Are the spatial and temporal scales biologically meaningful?
Paz-Vinas et al. focus their critique on our time series analyses. Our
analysis does conflate space and time to a certain extent, although we
used the smallest possible spatial scale (1000 km) with sufficient data
to test our hypothesis. We explain this (page 58) and make careful
statements not to over-emphasize these results (i.e., no consistent
effect of humans on animal IGD).
Whether 1000 km exceeds species’ dispersal capabilities is
species-specific. Geographic and anthropogenic barriers can further
fragment species’ ranges. Because there is no ‘right’ resolution, we
introduced a multi-scale approach to assess IGD patterns at the global
extent. Ideally, species traits, ranges and geological features should
be considered in future studies.
Paz-Vinas et al. selected fish in their comment, which in general have
unique dispersal abilities, and are the most ‘noisy’ taxa in our study
as a result of watershed constraints. We did not account for watershed
structure which is a shortcoming of our analysis, although fish
populations constitute only 11.4% of our temporal dataset. The purely
spatial analysis, that is the focus of our paper, assesses IGD patterns
at smaller spatial scales (sequences < 10 km apart) where the
risk of pooling sequences from isolated populations with divergent land
use is reduced. As we have discussed in our Appendix, there is little to
no spatial variation among sequences in the majority of populations:
“the mean great circle distance among sequences was 0 for
~50% of species in our dataset, and usually <
100 km for other populations (Fig. S1) ”. Thus, the “daisy-chaining”
issue is irrelevant for the majority of our analyses. We would have
liked Paz-Vinas et al. to acknowledge that the issue they raise only
applies to our time series analysis. Future macrogenetic studies should
be aware of the consequences of aggregating sequences into populations.
Regardless of the marker(s) chosen, or method of forming biologically
meaningful populations, Paz-Vinas et al.’s comment does not indicate
either way (positive or negative) how we purportedly over-interpret and
misinform readers on the effects of humans on IGD. Theory predicts
positive and negative effects of humans on IGD, so inconsistent trends
are not surprising.
We agree that macrogenetics will progress faster when researchers align
behind a set of best practices for data, analyses, and the communication
of uncertainties in our understanding of the patterns arising from data
limitations. We look forward to seeing future analyses as the field
tackles the problem of assessing human impacts on genetic diversity.