  In most circumstances, probability sampling is the only way to ensure
unbiased inference about population quantities where a complete census is not
possible. As we enter the era of 'big data', however, nonprobability samples,
whose sampling mechanisms are unknown, are undergoing a renaissance. We explain
why the use of nonprobability samples can lead to spurious conclusions, and why
seemingly large nonprobability samples can be (effectively) very small. We also
review some recent controversies surrounding the use of nonprobability samples
in biodiversity monitoring. These points notwithstanding, we argue that
nonprobability samples can be useful, provided that their limitations are
assessed, mitigated where possible and clearly communicated. Ecologists can
learn much from other disciplines on each of these fronts.
