A new study finds that large animals eat not only large seeds but small ones too – helping with seed dispersal. If they become extinct, this could have an impact on overall biodiversity in an ecosystem.
“Big animals do eat some big seeds from fleshy fruits,” says Si-Chong Chen, a PhD candidate at the University of New South Wales (UNSW) who participated in the study. “But the prediction is wrong because it overlooks the fact that big animals like buffalos, cows, deer and zebras also accidentally vacuum up hundreds of small seeds as they graze on short grassy vegetation.”
“We covered all vertebrate groups – fish, amphibians, reptiles, birds and mammals. And we included animals from many different areas – from the Arctic tundra to the tropical rainforests,” according to Chen.
Such rewilding projects are highly controversial and some conservation biologists are firmly opposed to them . It is certainly true that deliberate release of alien species into fragile island habitats for conservation purposes has very real potential to be spectacularly counterproductive and should be attempted only after very careful consideration, if at all. However, here we have an example where reintroduction of the historical seed dispersers of the focal tree is simply impossible, and thus the only other means of providing seed dispersal would be by employing humans as dispersers. This too is not without its potential risks to conservation and would require ongoing and substantial financial investment. Further, giant tortoises have large adult size and low reproductive rate. This implies that if the rewilding project is unsuccessful or counterproductive, removal of the entire population would be relatively easy. Moreover, their long lifespan means that it might be possible to initially introduce one sex only, and have several years monitoring the success of these individuals before the other sex is introduced and the population is allowed to expand. Rewilding much more mobile and fecund bat, bird or primate dispersers would be much less easy to both monitor and if necessary reverse. Alien tortoises have previously been introduced to other Indian Ocean islands as surrogates for extinct species, apparently without adverse effects, and in the case of Round Island have been implicated in the dispersal of seeds from previously dispersal-limited endemic trees .
Dispersal is a fundamental life-history process allowing organisms to reduce competition among kin and to colonize new, suitable habitats. In scenarios of current global change, dispersal is a particularly important life-history stage because it determines the future ranges of organisms, i.e. whether organisms can adjust their distribution to current and future changes in their abiotic and biotic environment. This general phenomenon is illustrated by post-glacial changes in the distribution of European tree species . Most animals are mobile for at least one stage of their development; by contrast, plants are essentially sessile with their only chance of dispersal being as seeds. Seeds are not self-powered and almost exclusively rely on some external agents to provide transport. Plants make extensive use of the self-powered movement of animals, as well as of abiotic agents such as air and water currents, to transport their seeds. Howe & Smallwood  estimated that 51–98% of canopy trees and 77–98% of sub-canopy trees and shrubs in neotropical forests attract animals as seed dispersers using fleshy fruits (i.e. fruits consisting of nutritional fruit pulp surrounding the seeds).
A final line of evidence of the importance of avian seed dispersers for plant conservation comes from manipulative studies where enhanced seed abundance and species richness have been demonstrated upon experimental provisioning of perches. This was demonstrated in regenerating Araucaria forest in Brazil by Zanini & Ganade , and in restoration of forests after landslides in Puerto Rico by Shiels & Walker .
(a) Islands might be particularly vulnerable to breakage of seed-dispersal mutualisms
It is important to acknowledge that passage of seeds through the gut influences the fraction of seeds that germinate and the distribution of germination times [16,17]. Clearly, dispersal can be costly if it reduces the likelihood of seeds germinating. Such a reduction can occur, for example, because passage through the gut can decrease the mechanical protection of seed coats . However, these costs can be offset, e.g. if seeds are deposited by animals in very favourable microhabitats. Gut passage can also affect the timing of germination through the effects on seed coats. The timing of germination can be critical for plants. A meta-study revealed that early emergence generally enhances plant fitness, particularly by increasing growth and fecundity . However, the effect of early emergence on plant fitness was greater under controlled conditions compared with field conditions, presumably because many other biotic and abiotic factors decelerate seedling growth in the field. Thus, Verdú & Traveset  suggest that selection on emergence time will vary in time and space slowing the overall rate of evolutionary change. It is feasible that climate change can alter selection pressure on the timing of germination, and that it can also alter the costs and benefits in the interactions among plants and seed dispersers. Such effects will vary according to disperser species because these differentially influence germination time; for example, owing to differences in the degree of abrasion of seed coats or the amount of dung in which seeds are deposited . It is important to stress the gap in our current knowledge on the trade-offs shaping seed–dispersal interactions that impede realistic conclusions on how climate change will affect seed-dispersal systems.
The Balearic islands in the western Mediterranean Sea offer another illuminating system, because historically endemic lizards of the genus Podarcis have been important seed dispersers, but these have been driven extinct in some islands by introduced mammalian carnivores, at least one of which (the pine martin Martes martes) is also an important frugivore . Dispersal success of the native scrub species Cneorum tricoccon decreases significantly on islands without lizards. The effectiveness of the lizards is a little surprising at first, because they are territorial, thus reducing dispersal distances, are small-bodied and so have a short digestive time for an ectotherm (2–3 days), and also preferentially select small fruits that have small seeds. However, there appear to be counterbalancing advantages of lizard-based dispersal. Traveset & Riera  argue that seed predation is lower for lizard-dispersed seeds both because the seeds are encased in faeces for a time, and because dispersed seeds are at lower density than those under a parent tree and so are less likely to attract the attention of seed predators. More interestingly yet, seedlings of the obligate lizard-dispersed scrub Daphne rodriguezii on Menorca survive better when growing under vegetation than when growing in the open (presumably because of the heat stress and water loss associated with direct and strong illumination). Lizards habitually shelter from sun and predators under vegetation and so preferentially disperse seeds to suitable microsites.
1. Introduction: the importance of dispersal and of animal seed dispersers
Dispersal distance will be a complex function of retention time of seeds in the digestive system, the locomotive ability of the animal and its behaviour. The retention time of seeds is affected strongly by physiology, with flying endotherms having very short retention times (typically of the order of a few minutes to a few hours) and large ectotherms having very long retention times (typically one to two weeks). However, although birds and bats might have short retention times, they can still disperse seeds over distances of hundreds of metres (and in rare cases over thousands of kilometres and entire continents ) because flight speeds can be very high. The relative importance of different animal taxa for long-distance seed dispersal might also vary according to habitat type. In the open woodland of southern Spain, long-distance dispersal of Prunus mahaleb depended particularly on mammals while birds dispersed most seeds only short distances . Conversely, birds and bats achieved longer dispersal distances than mammals in a neotropical forest . The transit time of seeds through the gut will vary according to properties of the seed (with small seeds having generally longer retention times). Retention of a particular seed will also be strongly influenced by gut fullness at the time of passage, the make up of other constituents of the gut, and also the recent foraging history of the animal.
Giant tortoises also demonstrate that what makes a species a suitable agent for introduction as a seed disperser can be a complex mix of different physiological traits. Experiments with giant Aldabran tortoises show that they might be suitable surrogate dispersers for the critically endangered Mauritian endemic plant Syzygium mamillatum. Hansen et al.  demonstrated that (unusually) separation of the seeds from the fruit flesh reduces germination rates of seeds, and such separation occurs during passage through a tortoise. However, they conclude that despite this cost, tortoises still offered a net benefit in seedling establishment because they removed seeds from the vicinity of parent plants (often to considerable distances away, since gut passage times were of the order of three to four weeks), and because deposition in dung provided a nutrient supply to the seedling. This was seen as particularly valuable in a situation where much of the potential range of this plant has been adversely affected by heavy soil erosion. Conservation concerns relating to seed dispersal certainly extends beyond island ecosystems. Anderson et al.  argue that fish are important seed dispersers in the Amazonian basin, but that this ecosystem service is being substantially weakened by overfishing. They focus particularly on the large-bodied characid, Colossoma macropomum, local populations of which have fallen by as much as 90 per cent in recent decades. They argue that fish are very important dispersal agents because they provide directed dispersal towards suitable sites on the extensive seasonally flooded area. By contrast, wind and terrestrial vertebrates (such as birds and primates) will disperse many seeds to locations that never flood and thus do not receive the seasonal input of nutrients provided by floodwaters and necessary for many plants to flourish. Furthermore, in contrast to water-borne dispersal of buoyant fruit, fish can transport seeds upstream and between different tributaries. Estimated dispersal distances by fishes are relatively long, with 5 per cent of seeds projected to disperse 1700–2110 m from the parent plant. This is associated with the low ectothermic metabolism of the fish producing long seed retention times in the gut (of the order of 6–7 days), combined with relatively wide-ranging and fast travel compared with terrestrial ectothermic vertebrates, and rapid changes in available habitat owing to the seasonal nature of floodwaters. However, the negative impacts of overfishing on dispersal extend beyond simple reduction in numbers of dispersers. Firstly, overfishing has led to a dramatic change in the size structure of the fish population, with increasing bias towards small individual size. This is significant because seed retention times and thus dispersal distances increase with individual body size. Further, when local population densities are high, fish rapidly exhaust the fruits available from one source plant and move on to another. Such high movement rates increase the dispersal distances of seeds and increase the chance of seeds being deposited away from the parent plant. Conversely, when local fish density is low, a given individual will remain longer in the vicinity of a given fruiting plant, and its effectiveness as a disperser is thus reduced.
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Date and time: Tue, 11 Jan 2022 11:05:56 GMT