Sully - Animal-plant Interactions In Rain Forests

Suleman Raja Biology

Tropical Ecology and Conservation 3rd Year

Discuss the diversity and importance of animal-plant interactions in tropical forest ecosystems Introduction Inter-specific symbiotic relationships, beneficial to both the concerned species – also known as mutualisms, occur across a variety of ecosystems and bear essential importance to the survival and success of many species. Tropical forest ecosystems are no exception to this. It is the interactions between plants and animals which interweave the development and continued biological flow of tropical rainforests in many different ways. Of course, there are also plant to plant mutualistic relationships, as well as animal to animal mutualisms which form the mainstay of adaptive life for some species. Animal-plant relationships differ in that the linked support of life between plants and animals is what gives rise to the maintenance of a successful and diverse tropical forest ecosystem. Being at the root of all food chains, plants form the essential base of rainforest ecosystems. Perhaps the most important biological process for the survival of a forest is pollination. In rainforests, animals play a vital role in the facilitation of this process, as the abiotic factors which would normally allow successful pollination are limited in rainforests. Such factors mainly being wind facilitated pollination – because of the humidity and dense foliage in a tropical forest, pollination by wind–carriage is not a successful means of reproduction for plants. Furthermore, due to the density of tropical forests, wind pollination would not suffice to carry pollen far enough to spread and mix genes beyond a certain area. This is where the importance of animalplant interactions is imperative. It is the animals in a tropical forest ecosystem which are the main pollination agents for plants that make up the forest environment. Many different species are involved in the pollination of plants – helping the plants to reproduce and diversify genetically, whilst the animals benefit from food sources provided by plants such as pollen and nectar. Whilst it is not possible to cover all animal-plant interactions within the constraints of a written essay, this report will endeavour to provide a comprehensive assessment on the importance and diversity of animal-plant interactions in rainforests, giving a selection of different types of interactions / mutualisms and examples of them, drawn from field experience and reference to learned sources. Pollinator mutualisms Insects The biodiversity of a rainforest can be assessed by the direct diversity of indicator species such as insects. Rainforests support an incredible number of insect species – to the extent that a huge many remain undiscovered or unidentified. Insects are essential to the success and diversity of a tropical forest ecosystem as they act as the primary pollinators of nearly all plant species in the forest. Vast numbers of insect species use plants as a place of refuge or for food. Some examples of insects acting as

Suleman Raja Biology

Tropical Ecology and Conservation

3rd Year pollinators are shown by the following examples of animal-plant interactions in tropical forests: Bees – Bees are important pollinators not only in tropical forests but for almost all flowering plants present in a place where Bees also occur. Typically, Bees collect nectar from the flowers of flowering plants. In doing this, the body of the Bee must first brush past the anthers inside a flower, causing pollen to rub off onto the Bee’s hairy body. The hairs on the body act to keep the pollen there whilst the Bee is in flight. Therefore, when the Bee visits another flower, the pollen on its body, from another plant – rubs off onto the stigma or female sexual organ of another flower while the Bee pushes past this organ in order to obtain the nectar which is normally at the bottom of a flower. In tropical forests, Bees act in this way to pollinate flowering plants. Flying insects and animals are what these plants rely heavily on for pollination, as rainforests do not get exposed to sufficient wind in order to carry pollen through vast distances. The humidity levels are also too high for pollen to be successfully blown from plant to plant in the air. Fig wasp – This is a wonderful example of an extremely specialised pollination mutualism in tropical ecology whereby both concerned species depend on one another for their reproductive success and survival. Figs are plants in the genus Ficus, which have a unique closed inflorescence called a syconium, typically containing hundreds of flowers (Cook and West, 2005). Fig trees (genus Ficus) have 755 species worldwide – around 511 of which are within the Indo-Australasian tropics. (Rossiter, 2008)

The life cycle of the fig wasp starts inside a fig, where the female deposits her eggs. Upon hatching, the larvae develop within the gallad ovary, using the endosperm tissue of the ovary as a food source for growth. As adults, they chew their way out of the galls and into the fig’s main cavity. Here, the flightless male wasps fight to mate with the females that have emerged from the same clutch – whilst this occurs, the fruit is in it’s male phase, thus the bodies and legs of the wasps get exposed to pollen in the mating stage of the fig wasp lifecycle. After mating, males and females leave their natal fig – the males die at this stage, whereas the females fly away to deposit their eggs (oviposition) inside a new fig before they die starting the cycle over.

Suleman Raja Biology

Tropical Ecology and Conservation 3rd Year

The morphology of a fig is such, that in order to enter the fig, fig wasps must enter through the only opening – called the ostiole. The female wasps collect pollen from the anthers in its natal plant, before coming into contact with the female part of a new fig. This gives rise to very effective pollination because the wasp gathers sufficient pollen from its natal fig to pollinate the female reproductive parts of another fig effectively upon contact with them. When wasps enter, the fig is unripe and only the female part is active – thus avoiding self pollination. The female lays her eggs down the stylet into the ovary of the fig – in doing this she brushes the pollen of the natal plant from her legs onto the female part of the new fig. Adaptations that the fig wasp has for this lifestyle are a long, flattened and elongated head and thorax designed for easier entry to the fig via the narrow ostiole. In fact, entering the fig is still such a squeeze, that female fig wasps often lose their wings and antennae in doing so.

This animal-plant interaction bears very important implications to many birds, primates, bats and other animals as the fig produces fruit all year round, even when other plants are not, thus providing a very important food source to the aforementioned species. This provision of food at otherwise tricky times makes the fig a keystone plant species (Cook and West, 2005), as it is crucial in maintaining the populations and diversity of rainforest fauna. The interaction with the fig wasp keeps the reproductive cycle of the fig successful, whilst also providing a very effective means for the fig wasp to carry out its reproductive cycle also. Both species benefit, as well as other species – by the maintenance and continued production of figs. Plants of genus Rafflesia – Plants from this genus have flowers which are known to emit a repulsive odour of carrion – this is a mechanism which evolved in order to attract flies to the flower for pollination. A particular species found in the rainforests of Borneo, Sumatra and Malaysia - Rafflesia arnoldii, produces the largest individual flower in the world. The flower can grow to be of a diameter of up to three feet and produces a distinctive, putrid smell of rotting meat. The plant is rare as it grows as a parasite of the Tetrastigma vine, found only in primary forests. This highly specialised plant lacks any visible stems, leaves or roots – and grows in a way similar to fungi.

Suleman Raja Biology

Tropical Ecology and Conservation 3rd Year

The smell it omits is used to attract flies – which settle on the flower and carry pollen from one flower to another – given the rarity of the plant and the fact that it only lasts as a flower for a few days – pollination is a rare event, thus the population of this flower is relatively low. Flies make good pollinators as they seek out carrion and can cover large distances. The strength of the odour therefore plays an integral role in attracting flies for pollination. This is extremely important for Rafflesia as it relies on flies for reproductive success. Birds – Birds also interact with plants as pollinators – although to a lesser extent than the invertebrate pollinators. Bird-pollinated flowers are usually tubular, unscented flowers with a large store of nectar at the base – some birds feed on this nectar whilst others are attracted to the insects which are attracted to the nectar – in either instance, birds such as hummingbirds and sunbirds will collect pollen on their bodies and transfer this to other flowers that they visit. Bats – Another vertebrate pollinator. Rainforests are home to a vast array of Bat species – some of which feed on nectar and pollen. Most Bats that pollinate plants are from the family Pteropodidae – the fruit bats and flying foxes. It is such Bats, as well as some of the leaf-nosed bats that pollinate many plants in tropical forests. The flowers which are pollinated by Bats usually bloom by night, and omit musky or fruity odours to attract Bats to them. Furthermore, they are often large and strong enough to support a Bat. The plant and its pollinator often co –evolve to highly specialised specifics in order to maximise the mutualistic relationship between them. For example with bat-pollinated flowers, the flowers as mentioned, smell fruity and often lack any vivid colouration as they are nocturnal like the bats are. Therefore they need not express vivid colouration. In addition to this, the bats rely more on olfactory senses to locate flowers – as it is thought that bats lack cones in their retinas and thus no not rely on colour vision to seek out food sources. The importance of animal-plant interaction with regards to pollination are not just bound to the reproduction and survival of a forest – but also bears great economical importance for human beings – 1/3 of mouthfuls of food and beverages consumed by humans come from animal pollination. Moreover, products of insect pollination are worth $40 billion annually in the US alone. (Rossiter, 2008) The importance of animal pollination was demonstrated with the example of the West African oil palm Elaeis guineensis which was introduced to Malaysia. For years the palm was not able to survive here without hand pollination. Work by botanist R. Syed in the 1970’s drew the conclusion that this was due to the fact that Malaysia lacked populations of its specialised pollinator species – the pollinating Weevil Elaeidobius spp. When this species was introduced and released in Malaysia the oil palms here

Suleman Raja Biology

Tropical Ecology and Conservation

3rd Year flourished and spread considerably without the need for human intervention for pollination. The oil palm industry is now a multi-million pound industry in Malaysian plantations due to the introduction of its pollinator species – this demonstrates how important one species of animal can be to a plant from its interactions with it. Conservation of pollinators and plants is an issue of rising concern due to the loss of wild pollinators. This ‘pollination deficit’ crisis has come about due to the loss of habitat, spread of disease, pests and deforestation. “Humanity, for its own sake, must attend to the forgotten pollinators and their countless dependent plant species” (E.O Wilson) Seed dispersal mutualisms Seed dispersal is also a very important process carried out by animals in rainforests. Following pollination, plants also rely on their animal counterparts to disperse and spread fertilised seeds. The animals involved in seed dispersal benefit by gaining food sources in the process – and the plants benefit by successful distribution of their progeny. Seed dispersal occurs by invertebrates (ants, beetles, other insects) as well as vertebrates (birds, bats, rodents, other mammals). Myrmecochery is the term for seed dispersal by ants. Seeds of many plants contain the accessory organ known as elaiosomes which are rich in protein and attract ants. This is because ants will remove and carry the elaiosomes to their nests to feed them to their larvae. Once the larvae have eaten the elaiosomes, the adult ants will dispose of the inedible seed outside of the nest or under the ground – where it will germinate. Ants are considered the main invertebrate dispersers and many tropical plants rely on them to disperse their seeds. Vertebrate dispersers in tropical rainforests mainly consist of frugivorous birds and bats. Birds are normally attracted to brightly coloured fruit as a food source. The birds / bats that feed on such fruits will ingest its seeds whilst eating the pulp. The seeds are indigestible and when excreted, become dispersed in the otherwise dense forest. Orang-utans, gibbons and other primates are also very important seed dispersers in a similar way within tropical forests. Fruits grow in a variety of shapes and sizes, specific to different dispersers’ preferences. For instance, larger, bigger-seeded fruits are consumed by progressively fewer dispersers, and the largest depend on a few species of mammals and birds which are highly vulnerable to hunting, fragmentation and habitat loss (Corlett, 1998). This is why conservation of species is hugely important in the knock-on effect the loss of just a single species can have on other species including plants.

Suleman Raja Biology

Tropical Ecology and Conservation 3rd Year

Decomposition The diversity of animal-plant interactions does not stop at pollination and dispersal. Decomposition is also imperative to the nutrient cycling and success of a forest where dead organic plant matter inevitably accumulates. Some old living trees also contain dead wood structures within them and these provide food sources for saproxylic organisms which rely on dead organic matter in some part of their lifecycles. Aside from fungi and bacteria, animals which carry out decomposition in rainforest include Annelids (worms), Arthropods (woodlice), Molluscs (snails) and Insects. Termites - Prolific decomposers of dead wood and other plant matter such as roots and leaf litter are termites (order Isoptera) – these insects are found in huge numbers throughout tropical forests and play a vital role in the ‘tidying up’ of dead plant matter in the forest, on which they rely for food. The importance of termites and their interactions with plants is vital for keeping a forest free of dead matter, particularly large logs of wood which would normally take considerably long to decompose without the termites feeding on it. Defence and shelter An example drawn from the field exhibiting an exclusive and highly specialised mutualism between plant and animal is that of the Bornean plants in the genus Macaranga. The mutualism between the Macaranga plants and their ants is known as a Myrmecosymbiosis. The ants live inside the structures on the stems known as stipules, and in the hollow shoots of the plant, where the plant provides them with so-called food bodies rich in lipids, carbohydrates and proteins (Linsenmair, 2001). These are extrafloral nectaries from which the ants feed. In return for this energetically costly provision, the ants provide biotic defence / protection to the plant from herbivorous predictors such as caterpillars and other insects which may graze on the leaves of the Macaranga. The ants will patrol the plant and defend their biotic host by killing any intruders they come across on the plant. Some macaranga plants are specialised so as to host just one particular species of ant, by having waxy stems that are not climbable by other species due to their slippery texture. This relationship is very important for the concerned species, and studies show that ant colony size on a macaranga plant is indeed negatively correlated with leaf damage caused by herbivory (Linsenmair, 2001). The importance of this relationship to both species is clear - ants gain food and shelter, whilst the plant gains defence and thus better survival. This example further highlights how diverse and specialised animalplant interactions in tropical forests can be.

Suleman Raja Biology

Tropical Ecology and Conservation 3rd Year

Less mutualistic animal-plant interactions Some of the interactions between plant and animal are not beneficial to both parties concerned and thus not all animal-plant interactions should be considered as mutualisms. For example, rainforests are home to many species of predatory plants – which feed on insects by trapping and killing them. There are around 600 species of predatory plants (Rossiter, 2008) which divide into three sub-categories by trapping mechanism. These are the pitfall traps, snap traps, and flypaper traps. Flypaper traps – As the name suggests, these plants operate in a way similar to fly paper – they produce a sticky mucilage secretion which acts to entrap insects which either walk over the plant or land on it from flight. They are often found growing in bogs where nutrient levels are low and derive their nutrients from consuming and digesting the insects they trap. The multiple, independent evolution of carnivory in diverse plant families suggests that it is an adaptation to the low nutrient, bright, waterlogged habitats in which carnivorous plants occur (Ellison and Gotelli, 2001) These plants have evolved independently in five dicot families (Lentibulariaceae, Roridulaceae, Biblidaceae, Droceraceae and Dioncophyllaceae) The Droceraceae consists of three genera (Drosera, Aldrovanda and Dionaea) (Ellison and Gotelli, 2001) The Drosera genera are known commonly as the sundews and contain the most species worldwide (around 150 spp). The other two genera Aldrovanda and Dionaea are Snap traps - A very well known example of this is the Venus fly trap Dionaea muscipula.

These plants have tiny receptor hairs inside the cavity of the trap, which when triggered by the presence of an insect, cause the two discs forming the trap to snap shut – enclosing the insect and sealing the edges with interlocking spikes to secure the prey. Once closed, the Venus fly trap will consume and digest the prey by enzymatic breakdown and absorption before re-opening.

Suleman Raja Biology

Tropical Ecology and Conservation

3rd Year Pitfall traps – Finally, pitfall traps are also aptly named by what they do – the plant structure is like a pitfall – when insects fall into the fluid-filled cavity of the plant, they cannot escape due to the waxy inside walls of the pitfall – they therefore drown or perish within the bounds of the plant and are subsequently consumed by the plant. Pitcher plants of the genus Nepenthes are examples of tropical pitfall traps. They occur mainly in Borneo and the genus contains around 117 species (Rossiter, 2008). Locally, these plants are known as ‘Monkey cups’ due to their cup-like structure which holds water. Indeed, it has been observed that Monkeys will sometimes pick these plants and drink from them like a cup. However, the main interaction these plants have with animals is that of their prey species. Insects are attracted to nectaries located along the entrance to the pitcher. When they settle on the rim the small insects usually slip and fall in – where they are met by water and very waxy walls to prevent escape. Most prey insects are ants, termites or midges which drown and accumulate in the fluid. Furthermore, pitcher plants also play a part in mutualistic interactions with insects as well as the predator-prey interaction. Some invertebrates live as ‘infauna’ and inhabit the pitcher without dieing. These include mosquito larvae as aquatic filter feeders and predators, as well as midge larvae as detritus feeders. By inhabiting the pitcher, infauna species help with the break down of prey and thus provide a mutualism by providing the plant with nutrients as well as obtaining food and shelter for itself in the pitcher. Moreover, some pitcher species are colonised by swimming ants which inhabit the plant’s tendrils. These ants also form a mutualism with the plant in that they remove prey items from full pitchers – this causes the plants with ants to have fewer putrid pitchers which prevents the death of infauna by the prevalence of anoxic conditions in the presence of too many dead insects inside the pitcher. Some species of pitcher such as the massive Nepenthes rajah prey upon vertebrates such as birds, lizards, frogs and rodents such as mice and rats. (Rossiter, 2008). This is a rare animal-plant interaction whereby vertebrates are preyed upon by a plant. This further highlights how diverse animal-plant interactions can be within an ecosystem.

Suleman Raja Biology

Tropical Ecology and Conservation 3rd Year

Conclusion The aforementioned animal-plant interactions are just a few examples of such relationships in tropical rainforest ecology. The purpose was to provide an all round guide on the diversity and importance of these interactions as both a factor on the survival of the forests as well as implications to human beings. The types of relationships covered included pollinators, dispersers, decomposers, defence and shelter relationships and plant predation. It is stressed that these subheadings only touch on a selected number of types of animal-plant interactions in order to outline the diversity of animal-plant interactions and their importance. In tropical forests, there are countless mutualisms and animal-plant interactions which are very diverse and also serve huge importance to ecosystem structure and function.

References  Rossiter, S, 2008. Animal-plant interactions (Lecture, Brunei)  Bascompte, J and P Jordano, 2007. Plant-Animal Mutualistic Networks: The Architecture of Biodiversity: Annual Review of Ecology, Evolution, and Systematics. 38: 567-593  Boucher, DH et al, 1982. The Ecology of mutualism. 13: 315-347  Knudsen, J. T. & L. Tollsten. 1995. Floral scent in bat-pollinated plants: a case of convergent evolution. Bot. J. Linn. Soc. 119: 45-57.  Cook, J. M and S West, 2005. Figs and fig wasps: Current Biology Vol 15: 24  Corlett, R.T, 1998. Frugivory and seed dispersal by vertebrates in the Oriental (Indomalayan) Region: Biological review., 73: 413-448  Linsenmair, K. E et al., 2001. Adaptations to biotic and abiotic stress: Macaranga-ant plants optimise investment in biotic defence. Journal of Experimental Botany, 52(363), pp.2057-2065  Ellison, A.M and J Gotelli, 2001. Evolutionary ecology of carnivorous plants. Trends in ecology and evolution., 16 (11)