Predation

Effects of predation on morphology, distribution and

abundance1. Change in size structure of prey population (if predator prefers the largest individuals in a prey population)

Brooks and Dodson 1965 (over 1350 citations)

Lakes in North AmericaWhen fish were introduced there were huge changes- predators preferred the larger zooplankton- small zooplankton became dominant- large phytoplankton become abundant

Effects of predation on morphology, distribution and

abundance2. Decreases in overall diversity – if predators are very efficient at removing prey, they drive populations to extinction which reduces diversity

3. Increase in diversity – in simple systems with few prey species, one of which is a dominant competitor. If a predator prefers the dominant competitor it can reduce the number of the dominant competitors, allowing the inferior competitors to exist.

All three of these can occur in “ecological time” = one to a few generations

Effects of predation on morphology, distribution and

abundance4. Morphological modifications – inference from observationa. protective devices (spines on sea urchins; strong shells)

Effects of predation on morphology, distribution and

abundance4. Morphological modifications – inference from observation

b. mimicry – organisms that resemble unpalatable species (usually because they contain toxic compounds)

Effects of predation on morphology, distribution and

abundance4. Morphological modifications – inference from

observationc. crypsis – organisms match the color and shading of their habitats. This morphology is likely shaped by predatory pressure over time.

Artificial camouflageDecorator crabs put algae on their backs, which increases their survivalIn areas with Dictyota spp. (algae), crabs use this species for decoration, but rarely food

Inducible versus Constitutive defenses

A bryozoan makes spines when placed in contact with a predatory nudibranch.

A hydrozoan, Hydractinia, produces defense stolons armed with nematocysts when in contact with another colony.

Inducible Defense:

The conical (right) and bent (left) forms of the acorn barnacle Chthamalus anisopoma. The animal develops the bent form if predatory snails are present.

Mytilus edulis (Blue mussel)

Threat of predation leads to:• Thicker shells

• Leonard et al (1999)• Smith & Jennings (2000)

• Larger adductor muscle• Reimer & Tedengren

(1996)• Increased gonad ratios

• Reimer (1999)• Increased byssus volume

• Cote (1995)

Predation: Indirect Effects• Non-lethal effects

– Injury by browsing predators – Trait-mediated indirect interactive effects

(TMII)• Risk averse foraging• More shelter dwelling in the presence of predators• Can produce larger effects than consumption does

– Trophic cascades

Predation: Indirect Effects

• Non-lethal effects– Injury by browsing predators – Trait-mediated indirect effects (TMII)

• Risk averse foraging• More shelter dwelling in the presence of predators• Can produce more dramatic effects than actual

predation does– Trophic cascades

Dugongs can modify the structure of seagrass beds through their foraging

Tiger sharks cause dugongs to change habitats, which can affect seagrass communities

Predation: Indirect Effects

• Non-lethal effects– Injury by browsing predators – Trait-mediated indirect effects (TMII)

• Risk averse foraging• More shelter dwelling in the presence of predators• Can produce more dramatic effects than actual

predation does– Trophic cascades

Trophic Cascade in Kelp Forests• When the keystone sea otter is removed, sea

urchins overgraze kelp and destroy the kelp forest

Figure 5.15b

Emergent Multiple Predator Effects (MPEs)

• Types of interactions among predators (Soluk and Collins, 1988):– Neutral: predators do not affect one another’s rates

of prey consumption– Negative (interference): combined prey

consumption less than neutral values MPE– Positive (facilitation): combined prey consumption

greater than neutral values MPE