Earlier springs translate into shifts in nutritional availability
Compared with terrestrial insects, aquatic insects are rich in omega-3 long-chain polyunsaturated fatty acids. We show that aquatic insect phenology is advancing more rapidly than bird phenology and birds that get fatty acids from aquatic insects may experience nutritional phenological mismatches.
From spring flowers blooming when snow used to cover the ground to migratory birds arriving on their breeding grounds and laying their eggs earlier, phenological advancements are one of the most consistent and widespread responses of organisms to increasing global temperatures. Numerous studies have documented these shifts in timing, often finding that organisms at lower trophic levels, such as plants or insects, are advancing faster than those at higher trophic levels, creating mismatches between consumers and their resources. Though we know that consumers need a diversity of nutrients like vitamins, fatty acids, and minerals as well as energy, past studies looking at phenological mismatches have typically focused on the timing of resources in terms of biomass or energy, rather than on the timing of specific nutrient availability.
In our recent study in Current Biology, we wanted to know if consumer demand for and the seasonal availability of specific nutrients were becoming mismatched due to climate change. We asked how the timing of emergent aquatic insects (i.e., those with a larval stage in water), terrestrial insects, and insectivorous birds had shifted from the late 1980s to the mid 2010’s. We answered this by taking advantage of a massive dataset of nearly 30 years of daily insect biomass and composition collected by one of the senior authors (with the help of countless Cornell University undergraduates) in Ithaca, NY as well as publicly available citizen science-generated data on local bird breeding from Nestwatch. What we found is that while everyone is advancing, insects are advancing almost twice as rapidly as birds. We then asked how changes in insect phenology changed the phenology of nutrients available for insect-eating birds to feed their chicks.
Specifically, we focused on the availability of omega-3 long-chain polyunsaturated fatty acids or n-3 LCPUFA. These fatty acids are important for many animals, including humans, and play numerous critical roles in metabolism, especially in the immune, cardiovascular, and nervous systems. Whether you’ve eaten salmon or chia seeds as superfoods, n-3 LCPUFA are the reason why such foods are considered so super. While humans and a number of other animals are technically capable of taking a DIY approach to getting some of their n-3 LCPUFA, by synthesizing these fatty acids from their shorter-chain precursors, it is typically much more efficient to get n-3 LCPUFA by eating them.
Part of the reason that we chose to concentrate on n-3 LCPUFA in our study because we previously found that these fats were beneficial for the chicks of several of the insect-eating birds species that we included in our study. In past studies, we found that Tree Swallow (Tachycineta bicolor) and Eastern Phoebe (Sayornis phoebe) chicks put on more weight in experimental settings when we fed them controlled diets that contained more n-3 LCPUFA compared to diets containing more of the n-3 LCPUFA precursor. We also found that wild Tree Swallow chicks were not very good at making n-3 LCPUFA from precursors.
The other reason that we focused on n-3 LCPUFA is that there is a fundamental dichotomy in the available of these fats between aquatic and terrestrial ecosystems stemming from differences in the fatty acid composition of primary producers: algae in both freshwater and marine systems are often rich in n-3 LCPUFA whereas most terrestrial plants contain little to none n-3 LCPUFA. Aquatic and terrestrial insects also reflect these differences and we found that emergent aquatic insects were the dominant source of n-3 LCPUFA for insectivorous birds. Consequently, we found that the phenology of n-3 LCPUFA, our nutrients of interest, was driven by emergent aquatic insect phenology. Thus, while both aquatic and terrestrial insects were advancing their phenology, the advancement that mattered most in terms of n-3 LCPUFA availability was that of aquatic insect phenology.
Importantly, like previous studies in other riparian ecosystems, we found that emergent aquatic insects peaked in availability during the spring, when many birds typically fed their young insects, whereas even with phenological advancement, terrestrial insects continued to be available throughout the summer. This means that insectivorous birds that can’t keep pace with aquatic insects may still be able to find food for their young, but that the food that is available later in the season doesn’t contain nearly as much as early foods. In particular, we found that insectivorous birds that breed earlier like Eastern Bluebirds (Sialis sialis) now likely have more n-3 LCPUFA-rich insects to feed their young compared to the 1980’s while later breeders like Purple Martins (Progne subis) have fewer aquatic insects to feed their chicks. Interestingly, earlier breeding birds like Bluebirds have had relatively stable populations in our study area over the past several decades whereas later breeding species such as Martins have been declining. While our study wasn’t designed to directly answer whether or not nutritional phenological mismatches are responsible for these declines, it does suggest that mismatches between bird demand for nutrients and insect nutritional phenology probably aren’t helping the situation.
- How similar are rates of aquatic insect, terrestrial insect, and bird phenological advancement at larger spatial scales? Where are aquatic and terrestrial insects advancing at similar rates and where are they most offset? Are birds and other insectivores generally keeping pace with aquatic insects, with terrestrial insects, or with neither?
- How is climate change altering the nutritional composition of aquatic insects either through shifts in insect species composition or changes in the nutritional composition of individual species? For example, we know that aquatic insects from different feeding guilds and at different trophic levels vary in their fatty acid composition. Therefore, shifts in species composition due to climate change may dramatically alter nutritional composition, especially if these shifts are in terms of feeding roles and/or trophic length. We also know that the n-3 LC-PUFA content of algae at the base of freshwater food webs tend to decrease with warmer temperatures. Thus, even without species turnover, the nutritional composition of primary producers as well as higher order consumers like insects is likely shifting with warming.
- How common are nutritional phenological mismatches in other ecosystems? We know that phenological mismatches are occurring in many food webs and we also know that many consumers have very specific nutritional requirements. It’s time to start looking at these things together to understand if key nutrients are still available when consumers need them.