```bash
#!/bin/bash

# Default threshold is 80%
THRESHOLD=${1:-80}

# Check if the argument is a valid number
if ! [[ "$THRESHOLD" =~ ^[0-9]+$ ]]; then
    echo "Error: Threshold must be an integer."
    exit 1
fi

# Get disk usage for all mounted filesystems, excluding tmpfs, devtmpfs, etc.
# We use df -P to ensure POSIX output format (one line per filesystem)
# We skip the header line (NR>1)
df -P | awk -v threshold="$THRESHOLD" 'NR>1 {
    # $5 is the Use% column, remove the % sign
    use = $5
    gsub(/%/, "", use)
    
    # Check if usage exceeds threshold
    if (use > threshold) {
        print "WARNING: Filesystem " $6 " is at " $5 " usage (threshold: " threshold "%)"
    }
}'
```