Interspecific predation as a mortality factor among overwintering spiders.

Area of Science:

• Arachnology
• Ecology
• Behavioral Ecology

Background:

• Spruce-living spiders exhibit natural activity during winter in SW Sweden.
• Philodromus spp. and Pityohyphantes phrygianus dominate active winter spider populations on spruce trees.
• These dominant species are potential predators of other overwintering spiders.

Purpose of the Study:

• To assess the significance of interspecific predation among spiders during winter.
• To determine the impact of spider size and species on winter mortality rates.
• To investigate the role of winter foraging and density on spider fitness and predation dynamics.

Main Methods:

• Field observations of natural spider activity on spruce trees.
• Laboratory experiments conducted at +4°C to simulate winter conditions.
• Comparative mortality assessments of small (<2.5 mm) versus large (≥2.5 mm) spiders under controlled conditions.

Main Results:

• Small spiders experienced 58% mortality when housed with large spiders, versus 3% mortality for large spiders.
• Erigninae spp. were identified as particularly vulnerable among small spiders.
• Intraspecific predation occurred among large P. phrygianus, but with less population impact than predation on small spiders.
• Winter foraging led to significant weight gain in subadult P. phrygianus, suggesting increased fitness.

Conclusions:

• Interspecific predation is a crucial mortality factor for overwintering spruce spiders, especially at high densities in early winter.
• Spider size is a critical determinant of predation risk, with smaller individuals being highly vulnerable.
• Winter foraging behavior can enhance the fitness of surviving spiders, potentially influencing population dynamics.