#41: Part 1, Mechanisms of change in fish populations: Patterns in common trend monitoring data
Dan Isaak
Are we sitting on some of the answers?
Hi Everyone,
So this time we’re going to start a new mini-module looking at the various mechanisms by which fish populations may adjust to climate change. After all, those big distributional changes & fish-sky falling scenarios projected for this century (Blog #33) must ultimately be rooted in myriad smaller scale processes that we’ll need to understand to make accurate predictions. As we take this next step, the potential exists for big synergies to occur between much of what we already know about fish populations at relatively limited spatial and temporal scales and the meso-scale models that downscale climate change effects to stream temperature (Blogs #7, #40) and hydrology (Blog #20). As we make this next set of linkages, we’ll be putting the final pieces in place to create a system for translating the global to the local (graphic 1).
But how to accomplish this linkage? Here, it may be useful to re-examine & expand the traditional view of population dynamics and their relationship to environmental relationships. In the traditional view (at least to my simple brain), a population’s size, it’s inter-generational growth rate (lambda), and long-term persistence probability is determined by the sum of the BIDE processes (birth + death + emigration + immigration). Each of those processes is, in turn, related to the environment as dictated by the size, quality, distribution, and temporal variability of suitable habitats. And that last bit—temporal variability—is key, because anyone that hunts, fishes, or otherwise closely tracks population abundances in their favorite haunts knows there are good years (lots of juvenile recruitment and growth) and bad years (not much of either) for populations. Get a string of consecutive good years and there seem to be critters everywhere, or consecutive bad years and they’re nowhere.
So the temporal variability of habitat conditions at a site needs to mesh with a species biology & life cycle requirements for things to work. Habitats where these things come together consistently tend to support larger, or more productive, populations; habitats where things mesh poorly tend to have smaller or less productive populations. Moreover, climate, as manifest by short- (intra- and interannual) and long-term (decadal and century) variability in stream temperature and flow conditions, is a ubiquitous & strong determinant of temporal variation in habitat conditions, so we’d expect to see relationships with the BIDE processes if we look closely.
A nice paper that articulates these concepts more coherently than I is Jackson and colleagues (Available here: http://www.cakex.org/sites/default/files/PNAS.pdf) titled “Ecology & the rachet of events, climate variability, niche dimensions, and species distributions” I think “rachet” is a perfect term for thinking about the incremental changes that occur through time as climate bounces around and population dynamics adjust accordingly (graphic 2). Another good one is Glenn and colleagues (attached) that uses time-series monitoring data for owl populations to examine the effects of short-term climatic variation on population parameters like lambda, survival, and recruitment.
And if it’s the case that useful information regarding bioclimatic linkages can be extracted from time-series monitoring data, then we are indeed sitting on a goldmine from whence a better understanding of climate effects on fishes may be developed because there are many, many sites where annual abundance & species composition monitoring has been done for years. As fishy illustration of this fact, we’ll highlight two examples by our European colleagues. In the first study by Clews and colleagues (attached), monitoring data on the density of brown trout and Atlantic salmon across the Welsh River Wye were compiled over a 20 year period. The authors then described relationships between the abundance of juveniles and a suite of the prior year’s climatic conditions (graphic 3). Generally speaking, juvenile abundances were higher when the preceding year’s climate was cool & wet, and abundances were lower when it was warm and dry. That being the case, one could surmise that if climate projections for the River Wye indicate warmer/drier summers will become more common in 50 years, then these species will fare poorly. Although more years of biological monitoring data are always good, an attractive feature of this approach is that it doesn’t require decades of data to yield useful results. As few as several years (i.e., 4 - 5) of monitoring might be sufficient to provide insights regarding future population sensitivity if those years encompassed a wide range of climatic conditions.
In the second fishy example, Almodovar and colleagues (attached) estimated the abundance of brown trout within 19 reaches each year during a 12 year period from 1993-2004 in northern Spain (graphic 4). Concomitant with a long-term warming trend across the region, an overall decline in the abundance of brown trout was observed. There was important spatial variation in this trend, however. Populations in reaches at the lowest elevations and warmest temperatures declined rapidly whereas those at high elevations and cold temperatures showed no trend. Thus, the monitoring data from cold streams served as an experimental control, and the study provides nice empirical support for the general response pattern we’d expect in populations of ectothermic organisms arrayed along a temperature gradient subject to climate warming. There are several other aspects to this comprehensive study that are worth checking out, including some evidence of warm-water species expanding their distributions upstream, use of the brown trout data to validate a bioclimatic model that is subsequently used to make future projections, and means of assessing and discounting potential confounding factors like changes in angler pressure or habitat quality unrelated to temperature. It’s worth a detailed read.
So that’s it for now. In several subsequent blogs we’ll continue this theme working through the various means by which climate change may affect local habitat conditions and patterns and processes in fish populations. Once we’ve forged that last link in our global fish system, we’ll be ready to start the penultimate Climate-Aquatics management module.
Until next time, best regards,
Dan