function RemoveAliensInArea(surface, area)
    for _, entity in pairs(surface.find_entities_filtered{area = area, force = "enemy"}) do
        entity.destroy()
    end
end

function ReduceAliensInArea(surface, area, reductionFactor, evolution)
    for _, entity in pairs(surface.find_entities_filtered{area = area, force = "enemy"}) do
        if (math.random(0,reductionFactor) > 0) then
            entity.destroy()
        end
    end
    -- Downgrade all huge worms
    if (evolution<0.75) then
        for _, entity in pairs(surface.find_entities_filtered{area = area, name = "behemoth-worm-turret"}) do
                surface.create_entity({
                    name="big-worm-turret",
                    position=entity.position
                })
                entity.destroy()
        end
    end
    -- Downgrade all big worms
    if (evolution<0.5) then
        for _, entity in pairs(surface.find_entities_filtered{area = area, name = "big-worm-turret"}) do
                surface.create_entity({
                    name="medium-worm-turret",
                    position=entity.position
                })
                entity.destroy()
        end
    end
    -- Downgrade all medium worms
    if (evolution<0.25) then
        for _, entity in pairs(surface.find_entities_filtered{area = area, name = "medium-worm-turret"}) do
                surface.create_entity({
                    name="small-worm-turret",
                    position=entity.position
                })
                entity.destroy()
        end
    end
end

local function removeAlienDecoratives(surface,area)
    surface.destroy_decoratives( { area=area, name= {"shroom-decal","worms-decal","enemy-decal","enemy-decal-transparent"} } )
end

local function scaleDownBiters(surface,area,evolution)
    local reduction = 40
    if (evolution>=0.1) then
        reduction = 30
    end
    if (evolution>=0.2) then
        reduction = 20
    end
    if (evolution>=0.35) then
        reduction = 10
    end
    if (evolution>=0.5) then
        reduction = 5
    end
    if (evolution>=0.65) then
        reduction = 3
    end
    if (evolution>0.8) then
        reduction = 1
    end

    local dice_roll = math.random(0,reduction)
    if ( dice_roll > 2) then
        RemoveAliensInArea(surface,area)            -- kill all "enemy" objects in this chunk
        removeAlienDecoratives(surface,event.area)
    elseif (dice_roll > 0)then
        ReduceAliensInArea(surface,area,2,evolution) --downgrade worms and kill some nests
    else
        ReduceAliensInArea(surface,area,0,evolution) --only downgrade worms
    end
end