Thresholds in songbird occurrence in relation to landscape structure.

Area of Science:

• Ecology
• Conservation Biology
• Landscape Ecology

Background:

• Ecological theory predicts abrupt changes in processes below critical habitat thresholds.
• Habitat fragmentation can decrease colonization and increase extinction rates, impacting species occurrence.
• Understanding these relationships is vital for species demography and conservation.

Purpose of the Study:

• To test for threshold effects of habitat amount on bird species occurrence at multiple landscape scales.
• To identify critical habitat levels influencing species presence and potential fragmentation impacts.

Main Methods:

• Segmented logistic regression was used to analyze the occurrence of 15 bird species.
• Habitat availability was quantified using spatially explicit distribution models at radii from 150m to 2000m.
• Thresholds were identified based on the amount of suitable habitat at landscape scales (>=500m radius).

Main Results:

• Ten of 15 species showed significant responses to habitat amount at landscape scales.
• Nine species exhibited threshold effects, with six showing asymptotic thresholds where habitat loss effects intensified at low habitat amounts.
• Identified landscape thresholds ranged from 8.6% to 28.7% suitable habitat.
• Two species demonstrated fragmentation thresholds, preferring larger patches only when overall habitat was scarce.

Conclusions:

• Threshold models effectively predicted species occurrence for most affected species.
• Conservation strategies should incorporate identified landscape habitat thresholds.
• Findings highlight that landscape-scale studies in high-habitat regions may not generalize to fragmented landscapes with low habitat availability.