascading impacts on biodiversity as a whole. Characterizing how edges affect vital biotic interactions such as seed dispersal by frugivores is important for better understanding potential mechanisms that drive species coexistence and diversity within a plant community. Here, we investigated how differences between frugivore communities at the forest edge and interior habitats of a diverse tropical rainforest relate to patterns of animal-mediated seed dispersal and early seedling recruitment. We found that the lemur communities across the forest edge-interior gradient in this system showed the highest species richness and variability in body sizes at intermediate distances; the community of birds showed the opposite pattern for species richness. Three large-bodied frugivores, known to be effective dispersers of large seeds, tended to avoid the forest edge. As result, the forest edges received a lower rate of animal-mediated seed dispersal compared to the interior habitats. In addition, we also found that the seeds that were actively dispersed by animals in forest edge habitats were smaller in size than seeds dispersed in the forest interior. This pattern was found despite a similarity in seed size of seasonally fruiting adult trees and shrubs between the two habitats. Despite these differences in dispersal patterns, we did not observe any differences in the rates of seedling recruitment or seed-size distribution of successful recruit species. Our results suggest that a small number of frugivores may act as a potential biotic filter, acting on seed size, for the arrival of certain plant species to edge habitats, but other factors may be more important for driving recruitment patterns, at least in the short term. Further research is needed to better understand the potential long-term impacts of altered dispersal regimes relative to other environmental factors on the successional dynamics of edge communities. Our findings are important for understanding potential ecological drivers of tree community changes in forest edges and have implications for conservation management and restoration of large-seeded tree species in disturbed habitats.
Specificity in the ecology and evolution of plant defense against herbivores: Identity and history
(2013-12-04) Carrillo, Juli; Siemann, Evan; Dunham, Amy E.; Miller, Thomas E.; Bartel, Bonnie
Biotic interactions can structure communities, drive succession, and account for patterns of biodiversity, yet we currently know little about how multiple biotic factors interact to influence traits in ecological and evolutionary time. Moreover, variation in biotic interactions among native and introduced plant ranges may account for the spectacular success of some invasive species. For example, in novel environments plants may experience ecological release from coevolved specialist enemies and may in turn evolve allocation away from costly defense towards competitive ability. This assumes that plants face a fundamental tradeoff between growth and defense, but we predict that this tradeoff depends on the biotic environment and specificity of plant responses to herbivores. I examined variation in growth and herbivore defense allocation in a model woody invader, using native and invasive populations of Triadica sebifera that differed in historic herbivore pressure. Greenhouse studies demonstrated that populations from the native range with a history of intense herbivory invested more in indirect herbivore resistance (extrafloral nectar production) than invasive populations. Additionally, the induction of indirect defense depended on herbivore feeding mode, suggesting tradeoffs among defenses against multiple herbivores. Further, I showed that even similarly feeding generalist herbivores can induce specific defense responses of plants, by demonstrating a loss of specificity in invasive populations compared to native populations, despite higher tolerance of herbivory overall. Together, these patterns suggest that selection for competitive ability may result in correlated selection for increased tolerance but decreased resistance of herbivory and reduced specificity of defense responses. I tested mechanisms causing this pattern of reallocation by examining the relative contributions of selection from herbivores and competitors in shaping herbivore defense in a different model plant species, common mustard. I used an experimental evolution approach to generate lines that differed in historical herbivore identity and competitive intensity. I detected that both herbivore and competitive history influence plant resistance to contemporary herbivores, and revealed a strong role for competitive history in contemporary plant defense. Understanding how variation in biotic interactions contributes to plant success is one way to predict the traits likely to evolve within a given selective environment and their ecological effects.
Using scalar models for precautionary assessments of threatened species
(Blackwell Publishing, 2006-10) Bridges, Todd S; Akcakaya, H. Resit; Dunham, Amy E.
Scalar population models, commonly referred to as count-based models, are based on time-series data of population sizes and may be useful for screening-level ecological risk assessments when data for more complex models are not available. Appropriate use of such models for management purposes, however, requires understanding inherent biases that may exist in these models. Through a series of simulations, which compared predictions of risk of decline of scalar and matrix-based models, we examined whether discrepancies may arise from different dynamics displayed due to age structure and generation time. We also examined scalar and matrix-based population models of 18 real populations for potential patterns of bias in population viability estimates. In the simulation study, precautionary bias (i.e., overestimating risks of decline) of scalar models increased as a function of generation time. Models of real populations showed poor fit between scalar and matrix-based models, with scalar models predicting significantly higher risks of decline on average. The strength of this bias was not correlated with generation time, suggesting that additional sources of bias may be masking this relationship. Scalar models can be useful for screening-level assessments, which should in general be precautionary, but the potential shortfalls of these models should be considered before using them as a basis for management decisions.
Vertebrate seed dispersers maintain the composition of tropical forest seedbanks
(Oxford University Press, 2015) Wandrag, E.M.; Dunham, Amy E.; Miller, R.H.; Rogers, H.S.
The accumulation of seeds in the soil (the seedbank) can set the template for the early regeneration of habitats following disturbance. Seed dispersal is an important factor determining the pattern of seed rain, which affects the interactions those seeds experience. For this reason, seed dispersal should play an important role in structuring forest seedbanks, yet we know little about how that happens. Using the functional extirpation of frugivorous vertebrates from the island of Guam, together with two nearby islands (Saipan and Rota) that each support relatively intact disperser assemblages, we aimed to identify the role of vertebrate dispersers in structuring forest seedbanks. We sampled the seedbank on Guam where dispersers are absent, and compared this to the seedbank on Saipan and Rota where they are present. Almost twice as many species found in the seedbank on Guam, when compared to Saipan and Rota, had a conspecific adult within 2 m. This indicates a strong role of vertebrate dispersal in determining the identity of seeds in the seedbank. In addition, on Guam a greater proportion of samples contained no seeds and overall species richness was lower than on Saipan. Differences in seed abundance and richness between Guam and Rota were less clear, as seedbanks on Rota also contained fewer species than Saipan, possibly due to increased post-dispersal seed predation. Our findings suggest that vertebrate seed dispersers can have a strong influence on the species composition of seedbanks. Regardless of post-dispersal processes, without dispersal, seedbanks no longer serve to increase the species pool of recruits during regeneration.

Browsing by Author "Dunham, Amy E."

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