Seed Dispersal by Primates in Asian Habitats: From Species, to Communities, to Conservation

A review of great ape behavioural responses and their outcomes to anthropogenic landscapes

Industrial expansion has brought humans and wildlife into closer contact, and added novel, complex dimensions to human-wildlife relationships. The seven great apes (chimpanzee, Bornean orangutan, Sumatran orangutan, Tapanuli orangutan, Eastern gorilla, Western gorilla, bonobo), the closest extant relatives to humans, have experienced substantial population declines resulting from anthropogenic activities. The effect of human activity on great ape behavioural ecology is therefore an emerging field of inquiry in primatology which has historically been minimally considered. This review explores how wild great apes respond behaviourally to human activities and environmental changes, synthesizing current knowledge and addressing potential outcomes and risks. Using precise search criteria, we found 96 studies documenting changes in great ape behaviour in response to human activity, and despite their broad geographic distribution, we found common patterns and responses across species to increasing human influence. Literature documented shifts in existing behaviour (57), the generation of novel behaviours (53) or reported both (15). Forty-three studies (45%) included direct (23) or indirect (20) assessment of the consequences of these behaviours. Only one study modelled a widespread loss of existing behaviours. The majority of studies included chimpanzees (67), followed by orangutans (19) and gorillas (19), and only 2 included bonobos. We found that the most frequently documented drivers of behavioural responses to anthropogenic activity were wide-scale land-use conversions in ape habitats. In response, apes have adopted crop foraging, and altered nesting behaviour, range use, and social strategies. While these responses appear to allow survival in the immediate sense, they may expose individuals to more risks in the long term. Analysis revealed that under many contexts changing great ape behaviour is putting strain on the human-ape relationship, resulting in injury, harassment, and even the killing of apes. We found examples of tolerant relationships between humans and apes shifting towards conflict, potentially worsening the conservation crisis and inviting inquiry into tolerance thresholds among human communities. We emphasize the importance of community-engaged strategies for reducing competition over resources and conclude that great ape behavioural responses to human activity must be interpreted through a locally specific lens.

Establishing the relationship between non-human primates and mangrove forests at the global, national, and local scales

Global and spatially explicit information about the interaction between habitat and wildlife species is critical to enhancing conservation efforts. Despite the recognized importance of mangrove forests to non-human primates, the relationship between the two lacks understanding. To counter this, we created the MangPrim-21 database to map and measure the locations of interactions between all non-human primates and all mangrove forests globally. We report our findings across the global, national, and local scales for all inventoried non-human primates and all inventoried mangrove forests. Globally, we find that half of all non-primates potentially use mangrove forests, and more than half of the global mangrove forest falls within the delineated range of at least one non-human primate species. Nationally, we find that Indonesia, Madagascar, Brazil, Cameroon, and Malaysia likely have the most non-human primate and mangrove forest interactions. At the subnational level, we find that several discrete locations in Kalimantan are critical to both mangrove forests and non-human primates. The MangPrim-21 database provides a globally consistent and locally applicable database of non-human primate and mangrove forest interactions. The results presented have broader implications for non-human primate and mangrove conservation and global actions to protect both. Additionally, our results raise questions about the idea that non-human primates primarily use mangrove forests as a refuge from human encroachment and habitat degradation.

Passive directed dispersal of plants by animals

Conceptual gaps and imprecise terms and definitions may obscure the breadth of plant-animal dispersal relationships involved in directed dispersal. The term 'directed' indicates predictable delivery to favourable microsites. However, directed dispersal was initially considered uncommon in diffuse mutualisms (i.e. those involving many species), partly because plants rarely influence post-removal propagule fate without specialized adaptations. This rationale implies that donor plants play an active role in directed dispersal by manipulating vector behaviour after propagule removal. However, even in most classic examples of directed dispersal, participating plants do not influence animal behaviour after propagule removal. Instead, such plants may take advantage of vector attraction to favourable plant microsites, indicating a need to expand upon current interpretations of directed dispersal. We contend that directed dispersal can emerge whenever propagules are disproportionately delivered to favourable microsites as a result of predictably skewed vector behaviour. Thus, we propose distinguishing active and passive forms of directed dispersal. In active directed dispersal, the donor plant achieves disproportionate arrival to favourable microsites by influencing vector behaviour after propagule removal. By contrast, passive directed dispersal occurs when the donor plant takes advantage of vector behaviour to arrive at favourable microsites. Whereas predictable post-removal vector behaviour is dictated by characteristics of the donor plant in active directed dispersal, characteristics of the destination dictate predictable post-removal vector behaviour in passive directed dispersal. Importantly, this passive form of directed dispersal may emerge in more plant-animal dispersal relationships because specialized adaptations in donor plants that influence post-removal vector behaviour are not required. We explore the occurrence and consequences of passive directed dispersal using the unifying generalized gravity model of dispersal. This model successfully describes vectored dispersal by incorporating the influence of the environment (i.e. attractiveness of microsites) on vector movement. When applying gravity models to dispersal, the three components of Newton's gravity equation (i.e. gravitational force, object mass, and distance between centres of mass) become analogous to propagules moving towards a location based on characteristics of the donor plant, the destination, and relocation processes. The generalized gravity model predicts passive directed dispersal in plant-animal dispersal relationships when (i) animal vectors are predictably attracted to specific destinations, (ii) animal vectors disproportionately disperse propagules to those destinations, and (iii) those destinations are also favourable microsites for the dispersed plants. Our literature search produced evidence for these three conditions broadly, and we identified 13 distinct scenarios where passive directed dispersal likely occurs because vector behaviour is predictably skewed towards favourable microsites. We discuss the wide applicability of passive directed dispersal to plant-animal mutualisms and provide new insights into the vulnerability of those mutualisms to global change.

The Influence of Seed Characteristics on Seed Dispersal Early Stages by Tibetan Macaques

There are numerous ecological and evolutionary implications for the ability of frugivores to predate on fruits and consume or disperse their seeds. Tibetan macaques, which are considered important seed predators, typically feed on fruits or seeds. However, systematic research into whether they have a seed dispersal function is still lacking. Endozoochory allows seeds to disperse over greater distances by allowing them to remain in the animal’s digestive tract. Consumption of fruit may not imply effective seed dispersal, and the physical characteristics of seeds (e.g., size, weight, specific gravity, etc.) may influence the dispersal phase’s outcome. We conducted feeding experiments with three captive Tibetan macaques (Macaca thibetana) and nine plant seeds to determine the influence of seed characteristics on Tibetan macaques’ early stages of seed dispersal. The results revealed that the percentage of seed destruction (PSD) after ingestion was 81.45% (range: 15.67−100%), with the PSD varying between plant species. Among the three passage time parameters, the transit time (TT) (mean: 18.8 h and range: 4−24 h) and the time of seed last appearance (TLA) (mean: 100.4 h and range: 48−168 h) differed significantly between seed species, whereas the mean retention time (MRT) (mean: 47.0 h and range: 32−70.3 h) did not. In terms of model selection, PSD was influenced by seed size, weight, volume, and specific gravity; TT was influenced by seed-to-shell investment rate, weight, volume, and specific gravity; and TLA was influenced only by seed size. These findings imply that seeds with a smaller size, specific gravity, volume, and greater weight pass more easily through the monkeys’ digestive tracts. Particularly, seeds with a mean cubic diameter (MCD) of <3 mm had a higher rate of expulsion, larger volume, and weight seeds pass faster, while smaller remained longer. Tibetan macaques, as potential seed dispersers, require specific passage time and passage rates of small or medium-sized seeds. Larger and heavier seeds may be more reliant on endozoochory. Tibetan macaques have the ability to disperse seeds over long distances, allowing for gene flow within the plant community.

The Current Status of the World's Primates: Mapping Threats to Understand Priorities for Primate Conservation

Over the past decades, primate populations have been declining. Four years ago, >60% of species were listed as threatened. As the rate of loss accelerates and new IUCN assessments are being published, we used IUCN Red List assessments and peer-reviewed literature published within the last 5 yr to evaluate the status of primates globally, by region and by taxonomic group. We also examined the main factors affecting a species’ conservation status to determine if we could predict the status of understudied species. We found that 65% of species are in the top three IUCN Red List categories (Vulnerable, Endangered, Critically Endangered). Globally, the main threats to primates are Biological Resource Use, including Hunting & Logging, and Agriculture. The impact of these threats varied by region and taxon. Our model showed that Malagasy and Asian primates, and those affected by Agriculture, Human Disturbance, and Climate Change were more likely to be considered at risk of extinction. The model’s predictive probability, however, was low. Our literature analysis showed that some threats, especially climate change and disease, affected more species than indicated by the IUCN Red List. As we move into the next decade, we must continue tackling hunting and agricultural expansion but also be vigilant about emerging threats. We must also aim to regularly test the effectiveness of mitigation strategies, evaluating their long-term adoption and their impact on primates; as well as to increase communication between researchers and applied conservationists to ensure IUCN assessments include current and emerging threats.

Ecological consequences of herbivory by Japanese macaques (Macaca fuscata) on succession patterns of tree assemblages: A case of snowy regions in Japan

Apart from frugivory, we have limited knowledge of the ecological consequences of primate herbivory. We aimed to ascertain the effects of spring folivory and winter bark/bud herbivory by Japanese macaques (Macaca fuscata) on tree species and succession patterns of cool-temperate forests with heavy snow. To evaluate the impact of herbivory on individual trees, we assessed the growth and mortality of trees consumed by simulating herbivory on nine tree species over 4 years. Additionally, we assessed the cumulative impacts of bark/bud herbivory observed at the tree community level by monitoring the patterns of natural herbivory for almost a decade and evaluating the structure of tree assemblages in places with different cumulative impacts of herbivory. The results of simulated herbivory showed that the mortality caused by both spring and winter herbivory was limited (<20%) for almost all tree species monitored; however, the simulated folivory led to delayed tree growth and/or weakening of tree architecture. In contrast, the simulated bark/bud herbivory sometimes resulted in overcompensation of the tree consumed. The multiyear monitoring of natural herbivory demonstrated that, while bark/bud herbivory did not reduced the diversity and biomass of tree assemblages, the cumulative impacts of natural herbivory could have affected the tree succession pattern, resulting in increasing the availability of bark/buds preferably fed by macaques. The key cause for this feedback effect of herbivory on available foods of macaques might be heavy snow conditions, which could physically and physiologically restrain the excessive bark/buds herbivory by macaques.

The Critical Importance of Old World Fruit Bats for Healthy Ecosystems and Economies

Despite extensive documentation of the ecological and economic importance of Old World fruit bats (Chiroptera: Pteropodidae) and the many threats they face from humans, negative attitudes towards pteropodids have persisted, fuelled by perceptions of bats as being pests and undesirable neighbours. Such long-term negativity towards bats is now further exacerbated by more recent disease-related concerns, particularly associated with the current COVID-19 pandemic. There remains an urgent need to investigate and highlight the positive and beneficial aspects of bats across the Old World. While previous reviews have summarised these extensively, numerous new studies conducted over the last 36 years have provided further valuable data and insights which warrant an updated review. Here we synthesise research on pteropodid-plant interactions, comprising diet, ecological roles, and ecosystem services, conducted during 1985-2020. We uncovered a total of 311 studies covering 75 out of the known 201 pteropodid species (37%), conducted in 47 countries. The majority of studies documented diet (52% of all studies; 67 pteropodid species), followed by foraging movement (49%; 50 pteropodid species), with fewer studies directly investigating the roles played by pteropodids in seed dispersal (24%; 41 pteropodid species), pollination (14%; 19 pteropodid species), and conflict with fruit growers (12%; 11 pteropodid species). Pteropodids were recorded feeding on 1072 plant species from 493 genera and 148 families, with fruits comprising the majority of plant parts consumed, followed by flowers/nectar/pollen, leaves, and other miscellaneous parts. Sixteen pteropodid species have been confirmed to act as pollinators for a total of 21 plant species, and 29 pteropodid species have been confirmed to act as seed dispersers for a total of 311 plant species. Anthropogenic threats disrupting bat-plant interactions in the Old World include hunting, direct persecution, habitat loss/disturbance, invasive species, and climate change, leading to ecosystem-level repercussions. We identify notable research gaps and important research priorities to support conservation action for pteropodids.

Correlated evolution of diaspore traits and potential frugivore-mediated selection in a fleshy-fruited tropical lineage (Artabotrys, Annonaceae)

Dispersal syndromes are often defined by reference to fruit traits that are associated with distinct frugivore guilds. Studies rarely examine the relationship between seed traits and frugivores or test the alternative hypothesis that traits are shaped by climatic variables. We assess whether the evolution of seed size and physical defense are correlated with dispersal-related traits and climatic variables in Artabotrys, a fleshy-fruited tropical lineage. Diaspore traits and WorldClim bioclimatic variables were compiled for 43 species. Correlated evolution was evaluated using phylogenetic regression and model-fitting approaches. The best-fitting multioptima Ornstein-Uhlenbeck model suggests that lineages with smooth testa and thin pericarp (SP) have evolved toward smaller seeds with a thinner testa, whereas lineages with rough testa and/or thick pericarp have evolved toward larger seeds with a thicker testa. A smooth testa facilitates spitting and/or swallowing of intact seeds while fruits with thin pericarp may be preferentially consumed by frugivores with less destructive oral processing, enabling lower investment in seed physical defense in SP lineages. Moreover, small seeds are more likely to be swallowed intact with a food bolus. The effect of climate on seed size and physical defense is equivocal and warrants further investigation.

Limited directed seed dispersal in the canopy as one of the determinants of the low hemi-epiphytic figs' recruitments in Bornean rainforests

Ficus species are keystone plants in tropical rainforests, and hemi-epiphytic figs play a notably important role in forest ecosystems. Because hemi-epiphytic figs have strict germination requirements, germination and establishment stages regulate their populations. Despite the ecological importance of hemi-epiphytic figs in the rainforests, seed dispersal systems by fig-eating animals under natural conditions remain unknown because of the difficulty in tracing the destiny of dispersed seeds in the canopy. Therefore, seed dispersal effectiveness (SDE) has never been evaluated for hemi-epiphytic figs. We evaluated the SDE of hemi-epiphytic figs using qualitative and quantitative components by three relatively large-sized (> 3 kg) arboreal and volant animals in Bornean rainforests that largely depend on fig fruits in their diets: binturongs Arctictis binturong, Mueller’s gibbons Hylobates muelleri, and helmeted hornbills Rhinoplax vigil. The SDE values of binturongs was by far the highest among the three study animals. Meanwhile, successful seed dispersal of hemi-epiphytic figs by gibbons and helmeted hornbills is aleatory and rare. Given that seed deposition determines the fate of hemi-epiphytic figs, the defecatory habits of binturongs, depositing feces on specific microsites in the canopy, is the most reliable dispersal method, compared to scattering feces from the air or upper canopy. We showed that reliable directed dispersal of hemi-epiphytic figs occurs in high and uneven canopy of Bornean rainforests. This type of dispersal is limited to specific animal species, and therefore it may become one of the main factors regulating low-success hemi-epiphytic fig recruitment in Bornean rainforests.

Seed fate and seedling recruitment in monkey latrines in rustic cocoa plantations and rain forest in southern Mexico

Primates are important seed dispersers in natural ecosystems and agro-ecosystems, but the latter scenario remains under-studied. The degree to which primates favour plant regeneration greatly depends on post-dispersal processes. The main objective of this study was to compare patterns of seed/seedling fate and seedling recruitment in two habitats of the black howler monkey (Alouatta pigra Lawrence 1933), rustic cocoa and rain forest, and two types of seed-deposition locations, monkey latrines and control locations. Field experiments were carried out within the non-overlapping home ranges of six monkey groups, three in cocoa and three in forest. Seed and seedling fates were assessed for one focal tree species, Brosimum lactescens. The probabilities of seed survival (0.52), germination (0.72), seedling establishment (0.73) and early seedling survival (0.38) were not affected by habitat or seed-deposition location. Late seedling survival was similar in the two habitats but was higher in control locations (0.22) than in latrines (0.09). In cocoa, 4641 seedlings of 59 species were recorded, in forest 3280 seedlings of 68 species. Seedling recruitment was similar in both habitats, but latrines had higher densities than control locations. The importance of agro-ecosystems with low management intensity for the maintenance of ecological processes in anthropogenic landscapes is discussed.