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Insects in Tropical Rainforests

Various factors may have promoted insect (particularly beetle) diversity in tropical rainforests, most notably:

• the spatial heterogeneity of forest resources;
• the inter-relationships between "predators" and "prey", particularly the co-evolution with angiosperms;
• forest refugia in the Neotropics.

Other factors include:

• stochastic localdisturbances;
• geographical barriers;
• variation in flooding cycles;
• soil type and nutrient levels.

In fact, the various theories that try to explain high species diversity in tropical rainforests are not mutually exclusive, and may interact over a range of temporal and spatial scales. [Haffer 1997] and [Hill & Hill 2001] provide a review of these current theories.

This enormous diversity has been the subject of considerable research effort since the mid-1980s, and is reflected in the many different techniques required to sample insect diversity.

Beetle to Insect Ratios

• How much insect diversity in tropical rainforests is due to beetles?

[Erwin 1982] , based on a fogging study in Panamanian tropical forest, suggested that beetles represent about 40 % of arthropod species found in the forest canopy.

Other researchers have come up with lower figures from 7 to 33 % [Southwood, Moran & Kennedy 1982] , [Stork 1988] , [Hammond 1992] .

• What is a reasonable compromise figure?

Insect Diversity in Forests

[Hammond 1992] reported that a tropical rainforest in Indonesia was five times richer in insect species numbers than a temperate wood in England.

The tropical forest yielded 412 beetle species pertrap, compared to 83 per trap in the temperate wood.

• Are levels of insect diversity similarly high in New World and Old World tropical rainforests?

• Where does this hyperdiversity reach its peak, and why?

Spatial heterogeneity in forests

Spatial Heterogeneity and Adaptation

Tropical rainforests are spatially heterogeneous and offer an enormous range of micro-habitats and micro-climates.

The small size, adaptability and motility of insects allow them to exploit these habitats to the full.

Canopy and emergent trees in a tropical rainforest in Borneo

Forest Canopy

The forest canopy forms the upper layer of the rainforest, interrupted by occasional emergent trees.

• What adaptations do insects have to live in the forest canopy?
  

Forest epiphytes in a tropical rainforest in Borneo

Forest Epiphytes

Epiphytes are widespread in on the tree trunks of tropical rainforests.

• What unusual microhabitats are associated with epiphytes?
  

Forest understorey in a tropical rainforest in Borneo

Forest Understorey

The forest understorey is a diverse habitat.

The shrub and herb layers provide alternative resources and habitats to the main forest trees.

Gaps in the understorey providing yet more habitats for insects.

Forest Floor

The forest floor provides a range of microhabitats.

Leaf litter and soil provide a range of microhabitats for insects.

Dead and decaying wood, fungi and other invertebrates provide food resources for insects.

Motility and Niche Use

[Erwin 2001] says that terrestrial arthropods can be found in "restaurants", "hotels" or "in transit" between the two.

• What examples are there of insects that move between such sites?

• What proportion of arthropods live in the forest canopy compared to the forest floor?

[Erwin 1982] thought twice as many arthropods lived in the canopy compared to other forest layer niches, but other researchers think this figure is too high [Hammond 1992] , [Stork 1988] , [May 1990] .

Predator / Prey Inter-relationships

Inter-relationships between "predators" and "prey" can be a major evolutionary force driving diversity.

With flowering plants and insects, this co-evolution has been both morphological and biochemical.

Co-evolution has also occurred between insect parasitoids and their hosts, and between insect predators and prey.

Insect and Plant Co-evolution: Phytophagy

According to [Farrell 1998] , diversity of beetles is closely related to that of the angiosperm plants, principally through the great diversity ofphytophagous beetles.

• What evidence is there for this, and how might this co-evolution have occurred?

[Hespenheide 2001] questions whether this explanation alone is enough to explain beetle diversity, given the proportion of non-phytophagous beetle groups that exist.

Insect and Plant Co-evolution: Host-specificity

Different plant species may support specialisedinsects not found on other plant species.

As a result, the high species richness of insects in tropical forests may be a consequence of high plant species richness, the number of insect species found per host plant species, and high levels of host-specificity.

What factors might explain tropical insect host-specificity? (See: [Basset 1992] and [Mawdsley & Stork 1997] for suggestions.)

• What levels of host-specificity are seen in phytophagous insects?

[Erwin 1982] suggested that up to 14 % of beetle species fogged from a single tropical tree were host-specific.

However, levels of host-specificity may be highly variable [Mawdsley & Stork 1997] , and lower than Erwin's estimate. [May 1990] and [Stork 1991] suggested 5 % would be a more realistic general figure for host-specificity.

Forest Refugia

According to [Haffer 1969] , cyclic patterns of climate change during the Pleistocene brought about fragmentation of the Neotropical forests, creating temporary local "islands" of forest between patches of grassland.

These forest islands became refugia for forest species during the drier climate periods, and became temporal centres of endemism, where rapid speciation occurred.

During moister periods, when the forests expanded, the refugia also served as centres of dispersal for the forest species.

Research Effort and Biodiversity

The provocative estimate of 30 million arthropod species by [Erwin 1982] sparked off a great proliferation in studies of insect diversity in the tropical rainforests, epitomised by [Stork et al. 1997] .

It also fuelled a near exponential growth of interest in "biodiversity" as a concept, and in particular the desire to estimate global species richness before the majority of it disappears.

Sampling Insect Diversity

Sampling is a prerequisite for estimating the number of insect species in any habitat.

• Might estimates of hyperdiversity be influenced by methods of sampling and extrapolation?

Different insect groups live in different habitats in rainforests, and exhibit different behaviours. These different behaviours will influence their "trapability" under different sampling regimes, and so impact on any estimate of species richness and diversity.

See [Basset et al. 1997] for a review on sampling arthropods in tree canopies.

Pitfall Traps

Ground-dwelling invertebrates are most commonly sampled using pitfall traps sunk into the ground.

This technique is particularly effective at collecting beetles and spiders.

The trap is normally part-filled with a preservative, which ensures that the catch does not decompose in the warm tropical conditions, and sometimes an attractant.

Leaf Litter

A variety of techniques are used to collect insects from leaf litter, soil, moss, bark, dung and other organic debris.

All work on the principle that insects in these substrates are dependent on moisture and will move away from desiccating influences, dropping into a funnel of preservative below.

Light traps

Many types of insect are attracted to light at night.

Simple light traps comprise a light source behind a suspended white sheet, whilst more elaborate devices use mercury vapour lamps and a collecting chamber.

• Why might samples from light traps be unrepresentative?
  

Sweep Nets

Insects on low-lying vegetation may be sampled using a sweep net.

This sampling method works particularly well for bugs, beetles, flies, grasshoppers, bees and wasps.

Beating trays are used to collect insects clinging to bushes and low branches.

• What are the pros and cons of sweeping and beating?
  

Flight traps

The majority of insects fly upwards when they encounter any sort of obstacle.

The Malaise trap exploits this response by funnelling flying insects, in particular flies, wasps and moths, upwards into its tent-like roof and then into a collecting pot.

Flight interception traps work on the principle that some flying insects, particularly beetles, drop out of the air when they hit an obstacle.

The traps usually comprise a mesh or Perspex screen suspended vertically above collecting trays containing preservative.

Fogging

Insecticide fogging is used to collect insects en masse, usually from a single bush or from the tree canopy.

A fog of knockdown insecticide envelops the canopy, temporarily stunning all of the insects, making them drop out of the tree.

Collecting trays are arranged under the canopy. These collect a sample of the insects which fall from the canopy.

• What are the possible advantages and disadvantages of this sampling method?