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Urban ecosystems

Woodland invertebrates

Feeding relationships make a good basis for comparing the invertebrate communities which live in the tree canopy with those on the woodland floor.

Comparing the canopy and the floor

1. The food web

The main primary producers in both habitats will be the trees and shrubs. In addition, algae, lichens, mosses and liverworts growing on the trunks and branches may also form a food source, as well as living plants in the ground flora.

Many of the primary consumers on the woodland floor will be detritivores feeding on leaf litter and other dead plant remains. In the canopy however they will mainly be herbivores, feeding on tree leaves, algae, lichens, mosses and liverworts. A number of animals in the tree canopy may be temporary visitors using the canopy as a resting place and not feeding there. They will, however, form a food source for secondary consumers.

The secondary consumers in both habitats will come from related groups but may show different feeding adaptations in the two habitats. For example, spiders living in the canopy are often web-spinners; those living on the woodland floor, like the wolf spider, are hunters.

The top carnivore (the carnivore at the top of the food web) in the tree canopy, is most likely to be a bird (e.g. sparrowhawk). On the woodland floor the top carnivore may be either a mammal such as the fox or a bird.

2. Biodiversity

The biodiversity of invertebrate species in both communities will partly depend on the plant species present, both in the tree and shrub layers as well as in the ground vegetation. The greater the variety of species, the higher biodiversity of animal species.

dog's mercury
Dog's mercury
dog's mercury flea beet
Dog's mercury flea beetle

The different palatability of the leaves of plant species to invertebrates also affects biodiversity. For example, dog's mercury (Mercurialis perennis), a plant which grows in the ground vegetation, is toxic to most invertebrates. Only the dog's mercury flea beetle (Hermeophaga mercurialis) is commonly found feeding on it, so where dog's mercury is dominant there will be few herbivores feeding on the ground flora.

Amongst the trees, birch is generally more palatable than beech and leaf litter from trees like the beech may be even less palatable.

3. Seasonal changes

pupa

The caterpillars feed on willowherbs in woodland clearings
and survive through the winter as pupae in the leaf litter

Seasonal changes occur in both the canopy and the floor. However, in deciduous woodland, they are likely to be most marked in the canopy community. As leaves become old and tough and eventually fall off the trees, the food source for many of the primary consumers in the canopy disappears. Individuals may die, the species surviving the winter as eggs or pupae; others hibernate or migrate to warmer climates. These changes in the number and types of primary consumer will result in changes in the whole canopy food web.

Although many of the plants in the ground flora die back in winter the detritivores in the soil and leaf litter community have a plentiful supply of food. In cold weather they move deeper into the litter and soil layer making it more difficult for larger predator such as birds and small mammals to find their prey. Eggs, pupae and even adults may be found hibernating in the litter and soil. These may include species more commonly associated with the canopy in summer months.

4. Adaptations

On the woodland floor many of the invertebrates are susceptible to losing water from their bodies and drying out. They tend to hide in cool damp places during daylight hours and become more active at night. In the tree canopy where it is more often warm and sunny a higher proportion of animals are likely to be fast-moving winged species with more resistance to drying out. They are often active during the daytime and rest at night.

large yellow underwing moth
A large yellow underwing moth well camouflaged
amongst the leaf litter on which it typically rests

green silverlines moth
A green silverlines moth blending in with its green leaf canopy background

Camouflage colours provide important protection from predators. In the soil and leaf litter therefore invertebrates are often a mixture of brown, grey or other colours which blend with the leaf litter. In the tree canopy, green is a commonly found colour amongst invertebrates. Warning colours and mimicry are found in both habitats.


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