Sources of asynchrony in a yucca aphid (Aphis yuccae) metapopulation

Background/Question/Methods Synchrony of population dynamics within a metapopulation can decrease metapopulation persistence. If populations decline synchronously there is an increased probability of synchronous population collapse and a decreased probability of a rescue effect, as declining populations are a source of fewer dispersing individuals. I followed populations of aphids Aphis yuccae, ladybugs (Coccinella septempuncuata, Hyppodamia convergens), and ants on inflorescences of 107 Yucca glauca plants for the duration of the flowering season. The spatial location of all flowering yuccas within the 0.75-acre study was determined with 15cm accuracy using high definition Treble GPS. The study site was located in a short grass prairie in the foothills region of central Colorado.I report on the synchrony among aphid populations between pairs of nearest-neighbor populations. Synchrony was measured as a correlation coefficient of log-transformed aphid population counts. Only unique nearest-neighbor pairs were included in the analysis. Results/Conclusions The average correlation coefficient among the nearest-neighbor populations was r=0.205 ranging from r=-0.790 to r= +0.929, however the distribution was strongly bi-modal with the first mode at r=-0.150 and second mode at r=0.50. The strength of the correlation weakly increased with increasing distance between the nearest neighbors (R2=0.053, p=0.062, n=67, linear regression). The strength of the correlation between the nearest-neighbor populations decreased relatively more strongly with increasing difference between days of colonization of the two populations (R2=0.37, p<0.0005, n=67, linear regression). When populations were colonized on the same day the average between-population correlation was r=0.410. The effect of distance and difference between colonization days interacted with each other. For plants colonized on the same day the strength of the between-population correlation was unrelated to the between-population distance (R2=0.006, p=0.778, n=16, linear regression). On the other hand, for plants colonized on different days the strength of between-population correlation increased with increasing distance between the populations (R2=0.09, p=0.033, n=51, linear regression). The data indicate that there are relatively strong endogenous dynamics that are very similar at all population sites --- populations that are colonized at the same day have synchronous dynamics irrespective of between-population distances. The data also indicate a weak source of local asynchrony --- populations that are colonized on different days have weaker synchrony if they are closer to each other. Possible source of this decreased synchrony may be coccinelid beetles, which are more prevalent on aphid populations that are closer to each other.