This study was initiated in the summer of
2013 as part of an ongoing M. Sc. project, and will undertake assessing the
effects of natural disturbance and fire management practices on the boreal lake
islands of northern Saskatchewan. This work will study how habitat fragmentation and disturbance history interact to affect underlying patterns of biodiversity, and inform expectations of what constitutes healthy
biodiversity and ecological integrity in boreal lake environments.
Background
According to the Theory of Island Biogeography, species diversity is highest on islands that are large and nearest mainland (MacArthur & Wilson 1967). However, due to their size, large islands are struck by lightning more often and burn more frequently than small islands (Wardle et al. 1997). Classic island biogeography does not consider the effects of wildfire on island diversity, or the role of wildfire in structuring boreal island communities.
Low productivity and turnover rates in the boreal forest mean that if wildfire occurs too often, burned areas will be dominated by rapidly reproducing species (r-selected). Conversely, if wildfire occurs infrequently, conditions will favor species that are good competitors for limited resources (K-selected). Disturbance regimes that are neither too rare nor too frequent allow maximum diversity because they favor multiple life-history strategies (see Intermediate Disturbance Hypothesis, Connell 1978; Wilkinson 1999). If fire frequency is higher on large islands, local extinction might occur, and thus result in lower diversity on large islands.
According to the Theory of Island Biogeography, species diversity is highest on islands that are large and nearest mainland (MacArthur & Wilson 1967). However, due to their size, large islands are struck by lightning more often and burn more frequently than small islands (Wardle et al. 1997). Classic island biogeography does not consider the effects of wildfire on island diversity, or the role of wildfire in structuring boreal island communities.
Low productivity and turnover rates in the boreal forest mean that if wildfire occurs too often, burned areas will be dominated by rapidly reproducing species (r-selected). Conversely, if wildfire occurs infrequently, conditions will favor species that are good competitors for limited resources (K-selected). Disturbance regimes that are neither too rare nor too frequent allow maximum diversity because they favor multiple life-history strategies (see Intermediate Disturbance Hypothesis, Connell 1978; Wilkinson 1999). If fire frequency is higher on large islands, local extinction might occur, and thus result in lower diversity on large islands.
If island size is influencing the frequency of disturbance, this will have important consequences for the biota. In the case of beetle diversity, the role of wildfire may be indirect. For example, wildfire may determine the vegetation structure and composition on islands, which will in turn influence habitat availability for beetles. If large islands are burning more frequently, we could expect more “early” successional plant species on large islands. Conversely, small islands should have more “late” successional species because of the lower frequency of wildfire.
In a study of boreal islands (<0.1 vs. 1-10 ha) in Sweden, Wardle (2002) found higher plant diversity on small islands and attributed it to higher fire frequency on large islands (every 375 vs 1750 yrs, Wardle et al. 1997). According to the fire database compiled by Stocks et al. (2002), the region of Lac La Ronge burns on average every 66-100 years. However, Stocks et al. (2002) considered large fires (>200 ha) on mainland and did not include fire intervals for islands or account for the effects of island size.
The positive relationship between species and area is well established in Ecology and is depicted as the "Classic" line in this conceptual graph. Generally, an increase in area is accompanied by an increase in species diversity. However, given the higher frequency of wildfire on large islands, wildfire may alter the typical species-area relationship. If wildfire is causing local extinction on large islands, we may observe a loss in diversity as area increases, as depicted by the "Expected" line in this conceptual graph.
Objectives
1. Quantify carabid beetle diversity on the mainland and islands of Lac La Ronge.
2. Characterize the vegetation structure and composition of sites where carabids were collected.
3. Calculate the average fire interval for the islands of Lac La Ronge.
For the purposes and scope of this project, I will be focusing on objectives 1 and 2. The information required to calculate fire interval will be collected in the coming field season, however, if wildfire frequency varies with islands size, we should see differences in the vegetation structure and composition between large and small islands (see Wardle 2002).
1. Quantify carabid beetle diversity on the mainland and islands of Lac La Ronge.
2. Characterize the vegetation structure and composition of sites where carabids were collected.
3. Calculate the average fire interval for the islands of Lac La Ronge.
For the purposes and scope of this project, I will be focusing on objectives 1 and 2. The information required to calculate fire interval will be collected in the coming field season, however, if wildfire frequency varies with islands size, we should see differences in the vegetation structure and composition between large and small islands (see Wardle 2002).
Carabid beetles are good candidates for island studies because they are diverse (349 species in Saskatchewan; Bousquet 2013), show a wide range of habitat preferences, and vary in dispersal capability based on their wing morphology (Den Boer 1970).
The presence of full wings (macropterous) or reduced/absent wing (brachypterous) varies by species and even within species (dimorphic; Den Boer 1970). Furthermore, species with different wing morphologies can be found in the same location, allowing for comparisons of dispersal capability (Den Boer 1970).
This image illustrates the different wing morphologies of the wing-dimorphic carabid Agonum retractum. On the left, the wing is reduced (red circle) or "brachypterous". On the right is another A. retractum with fully developed wings or "macropterous". Recording the wing morphology of carabids collected at my sites might provide clues as to whether carabids are moving between islands.
The presence of full wings (macropterous) or reduced/absent wing (brachypterous) varies by species and even within species (dimorphic; Den Boer 1970). Furthermore, species with different wing morphologies can be found in the same location, allowing for comparisons of dispersal capability (Den Boer 1970).
This image illustrates the different wing morphologies of the wing-dimorphic carabid Agonum retractum. On the left, the wing is reduced (red circle) or "brachypterous". On the right is another A. retractum with fully developed wings or "macropterous". Recording the wing morphology of carabids collected at my sites might provide clues as to whether carabids are moving between islands.
The proportion of macropterous (%M) individuals within a population has also been used as an indicator of population age (Zalewski 2004). In Pterostrichus melanarius, macroptery is much higher among newly established populations but shifts over time to the dominant, less mobile, brachypterous form. On lake islands, %M is higher on small islands because of greater extinction rates (Zalewski 2004), as predicted by island biogeography theory. In the current study, a higher %M among dimorphic species might be an indicator of younger populations or instable, possibly as a result of extinction caused by fire.
Disclaimer
This website was created as part a class project for RenR 690 at the University of Alberta. Any results discussed are preliminary and may be based on modified data sets. Please contact the author for more information.
This website was created as part a class project for RenR 690 at the University of Alberta. Any results discussed are preliminary and may be based on modified data sets. Please contact the author for more information.