Changes in seed abundance driven by overgrazing and rainfall variability trigger variations in the diet of a small harvester ant

Abstract

Some species are severely affected by global change, whereas others can survive or even thrive under the same circumstances. Species with more flexible behaviours can better cope with changes in the environment than those with more stereotyped behaviours; therefore, knowing the limits of behavioural flexibility is critical to predict their responses to global change. Livestock grazing is the most widespread land use in deserts and a major driver of vegetation change. In turn, unpredictable and sporadic rainfall can increase the negative impact of grazing. The interaction of both factors is more relevant nowadays as a consequence of climate change. In the central Monte desert, cattle grazing and inter-annual variations in rainfall reduce the abundance of seeds preferentially consumed by granivorous animals. There we analysed the dietary flexibility of the specialist harvester ant Pheidole spininodis in response to inter-annual variations in seed availability during four consecutive years at sites with contrasting grazing pressure. Pheidole spininodis expanded its diet by including other items and decreasing seed consumption as seed availability decreased over the years, but this response did not differ between grazing conditions, probably because the drastic reduction in seed abundance at the regional scale blurred the contrast between grazed and ungrazed sites. Our results highlight the potential dietary flexibility of a specialist species that, under adverse conditions, can modify its diet. Future studies should investigate whether these changes in the diet of P. spininodis in contexts of low resource availability may have negative effects on its survival, as previously reported for other granivorous species.