And now a new season begins, the pure November season of the russet earth and withered leaf and bare twigs and hoary withered goldenrods, etc.
But one angler’s minor annoyance is another critter’s feast. Massive quantities of leaves find their way into streams this time of year, and the dinner bell has rung for the shredders - the name we give to the “functional feeding group” of leaf-chomping insects, most of which are stoneflies, caddis flies, or craneflies.
Many of these hatch from egg to larval stage, appetites ready, just as the leaves arrive. This fortuitous timing ensures a plentiful food supply for the newborns,
and they’re off to the races; eating and growing their way towards eventual adulthood, fresh air, flight and sex. Who knows? Some of them may someday come to rest on the very trees whose former leaves nourished them through their youth.
There’s some concern that climate change may put a wrinkle in this cycle – that a disruption in timing of either the seasonal leaf fall or insect egg hatching (or both), could separate the young larvae from their food supply. There’s a growing body of research documenting timing changes for a variety of plant and animal communities in the spring: e.g. in bird migrations, buds bursting, emergence dates for several mayflies and other insects targeted by anglers. These changes may be messing up the trophic balance. Those eating and those being eaten are not meeting up like they used to (ditto for pollination and other useful life functions). Bad news for balanced ecosystems.
Less is known about fall phenology shifts. A brief and selective ramble through the literature reveals these research nuggets:
- Some tree species appear to be holding on to their leaves longer than heretofore; others are exhibiting no noticeable change.
- Invertebrate larvae commonly have a thermal comfort zone. Up to a point, warmer temperatures induce growth spurts; exceed this, and growth (and survival) rates diminish. Warmer winter stream temperatures are known to expedite some spring-summer hatches. In summer, the thermal influence is more ambiguous. Many species hunker down in warm and low water conditions by going into diapause (“quiescent periods in a resistant life stage for avoiding inclement periods of excessively high or low temperatures”). Eggs are one such diapause stage. Less commonly, diapause can also occur larval and pupal stages. Will diapause last longer with warmer summers? For which species?
- For a number of species - plant and animal – phenology governed more by photoperiod (amount of light and dark each day) than by temperature. As yet, we have an imperfect knowledge of which species fall into either camp.
To add one more wrinkle, studies have shown that several invertebrate species exhibit preferential feeding on different tree species. For instance, hickory and white ash seem tastier – or more nutritious – than red oak or American beech.
Interesting findings all, but it’s something of a puzzle to piece them all together. The picture of coming climate change impacts to trout streams is only slowly coming into focus. It is becoming clear that there’s a lot more to climate change than the headline grabbers of heat waves, storm surges and bad weather. It behooves us to be observant, to compare notes with researchers in other disciplines.
And to keep fighting the good fight. It seems like every week, someone’s discovered a new climate impact we hadn’t known before. With every new bit of bad news, the temptation is to give in to doom and gloom. But that’s a losing strategy. Sure, we’ve dealt ourselves a bad hand, but that’s no reason to quit the game. We might as well recognize that the fight against climate change is a war of attrition, and that much of the battle involves performing triage on our trout streams. We’ll lose some waters, we’ll save some others, and restore a few more.
Don’t give up the fight, and don’t give up on the fishing. Treasure every moment astream; every leaf, bug, fish we have the good fortune to come across.