TROPICAL FORESTS: THEIR RICHNESS IN COLEOPTERA AND OTHER ARTHROPOD SPECIES

1. TROPICAL FORESTS: THEIR RICHNESS IN COLEOPTERAAND OTHER
ARTHROPOD SPECIES
TERRY L. ERWIN, 1982, The Coleopterists Bulletin, 36(l):74-75
Presenters:
SURESH R. JAMBAGI- PAMB 0021
VINODA PATIL- PAMB 0022

2. How many species of arthropods…???
 Bates (1892): ˃700 species of butterflies within an hour walk of his
home in Para, Brazil
 Many have guessed: 1.5 to 10 million
 Peter Raven (Missouri Botanical Garden) and Bates – Think tanks
 TERRY L. ERWIN’s calculation

3. TERRY L. ERWIN
American entomologist with the Smithsonian Institution
December 1, 1940 – May 11, 2020
Secretary: Society of Systematic Biologists (1973-1975)
Editor in chief: ZooKeys
 Described over 20 genera and more than 400 species of
insect

4. Tropical forest
 Forested landscapes in tropical regions:
i.e. land areas approximately bounded by
the tropic of Cancer and Capricorn.
 Moist deciduous, semi-evergreen
seasonal.
 Location: tropical seasonal forest of
Panama.

5. Luehea seemannii
 Medium-sized seasonal forest evergreen tree
 Open canopy, large and wide-spaced leaves
 Can grow 10 - 40 meters tall
 Medicine and source of wood
 The largest trees found in wet lowland forests

6. Terry Erwin’s Estimate
Methodology
 19 trees of Luehea seemannii
 Collected all insects from the
canopy of these 19 trees – counted
only beetles

7. Fogging Technique
 A fogging machine is hauled into the forest canopy
and disperses a warm cloud of insecticide.
 The cloud rises through the tree canopy and droplets
of insecticide (diluted with diesel to make the cloud
visible) are dispersed in the exhaust fumes of the
machines engine.
 The cloud has a high knockdown effect.
This causes most invertebrates to quickly fall out of
the trees.

8. Collecting Trays
 Collecting trays are arranged on the
ground under the canopy.
These collect a sample of the fogged
insects that fall from the canopy
 For 3 seasons

9. Calculations
Assumption: 1
1 ha of tropical forest: 70 tree species
 There can be as many as 245 species of trees in one hectare of rich
forest in the tropics, often some of these in the same genus.
 Usually there are between 40 to 100 species.
 He used 70 as an average number of genus-group trees where host-
specificity might play a role with regard to arthropods.

10. No. of tree (Luehea seemannii) sampled: 19
Assumption:2
Number of host specific species: 163
Trophic group # Species
% Host-specific
(estimated)
# Host-specific
(estimated)
Herbivores 682 20% 136.4
Predators 296 5% 14.8
Fungivorcs 69 10% 6.9
Scavengers 96 5% 4.8
TOTAL 1,200+ 162.9 ≈ 163 (13.5%)
Erwin’s assumption
Other 86.5 % as transient species, merely resting or flying through Luehea trees

11.  Number of host specific beetle species per hectare: (70×163) = 11,410
 Plus the remaining transient species: (1200-163)= 1,038
 Number of beetle species per hectare: (11 410+1038) = 12,448
Assumption: 3
Beetles constitute 40% of all Arthropod species
 Number of arthropod species per hectare: (12,448×100)/40 = 31,120
Assumption: 4
Canopy fauna to be at least twice (2/3rd
) as rich as the forest floor
 He added 1/3rd
more to the canopy figure to obtain a grand total
Grand total: (31120×1/3)+ 31,120= 41,389 arthropod fauna/ha

12. Assumption: 5
There are an estimated 50,000 species of tropical trees (R. Howard, via R.
Eyde)
 Tropical forest insect species, for the most part, are not highly vagile and have
small distributions (Mostly host specific).
 163 host specific beetle species: (40%)= 8,150,000
 So the total no. of canopy insect species: [(100*8,150,000)/40] = 20,250,000.
 + 1/3 the no. of ground species = 30,000,000
So, as per Erwin’s estimation, the total no. of arthropod species = 30 million

13. Critics for Erwin’s assumptions/estimate…!!
1. How valid is the figure that beetles form 40 % of all canopy arthropod species?
Southwood, Moran &
Kennedy 1982
Found 7 % of the insect species in British trees to
be beetles, and 16 % in South African trees.
Stork 1988
23 % of the species in an arthropod sample from
tree canopies to be beetles
Hammond 1992
Beetles represent 33 % of all insects in tropical
forest canopies
Clearly the proportion of the arthropod community that is beetles varies from study to study

14. 2. Erwin assumed that 2/3rd
of arthropod species associated with a tropical forest tree lived in the
canopy and one third lived elsewhere.
Hammond 1992
Suggested that more species are likely to be found in the ground fauna
than in the canopy.
Stork 1988
Nearly 70 % of the sample came from the leaf litter; only 14 % of
individuals came from the canopy
Reliable data need to be obtained for the number of species (rather than individuals) found in
different parts of a tropical forest tree

15. 3. Is it reasonable to scale up from just a single tropical tree species sample to
get a total number for arthropods found in all tropical tree species?
 Because
- The same tree species may host different insect species in different parts of its range.
- The same insect species may specialize on other tree species in other parts of its
range.

16. THANK YOU