Resolve a Contradiction Practice Test

Over the last decade, observers at a network of long-term monitoring sites have documented a steady decline in the number of individuals of a particular forest-dwelling songbird within a large reserve. Point counts conducted during breeding season show fewer territorial males each year, and fledgling surveys indicate reduced nesting success in fragmented patches along the reserve's edges. By standard measures of abundance, the local population appears to be shrinking. Yet laboratory analyses of blood samples taken from captured birds tell a different story about the population's composition: genetic diversity, as measured by heterozygosity across several neutral markers, has increased over the same interval. Conservation biologists often worry that small, isolated populations will lose genetic variation, making the simultaneous trends—fewer birds but more genetic diversity—seem paradoxical. Field biologists note that the reserve has experienced bouts of drought and a series of intense storms that downed canopy trees, altering microhabitats and concentrating predators along new edges. At the same time, satellite imagery shows continued loss of similar forest types in agricultural districts surrounding the reserve, including to the north and west. The monitoring team has not observed a corresponding rise in reproductive success within the reserve that could account for the genetic pattern. How, then, can a population that is numerically declining display rising genetic diversity? An adequate explanation must reconcile the decline in local counts and nesting success with the increase in measured genetic variation among the birds that remain within the reserve.

Five years after a major river-restoration initiative, county officials released a report celebrating sustained improvements in water clarity. Monitoring data show that average turbidity has fallen each year since upgrades to the municipal treatment plant reduced suspended solids in the effluent, and bank stabilization projects have curtailed erosion during storm events. Kayakers praise the newfound transparency; local media circulate photographs of rocks visible several feet below the surface. Yet biologists conducting seasonal fish surveys have documented a decline in several trout species that, in angler lore and fisheries manuals alike, are described as favoring clear, cool streams. The survey methodology, unchanged for more than a decade, indicates population densities down roughly twenty percent from pre-restoration baselines.

Some commentators suggest that the trout decline reflects normal population cycling, but comparative records from nearby, unaltered tributaries show no similar pattern. Others posit that the river's improved clarity may make trout more visible to predators, though raptor counts in the corridor have not spiked and refuge cover remains. A third hypothesis points to thermal stress, but continuous temperature loggers reveal that summertime peaks remain within historical ranges, and the restoration added shading vegetation along several reaches. The paradox persists: a hallmark environmental metric—clarity—improves in a way that should, if anything, benefit sight-feeding trout, while those very trout become scarcer.

Restoration projects often involve multiple interventions, and the county's effort was no exception: while sediment was reduced, so were certain nutrient inputs once carried by older, less effective wastewater treatment. The immediate visual result is water that looks "healthier," yet the food web may respond in counterintuitive ways. Aquatic invertebrates that trout consume—mayfly, stonefly, and caddisfly larvae—depend on organic matter and nutrients that fuel algal growth and biofilm production. If the base of the food chain contracts, trout may face calorie deficits even in crystal-clear pools. Thus, to make sense of both the clarity gains and the trout decline, one must look beyond visibility alone and consider the energetic underpinnings of the river's ecology.